This PDF file contains the front matter associated with SPIE Proceedings Volume 11552, including the Title Page, Copyright information, and Table of Contents.

The giant laser device used in inertial confined fusion (ICF) experimental research is the largest optical engineering ever built by humans. It requires thousands of large-diameter optical components, especially for optical components with diagonal dimensions close to or exceeding 1 meter, which leads to the manufacturing is extremely difficult. Wavefront characteristics are one of the key parameters of meter-size optical components. Since different degrees of wavefront error are introduced during the material preparation, manufacturing and coating processes, it requires precise measurement and precise control throughout the manufacturing process. In this paper, the research work on key problems such as measurement accuracy, measurement aperture and measurement efficiency in the wavefront error detection of meter-size optical components is carried out and summarized.

A new FFT wavefront reconstruction algorithm is proposed in this work, which can retrieve the test wavefront without spectrum leakage error. Since only part of the test wavefront interferes with its copy wavefront in the lateral shearing interferometer, the sizes of the sheared wavefronts are usually smaller than the test wavefront. To solve the issues introduced by the incompatible sizes of the sheared wavefronts, we used the measured information in the sheared wavefronts to compensate the missing parts based on the periodicity of the Fast Fourier Transform. This procedure can be applied under an arbitrary shear amount. Since no estimation is required during the extension, the accuracy of the reconstructed wavefront can be improved. We also provide an algorithm to estimate the lost information due to the illposed problem of reconstructing wavefront from the sheared interferometric data base on the proposed algorithm. The detail of the FFT processing and the sheared wavefront extension algorithm is given in this work. Some test is performed to evaluate the performance of the proposed algorithm. The test result shows that this algorithm is capable of reconstructing the test wavefront without introduces extra error. The estimation of the lost information based on our algorithm is convenient to be applied in the frequency. When the shear ratio is less than 25%, the estimation can always give a better result. The accuracy of the reconstructed wavefront is improved by up to 14.3% after estimation.

Micro Systems Technologies (MST), MicroElectroMechanical Systems (MEMS) and Additive Manufacturing (AM) have been playing important roles in different industrial sectors such as semiconductor, precision engineering, aerospace, oil and gas and etc. Besides continuing effort for MEMS-based measurement techniques, related effort in MEMS measurement is also applicable to non-MEMS-based manufacturing such as AM. This paper introduces a micro-scale surface metrology at the National Metrology Centre based on a large scanning range laser confocal microscope for measuring surface structures. A laser confocal probe projecting a single laser spot onto the testing surface is fixed above the sample rather than a laser spot scanning over the testing surface for an area measurement in conventional laser confocal microscopy. There are two unique features in this configuration: 1) its optical axis can be always perpendicular to the test surface and no need to compensate/correct the warpage effect because of the laser deflection for scanning purpose and 2) a large scanning area can be flexibly introduced by a separate 2-dimensional moving stage. Corresponding measurement working principle and system instrumentation and configuration are described. These measured features and structures include dimensional and surface functional parameters as in material removal/wear, cutting tool edge, laser marking, micro-channel in microfluidic device, lens array mold, surface flatness/deformation of super-low reflection (less than 2%) surface, low reflection rock surface and relatively rough AM surfaces. It successfully demonstrated the laser confocal microscope is applicable to surface features’ measurement on both smooth and rough surfaces in order to provide proper quality control feedback to related manufacturing.

Aiming at the problem that the center extraction accuracy of the circular coding target is easily affected by camera angle, a method to extract the center based on the radial line fitting of the coding ring is proposed in this paper. Firstly, the radial straight line edge of the circular coding target ring is obtained by edge extraction after image preprocessing, The extracted sub-pixel edge points set is mapped to the parameter space into curves, and the intersection points between all curves are found. Finally, the RANSAC (Random Sample Consensus) algorithm is used for fitting the line to the above points, and the actual center is obtained by mapping the fitted line to the original coordinate space. Compared with the ellipse fitting method, the reprojection error of the camera calibration by the extracted targets centers is reduced by 20%, and the calibration result is more accurate and reliable.

Estimation of phase and its derivatives from fringe patterns, commonly referred to as fringe analysis is an important step in many non-destructive optical measurement techniques such as Fringe projection, Electronic speckle pattern interferometry (ESPI) and Digital holographic interferometry (DHI). Some of the applications of these techniques include deformation analysis, stress analysis, profilometry and defect detection. Here, the quantities of interest like displacement, stress and refractive index are encoded in the form of phase and phase derivatives in the recorded fringe patterns. In dynamic systems, large number of such fringe patterns are recorded necessitating the use of reliable and fast methods for fringe analysis. In this work, we proposed a GPU assisted subspace based method called multiple signal classification (MUSIC) for estimating both the phase and its derivatives. The method relies on noise subspace to calculate a polynomial equation whose roots are computed numerically. Both the phase and its derivatives are computed by selecting a relevant root according to the stability conditions. The performance of the proposed method was tested using 250 simulated fringe patterns with gradually increasing phase at signal to noise ratio of 5 dB. Collective processing of all 250 fringe patterns using Python’s Numpy library took approximately 14535 seconds whereas the graphics processing unit (GPU) had taken only 260 seconds, thus resulting in substantial reduction in execution time. These results show that the proposed method is robust against noise and the use of GPU makes it computationally very efficient.

We introduce our recent work in deep-learning-based fringe projection profilometry (FPP), including phase retrieval, phase unwrapping, geometric constraint, color fringe demultiplexing, and high-dynamic-range imaging.

Ensuring high quality standards at a competitive cost through rapid and accurate industrial inspection is a great challenge in the eld of intelligent manufacturing. Three-dimensional (3D) optical quality inspection technologies are gradually widely applied for surface defect detection of complex workpieces because of its non-contact, high- accuracy, digitization and automation. However, the contradiction between cost and eciency, dependence on additional position hardware, and compromised detection strategies remain the urgent obstacles to overcome. In this work, we propose a fast 3D surface defect inspection approach based on fringe projection pro-lometry (FPP) for complex objects without any auxiliary equipment for position and orientation control. Firstly, a multi- view 3D measurement based on geometric constraints is employed to acquire high-accuracy depth information from dierent perspectives. Then, a cycle-positioning-based registration scheme with the establishment of the pose-information-matched 3D standard digital model is proposed to realize rapid alignment of the measured point cloud and the standard model. Finally, a minimum 3D distance search method is driven by a dual-thread processing mechanism for simultaneous scanning and detection to quantify and locate 3D surface defects in real time. To validate the proposed inspection approach, a software that combines 3D imaging, point cloud registration, and surface defect calculation is developed to perform quality inspections on complicated objects. The experimental results show that our method can accurately detect the 3D surface defect of the workpiece through more economical hardware and more convenient means in real time, which is of great signicance to intelligent manufacturing.

Eliminating the effective phase ambiguity with as few fringe patterns as possible is a huge challenge for fringe projection profilometry (FPP). The stereo phase unwrapping (SPU) technologies based on geometric constraints can achieve phase unwrapping without projecting any additional patterns, which maximizes the efficiency of the absolute phase retrieval. However, when high-frequency fringes are used, the phase ambiguities will increase which makes SPU unreliable. The adaptive depth constraint (ADC) method can increase the robustness of SPU, but it is difficult to deal with scenarios without the priori depth guidance. In this work, we propose a stereo phase unwrapping method based on feedback projection to robustly unwrap the wrapped phase of dense fringe images. Aiming at the problem that the ADC is too dependent on the last measurement result, a simple and effective deep anomaly detection strategy is proposed. After determining the reconstruction error, through the proposed fully automatic projection feedback mechanism, the absolute depth of the object can be quickly obtained to correct the dynamic depth range of the ADC, thus guiding the acquisition of high-quality 3D information. Experiments prove that this approach can achieve high-speed, real-time, and high-resolution 3D measurement with a measurement speed of 30 Hz under the premise of using two perspectives.

The phase measuring deflectometry is a powerful technique for the in-situ measurement of of complex optics. Its measurement accuracy is comparable with conventional interferometry, but with higher flexibility, stability and efficiency. The three main challenges in the deflectometric measurement, namely the position-angle uncertainty in calculating the pixel correspondences, height-slope ambiguity in specifying the normal vectors, and rank deficiency in surface reconstruction are analyzed. Some significant error factors and effective solutions are introduced. The measuring accuracy of complex surfaces can achieve a level of 100 nm RMS.

Rotary optical encoders are widely used in all areas of industry, robotics and special equipment. The accurate measurement of their characteristics is a necessary task for their successful implementation. The report attempts to find a unified approach to determining the accuracy of rotary encoders. The proposed approach is based on statistics, on the reliability of the code, which characterizes the probability that the measured code corresponds to the specified code. To find the reliability of the code, it is necessary to measure angle of the turn using some goniometric device and compare the measured values with the codes of encoder. The report considers the laser dynamic goniometer for determining the error of absolute angle encoders, which allows measurements in a wide range of rotation rates with high accuracy. The measurement is performed at certain moments of time, depending on the electrical interface of the encoder. In the case of a parallel interface, the moments of time are determined by the moment when the code of the encoder changes. This data collection allows to get a General set of data. In the case of an encoder, where the sequential principle of reading the code, it is not possible to receive the General set in a wide range of rotation rates. The report analyzes the situation discussed above in detail. The report presents experimental data obtained on a laser dynamic goniometer and the results of calculating the reliability of the code.

The use of sparse aperture can reduce the size and weight of the large aperture telescope. The sphere or aspheric surface commonly used is difficult to increase the field of view of the system and improve the image quality. Compared with spherical or aspherical surfaces, optical freeform surface has more design freedoms. This paper designs a two-mirror sparse aperture telescope. The primary mirror is made of three sub-mirrors arranged in the Golay3 configuration while the primary is a freeform surface defined by Zernike polynomials. The results show that the full field of view increases up to 0.32° in the optical system when the primary mirror uses a freeform surface. The image quality meets the requirements form its modulation transfer function.

The evolution of a vector optical field (VOF) with a spiral phase and azimuthally-variant polarization distributions in the field cross-section in a Parity-Time (PT) symmetric system is demonstrated. The evolutions of the Stokes parameters of azimuthally-variant VOF in PT symmetry system are obtained by the split-step finite-difference method. Numerical results show that the coherent superposition of two orthogonal polarization components leads to the occurrence of the linear-circular polarization conversions in the periphery of the VOF during propagation in a PT symmetric system. Moreover, the optical intensity and the circular polarization components generated by transmission is not uniform during propagation in a PT symmetric system due to the gain and loss effect of PT symmetry. These results provide a new approach for the manipulation of a VOF with different modulation factors (refractive index, gain/loss, and lattice period) at different propagation distances in the PT symmetric system.

Null-indicator (NI) is an optical device setting up a reference direction in space for high-precision angle measurements by means of dynamic goniometry. During continuous rotation of the optical object under test (e.g. optical polygon or gauge) together with the angular scale the NI registers moments when reflecting faces of the object are normal to the optical axis of the NI. At these moments NI yields electronic logical pulse which triggers the readout from the angular scale. Conventionally interferometric null-indicators were considered as the most accurate, though they have few significant drawbacks. The interferometric NI is very sensitive to the quality of the reflecting surface and other factors that may bring aberrations to the NI emitted wavefront. This makes the interferometric NI suitable only for laboratory applications with test objects of high quality. Recent development has shown that autocollimating NI with digital signal processing can reach the same level of random error as the interferometric one. But during testing of the experimental model of the autocollimating NI authors have encountered systematic error that needed additional study. The encountered error was related to the adjustment of certain element of the NI optical scheme and the object under test relatively to the axis of rotation. The report presents theoretical and experimental study of the influence of aforementioned factors on the accuracy of setting the reference direction by the autocollimating NI. Also authors give recommendations on the adjustment procedure of the dynamic goniometer utilizing autocollimating NI in order to reach required measurement accuracy.

Photogrammetry technology has been gradually applied in the development of large satellite antenna, and the experimental evaluation method for deformation measurement of large satellite antenna proposed in this paper is derived from the development task of large flexible antenna, which needs to conduct reflector measurement in a vacuum and cryogenic environment based on the principle of photogrammetry.Therefore, it is necessary to establish an evaluation method for large satellite antenna deformation measurement based on photogrammetry.The repeatability and accuracy of the measurement results can be judged by the method.In this paper, a method of measuring repeatability and measuring accuracy is proposed, and the measurement results are evaluated by defining the concepts of single point measuring evaluation factor and length measuring evaluation factor.This evaluation method has been successfully applied to the measurement of large antenna deformation in vacuum and cryogenic environment.

To achieve the high precision absolute distance measurement of large aperture remote sensor structure, a large range and high precision distance measuring method based on the dual optical frequency combs was presented. Two femtosecond combs with repetition frequency locked were used in the system, and the repetition frequency difference between combs had great influence on ranging accuracy. To measure the stability of the remote sensor structure, the Michelson interferometer optical structures have been launched. The experiment results demonstrated that the system was capable of large range and high accuracy distance measurement. For a continuous measurement of 2.6m distance, the ranging accuracy was 5×10^-7. The system has applied in stability measurement of a 1.8m length main structure, and the test result has demonstrated the structure stability was less than 2μm.

The wavelength range of a femtosecond laser system can be extended to cover 190 nm ~ 4000 nm via optical parametric oscillation and various harmonic generation processes starting from a Ti:Sapphire mode-locked femtosecond pulsed laser. The laser beams can be well adjusted to be coupled to a galvo scanning system to generate a large-area, uniform light field which is highly suitable for the bi-directional reflectance distribution function (BRDF) measurement of the surface of a large-size objective. The scanning route was carefully designed with a few sub-cycles to provide the same illumination time over different spots on the measurement surface. The irradiance of the light field @ 532 nm was measured using a silicon photodiode with an integration time in the range of 0.1 s ~ 10 s and the non-uniformity was measured to be < 4% over a 60 mm × 60 mm area. The integration time of the radiance sensor for the BRDF measurement sensor was adjusted in the range of 0.1 s ~ 10 s, as well, and the BRDF of a 60 mm × 60 mm surface can be achieved with a standard uncertainty of 5%.

Defect identification for quality control and industrial inspection necessitates the need for novel techniques in the field of non-destructive testing and experimental mechanics. Optical interferometric techniques are quite popular for non-destructive testing, and hence they are extensively used to locate and identify defects from fringe patterns. A rapid variation of the fringe density in the vicinity of the defect’s region serves as a means for the detection algorithms to identify them. With the advent of machine learning over recent years, it has paved way for algorithms with automatic detection, thereby, eliminating the problem of manual thresholding. In this paper, we propose an elegant technique which relies on computing windowed Fourier spectrum of the fringe pattern at a given spatial frequency and subsequently utilizing this spectrum with a GPU accelerated Support Vector Machine (SVM) algorithm for classification of a defect and a non-defect region. The windowed Fourier spectrum of the fringe pattern serves as a feature vector for the GPU accelerated SVM algorithm which internally performs a pixel classification, thereby producing a binary output of the defect. The performance of the proposed technique is tested on computer-generated fringe patterns at severe noise levels. A machine learning model is trained using the windowed Fourier spectrum of a 1024 x 1024 fringe pattern which is corrupted with an additive Gaussian noise at a signal to noise ratio of 5dB. The best set of hyper-parameters are deduced from the validation data and the proposed method is tested on the fringe patterns of size 1024 x 1024 which contain temporally varying defects. The results indicate that the proposed method is computationally efficient, robust against noise, and also capable of automating the defect identification problem.

Owing to the capability of noncontact measurement, optical profilometry is required for measurement of cylindrical openings for thin transparent objects, which are easily deformed and damaged by the contact of mechanical probes. In our previous work, we have developed optical profilometry for opaque and translucent objects. During measurement, disk shaped beam illuminates across the objects, and the intersection of beam with the objects is collected by an image sensor. When the surface of objects is rough enough to scatter light, the cross-sectional profile images are detectable by the sensor. The transparent objects have smooth surface so that reflection from the surface is much stronger than scattering. Consequently, the capture of intersection image becomes very difficult. In this work, we introduce the method to measure dimension of cylindrical openings for transparent objects. To generate the cross-sectional profile, the disk shaped beam illuminates the objects obliquely. The dynamic range of the oblique/ tilted angle was investigated for transparent objects measurement. Several transparent objects with cylindrical openings were employed for practical measurement. Measurement results showed the wall thickness strongly affects the measurement accuracy.

The spherical target is the standardization target of the laser scanner performance evaluation commonly used by international metrology institutions. How to obtain the spherical center coordinates of the target by using a higher precision class instrument is an important prerequisite for realizing the spatial performance calibration of the laser scanner. A split target set is proposed, which can be concentric with the laser scanner target by machining and adjusting. The extended uncertainty of the reference distance formed by the split target sets can reach 0.07 mm under the effective length of 3 m, which can be effectively applied to the spatial distance indication error calibration of the laser scanner.

The Physikalisch-Technische Bundesanstalt is currently developing a form measurement system for optical surfaces with diameters up to 1.5 metres, and local radii of curvature larger than 10 m. Moreover, the surfaces can have a radial symmetrical form, or a freeform shape. This contribution presents an optomechanical simulation of the measurement system to give hints about the achievable accuracy. The simulated system uses a Fizeau interferometer with an aperture of 100 mm for subaperture stitching interferometry. An advanced subaperture stitching method (called angle- and distance-assisted subaperture stitching, ADASS) is used to reconstruct the absolute topography of a virtual specimen. The topographies of the single subaperture measurements are positioned in space by using the angular position of the interferometer and the distance to the specimen. Thereafter the remaining height difference of the neighbouring subtopographies in their overlap region is corrected. The resulting surface form is fitted to the Zernike polynomials. In the simulation, a parabolic surface with a radius of curvature of 70 m was used as the virtual specimen. The reconstructed absolute surface form has a deviation of 418 nm (RMS) from the input surface over a diameter of 1.5 m.

In order to solve the problem of coherent noise in the measurement of laser interferometer with point source, the method of suppressing coherent noise with ring source is described. The radiation intensity of the central noise point in the interference field under the illumination mode of the ring source is analyzed. In this paper, an optical system based on annular lens is used to generate ring source, and a Fizeau interferometer based on ring source illumination is built in ZEMAX Non-sequence mode. The simulation results show that the radiation intensity of the interference field noise point under the ring source illumination mode reaches 9.8357w/cm2, and the visibility of fringe is low. After adding the bulk scattering element with an average optical path of 0.2 and a scattering angle of 3°, the interference field radiation intensity becomes more uniform, and the fringe visibility was obviously improved.

The interaction phenomenon between light and turbid media like tissue is mainly characterized of absorption and scattering. The absorption coefficient (μa) and reduced scattering coefficient (μ's) of biological tissue can provide useful information about the composition concentration and structure features. Spatial frequency domain imaging (SFDI) is a relatively new and effective technique for quantitative optical property mapping of turbid media, with unique advantages of noncontact and wide-field detection. In this study, a self-developed SFDI system was used for optical property detection of phantom and meat tissue. System calibration and error correction was realized by using a series of self-made phantoms that covering a wide range of absorption and reduced scattering. Optical properties of turbid media samples of different meat tissue (pork tenderloin, pork hind leg muscle, beef sirloin, beef rump steak, chicken breast and chicken drumstick) were estimated based on spatial frequency dependent diffuse reflectance. The results showed that suitable system calibration and error correction can help to improve the system performance and detection accuracy. It’s found that the optical properties (μa and μ's) of different species of meat were distinctively distinguished. The classification accuracy of using μa and μ's as features was higher than the result of using reflectance as the feature, except for chicken breast and thigh, which indicated the advantage of SFDI in measuring the properties of meat samples compared to traditional spectroscopy. Then the estimated optical properties were furtherly applied for sample classification. Classification accuracies of different meat species and parts were much higher by using SVM compared with using KNN. The classification accuracy was as high as 96.7% for three meat species classification, and 90.8% for six set of meat samples classification. Future studies could be focused on multi spectral imaging of optical properties of meat tissue as well as other biological tissue to estimate related physical or chemical properties quantitatively.

Speckle projection profilometry (SPP), which is highly suited for dynamic 3D acquisition, can build the global correspondences between stereo images by projecting a single random speckle pattern. But SPP suffers from the low matching accuracy of traditional stereo matching algorithms which limits its 3D measurement quality and precludes the recovery of the fine details of complex surfaces. For enhancing the matching precision of SPP, in this paper, we propose an end-to-end speckle matching network for 3D imaging. The proposed network first leveraged a multi-scale residual subnetwork to synchronously extract feature maps of stereo speckle images from two perspectives. Considering that the cost filtering based on 3D convolution is computationally costly, the 4D cost volume with a quarter of the original resolution is established and implemented cost filtering to achieve higher stereo matching performance with lower computational overhead. In addition, for the dataset of SPP built for supervised deep learning, the label of the sample data only has valid values in the foreground. Therefore, in our work, a simple and fast saliency detection network is integrated into our end-to-end network, which takes as input the features computed from the shared feature extraction subnetwork of the stereo matching network and produces first a low-resolution invalidation mask. The mask is then upsampled and refined with multi-scale multi-level residual layers to generate the final full-resolution mask. This allows our stereo matching network to avoid predicting the invalid pixels in the disparity maps, such as occlusions, backgrounds, thereby implicitly improving the disparity accuracy for valid pixels. The experiment results demonstrated that the proposed method can achieve fast and absolute 3D shape measurement with an accuracy of about 100um through a single speckle pattern.

Remote sensing imaging is disturbed by clouds frequently and produces images with low contrast and poor resolution. Algorithms removing clouds from single remote sensing images are expected to improve their qualities and aroused worldwide interests. Compared with multi-image methods, cloud removal algorithms from single images reduce the demands for weather and improve the image collecting flexibility. In this paper, a human-computer interactive system for cloud removal from single images was developed according to the proposed cloud removal algorithm. Firstly, the distributions of clouds and sceneries in the contaminated remote sensing images were discussed. Features of the dual tree complex wavelet transform (DTCWT) to split image frequencies were investigated after the principle of DTCWT was analyzed. By combining the frequency features of DTCWT sub-bands and the frequency distributions of the remote sensing images, the cloud removal algorithm based on DTCWT was proposed, and its procedures were described completed. Then, a human-computer interactive system for cloud removal was developed in order to improve the cloud removal processing conveniences and achieve the best performance. The development schemes were introduced. The system structures were illustrated. The design methods of the camera driver and video signal output program were described. Communication streams between modules and human-computer interaction interfaces were designed further according to the cloud removal algorithm based on DTCWT. The system has a very good flexibility which permits the input of most parameters from users and is expect to achieve a best performance. Finally, the device of the system was completed and tested. Image acquisition and cloud removal processing experiments were implemented by applying the human-computer interactive system. Cloud removal effects by adjusting parameters through the human-computer interaction interfaces were compared and evaluated. It proved that the designed system has a pleasant human-computer interaction ability and satisfactory cloud removal performance.

In this paper, we proposed an aero-engine turbine blade measurement technique based on photometric stereo, which can reconstruct the blade surface quite accurately and efficient from the images captured under multi-illuminations. In this work, a measurement system is designed by using 8 LEDs as the point light sources, a light propagation process under the point light illumination is developed. Based on this process, a normal estimation method is provided for the reconstruction process. The system calibration is implemented on a USAF resolution target manufactured by Edmund, which proves the method is capable of performing the surface recovery with the repeatability error down to 0.0272mm. The experiments are performed through a turbine blade from aero-engine, where the comparison result is also provided by using a commercial portable laser scanner from Creaform, which shows the better performance of the methodology presented in this paper.

The underwater optical polarization imaging with active light source illumination is studied. The mechanism and modeling of active polarization detection process with the linear polarization are demonstrated through theoretical and experimental analysis. The corresponding experiments were carried out for different objects with different distances and materials in water with different turbidity. In particular, the imaging effects with different wavelengths of light sources were compared at low turbidity, the imaging effects with 532 nm wavelength light are better than that of 633 nm wavelength. These results may benefit the underwater optical polarization imaging for achieving more diversified and flexible functions.

The article is devoted to the study of the opportunities of evaluating the vitreousity of malted barley using machine vision and image processing. With the help of the developed hardware and software complex, experimental studies were conducted on samples of three different barley varieties. As a result, the optimal shooting mode was selected and an algorithm for processing digital images of barley grains was developed to determine the number of vitreous grains in the batch. In addition to classifying grains according to their vitreousity, the proposed approach also allows to evaluate the sample's uniformity and, thus, to identify higher-quality barley. As a result of additional research, it was found that the grain orientation adds an error of no more than 5%. High repeatability of results and the accuracy of the algorithm are characterized by variation coefficient which is 1.1%.

Phase measuring deflectometry (PMD) is a superior technique to obtain 3D shape information of specular surfaces because of its advantages of large dynamic range, non-contact operation, full-field measurement, fast acquisition, high precision and automatic data processing. This paper reviews the recent advance on PMD. First, the basic principle of PMD is introduced following several fringe reflection methods. Then, a direct PMD (DPMD) method is presented for measuring 3D shape of specular objects having discontinuous surfaces. The DPMD method builds the relationship between phase and depth data, without gradient integration procedure. Next, an infrared PMD (IR-PMD) method is reviewed to measure specular objects. Because IR light is used as a light source, the IR-PMD method is insensitive to the effect of ambient light and has high measurement accuracy. The following will analyze the effects of error sources, including nonlinear influence, lens distortion of imaging and projecting system, geometric calibration error, on the measurement results and evaluate the performance of the 3D shape measurement system. Finally, the future research directions of PMD will be discussed.

Complicated and time-consuming system calibration is indispensable for existing accurate three-dimensional reconstruction methods. To address this problem, this paper presents a simple and accurate optimized 3D reconstruction model based on a coaxial structured light system (CSLS). Compared to the traditional structured light system, CSLS contains a perspective projection unit and a telecentric imaging unit. Instead of obtaining the complicated structured light system parameters, the proposed model only requires the retrieval of a product matrix which could be easily calibrated by using an ordinary white plane. The influence of geometric distortion in the projector lenses has also been considered to further improve the measuring accuracy. Experiments were carried out to verify the validity of the proposed method.

Most 3D body scanners adopt a structure with multiple scanning sensors on the fixed frame to perform synchronous scanning from multiple directions of the human body. The position and attitude parameters of the multiple scanning sensors are the key to determining the 3D model stitching, and they are often calibrated when the scanner is installed. Any changes in these parameters of sensors will make model stitching errors and affect scanning accuracy. This paper studies a method for judging faults of 3D body scanner based on standard sphere. By scanning a standard sphere and observing the splicing deformation of the 3D model, it can determine whether the scanner's structure has changed, and identify which sensor has moved or rotated. Then the user can be guided to choose the appropriate calibration steps to compensate the scanner. Experiments prove that this method is a quick and effective intermediate check method of the 3D body scanner.

A CMM with multiple probing systems have the power to deliver tremendous benefits to most notably manufacturing, and have the advantage of high automation, high integration and high precision. According to the ISO standard 10360-9, multiple probing system location error should be identify and calibrated before use. In this paper, a location error self-calibration model was established based on a composite artifact. The location error of individual probing system can be respectively determined reference to the position of the probe configuration. An error separation procedure was introduced to correct this location error. A series of representative experiments were performed on a commercial combined probing system produced by our partner. The experimental results show that multiple probing systems location error was effectively reduced from 4.5μm to2.8μm. Also, this calibration evaluation is very apparently practical outside a laboratory due to its simple, portable, low-cost, and rational procedure.

Nowadays, in industrial area, many mechanical part manufactures are applying 3D optical scanner in their production shop to do part online inspection, or in their coordinate measurement laboratory to obtain crucial part dimensions. The high demands of the 3D optical scanner make it necessary for researchers to create convenient and stable device for calibrating this kind of contact-less measurement instrument. In a former study, we introduced a plate with standard spheres as a calibration device, which showed a good test result when applied on a 3D optical scanner. However, we decide to do more research on related area. The first purpose of this paper is to present an improved calibration device based on our former study and daily work experience. The new device has a wider test range with less spheres, and it’s structure is more portable, stable and more convenient to provide spatial positions. Results showed that the new device performed well in data stability and is quite easy to practice. The second purpose of this work is to study related influence factors on the 3D optical scanner calibration process. We investigated factors such as temperature, points cloud density, reflection patch density and numbers of images stitching. The results showed that those factors should be limited in proper conditions to ensure an acceptable calibration of 3D optical scanner.

The passive depth map recovery based on light field data always suffers from low accuracy and high computational burden due to the noise in sub-aperture images and the narrow disparity between sub-aperture views. To overcome this problem, in this paper, temporal digital fringe projections are used to guide the accurate depth map recovery with light field camera. Since the temporal structured light field casts a set of high signal-to-noise-ratio projections onto the object, the depth map of sub-aperture image can be recovered pixel-by-pixel independently. Therefore, the accuracy of the depth map recovery could be significantly improved. In our propose method, the sub-aperture images with temporal fringe projections are extracted from structured light field information, wrapped phase maps of sub-aperture images are obtained from phaseshifting fringe analysis, the absolute phase map and depth map are recovered from a group of wrapped phase maps with temporal phase unwrapping. In our experiment, when the distance between the light field camera and reference plane is 36 cm, the measurement error of the proposed method is within 0.2 mm. The experimental results validate the effectiveness of the proposed method.

This paper mainly studies the laser interferometer by PC control means, to adjust optical measurement control device, make up for the traditional laser interferometer manually adjust the shortcomings of low precision, difficult to quantitative control. This system by a Windows forms application control commands sent, by PL2303HXD level conversion module to CC2530 MCU, MCU control action accordingly according to the command to complete. By optical measurement adjustment device with high precision motor mechanical parts combination, can adjust the measurement parameters, to meet the requirements of the environment and the measurement accuracy of a laser interferometer. It has the characteristics of quantitative control, convenient operation, reduces the artificial adjusting the air flow and vibration influence on measuring, make sample detection more accurate and efficient.

Aiming at the problems that traditional interferometers need to manually adjust mechanical devices and the degree of automation is low, a wireless measurement and control system with zigbee-CC2530 as the core is designed. The system use two CC2530 nodes as radio frequency signal transceivers to implement wireless control functions in a point-to-point communication manner. At the same time, the Microsoft basic class library MFC(Microsoft Foundation Classes)is used to design the host computer software to control the operation of the precision motor through serial communication, and the corresponding travel limit solution is designed based on different conditions in the narrow and compact space of the interferometer, so that the precision motor can meet the measurement and control requirements for fixed-point braking,improve the measurement efficiency and accuracy of the interferometer.

The transmission sphere(TS) can only be used with a laser interferometer at a specific wavelength. This paper proposes a design method that can make the TS work at multiple wavelengths. First, use the achromatic form to design the TS so that it can be used at two wavelengths at the same time, and then adjust the interval between the TS’s lens to make it at the other two wavelengths, so that the TS can be used of 4 wavelengths in total. Using this method, a TS with a diameter of 101.6mm and F# of 3 is designed. The final TS can work at the wavelengths of 532nm, 721nm, 660nm and 570nm, and the accuracy of the transmission wavefront at each wavelength meet measurement requirements.

A kind of Michelson interference structure is used to improve the round-trip phase correction method and realize the feedback phase stabilization control. In the system, the phase shifter pre-compensation control, 1/4 wave plate debirefringence and Faraday rotator de-polarization sensitivity effectively avoid the polarization fading phenomenon existing in photoelectric detection by external factors, and realize the signal arm carrying phase jitter. At the same time, the AOFS driven by the intermediate frequency signal realizes the construction of the reference arm of the interferometer. Through photoelectric detector to beat frequency of two arms mutual interference signal, the beat frequency signal and intermediate frequency reference signal phase matching, can get the phase perturbation of optical fiber transmission system, the phase discriminator output signal processing can be used as the bias current of SOA control signal, realize the purpose of the feedback control signal phase, has solved the traditional approach to the problem of high frequency signal phase is not convenient to monitor.

A comparison measurement of absolute power responsivity of silicon photodetector between He-Ne laser and supercontinuum white light source has been realized recently. The power responsivity of silicon trap detector at 632.8nm has been transferred from NIM’s primary optical power standard-the absolute cryogenic radiometer. A 0.07% difference was present for the two different light sources at the same wavelength. Also, a series of experiments related to the comparison have been done including measurement of source power stability, detector polarization dependence, uniformity, and angular dependence. At last, the measurement uncertainty of the comparison is analyzed.

The theory of compound zoom system and its influence and control on aberration are the basis for the design of such systems and have practical value in engineering. The principle, modeling and error analysis are analyed. Based on the system configuration of the compound zoom system, moreover, this paper advances the algorithm analysis.

In recent years, machine vision technology are widely used in the industrial production process. In this paper, We studied two straight line extraction methods. Traditional method generally use the canny algorithm and sub-pixel edge detecting algorithm to detect sub-pixel edge of the image, and then use least-squares method to fit the geometric information of the edge of the image and fulfil measurement. It was found that the image collected in the actual measurement environment is often affected by the environment and produces one or other interference information, such as dust and hair interference, which affects the extraction of image edges and the accuracy of the measurement, resulting in measurement failure. We search the sub-pixel precision edge by the caliper tool method, and then use the method of RANSAC to fit straight line and get the corresponding geometric information. Finally, the distance information of the two straight line sub-pixel edges is obtained by distance calculation, and compared with the traditional method. Through the comparison of experimental data, the caliper tool method has significantly improved the measurement accuracy and robustness of the system, and achieved a better result.

Coplanar waveguide is one of the key devices in the electro-optic sampling system, which directly affects the signal modulation effect of the system. In this paper, the LiTaO₃ coplanar waveguide (characteristic impedance is 50Ω) is numerically analyzed and simulated. Firstly, the effective permittivity and characteristic impedance of coplanar waveguide are calculated by the elliptic integral method of conformal transformation. The relationship between the characteristic impedance and the structural parameters (dielectric thickness, central electrode width and dielectric bottom plate width) can be obtained by numerical analysis. Secondly, using the electromagnetic simulation software to simulate the electromagnetic characteristics of the designed coplanar waveguide, the results could be seen as the guidance to acquire the optimal structural design parameters. Accordingly, the essential parameters of designed LiTaO₃ coplanar waveguide characteristic impedance, can meet the design requirements of the electro-optic sampling system, for transmission parameters and attenuation parameters.

Surface shape of precision ring is critical to its performance. To require the whole surface information, more requirements are needed for traditional measurement. A Fizeau type interferometric system is proposed, with a 90° conical mirror the whole cylindrical inner surface can be achieved through one measurement. Considering the introduced errors from the real experiment condition, it is almost impossible to get the real surface information from the inner surface. Any small angular deviation from the 90° cone and misplacement from its ideal position may result in large measurement errors. These errors are hard to be removed before their origins are figured out, also their influence on the measurement results are understood. The principle of the testing setup, the theoretical analysis of main errors based on a cylindrical coordinate, computer simulations by MatLab, and experimental validation are presented in this paper.

With high quantum efficiency, low capacitance, low dark current and wide bandwidth, InGaAs detectors has lots of applications in optical radiation measurement, optical communication and remote sensing. This paper using a spectral comparison facility based on supercontinuum white light source and double monochromator, in which a reference InGaAs trap detector is used for the relative spectral responsivity calibration of a planar InGaAs detector. Calibration of the same detectors using tungsten lamp as light source has also been done and shows a good consistency, which proves the feasibility of relative spectral responsivity calibration using supercontinuum white light sources.

In fringe projection profilometry, phase unwrapping has long been a critical issue. Unwrapping methods can be classified as spatial unwrapping methods and temporal unwrapping methods. The spatial unwrapping method applies only to the surface of continuous objects. The temporal unwrapping method is more widely used and can be used on discontinuous or isolated objects. Nevertheless, the temporal unwrapping methods are suffering from time-consuming, such as the large number of pictures required (standard temporal unwrapping method, phase shift plus Gray code method), and high algorithm complexity (such as periodic encoding method, Fourier transform Method) Etc. Therefore, we propose a temporal unwrapping method using only three projection patterns. In this method, two linear gray scale increasing and decreasing pictures are used to obtain the cores global phase map and uniform illumination background. Another sine fringe image and the above uniformly illuminated background image are used to obtain the wrapped phase. Then the absolute phase can be achieved with the coarse global phase distribution and the wrapped phase. Experimental results prove that this method can measure three-dimensional scenes containing isolated objects.

Three-dimensional digital image correlation (3D DIC), which combines binocular stereo vision and digital image cross-correlation matching technology, can be used to restore the three-dimensional and deformation information of the object under test. 3D DIC can be accomplished by matching the subset in the left(right) image with that in the right(left). The size of the matching window is found to be critical to the measurement accuracy. Nevertheless, when the subset is small, the measurement accuracy and resolution are high but very sensitive to noise. In contrast, for large subset the measurement accuracy and resolution are lower, while the measurement is more robust to noise. To combine the advantages of high precision and robustness, the Spatio-temporal cross-correlation method is proposed in this paper. A set of speckle patterns are projected onto the objects under test. Instead of the way constructing subsets from spatial neighbor points, the way used in conventional DIC, both spatial and temporal neighbor points are utilized to construct subsets with rich information and strong characteristics. To implement the proposed scheme, we use a mechanical galvanometer to realize the projection of sequential speckle patterns and construct a stereo vision system to realize the three-dimensional reconstruction. The sub-pixel matching algorithm is used to improve the accuracy of stereo matching and 3D reconstruction. Simulations and experiments are carried out to verify the feasibility and success of the proposed scheme and system.

We analyze in detail a scheme of the comb-calibrated frequency-modulated continuous-wave (FMCW) laser and study the accuracy of the laser frequency measurements. In this scheme the moment when the tunable laser frequency crosses the reference comb lines is obtained by filtering the heterodyne signal between the frequency comb and the tunable laser with a narrow band-pass filter. We show that the accuracy of the measured instantaneous frequency depends on the frequency sweep speed, bandwidth filter parameters and total phase noise of the laser and applied frequency comb. In this work we present the optimal ratio of frequency sweep and filter bandwidth for the given total phase noise and type of narrowband filter providing the highest frequency calibration accuracy.

This paper, originally published on 10 October 2020, was withdrawn per author request.

Surface defects and texture features have a significant effect on the opticalcal properties of advanced optical components. However, most precision optical components have non-stochastic surfaces, so the current defect identification algorithm needs to be further improved to meet the quality inspection requirements for non-stochastic surfaces. In this paper, the scheme of non-subsampled contourlet transform is applied to identify feature of non-stochastic surfaces. A concrete analysis of sparse representation and feature identification about the non-subsampled contourlet transform were presented. The effectiveness of the method is proved by simulation results and experimental examples.

When implementing the phase shifting profilometry to reconstruct the object, phase unwrapping is a critical step when the object with height jump is reconstructed. In this paper, a new method encoding the phase shift value is proposed to unwrap the phase map by two-frequency unwrapping method. Only four fringe patterns are required and the phase value of the low frequency is encoded into the phase shift value of the projected fringe patterns. The effectiveness of the proposed method is verified by the simulations.

During the transmission of the thermal radiation in the atmosphere, the radiant energy is attenuated due to physical processes such as refraction, absorption, and scattering, which causes a change in the detection capabilities of the thermal imaging system. As a key performance of thermal imaging system, the inspection of the operating range is of great significance to the theoretical research and practical application of thermal imaging systems. Actually, the environmental condition that does not match the specified test condition when testing the operating range which are not encountered. It's complex to judge whether the thermal imaging system meets the index requirements by the test result directly. By modeling the target, analyzing and comparing the spectral transmittance under different atmospheric conditions, the paper introduces a method for judging the operating range of the infrared thermal imaging system when inspecting environmental deviates from the specified environment proposed.

Aperture is widely used in optical instruments, for which the area is of importance especially for instruments measuring radiometric or photometric quantities. To measure the radiometric area of the aperture, an optical method using a power stabilized laser to scan the aperture was proposed and investigated by many metrology institutes worldwide. The National Institute of Metrology (NIM) of China has built an apparatus to measure the radiometric area aperture by using the optical method. The scheme of experimental setup and related issues are discussed in this work. Uncertainties are analyzed for an aperture with nominal 6mm diameter. Approaches for further improvements are also discussed.

A new comparator facility for spectral responsivity calibration of InGaAs photodiode based on cryogenic radiometer fundamental has been set up at the National Institute of Metrology of China (NIM). The comparator employed a stable supercontinuum light source, a prime-grating monochromator, a reliable 5-axis stage and some necessary optics as its main construction components. The supercontinuum source, monochromator and some beam path were also commonly utilized by the cryogenic radiometer, which was designed to minimize the deviations to the calibration procedure introduced by these components. The spectral responsivity of the InGaAs photodiode in the spectral range 900nm- 1600nm was determined. The repeatability obtained by this new comparator is one decade better than that of former InGaAs measurement facility. NIM has been better supported to finish the measurements of transfer detectors of key comparison of CCPR-K2.a.2016 by this new and accuracy comparator.

A self-calibration method for camera using two views of unknown-structure planar scene is introduced. The planar scene is common in the environment and can be easily identifiable outside the lab. Firstly, two orientation- and scale-covariant features, which can be provided by the SIFT feature detector, is used to estimate the homography of two views. Then the homography is decomposed into the camera parameters. A RANSAC scheme is adapted to cope with the outliers of SIFT correspondences. Finally, the camera parameters are optimized with a non-linear parameter optimization using the inliers of two views. This method calibrates the camera parameters and recovers the planar scenes simultaneously. Real scene data experiment demonstrates that the proposed method is easy to operate and provides the reliable calibration results for non-expert users.

Deformable mirror (DM) is the most main wavefront corrector in adaptive optics, which can be used to compensate optical aberrations through changing the reflective mirror’s surface frequently. However, a commercial piezoelectric DM can’t have an ideal flat initial surface under zero-voltage condition due to limitation of thin mirror fabrication and support structure of actuators behind of mirror. Optical aberrations generated by this initial distortion will seriously attenuate the performance of DM’s close-loop control, so a flat-surface calibration of mirror needs to be carried out before DM properly correct optical aberrations. In order to properly control the optical figure of the DM we have to obtain an interactive matrix which is the response of optical surface to the DM actuator’s stroke. We measured a serious of surface phase data of OKO 109-channel DM through self-collimation using a ZYGO-GPI interferometer directly, then construct the interactive matrix by zonal and modal methods. After several close-loop iterations, the initial RMS surface error of OKO 109-channel deformable mirror, 1.506λ has been remarkably reduced to 0.145λ.

Optical interferometric techniques have come to forefront in precision metrology applications such as surface profilometry, deformation analysis and defect testing. Reliable phase measurement from a recorded fringe pat- tern is the primary goal in most of these interferometric techniques. A primary constraint for accurate phase measurement is non uniform intensity fluctuations in fringe patterns caused by irregular illumination, uneven reflectivities and pixel defects. This problem is further aggravated in dynamics studies with multiple image capture where the intensity fluctuations can vary with time and thus lead to several fringe patterns getting affected. In this paper, this problem is investigated using the second order optimization framework along with the total variation regularization on a graphic processing unit (GPU). In our study, a series of fringe patterns are simulated with additive white Gaussian noise at signal to noise ratio of 20 dB. To induce fringe corruption due to the non uniform intensity conditions, the object wave amplitude is varied using an ellipse shaped filter which enables brighter conditions or stronger amplitudes inside the elliptical boundary and relatively darker conditions or weaker amplitudes outside of it. For time varying studies, we varied the size of the filter sequentially to gener- ate a series of interferograms with temporally varying non-uniform intensities. Further, orientations of elliptical illuminations are changed to verify the robustness of the algorithm. For processing a series of interferograms each with size of 512 by 512 pixels with dynamic range of 10 (ratio of brightest to darkest amplitude), we obtained root mean square error less than 0.25 radians for every interferogram using the proposed method. The results show that the proposed method is robust for handling non-uniform intensity corruptions in fringe patterns for dynamics based investigations.