This PDF file contains the front matter associated with SPIE Proceedings Volume 11056, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Time averaging technique applied to different interferometric methods is one of the most commonly used measurement technique in vibration testing. The information on amplitude of vibration is encoded in so called fringe envelope function described by the J₀ Bessel function in case of harmonic motion. In the presented paper the author proposes novel solution for amplitude distribution evaluation. It is based on the analysis of the Bessel pattern modulation distribution. For modulation evaluation and Directional Spatial Carrier Phase Shifting (DSCPS) method are used. Conducted error analysis prove usefulness of the proposed approach.

The optical interferometry is a non-contact dimensional measurement technique which is capable of ultra-high-precision measurements. Fundamentally, it provides the optical path difference instead of the geometrical path difference. For thickness measurements of glass panels, the physical thickness can be extracted from the optical thickness when the refractive index of the glass panel is precisely given. Otherwise, the precision of an optical interferometer cannot be maintained owing to errors in the refractive index. To overcome this problem, several studies based on optical interferometry for simultaneously measuring the physical thickness and refractive index have been proposed and realized. For in-line inspections, the vibration problem becomes serious with increased dimensions of thin glass panels. When delivering large glass panels, a large amount of vibration is inevitable. In this paper, a transmission-type spectral-domain interferometer for determining physical thicknesses and group refractive indices of large glass panel, which can be operated even under vibration conditions is introduced. For an in-line inspection, large tilt angles of glass panels are created by swing motion when delivering these glass panels at a high-speed. Even if the proposed method determines physical thickness values successfully under the severe vibration condition used here, the measurement error caused by the vibration effect should be investigated and analyzed to correct the measured thickness values. To do this, a theoretical analysis of the error was performed by mathematical modeling. Moreover, the error of the physical thickness was experimentally analyzed at various tilt angles of the large glass panel. The uncertainty was evaluated to be about 436 nm based on the results of these investigations.

Time averaging interferometry is one of the techniques that allows to investigate a dynamic behaviour of MEMS elements. The information of the maximum amplitude of vibration is encoded in the argument of Bessel function. Many different approaches enable evaluation of this value. Due to the fact that accuracy of the result depends on the quality of the input data, Bessel function of a good quality must be calculated first. Wide range of modulation distribution calculation methods enable to obtain absolute or square value of J₀. These results are more difficult in processing than pure Bessel function because of gradient function discontinuity or poor SNR. For ensuring smaller errors one may normalize absolute values of J₀ by the sign of the function. In this paper, several different algorithms for determining the sign of Bessel function were investigated and compared. For each approach accuracy of the method was calculated. In the end, the best solution was found.

High demand of optical inspection is increased to guarantee manufacture and product quality in industries. To overcome limitations of the manual defect inspection, machine vision inspection is needed to efficiently and accurately screen the undesired defects on various products. Recently, the transparent substrate is becoming widely used for manufacturing optics and electronics products. For high-grade transparent substrates, development of machine vision inspection has increased its importance for inspecting defects after production. To perform machine vision inspection for the transparent substrate, the exposure procedure and analysis of the capturing image are critical challenges due to its properties of reflection and transparency. However, conventional machine vision systems are performed for optical inspection based on two-dimensional (2D) intensity images from the camera-based photography without phase and depth information, and may decrease inspection accuracy as well as defect classification. Conversely, instead of the 2D intensity image by camera-based photography with complicated algorithms and time-consuming computation, digital holography is a novel three-dimensional (3D) imaging technique to rapidly access the whole wavefront information of the target sample for optical inspection and complex defect analysis. In this study, we propose digital holographic imaging of transparent target sample for optical inspection in learning-based pattern classification, which a novel complex defect inspection model is presented for multiple defects identification of the transparent substrate based on 3D diffraction characteristics and machine learning algorithm. Both theoretical and experimental results will be presented and analyzed to verify the effective inspection and high accuracy.

In digital holography, the field of view (FOV) and lateral resolution are limited by the pixel pitch and sensor dimensions, respectively. A large numerical aperture can be synthesized to increase the FOV and spatial resolution by coherently combining low resolution holograms obtained for different illumination and/or observation directions. This is known as Synthetic Aperture Interferometry (SAI) and in this work we describe the design, construction, calibration and testing of high numerical aperture compact coherent imagers (CI) which constitute the optical building block of a multi-sensor SAI array. The CIs consist of a photodetector array, a highly divergent reference beam close to it and an aperture that acts as a spatial filter to prevent aliasing of the digital holograms. We explore different optical designs to produce a highly divergent reference beam close to the sensor, including bulk optics, micro-optics, and ion beam milled optical fibres. An optimization approach is used to characterize the reference wavefront for accurate digital reconstructions of the scattered field first at the aperture plane and then at the object plane. The performance of a compact CI is demonstrated by reconstructing an object 76 mm wide at 80 mm from the sensor, which corresponds to a numerical aperture NA>0.5.

The high-speed topography of both specular reflective and diffuse reflective surfaces with one single instrument represents a major challenge in optical metrology. Surface metrology techniques exist, which can cope with both types of surfaces, such as white light interference microscopy (WLI) and confocal microscopy (CM). However, the measurement is very time-consuming ranging from multiple seconds to minutes depending on the field of view and resolution. This is due to the fact, that WLI and CM are highly informative, revealing all surface parameters such as roughness, waviness, and shape. Structured illumination, on the other hand, recovers the shape only, but is restricted to the inspection of rough scattering surfaces. Moreover, a projection device, often implemented via a spatial light modulator, and a certain angle between projection and observation direction are necessary to enable the application of triangulation. In that manner, it is not only limited by the type of surface but also is quite bulky and heavy. Dual wavelength holography can overcome the aforementioned shortcomings. It can be applied on rough scattering and specular reflective surfaces. In addition, the measurement range and the axial resolution can be adjusted via the choice of the dual wavelength pairs. Moreover, a flexible and light weight setup can be enabled, without the necessity to employ imaging lenses due to the recovery of complex information (amplitude and phase) and numerically refocusing the recorded wavefield to the object plane. In the last years, Vertical Cavity Surface Emitting Lasers (VCSEL) have attained more and more attraction. Commonly they are used in the form of multimode VCSEL arrays and have for instance been applied for face recognition in smartphones. However, single mode VCSELs do likewise exist. They offer excellent coherent properties, with a coherence length of a few tens of cm, an isotropic angular radiation profile and dimensions < mm, allowing a very compact and lightweight setup arrangement. Moreover, the wavelength can be tuned within a few nm via changing the current applied to the VCSEL or the temperature, which makes them extremely attractive for dual wavelength optical metrology. The only downside of single mode VCSEL is the limited power of not more than a few mW, which restricts the investigated field of view to a few mms. In this paper, we describe the application of VCSEL for the dual wavelength digital holography. The surfaces investigated range from diffuse scattering to specular reflective. At first, the spectral response of the VCSEL is determined in a calibration process. A wavemeter with sub-Angstrom spectral resolution has been employed to investigated stability and repeatability of the wavelength. For the sake of a compact and lightweight setup and in order to take advantage of the little power of the VCSEL, an off-axis arrangement has been chosen. The shape measurements are taking on referenced surfaces. The measurement system, the evaluation of the data and an error analysis based on the referenced surfaces will be shown and discussed in this paper.

Digital holography interferometry (DHI) and digital holographic interference microscopy (DHIM) are tools that provide whole field information of the wavefront interacting with the object. This imaging modality can be an ideal tool for quantitative phase imaging of technical objects such as semiconductor samples. The phase information numerically reconstructed from the holograms can lead to extraction of the thickness/height of the sample. Usually DHI and DHIM are used in transmission mode for determination of optical thickness of the sample under investigation. In reflection mode this imaging techniques provide height of the sample. For the semiconductor industry determination of height/thickness of object structures as well as the quantification of defects in the object structures is an important issue. The thickness and defect determination can be that of semiconductor thin films, micro/nano-pillars, LED displays, liquid crystal panels, the cover glasses used for protection of these structures etc. Digital holographic interferometric method (both reflection and transmission) can be used to design devices that can act as a fast, single shot technique for quantitative phase imaging of such samples. Such devices can provide more information about the sample compared to intensity based measurement systems. Also compact the digital holographic interferometric systems can be deployed in the manufacturing line of such devices to provide real time information. We are involved in the design and development of digital holographic devices for inspection of semi-conductor wafers, thin films, displays and glass plates covering such samples. We have implemented digital holographic devices in the lens-less mode (in the case of DHI) and also with the use of an imaging lens (in the case of DHIM) both in reflection and transmission mode. DHI provides field of view equivalent to the sensor size, while DHIM technique was implemented with different magnifications, thereby providing varying field of views of the sample. Also in the case of DHI a propagation from the hologram plane (the plane at which the digital array for recording the hologram was situated) to the best focus plane (object plane) was realized by numerical implementation of diffraction integral. In DHIM, the digital array used for recording the hologram was at the image plane of the magnifying/de-magnifying lens. So the whole numerical reconstruction process reduced to Fourier fringe analysis, making the technique less computationally exhaustive, fast and quasi real-time. The developed devices were calibrated using known objects and then tested on different samples. The obtained results are found to be encouraging. In this paper, we describe our efforts in design, development and fabrication of digital holographic devices for the inspection of semiconductor samples.

With their high capacity and low cost, magnetic disks remain competitive for digital data storage. The low flying heights of the read/write head in the single nm-range place stringent requirements on the disk substrate topography. Interferometric surface metrology can provide the required topography data. For the measurement of state-of-the-art disk substrates with Angstrom-level waviness, the interferometer noise needs to be extremely low. A robust and cost-effective approach uses an LED as a light source providing temporally and spatially low-coherence illumination. The low temporal coherence leads to axially localized fringes, and clean single-surface fringes and topography maps are obtained even for transparent substrates. The low spatial coherence suppresses coherent noise, or speckle noise, in the fringe intensities and topography maps. Coherent noise cannot be determined by a simple repeatability measurement, since it is stable over some time for a given disk alignment. It does not appear in difference maps, but nevertheless is present in all acquired maps. An upper limit of low-level coherent noise is determined by looking at speckle decorrelation with increasing tilt of the test surface. In this presentation, the coherence issues are discussed together with the characterization of coherent noise and waviness filtering. Disk measurement examples are shown where the data were acquired with OptoFlat, an LED-based interferometer newly developed for the measurement of flat surfaces like disks and wafers.

In the measurement of three-dimensional structure beyond the diffraction limit of the objective lens of the optical system using the speckle interferometry, it was reported that the shape of a measured object can be measured by detecting the change of phase distribution generated by a lateral shift instead of analyzing the focal image of the measured object. In this paper, it is investigated that the three-dimensional shape measurement can be realized by detecting the phase change of only the zeroth-order diffraction light by the lateral shift of the object using experimental results. As a result, it is confirmed that the shape information of the object is included in the zeroth-order diffraction light and that the three-dimensional shape can be measured by using only zeroth-order light without any higher order diffraction lights.

In this work, using our range-resolved interferometry (RRI) signal processing technique, we present a novel approach to multidimensional displacement measurements using only a single optical access port and very simple optical setup. By utilising surface reflections from a stage-mounted moving beamsplitter and two orthogonal stationary reference mirrors, two interferometers for the two Cartesian measurement directions are formed. With RRI, the interferometric phase signals of both interferometers can be independently demodulated, allowing simultaneous measurements of displacement in both dimensions using a single continuous-wave laser diode source and a single photodetector. In this paper, the capabilities of this approach are demonstrated using a proof-of-concept experiment with a multidimensional Piezoelectric stage performing a variety of stage movements. Measurements of displacements over a nominal stage working range of ±50μm are presented, demonstrating independent, simultaneous displacement measurements of two dimensions. The presented measurements show nanometer-level displacement resolutions with typical noise densities of 0.02 nm/√Hz over a 21 kHz bandwidth. It is thought that this approach could offer an interesting alternative to existing interferometric techniques for multi-dimensional metrology, benefiting from both simplicity and cost-effectiveness whilst maintaining the advantages that make optical techniques attractive to scientific and industrial applications.

The need for freeform measurement with sub-nanometer precision is increasing with the demand for freeform optics with only single nanometer or less of figure error and surface roughness in numerous fields. Since there are no method to meet the demand, therefore we developed a non-contact three-dimensional nanoprofiler based on normal vector tracing method with the light straightness and absolute-calibrated goniometers as core concepts. The nanoprofiler achieved the sub-nanometer repeatability at the figure measurement of spherical, aspherical, cylindrical, and patterned-flat mirror. However, our numerical analysis about the systematic errors of nanoprofiler revealed that the mismatch between the recognized and the actual optical path length L, the distance between the detector and the sample in nanoprofiler, causes the assembly and motion errors such as the second-order aberration. Therefore, we developed a tandem white light interferometer which references a high-precision linear encoder as a standard of displacement, in order to measure the physical distance of L absolutely. We expect that the second-order terms of systematic errors of nanoprofiler will be decreased below than 0.1 nm or less by measuring the absolute length of L with the uncertainty of 0.1 μm or less. The first interferometer fringe was obtained for the measuring test.

Many applications of precision interferometry suffer from uncertainties in knowing the values of the air refractive index or the laser wavelength and often require the use of complex setups or costly components to mitigate these effects. In this work, using range-resolved interferometric measurements, we show how differential measurements along a single optical beam can be used to perform displacement measurements with drastically reduced systematic errors. In a proof-ofprinciple experiment, a semi-transparent target mounted on a moving stage is placed centrally between a window and a mirror, forming two air paths around the target that are arranged to be of nominally equal length. The evaluation of the interferometric phase signals at multiple locations along the measurement beam then allows the simultaneous measurement of changes in the length of the two air paths on either side of the target. The difference of the air path length changes yields the desired target displacement measurement, while influences that are common to both air paths such as air refractive index changes or laser wavelength drift are strongly suppressed.

A new demodulation method for sinusoidal frequency/phase modulation (SFM/SPM) interferometers using an artificial harmonic series signal and a phase-locked loop (PLL) is proposed in the paper. Utilizing a laser diode (LD) as the light source and an electric optic modulator (EOM), frequency/phase modulations of the LD light can be achieved by modulating the LD injection current or the driving voltage to the EOM. The SFM/SPM interference signals for the displacement measurement has have the form of cos(msinω_mt+θ), where m, ω_m and φ are a modulation index, a modulation frequency and a phase change due to optical path movement, respectively. Therefore, the SFM/SPM interference signals is composed by a series of harmonics of the modulation frequency. The two adjacent (2k and 2k+1: k is integer) harmonics include the displacement information. By using the two harmonics, a Lissajous diagram can be drawn to obtain the displacements. In the conventional way, the two harmonics can be demodulated by two lock-in amplifiers. In this paper, we propose new demodulation method for SFM/SPM interferometers using an artificial hormonic series signal and the PLL, which are installed in field programable gate array (FPGA), to obtain sub-picometer (μrad) resolution. An artificial hormonic signals with the form of sin(msinω_mt+θ) can be made virtually by the FPGA, where θ is the target phase controlled by the PLL. The output signal from the interferometer will be firstly digitized by the FPGA, and the digitized output signal and the artificial harmonic series signal are multiplied by a mixer, then the mixer output signal is inserted to a low pass filter (LPF). Then the LPF output signal should be -sin(φ-θ)~-(φ-θ) (if (φ-θ) is sufficiently small). In the system, modulation index should be fixed to some value. To set the LPF output signal (-(φ-θ)) to null, the target phase θ of the virtual artificial harmonic signal is controlled by the PLL. In the paper, we discuss the principle, experimental system and results.

Shearography nondestructive inspection (NDI) based on the use of thermal stress in studied in view of detecting defects in composite materials. In particular, we would like to extract the content of information that is present in the deformation due to the evolving deformation during the variation of the stress. For that a good approach is to try to apply processing techniques which are largely applied in thermography NDI for the same purpose. We will discuss the necessity of pre-processing the shearography data to make them compatible with thermography inspired processing techniques. After that we will present the statistical analysis based on principal components, and we will discuss the possibility of deterministic analysis based on the temporal behavior during and after heating.

Speckle shear interferometry, or shearography, has been more and more frequently used in the aerospace and oil and gas industry for in-field nondestructive inspections of flaws in composite materials. Nowadays new applications have emerged demanding the ability to operate in harsher environments, requiring more robust systems to meet this type of application. A recent modified shearography device allows multiple and simultaneous measurements with different shearing directions on a single grabbed image. This work proposes a robust integration algorithm by error minimization to obtain full-field displacement measurement. Simulated images are used to validate the effectiveness of the integration algorithm. Further, experiments are performed on a clamped circular plate with uniform loading. The proposed algorithm leads to a more accurate estimate of defect size measurement in composite materials.

Delamination in carbon fiber reinforced polymer (CFRP) laminated composites is an important problem for industry. Nondestructive inspection (NDI) methods aim at locating such defects. Shearography is a full-field NDI method that could be considered to detect them. Reference plates for assessing performances of NDI methods for detecting delamination use artificial defects of several types introduced in reference CFRP matrices. Although they are standardized for usual ultrasound testing, these artifacts are not necessarily adequate for shearography. We have studied this problem by comparing shearography experiment and simulations by finite element analysis. We show the convergence with experiments on the case of flat bottom hole artifacts. Then we discuss the adequateness of other artifacts through simulations.

This work presents the design and the latest experimental results on the surface strain measurements during an impact event obtained with the EXTREME high-speed shearography instrument. The shearography technique is used in this project to provide a quantitative measurement of the surface strain development at the first moments of the impact event (μs time scale) which may reveal the initiation of the failure mechanisms in composite materials. Experimentally measured surface strain components over the field of view will be used as input and validation data for new numerical and analytical models of the impact response of composites. The new configuration of the shearography instrument realises measurements of the in- and out-of-plane surface strain components to improve coupling with the numerical models. Two viewing directions (shearing interferometers) with a double-frame approach are used to capture the interferograms during the impact. The interferometers realise a double-imaging Mach-Zehnder scheme for the spatial phase-shifting with independent control of the shearing amount and the carrier frequency. The set of technical parameters of the developed shearography instrument makes it one of the most extreme applications of shearography for material characterisation. The framework for this work is the “EXTREME Dynamic Loading – Pushing the Boundaries of Aerospace Composite Material Structures” Horizon 2020 project.

A non-contact optical technique employing dual-wavelength laser speckle is investigated for absolute angle measurement. The approach uses the separation of the speckle patterns formed by two closely spaced illumination wavelengths to determine the angle of a surface. Autocorrelation is performed on a single exposure containing both speckle patterns to find their relative displacement, which is directly related to the absolute surface angle. This absolute angle determination offers an advantage over previous techniques using laser speckle that require a reference image. The underlying theory linking the speckle pattern displacement and the surface angle is presented, along with a proof-of-concept sensor. Experimental results from the sensor confirm the validity of the theory, with measurements demonstrating a mean difference from applied angles of 0.136°.

Nanometre accuracy and resolution metrology over large areas is becoming more and more a necessity for the progress of precision and especially for nano manufacturing. In recent years, the TU Ilmenau has succeeded in developing the scientific-technical basics of new ultra-high precision, so called nanopositioning and nanomeasuring machines. In further development of the first 25 mm machine, known as NMM-1 from SIOS Meßtechnik GmbH, we have developed and built new machines having measuring ranges of 200 mm x 200 mm x 25 mm at a resolution of 20 pm and enable measuring reproducibility of up to 80 pm. This means a relative resolution of 10 decades. The enormous accuracy is only made possible by the consistent application of error-minimum measurement principles, highly accurate interferometric measurement technology in combination with highly developed measurement signal processing and comprehensive error correction algorithms. The probing of the measurement objects can optionally be carried out with the aid of precision optical, interference-optical, tactile or atomic force sensors. A complex 3D measurement uncertainty model is used for error analysis. The high performance could be demonstrated as an example in step height measurements with a reproducibility of only 73 pm. The achieved resolution of 10^-10 also presents new challenges for the frequency stability of the He-Ne lasers used. Here, the approach of direct coupling of the lasers to a phase-stabilized optical frequency comb synchronized with an atomic clock is pursued. The frequency stability is thus limited by the relative stability of the RFreference to better than 4•10^-12 (1s).

Increasing miniaturization requires improved and highly miniaturized optical 3D metrology systems. In this paper a basic measurement principle and a proposed optical design of a highly miniaturized endoscopic spatial confocal point distance sensor are presented. The sensor uses a, to our knowledge new technique called spatial confocal point distance measurement. A special feature of the proposed sensor design is the high degree of miniaturization, through femtosecond direct laser writing and the use of optical fiber bundles, which enable an endoscopic application.

This article presents the evaluation of the optical performance of a new high-frequency focal-distance-modulated confocal point sensor. While maintaining the known advantages of the confocal measurement principle, the sen- sor represents an innovative combination of a fibre-coupled confocal illumination and detection with a tuneable, acoustically driven gradient-index fluid lens (TAG lens) for the modulation of the focus distance and a novel signal processing utilizing a lock-in amplifier. The new arrangement is able to achieve an approximately linear characteristic curve for the optimised feedback control of nano coordinate measuring systems (CMS) in scanning sample mode. This article emphasises the implementation and use of the sensor in nano CMS (NMM-1) and the advantages of the new signal processing. Measurements on different resolution standards are conducted and compared with the focal-distance-modulated sensor and without focus-distance-modulation as conventional confocal microscope (CCM).

Between 50 to 60% of tasks in manufacturing metrology are measurements of cylindrical parts. The tasks are accomplished by roundness testers, which feature high-precision displacement sensors and rotary tables. This paper describes a high-precision method for evaluating roundness and diameter of the cylindrical parts, where a novel laser Doppler sensor and error separation techniques are employed. Nowadays, the most commonly employed sensors in the roundness testers are still contact stylus, which might damage the machined surface. Non-contact capacitive sensors offer sub-nanometer accuracy, but suffer from low lateral resolution. Therefore, we employ a multi-functional optical sensor, the laser Doppler distance sensor with phase evaluation (P-LDD sensor), for the roundness measurement. The P-LDD sensor offers a high lateral resolution, low uncertainty, and also, can determine the diameter simultaneously. Apart from the distance sensor, the rotary table also plays a critical role in the roundness measurement. Its error motion, always leads to systematic deviations. Error separation technique (EST) can separate the spindle error motion from the roundness, thus, cancelling the systematic deviation. Regarding this technique, substantial research effort has been paid, especially into the harmonic suppression problem, which has long been regarded as the dominant factor affecting the measurement accuracy. Nevertheless, even today the ESTs are only sparsely represented in industry and still under research and development. We suspect that a shift of the research focus from the harmonic suppression problem to the measurement uncertainty propagation will yield the foundation for an eventual solution to the measurement accuracy problem, and thus, bring a new paradigm for the EST. Therefore, by means of the stochastic spectral method, we analytically derive the propagation law of the measurement uncertainty within the two-step error separation method (TSM), which is subsequently validated by Monte Carlo simulation. Based on the propagation law, three improved TSMs are further put forward for reducing the uncertainty propagation: the angle-optimized TSM, the hybrid TSM, and the fusion TSM. In the angle-optimized TSM, the angle is optimized to minimize the measurement uncertainty. In the hybrid and the fusion TSMs, two measurements are performed first under different angles; then, the two obtained estimations are hybridized or fused in the harmonic domain, which decreases the measurement uncertainty significantly. Finally, by using the P-LDD sensor and the improved TSMs, test measurements are performed and the results are discussed.

This Conference Presentation, "Advanced methods for optical nanometrology," was recorded at SPIE Optical Metrology 2019 held in Munich, Germany.

We simulate the image generated by a microsphere residing in contact on top of an exposed Blu-ray disk surface, when observed by a conventional microscope objective. While microsphere lenses have been used to focus light beyond the diffraction limit and to produce super-resolution images, the nature of the light-sample interaction is still under debate. Simulations in related articles predict the characteristics of the photonic nanojet (PNJ) formed by the microsphere, but so far, no data has been published on the image formation in the far-field. For our simulations, we use the open source package Angora and the commercial software RSoft FullWave. Both packages implement the Finite Difference Time Domain (FDTD) approach. Angora permits us to accurately simulate microscope imaging at the diffraction limit. The RSoft FullWave is able to record the steady-state complex electrical and magnetic fields for multiple wavelengths inside the simulation domain. A microsphere is simulated residing on top of a dielectric substrate featuring sub-wavelength surface features. The scattered light is recorded at the edges of the simulation domain and is then used in the near-field to far-field transformation. The light in the far field is then refocused using an idealized objective model, to give us the simulated microscope image. Comparisons between the simulated image and experimentally acquired microscope images verify the accuracy of our model, whereas the simulation data predicts the interaction between the PNJ and the imaged sample. This allows us to isolate and quantify the near-field patterns of light that enable super-resolution imaging, which is important when developing new micro-optical focusing structures.

Observation of nanoscale elements through an optical microscope is often restricted by the resolving power of the optical system. Indeed, a white-light microscope allows the visualisation of objects having a size that is only just greater than half of the wavelength of the illumination used, in ideal cases, such as features of MOEMS- based components. In reality, imperfections or misalignment of the optical components makes this resolution limit worse. In 2011, Wang et al. introduced experimentally the phenomenon of two-dimensional super-resolution imaging through a glass microsphere. They showed that microsphere-assisted microscopy distinguishes itself from others by being able to perform label-free and full-field acquisitions. In addition, with only slight modifications of a classical white-light microscope, microsphere-assisted microscopy makes it possible to reach a lateral resolution of a few hundred nanometers. Recently, we successfully demonstrated the label-free combination of microsphere- assisted microscopy with interferometry. This work aims to compare performance of 2D imaging (microsphere- assisted microscopy) with 3D imaging (microsphere-assisted interference microscopy).

Scanning White Light Interferometry is a non-contacting method for three-dimensional (3D) surface characterization that provides Angstrom level vertical resolution and diffraction limited lateral resolution. This lateral resolution can be improved by implementing a photonic nanojet (PNJ) generating structure. The new method - Photonic Nanojet Interferometry (PNI) allows nanometer vertical resolution and lateral resolution better than 100 nm. In this work, a new design of a PNI system is proposed. The PNJ generating structure is a high refractive index microsphere embedded in a polymer material. We model the entire PNI objective in commercial software (Rsoft FullWAVE) and choose optimal parameters for the construct in such a way, that the working distance (FoV) is maximized while the width of the PNJ is kept below the diffraction limit. To test the new system, we imaged the data layer of a recordable Blu-ray Disc (BD). The results show that the proposed interferometer has two times higher magnification and two times larger field of view compared to the previous design featuring a 11 μm melamine formaldehyde micro-sphere. The new design also increases the fringe contrast by 1.5 times and provides easier handling of big samples by allowing them to be scanned.

This contribution considers the application of pulsed LED illumination, synchronized with the exposure gap of a CMOS Bayer camera and an oscillating reference mirror, to record two π/2 phase shifted interferograms for quadrature based phase retrieval. Measurements of a superfine roughness standard in lateral motion are presented, demonstrating the capability of the setup to record the surface topography of moving objects. The application of the interferometric measurement data to calibrate a topography measuring wave front sensor, which is employed to support surface unwrapping is also discussed.

We describe a full field heterodyne interferometry system where the object beam is shifted by a high frequency with respect to the reference beam. The optical path difference between the object and reference beam is encoded in the phase of the envelope of the interference signal which is at the difference frequency. Conventionally, high frequency heterodyne interferometry is restricted to measurement of a single (or a few) point(s) as is the case in displacement measuring interferometers for precision machine tool axis feedback. This is because of the problem of demodulating the signal phase simultaneously for many pixels comprising a full-field image. This problem is overcome here by exploiting the capability of the special pixel structure employed in a Time of Flight (ToF) camera. This structure enables the measurement of the envelope phase of an optical signal at every pixel with respect to an electronic reference at the same frequency. ToF cameras are designed to measure the distance to an object in its field of view by detecting the phase delay, due to time of flight, of reflected light from a modulated source synchronized to the camera. In the described experimental interferometer, a Twyman-Green architecture is used with an acousto-optic modulator to produce interfering beams with a difference frequency of 20MHz. The image detector is a modified ToF camera based on a Texas Instruments OPT8241 sensor. The interferometer directly outputs a wrapped optical phase map with 12-bit resolution (equivalent OPD resolution 0.15 nm) at greater than 50 frames per second with no post-processing. The phase reconstruction is highly insensitive to the reference/object beam intensity ratio or to environmental noise.

This article reports a novel GPU-based 2D digital image correlation system (2D-DIC) overcoming two major limitations of this technique: It measures marker-free, i.e. without sample preparation, and the sampling rate meets the recommendations of ASTM E606. The GPU implementation enables zero-normalized cross correlation (ZNCC) calculation rates of up to 25 kHz for 256 × 256 pixel ROIs. This high-speed image processing system is combined with a high-resolution telecentric lens observing a 10 mm field-of-view, coaxial LED illumination, and a camera acquiring 2040 × 256 pixel images with 1.2 kHz. The optics resolve the microstructure of the surface even of polished cylindrical steel specimen. The displacement uncertainty is below 0.5 μm and the reproducibility in zero-strain tests approximately 10^-5 (1 σ) of the field-of-view. For strain-controlled testing, a minimum of two displacement subsets per image are evaluated for average strain with a sampling rate of 1.2 kHz. Similar to mechanical extensometers, an analogue 0-10V displacement signal serves as a feedback for standard PID controllers. The average latency is below 2 ms allowing for cycle frequencies up to 10 Hz. For strain-field measurement, the number of ROIs limits the frame rate, e.g., the correlation rate of 25 kHz is sufficient to evaluate 10 images per second with 2500 ROIs each. This frame rate is still sufficient to compare the maximum and minimum strain fields within a cycle in real-time, e.g. for crack detection. The result is a marker-free and non-contact DIC sensor suitable for both strain-controlled fatigue testing and real-time full-field strain evaluation.

Focus Variation (FV) has been successfully employed for the three-dimensional measurement of rough surfaces. The technique relies on scanning the sample under inspection across the depth of focus of a high numerical aperture microscope objective, while computing the local contrast of its surface. Only those samples with sufficient texture will provide a usable axial response to compute its height location, limiting the application of Focus Variation to optically rough surfaces. Active illumination Focus Variation (AiFV) introduces an artificial texture on the field diaphragm position which is superimposed onto the surface. The benefit is a usable axial response, even when scanning an optically smooth surface, while minimizing the evaluation window of the focus operator close to the spatial autocorrelation length of the artificial texture. In this paper, we show the development of an Active illumination Focus Variation on an existing confocal microscope using Microdisplay Scanning technology. We analyzed the performance of AiFV on smooth surfaces with low frequency components, such as traceable Step Height or Type B2 roughness standards. Higher frequency samples such as random direction roughness standards or high-resolution targets are affected by the lateral resolution loss inherent on the AiFV technique. In this paper, we compare the lateral resolution limit of AiFV and Confocal Microscopy with the use of a Siemens Star specimen for a range of microscope objectives with numerical apertures from 0.3 to 0.95. Its influence on the computed ISO 25178 parameters on random surfaces is shown.

The functional behavior of sealing surfaces and bearings depends on the texture direction with respect to the axis of rotation of a machined part, known as the twist or lead angle. We present our lead angle measurement solution using interference microscopy, multi-axis staging, and advanced software for determining surface texture direction and cylinder rotation axis.

Areal surface topography measurement is becoming more and more widespread. In this context, increased activities in the field of standardization can be observed in order to ensure comparable measurement results. A current hot topic in the field of standardization is areal calibration, which is going to be defined in the upcoming standard ISO 25178-700. Besides, the metrological characteristics of the ISO 25178-600, representing the properties to be calibrated, as well as the corresponding material measures of the ISO 25178-70, used for imaging, are required for a comprehensive standardization of areal calibration. It can be expected that the application of the calibration guidelines is challenging as no easy-to-implement guidelines for the evaluation routines will be included. In this paper, the standards required for areal calibration are briefly presented and their metrological properties are described. A user-friendly implementation of the algorithms required for the evaluation will be discussed in detail. The basic metrological characteristics to be evaluated are the flatness deviation, measurement noise, the topography fidelity, the amplification coefficients and linearity deviations of all axes, the x/y mapping deviation and the topographic spatial resolution. With Two-Photon laser lithography, all required material measures to calibrate a measurement device according to ISO 25178 can be printed on one single calibration artefact. Based on this universal artefact, user-oriented evaluation routines to determine the aforementioned metrological characteristics are introduced and a software package implementing the algorithms and supporting the user effectively during the holistic calibration of areal surface topography measuring instruments is described.

Characterizing the surface of microlenses by optical profilers has the important advantages of measurement speed, flexibility and automation. Nevertheless, the accuracy of such characterization is limited by error occurring in non-flat measurements. Here, we propose a method that uses multiple measurements of a single reference ball combined with a machine learning algorithm that fits the experimental data to correct the measurements. The success of the method is demonstrated by showing that the residual error after correction reaches 20 nm RMS. Such results extend greatly the quality of microlens characterization by optical profilers.

Low-coherence interferometry is combined with confocal scanning to provide remote refractive index and thickness measurements of transparent materials. The influence of lens aberrations in the confocal measurement is assessed through investigation of the axial point-spread functions (APSFs) generated using optical configurations comprised of paired aspherics and paired achromats. Off-axis parabolic mirrors are suggested as an alternative to lenses and are shown to exhibit much more symmetric APSFs provided the system numerical aperture is not too high. Refractive index and thickness measurements are made with each configuration with most mirror pairings offering better than twice the repeatability and accuracy of either lens pairing.

By combining classic differential interference contrast (DIC) with the chromatic confocal principle, we show that phaseshifting calibration can be avoided in DIC by using spectral information induced by the investigated sample. The created spectral fringe can be further used to unwrap the phase. This unwrapping is limited by the spectral resolution of the spectrometer. Therefore, the depth-difference around a single measurement point can be determined instantaneously. To reconstruct the depth profile, the integration of a depth-gradient is necessary. By combining the depth information of the chromatic confocal carrier signal with the differential depth information of the carried DIC signal, the accumulation of measurement uncertainty can be reduced. To our best knowledge, the proposed chromatic confocal differential interference contrast (CCDIC) is a novel profile reconstruction principle. To verify the feasibility of the CCDIC, a prototype probe with an adjustable shear and phase has been developed. Preliminary experiments achieve sub-micrometer depth resolution. A current challenge requiring further work is the stable unwrapping of the phase-difference by spectral frequencies.

This Conference Presentation, "Deflectometry," was recorded at SPIE Optical Metrology 2019 held in Munich, Germany.

High-resolution optical inspection techniques typically require expensive imaging systems. One possible approach for reducing the overall cost of such systems is to combine optics of limited performance with digital image restoration techniques. In this contribution, we introduce a new hybrid system based on the combination of a field-programmablegate-array (FPGA), a large telecentric lens with strong field dependent chromatic aberrations and different types of object-adapted projections for 2D and 3D inspection tasks. The optical design tries to minimize cost and weight by replacing the – typically very large – first refractive group of the object-sided telecentric lens by one thin diffractive optical element. This approach can correct the field-dependent aberrations very well. Unfortunately, strong chromatic aberrations are unavoidable. For restoration, we use digital image processing by means of advanced deconvolution algorithms. The necessary number of operations for the strong fielddependent aberrations is extremely high. We combine algorithmic approaches (convolutions using singular-value decomposition) and parallelization directly on the FPGA to achieve live restoration (15 Hz) for a custom-built image sensor with three megapixels. The power consumption is low and a compact processing unit is realized. This hybrid (digital-optical) imaging system is used for 2D- and 3D inspection based on adaptive triangulation. One approach is based on inverse fringe projection. Ray-tracing based on the sensor setup and a reference CAD-model of the part to be tested is used for computing a fringe mask. This mask is inscribed on a spatial light modulator, which then projects the fringes towards the object. If the object under test is without faults, high-frequency equidistant straight-line fringes will result on the image sensor and low height measurement uncertainties at low lateral resolutions, even for non-cooperative objects, are achievable. The importance of a large depth-of-field and constant image-scale along the optical axis for the inverse fringe projection lead to the decision of a telecentric objective.

Within the Collaborative Research Centre 1153 Tailored Forming a process chain is being developed to manufacture hybrid high performance components made from different materials. The optical geometry characterization of red-hot workpieces directly after the forming process yields diverse advantages, e.g., the documentation of workpiece distortion effects during cooling or the rejection of deficient components in an early manufacturing state. Challenges arise due to the high components temperature directly after forming (approximately 1000°C): The applied structured light method is based on the triangulation principle, which requires homogeneous measurement conditions and a rectilinear expansion of light. This essential precondition is violated when measuring hot objects, as the heat input into the surrounding air leads to an inhomogeneous refractive index field. The authors identified low pressure environments as a promising approach to reduce the magnitude and expansion of the heat induced optical inhomogeneity. To this end, a vacuum chamber has been developed at the Institute of Measurement and Automatic Control. One drawback of a measurement chamber is, that the geometry characterization has to be conducted through a chamber window. The sensors light path is therefore again affected - in this case by the window’s discrete increase of refractive index, and also due to the different air density states at sensor location (density at ambient pressure conditions) and measurement object location (density at low pressure conditions). Unlike the heat induced deflection effect, the light path manipulation by the window and the manipulated air density state in the chamber are non-dynamic and constant over time. The reconstruction of 3D geometry points based on a structured light sensor measurement directly depends on the mathematical model of detection and illumination unit. The calibration routine yields the necessary sensor model parameters. The window light refraction complicates this calibration procedure, as the standard pinhole camera model used for entocentric lenses does not comprise enough degrees of freedom to adequately parametrize the pixel-dependent light ray shift induced by thick vacuum windows. Telecentric lenses only map parallel light onto a sensor, therefore the window induced ray shift is constant for all sensor pixels and can be directly reproduced by the so-called affine camera model. In this paper, we present an experimental calibration method, and corresponding calibration data and measurement results for a structured light sensor with and without measurement window. The sensor comprises a telecentric stereo camera pair and an entocentric projector. The calibration of the telecentric cameras is conducted according to the well-known affine camera model. The projector is used as feature generator to solve the correspondence problem between the two cameras. The calibration data illustrates that the window refraction effect is fully reproduced by the affine camera model, allowing a precise geometry characterization of objects recorded through windows. The presented approach is meant to be used with the aforementioned vacuum chamber to enable a geometry characterization of hot objects at low pressure levels.

Structured light projection techniques based on diffuse reflection are widely used for accurate, fast, contactless, and nondestructive optical 3D shape measurements. It cannot be utilized to measure uncooperative materials, i.e., materials with optical properties such as being glossy, transparent, absorbent, or translucent. Recently, it was shown that 3D reconstruction of an uncooperative object can be performed by a two-step process for each camera image pair. In the first step, the object absorbs a projected thermal pattern, e.g., in the long-wave infrared range. In the second step, after energy conversion, the object surface reemits light according to Planck’s law. Whereas the irradiation can be performed by a CO₂ laser at 10.6 μm, the detection of the reemitted light can be carried out by mid-wave infrared (MWIR) cameras sensitive in the wavelength range between 3 and 5 μm. In order to achieve accurate 3D results in a short measurement time, the projection parameters like radiation intensity and illumination time as well as the projection patterns have to be optimized depending on optical and thermal material properties (e.g., complex spectral refractive index, thermal conductivity, specific heat capacity, emissivity). Therefore, we have developed a simulation tool based on the Beer-Lambert law (for absorption of the irradiation) and on the heat diffusion equation (for the illumination-induced thermal pattern on the object surface). In this contribution, we present our simulation tool and several simulation results. We apply our tool to investigate the projection parameters and projection patterns for a given material and a specific total measurement time. Finally, we experimentally verify the theoretical results with our MWIR 3D sensor.

For the requirements of newly developed production processes like sheet bulk metal forming a previously developed measuring method exists for dimensional measurement of these manufactured parts. The presented multi-sensor approach allows the combination of several fringe projection sensors with different measurement resolutions and ranges to meet the requirements of the new production technology. The measurement setup includes a high precision hexapod to position the investigated workpiece inside the measuring volume. The measurement devices utilized are high precision fringe projection systems with 17 µm lateral and 1 µm vertical resolution. An additional overview sensor captures the whole measurement range of the hexapod. This paper presents two approaches that can be used for camera calibration and sensor registration of the fringe projection systems in a global coordinate system. A speckle pattern and a random dot pattern are placed on the hexapod to calculate the intrinsic parameters of the sensor camera and extrinsic parameters of the fringe sensor in one single step. To obtain metric lengths with the calibration process, a defined scale is included additionally on the pattern. The calibration process is executed by an automatic and random movement of the hexapod in the recording area of the sensors. Measurements show a direct transferability from the camera to the sensor coordinate system. This allows the measurement datasets to be directly merged together without the use of a separate registration routine.

We previously presented a technique calibrating a Multi-Aperture Array Projector (MAAP) for performing high-speed three-dimensional (3D) surface topology measurements with structured illumination. The MAAP achieves projection rates up to 3 kHz by projecting a sequence of aperiodic sinusoidal fringes through several distinct projecting aperture channels such that up to 330 3D measurements per second could be achieved. This rapid switching between several projecting channels overcomes the projection limit of conventional off-the-shelf single aperture Digital Light Processing (DLP) or Liquid Crystal on Silicon (LCoS) technologies that typically have projection rates on the order of 100 Hz. The proposed MAAP calibration technique re-enabled photogrammetric 3D measurements to be made using a single CCD or CMOS camera where a stereo-camera setup used to be required for triangulation. The focus to reduce a stereo-configuration to a single camera system opens possibilities to decrease cost as well as form factor of such a high-speed metrology system utilizing an MAAP. For such a photogrammetric optical metrology system, intrinsic camera calibration and extrinsic setup calibration are vital to obtain high measurement performance. With the pinhole camera model assumption, intrinsic calibration is typically carried out using the standard Zhang method. However, there is currently no such standard technique that can comprehensively perform the required extrinsic calibration. In this study, the main sources of measurement error arising from extrinsic calibration of the proposed technique are discussed and investigated. Several different extrinsic calibration techniques are proposed and the resulting measurement performance evaluated.

The 3D multispectral imaging system proposed in this contribution realizes simultaneous detection of contaminants and 3D localization. The imaging system is composed of a digital pattern projector, a stereo-vision setup of two multispectral filter wheel cameras, and an external light source. A calibration procedure is developed to estimate simultaneously stereo camera parameters and light source parameters. For an acceleration of image acquisition, the entire spectral range is split into two parts which are captured from two different camera views and merged using structured light. The usefulness of the proposed system is demonstrated with an example of cutting oil detection. For this the fluorescence effect is utilized, and specular reflections are filtered out based on the estimation of illumination geometry. Experimental results show that the surface areas with cutting oil could be reliably distinguished from workpieces using the proposed algorithms.

Full-field deflectometry, which combines high-precision and robustness to external perturbations, is well adapted for the characterization of high-precision freeform mirrors. Instead of measuring the surface height map like interferometry does, the instrument will estimate the surface slopes in two perpendicular directions. The principle of the method is to measure the angular distribution by applying spatial filtering. This method has been called phase-shifting Schlieren deflectometry Inspection of mirrors in terms of slopes instead surface height offers multiple advantages. In particular, deflectometry is well adapted for the detection of waviness, which is a mid-spatial frequency topography error. Waviness detection during the diamond turning process is critical since it is hard to remove afterwards by polishing. Keeping the mirror mounted in the lathe during the measurement of its shape will simplify the process since it will avoid misalignment when remounting the mirror in the lathe.

Coherent Fourier Scatterometry (CFS) is a scanning optical technique that is particularly suitable for nanoparticle detection. Inspection of wafer surfaces is one of the critical bottle-necks for high yield in the production of semiconductor chips. Ideally, inspection systems are required to work fast, be sensitive, and should not thermally damage the samples with an excess of illuminating light power. The sensitivity of detection of nanoparticles, attributed to the smallest size of the scatterer that can be detected, is severely limited by noise. The optical readout of the scatterometer consists of a bi-cell (a split photodetector) that collects the scatterred light from the surface to be inspected while the latter is scanned in the lateral direction (2D scan). The difference voltage signal resulting from integrating and subtracting the two halves of the bi-cell is recorded as a function of the lateral scanning position of the sample surface. The bi-cell has two functions: first, it allows us to acquire signals in a fast manner, and second, it eliminates effects due to substrate spurious reflections, which is usually a big issue in dark field based particle detection systems. In this paper, we present an extension of the original CFS detection system by incorporating a heterodyne technique to the detection system. We show the implementation of the new detector system as well as a comparative signal-to-noise ratio (SNR) gain studies that are used to determine the suitable frequencies and waveforms for both modulation and reference signals. We demonstrate the detection of polystyrene nanoparticles with a diameter of 80 nm, which were deposited on top of a silicon wafer, with high SNR at low illuminating light power. The experiments were performed with a diode laser at the wavelength of 405 nm. In this particular particle size, we have observed an improvement of the SNR of about 45 dB as compared to the original detection system of the CFS. Although the proposed heterodyne CFS technique already shows excellent performance for detection of polystyrene nanoparticles on silicon wafer, there is still room for improving the sensitivity towards even smaller particles, as discussed in the outlook and conclusions section.

Step-index fibers consisting of two (or more) concentric layers of different transparent materials are common in optical telecommunication technology. Our aim is to establish algorithms allowing for the determination of the parameters (e.g., diameters) of these layers by evaluation of the lateral scattering pattern originating under planewave monochromatic illumination perpendicular to the fiber axis. We investigate numerical simulations only, i.e. applying the corresponding parameter estimation algorithms to numerically generated scattering patterns with known geometry and optical parameter values. We consider three algorithms for parameter determination. The first is the iteratively regularized GaussNewton (IRGN) algorithm, which minimizes the norm ||F(q) − u∞||₂ of the difference between the pattern for the parameters qn and the input u∞, but only locally. However, local minima in the norm “landscape” are spaced periodically, which we utilize. The second approach treats parameter determination as the optimization problem of minimizing ||F(q)−u∞||₂ globally, subject to the Dividing Rectangles (DiRect) algorithm. The third approach hybridizes both optimization methods. The results show that the modified IRGN algorithm is considerably faster for cases where the scattering intensities are superimposed with no or little noise. In terms of precision, the DiRect algorithm and its hybrid variant perform slightly better. These algorithms also terminate quicker for increasing noise. This, however, highlights the general trade-off between calculation times and precision.

Scattered light level of optical components can severely impair SNR and overall performance of optical systems for imaging and spectrometry. It is therefore necessary to directly assess its angular distribution in terms of BRDF measurements which is, due to the extreme dynamics required for high quality optical surfaces, still a challenging task. In our contribution we will present a self-built scatterometer that is based on a Czerny-Turner geometry in conjunction with a CMOS-camera detector and a single mode fiber coupled 405 nm diode laser source. Our setup is, besides simple spherical mirrors, purely based on stock-components and both, cost-effective and simple to build. Considerations on system design for high resolution and minimized instrumental signature as well as a first breadboard experimental setup will be discussed. The scatterometer utilizes the sensor’s pixels for adaptive sub-slit resolution and 2d measurements in the close vicinity of the plane of incidence. It can cover BRDF-values of up to 14 orders of magnitude and reaches resolutions well below 0.01° which allows to gain useful insights about small-angle scattering that has in the past been difficult to experimentally address. First measurements of superpolished mirrors as well as holographic and mechanically ruled diffraction gratings will be presented. Simple formulae can be used to assign rotation angles to spatial frequencies and, for smooth surfaces with negligible particulate contribution of scattering, also to PSD values and band-limited RMS roughness.

The Centre Spatial of Liege (CSL) is involved from more than 10 years in the BTDF/BRRDF metrology of large Lambertian diffusers used for on-board calibration on space instrumentation. In this context, a dedicated automatized BSDF calibration facility has been developed, suitable for calibration of all types of diffusers (from industry to space applications). Accurate calibration of such systems induces constraining requirements on the calibration bench: manipulate large diffusers and mechanisms in class ISO5 environment; incidence beam divergence close to the Sun divergence; large spectral range coverage; measurement of very low diffused signals (highly stable, with low noise, straylight free set-up), high accurate measurement of attenuator and a high knowledge of all calibration properties for BSDF modelling. Based on previous heritage, recent improvements have been implemented in order to correct some defaults like mechanical stabilities (complete new design), specimen alignment (new iterative procedure), absolute error (super stable light bulb, improved filters wheel repeatability, optics to limit diffraction effects in UV, incidence beam non-uniformity compensation …). This paper presents the design of the improved bench, highlights critical parameters for BTDF/BRDF calibration and relates main improvements to reach todays performances. The current bench performances are illustrated by calibration campaigns results performed at CSL for Sentinel 2, Sentinel 3 and Sentinel 4 calibration assemblies.

In the past grazing incidence interferometry has been applied for plane, cylindrical, acylindrical and general rod-like surfaces using diffractive beam splitters. Here, we demonstrate that also rough convex steep rotational symmetric aspherics can be measured along one meridian in a single step using diffractive beam splitters and phase shifting techniques. Especially, rough surfaces can be measured with this method due to the large effective wavelength (here about 10μm) of the test. The extension to the whole surface can be attained by successive meridional measurements of the surface under test by azimuthal adjustments. The principle of the method is given, first simulated and experimental results are presented. The occurrence of the interferogram is discussed and the experimental evaluation on a single meridian including the unwrapped phase are shown. Furthermore the simulated results of aberrations caused by object misalignment and the first results for the elimination of adjustment aberrations are presented.

Tilted Wave Interferometry has been invented and developed over the last years as a flexible and very fast method to test precision aspheres and freeform surfaces . It measures surface deviations full field, with high lateral resolution, without any null compensator like CGH and without moving the tested part while measuring. The test of non-spherical optical components is a topic of high relevance for optics industry, as current optic designs rely heavily on those elements, since the small form factors and high performance of actual designs would be impossible with traditional spherical optics designs. As all precision components, aspheres and freeform surfaces need accurate testing for their production. Ideally, testing of components is closely integrated into the fabrication chain. Due to the high flexibility and measurement speed of the TWI of typically less than 30 sec it is well suited for this purpose. Due to the special illumination scheme, the first implementations of this new interferometer have been of Mach Zehnder type. In this contribution we demonstrate, how the tilted wave interferometer principle can be implemented in a Fizeau configuration. The benefit of this configuration against the Mach Zehnder configuration is the common path feature. Here, the reference beam and the measurement beam follow the same optical track inside the interferometer, making the interferometer much more robust against temporal environmental influences such as vibrations and air turbulences. At the same time, form tolerances of the interferometer components in the common path area can be relaxed. These advantages of Fizeau are well known. The multiple source illumination of the tilted wave interferometer however leads to the generation of multiple reference wavefronts that can be disturbing. We therefore present a new TWI implementation that avoids these problems. It relies on a new illumination design with four sets of illumination patterns that each generate their own reference wave. The new approach has been implemented in a lab setup and shows in first measurements the expected improvements in stability. We tested the system in extensive Monte Carlo simulations. The common path approach showed a reconstruction error of the test specimen of up to an order of magnitude lower compared the Mach-Zehnder configuration.

Freeform optics have emerged as a new tool for optical designers and integrators. Manufacturing innovations are gradually increasing availability of precision freeform optics. As optics manufacturers strive to improve quality and decrease cost, some focus is placed on improvements in the challenging metrology requirements for freeforms. One relevant technique to be discussed here is the use of deflectometry to measure mid-spatial frequency error in-process or in-situ during the manufacturing of freeform parts. Deflectometry can measure the mid-spatial frequency error on freeform parts orders of magnitude faster than traditional tactile metrology tools at similar or better accuracy.

This Conference Presentation, "Approaches for a destructive measurement method of subsurface damages," was recorded at SPIE Optical Metrology 2019 held in Munich, Germany.

This Conference Presentation, "Investigation of non-uniformity of classically polished fused silica surfaces via laser-induced breakdown spectroscopy," was recorded at SPIE Optical Metrology 2019 held in Munich, Germany.

Lenses and mirrors with freeform surfaces are the latest step in the evolution of optical components. However, the measurement of these components still challenges metrology. We have developed a gradient-based measurement technique that is able to measure freeform specular surfaces either if their form is known or not. The measurement of freeform surfaces is a challenge for every measurement system. Especially if the form of the surface is not known in advance. Our measurement system can measure continuous freeform surfaces with up to 10° deviation from a plane surface even if the surface model is not known in advance. Therefore, a ray, represented by a narrow laser beam, is targeted on the surface under test (SUT) under a certain angle. Affected by its slope, the surface reflects the ray in a new direction. This direction is measured by using a variation of Experimental Ray Tracing (ERT). This includes the measurement of the position of the reflected ray in two parallel planes. Calculating the difference of the position on these planes, the direction of the ray in relation to them can be calculated. Having the direction of the reflected ray, as well as the direction of the incident ray, one can determine the surface normal at the point of reflection. By moving the SUT, the incident ray targets on a different point on the SUT. Therewith, various points are investigated. Using appropriate integration methods, the surface can be reconstructed. Although, with the introduction of the incident ray under a certain angle comes the issue, that the point of reflection changes with the sag of the SUT. This leads to an unequal distant measurement grid of points of reflection even if the SUT has been moved to equal distant sample points. This shift has to be considered for the reconstruction of the surface. This issue is solved in different ways for known or unknown surfaces. For an unknown surface, the investigated sample points are transferred into a coordinate system where they are equal distant. This is the coordinate system of the incident beam. Performing the integration here and transferring the reconstructed surface back into the coordinate system of the SUT leads to the expected shift of the sample points. For a known surface, the expected surface form is taken into account to determine the sample point shift. Therewith, the difference between the measured surface normals and the expected normals can be calculated and the integration can be performed only on the normal residuals. By adding the residuals to the model, the surface can be reconstructed. The measurement technique described above has been implemented in an experimental setup. To show the abilities of this technique, we will show the process of the measurement of a known and an unknown surface using the same sample. The results will be evaluated and compared.

One of the problems that have manufacturers of aspheric and freeform surfaces is the local measurement of the shape, in order to ensure the performance of the surface. In this paper we present an alternative method to measure local radii of curvature of systems with symmetry revolution, using the Point Diffraction Interferometer technique (PDI). To implement this proposal a certified plane wavefront is used as reference light source, and the PDI as sensor element for measuring the local radii of curvature. We proposed to use a PDI due to its high sensitivity because is a common path interferometer and generate interference only when there is an only point a single convergence which is produced from an annular region of the surface, and the annular region is associated to each local curvature center. Experimental results are shown for one aspherical surface with different rates of change in their slopes for each region of the surface, showing the versatility of the proposal and its possible use, including free-form surfaces without symmetry of revolution.

Hyperspectral imaging is an established technique for process analysis capturing a two-dimensional spatial image and the spectral information for each pixel simultaneously. When moderate spectral resolution is sufficient, static imaging Fourier transform spectrometers (sIFTS) can offer a viable alternative to their scanning counterparts in the mid-infrared spectral range. Therefore, in this paper we present a sIFTS concept based on a single-mirror interferometer which shows no internal light losses and still works with extended light sources, achieving sufficient signal-to-noise ratios. The interferometer consists of a beam splitter, a plane mirror and a lens, which makes it both inexpensive and relatively easy to adjust. For a proof of principle we present a transmission measurement setup including a light source module, imaging optics and a single-mirror interferometer. The system achieves a spectral resolution of 12 cm⁻¹ in a spectral range from 2700 cm⁻¹ to 800 cm⁻¹ , respectively from 3.7 μm to 13 μm. The spatial resolution amounts to about 10.10 lp/mm, the results for a sample containing different polymers show good agreement with a laboratory FTIR spectrometer.

Measuring the propagation delay of optical signals reflected by the illuminated surfaces is an established approach to non-mechanical distance estimation and the basis for 3d laser scanning. We have in the past extended a technique using the intermode beat notes obtained from a femtosecond laser to a coherent ultra-broadband source. Using cooperative targets, we have shown that this may enable inline correction of atmospheric delays by using a multispectral configuration and exploiting atmospheric dispersion. In this work, we enhance the scope by providing a first demonstration of successful application to reflectorless measurements on natural targets. This extension is relevant because of two aspects: (i) the field of applications of reflectorless distance measurement is much wider (in particular through laser scanning) than for highly accurate measurements to prisms, and (ii) the approach offers the opportunity to simultaneously acquire delay and spectral signatures of both delay and power thus allowing to combine distance measurement with material probing. We present a table-top experimental set-up that uses a coherent femtosecond laser supercontinuum to probe the displacement and multispectral relative distance of various material samples on 5 spectral bands of 50 nm in the visible and near-infrared regions. Using integration times of about 30 ms, we have achieved a distance measurement accuracy of better than 50 μm with promising perspectives regarding scalability to practical distances. Comparative measurements on 5 different materials additionally yielded repeatable material-dependent profiles in the multispectral relative distances, whose combination with reflectance estimations may allow mitigating surface related uncertainties and remotely identifying materials.

Optical interferometry is one of the suitable methods which can be used to measure the physical thicknesses of microscale structures because this approach can measure optical path differences accurately with a non-contact method. In this paper, on the basis of the simultaneous measurement of the physical thickness and refractive index of an optically transparent plane-parallel plate, a spectral-domain interferometer capable of measuring the physical thickness and refractive index of separate layers in a step-shaped structure with two layers was proposed and realized. For a feasibility test, a microfluidic channel mold with two layers was selected as a sample. For verification of the measured thickness in a double-layered region, a contact-type surface profilometer equipped with laser interferometers on the x-y-z axes was used for a thickness comparison. However, it is never simple to compare measured thicknesses due to positioning errors and the different measuring sizes of each method. For these reasons, the corresponding thickness value was defined as an offset between height values at center points of the single-layered and double-layered region in a specific area of 5 mm × 5 mm. For an accurate determination of the offset, the slopes of the surface profile were removed. The assumption that the surface profile of the bottom layer in the double-layered region is very flat was applied to calculate the thickness from the measured surface profile, and this assumption was checked as to whether it is acceptable or not in this study. In conclusion, the physical thicknesses according to a surface profilometer and by the proposed method were determined to be 106.332 μm and 106.304 μm, respectively, in good agreement within the respective uncertainty values.

Due to excellent optical performances, two-dimensional materials have emerged as promising materials for applications like optoelectronic devices, photonic devices, and optical sensors. To better study the unique optical performances of 2D materials, spectroscopy techniques such as reflectance and transmittance spectroscopy, and Raman spectroscopy have been utilized for image acquisition and optical property analysis. Hyperspectral imaging (HSI), a combination of spectroscopy and imaging technique, has been used for characterization and property analysis of new materials. A 3D datacube with the wavelength as z-axis, plus spatial axes x and y, can be acquired, and the spectral information can be extracted for characteristic analysis. With the high demand for area imaging of 2D materials, a microscopic HSI setup with a LED light source working in the visible range was proposed for 2D MoS₂ imaging. The HSI imager using a reflection grating works in line-scanning mode in the range of 380-1000 nm. A 3D datacube of 2D layered MoS₂ was built and processed for thickness measurement and optical property analysis, including single-band analysis of the imaging area, spectral analysis of the interesting area, and comparison with the image acquired by a white-light microscope. Finally, general performances of hyperspectral imaging of 2D MoS2 in the visible range was analyzed and discussed for further optical applications

This manuscript discusses the realization of a measuring system, which monitors elementary composition of the weld seam during aluminum sheet welding. For this purpose, an aluminum sheet is welded to titanium, which is detected by the measuring system if the power of the welding laser is set too high during the welding process of aluminum. This measuring system is based on the principle of laser-induced breakdown spectroscopy (LIBS). A high-energy probing laser is used to irradiate a sample on the surface and a plasma is formed. This plasma emits material-specific characteristic radiation that enables conclusions to be drawn about the elementary composition of the sample. First, the spectra of the metals used in the welding process, aluminum and titanium, were collected by means of LIBS. The investigations showed that there are two peaks of high intensity for aluminum at a wavelength of approximately 395 nm. Titanium, on the other hand, has peaks of high intensity in the wavelength range of approximately 500 nm. For the separate detection of these characteristic peaks, a suitable measurement setup was created. This design allows spectral and temporal separation of the element-specific signals. An aluminum sheet welded to titanium was then examined at right angles to the weld seam in 30 μm steps. The results show an increased signal from the detector for the 500 nm wavelength range, indicating the presence of titanium. These results are supported by additional investigation using energy dispersive X-ray spectroscopy (EDX) and indicate material mixing in the center of the weld seam.

For the design and modelling of reactive flows, profound knowledge of temperature and species concentration is essential. Here, optical, non-invasive sensing techniques are frequently chosen, yet they often require elaborate experimental effort or inhibit other disadvantages. To circumvent these drawbacks, we developed a mobile, fiber-based sensor system, utilizing linear rotational Raman spectroscopy. This technique requires neither sampling from or tracers inside the reactive flow nor an external temperature or composition calibration. It simultaneously yields point-wise information on temperature and species concentration. To extract these quantities of interest the acquired, background-corrected spectra are matched to simulated spectra via a least-square fit algorithm. Such an approach constitutes an ill-posed inverse problem as multiple solutions could explain the measured data. Conventional least-square approaches only yield a set of parameters minimizing the residuum, but neglect uncertainties arising from the ill-posedness. Here, Bayesian inference offers many advantages: besides pointestimates it allows to determine the corresponding uncertainties. Furthermore, prior knowledge about quantities of interest or model parameters can be included in the evaluation to establish a more advanced analysis routine. Using these tools, the benefits and limits of the rotational Raman technique are evaluated by the investigation of a flame from a premixed methane/air laminar flat-flame burner regarding the flame temperature and species concentrations of the rotational Raman-active and, therefore, detectable gas species N₂, O₂ and CO₂. In addition, two different backgroundcorrection approaches are applied and compared using Bayesian inference and inter-parameter correlations.

Pump-probe microscopy is a well suited tool to follow the highly dynamic phenomena during laser material processing on a picosecond to microsecond scale. We present a pump-probe microscope, to monitor the phase and morphology of the silicon-dielectric interface during and after irradiation with focused high-intensity laser pulses. Laser ablation with ultrashort pulses is used to locally structure dielectric layers for contact formation on the surface of silicon solar cells. With the pump-probe microscope we can study the dynamics and the physical mechanisms of the local ablation process. A mode-locked 180 fs laser is used to illuminate the sample through a microscope objective at a wavelength of 515 nm. The same laser can be used to locally ablate the thin dielectric layer on the silicon substrate. The delay between ablation pulse (pump) and illumination pulse (probe) can be adjusted using an optical delay line with sub-picosecond precision. Alternatively other pump laser sources have been integrated using timing electronics and photodiodes, with a temporal resolution of around 1 ns. Because the system is fully automated, we can collect a large number of transient reflectance images at different points in time, which can then be combined to form a consistent video of the process. With a temporal resolution of 0.2 ps, we find a fast increase in reflectance due to melting of the surface at the beginning of the process. As the removal of the topmost layer begins, newton rings can be observed and evaluated to reconstruct the surface.

Metal processing industries utilize two types of functional coatings. Conversion coatings, based on Zirconium and/or Titanium, generate corrosion resistance and paint adhesion for aluminum surfaces. Another type of functional coatings are lubricants based on mineral oil, which act as corrosion protection as well as drawing and punching oil. Efficient process development and control requires the monitoring of the thickness of these functional coatings. In this article, we present a new optical setup, which uses a rotating polygon-scanning mirror in combination with laserinduced fluorescence to monitor the spatial distribution of lubricants and conversion coatings on metal sheets. In the presented setup, the beam of a 405 nm diode laser excites auto-fluorescence of the organic molecules inside the functional coatings. By using a fast rotating scanner mirror combined with a fast analogue digital conversion, the presented setup reaches data rates of 400 lines/s consisting of 1000 data points each. Installing the scanner system at a distance of 1200 mm above the metal sheets, realizes a field of view of 2200 mm. At strip speeds of 2 m/s, the distance between two scanner lines on the surface to be investigated is 5 mm. In addition to the description of the optical system, we present different approaches for the calibration of systems for inline fluorescence measurements. For the calibration of lubricant layers in the range down to one micrometer, the reference samples are weighted. To evaluate the limit of detection of the system we use a multiphase carbon analyzer. We show the calibration results for different lubricants and metal materials with different surface textures typically used in car body manufacturing.

Electrode manufacturing is one of the most critical processes within the energy storage production chain, as the electrodes coating and drying quality determines the later performance of the energy storage system in large part. Critical quality losses within the electrode coating are often optical visible, but cannot be automatically and inline inspected. Often small defects like agglomerates, capillary cracks or pinholes in the carbon or metal oxide based layers have to be detected at high web speeds. In the work described in this paper it was examined which kind of features and defects can be recognized by two optical methods: visual inspection and 3D measuring. A visual camera in conjunction with a white illumination and a 3D data laser line system has been used. Image processing algorithms are applied on the scanned data to detect pinholes and agglomerates. Also thickness of coating, gradient of the coating edge and form anomalies can be determined out of the produced data. Reliable evaluation of pinholes even of small sizes has been proofed. Particle agglomerations are more challenging. To achieve good enough data for 3D evaluation standard sensors are often insufficient, due to the necessary resolution and the production speed that requires high scanning frequency. In order to provide an automated, digitalized production and inspection system, the devices were coupled to a central experiment management system. Their operation is controlled non-locally in the cloud and synchronized with the execution of experiments. The acquired raw data is stored for later evaluation and long-term archiving.

The quality and stability of materials or products containing natural fibres (textiles, fleece, fibre-reinforced materials) is mainly determined by the length of the employed natural fibres. To this day, the only possibility for determining the length distribution of natural fibres in a fibre bundle consisting of irregularly arranged fibres is to either manually sort and classify the single fibres or use a measuring instrument called Almeter. The device has not been produced for 20 years and is getting outdated with respect to hardware-interface and operating system compatibility. However, more than 500 of these devices are still in use all over the world. In this paper we present a novel method and device for optically determining the length distribution of fibres and fibre bundles with lengths greater than 5 cm. A fibre or fibre bundle consisting of fibres with varying lengths is put on a needle bed where a gripper pulls the fibres at their one end. During this draw-out process a laser line is projected onto the fibres and the reflected light is detected by a line sensor taking grey-scale pictures with a frequency of (currently) 8 kHz, but this can be easily increased by a factor of 3 or 4 with current line sensor technology. The image stack can be analysed, and the lengths of the fibres may be determined. To date, owing to the dimensions of the pre-existing apparatus that has been modified, the maximum length of fibres to be measured is 350 mm. The resolution of the optical device has been proven to be 20 microns. In addition to that, the distribution of the width of single fibres can be determined, as well. The measuring method may be easily extended to other measurement parameters, such as fibre colour. An additional advantageous feature of the novel measuring method is the possibility to adjust the resolving powers in two perpendicular directions to given desired values independently.

We present new imaging techniques for the detection and classification of particulate contamination on structured surfaces. This allows for cleanliness inspection directly on the sample. Classical imaging techniques for particle detection, such as dark-field imaging, are typically limited to flat surfaces because structures, scratches, or rough surfaces will give similar signals as particles. This problem is overcome using stimulated differential imaging. Stimulation of the sample, e.g. by air blasts, results in displacement of only the particles while sample structures remain in place. Thus, the difference of images before and after stimulation reveals the particles with high contrast. Cleanliness inspection systems also need to distinguish (often harmful) metallic particles from (often harmless) nonmetallic particles. A recognized classification method is measuring gloss. When illuminated with directed light, the glossy surface of metallic particles directly reflects most parts of the light. Non-metallic particles, in contrast, typically scatter most of the light uniformly. Here, we demonstrate a new imaging technique to measure gloss. For this purpose, several images of the sample with different angles of illumination are taken and analyzed for similarity.

Pipelines usually transport hydrocarbons in the oil and gas industry. Small amounts of salt water may be present, which can cause corrosion from the inside to outside. Severe corrosion may produce through holes in the pipe wall. External protective layers of fiber reinforced plastics has been applied to postpone the need to stop production to perform a definitive repair, but do not stop the internal corrosion. It is very important to monitor the dimensions of through holes hidden by the protective layers of composite materials, which cannot exceed critical size and compromise pipe safety. Classical methods, such as ultrasound, do not give very reliable answers when composite materials and steel are combined. There is a great demand for a reliable and practical in field non-destructive inspection method. This work brings a hybrid and reliable solution, which combines a new configuration of a portable one-shot shearography system with finite element methods, resulting in a portable and easy to apply solution. The paper describes the principle of the portable one-shot shearography system that is able of simultaneously measuring in three shearing directions[1]. Considerations and modeling of finite element to determine the response of the composite material to the variation of the internal pressure of the duct are discussed. The report details how the experimental data and numerical model results are combined through an iterative process to determine the diameter of the hidden hole. Instead of integrating the experimental results of shearography, the authors differentiate the results of finite element in the same directions as the one-shot shearography system does simultaneously. The minimum of the square error results in an estimation of the hidden hole diameter. Experiments were performed with 150 mm diameter tubes with 20 to 50 mm diameter holes hidden by 6 to 24 mm thick glass fiber reinforced plastic protective layers. Smaller thickness and bigger diameters led to better results. Mean deviations of the order of 10% were found for the whole set of tests. Although the authors consider it is possible to improve the results, deviations of the order of up to 10% are very acceptable results and represent a significant improvement over the classic methods used today. The robustness of the optical system, the ease of use of the algorithm and the level of uncertainty already achieved make the authors believe that the techniques developed here achieve the requirements and will be very useful for the oil and gas industry demands.

Air-coupled ultrasound (ACU) is already an established method for the non-destructive failure inspection of carbon fiber reinforced polymers (CFRP). In the through-transmission setup, plate-like structures are placed between the ultrasound (US) source and the receiver. The ultrasonic wave propagating through the material is observed; deteriorations inside the material such as defects alter the captured signal. Such defects can be delaminations, cracks, thickness changes or porosity. In the measurement setup chosen, conventional piezoelectric transducers and receivers are replaced by laser-based components. On the excitation side a nanosecond laser pulse, illuminating the plate surface, was used to induce ultrasonic waves (thermal regime) directly into the specimen. On the receiver side a laser-based optical microphone was tested. This membrane-free microphone detects the refractive index changes of the air, when the ultrasound propagates through the miniature Fabry-Pérot etalon. Using this new measurement setup, C-scans of CFRP plates were performed containing impact damage, delaminations and blind holes. In comparison to conventional aircoupled testing methods, our method is sensitive over a broader frequency range, has better signal-to-noise ratio (SNR) and a smaller acoustic aperture. This allows obtaining a more detailed image of a specimen including defects.

Cutting edges are of great importance in industry and especially in mechanical engineering. However, like other components, they wear out over time. The contour and in particular the cutting edge itself can be damaged over time or by other occurrences and be defective. If the traces of use or defects are small, they can be corrected by reworking. This means that the cutting edge can still be used by post-processing. To achieve this, it is necessary to measure the cutting edge. Subsequently, the error must be evaluated. This error should indicate whether and how far the cutting edge must be reworked. In order to carry out such an evaluation, ideal references of the cutting edge are necessary. If an ideal geometry of the cutting edge is available as a computer-aided design model, the evaluation is trivial. However, this only exists in very rare cases. Often the reference geometry must be formed on the basis of one measurement. This paper presents a possibility of reconstructing cutting edges and therefore a rating of this cutting edge. The reconstruction is based on neuronal networks, more precisely by convolutional neuronal networks.

The inspection of defects is an important task in many industrial sectors: from metals to plastics, passing through glass and other materials, these products need to satisfy some aesthetical and quality requirements. Flaws can arise in many different forms: spot of different color, crack, incompleteness, excess and/or lack of material are just some examples of defects deriving from the industrial manufacturing process, which can lead to discard the component or the piece examined. These defects are recognizable by the human eye, but some issues like fatigue, illness of the operator and incorrect lighting of the samples can be tough obstacles in obtaining the right selection of the pieces. To detect faulty pieces and in order to avoid wasting compliant pieces instead, a computer based visual inspection system has been designed and implemented. As benchmark samples we adopt the outer lenses of automotive rear lamps. The surface of an outer lens needs an extreme precision manufacturing procedure and the absence of defects is essential for the quality of the final product. The aim of the work involves the ideation and commissioning of a setup to extract and analyze information about the flaws present in an outer lens, exploiting different image processing techniques depending on the nature of the defects.

The perception of a display’s content can be negatively influenced due to surface properties and variable ambient light conditions. Conventional measurement methods, such as DIN EN ISO 2813, provide limited information on the relationship between gloss values and gloss perception. Generally, there are no further statements about scattering or disturbing reflections and their correlation to perception. This work presents the development of a display measuring device which can measure several optical parameters simultaneously as well as in a spatially resolved manner. The measuring principle was oriented on the DIN EN ISO 2813 standard with measuring reflection geometries of 20° and 60° relative to the normal. The determined objective properties were correlated with the subjective perception of the end user using a laboratory study. In this study, subjects assessed the reflections from the displays in terms of gloss and scatter on a scale of 1 (minimal) to 5(maximal). The influence of different settings of viewing positions, ambient brightness and display brightness was investigated. For the comparison between objective and subjective data, the subjective scale was related to the objective measured value for gloss and dispersion. A correlation between objectively measured data and the subjective perception of optical properties of displays was observed. This correlation occurs mainly in extremely glossy and matt displays.

The wavefront of coated optics is one of critical performances. Due to the interference between the coating layers, the measurement results will be totally different if the measurement wavelength is different from the working wavelength. However, all of the commercial interferometers have single measurement wavelength, which can’t treat the optical coatings working at various wavelengths. A wavelength-switchable interferometer (WSI) capable of detecting wavefront information in a wide wavelength range of 488-1064 nm is proposed in this paper. The principle of design and performance of the system are given in detail. Some typical measurement applications, such as reflection plate and optical filters will also be presented.

This letter reports a novel method for light field three-dimensional measurement by using unfocused plenoptic cameras. A light field metric model, based on which nonmetric depths in the image space can be mapped to metric dimensions in the object space, was established. Furthermore, with the aid of a three-dimensional measurement system, light field metric calibration was carried out by determining light rays with spatio-angular parameters and set up the depth relationship between the object and image spaces in accordance with the light field image properties.

Laser radiation for a high precision measurement such as nanostructure measurement needed to be clean of wavefront aberrations. However, only expensive lasers with stabilization can achieve such robust parameters. The emission is needed to be corrected via adaptive optics. Nevertheless, high precision measurement of wavefront aberrations is necessary to high precision correction. The present work is dedicated to the problem of computer-generated Fourier holograms application for measurement of optical wavefront curvature with high precision. Mathematical modeling and experimental results of the method with the comparison of different approaches are presented.

White light scanning interferometry (WLSI) is a fast, noncontact, high-precision method to measure three-dimensional (3D) surface profile and extensively used in roughness measurement of ultra-precision machined surface. However, due to Rayleigh criterion, the lateral resolution of WLSI is limited to hundreds of nanometers. It is hard to measure rough surfaces with delicate details that adjacent distance less than lateral resolution. Also, WLSI can’t measure profiles with large surface gradient for no light reflected and received by objective lens. In this work, with a proposed simulation measurement model, surface gradient error and lateral resolution error on measuring result of WLSI is studied by simulating the response characteristics of sinusoidal signal, square signal, sawtooth signal and actual surface profile of grinding silicon wafer measured by AFM respectively. The effectiveness of the simulation model is verified by comparing the simulation results with the experimental results. The mechanism of surface gradient error and lateral resolution error is revealed from the perspective of simulation analysis, which has certain guiding significance for the future research of error analysis on white light scanning interference.

The SS-OCT system was constructed using a swept laser source and a compact Michelson interferometer. The outputs of sweep source include laser signal and k-trigger signal. The laser signal realizes spectral interference and the k-trigger signal realizes acquisition interferometer signal with uniformly wavenumber interval. However, there are two problems. The first one is that the output wavenumber of the source changes non-linear with time. To solve this problem, a wavenumber phase equalization algorithm is proposed. The second problem is that the unfixed delay between the spectral calibration signal k-trigger and the OCT signal, which makes the result of spectral calibration incorrectly. Aiming at the asynchronization between the acquire OCT signal and the k-trigger signal, an algorithm of k-clock delay correction based on cross correlation is adopted to correct the delay. The SS-OCT system could be used for accurate measurement the curvature of the laser focusing lens due to its excellent advantages such as non-destructive, high resolution and high inspection speed. The 2D tomography maps of a laser focusing lens are measured by this system and real-time images are obtained.

Positioning an interferometric sensor with respect to the specimen requires a precise and expensive motion system to retain the sensors accuracy. A cost-efficient and precise setup requires a compensation of any run-out that the motion system induces. This contribution demonstrates that run-outs of a rotational axis can be compensated by implementing a low-cost interferometric point sensor to a previously presented interferometric line-scan system. Furthermore, the setup is extended by an evaluating algorithm that is capable of evaluating the line-sensor’s measurement data in real-time, using either a CPU or a GPU. An improved mechanical design of the interferometric sensor is introduced. It consists of fewer mechanical parts compared to previous versions, thus making the sensor more efficient in production and more robust against vibrations. An up-to-date high-speed line camera with a length of 4,000 pixels and line rate of up to 200 kHz increases the measurement rate of the sensor to up to 2,000 height values per second per camera pixel, enabling the sensor to evaluate 8 million height values per second.

Dynamic speckle analysis is an effective approach for industrial inspection of speed of processes. The relevant information is retrieved by statistical processing of temporal sequences of speckle patterns formed on the surface of diffusely reflecting objects under laser illumination. The measurement output is a 2D activity map, which shows regions of higher or lower activity in the object. Following a process in time requires a great number of images, in which the recorded values of intensity are of minor importance. In view of the dynamic speckle measurement specifics, we propose in the paper coarser quantization of the raw speckle data as an approach for data compression. Its efficacy is proved by processing numerical and experimental speckle patterns. Decreasing the number of the quantization levels down to 32 causes no distortions in the activity map. Further decrease of the quantization level number is applicable only to data acquired at uniform illumination and equal reflectivity across the object surface.

The combination of engineering geodesy methods with computer processing of digital images on the matrix field of analysis has found wide application in various automatic systems for monitoring the mutual spatial position of elements. The use of stereoscopic schemes allows to control the spatial position of the railway track relative to the brands placed on the supports of the contact network. When moving along the path in such schemes, additional errors arise due to blurring on the photo-receiving matrix fields. The article discusses the methodology for estimating additional errors due to the movement of the stereoscopic optical-electronic system relative to fixed reference marks. Ways to reduce additional errors from the speed of movement by adaptive control of the size of the input apertures and the exposure time of images on photo-receiving matrix fields are proposed.

The measurement of the spectral diffraction efficiencies of a diffraction grating is essential for improving the manufacturing technique and for assessing the grating’s function in practical applications. The drawback of the currently popular measurement technique is its slow speed due to the hundreds of repetitions of two kinds of time-consuming mechanical movements during the measuring process. This limitation greatly restricts the usage of this technique in dynamic measurement. We present here a motionless and fast measurement technique for obtaining the spectral diffraction efficiencies of a plane grating, effectively eliminating the aforementioned two kinds of mechanical movement. We estimate that the spectral measurement can be achieved on a millisecond timescale. Our motionless and fast measuring technique will find broad applications in dynamic measurement environments and mass industrial testing.

Digital video cameras are the main component of both visual and measuring optoelectronic devices. The parameters and characteristics of video cameras can be varied significantly for each instance. Since the characteristics of a video camera mostly determine the characteristics of the entire device, it is important to monitor them. This will ensure the stability of the characteristics of video cameras, and consequently, image stability and improvement of measurement accuracy in the case of using video cameras in optical-electronic measuring devices. This paper presents an experimental test bench designed to study the parameters of serial video cameras based on CMOS matrix photodetectors. Method are proposed for determining such camera parameters as irregularity of photosensitivity around the site, as well as change in the signal-to-noise ratio with variations in the level of exposure, exposure time and amplifier coefficient.

In this paper we discuss a near real time digital holographic imaging algorithm achieving ∼4 fps operation speed on a common central processing unit. The hologram recording is performed in the off-axis geometry in the transmission mode. The algorithm follows a standard angular spectrum method routine and utilizes experimental calibration of the optical instrument for aberration correction. The main limiting factor is related to the size of the initial hologram and its Fourier transform (∼57% of the total execution duration). The performance of the approach is tested on different transparent and semi-transparent samples for reconstruction of sample topography and object in-depth allocation.

The diameter of a main mirror of the large radiotelescope RT-70 (Suffa) is value 70 meters and it contains 1200 metal panels. The deformation of the radio telescope construction moves the reflecting panels, so it is necessary to measure these deviations from theoretical 3-D parabola. The permissible measuring error is not more 0.1 mm for distance 35 m. Following issues dealing with this problem are described in this article: 1) the new scheme of optic-electronic system for measuring the shift of the reflecting panel; 2) the problems of the of the multi-matrix measuring block designing. The great attention during the research was paid to the experimental approval of the theoretical results. The experimental setup had the following parameters: infrared emission diode by power 15 mWt as sources of radiation on the panel of mirror; the objective by the focal length 450 mm as aperture of receiver video-camera, the CMOS matrix receiver by type OV05610 Color CMOS QSXGA with 2592*1944 pixels and one pixel size (2.8*2.8) μm² produced OmniVision as image analyze. The experimental error measurement was 0.07 mm at the range 20 mm on a working distance 20 m, which allows measuring the deformation of radiotelescope construction with the mirror diameter 70 m.

In this paper, a micro-opto-mechanical polymeric cantilever based Fabry Perot Interferometer(CFPI) is reported as an optical fiber sensor. To be able to fabricate the sensor, a mold for the polymeric cantilever was fabricated using poly methyl methacrylate (PMMA) and with which a polydimethylsiloxane (PDMS) cantilever was constructed. After releasing the cantilever from the mold, cantilever and SMF were integrated together. As a result, a cavity was formed between SMF cleaved end and cantilever surface. When vibration is applied, the distance between the cantilever free end and cleaved end SMF changes, consequently the optical patch length of the sensor structure altered dynamically. Here, the effect of different frequencies and their amplitudes on the sensor spectrum is studied by using FFT and STFT analysis method.

We have generated soft X-ray pulses at the wavelength around 30 nm and 10 nm using high-order harmonic generation (HHG) in Ar and Ne gas targets respectively with low repetition rates, hundred-terawatt-level laser system in Shanghai Institute of Optics and Fine Mechanics (SIOM). The shortest wavelength of our generated harmonics is 9.8 nm at the 81th harmonic with the Ne gas target. Around the wavelength of 13 nm, the output energy of the 59th harmonic (13.5 nm) and the 61th harmonic (13.1 nm) reaches 10 nJ per pulse. This highly coherent extreme ultraviolet (XUV) source can be served as a potential seed for the free-electron laser (XFEL) with the method of laser wakefield acceleration (LWFA). Moreover, the 13 nm HHG source can be applied to coherent diffraction imaging (CDI) and XUV lithography.

Carbon fiber reinforced plastics (CFRP) are composite materials which are an interesting alternative to metal alloys in fields such as oil, aerospace, automotive, since CFRP have mechanical properties like metals but with a fraction of their weights. However, these materials have typically a highly anisotropic behavior, which may hinder the characterization of their integrity for example when subjected to an impact, because of its stochastic nature. Non-destructive testing (NDT) methods are interesting for integrity assessment, as they can evaluate the damage extension without affecting any part characteristics. Optical lock-in thermography (OLT) is an convenient NDT inspection alternative since it is a depth-wise method in which one can set different loading frequencies, leading to different scan depths. Pre-processing techniques like Principal Component Analysis (PCA) and Empirical Mode Decomposition (EMD) can be used to more accurately evaluate the damaged area. Their dimensionality reduction capability is highly desired as OLT images of CFRP laminates do not only show the defect, but also undesired information such as changes of background radiation, noise and the disposition of the fiber tissue. Traditional feature extraction methods must be highly tuned to obtain useful results. PCA and EMD methods may be considered non-supervised approaches, making them useful for a wide variety of inputs. However, PCA and EMD have their own natural limitations when being applied to images. While PCA may require high computational effort because of mathematical manipulation of matrices due to the mathematical manipulation of large matrices, problems due to mode superposition may occur if EMD original method is applied on images. In this sense, in this work it has been performed a comparison between PCA, with a new input vector architecture used to mitigate its problem with matrix dimensions, and a derivation of EMD method, called Multi-dimensional Ensemble Empirical Mode Decomposition (MEEMD), applied to prevent the previously mentioned superposition but without a higher computational effort, when segmenting OLT phase images. Outputs in both cases are binary masks with an estimation of the defect region, which were compared to a ground truth manually defined by a specialist. Matthews Correlation Coefficient (MCC) was chosen as segmentation comparison metric instead of F-score, since the last one does not take true negatives into account. The main difference during the application of PCA and MEEMD is that MEEMD yields two kinds of results, one for each frequency image and one average for the whole frequency set, whereas PCA gives only results in relation to the average. This study shown that PCA has better performance than MEEMD when the average results are compared, but MEEMD provides better results if onlt the best image per frequency set is used. Both PCA and MEEMD can yet provide interesting results for three-dimensional reconstruction with OLT images, thus further investigation of such techniques is desirable.

Position determination is key in many aspects of automation, production technologies, and QA/QC. Optical technologies are used in several position sensing technologies, such as single-axis incremental encoders based on grating scales and interferometers. A lesser known approach for position sensing is based on imaging systems, where the position scale is based on semi-regular patterns read by a camera equipped with suitable optics. Specific algorithms extract the in-plane coordinates of the patterns that are conjugated with the center of the read head, along with the angle that corresponds to the orientation of the patterns. Consequently, it is possible to turn a machine vision camera into an encoder with 3 degrees of freedom: in-plane position x, y, and in-plane orientation φz. Through a proper design of the 2-dimension scales, proprietary decoding software, and adequate implementation, we have been able to design and demonstrate absolute position determination with the nanoGPS Oxyo technology, with nm resolution on x, y, and accuracy better than 100nm over 80mm. The resolution on the orientation has been found to be better than 10μrad on φz. As an example, we have shown that this system can be extremely efficient for the performance assessment of a microscopy stage. The nanoGPS Oxyo technology can be used to turn any camera into an absolute position encoder. This can be useful for a variety of purposes, such as checking trajectories, or checking the repositioning capabilities, or checking cameras axis alignment are with machine movement axis, etc. These tasks can be performed either on a QA/QC basis (in this case the scale is placed during the QC operation, and removed after), or on a permanent basis (in this case the scale should be integrated to the machine).

In this paper, nonlinear noises that characterize the performance of a long-haul optical fiber sensing system were investigated. In a 50 km transmission system, when stimulated Brillouin scattering (SBS) occurs seriously, the phase noise of the interferometer increases from -102dB (0dB=1rad/sqrt(Hz)) to -84dB due to the enlargement of the laser linewidth and the deterioration of the signal-to-noise ratio (SNR). While the phase modulation (PM) and the Phase-generated carrier (PGC) modulation to the laser frequency are applied simultaneously, the suppression of SBS is 35dB and 10dB respectively in the backscattering spectra and the interferometric phase noise caused by SBS is completely eliminated. When the input power continues to increase and exceeds the modulation instability (MI) threshold, the system performance also deteriorates significantly. The forward output spectra of the 50 km optical fiber and phase noise of the interferometer are measured. The results show that with the increase of the injection power, the increase trend of the MI component in the total power of the spectrum is approximately consistent with that of the phase noise. It can be concluded that the phase noise introduced by MI is mainly caused by the increase of light intensity noise and the deterioration of optical SNR. Therefore, in order to reduce the impact of MI in the sensor system, it is needed to avoid the generation of serious MI as far as possible, and then the ultra-narrow band filter should be used to filter the MI sideband for the improvement of the system SNR.

We describe a wafer prober integrated with an optical probe for wafer-level inspection of photonic integrated circuits. The design of the electric and photonic circuit was optimized for wafer-level inspection. The customized prober and circuit design enabled us to perform high-volume and high-speed inspection of over 400 elements, and sufficiently reliable results were obtained. It took about 10 sec. to evaluate the propagation loss of an element. This technology will be a key to reducing the costs of photonic devices.

In this paper a novel optical fiber interferometer structure for ultra-large dynamic range detection is proposed. The structure combines conventional 3×3 interferometer with optical differential 3×3 interferometer. And the sensing fiber of the conventional interferometer is used as the transmission fiber of the differential interferometer while sensing. When the external signal acts on the sensing fiber, the conventional coherent detection and differential coherent detection can be carried out simultaneously. Conventional interferometer is used to detect the normal phase change of interferometric signals. However differential interferometer can detect the difference of the phase change, that is, the undistorted phase compression signal. Then the actual signal waveform can be obtained by integrating the compressed signal, so that the detection of large signal can be realized. The simulation analysis and experimental results show that the dynamic range of 200dB can be obtained within 20Hz-10kHz band. The structure of combined interferometer uses continuous light injection and has the advantages of simple structure and low cost. It can be used to detect wide-band and ultra-large signal and has good application prospects.

Holographic tomography (HT) enables measurement of three-dimensional refractive index distribution of transparent micro-objects by merging information from multiple transmitted waves corresponding to various illumination directions. HT has proven its great potential in technical inspection and biomedical studies; nonetheless, its further progress is hindered by inability of the standard reconstruction algorithms to account for multiple scattering. This limitation has been recently addressed with a few novel reconstruction approaches. In those techniques the tomographic reconstruction is iteratively improved by minimizing discrepancy between the experimentally acquired transmitted fields u_E(x,y) and the analogical data u_q(x,y) obtained via numerical propagation of the incident beams through the current refractive index estimate n_q(x,y,z). The accuracy of these multiple-scattering reconstruction methods depends primarily on two features: (1) the forward model that allows computing the transmitted fields u_q; (2) the feedback mechanism that converts u_E - u_q discrepancy into the reconstruction upgrade n_q+1=n_q+Δn_q+1. In our work, we address the first issue with the wave propagation method that represents a reasonable trade-off between accuracy and time of computation. The paper focuses primary on the second issue, i.e. the feedback mechanism, that considerably influences the performance of the multiple-scattering reconstruction methods. In our work, we cross-analyze two feedback solutions, i.e. the gradient descent and the forward backward method. The performance of these solutions is tested via numerical simulations on different types of samples: step-objects representing technical samples and gradient structures emulating biological specimens. Our study investigates accuracy of the reconstruction, time of computation as well as stability and flexibility of the feedback method.

While industry 4.0 thrives, the needs related to three-dimensional assessment of industrial products growths in complexity. Frequently, this complexity cannot be achieved by one single sensor without outstandingly escalating procedural expenses, which can be prohibitive for many applications. Photogrammetry is an imagebased three-dimensional measurement technique which can offer good metrological results, although presenting limitations specially when using the object texture for point correspondence. Employing an infrared camera can imply in advantages when measuring components with low number of surface features or translucent workpieces, but generally they do not have resolution enough to properly reconstruct the object with typical photogrammetry techniques. This scenario is proper for using data fusion of two or more sensors, resulting in a more informative point cloud. Therefore, it is proposed in this work the three-dimensional measurement of a transparent workpiece using data seized from a visible-light camera and an infrared camera. In the proposed approach, a pixel-level image fusion technique based on two-dimensional wavelet decomposition as a step of the registration process is used to combine images from these devices. This procedure is compared with the reconstruction of the object using only the infrared images and only the visible-light images. The results show a more complete point cloud using data fusion compared to use only visible-light or infrared images.

The article describes the general models, the basic principles of work and features of the construction of the photometric unit. Various factors affecting the photometric module are described. A method for measuring the overall energy calculation of the optical system of a photometric module has been proposed. Research has been conducted to develop methods for determining the sensitivity characteristics of a photomultiplier, finding ways to reduce the impact of the photomultiplier’s sensitivity non-uniformity on the area when diamonds are detected. As a result of experimental studies, dependences were obtained from which it can be concluded that the photomultiplier’s sensitivity non-uniformity affects the process of extracting diamond-containing ore.

Stereoscopic video endoscopes are widely used for remote visual inspection and precise three-dimensional (3D) measurements in industrial and biomedical applications. The reconstruction of 3D points from the corresponding image points requires calibration procedure which accuracy affects the measurement uncertainty. We propose to perform an optimal choice of the calibration technique and the calibration target parameters using the computer simulation at the design stage. The effectiveness of this approach is demonstrated via the design of self-developed miniature prism-based stereoscopic system. We simulated acquisition of the calibration and measurement data using optical design software. The conventional calibration technique requiring many positions of the flat target with arbitrary displacements and rotations was compared with another one, which uses the translation stage to provide pure translation of the target. We analyzed the impact of the translation uncertainty, the number of positions, the number of targets and the uncertainty of image point coordinates on the uncertainty of calibration parameters and 3D measurements. We have shown that the second technique could provide the same calibration accuracy as the first one with less number of images. The results of computer simulation were confirmed experimentally using the prototype of the self-developed stereoscopic endoscope. The proposed approach may be used to optimize calibration techniques and reduce a cost of calibration equipment for various stereoscopic measurement systems.

The metrological maintenance of many measurement tasks requires measuring spatial position of some control objects relative to rigid base. The microwave radio-telescope development requires high-precision control for the position of the counter-reflector in relation to main mirror. The construction elements weight and thermal deformation leads to changes of position and linear shift of each planar section relative to theoretical parabola. Nowadays international project Millimetron led by the Russian Space Agency is developing. In case of originality of Millimetron construction is necessary to realize special system for controlling its segments’ positions. The triangulation method is chosen to solve this problem. This method means CMOS cameras, placed on the base ring close to the top of main mirror. On every segment there are placed three emission diodes, determining its spatial position. In accordance to the method video camera measures the LED vision angles. Every video camera measures position of two adjacent parts of main mirror. Measuring channel consists of two nearby cameras with crossed angle fields of view. The experimental research of the structure of the system error was made with the made computer model. There are two parts described in this article. First one is discussion on algorithm of detection images of control points and calculation its coordinates. Second one is about analysis are the primary errors influencing the error of linear and angular position measurements of control object. The results of computer modeling and research of experiment sample are discussed

This paper is devoted to the research of solid-state matrix photomultiplier’s polarization-optical parameters. Maim parameters of SiPM detectors depend on photodetector’s sensitivity. As an object of study, a silicon photomultiplier ARRAY-C 60035-4P was chosen. Detector consists of 4 photosensitive sites. SiPM contains avalanche photodiodes separated from each other by elements that do not participate in the formation of the useful signal and are used for pacifying secondary optical signal. In this work experimental studies of the state of radiation’s polarization reflected from the surface of SiPM matrix’s active regions are performed using ellipsometer LEF-3F-1. The methodic used is a zero method of determining the polarization angles. During experiment the contractions of ellipsometric angles were determined. The experiment was carried out at four angles of incidence on the surface of the receiver, which corresponds to a set of reflective characteristics of a silicon photoelectric multiplier. Using this data, the estimation of distribution of the reflection and transmission coefficients becomes possible, as well as the sensitivity distribution over the different sites of the SiPM.

A hole diameter measurement is a common task in mechanical engineering metrology. Inner hole dimension is usually measured with contact gauges touching the inner wall. Another approach is the hole diameter estimation by a contact or optical measurement of the diameter at the top or bottom of the hole. Such approach does not allow for the measurement of the real geometry of the hole inner walls. As the hole diameter decreases, small geometry variations along the hole axis lead to a large relative error between the actual and measured diameter value. We assessed several different optical schemes allowing for an optical measurement of a hole diameter by reflection of light towards the hole inner wall. We selected and tested a reflection optical laser triangulation approach for measurement of side wall of a recess and we analyzed the allowable depth of a bore enabling the hole diameter estimation by scanning of the inner wall along the hole axis.

In this paper, we investigate the effect of acetone gas concentration on the transmission spectrum of a micro polymeric tapered optical fiber sensor. To study the effect of acetone concentration, the swelling effect of polydimethylsiloxane (PDMS) is studied. Presented sensor demonstrates acetone concentration sensitivity of ~4.1×10^-4 nm/ppm in the range of 0 and 17600 ppm. Considering our interrogation unit resolution, minimum detectable concentration around ~4.8 ppm can be achieved.

As a summary of of the authors' previous paper of Ref 1, we describe a new scheme of a Linnik interferometric configuration based on spectrally-resolved white-light interferometry for simultaneous measurement of top surface and its underlying film surfaces in multilayer film structure. Our proposed technique enables accurate measurements of the phase and reflectance over a large range of wavelengths using the iterative least-squares phase-shifting algorithm by suppressing critical phase shift errors, and it provides a better measurement result than conventional methods. To verify our method a complex multilayer film was prepared and we measured it, and compared with well-known conventional techniques. Comparison results show our new method successfully works well with high precision as same as existing methods.

The results of optical diagnostics of physical processes occurring on the surface of Nickel alloy powder melt in selective laser melting (SLM) technology are presented. The independent registration of the dynamics of the brightness temperature and the fraction of the laser radiation reflected from the surface of the melt was carried out. The dependences of the surface temperature of the melt averaged over the time of observation on the specific volumetric heat input were obtained for the two values of the width of the generated paths. A multiwave method of optical diagnostics is proposed, including monitoring of laser radiation reflected from the melt surface. It is shown that amplitude pulsations of the reflected radiation induced by fluctuations of the relief of the melt surface associated with dynamics of the surface temperature of the melt: the number of ripples on the waveform of the reflected radiation increases at the stages of decline of the brightness temperature. This observed phenomenon shows the relationship between changes in surface topography and convective processes in the melt during laser heating. Analysis of the signals determined by the reflection of laser radiation from the melt surface allows to determine the moments of change in the intensity of heat and mass transfer in a shorter time than when controlling the melt temperature. The results of the study can be used in the development of methods and tools for monitoring and operational control of the SLM process.

Experimental results of on-line monitoring of temperature on the surface of a melt of stainless steel 304 in laser metal deposition technology with an inclined position of the sensor relative to the laser beam are presented. The additive synthesis setup uses the heating of a coaxial flow of a gas-powder mixture in the field of focused laser radiation from a fiber-optic ytterbium laser with a continuous power up to 400 W. To control the temperature on the surface of the melt, a multichannel pyrometer in the SWIR brightness pyrometer mode was used. The spatial resolution of the optical system was approximately 100 μm, and the speed of the sensors was at least 5 kHz. A multi-wave optical diagnostics method is proposed, including monitoring of laser radiation reflected from the melt surface. The features of optical diagnostics of the surface of the melt in the technology of laser metal deposition are given. The results of measuring the temporal temperature behavior as a function of the scanning speed and the distance from the substrate to the laser focus are presented. The spectrum of temperature pulsations T* during the formation of the track is calculated. It is shown that as the scanning speed increases, the pulsation spectrum of T* shifts toward higher frequencies in proportion to the speed. It is also shown that the temperature T* and the mass productivity of track formation increase with a change in the distance from the substrate to the laser focus from 10 to 11 mm, and also with an increase in the powder mass flow rate from 8.4 g/min to 15.6 g/min

During the manufacturing process of heavy forgings, simple contact measuring techniques are still used to check the dimensions, therefore an optical measuring system is in demand. In this paper, a camera calibration method for the passive measuring system, which is being developed in collaboration with an industrial partner, is proposed. Our approach is based on space resection and works with robust coded targets, which are distributed in the field of view. The coordinates of targets are measured using TRITOP (GOM) measuring system. This solution allows to build a large calibration field, without a need of large calibration objects. The camera calibration works in 2 steps - at first, the intrinsic parameters of the camera, including lens distortion, are calibrated. These parameters are considered as stable, due to the use of special camera covers. Multi-image version of the calibration method and dense field of calibration targets are used. The second step is performed from every image and employs a single-image extrinsic camera parameters calibration method. Only a few coded calibration targets, mounted on stable objects in the scene, are required. The calibration method was tested in industrial conditions. The method showed great results, the average reprojection error was under 0.1 px. The effect of thermally affected zone on the calibration process is discussed.

Our paper presents the main results of the research and development of an optical scheme of the autocollimator to control the deflector rotation angles. We have performed a literature review of existing schemes basic devices, the peculiarities of their work, the scope, the analysis of the advantages and disadvantages of existing devices. The main difficulty in the development of the optical scheme of the autocollimator in our study is the high requirement for the accuracy of the calculation of the deflector rotation angles and the processing speed of the measurement data. We have sold this problem by reducing the size of the image of the autocollimation spot to a few micrometers and the normal distribution of energy within the image in the entire range of angular fields of the device. We are presenting the original methods of aberration correction of the autocollimator optical system due to the use of optical surfaces with special properties.

To keep up with Moore’s law in future, the critical dimensions of device features must further decrease in size. Thus, the nano-electronics and nano-optics manufacturing is based on the ongoing development of the lithography and encompasses also some unconventional methods. In this context, we use the Nanopositioning and Nanomeasuring Machine (NPMM) to generate features in resist layers by means of Direct Laser Writing (DLW),¹ Field Emission Scanning Probe Lithography (FE-SPL)² and Soft UV-Nanoimprint Lithography (Soft UV-NIL)³ with highest accuracy. The NPMM was collaboratively developed by TU Ilmenau and SIOS Meßtechnik GmbH.⁴ The tool provides a large positioning volume of 25 mm × 25 mm × 5 mm with a positioning resolution of 0.1 nm and a repeatability of less than 0.3 nm over the full range. Previously a single electron transistor (SET) working at room temperature generated by FE-SPL has been demonstrated.⁵ However, the throughput is limited because of the serial writing scheme making Tennant’s law (At ∼ R5 ) valid.⁶ Here, At is the areal throughput and R the lithographic resolution. Thus, patterning of the whole NPMM positioning area by FE-SPL is very time consuming. In order to address this problem, different strategies and/or combinations are conceivable. In this work a so-called Mix-and-Match lithography is conducted. A fast generation of structures in the sub-micron range is possible by means of DLW. By this, features such as electrical wires, contact patches for bonding or labels are generated in resist. Subsequently, we use FE-SPL in order to define the actual nano-scaled features for quantum or single electron devices. In combination, DLW and FE-SPL are maskless lithography strategies, hence, offering completely novel opportunities for rapid nanoscale prototyping of largescale resist patterns. An explanation of this technique is given in a previous publication.⁷ Furthermore, after reactive ion etching, the sample can be used as template for Soft UV-NIL, thus resulting in a high-throughput process chain for future quantum and/or single electron devices.

In this work, a method based on vortex operator, for removal of monotonically increasing or decreasing phase ambiguity in the retrieved phase by the Riesz transform method in digital interferometric techniques is presented. Since in digital interferometric techniques, the phase extraction methods and algorithms are essential because these techniques are being continuously employed in many scientific, industrial, and engineering applications to measure various physical parameters which are encoded as the phase of the fringe pattern. There exist many methods/algorithms for phase extraction from the fringe pattern such as temporal phase-shifting, spatial phase-shifting, fast Fourier transforms (FFT) method, wavelet transform, Hilbert transform etc. In recent years, phase extraction from a single fringe pattern by using the Riesz transform method is developed because of its several advantages. However, the retrieved phase by Riesz transform is affected by π shifts due to the lack of discrimination between positive and negative spatial frequencies. This problem could be resolved by using a vortex operator which filters the data in the frequency domain. We present here some simulated results demonstrating the removal of phase ambiguity in the retrieved phase by Riesz transform method.

This paper proposes the use of an optoelectronic autocollimator constructed according to an auto-reflection scheme with a passive control element as a device for monitoring the supporting structures of infrastructure facilities. The results of the possibility of applying the autoreflection scheme for solving this problem, information about the control element and solving problems arising from its use are presented in this paper. The control element does not require a power supply at the control point and can be installed anywhere in the structure. The theoretical results are backed by experimental results. The main feature of the proposed solution is a single-channel system with reduced dimensions and weight in comparison with traditional schemes. The device allows you to register the reflected radiation from the passive control element installed on the monitored object and to determine the angular displacement along the axes OX and OY and linear displacements along the same axes in real time. The proposed solution in comparison with analogues has a smaller weight and size, is able to control a larger number of coordinates and is not inferior in accuracy while having a large metrological range by reducing the area of inoperability and features of the auto-reflection scheme.

This article reflects the application of optical spectroscopy methods in automatic control systems of combustion process in thermal power plants and different transport systems engines. Forming of controlled states and error signals is carried out by means of the multichannel optical spectrometer where the spectral decomposition is based on a resonance phenomenon in the narrow-band optical interference filters on the measurement data of the electromagnetic radiation spectrum as one of the results of the combustion process. This device carries out the contactless spectrum analysis in the predetermined optical ranges using a set of narrow-band interference optical filters tuned to certain wavelengths and fiber-optical system using for analyzed optical radiation transmitting. This radiation carries the spectroscopic information about the controlled combustion process. Using fiber-optical system excludes the negative impact of various extreme factors on the spectrometer operation as part of the error signal forming system. The special principle of the developed multichannel optical spectrometer construction allows applying not only one optical fiber but a fiber bundle. That increases the sensitivity of the device without reducing of its resolution. The novelty of the developed device is confirmed by the patent of the Russian Federation. This paper presents the results of the development of the multichannel optical spectrometer set-up and the results of its experimental research. The experimental research is carried out to determine the spectral line of copper (Cu) when burning copper powder.

Segmented and deployable primary mirror telescope is adopted to realize a higher resolution observation. Meanwhile the cophasing error is introduced. The piston error between the segments should be smaller than λ/20 RMS to achieve a diffraction-limited imaging. However the initial piston error is about 200 μm. A high-accurate piston error measurement with a large capture range is needed. We propose a method to simultaneously detect the multi-piston errors between segments with a high accuracy in a large capture range. A mask with a sparse sub-aperture configuration is set in the exit-pupil plane of the telescope to sample the wave from the segments. The relation between the piston error of any two segments and the amplitude of the modulation transfer function (MTF) sidelobes (MTF_nph) is derived according to the Fourier optics principle. The piston error can be retrieved by this relation after measuring the MTF_nph. Simulation and experiments have been carried out to validate the feasibility of the method. The results state that this method's capture range is the operating light’s coherence length, the accuracy is 0.026λ RMS (λ = 633 nm). The MTF model of a mask with sparse multi-subaperture configuration is established. The arrangement rules, to avoid the sidelobes overlapping, are obtained. The mask with a sparse 18 subaperture configuration is designed, which makes the MTF sidelobes distribution non-redundant. Consequently, just a mask with a sparse multi-subaperture configuration is needed, simultaneous detection of the multi-piston errors can be realized in term of this method.

We describes the status of AO test bench, which is developing at the Adaptive optics Lab, V.E. Zuev Institute of Atmospheric Optics of the Siberian Branch of the Russian Academy of Sciences (IAO SB RAS), Tomsk, Russia to simulate predictive algorithms of wavefront adaptive correction. The description of the optical and mechanical design, components AO bench, and the working principle and first experimental results are presented. The current AO test bench consists of laser source, two deformable mirrors with 59 actuators and 56 mm diameter (Visionica Ltd., Russia), two tip/tilt mirrors (IAO SB RAS, Russia), Shack-Hartmann Wavefront Sensor (WFS), which we specially designed, and a science camera for the evaluation of the performance. The user derived aberrations are introduced using a one deformable mirror and corrected by another deformable mirror. The tip/tilt mirrors are used for predictive control of the low-order wavefront aberrations related such as vibrations.

The presented x-y stage measurement unit uses 2 linear cross mounted stages, where the stages are mounted on a heavy aluminum holder to ensure low vibrations and precise repeatable positions. The travel range in x and y direction is 52 mm resulting in a 52 mm x 52 mm scan area. The lowest step size in x and y direction is ▵x = ▵y = 0.2 μm. The software LabView controls with a computer the 2 stages and as well as the detector to ensure a fully automated set-up. After implementing, mounting and testing all components of this measurement unit repeatability tests were performed with a red LED array as source. 40 different measurements (step size ▵x = ▵y = 1.0 mm over full 52 mm x 52 mm) were taken under same conditions. A statistical analysis, results in a low uncertainty of < 0.06 % (1 standard deviation) as maximum deviation. This shows the high precision reached with this developed fully automated measurement unit..

In this study, we demonstrate reduction of speckle noises of digital holography using speckle correlation properties in the longitudinal direction. In this method, it is assumed that digital holograms of diffuse objects are recorded on an image sensor. Using multiple holograms recorded by moving the image sensor to the longitudinal direction, the speckle noise of a reconstructed image is reduced and therefore the image quality is improved by the proposed method.

The optical characteristics of KDP/DKDP crystals, including thermal absorption and nonlinear absorption, were investigated through thermal lens method and Z-scan method. It is found that the thermal absorption in KDP crystal behaves anisotropy at 355nm and 532nm and nonlinear absorption anisotropy in KDP/DKDP crystals is also found at 520nm. For KDP and DKDP samples, the thermal absorption coefficient of crystals with different orientations is slightly different. The four-photon absorption coefficient γ of z-cut KDP sample is larger than that of other orientations. The relationship of nonlinear absorption coefficient γ is z<II<I for KDP sample, while II≈z for DKDP sample.

Due to the continuing trend to minimize micro structures on technical surfaces the requirements on measuring sensors are also increasing. This includes the lateral and axial resolution as well as the measuring speed. The latter is provided by using optical sensors like confocal microscopes, laser and coherence scanning interferometers. However, artifacts may occur in the measurement result leading to an erroneous reproduction of the surface to be measured. In order to investigate these artifacts and to optimize the setup of optical sensors the measuring results of different optical sensors can be compared among each other as well as with further sensors by using a multisensor measuring setup. Important characteristics of topography sensors are their axial accuracy and the lateral resolution. For this purpose, a standardized repeatability is determined for each sensor. In addition, measurements on technical surfaces to investigate the lateral resolution are presented.

We propose the method of measuring the refractive indices (RI) of transparent films using White Light Scanning Interferometry(WLSI). Fourier-transformed interferogram is represented as a DC-term and the sinusoidal term whose amplitude is the product of the sample reflectance and the energy term which is determined by the reference mirror and a light source. Once we get the interferogram data of the reference sample whose RI is known, we can obtain the energy term of the WLSI system. After obtaining the energy term of the WLSI system, we measure the interferogram data of the sample whose RI is unknown under the same experimental conditions. Combining these energy terms and the interferogram, we get the RI of the sample for each wavelength.

This paper reports on the development of an end-effector mounted, multi degree-of-freedom positioning sensor capable of measuring the (x,y,z) movements relative to an object. The instrument termed wPOS (workpiece Positioning Sensor), combines two complimentary non-contact optical techniques; laser speckle correlation (LSC) and Range-Resolved Interferometry (RRI). Laser speckle correlation is used to measure the in-plane position (x,y) relative to a reference point, while the RRI system provides an out-of-plane (z) absolute range measurement and correction of the in-plane LSC measurement for varying working distance. The concept and operating principles of the instrument is described along with details of the prototype sensor implementation and exemplar results showing accuracies of <30μm after 0.75m lateral travel and repeatability between measurements of ~7 μm.

Retroreflective traffic markings are frequently used on roadways to provide guidance to drivers as supplements to regular markings. Portable retroreflectometers are widely used to measure the photometric characteristic of retroreflective traffic markings at present. Portable retroreflectometers include an internal light source and photoreceptors. It is based on the substitution method. Substitution relies on the use of calibrated reference plate. The traffic marking has a low coefficient of retroreflected luminance. It is difficult to measure the coefficient by direct measurement method. The direct measurement method cannot assign measurement values to the reference plates. The paper proposes a new measurement method to solve the problem of measuring the photometric characteristic of the retroreflective traffic marking. It is called the expanded direct luminous intensity method, and it is different from the four methods in JT/T 690 Test Method for Photometric Characteristics of Retroreflectors. The expanded direct luminous intensity method based on CIE angular reference system. A calibrated standard source A illuminates the specimen at a distance of 15 m from the specimen. And a calibrated low-light illuminometer is used to measure the retroreflectivity of the specimen. The paper built a standard system according to this method. The measurement uncertainty of the system is 3.1% while k is 2. After comparing with several different kinds of portable retroreflectometers, the results were satisfactory. Studies have shown that this method and standard system can not only calibrate the reference plates, but also measure the photometric characteristic of retroreflective traffic markings specimens.

In a structured light-based 3D scanning system, the overall 3D information of to-be-measured objects cannot be retrieved at one time automatically. Currently the 3D registration algorithms can be divided into the auxiliary objects-based method and the feature points-based method. The former requires extra calibration objects or positioning platforms, which limits its application in free-form 3D scanning task. The latter can be conducted automatically, however, most of them tried to recover the motion matrix from extracted 2D features, which has been proved to be inaccurate. This paper proposed an automatic and accurate full-view registration method for 3D scanning system. Instead of using the 3D information of detected feature points to estimate the coarse motion matrix, 3D points reconstructed by the 3D scanning system were utilized. Firstly, robust SIFT features were extracted from each image and corresponding matching point pairs are achieved from two adjacent left images. Secondly, re-project all of the 3D point clouds onto the image plane of each left camera and corresponding 2D image points can be obtained. Filter out correct matching points from all 2D reprojection points under the guidance of the extracted SIFT matching points. Then, the covariance method was adopted to estimate the coarse registration matrix of adjacent positions. This procedure was repeated among every adjacent viewing position of the 3D scanning system. Lastly, fast ICP algorithm was performed to conduct fine registration of multi-view point clouds. Experiments conducted on real data have verified the effectiveness and accuracy of the proposed method.

Traditional optical 3D shape measurement methods, such as light stripe triangulation, binary coding, and fringe projection, cannot acquire complete and correct 3D measurement results in the presence of interreflections. In this research, a 3D shape measurement method in the presence of interreflections based on light stripe triangulation is presented. The wrong measurement results caused by interreflections are excluded by the geometric constraints introduced by an additional camera. Each 3D point reconstructed by light stripe triangulation is projected onto the image plane of the additional camera to determine whether the 3D point is correct measurement result. Experimental results demonstrate that the proposed method can measure 3D shape in the presence of interreflections.

Laser spot detection is an important step of laser triangulation and limits its accuracy. Common methods to determine the center position include circle fitting, the Hough transformation, the gray centroid method or Gaussian fitting. As these algorithms were often tested in different set-ups and under various conditions, they generally lead to diverse results. The aim of this contribution is to investigate the algorithms in a more objective and realistic approach. After a short introduction to laser triangulation, basic information about laser spot center detection is given. Fundamental limitations of the laser spot detection are then considered and analyzed. The investigations are followed by evaluations of measurements in a real laser triangulation setup. Through this approach we could show that the influence of the spatial quantization and the quantization due to the limited bit depth of the detector can be in a similar range as the deviation due to speckle noise in the image. By using adapted image processing steps, the performance of the laser spot center determination could be improved significantly.

In this work we propose a holographic approach for accurate characterization of thickness of transparent objects. The proposed method is based on recording a series of fully-coherent holograms, which are generated with varying tilt of object plane wave illumination. The captured holograms are numerically processed to obtain the corresponding complex fields, which are used to produce the longitudinal coherence function. This function allows to measure the absolute thickness of transparent parallel plates using highly monochromatic light source. The conclusions of this work are supported with results of numerical simulations.

Nonlocal means (NLM) and its variant filtering methods such as nonlocal means-average (NLM-av), nonlocal meanslocal polynomial regression (NLM-LPR), and nonlocal means-shape adaptive patches (NLM-SAP) for speckle noise reduction in digital speckle pattern interferometric (DSPI) fringes are presented. The performance of these filtering methods is appraised by several criteria such as peak signal-to-noise ratio (PSNR), the quality index (Q), and the edge preservation index (EPI). The obtained filtering results corroborate the effectiveness of NLM and its variant filtering methods for speckle noise reduction in DSPI fringes.

Frequency-Modulated Continuous-Wave (FMCW) technique has been introduced into digital holography (DH) using an injection-current-induced frequency modulation of a laser diode (LD) as a light source. Since the frequency of beat signals observed in the intensity variation of holograms captured by a high speed camera depends on the optical path length difference between the reference and the object light waves, the use of FMCW technique in DH can achieve selective reconstruction of an object located at a desired distance. As a preliminary consideration, both experimental and calculational studies were conducted for selective reconstruction of two objects at different positions. In addition, a polarization state of object wave transmitting through a PMMA sample was investigated using the technique in which the object wave interfered with two orthogonally linearly polarized reference waves having the different optical path lengths.

The paper describes a hybrid method for estimating flow-velocity vector fields from Particle Image Velocimetry (PIV) images that combines a cross-correlation technique with a multiresolution estimation based on optical flow with the aim of obtaining the highest possible spatial resolution. The method offers the possibility to determine one vector per seeding particle. The manuscript examines accuracy of the estimates compared to other known methods using various standard test images. Experimental results are also presented.

The present report is devoted created interferometer for large convex optical aspheric surfaces. The testing method is also presented. It is based on an orthogonal ray scheme, where the convex testing surface is illuminated by a collimated beam, that is perpendicular to the optical axis of the surface. The interference pattern is formed on a screen by the interaction of the reference beam passing over the test surface and the object beam reflected from the test surface. The interference pattern is a system of quasi-straight fringes, containing information about the shape of the tested surface as the distance between those fringes. The experimental data obtained on the interferometer and presented.

Signal detection stability is very important for vector fiber-optic hydrophone and hydrophone array, because the instability of the demodulated signal directly leads to the target azimuth estimation error and the degradation of system performance. In this paper, a method to achieve high-stability signal demodulation for interferometric vector fiber-optic hydrophone is studied. A parameter estimation and demodulation parameter compensation method for phase generated carrier demodulation system is proposed based on elliptic curve parameter fitting algorithm. An elliptic curve is constructed using the second and third frequency of the reference interference signal. The ellipse curve fitting algorithm is introduced to estimate the distortion parameter of the modulation and demodulation system by fitting the value of each elliptic curve parameter. By compensating the PGC demodulation for the tested signal with the estimated parameters, the instability of demodulation system caused by PGC modulation depth variation and additional modulation intensity of the light source can be effectively reduced. The feasibility of the method is verified by simulation experiments and actual system experiments. High stability signal detection is realized using the proposed method, which can effectively improves the detection effect of the vector fiber-optic hydrophone array.

This work describes a method for an experimental determination of a paraxial back focus position and a paraxial focal length of optical systems. It is analyzed an influence of spherical aberration on the value of the measured effective focal length of an optical system and the method is proposed for an elimination of this influence and the determination of the paraxial back focus position and the paraxial focal length of a lens from its effective focal length and the Strehl definition measurements.

During this article, we have reviewed an application of optical spectroscopy in the control of physical and technological processes, which include: monitoring combustion processes, monitoring a rocket engine state, the monitoring process of melting metals, dyeing textile materials, etc. We have given a solution for contactless optical spectroscopy to control these processes. Contactless spectroscopy means the absence of direct contact between the resolving system of the spectral device and analyzed radiation. This was done by using optical fiber as a transmission system. In this case, spectral devices are the traditional diffraction grating device and a multichannel optical spectrometer. There is giving schemes for creating control devices based on these spectral devices, and their comparison is made. Results of hightemperature process experimental researches of NaCl burning and recording the spectral line of Na in spectrum flame of gas-jet. That researches accomplished via developed laboratory models of control devices based on spectral devices.

This study reports a unique optical system for experimental, laboratory-level testing of light scattering methods for noninvasive characterization of optical fibers. This new modular system comprises of various optical, mechanical, electrical and software components enabling the control, detection, and analysis of the measurement results. Practical measurements are investigated to explore an inverse relationship between scattering data from the vicinity of a rainbow and fiber diameter/refractive index estimates.

The principle of construction and design of the optical control system of the thickness of thin-film interference coatings applied in vacuum with the low-cost realization has been developed. The optical control system is built on the principle of measurement directly on the product - direct control at one wavelength (monochromatic). Model, allowing to evaluate technical capabilities of introduced system was made.

Control of the position of the object is an important task in the industry. For this task, use special devices that are encoders. Encoders are needed for feedback when moving, or building coordinate systems. Unfortunately, for solving some problems, absolute angular encoders have not suitable dimensions. This is due to the use of code disks in systems with good accuracy. The design of the encoder is proposed on the basis of position-sensitive photodetectors, which implies smaller dimensions while maintaining accuracy characteristics. The installation unit of the photodetector has the ability to adjust the alignment of the center of the photodetector and the axis of rotation of the shaft, since according to the study it was found that this error has the greatest impact on accuracy. The brand illumination system is developed. The brand is projected onto the photosensitive surface of the photodetector. Replacement of the photodetector provides for the replacement of the node setup of the sensor.

The heavy speckle noise in speckle fringe patterns makes it difficult to extract phase from those interferograms. Therefore filtering is essential and necessary. The windowed Fourier transform (WFT) is an effective method to fulfill this task. However, when the WFT is employed, several parameters, such as the window size, the threshold and the passband, need to be chosen carefully. In this paper, the filtering performance of WFT with different combination of those parameters is investigated by simulation firstly. Based on the comprehensive analysis, we then propose a fully adaptive windowed Fourier transform filtering method. In our method, the window size was determined with the help of wavelet transform, then the threshold was dynamically adjusted according to the amplitude of the window Fourier ridge. As a result, the optimum spectrum containing the fringe information could be obtained. At last, the filtered image can be obtained by inverse Fourier transform of such spectrum. The experimental results are presented to validate the method’s potential. The results were also compared with those obtained using contour window filtering and second-order directional partial differential equation filtering method. It can be seen that contour window filtering and second-order directional partial differential equation filtering could damage fringe structure because of deviation on fringe orientation calculation. While our proposed method can preserve the fringe structure as much as possible. As a result, our adaptive WFT method performs better than the other two methods. It provides an alternative way to suppress the speckle noise in the actual applications.

Optical alignment of the mirror components in a space telescope is an important process for obtaining high optical resolution and performance of the camera system. The alignment of mirrors depends on the external coordinate frame using a cube mirror, and a relative coordinate mapping between the mirror and the cube mirror before optical system integration is a prerequisite step. Therefore, in order to align the spacecraft camera mirrors, the relative coordinates of the vertex of each mirror and the corresponding cube mirror must be accurately measured. This paper proposes a new method for finding the vertex position of a primary mirror by using an optical fiber and alignment segments of the computer-generated hologram (CGH). The measurement system is composed of an optical testing interferometer, a multimode optical fiber. We used 2 theodolites to measure the relative coordinates of the optical fiber located at the mirror vertex with respect to the cube mirror, and achieved a measurement precision of less than 25um.

This paper presents a new method for measuring target reflectivity. Different from the traditional direct measurement method, it is an indirect target reflectance measurement method based on the output voltage of the radiometer. The principle is that the target emissivity is first measured by a simple and convenient method, which use a radiometer to measure the blackbody, metal and target radiation voltage of the same size at the same temperature. Then the mathematical relation between reflectivity and emissivity is used to calculate the reflectivity of target. Experiments are designed to verify this method, a metal plate coat with stealthy nano-materials is selected as the measured target. The experimental results were compared with the standard arch method, indicated that maximum error is less than 5%. This is to say that the cost of the measurement scheme is reduced and the applicability is increased under the condition of ensuring the measurement accuracy.

We construct highly repetitive low-coherence interferometer using time-stretch technique and confirm its basic characteristics. The experimental system consists of a mode-locked laser diode (MLLD), a time-stretcher, an optical interferometer, a photodiode (PD), and a real-time oscilloscope. The ultra-short pulse from MLLD was passed through a dispersion flat fiber to generate supercontinuum light with 23.5 nm wavelength bandwidth. Repetition frequency of the laser pulse is set to be 10 MHz by the LiNbO3 modulator, and then it feeds to a time-stretcher composed of a dispersion compensation fiber (DCF) with a wavelength dispersion of 8959 ps². The pulse width after passing through the timestretcher is 28.2 ns. The fiber-optic Michelson interferometer consists of a 50:50 optical fiber coupler, two collimating lenses, an objective lens and two reflective mirrors. Interference signals are detected by a photodiode (32 GHz) and recorded by a real-time oscilloscope (16 GHz, 50 GS/s). The temporal profile of the recorded interference signal is converted to an optical frequency profile. The optical path length difference is determined by Fourier transform of the spectrum. We demonstrate a preliminary measurement on the experimental system. The calculated path length difference agrees well with the actual set values. It is confirmed that the optical path length difference can be measured at a high repetition rate of 10 MHz. It is shown that the degradation of the interference signal can be prevented by considering the second order of the group delay of the DCF.

Phase-shifting (PS) is a well-established technique for phase retrieval in interferometry that requires a series of intensity measurements with known or unknown phase-steps. Contrast information is useful for evaluating the quality of the collected data. The objective of this work is determinate an algorithm to calculate the contrast function in the case of arbitrarily spaced phase steps.

The extreme value of interference (EVI) algorithm is a very fast and efficient method for the fringe pattern phase demodulation. It requires only two arbitrarily phase-shifted frames in which the phase shift between interferograms can be determined by searching the maximum and the minimum of the normalized interference patterns, then the measured phase is obtained by an arctangent function. Compared with other two-frame demodulation algorithms, the EVI algorithm has great advantages. Firstly, the EVI algorithm is simple and the calculation speed is fast. Secondly and more importantly, it works very well even if the number of fringes of the interferogram is less than one. However, to make this method work, the fringe should be normalized in advance, which is sometimes not a satisfactory requirement. The effects of uneven background terms, modulation amplitude variations, and random noise in the fringe pattern will make the normalization of the fringes extremely complex. Therefore, by employing the HilbertHuang transform (HHT) based prefiltering in this paper, the background intensities and modulation amplitudes of the two interferograms are suppressed and normalized respectively. Then, phase demodulation is implemented using the EVI method. Because of the HHT process, the demodulation result is greatly improved in plenty of situations. Both simulation and experimental studies have shown that the proposed improved method makes it easier to determine the phase distribution with high precision even under complex conditions.

The freeform optical surfaces are the advanced optical elements being used in the optical systems ranging from the illumination system, head up display and ophthalmic systems. So far the metrology is not well established for freeform surfaces.There are interferometric, profilometry, deflectometry and slope measurement techniques used to measure the freeform surfaces. Due to non-rotationally symmetric nature of freeform surfaces, slope measurement systems like Shack Hartman Sensors (SHS) are being explored for the measurement of freeform wavefronts. The spatial resolution of Shack Hartmann sensor is limited by the size of the lens lets used in the sensor which is typically 100 μm to 200 μm. The self-imaging based sensing uses a periodic structure which can be replicated under collimated illumination at certain distance known as Talbot distance. If there is a wavefront other than collimated light, the deviation in self-imaging pattern is observed, and this deviation can be utilised for wavefront measurements. Being a smaller pitch of the periodic structure, a high resolution data is obtained. In the present study, we have proposed a high resolution system for measurement of freeform surface using self-imaging based technique, which is having advantage of higher spatial data as compared to Shack Hartman Sensor. A simulation study is carried out and demonstrated the improved performance of the proposed sensor as compared to SHS.

The proper functionality of microelectromechanical systems (MEMS) is governed by characterisation methods reliant on optical metrology. Static characterisation methods like interferometry are well established. The standard dynamic characterisation method is Laser Doppler Vibrometry (LDV), which can be used for areal measurements by merging data from pointwise measurements. However, the increasing complexity of MEMS structures is raising the demand for more efficient spatial dynamic analyses. Phase-shifting interferometry (PSI) is state of the art in high-resolution topography acquisition techniques. By equipping interferometers with a stroboscopic illumination source, it is possible to perform dynamic analyses of periodically oscillating samples by synchronising the illumination flashes to the sample motion. Although this is a known approach, interferometers with this technology embedded are scarce and restricted to lower bandwidths. This paper introduces an external standalone stroboscopic illumination module, which can be installed onto the interferometer objective through optical fibres. Firstly, fast-switching power-LEDs were selected as illumination sources. Optical pulse length as short as 15 ns was realized, leading to a bandwidth of some tens of MHz. Micromirror arrays (MMAs) developed at the Fraunhofer IPMS were used as test samples to prove the concept. For MMAs switched at 2 kHz, overshoot and oscillation frequency were determined. The main advantage of the stroboscopic interferometric method is in recording thousands of points simultaneously, in contrast to the LDV. This is equivalent to a performance increase in the order of tens or even hundreds of times.

The accurate determination of the contact area between an instrumented indenter and the material under testing is important for material property measurements. Usually, it is masked by the pile-up or sink-in phenomenon. Due to the wide range of materials available in the industry, different amounts of pile-up would be generated depending on the grade of fragility of the material. This paper presents the sensitivity evaluation of dual-wavelength image-plane digital holography in order to identify these different degrees of pile-up. It was observed that the interferometer was able to identify pile-up with a height of 3μm. Additionally, this result showed a good concordance with the measurement of the same part by using a Focus-Variation Microscope (FVM). Considering the application for in-situ measurements, corrections to deal with the misalignment between the interferometer and the evaluated surface was proposed and tested. Results showed that the setup is able to work with misalignments, which will produce up to 13 fringes in the measured phase.

The work is devoted to the study of the installation for measuring the characteristics of compensation couplings by the method of autocollimation. The sources of errors of instrument couplings, their numerical characteristics are considered. Also analyzed coupling designs. Experimentally investigated various instrument couplings, measured kinematic error.

Freeform optics is the next generation optics with no rotation symmetry about any axis. The fabrication and metrology of freeform optics are not possible by conventional techniques. Due to non-symmetric nature, it is more critical to align the freeform surface during fabrication and metrology. Fabrication and metrology accuracies of the freeform optics are mainly limited due to alignment errors at all the stages of development process. In this paper, effects of alignment errors on quality of freeform optics during of fabrication and metrology are studied. It is found that alignment errors have significant contribution on quality of freeform optics development. Different types of fiducials and their importance and utilization are discussed. Further, a strategy for effective alignment of freeform optics is proposed.

Simple and cost-effective immersion method for freeform optical surface (FFS) measurements is proposed. Method is based on the determination of the borderline curve formed between the contacted immersion liquid and the controlled surface. Proposed method is suitable for measuring polished and rough surfaces, convex and concave freeform, aspheric, spherical surfaces, prisms and another optical and non-optical 3D objects having freeform surface.