Conference Presentation for "Computational Imaging: Going beyond the limits of conventional lenses using computation"

Near-infrared imaging InGaAs sensors show lower performances in term of noise and sensitivity compared to silicon based cameras. Image frequency conversion from near-infrared to visible wavelengths by nonlinear parametric sumfrequency mixing in a χ⁽²⁾ medium should increase detection performances in active imaging applied to long range target identification. For such applications, both energy conservation and phase matching conditions are ideally suited to efficient upconversion. Nevertheless, the available resolution still hampers the development of upconversion imagers.

In this paper, we upconvert images provided by 1.5 μm collimated continuous wave lasers illuminating resolution targets and small objects. Using a 2.7 nm wide pump spectrum at 1064 nm, we resolve 56x64 spatial elements whereas we obtained only 16x19 spatial elements with a narrow spectrum pump laser at 1064 nm with the same beam diameter and 8x8 spatial elements with a 0.5 mm thick crystal. These results are compatible with long range target recognition. A laboratory scale experiment of active imaging of diffusive objects is shown as an illustration.

Unlike photographic image sensors with infrared cutoff filter, low light image sensors gather light over visible and near infrared (VIS-NIR) spectrum to improve sensitivity. However, removing infrared cutoff filter makes the color rendering challenging. In addition, no color chart, with calibrated infrared content, is available to compute color correction matrix (CCM) of such sensors. In this paper we propose a method to build a synthetic color chart (SCC) to overcome this limitation. The choice of chart patches is based on a smart selection of spectra from open access and our own VIS-NIR hyperspectral images databases. For that purpose we introduce a fourth cir dimension to CIE-L*a*b* space to quantify the infrared content of each spectrum. Then we uniformly sample this L*a*b*c_ir space, leading to 1498 spectra constituting our synthetic color chart. This new chart is used to derive a 3x4 color correction matrix associated to the commercial RGB-White sensor (Teledyne-E2V EV76C664) using a classical linear least square minimization.. We show an improvement of signal to noise ratio (SNR) and color accuracy at low light level compared to standard CCM derived using Macbeth color chart.

We present an embedded imaging approach based on low cost sensors that span a long spectral range in the infrared. A system has been implemented with 12 apertures that combine unique uncooled FPAs in the mid infrared domain -2 to 5 microns wavelength- with very low cost microbolometers in the thermal infrared -7 to 14 microns wavelength-. Both FPA technologies are uncooled and low cost, manufactured as monolithic devices. The system is made of two modules, one LWIR, other MWIR. Each module has a system-on-chip GPU/ARM board that carries out all the image processing required for image reconstruction. This includes the calibration of the system, the registration of the images acquired with the many apertures, and the reconstruction of the super resolved image. Besides, the board performs all the operations and transformations required for noise correction. The output of each of the modules is a video stream at 30 frames per second. Each frame is a super resolved image with a resolution 2.5x compared to the images acquired by the FPAs used. Furthermore, the modules may be integrated and the acquired images combined in a single one in the embedded processing boards. Moreover, the boards may also combine and fuse this output with a visible range video stream. The use of low cost FPAs facilitates the deployment in a broad range of applications that an benefit from imaging in the infrared, particularly in the MWIR range in which existing commercial cameras based on hybrid technology are very expensive. The system is being tested in different applications, including surveillance in variable lighting conditions and monitoring in firefighting scenarios

We propose a technique based on a transmission grating placed in front of an imaging system (e.g. a telescope) mounted on a frame that can be rotated around the optical axis. The grating creates, for each point of the source image (e.g. a star), at the focal plane, an image composed by the undistorted image of the star plus symmetrical dispersion images of several diffraction orders. The grating is rotated and several images are captured for different angular positions of the same. By analyzing the different images obtained for a different grating angle, it is possible to build the hyperspectral cube. The advantages of this method is its simplicity, extreme compactness and low cost making it suitable both for amateur astronomy and low budget science laboratory. We will present preliminary experimental results along with a discussion about the achievable spectral and spatial resolution and photon collection efficiency as a function of different type of gratings and of the number of the captured pictures. Furthermore, we present the result when the method is applied to extended non-punctiform light sources.

One of the most peculiar features of imaging systems is the trade-off between resolution and depth of field. Resolution can be improved by increasing the numerical aperture of the imaging system. However, the range of distances that can be put in sharp focus in a single shot decreases with the square of the numerical aperture. Plenoptic imaging (PI) devices are able to retrieve both spatial and directional information from the scene of interest, usually by placing a microlens array in front of the camera sensor. This feature entails the possibility to refocusing planes of the scene in a much wider range than the natural depth of field of the system, and also to change the point of view on the scene. Though plenoptic imaging is one of the most promising techniques for 3D imaging, its advantages come at the expense of spatial resolution, which can no longer reach the diffraction limit. We experimentally demonstrate that correlations of chaotic light can be exploited to overcome the inverse proportionality between depth of field and resolution, and perform plenoptic imaging at the diffraction limit. We retrieve images by correlating intensity fluctuations at different points of two parts of a sensor, which register spatial and angular information, respectively. Hence, our Correlation Plenoptic Imaging (CPI) protocol does not add any limitation to the native resolution of the imaging system. We show the experimental refocusing, through the CPI procedure, of widely out-of-focus parts of a transmissive test target. Moreover, we determine and test the theoretical limits of CPI in terms of resolution and depth of field, quantifying the improvement with respect to standard imaging and classical PI. We finally comment on future perspectives.

We propose to add an optical component in front of a conventional camera to improve depth estimation performance of Depth from Defocus (DFD), an approach based on the relation between defocus blur and depth. The add-on is an afocal doublet, which adds chromatic aberration to the global system. This overcomes ambiguity and dead zone, which are the fundamental limitations of DFD with a conventional camera since the blur become unambiguous and measurable for each depth. In this paper we present the principle of the add-on, a theoretical performance model and experiments on real prototype to illustrate the improvement of depth estimation performance with the proposed add-on.

Automated visual inspection of transparent objects is important for many industrial fields. Especially the detection of scattering impurities inside complexly shaped transparent objects is a demanding task. Usually, so-called dark field approaches are employed in this case. However, these methods often fail due to direct reflections of the light sources, e.g., at the test object's surface which cannot be distinguished from signals of real material defects. This paper introduces an inspection approach which captures images at different illumination modalities and fuses them while optimizing the signal-to-noise ratio. Two fusion strategies are presented, which employ prior knowledge in order to obtain optimized inspection images. The signal component of the observed images is defined as the signal corresponding to visualized defects. Conversely, all light reaching the sensor due to scattering or reflections caused by the test object's geometry is regarded as noise. The signal values and noise values depend on both the pixel position and the respective illumination source. Prior knowledge about the signal and noise components allows to estimate the spatially resolved SNR for every illumination channel. The images resulting from the fusion step show scattering material defects with high contrast whereas surface reflections are nearly completely mitigated by the SNR-optimized fusion strategies. Several experiments state the performance of the presented approaches.

In remote sensing, a common scenario involves the simultaneous acquisition of a panchromatic (PAN), a broad-band high spatial resolution image, and a multispectral (MS) image, which is composed of several spectral bands but at lower spatial resolution. The two sensors mounted on the same platform can be found in several very high spatial resolution optical remote sensing satellites for Earth observation (e.g., Quickbird, WorldView and SPOT)

In this work we investigate an alternative acquisition strategy, which combines the information from both images into a single band image with the same number of pixels of the PAN. This operation allows to significantly reduce the burden of data downlink by achieving a fixed compression ratio of 1/(1+b/p²) compared to the conventional acquisition modes. Here, b and p denote the amount of distinct bands in the MS image and the scale ratio between the PAN and MS, respectively (e.g.: b = p = 4 as in many commercial high spatial resolution satellites). Many strategies can be conceived to generate such a compressed image from a given set of PAN and MS sources. A simple option, which will be presented here, is based on an application of the color filter array (CFA) theory. Specifically, the value of each pixel in the spatial support of the synthetic image is taken from the corresponding sample either in the PAN or in a given band of the MS up-sampled to the size of the PAN. The choice is deterministic and done according to a custom mask. There are several works in the literature proposing various methods to construct masks which are able to preserve as much spectral content as possible for conventional RGB images. However, those results are not directly applicable to the case at hand, since it deals with i) images with different spatial resolution, ii) potentially more than three spectral bands and, iii) in general, different radiometric dynamics across bands. A tentative approach to address these issues is presented in this work. The compressed image resulting from the proposed acquisition strategy will be processed to generate an image featuring both the spatial resolution of the PAN and the spectral bands of the MS. This final product allows a direct comparison with the result of any standard pan-sharpening algorithm; the latter refers to a specific instance of data fusion (i.e., fusion of a PAN and MS image), which differs from our scenario since both sources are separately taken as input. In our setting, the fusion step performed at the ground segment will jointly involve a fusion and reconstruction problem (also known as demosaicing in the CFA literature). We propose to address this problem with a variational approach. We present in this work preliminary results related to the proposed scheme on real remote sensed images, tested over two different datasets acquired by the Quickbird and Geoeye-1 platforms, which show superior performances compared to applying a basic radiometric compression algorithm to both sources before performing a pan-sharpening protocol. The validation of the final products in both scenarios allows to visually and numerically appreciate the tradeoff between the compression of the source data and the quality loss suffered.

Single-pixel imaging based on structured illumination and compressed sensing has opened a new way to compress massive imaging data volume and significantly reduce the cost of image sensor without sacrificing imaging quality. However, conventional structured illumination methods based on digital micro-mirror device (DMD) or a liquid crystal based spatial light modulator (SLM) fall short in fresh rate, making it a real challenge for high-speed imaging applications, which are however of paramount importance in studying dynamic phenomena in living cells, neural activity, and microfluidics, and capturing important rare events. In this work, we propose and demonstrate a new approach for ultrafast (20 Mfps) structured illumination single-pixel imaging using light beam speckles out of a multimode fiber due to multimode interference. Our experimental results show that the excited high-order modes, and hence the multimode interference, are strongly wavelength-dependent. Update of the random speckle patterns can be easily obtained by sweeping the incident wavelength. Ultrafast wavelength sweeping is achieved by stretching ultrafast optical pulses from a mode-locked laser using chromatic dispersion. Extremely broad bandwidth and small wavelength step guarantee a good number of illumination patterns. By measuring multiple dot products of a sparse image with a set of known speckle based random, the image can be reconstructed using an L1 minimization algorithm. The most significance of this completely new design is that multiple (up to thousands) structured illumination measurements can be carried out within a single pulse period, enabling ultrafast pulse-by-pulse imaging. Moreover, thanks to structured illumination and compressed sensing, the proposed structured illumination single-pixel imaging system offers much higher imaging resolution than existing ultrafast photonic time stretch imaging systems for the same captured data size.

Diffuse Optical Tomography (DOT)is a powerful tool for the reconstruction of optical properties inside a diffusive medium, such as biological tissues. In particular, in the last years, techniques based on structured light illumination and compressive sensing detection have been developed. In this work a time-resolved system based on structured light illumination and compressive detection has been developed and used for DOT. Moreover, a data-driven algorithm for optimal pattern generation based on the Singular-Value Decomposition has been implemented and validated.

We present the design, calculations and simulations of high-resolution concave-VLS-grating-based soft X-ray and VUV spectrographs, as well as a plane VLS grating instrument. We have designed a normal-incidence imaging VLS grating spectrograph for a 820 – 1690 Å spectral interval and a series of grazing-incidence VLS spectrographs with imaging capabilities. The experimentally recorded spectral images of laboratory laser plasmas were obtained with the aid of a VLS spectrometer based on a concave aperiodic multilayer mirror and a plane VLS grating. Two modifications of this spectrometer were implemented with two different VLS gratings. These modifications exhibit spectral resolution of 500 and 800 over the 125 – 300 Å spectral waveband. Spatial resolution corresponds to double CCD-detector pixel size.

Nowadays, the stereoscopic endoscopy is a widely used tool for precise three-dimensional (3D) measurements of hard-to-reach elements in industrial and biomedical applications. The most common approach for its implementation is the utilization of prism-based optical tips which allow to acquire two images from different viewpoints on a single sensor. Stereo video endoscopes are typically equipped with a wideband white light source, but contrast visualization of the inspected object and, therefore, accurate quantitative characterization of its parameters often requires narrow band spectral imaging. We show that the standard geometrical calibration may lead to significant measurement errors when obtained using white illumination and applied to narrow band images. In order to overcome this, we propose the new calibration procedure based on a proper choice of a few spectral bands for calibration and interpolation of the calculated parameters. Results of multiple experiments show that the proposed technique fosters the measurement accuracy increase.

I review our latest off-axis holographic compression techniques for quantitative phase microscopy of dynamic cells. Offaxis holography allows quantitative acquisition of live cells without staining, by reconstructing their quantitative phase profile from a single camera exposure. In this technique, one of the interfering beams is slightly tilted relative to the other beam, creating separation of the field intensity from the two conjugate wave front terms in the spatial-frequency domain. We showed that this encoding leaves a lot of empty space in the spatial-frequency domain, into which additional information can be compressed. This compression can be done using optical multiplexing of up to six complex wave fronts into a single camera plane, where each pair of sample and reference beams creates an off-axis hologram with a different interference fringe direction that positions the wave fronts in the spatial frequency domain without overlapping with any other term. This new holographic compression approach is useful for various applications, with focus on quantitative phase acquisition of fast cellular dynamics, including imaging cells during rapid flow. I present several experimental systems that implement this holographic compression approach, and review various applications.

This paper proposes digital holographic microscopy applied to the study of the production of lipids in Chlorella vulgaris microalgae. We propose to use several wavelengths to establish a chromatic phase contrast being the signature of the presence, or absence, of lipids in a set of microalga culture. In order to achieve this goal, the 3-color microscope has to be perfectly calibrated so that a point-to-point chromatic contrast can be evaluated without any distortion due to aberrations. The analysis of chromatic phase contrasts leads to measure the experimental probability density functions of the chromatic phase contrast distribution. Statistical analysis of the phase distribution in stressed cells and for not stressed cells reveals a difference in the probability densities. These experimental curves are fitted with two different theoretical modeling, one for the stressed algae (production of lipids), and one other for not-stressed algae. Fitting to experimental data demonstrates that the proposed approach opens the way for an in-situ and non-destructive experimental method to evaluate the production rate of lipids from Chlorella vulgaris algae.

Tomographic Diffractive Microscopy (TDM) is an advanced digital imaging technique, extending the concept of Digital Holographic Microscopy (DHM), which provides quantitative information about the index of refraction distribution within the observed sample, by recording of multiple holograms under varying conditions of illumination, then applying numerical inversion procedures to reconstruct a 3-D image of the specimen under consideration. After a short recall of DHM applications and limitations, various implementations of TDM are described, highlighting their respective advantages and. drawbacks. To conclude, some perspectives and challenges for this imaging modality are presented.

This paper reports on advances in optical coherence tomography (OCT) for application in dermatology. Full-field OCT is a particular approach of OCT based on white-light interference microscopy. FF-OCT produces en face tomographic images by arithmetic combination of interferometric images acquired with an area camera and by illuminating the whole field of view with low-coherence light. The major interest of FF-OCT lies in its high imaging spatial resolution (∼ 1.0 μm) in both lateral and axial directions, using a simple and robust experimental arrangement. Line-field OCT (LFOCT) is a recent evolution of FF-OCT with line illumination and line detection using a broadband spatially coherent light source and a line-scan camera in an interference microscope. LF-OCT and FF-OCT are similar in terms of spatial resolution. LF-OCT has a significant advantage over FF-OCT in terms of imaging penetration depth due to the confocal gate achieved by line illumination and detection. B-scan imaging using FF-OCT requires the acquisition of a stack of en face images, which usually prevents in vivo applications. B-scan imaging using LF-OCT can be considerably faster due to the possibility of using a spatially coherent light source with much higher brightness along with a high-speed line camera. Applied in the field of dermatology, the LF-OCT images reveal a comprehensive morphological mapping of skin tissues in vivo at a cellular level similar to histological images.

Phase retrieval reconstruction is a central problem in digital holography, with various applications in microscopy, biomedical imaging, fluid mechanics. In an in-line configuration, the particular difficulty is the non-linear relation between the object phase and the recorded intensity of the holograms, leading to high indeterminations in the reconstructed phase. Thus, only efficient constraints and a priori information, combined with a finer model taking into account the non-linear behaviour of image formation, will allow to get a relevant and quantitative phase reconstruction. Inverse problems approaches are well suited to address these issues, only requiring a direct model of image formation and allowing the injection of priors and constraints on the objects to reconstruct, and hence offer good warranties on the optimality of the expected solution. In this context, following our previous works in digital in-line holography, we propose a regularized reconstruction method that includes several physicallygrounded constraints such as bounds on transmittance values, maximum/minimum phase, spatial smoothness or the absence of any object in parts of the field of view. To solve the non-convex and non-smooth optimization problem induced by our modeling, a variable splitting strategy is applied and the closed-form solution of the sub-problem (the so-called proximal operator) is derived. The resulting algorithm is efficient and is shown to lead to quantitative phase estimation of micrometric objects on reconstructions of in-line holograms simulated with advanced models using Mie theory. Then we discuss the quality of reconstructions from experimental inline holograms obtained from two different applications of in-line digital holography: tracking of an evaporating droplet (size~100μm) and microscopy of bacterias (size~1μm). The reconstruction algorithm and the results presented in this proceeding have been initially published in [Jolivet et al., 2018].¹

We propose a new detection scheme for Spectral Optical Coherence Tomography (SOCT) that allows for a single shot depth-dependent visualization of spectroscopic properties of imaged objects. Compared to commonly used methods based on short time Fourier transformation or recently proposed technique based on Spectral and Time domain Optical Coherence Tomography (STdOCT) [1] it offers increased sensitivity and resistance to motion artefacts. The proposed method, called single shot Spectral and Time domain Optical Coherence Spectroscopy (ssSTdOCS), is based on spectroscopic version of STdOCT, but uses a 2D detector in the spectrometer and thus allows for registration of a complete data set in a single acquisition event. Originally we proposed STdOCS as an alternative for commonly used methods based on short time Fourier transformation. The method utilizes spectral OCT setup with a spectrometer equipped with linescan CCD or CMOS camera as the detector. During the measurement continuous change in optical path difference between the two arms of OCT interferometer is introduced by an optical delay line in the reference arm. Resultant sequence of spectra is subject to 2D Fourier transformation that provides the representation of the OCT signal in “Doppler frequency” – “depth” space. The signal along Doppler frequency axis has envelope corresponding to the spectrum of the light coming back from particular depth in the object. In this aspect it can be regarded as spatially resolved Fourier transform spectroscopy. The measurement scheme required by STdOCS is sensitive to the internal motion of the imaged object, since the resultant 2D data set is built from the interference spectra acquired in time. To avoid this problem we propose here a modification of the measurement instrument. To acquire the whole 2D data set used in STdOCS we propose a spectrometer equipped with 2D array sensor. The broadband light from the superluminescent diode is collimated and brought to the input of Mach-Zehnder interferometer. In the object arm the beam is focused by the objective lens in the object plane and the light scattered within the object is brought to interference with the reference beam in the output beamsplitter. The beams from reference and object arms of the interferometer are incident on the diffraction grating at relative angle in the plane determined by the grating lines and the optical axis. In the perpendicular plane the diffraction angles were the same for both beams. In the reference arm the pair of kinematic mirrors provides the possibility of precise adjustment of the angle between the beams. The first diffraction order is focused on the CCD camera with the use of cylindrical achromatic doublet. In the resulting 2D interferogram each horizontal row contains interferometric spectra with different optical path differences between the optical paths of the reference and object beams resulting from different angles of incidence at the CCD plane. This single-shot exposure event provides data sufficient for STdOCS analysis scheme. We show that the effective quantitative measurement of the depth-dependent absorption spectra of indocyanine green solution placed in glass capillary is possible with the use of our method. 1. Maciej Szkulmowski, Szymon Tamborski, Maciej Wojtkowski, "Spectroscopy by joint spectral and time domain optical coherence tomography," Proc. SPIE 9312, Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XIX, 93122P (2 March 2015);

The study of bioimaging with controlled depth using upconversion nanoparticles under near-infrared excitation was performed in this work. Monte Carlo simulation was performed to determine optimal distance between the fiber - source of laser radiation, and the receiving fiber for obtaining the signal from maximal depth in biological tissue. Also theoretical modeling of the spatial distribution of diffusely scattered radiation inside the tissue depending on wavelength is presented. Penetration depth for wavelengths corresponding to the upconversion luminescence was calculated.

Experimental modeling was carried out on phantoms of biological tissues simulating their scattering properties as well as accumulation of the investigated nanoparticles doped with rare earth ions. Measurements were performed using NaGdF₄ nanoparticles doped with Yb³⁺, Er³⁺ and Tm³⁺ rare earth ions, which demonstrated several luminescence bands from the blue (475nm) to the near-infrared (800 nm) regions of the spectrum under 980 nm excitation. The different penetration depth of various wavelengths in biotissue allows us to estimate the depth from which the signal was obtained using luminescence intensity ratio (LIR). Due to non-linearity of upconversion process, pumping power dependences of luminescence intensity was taken into account. The number of involved photons for each spectral band was estimated and intensity ratio of emission bands was calculated. Based on calculations and experimental measurements, the theoretical and experimental luminescence intensity ratio for different depths was estimated. The experimental study was performed on biological tissue phantoms containing Lipofundin® with red blood cells and has shown good agreement with calculations. The use of theoretically calculated LIR allows us to solve the inverse problem and estimate the depth from which the signal was obtained.

Recent technological developments in ultrafast laser physics have permitted to make new kind of nonlinear microscopies, as microscopy based on Stimulated Raman scattering. These techniques are based on vibrational contrast mechanism for imaging with high sensitivity, high spatial and spectral resolution and 3D sectioning capability. The interest in the study of lipids and the possibility to image lipid droplets, thanks to their isolated Raman peaks associated with vibrational C-H bond, have encouraged investigation and identification of lipid structures inside cells, taking advantage of Stimulated Raman Scattering (SRS) imaging. In this work, we report and discuss label free images on biological environmental and structural analysis, to detect lipid microstructures inside adipocyte cells.

We present a partial application space of the metrology method referred to as through-focus scanning optical microscopy (TSOM), with most number of favorable attributes as a metrology and process control tool. TSOM is a NIST-developed, high-throughput and low-cost optical metrology tool for dimensional characterization with sub-nanometer measurement resolution of nano-scale to microscale targets using conventional optical microscopes, with many unique benefits and advantages. In TSOM the complete set of out-of-focus images are acquired using a conventional optical microscope and used for dimensional analysis. One of the unique characteristics of the TSOM method is its ability to reduce or eliminate optical cross correlations, often challenging for optical based metrology tools. TSOM usually has the ability to separate different dimensional differences (i.e., the ability to distinguish, for example, linewidth difference from line height difference) and hence it is expected to reduce measurement uncertainty.

TSOM is applicable to a wide variety of target materials ranging from transparent to opaque, and shapes ranging from simple nanoparticles to complex semiconductor memory structures, including buried structures under transparent films. TSOM has been successfully applied to targets ranging from one nm to over 100 μm (over five orders or magnitude size range). Demonstrated applications of TSOM include critical dimension (linewidth), overlay, patterned defect detection and analysis, FinFETs, nanoparticles, photo-mask linewidth, thin-film (less than 0.5 nm to 10 nm) thickness, throughsilicon vias (TSVs), high-aspect-ratio (HAR) targets and others with several potential three-dimensional shape process monitoring applications such as MEMS/NEMS devices, micro/nanofluidic channels, flexible electronics, self-assembled nanostructures, and waveguides. Numerous industries could benefit from the TSOM method —such as the semiconductor industry, MEMS, NEMS, biotechnology, nanomanufacturing, nanometrology, data storage, and photonics.

Flow cytometry is a well-established technique that is widely applied in numerous fields, including pathology, pharmacology, immunology, marine biology, plant biology, and molecular biology. Conventional methods for flow cytometry fail to accurately detect cellular phenotypic characteristics due to a limited number of variants and the lack of spatial metrics. Imaging-based cell analysis methods, such as high-content screening and imaging flow cytometry, are advantageous over those univariate or few-variate methods because they offer the capability of acquiring multi-dimensional information of single cells, from which cellular characteristics can be detected with high accuracy and high specificity. However, currently available imaging flow cytometry methods suffer from low throughput which is mainly limited by the imaging techniques, or specifically, the frame rate of the commercial imaging sensors, such as CCD or CMOS sensors. In order to address these problems, here we present optofluidic time-stretch microscopy with extremely high throughput which is capable of acquiring bright-field images of large populations of cells with a high spatial resolution of 780 nm and a high throughput of >1 million cells/s, which is two orders of magnitude higher than conventional univariate or few-variate flow cytometry methods and three orders of magnitude higher than other imaging flow cytometry methods. This is made possible by integrating an optical time-stretch microscope with a hydrodynamic-focusing microfluidic device. In addition, we apply machine-learning algorithms to the acquired images to extract multiple morphological features from each cellular image to identify and classify the cells in a label-free manner with accuracy higher than 90%, which is comparable with the fluorescence-based methods. Specifically, we experimentally performed optofluidic time-stretch microscopy to detect K562 cells (leukemia cell line) spiked in whole blood samples which were treated with different concentrations of anti-cancer drugs. In the experiment, more than 10,000 high-quality images of K652 cells were acquired for each concentration of the drug. With machine-learning-based image processing and analysis, 548 morphological features were extracted from each image to comprehensively evaluate its cellular phenotypes and hence dose-dependent morphological changes of the cells caused by the drug treatment. We further confirmed the dose-dependent results in different experimental trials where the cells were treated with the drugs for different time spans. This is potentially applicable for research of cellular drug responses directly with whole blood, hence, beneficial to drug discovery and drug screening. With such high throughput, high performance and good compatibility with existing techniques, we believe that optofluidic time-stretch microscopy with extreme throughput will revolutionize the flow cytometry field and play an integral role in high-throughput, high-accuracy, and label-free cell screening in the future.

Glass formation and glassy behavior remain areas of investigation in soft matter physics with many aspects which are still not completely understood, especially at the nanometer size-scale and close to the glass transition temperature. In the present work, we show an extension of the “nanobubble inflation” method developed by O’Connell and McKenna [Rev. Sci. Instrum. 78, 013901 (2007)] which uses an interferometric microscope (white light scanning interferometry method) to measure the surface topography of a large array of 5 μm sized nanometer thick films. These so-called free-standing films are subjected to constant inflation pressure during which the nanobubbles grow or creep with time. Measurements of multiple bubbles in real time are possible via the technique of Phase Shifting Microscopy (PSM) thanks to the fast acquisition and processing of interferometry. This has been implemented using in-house developed LabVIEW based software combined with the IMAQ Vision module. Moreover this technique has the advantage, over the AFM method of O’Connell and McKenna, to be a true non-contact technique. Using this optical configuration, there is no substrate interaction to affect the polymer chains. Here we demonstrate the method using ultra-thin films of both poly(vinyl acetate) (PVAc) and polystyrene (PS) and discuss the capabilities of the method in comparison to AFM, with its advantages and disadvantages. The viscoelastic responses of the nanobubbles are determined by measuring their time-dependent diameters and then by extracting both the stress and strain time-dependent components (here the history of the polymer films has to be taken into account). We show that the results from experiments on PVAc are consistent with the prior work on PVAc. However high stress results with PS show signs of a new non-linear response regime that could be related to the plasticity of the ultra-thin film. Our homemade setup used to apply stresses on the films is also described. This first allows the control of both temperature, using a Peltier ring which surrounds the sample, and pressure, using gas flow linked to a manometer. Then major improvements of our setup in order to solve small experimental issues are described. Finally, plans for further improvements to the cell are explained for future experiments.

Multiphoton microscopy (MPM) is a recent method of imaging especially adapted to the imaging of samples of life sciences thanks to its ability to generate 3D images, with an interesting contrast and a low level of photodamage thanks to the range of wavelengths involved in the near infrared. This last point is crucial in the field of laser source development. Indeed, it has been recently identified that many new laser sources are adapted in their parameters to generate images by a multiphoton process. This results in the recent and fast increase of the quantity of laser sources especially dedicated to MPM with sometimes a focus on a specific multiphoton process. This article is an updated review of the laser sources involved in MPM in order to complete the previous one already published in 2017. Now, a focus on the new laser sources that can be listed during the two last years is proposed. We can see that a ten of drastically different and new laser sources are listed during the two last years. Is MPM dedicated to biomedical application a sufficiently broad topic with a sufficiently high level of market allowing to warrant such high level of investment in research-time and in laser development with the highest performances? Would not there be other scientific fields requiring such level of investments? Are these laser performances really identified and considered at their true level by the scientists who need MPM?

Conventional optical microscopes, such as brightfield, darkfield, phase contrast or differential interference contrast microscopes are partially coherent imaging systems. Imaging in a partially coherent system was first analyzed by Hopkins only in 1953. He propagated the mutual intensity through the optical system, but did not give an expression for the mutual intensity of the image itself. The mutual intensity is a four dimensional (4D) quantity that contains information about the modulus and phase of the image wave field, which depends on the object’s complex refractive index in 3D. The mutual intensity is related to other representations such as the Wigner distribution function (WDF) and ambiguity function. Explicit expressions for different phase space representations of the image wave field are given. The expressions separate into system and object dependent parts. In addition, explicit relationships between the defocused partially coherent cross-coefficient and phase space representations in the image plane are derived.

A model describing the signal generation in chromatic confocal imaging is presented here. It can be used to understand the signal development process accounting for wave-optical phenomena using scalar wave theory. The influence of the optics in terms of aberrations, the specimen in terms of roughness and further parameters on the signal generation process will be investigated. Moreover, the possibility to adapt the model to investigate other spectral imaging systems, such as chromatic confocal spectral interferometry will also be shown.

Because the inverse problem in diffuse optical tomography (DOT) is highly ill-posed in general, appropriate regularization based on prior knowledge of the target is necessary for the reconstruction of the image. The total variation L1 norm regularization method (TV-L1) that preserves the boundaries of a target is known to have excellent result in image reconstruction. However, large computational cost of the TV-L1 prevents its use in portable applications. In this study, we propose a dimension reduction method in DOT for fast and hardware-efficient image reconstruction. The proposed method is based on the fact that the optical flux from a light source in a highly scattering medium is localized spatially. As such, the dimension of a sensitivity matrix used in the forward model of the DOT can be reduced by eliminating uncorrelated subspaces. The simulation results indicate up to 96.1% reduction in dimensions and up to 79.3% reduction in runtime while suppressing the reconstruction error below 2.26%.

The mathematical phenomenon of super-oscillation, in which a spectrally bound function oscillates locally at a rate faster than its fastest Fourier component, has found use in both theoretical and applied areas of optical research. We show the existence of a complementary phenomenon we term sub-oscillation, in which a spectrally lower bound limited function oscillates locally at an arbitrarily low frequency beyond the lower band limit. We construct a spatially sub-oscillatory optical beam to experimentally demonstrate optical super defocusing i.e. a very fast, exceptional, expansion of a partially blocked light beam. The relevance of super-oscillations to varied fields such as quantum measurement, optical beam shaping and super-resolution, particle manipulation, electron beam shaping and radio frequency antenna design, suggest that sub-oscillations could find interesting uses in varied fields as well. Our demonstration of super defocusing by itself might be relevant for optical dark-field microscopy. [1] Y. Eliezer and A. Bahbad, Optica 4, 440 (2017)

Super-resolution (SR) is an effective approach to enhance image spatial resolution. Although many SR algorithms have been proposed by far, little progress has been made to improve resolution for a noisy image. Conventional approaches always adopt the denoising step before applying the SR method to noisy low-resolution images. However, some high-frequency details lose during the denoising step and cannot be restored by the following SR step. Therefore, motivated by the success of deep learning in different computer vision missions, we propose a novel method named Denoising Super-Resolution Deep Convolutional Network (DSR-DCN), to combine both denoising and SR step in a single deep model. The proposed deep model straightly learns an end-to-end mapping from noisy LR space to the corresponding HR space. To equip the proposed network with the capability of blind denoising, Gaussian noise, with a range of standard deviation instead of constant value, is added to each patch of the LR space during training. Experiment results demonstrate that DSR-DCN achieves superior performance and better visual effects than the conventional approaches.

When light passes through a disordered medium, its wavefront is scrambled, resulting in a seemingly random speckle pattern. In the multiple scattering regime, it is commonly assumed that this randomization removes any memory about the original wavefront, effectively destroying all its information content. But as linear elastic scattering is a purely deterministic process, information is not destroyed, but just hidden and redistributed within these patterns. We present an experimental observation of the correlations between reflected and transmitted speckle patterns, which indicate that information can survive even very strong scattering. We show that there are two distinct contributions to the correlation function - a narrow positive peak and a broad negative dip, which depend in a different way on the system parameters. We study the dependence of this correlation on the thickness of the scattering medium and the mean free path of the light in the sample, probing different regimes from ballistic to diffusive scattering. We propose an experimental procedure, based on the ghost imaging technique, that allows to use this correlation for imaging of the objects hidden behind the scattering media.

We present a novel approach for imaging through turbid media that combines the principles of Fourier spatial filtering with single-pixel imaging. We compare the performance of our single-pixel imaging setup with that of a conventional system. We conclude that the introduction of Fourier gating improves the contrast of images in both cases. Furthermore, we show that single-pixel imaging fits better than conventional imaging in vision through turbid media by Fourier filtering.

For different kinds of applications, mainly focused on probing/imaging of complex media, management of light scattering remains a challenge. Numerous studies have proven that the statistical laws followed by the intensity patterns and the polarimetric behaviour of the scattered field at the speckle scale provide a key solution to discriminate between surface and bulk scattering. It was shown for totally diffusing media that surface and bulk scattering can be seen as extreme scattering regimes. For these two regimes, exact electromagnetic calculation can be performed, but this requires a deterministic knowledge of the medium (surface profile, inhomogeneity function) under study. Furthermore these exact models are time and memory consuming, which reduces the analysis to samples with small dimensions. Within this framework statistical optics provide solutions which allow to simplify the problem. As an illustration, in previous studies our group recently proposed to use random phasors matrices to predict the statistical behaviour of surface and bulk scattering patterns in terms of intensity and polarimetric parameters. These phenomenological results were validated by comparison to both exact electromagnetic theories and experimental data, with high agreement. However in these works only the extreme scattering regimes were addressed while they involve zero or total correlation of the scattering coefficients. In this paper we show how to extend the analysis to intermediary scattering regimes, with a comparison to experiment. Furthermore we propose a simplified model involving a unique correlation parameter in the scattering matrix of a strongly disordered medium. Comparison with experiment emphasizes the validity of this model to predict the statistical behavior of the speckle patterns and their polarization properties, as well as spatial depolarization and temporal repolarization. The parameter is connected with the weight of multiple scattering and allows to consider transition between surface and bulk scattering. This statistical approach provides great help to analyze scattering media in the absence of electromagnetic theories.

We investigate ways to improve image resolution and contrast in scatter-plate microscopy by image deconvolution and speckle pattern manipulation. Scatter-plate microscopy uses a single diffusively scattering element instead of a complex lens system to record high resolution images with almost arbitrary magnification. The image of the sample is acquired by cross-correlating the speckle pattern of a point source and the speckle pattern of the incoherently illuminated sample. The working principle is restricted by the finite range of the optical memory effect and by the quality of the light source used to approximate a point source (in our case a single mode fibre). With a deconvolution method using the autocorrelation of the point source speckle pattern as the filter function, describing the relationship between the acquired image and the original object, it is possible to compensate partially the deviation of the used point source from an ideal one. The influence of the restricted range of the memory effect can be reduced by manipulating the sample’s speckle pattern.

The combined approach including experiments (Mueller polarimetry) and theory (differential Mueller matrix formalism) for the studies of anisotropic scattering media was tested on the model system of human skin. Custom-build Mueller polarimetric microscope was used for the studies of histological cuts of full-thickness skin equivalents generated from epidermal keratinocytes forming a multilayered epidermis on top of collagen I hydrogel with dermal fibroblasts. The sets of fixed unstained tissue cuts of different thicknesses (5μm - 30μm) were measured in transmission configuration. The values of polarimetric (dichroism and retardance) and depolarization parameters were calculated by applying pixel-wise the logarithmic decomposition of Mueller matrices. The parabolic dependence of depolarization parameters and linear dependence of polarimetric parameters on thickness, as predicted by theory, was confirmed by measurements. It proves that phenomenological modeling of complex anisotropic scattering medium (e.g., biological tissue) may effectively disentangle the polarimetric and depolarizing properties of the system understudies and may be used for analysis and diagnostics of tissue.

This paper discusses on off-axis digital holography applied to quantitative imaging of flows. The close link between phase measurement and measurands of interest is discussed. Numerical processing of digital holograms and phase retrieval is summarized. The paper presents different architectures that can be found in literature according to different operating modes: speckle-free mode, speckle mode, reduced-sensitivity, and combined approaches.

Lensless color microscopy is a recent 3D quantitative imaging method allowing to retrieve physical parameters characterizing microscopic objects spread in a volume. The main advantages of this technique are related to its simplicity, compactness, low sensitivity of the setup to vibrations and the possibility to accurately characterize objects. The cost-effectiveness of the method can be further increased using low-end laser diodes as coherent sources and CMOS color sensor equipped with a Bayer filter array. However, the central wavelength delivered by this type of laser is generally known only with a limited precision and can evolve because of its dependence on temperature and power supply voltage. In addition, Bayer-type filters of conventional color sensors are not very selective, resulting in spectral mixing (crosstalk phenomenon) of signals from each color channel. Ignoring these phenomena leads to significant errors in holographic reconstructions. We have proposed a maximum likelihood estimation method to calibrate the setup (central wavelength of the laser sources and spectral mixing introduced by the Bayer filters) using spherical objects naturally present in the field of view or added (calibration objects). This calibration method provides accurate estimates of the wavelengths and of the crosstalk, with an uncertainty comparable to that of a high-resolution spectrometer. To perform the image reconstruction from color holograms following the self-calibration of the setup, we describe a regularized inversion method that includes a linear hologram formation model, sparsity constraints and an edge-preserving regularization. We show on holograms of calibrated objects that the self-calibration of the setup leads to an improvement of the reconstructions.

We propose a new algorithm for absolute phase retrieval from multiwavelength noisy phase coded diffraction patterns in the task of surface contouring. A lensless optical setup is considered with a set of successive single wavelength experiments. The phase masks are applied for modulation of the multiwavelength object wavefronts. The algorithm uses the forward and backward propagation for coherent light beams and sparsely encoding wavefronts which leads to the complex-domain block-matching 3D filtering. The key-element of the algorithm is an original aggregation of the multiwavelength object wavefronts for high-dynamic-range profile measurement. Numerical experiments demonstrate that the developed approach leads to the effective solutions explicitly using the sparsity for noise suppression and high-accuracy object profile reconstruction.

Quantitative phase imaging (QPI) became an important technique for label-free biomedical imaging suitable particularly for observation of live cell and dry-mass profiling. Extension of this technique to objects immersed in turbid medium is highly desirable with respect to the need of non-invasive observation of live cells in real 3D environments. Coherencecontrolled holographic microscopy is capable of QPI through turbid media owing to coherence gating induced in transmitted-light geometry by low spatial coherence of illumination. Using this approach, QPI of object in turbid medium can be formed both by ballistic and multiply scattered photons. Moreover, the particular QPIs formed by ballistic and scattered photons can be superimposed thus yielding synthetic QPI of substantially improved image quality. We support the theoretical reasoning of the effect by experimental data.

The incoherent imaging properties of ptychography are discussed in this paper. Usually a coherent light source is employed in ptychography for the recording of the diffraction patterns. However, in combination with a curved illumination it is possible to obtain an image quality of the reconstructed images that is equal to the one known from incoherent imaging. The underlying principle and results to demonstrate these findings are presented in this paper. Moreover, it will be shown that consequently not only the coherent speckle noise but likewise the resolution can be increased.

Tomographic diffractive microscopy (TDM) is an imaging technique which allows for recording the complex optical index of unlabelled specimens. It is based on diffraction theory with a spatially coherent illumination and interference demodulation. Different methods have been developped like illumination rotation with fixed sample or sample rotation with fixed illumination. However this last technique is difficult to set up. Hence, we propose a novel reconstruction technique applicable to axisymmetric unlabelled specimens. It consists in a numerical rotation of the Ewald cap of sphere generated by a zero-degree illumination on the sample. Due to the specimen symmetry, we show that the Fourier space can be filled in the direction perpendicular to the axis of symmetry.

A critical issue associated with the clinical translation of fluorescence molecular imaging relates to the reproducibility of the collected measurements. In particular, images acquired from the same target using different fluorescence cameras may vary considerably when the employed systems have markedly different specifications. Methods that standardize fluorescence imaging are therefore becoming necessary for assessing the performance of fluorescence systems and agents and for providing a reference to the data collected. In the work presented herein we propose a composite phantom for integrating multiple targets within the field of view of a fluorescence camera. Each quadrant of this phantom resolves different fluorescence features: (1) sensitivity as a function of the optical properties; (2) sensitivity as a function of the depth from the top surface; (3) resolution of the fluorescence and optical imaging; and (4) cross-talk from the excitation light. In addition, there exist structures in the phantom for assessing homogeneity of the incident illumination. In order to validate our main hypothesis that standardization of fluorescence imaging systems is feasible through imaging such a phantom, we employed two systems of different specifications and quantified all relevant performance metrics. The derived results showcase the feasibility of fluorescence cameras calibration. Additionally, we demonstrate a methodology of comparing fluorescence cameras by means of benchmarking scoring. We expect that such approaches will boost the clinical translation of fluorescence molecular imaging and will allow for the investigation of novel fluorescence agents.

The introduction of optical imaging by using near-infrared (NIR) light shines new light in the field of (oncologic) surgery. The use of non-specific fluorophores, such as Indocyanine Green (ICG) and Methylene Blue (MB) have already shown its value for different applications during image-guided surgery. Both ICG and MB are currently the only fluorophores approved by regulatory agencies for off-label use. ICG improves visibility of several solid tumors, sentinel lymph nodes, biliary ducts and can be used to evaluate tissue perfusion. MB could be used for ureter imaging and neuroendocrine or thyroid tumors detection. Recently, a shift to molecular imaging was made by the introduction of new NIR fluorophores (IRDye-800CW, ZW800-1), which could be conjugated to tumor or structure specific targets, such as proteins, antibodies, antibody fragments or nanoparticles. Several clinical trials showed detection of both tumor and metastases in patients with head-and neck, colorectal, pancreatic, ovarian, and renal tumors. Furthermore, nerve and ureter specific agents are (pre-)clinically evaluated, however more research is necessary to make these agents clinically available. Limitations of using NIR fluorescence imaging during surgery are the lack of quantification of fluorescence signals and limited penetration depth. Further optimization of NIR fluorescence imaging and evaluation of the clinical benefit for the patient are necessary steps to make NIR fluorescence guided surgery general applicable into surgical daily practice.

Near-infrared fluorescence imaging methods have great advantages. They are fast and noninvasive. Using near-infrared photosensitizers, such as indocyanine green, it is possible to evaluate blood flow in real time. These methods find their application in problems of engraftment of skin grafts¹ and in the evaluation of the severity of peripheral arterial disease during indocyanine green fluorescence angiography procedure² . Irradiation and registration of fluorescent images and video was performed using developed video system that consists of a 785-nm laser diode, a broadband source diode, a beam splitter with a dichroic mirror and two digital CMOS cameras for recording color and luminescence images. Indocyanine green fluorescence angiography procedure was performed in 10 diabetic patients with critical lower limb ischemia. To evaluate the soft tissue perfusion of the foot diabetic patients via fluorescence angiography the following parameters have been used: T_0m - time to reach maximum intensity after intravenous injection of the indocyanine green; T_Im- the onset of fluorescence intensity in the area of interest; I_m - the level of the maximum intensity. Regions of interest are different parts of foot near foot. Fluorescence angiography parameters were compared before and after ercutaneous transluminal angioplasty of lower limb arteries. Fluorescence images show the spread of indocyanine green in skin tissue. The degree of the formation of new blood vessels can be evaluated by intensity of fluorescence. Data from all ROI with different fluorescence angiography parameters was collected. There were significant difference in T_0m and T_Im in different ROI more than 10 sec.

Nowadays imaging the early liver metastases has to be improved in order to have an easier setup than MRI or to be more discriminant than ultrasound between healthy and diseased tissues. Acousto-optic imaging could solve these issues by coupling itself with ultrasound modality: the additional optical contrast would suppress the indetermination on the health of the biological tissue. Acousto-optic imaging is a multi-wave technique which localizes light in very scattering media thanks to an acoustic wave: the acousto-optic effect creates frequency-shifted light, carrying local information about the insonified volume. The central challenge of acousto-optic imaging is the detection of the frequency-shifted light, because there are only very few modulated photons and they create a speckle pattern. We choose to explore the detection by spectral filtering using the spectral hole burning process in rare earth doped crystal [1]. Spectral hole burning consists in creating a sub-MHz-wide transparency window in the wide absorption spectrum of a rare earth doped crystal: the crystal becomes transparent at the wavelength of the spectral hole and thus can filter the modulated light. This filtering technique is intrinsically immune to speckle decorrelation and therefore well adapted to in vivo imaging. We use a YAG crystal doped with thulium ions under a magnetic field which increases the lifetime of the spectral hole from 10ms to longer than a minute. We have undertaken a spectroscopic study to optimize the hole preparation sequence. The long lifetime simplifies the optimization of fast imaging sequences, making real-time acousto-optic imaging reachable. We will present the first acousto-optic images achieved with a long-lived spectral filter in Tm:YAG, in a scattering medium. [1] Li, Y., Zhang, H., Kim, C., Wagner, K. H., Hemmer, P., & Wang, L. V. (2008). Pulsed ultrasound-modulated optical tomography using spectral-hole burning as a narrowband spectral filter. Applied physics letters, 93(1), 011111.

In this paper, a hybrid method of in-line and off-axis digital holography is proposed. Off-axis digital hologram and in-line hologram are recorded. The approximate phase distributions in the recording plane are obtained by constrained optimization approach from the off-axis hologram, and they are used as the initial value in the iterative procedure of the phase retrieve of in-line hologram. For static objects, the proposed method can be implemented with a single laser and dual shots. For dynamic objects, it can be implemented with dual-wavelength lasers and a single shot. It can achieve high-resolution, wide field-of-view holographic imaging.

We present a phase imaging system using a novel non-interferometric approach. We overcome the limitations in spatial resolution, optical efficiency, and dynamic range that are found in Shack-Hartmann sensors. To do so, we sample the wavefront using a digital micromirror device. A single lens forms a time-dependent light distribution on its focal plane, where a position detector is placed. Our approach is lenslet-free and does not use any kind of iterative or unwrap algorithm to recover the phase information. The validity of our approach is demonstrated by performing both aberration sensing and phase imaging of transparent samples.

Phase Diversity (PD) is an unconventional imaging technique which uses two or more distorted views of an object to perform wavefront sensing and/or to clarify the object. In 1990 it was used to remove the flaw in the Hubble Space Telescope. A major advantage of PD is that it needs no auxiliary hardware, like a guide star or a Shack-Hartmann wavefront sensor. We review a personal path in the discovery and the use of phase diversity. That path started in 1974 with the removal of raster lines in digital-based satellite images and has led to the real-time removal of aberrations in high-performance, ground-based telescopes, among other applications. PD could be used in cell phones which maintain good image quality by changing the camera's optics. The recorded video and the changes in the camera's optics are the necessary observations needed to improve the video, with software called sequential diversity imaging.

When light propagates through highly disordered media such as biological tissue, multiple scattering prevents light from reaching depths much larger than the transport mean free path, making the material opaque. This poses a problem for Raman spectroscopy in biological media, where in order to obtain spontaneous Raman signal they need to work in the superficial region of the material or increase the pump power, which is not always a safe option. In this work we show that wavefront shaping techniques can significantly increase light penetration through opaque media, thus allowing detection of Raman scattered light of materials deep inside an opaque medium, avoiding the increase of the pump power. Wavefront shaping techniques are capable of manipulating the amplitude and/or phase of a light beam, which allows control over light propagation through such media. Wavefront shaping was originally proposed to focus light through a scattering material [1], and was recently shown capable of increasing light penetration in very thin (8 μm) scattering layers [2]. But efficiently delivering light at larger depths (~100 μm) in strongly scattering material, with optical densities comparable to thick biological tissues, is still an unsolved problem. In this work we use a fast Digital Micromirror Device (DMD) to control the phase profile of the pump light incident on the sample, made of two layers of different strongly scattering materials (TiO2 and Hidroxyapatite). Using the transmitted light as feedback, an iterative algorithm adapts the phase pattern controlled by the DMD maximizing the penetration depth of the incident light. A spectrometer collecting the reflected light quantifies the depth at which the pump light is reaching by analysing the spontaneous Raman signal of the inner layer of the sample. We show an increase of 40% in the Raman signal collected from the inner material when the wavefront is optimized, equivalent to 40% deeper penetration of the pump light, given the linear characteristics of spontaneous Raman scattering. This result shows the usefulness of wavefront shaping techniques to increase the penetration depth of light, improving the applicability of Raman spectroscopy in thicker materials. This is of enormous interest in the fields of non-invasive breast cancer diagnosis, light-activated cancer drugs or white LEDs where penetration depth of light is limited by scattering and applied power needs to be in the safe regime. [1] I.M. Vellekoop, and A.P Mosk, Optics Letters 32, 2309 (2007). [2] W. Choi, et al. Scientific Reports 5, 11393 (2015).

CIAO is a Compact Innovative Adaptive Optic system for 0.5 to 2 m telescopes. Pic du Midi observatory is known for the quality of its planetary images thanks to the site quality and the long experience of the team. The technique is still improving; our last step is the development of an adaptive optics system. This compact and affordable system, developed in collaboration with Observatoire de Paris could also interest other observatories with telescopes in the 0.5 to 2m range. We are able to analyze the wave front of the target at 1000 Hz, with a rejection bandwidth of correction up to 100 Hz on bright targets. On fainter targets, the system can still run slower, it is used in this case as an active optics to compensate the static defaults of the telescope or slow evolution of the air turbulence. The first tests on the sky were at the end of October. The device worked as expected and the results are very encouraging. We have been able to improve the image quality by a factor 4 on several targets such as stars and the planet Mars. We will present in this conference how an adaptice optics system works, the prototype architecture and how we implemented it on the Nasmyth focal plane of the 1m diameter telescope at Pic du Midi. Many results will be presented including video showing in realtime the gain of the adaptive optics. The best known images of Mars in this orbit situation will be shown. We thank PNP (Programme National de Planétogie), Paris Observatory scientific Council and IMCCE (Institut de Mécanique Céleste et de Calcul des Ephémérides) for their financial support and Jean-Luc Dauvergne for his help on the project.

This work shows the interest of combining polarimetric and light-field imaging. Polarimetric imaging is known for its capabilities to highlight and reveal contrasts or surfaces that are not visible in standard intensity images. This imaging mode requires to capture multiple images with a set of different polarimetric filters. The images can either be captured by a temporal or spatial multiplexing, depending on the polarimeter model used. On the other hand, light-field imaging, which is categorized in the field of computational imaging, is also based on a combination of images that allows to extract 3D information about the scene. In this case, images are either acquired with a camera array, or with a multi-view camera such as a plenoptic camera. One of the major interests of a light-field camera is its capability to produce different kind of images, such as sub-aperture images used to compute depth images, full focus images or images refocused at a specific distance used to detect defects for instance. In this paper, we show that refocused images of a light-field camera can also be computed in the context of polarimetric imaging. The 3D information contained in the refocused images can be combined with the linear degree of polarization and can be obtained with an unique device in one acquisition. An example illustrates how these two coupled imaging modes are promising, especially for the industrial control and inspection by vision.

We introduced two continuous-wave terahertz iterative phase-contrast imaging methods. In-line digital holography has the capability to reconstruct the amplitude and phase distributions simultaneously. It is a non-destructive, high-resolution, full-field dynamic phase-contrast imaging technique. Ptychography can reconstruct the complex amplitude distribution of the transmission object from the overlapped diffraction patterns. Both methods can achieve phase-contrast imaging, and are suitable for terahertz region. In this paper, both Gabor in-line holographic and ptychographical configurations are investigated from algorithms to experiments. For in-line holography, the use of extrapolation, synthetic aperture, sub-pixel shifting and multi-plane imaging are introduced to improve the resolution and reconstruction accuracy. For ptychography, we obtained the ptychographical reconstruction results of a polypropylene alphabet sample, which provides a new imaging method for terahertz phase-contrast imaging.

We have developed a method of the terahertz (THz) solid immersion microscopy for the reflection-mode imaging of soft biological tissues. It relies on the use of the solid immersion lens (SIL), which employs the electromagnetic wave focusing into the evanescent-field volume (i.e. at a small distance behind the medium possessing high refractive index) and yields reduction in the dimensions of the THz beam caustic. We have assembled an experimental setup using a backward-wave oscillator, as a source of the continuous-wave THz radiation featuring λ= 500 μm, a Golay cell, as a detector of the THz wave intensity, and a THz SIL comprised of a wide-aperture aspherical singlet, a truncated sphere and a thin scanning windows. The truncated sphere and the scanning window are made of high-resistivity float-zone silicon and form a unitary optical element mounted in front of the object plane for the resolution enhancement. The truncated sphere is rigidly fixed, while the scanning window moves in lateral directions, allowing for handling and visualizing the soft tissues. We have applied the experimental setup for imaging of a razor blade to demonstrate the advanced 0:2λ resolution of the proposed imaging arrangement. Finally, we have performed imaging of sub-wavelength-scale tissue spheroids to highlight potential of the THz solid immersion microscopy in biology and medicine.

Terahertz pulsed imaging has attracted considerable interest for revealing the stratigraphy and hidden features of art paintings. The reconstruction of the stratigraphy is based on the precise extraction of THz echo parameters from the reflected signals. Several historical panel paintings and wall paintings have been well studied by THz reflective imaging, in which the detailed stratigraphy has been successfully revealed. To our knowledge, however, the stratigraphy of oil paintings has not been clearly uncovered by THz imaging, since the paint layers in an oil painting on canvas, especially for the 16th and 17th century art works, are usually very thin (~10 μm) in the THz regime. Therefore, in order to improve the performance of THz imaging, advanced signal-processing techniques with higher depth-resolution are still needed. In this study, THz reflective imaging is employed to reveal for the first time the detailed stratigraphy of a 17th century Italian oil painting on canvas. The paint layers on the supporting canvas are very thin in the THz regime, as the THz echoes corresponding to the stratigraphy totally overlap in the first cycle of the reflected THz signal. THz sparse deconvolution based on an iterative shrinkage algorithm is utilized to resolve the overlapping echoes. Based on the deconvolved signals, the detailed stratigraphy of this oil painting on canvas, including the varnish, pictorial, underdrawing, and ground layers, is successfully revealed. The THz C- and B-scans based on the THz deconvolved signals also enable us to reveal the features of each layer. Our results thus enhance the capability of terahertz imaging to perform detailed analysis and diagnostics of historical oil paintings on canvas with foreseen applications for the study of the artist’s technique and for authentication.

We present a study on the resolution limits and resolution factors of terahertz (THz) ptychography. Simulations of a binary amplitude object show that ptychography shares the same intrinsic resolution factors with digital holography, i.e. it is diffraction-limited. Reconstructions of amplitude and phase objects obtained from holographic and ptychographic experiments are comparable. A lateral resolution of around one wavelength λ is achieved on an amplitude object, while a depth resolution of around λ/5 is reported on a weakly diffracting phase object. THz ptychography is expected to complement THz holography for imaging biological samples and THz transparent specimens.

In this proceeding, we present the description of the numerical approach for resolution enhancement, field of view widening and noise reduction in pulse time-domain holography. The approach comprises iterative procedure of the recorded hologram self-extrapolation into wider spatial area, and consecutive ‘self-healing’ of an object. The concept has been proofed on a synthetically generated pulse time-domain holograms. The proposed method is sought after, especially in THz range, where the distance between the object and the hologram lies in the order of several tens of wavelengths, and the detector sizes are usually limited, and with minor modifications can be applied for other THz holographic approaches.

In the International Thermonuclear Experimental Reactor (ITER) Project, under construction in southern France, there will be a need for continually measuring the erosion at the wall, after the machine starts operating. A multiple wavelength interferometric technique based on digital holography is proposed for the erosion measurement. This technique has the ability to tackle the challenging environmental conditions within the reactor by a long distance measurement, where a relay optic will be used for imaging the investigated surface on the detector. We will show that shape measurements of objects located at a distance of more than 13 m from the measuring head can be carried out by the multiple wavelength interferometric technique. A depth accuracy of ±10 μm is achieved.

Two examples are presented of interferometric measurements in hostile environments. In the first example a modified shearing interferometer. The optical approach is novel in that while being based upon conventional interferometry; modifications provide a whole field, non-invasive, quantitative measurements of density gradients of 300Atmos in a supercritical CO2 flow. This an almost 2 orders of magnitude greater than previously presented by an interferometric approach. The combination of the area of view (40mmx70mm) and the strong localised density gradient within the flow was beyond the image capture resolution of conventional interferometry. This necessitated the need for a modified Twyman-Green interferometric shearing approach. The images recorded had a resolution of 5,000x3,000 pixels and provided sub-wavelength measurements, with an average of 10 pixels/fringe. A Fourier based phase unwrapping approach has been used to measure the differential path change in the gas to 1/20 of an optical wavelength. The phase unwrapping process used was critical in its ability to successfully process images of 15M pixels to an accuracy of 0.05kg/m3. The results from the unwrapped images obtained have been numerically integrated and validated against conventional pressure measurements. They have also been compared with numerical prediction. The second example is a high resolution holographic interferometric measurement of a propane flame. To increase the burning efficiency of the flame a vortex mixing process had been augmented in the burner between mixing gases of propane and air. This experimental requirement was both to image the high degree of turbulence and provide a whole field visualization of the complete flame. This was achieved using double exposure, image plane, pulse laser, holographic interferometry. The optical resolution of the technique both preserved fringe data and provided a near instantaneous whole field visualization of the burning process. The burner was 0.1m in diameter and the flame extent was 0.2m long. In this case a photographic reconstruction has been made using a conventional CCD camera. To increase the digital image resolution the camera was mounted on a traversing system and moved in a step and repeat manner, each image being 4,000x3,000 pixels. The 20 individual frames were stitched to create a contiguous 240Mbytes image. This resolution was essential in visualizing the whole complex highly turbulent burning structure of the flame visual. However, initially such high-resolution image presented an intractable processing problem, conventional, Fourier processing had insufficient spatial bandwidth to be applicable. However, recently a globally searching algorithm has yielded a solution. Further, a tomographic reconstruction approach has been applied to the resulting unwrapped map which utilizes the overall symmetry in the flame to augment a simple transform.

The presence of oceans on the outer solar system moons Europa and Enceladus poses the question of whether microbial life might be present on those bodies. One approach to answering that question could be a very compact, lightweight and robust microscope that is capable of rapidly imaging the contents of a 3-dimensional sample volume. To this end, we have been developing deployable digital holographic microscopes, which can also be used in the short term for terrestrial field work. A very stable “common-mode” microscope, in which two adjacent beams share several optics, has shown very good imaging performance, and has been deployed successfully to several field sites, including Greenland and Alaska. A more compact approach is our version of the lensless digital holographic microscope, which uses gradient index rod lenses to produce a pair of high numerical-aperture input beams. Sub-micron resolution has been obtained with both systems, and further performance improvements are possible.

A hardware-software complex for non-contact investigation of marine particles is presented. The complex is based on digital holography principles and can be immersed in water, for example, to study plankton in a habitat. Special features of a submersible holocamera (or DHC sensor) are considered. Results of approbation of the complex during the Mission in the Kara Sea are presented. A new DHC sensor design is discussed.

In situ detection, tracking, localization and identification of undersea marine life in their natural environment is an important aspect of marine biology, fisheries management, ecology and environmental impact studies in the vicinity of undersea infrastructure. However due to the challenging optical characteristics of the underwater environment, mainly due to attenuation and scattering, it is not operationally effective to observe marine life using conventional approaches, such as underwater cameras and lights operating in the visible spectrum. Images often appear dim and blurry and increasing the photon output of the flood lamps or strobes does not solve the issue, instead leading to the formation of image hotspots, and in turbid conditions also reducing image contrast and resolution due to increased back-scattering and blur/glow field effects due to increased forward-scattering. Perhaps more importantly, the introduction of bright broadband lighting into the underwater environment is known to induce behavioral changes in the animals being studied. The MEMS-based serial LiDAR (Light Detection and Ranging) detection and imaging system that was recently developed uses red (638 nm) pulsed laser diode illumination to be invisible and eye-safe to marine animals. Furthermore it has the potential to be very compact, and cost-effective. The equipment is designed for long-term, maintenance-free operations. It generates a sparse primary dataset that only includes detected anomalies, with dense identification-quality dataset being triggered within a scan cycle, thus allowing for efficient, real-time, automated, low bandwidth animal detection, classification and identification. This paper outlines the operating principles of the detection and imaging optical and electronic architecture, with an example of recent results obtain in turbid coastal conditions.

Imaging measurements of transient flow behavior in harsh environments are a demanding task in industrial development, e.g. regarding the reduction of pollutant and sound emissions of jet engines by means of lean combustion. Such processes can exhibit instabilities based on flame oscillations due to thermo-acoustic fluctuations of pressure and heat release rate, which potentially result in failure of the device. We present a characterization of the combustion by non-invasive measurements of the heat release rate inside the flame and the flow velocity at the combustor outlet. Additionally, measurements of local sound pressure fluctuations are presented. Simultaneous, seedingless, optical measurements of these quantities can be achieved using multiple laser-vibrometers and signal correlation [1, 2]. The measurement is based on the linear relation between the heat release rate or sound pressure and the change of the refractive index, which can be detected integral along the laser beam. Using multiple laser-vibrometers it is furthermore possible to measure the flow velocity due to the phase delay between two vibrometer signals and the known distance between the measurement positions. However, simultaneous field measurements have to be accomplished in order to detect the transient spatio-temporal flow behavior of the combustion and to enable tomographic reconstruction of the instable process. In order to overcome these limitations, a high-speed camera-based laser-vibrometer (CLIV) is designed for non-invasive seedingless measurements of the flow velocity and heat release rate inside premixed flames. The novel imaging CLIV offers single pixel resolution with measurement rates up to 40 kHz at an image resolution of 110 x 110 px, resulting in a high data rate of 0.5 GHz. A discussion of the traceability to SI units and the measurement uncertainty is presented. Spatially resolved simultaneous measurements of the heat release rate and its temporal fluctuations inside a swirl-stabilized flame are demonstrated. Subsequently, the calculation of the flow velocity field inside the flame is performed by means of signal correlation between different pixel data. Additionally, local sound pressure fluctuations are detected using the novel system in combination with tomographic reconstruction. References [1] J. Gürtler, F. Greiffenhagen, J. Peterleithner, J. Woisetschläger, D. Haufe, J. W. Czarske, “Noninvasive seedingless measurements of the flame transfer function using high-speed camera-based laser vibrometry”, SPIE Optical Metrology, Optical Measurement Systems for Industrial Inspection, Proceedings pp. [10329-66], Munich, Germany, 26.06. – 29.06.2017 [2] S. Köberl, F. Fontaneto, F. Giuliani, J. Woisetschläger, “Frequency resolved interferometric measurement of local density fluctuations for turbulent combustion analysis”, Meas. Sci. Technol., 21:035302 (10pp), 2010.

We demonstrate the possibility to realize supervised machine learning for a cell detection task without having to manually annotate images through the sole use of synthetic images in the training and testing steps of the learning process. This is successfully illustrated on 3D cellular aggregates observed under light sheet fluorescence microscopy with a shallow and deep learning detection approach. A performance of more than 90% of good detection is obtained on real images.

Accurate calculation of diffraction field emitted from a three-dimensional object paves the way to reconstruct more reliable information related to three-dimensional objects. As a result of this, we can analyze the structure of cells and perform precise measurements in non-destructive tests. Furthermore, accurate calculation of the diffraction field will help to improve the results in many applications of holography. In this work, we propose an accurate diffraction field calculation method which is based on L1-norm minimization and achieved successful results.

X-rays and e-beams are used for high-resolution imaging where the requirement for highly accurate lenses is one of the limiting factors. Hence, lensless imaging is preferred. Coherent Diffractive Imaging (CDI) is a lensless imaging technique which uses intensity patterns in the far-field to obtain the complex amplitude of the object. Ptychography, an emerging area of research, belongs to the class of CDI techniques. In ptychography, the object is partially illuminated by a scanning probe and the intensity patterns in the far-field for each probe position are recorded and used to reconstruct the object. One of the important aspects to the success of this method is the overlap between the adjacent probes and the accurate knowledge of the probe positions. Recently, we proposed a new method to correct the probe positions which uses the gradient of intensity patterns in the far-field. Here, we propose another method to correct probe positions.

In this contribution we present a novel one-stop-shop solution providing comprehensive, robust and automatic singleframe fringe pattern analysis for quantitative phase imaging. It is based on the modified variational image decomposition (mVID) algorithm and the Hilbert spiral transform. The VID concept is applied to tailor input data for efficient Hilbert spiral transform (HST). It returns the fringe-signal which is in quadrature to the input VID-filtered zero-mean-value fringe pattern. Both fringe-signals form the 2D complex analytic fringe pattern with phase and amplitude clearly defined by angle and modulus. Additional means for mVID-based compensation of characteristic phase errors are to be provided. The performance of the proposed novel mVID-HST technique is tested on simulated and experimental data. Its versatility and data-driven nature is emphasized processing off-axis, slightly off-axis and on-axis holograms.

The actual 3D shape of free formed transparent objects is of interest in different fields of research and production. Measuring shapes just by looking through an object is a challenging task for any hard- and software concepts. We report a stereo imaging system that computes the topography of the surface and the refractive index of freeform lenses. The system takes a set of images from different viewpoints of various patterns in a fix environment. Based on this 2D images, an optic design model is built up with the sample as unknown optical element. Then, the shape of the sample will be modified until a cost function is minimized. In a first approach the sample is described as a spherical lens. In this case two radii of curvature and a refractive index of the material can be determined. In a more general case, we discuss an approach to calculate the full topography of one surface of the sample from the recorded data. Hereby, an iterative method to calculate the topography will be demonstrate. To qualify the performance of this principle, different free formed optics were measured and the deviation to reference measurements will be shown. This work shows the advantages of combining methods from different disciplines as optical engineering, optics design and computer vision.

Acute lymphoblastic leukemia (ALL) is a type of cancer caused by the disordered growth of white blood cells, known as lymphomas, which are formed in the bone marrow and rapidly diffuse through the bloodstream; affecting other organs and eventually leading to death. Diagnostic tests currently implemented require a cytometric analysis of bone marrow extraction or peripheral blood marking for counting of blastic cells present in peripheral blood via light microscopy. These techniques are invasive, requiring labeling and alteration of cells. Therefore, an alternative technique is sought by implementing a diffraction phase microscope (DPM), which allows the measurement of the size and refractive index variations of the cells in a non-invasive way, looking for the detection of blast cells in human peripheral blood. In a first phase it is necessary to distinguish blastic cells from other mononuclear cells such as T and B lymphocytes and monocytes. The present work describes the implementation of the technique in order to establish parameters of population differentiation, morphometric and refractive index of mononuclear cells and blast cells in a single blood sample. For this purpose it is described the process of separation of peripheral blood mononuclear cell populations and cells from two diseased donors are analyzed. Consequently, the DPM technique is validated as a differentiation parameter, opening the way to the possibility of validating it for the diagnosis of ALL in the analysis of a sample of human peripheral blood.

Optical diagnostic methods were used to study the physical processes occurring on the surface of the melt in the technology of selective laser melting (SLM) of metallic powders. Independent registration of the fraction of reflected laser radiation and thermal radiation from several points of the surface of the melt were carried out simultaneously. This made it possible to distinguish changes in the surface relief and subsurface processes of heat and mass transfer during laser heating. It is shown that the frequency and amplitude characteristics of signals obtained by optical diagnostics make it possible to identify the moments of intensification of convective heat and mass transfer. The results of the research can be used to develop methods and tools for on-line monitoring and control of the SLM process.

Photoacoustic (PA) imaging is one of the fastest growing imaging technologies nowadays in both research and clinical applications, especially due to its unique capability to visualize blood vessels. The PA microscopy (PAM) is classified into two types: optical-resolution PAM (OR-PAM) and acoustic-resolution PAM (AR-PAM). OR-PAM image has a point spread function (PSF) much smaller than AR-PAM because it uses a tightly focused optical beam and the PSF is determined by the optical focus. In contrast, AR-PAM uses an unfocused optical illumination to excite a relatively large area and detects the PA signal from a small area determined by its acoustic focus. Because ultrasound is less scattered than light in biological tissue, AR-PAM can achieve deeper imaging depth than OR-PAM at the expense of image resolution. Due to the limited resolution and imaging depth scale of each PAM type, it is challenging to image vessels in various area of small animals. In this study, we demonstrated in vivo OR-/AR-PAM imaging of blood vessels in various areas such as eye, ear, and hind limb by using a single commercial PAM system. Additionally, we quantified micro-vessel density (MVD) of the mouse eye and ear images, and applied a synthetic aperture focusing technique (SAFT) to correct the distorted PA signal at the out-of-focus in AR-PAM image. As a result, we have demonstrated multiscale PAM imaging of small animal vasculature in various areas with vessel quantification and resolution enhancement, so we believe that this multiscale PAM imaging technique would be helpful in biology research such as ischemia and neovascularization.

This paper presents a method for determining the degree of misalignment between a LED and its epoxy encapsulation in a simulated environment. The misalignment is determined by analyzing the light emitted by the optoelectronic component and transmitted through a grid of holes. The results obtained are compared with simulated and experimental registers wherein there is no misalignment between the LED and the epoxy but a displacement of the whole optoelectronic component perpendicularly to the observation axis. We prove that the method used allows us to distinguish between the existence of misalignment inside the optoelectronic component and a simple displacement between the optoelectronic component and the observation axis.

Among the various configurations that may be used in digital holography, the original in-line “Gabor” configuration is the simplest setup, with a single beam. It requires sparsity of the sample but it is free from beam separation device and associated drawbacks. This option is particularly suited when cost, compact design or stability are important. This configuration is also easier to adapt on a traditional microscope. Finally, from the metrological point of view, this configuration, combined with parametric inverse reconstructions using Lorenz-Mie Theory, has proven to make possible highly accurate estimation of spherical particles parameters (3D location, radius and refractive index) with sub-micron accuracy. Experimental parameters such as the defocus distance, the choice of the objective, or the coherence of the source have a strong influence on the accuracy of the estimation. They are often studied experimentally on specific setups. We previously demonstrated the benefit of using statistical signal processing tools as the Cram´er-Rao Lower Bounds to predict best theoretical accuracy reachable for opaque object. This accuracy depends on the image/hologram formation model, the noise model and the signal to noise ratio in the holograms. In a co-design framework, we propose here to investigate the influence of experimental parameters on the estimation of the radius and refractive index of micrometer-sized transparent spherical objects. In this context, we use Lorenz-Mie Theory to simulate spherical object holograms, to compute Cram´er-Rao Lower bounds, and to numerically reconstruct the objects parameters using an inverse problem approach. Then, these theoretical studies are used to challenge our digital holographic microscopy setup and conclude about accuracy, limitations and possible enhancements.

The spread of micro-optical elements fabrication by femtosecond 3D nanolithography is limited due to lack of available optically resilient photopolymers. We have conducted S-on-1 laser-induced damage threshold (LIDT) measurement experiment with two different photopolymers, which demonstrated varying damage probability distribution after which suitable linear approximation technique was not evident. Moreover, different optical damage mechanisms were present during laser irradiation of diverse photopolymers that featured different chemical composition and physical properties. Seeking to determine reliable LIDT evaluation method which would provide trustworthy results for all the investigated materials, we have used three different ISO standard-like approaches. Results comparison for two different cases was conducted concluding that linear approximation of input fluencies range where optical damage probability is 0%<P <100% provides the lowest LIDT value for discussed examples, at the same time showing consistent and suitable for practical applications results.

We have numerically optimized several broadband aperiodic normal-incidence multilayer mirrors based on Sb/B₄C for the 85 – 125 Å spectral domain for spectroscopy applications below the silicon L-edge (λ = 125 Å). Three multilayer mirrors were then synthesized. The designed multilayer structures were optimized for a maximum uniform reflectivity in the 90 – 100, 95 – 105 and 100 – 120 Å wavebands, respectively. All the Sb/B₄C multilayer mirrors were synthesized via magnetron sputtering in argon medium. The multilayers reflection spectra were evaluated with the use of a laboratory XUV spectrograph employing a laser-plasma radiation source, the mirror under study, a transmission diffraction grating and a backside-illuminated CCD matrix. The experimentally recorded spectra were compared with the theoretical ones. Numerical calculations of aperiodic Sb/B₄C multilayer structures with different layer densities are presented. Effects of lower densities and small random variations of the individual layer widths on the reflection spectra are discussed.

The design and construction of a Full-Stokes Imaging polarimeter is presented. The device uses a telescope optical system and a polarization state analyzer (PSA) to obtain polarimetric images on a CCD. The PSA employs two liquid crystal variable retarders (LCVR) and a linear polarizer to measure the four Stokes parameters. The Stokes polarimetry method used in this paper is based on the application of six combinations of retardance values on the LCVRs. A well-known method is used to extract all the Stokes vector parameters from this intensity data. Due to experimental errors, a calibration is necessary. The calibration method used in this paper, also calculates the errors in the experimental set-up by fitting the experimental intensity measurements for the calibration samples to a theoretical polarimeter with errors. In this case, we used incident 45° polarized light to control the output polarization, and six calibration samples. The errors calculated in the method include the axes alignment errors and the errors in the retardance values of both LCVRs. The acquisition of Stokes images used a telescope optical system with a CCD camera.

In this work, we report the achievement of images obtained with radial Walsh filters. Derived from Walsh functions, radial Walsh filters are phase binary diffractive optical elements characterized by a set of equal-area concentric rings that take the phase values 0 or π, corresponding to +1 or -1 transmittance values of the corresponding Walsh function. Then, a radial Walsh filter can be re-interpreted as an aperiodic zone plate with self-similar multi-focusing properties under monochromatic illumination and, therefore, multi-imaging capabilities. We have implemented these unconventional lenses with a spatial light modulator and the first images obtained with this type of lenses are presented and evaluated.

This paper is devoted to investigating the application of different dynamic light structures generated by a self-calibrated Liquid Crystal on Silicon (LCoS) display for microparticle manipulation. Two major studies based on implementing different DOEs, to thoroughly characterize the LCoS display and to achieve optical-inspired particle manipulation, are proposed, respectively. On the one hand, we dynamically introduced two diffractive lens based patterns (the Billet-lens configuration and the micro-lens array pattern) on the LCoS display, from which the self-calibration of the studied device is implemented. In this case, both the phase-voltage relation and the surface profile were determined and optimized to the optimal performance for microparticle manipulation. On the other hand, we performed the optical manipulation of microparticles by addressing configurable three-dimensional light structures obtained from different phase driven split-lens configurations initiated by the same but optimized LCoS display. Experimental results demonstrated that, by addressing certain phase distributions on the LCoS display, the microparticle can be trapped in the light cones and manipulated by providing certain continuous split-lens configurations.

Despite the evolution of technologies, high-quality image acquisition systems design is a complex challenge. Indeed, during the image acquisition process, the recorded image does not fully represent the real visual scene. The recorded information could be partial due to dynamic range limitation and degraded due to distorsions of the acquisition system. Typically, these issues have several origins such as lens blur, or limited resolution of the image sensor. In this paper, we propose a full image enhancement system that includes lens blur correction based on a non-blind deconvolution followed by a spatial resolution enhancement based on a Super-Resolution technique. The lens correction has been software designed whereas the Super-Resolution has been both software and hardware (on an FPGA) implemented. The two processing steps have been validated using well-known image quality metrics, highlighting improvements of the quality of the resulting images.

Uncooled focal plane arrays have revolutionized the market of infrared technology in the last years, reducing the size, weight, power and cost of thermal detectors, and make them accessible to mass markets. This revolution has been lead by micro-bolometers, considered a mature technology for the long wavelength infrared range. However, there is a lack of uncooled detectors in the mid-wavelength infrared (2-5μm) range, one of the most interesting bands for gas identification and temperature measurements of hot objects. Here, we describe the development of a novel gating imaging system for the mid-wavelength infrared. The system is based on unique low cost uncooled PbSe focal plane arrays with a peak response around 3.6 um. The PbSe detectors present some challenges, such as low sensitivity (which limits their application for imaging objects at temperature above 200°C and low signal-to-noise ratio (mainly due to thermal drifts and electronic noise). On the other hand, the photonic detection technology enables high speed acquisition up to 10000 frames per second depending on the resolution. The proposed system combines a modulated aperture, turning the incident light on and off periodically, and a digital lock-in to demodulate the incoming signal and avoid noise in the unwanted region. Contrary to active lighting approaches, the whole scene is modulated at a high frequency and the signal processing is tailored to reduce the internal noise introduced by the detector, while keeping enough bandwidth to achieve video at 30 frames per second. Two imaging prototypes were built: a camera array with six detectors of resolution 32x32, and a camera array with three detectors with increased resolution (128x128) and sensitivity. The proposed technique significantly improves the SNR and reduces the thermal drifts of the PbSe detectors, enabling the imaging of objects at temperatures below 100°C. The camera array was tested in the field during a firefighting training, showing enhanced capabilities to see through fumes, detect fire hotspots and measure temperature robustly.

This paper provides a detailed system theoretical model of a plenoptic camera with the aim to provide in-depth understanding of the plenoptic data recording concept and its effects. Plenoptic cameras, also known as light field cameras, were firstly thought of in the beginning of the 20th century and became recently possible thanks to rapid development of processing hardware and the increase of camera sensor resolution. Despite being a new type of sensor, they are operated in the same way as conventional cameras, but offer several advantages. A plenoptic camera consists of a main lens and a lenslet array (microlens array) right in front of the detector. The microlens array causes not only the recording of the incident location of a light ray on the sensor, as it is done by a conventional camera, but also the incident direction. Such a record can be represented by a 4-D data set known as the light field. In fact, by inserting a microlens array any conventional camera can be transformed into a plenoptic camera. The plenoptic recording concept and the 4-D light field provide multiple advantages over conventional cameras. For example, a single recorded light field allows first, to reconstruct novel views with small changes in viewpoint, second, to create a depth map, and third, to refocus images after the data capture. Hence, the process of focusing is shifted from hardware to software. Last, but not least, plenoptic cameras allow an extended depth of field in comparison to a conventional camera and the use of a bigger camera aperture. Most of the mentioned advantages become particularly effective at close-range to an object. The German Aerospace Center performs research on plenoptic cameras for close-range imaging in space. Possible applications are for example robot vision with plenoptic cameras for robotic arm operations during on-orbit servicing missions or the use of plenoptic cameras on rovers in the course of exploration missions to other planets. Those application scenarios and the demanding conditions in space require thorough comprehension of plenoptic cameras. For this purpose, this paper shall provide a detailed model of plenoptic cameras, which allows to derive camera parameters and optimize them with particular attention to the user requirements and to generate synthetic data. The latter can be utilized to assess the evaluation algorithms, which are not mentioned in detail in this paper. The modeling of the plenoptic camera is mainly based on the theory of geometric optics expanded by elements of diffraction optics.

A Focused Plenoptic Camera in Galilean configuration is studied and its aberrations behavior is interpreted with the Nodal Aberration Theory (NAT). Sequential ray tracing is applied to individual optical channels constituted by the camera objective and individual decentered microlenses. The wavefront aberration field is retrieved at the exit pupil of the optical channels and is analyzed through the Zernike Fringe decomposition technique. Decentered optical channels show nodes in the field-dependence of different Zernike coefficients approximating the wavefront aberration field. The nodal behavior is a consequence of the loss of rotational symmetry in a decentered optical channel due to the displacement of a microlens with respect to the mechanical axis of the camera.

Plenoptic cameras provide single-shot 3D imaging capabilities, based on the acquisition of the Light-Field, which corresponds to a spatial and directional sampling of all the rays of a scene reaching a detector. Specific algorithms applied on raw Light-Field data allow for the reconstruction of an object at different depths of the scene.

Two different plenoptic imaging geometries have been reported, associated with two reconstruction algorithms: the traditional or unfocused plenoptic camera, also known as plenoptic camera 1.0, and the focused plenoptic camera, also called plenoptic camera 2.0. Both systems use the same optical elements, but placed at different locations: a main lens, a microlens array and a detector. These plenoptic systems have been presented as independent. Here we show the continuity between them, by simply moving the position of an object. We also compare the two reconstruction methods. We theoretically show that the two algorithms are intrinsically based on the same principle and could be applied to any Light-Field data. However, the resulting images resolution and quality depend on the chosen algorithm.

In this report, we discuss the interest of quality metrics for imaging and image processing of multi-views in light sheet fluorescent 3D microscopy. Various metrics of focus are tested on real and simulated data so as to automatically assess the informational quality of the images. Application of such metrics are given for several information tasks including online control of acquisition, fast registration or image fusion. Illustrations are given for typical samples of interest for in vivo imaging with light sheet microscopy such as spheroids or organoids. We point to the reader softwares freely available under FIJI which enable to test the computation of a basic quality metric, for registration and fusion.

Multiphoton microscopy (MPM) is a relatively recent tool involved in biological imaging. Its resolution is somewhat limited due to its physical principles, resulting experimentally in a deteriorated planar resolution of about few tenths of micrometers and a reduced axial resolution of a few micrometers. In this communication, we present a numerical approach taking the form of a processing pipeline for image restoration going from the raw images of 0.2 μm diameter fluorescent microbeads to the reconstructed ones. The strategy consists in estimating 3D Point-Spread Function (PSF) shapes in automatically selected volumes of interest, considering a 3D Gaussian profile of intensity. An automatic crop procedure selects areas of a few dozens of individual beads. Our algorithms, based on a variational approach for multivariate Gaussian fitting, allows us to identify evolutions of PSF dimensions along the 3 dimensions of the volume. A proximal alternating optimization method called FIGARO is employed to minimize a cost function serving to quantify the optimality of the process. The difficulty overcome thanks to this novel variational approach for multivariate Gaussian fitting lies in its ability to estimate parametrically Gaussian shapes in a 3D space, which leads to accurate models of the PSF. Thanks to our technique, the PSF measurements show a dimension in (X,Y,Z) of (0.21, 0.27, 1.49) μm. It is the first time to our knowledge that such a direct 3D measurement has been made possible. This process paves the way to a significant improvement of the resolution, perfectly suited to MPM.

In this contribution, intelligent identification of glaucoma from digital fundus images using stacked sparse autoencoder (SSAE) is proposed. The fundus images are initially converted to gray-scale and normalized w.r.t., background illuminance while maintaining contrast constancy across the dataset. Unfolded feature vectors from the pre-processed with proper rescaling and grays-scale converted fundus images are fed to SSAE for learning efficient feature representation and classification thereof using a softmax layer. A comparative evaluation highlighting the superiority of SSAE method with existing state-of the art techniques is presented to validate its efficacy in glaucoma detection. The proposed framework can be used as a clinical decision support system assisting ophthalmologists in confirming their diagnosis with high reliability and accuracy.

A variational mode decomposition (VMD) and local binary patterns (LBP) based features extraction from digital fundus images is proposed for glaucoma detection. The band-limited intrinsic mode images (BLIM’s) obtained by VMD, encompasses the varying spectral content embodying the non-linear and spatial non-stationary textural modulations in the fundus images. LBP feature descriptors apprehend the topographic tortuousness of the optical tissue fluids and substantiate the perturbations in intraocular fluid pressure (IOP) within the human eye which is caused due to glitches in the optical drainage system. Using artificial neural network, a classification accuracy of 95.2% is obtained on publicly available Medical Image Analysis Group (MIAG) dataset, which validates the suitability of the proposed framework in glaucoma identification.

The development of express method for assessing the state of skin graft by the spectroscopic properties of tissue components involved in the healing of the affected skin or healing of skin grafts was carried out in present work. The proposed method for assessing the state of the skin by the spectroscopic properties of tissue components (using photosensitizers, fluorescent dyes (methylene blue and IcG) and nanophotosensitizers aluminum phthalocyanine nanoparticles (NP-AlPc) applied locally) will evaluate the physiological condition of the skin and assess the degree and rate of engraftment or rejection while also controlling several biochemical and physiological parameters in the entire graft, or the whole area of the skin lesions. Such parameters include the oxygenation of hemoglobin in the tissue microvasculature; the blood supply level; blood flow and lymph flow; assessment of intracellular metabolism; assessment of the cellular respiration type (aerobic/anaerobic).To assess the extent of inflammation the spectrally sensitive to biological environment nanoparticles of aluminum phthalocyanine (NP-AlPc) were also used.

The surveillance industry has traditionally focused on the use of colour intensity images and then used computer vision methods to extract information. Deep learning methods have been demonstrated successfully but require significant computational resources. Fog and rain still present a problem to these methods. Other non-optical imaging technologies are available but the applications can be cost sensitive. Polarimetric cameras offer a solution to some of these problems. This paper presents a practical and low cost design that uses between two and four HD cameras with a wide field of view. This system has an automatic calibration stage that ensures the video frames are synchronised in time. To produce the Stoke parameters each pixel from one camera must be mapped to the others. To perform this, a homography matrix for each camera is automatically discovered and maps each video stream into the correct spatial coordinates. This attempts to use SIFT keypoint mapping but since each input image is a different polarisation state there are potentially a low number of keypoints so an additional check stage is introduced. Calibration results are presented along with example images, post process methods and feature extraction results.

In this paper, the applications of polarimetric imaging for rust preventing oil film detection and characterization are discussed. A three-channel polarimetric imaging system is introduced, which can obtained the degree of linear polarization images at one shoot. The experimental results show that the proposed three-channel polarimetric imaging system can identify the oil film on the steel strip quickly and effectively, which is a fast and reliable detection method.

Multi-band polarization imaging, by mean of analyzing spectral and polarimetric data simultaneously, is a good way to improve the quantity and quality of information recovered from a scene. Therefore, it can enhance computer vision algorithms as it permits to recover more statistical information about a surface than color imaging. This work presents a database of polarimetric and multispectral images that combine visible and near-infrared (NIR) information. An experimental setup is built around a dual-sensor camera. Multispectral images are reconstructed from the dual-RGB method. The polarimetric feature is achieved using rotating linear polarization filters in front of the camera at four different angles (0, 45, 90 and 135 degrees). The resulting imaging system outputs 6 spectral/polarimetric channels. We demonstrate 10 different scenes composed of several materials like color checker, high reflecting metallic object, plastic, painting, liquid, fabric and food. Our database of images is provided online as supplementary material for further simulation and data analysis. This work also discusses several issues about the multi-band imaging technique described.

We propose to simulate holograms with the vectorial Rayleigh-Sommerfeld diffraction integral. This new approach is compared with a classical Fresnel diffraction integral, and that is done through the comparison of the kernels. We define the limit where the Fresnel approximation fails to simulate holograms. We show that the vectorial Rayleigh-Sommerfeld diffraction integral is well adapted to simulate holograms in extremely near field conditions. This is particularly interesting when dealing with very small particles.

This paper presents the proof of principle of high-speed holographic measurements applied to solve an inverse problem in the domain of vibroacoustics. In order to get a robust and efficient set-up, a compact holographic interferometer which includes a Fresnel configuration equipped with a negative zoom for large surfaces and a diffractive optical element to improve the photometric efficiency of the set-up was developed. The vibration measurements from the digital holographic set-up are applied to the “RIFF” method which provides identification of the force distribution at the surface of the vibrating object, by solving a regularized inverse problem. Experimental results demonstrate the advantage provided by full-field measurements from multi-point holographic vibrometer with fine spatial and temporal resolutions.

A technique for data acquisition from digital holograms of particle ensembles, including preprocessing of the digital hologram, construction of a two-dimensional display of the holographic image of investigated volume, and segmentation and measurement of particle characteristics is considered. The proposed technique is realized in automatic regime and can work in real time. Results of the technique approbation using digital holograms of sand, plankton particles in water, and air bubbles in oil are presented.

Digital holographic microscopy (DHM) is a quantitative phase imaging (QPI) modality, which retrieves 3D object phase information. The quantification of subcellular features within the object is possible. Its single-shot hologram recording feature makes it suitable for real-time imaging applications. This paper discusses QPI capability of LED-based digital inline holographic microscopy (LDHM), which has gained much attention for its portability, cost-effective features. However, the twin image artifact is present in the inline setup. Though several twin image reduction and elimination methods are developed, the exact phase quantification is always a challenge. Original phase information may be lost after elimination of twin image. There is always a trade-off between twin-image elimination and QPI of inline microscopy setup. This paper discusses the QPI capability of LDHM in comparison with the conventional off-axis DHM. Further, the results of phase objects using both the methods are studied.

The article presents a two-stage scheme for obtaining volume color security holographic stereograms. These holograms are digital holograms. The H1 - hologram is recorded at the first stage. The final holographic stereogram is recorded at the second stage. The image of the H1-hologram is reconstructed in the plane of pupils of the observer's eyes, when 3-D images are reconstructed from holographic stereograms. The quality of the 3-D images reconstructed from hologram stereograms directly depends on the degree of blurring of the H1 image - the hologram. The article shows mathematical calculations describing the effects of the spectral and angular selectivities of a three-dimensional color security holographic stereogram on the process of reconstructing 3-D images (the degree of blurring of the H1 image-hologram). It is shown, that the quality of security elements, such as the "flip-flop effect", for this type of hologram, has a more pronounced effect in the vertical plane than in the horizontal plane. This is due to the large influence of the spectral and angular selectivities in the vertical plane, than in the horizontal one when reconstructing the images. Photos of images, reconstructed from a three-dimensional color security holographic stereogram, are also presented in the article. These photos confirm the correctness of the presented calculations.

The work developed and presented in this communication, relates to the restitution of frequency chirp of an interferometric signal deduced from a measured diffraction pattern relating to a spherical micro-particle. For this purpose, analysis were achieved by implementing a parametric method with a sliding window. These frequencies allows us to reconstruct the axial position of the corresponding object. The study, achieved in the far field approximation, allows us to validate preceding methods based on simulation results. The principle consists to generate optically in-line diffraction patterns of a spherical particle with radii of 39μm and measured with a microscope ZEISS. The collimated coherent light was generated from a He-Ne laser that the wavelength is λ = 632.8 nm. The generated diffraction pattern was recorded by using a 2D-CCD camera Ophir having 1024 x768 pixels with a pitch of 4.65 μm connected to a computer. Since the variation of the chirp frequency is linear, the knowledge of its variation slop, resulting from a linear fit, enables us to deduce the z-position of the particle. This is achieved with a resolution of 1.2 %.

Recently, a method for synthesizing a hologram of three dimensional (3D) objects from captured light field array is demonstrated. The 3D objects can be captured under incoherent light illumination using a micro lens array and their orthographic projection view images are generated from the captured elemental images. The synthesized orthographic projection view images are then multiplied by the corresponding phase functions and combined to form a digital hologram. For the first time, we analysis the performance of synthesized hologram under photon counting (low light imaging) conditions. The feasibility of this technique is experimentally verified by recording the orthographic projection images using a micro lens array and the reconstructed photon counted hologram is presented with varying photoncounting measurements.

Partial coherence is used in a plurality of applications, magnifying microscopic imaging, interferometric measurement, lithographic imaging, CGH based wave front shaping, interference lithography and space-bandwidth-limited wave front reconstruction, just to name a few. In some applications the primary light source is characterized by a limited coherence length and an extended angular spectrum of plane waves, which has to be narrowed, e.g. if an Excimer laser is used. Sometimes the angular spectrum of plane waves of the primary light source has to be increased in order to be practical. There are several possibilities in general, the primary light source can be used directly, the system has to be adapted or the coherence function Γ has to be tailored in order to provide the specific requirements. Almost all embodiments come with little changes of the light sources coherence properties only. For example, to use a spectral bandpass filter or to limit the size of the light source seem to be the standard solution for almost everything.

However, more advanced tailoring of the complex valued coherence function Γ leads to an increased image quality, e.g. in interferometers, but is not limited to this, reduces background noise, decouples Fizeau cavities or it enables complete new illumination and imaging system designs, which provide unique features. This aspect will be discussed herein. Furthermore, the propagation of the complex coherence will be taken into account. This is done in order to provide defined conditions in defined planes of imaging devices. In other words, the usage of the Wiener-Khintchin theorem and the van Cittert-Zernike theorem is just a part of the system analysis and system optimization, which has to be done. Although generic approaches are used, discrete light source layouts are strongly related to the discrete optical devices, which make use of them.

The specific tailoring of the complex coherence function, which is related to the space-bandwidth-limited reconstruction of wave front segments, which also can be referred to as space-bandwidth-limited CGH reconstruction, will be described in more detail. For this type of real time dynamic imaging two major problems - among several others - have to be solved. One problem is the huge computation power and the other one is the coherent retinal cross talk of adjacent image points, which are reconstructed in the image volume. The disclosed layouts of tailored secondary light sources are based on the Wiener-Khintchin theorem and the van Cittert-Zernike theorem. Both problems, which are mentioned above, can be solved. Tailored complex valued light sources reduce the required computation power by enabling reduced coherent overlay of sub-CGH areas. Furthermore, they reduce the coherent retinal cross talk of dynamic real time spacebandwidth- limited CGH reconstruction, which is used in advanced imaging applications, too. This results in an increased image quality of partial coherent wave field reconstruction based imaging.

Terahertz spectroscopy and imaging have proved to be a versatile tool for the studying of drugs and pharmaceutical analysis. Particularly, this method has been employed to map the coating layer of pharmaceutical tablets, to identify the polymorphism of active pharmaceutical ingredients (APIs), etc. In this paper, Terahertz (THz) hyper-spectral imaging using time-domain spectroscopy technique has been applied to image a commercial packaging of paracetamol tablets, showing the ability to penetrate through both cardboard of the box and plastic cover of the blister pack. The advantage of THz time-domain spectroscopy (THz-TDS) is that not only the dimensional imaging was captured but also the spectral information was registered. A comparison between THz spectrum of commercial and lab-prepared paracetamol tablets shows the presence of a certain amount of α-D-lactose monohydrate at the absorption peak of 0.53 THz. Any API or excipients can be detected if their fingerprints fall in the measureable range of the system. This work opens up a possibility of using THz-TDS to non-destructively control the quality of pharmaceutical products while they are still in their packaging. This potential capacity is important as for the identification of the APIs as well as changing of unwanted physical (and hence biological) properties during the shelf-life of the product.

In this proceeding, we discuss the method that allows for field of view and reconstruction quality enhancement of pulsed THz holograms recorded by matrix detectors that do not exceed the the object transverse dimensions, at distances, that are comparable with the object size. The method comprises the use of random phase mask situated between the object and the hologram, at the hologram registration process. The introduced phase variation levels out the input from closer and further (to the hologram pixel) points of the object, and thus improves overall reconstruction quality. Here, we study numerically this approach and demonstrate 4 times increase of the properly reconstructed object area, if compared to the undisturbed hologram recording, and consecutive increase of the correlation between the reconstructed and actual object from 0.34 to 0.82.

In molecular spectroscopy, imaging of supersonically expanded and jet-cooled molecular pulses to evaluate their time of flight and velocity is useful in obtaining photoionization spectra of higher resolution, well-resolved, and enhanced signal to noise ratio. In this paper, the ultraviolet output of frequency doubled OPO laser at 266 nm was employed to obtain highly-resolved (2+1) resonance-enhanced multiphoton ionization (REMPI) spectra for cooled molecular pulses of methyl iodide (CH₃I) sample seeded in helium gas. The recorded photoionization spectra were manipulated to study the shape, duration and structure of the jet cooled molecular beam pulses. A two meter time- of- flight (TOF) mass spectrometer was employed to identify and record the ions produced while varying the time delay between molecular pulses and laser shots. Imaging of CH₃I molecular pulses yielded lorentzian distributions of full width at base and flight time peak location corresponding to pulses duration of ~ 0.3 milliseconds and 931 m/s cooled molecules translational velocity, respectively.

A 5-ALA-induced fluorescence-based imaging device for guidance during surgery of malignant and non-malignant preliminary photosensitized tumors is presented. The setup fits existing clinical optical rigid and flexible endoscopes and operation microscopes. It consists of three light sources including white light, red light fluorescence excitation and blue light fluorescence excitation sources. The light from any combination of the latter sources is delivered to tissue using specially designed fiber optic light guide. Two cameras are used to acquire fluorescence and back reflected white light images: a gray-level camera for fluorescence in the far red range and a color camera for white light images. A dichroic mirror is implemented to spectrally split the light coming from tissue. Images from both cameras are processed into a computer with specially developed software where it can be displayed in different modes including overlaying or been used for image mosaicing which allows for increasing the intrinsic reduced field of view of endoscopes by providing highly resolved extended cartography. Experiments were carried out on phantoms and on patients in clinical conditions during surgery of brain and other tissues. Blue light excitation was more sensitive for thin tumors but red light excitation was more beneficial for solid tumors and for navigation in presence of slight bleeding.

Hyperspectral imaging is an optical technique that recently started being used in medical field. The correct extraction of spectral and spatial information from hyperspectral images depends on preprocessing, processing and analysis methods applied for an accurate diagnosis and monitoring medical treatments. A fundamental task in preprocessing hyperspectral images is the elimination of various types of noise generated by the hyperspectral systems. One of the major causes for the noise in a hyperspectral system is dark current noise. This type of noise arises from the temperature difference between environment and charge-coupled device of the hyperspectral camera. Electrons are generated over time and they are independent of the light falling on the detector. These electrons are captured by the potential wells of the charge-coupled device and counted as signal. The dark current noise removal can lead to an improvement in the performance of classification, target detection, anomaly detection and mapping methods, thus contributing to a better and more accurate diagnosis. Two denoising techniques - principal component analysis and minimum noise fractions were used until now in medical hyperspectral imaging applications. In this paper, the wavelet transform was proposed as a denoising technique for medical applications. The study was performed in both laboratory and clinical conditions. Two hyperspectral systems were used for the hyperspectral images acquisition of rabbit liver and a burn wound located on the posterior side of the patient left leg respectively using the same pushbroom hyperspectral camera but with two different scanning components (translation table and scanning mirror). The pushbroom hyperspectral camera acquires the image collecting the x-axis and λ information completely at the same time for a line on the y-axis. The two scanning components are used to move the sample (liver or patient leg) across the field of view of the hyperspectral camera so that the images are acquired line by line. The experimental results showed that the proposed denoising technique achieves better performance when applied to hyperspectral images acquired under laboratory conditions than in clinical situations. In conclusion, the wavelet transform could be considered a successful approach to denoising in laboratory hyperspectral measurements.

Accurate diagnosis of burns, mainly in terms of depth and healing potential, has still remained an unsolved clinical problem. Hyperspectral imaging, with its unique capabilities to simultaneously provide both spatial and spectral information, can be considered as a particularly useful tool in early diagnosis of burns by providing accurate and valuable information about injured biological tissues. In this study, the potential of hyperspectral imaging to generate burn characteristics maps was evaluated. Two supervised classification methods (spectral angle mapper and support vector machine) of the hyperspectral data were investigated and their classification accuracy was compared. The study was performed on a 24 hours old burn of the hand (superficial-partial and deep-partial thickness burn wound). A pushbroom hyperspectral imaging system was used to acquire the hyperspectral image of the burn wound within the wavelength range from 400 nm to 800 nm. The hyperspectral image was calibrated with respect to the white and dark reference images in order to minimize the influences of light intensity variations across the spatial scanning lines and the dark current in the hyperspectral system. Minimum noise fraction transform was used to determine the inherent dimensionality of hyperspectral data and to separate the information from noise before the calibrated hyperspectral image being analyzed using the spectral angle mapper and support vector machine classifiers. The accuracy of these two classification methods in mapping the skin burn characteristics was evaluated based on the classification accuracy assessment of the resulted skin burns characteristic maps. The results revealed that the overall classification accuracy of support vector machine classifier exceeded (overall accuracy = 91.94 % and Kappa coefficient = 0.902) that of the spectral angle mapper classifier (overall accuracy = 84.13 % and Kappa coefficient = 0.808). In conclusion, these preliminary data suggest that hyperspectral imaging combined with support vector machine classifier could play an important role in burn characterization and mapping.

Passive hyperspectral imaging (HSI) sensors are essential in many space-borne surveillance missions because rich spectral information can improve the ability to analyze and classify oceanic and terrestrial parameters and objects/areas of interest. A significant technical challenge is that the amount of raw data acquired by these sensors will begin to exceed the data transmission bandwidths between the spacecraft and the ground station using classical approaches such as imaging onto a detector array. In this paper, the Compressive Line Sensing (CLS) imaging concept, originally developed for energy-efficient active laser imaging, is extended to the implementation of a hyperspectral imaging sensor. CLS HSI imaging is achieved using a digital micromirror device (DMD) spatial light modulator. A DMD generates a series of 2D binary sensing patterns from a codebook that can be used to encode cross-track spatial-spectral slices in a push-broom type imaging device. A high sensitivity single-element detector can then be used to acquire the target reflections from the DMD as the encoder output. The target image can be reconstructed using the encoder output and the encoding codebook. The proposed system architecture is presented. The initial simulation and experimental results comparing the proposed design with the state-of-the-art are discussed.

The possibilities of forming optical images on horizontal extended atmospheric paths are explored. Two different methods for improving image quality are analyzed. It is known that at the present time the solution of the problem of long-range vision with super-high resolution is conducted along several independent lines, firstly, on the development of methods based on the classical technique of adaptive optics, i.e., by correcting the distorted wavefront itself, and, secondly, on the use of digital post-detection techniques, as well as, on the way to attract purely engineering solutions. These methods are applied, both for terrestrial systems, and for astronomical instruments. They can be instrumental (for example, adaptive correction, the use of polarization filters, receiver gating, etc.), mixed (adaptive correction and subsequent processing of images on computers or special processors) or program-algorithmic only. The analysis is carried out for systems operating on horizontal paths. This, first of all, is due to the fact that any horizontal path by the strength of turbulence far exceeds any astronomical one. On extended horizontal paths, in addition to phase distortions that cause the effects of jitter and blurring of the image, there are fluctuations in the intensity of the received radiation, which leads to the appearance of flickering effects of the image, as well as to the manifestation of ambiguity in describing the phase distortions of the optical wave. Numerical and analytical calculations are performed. Experiments were carried out on the atmospheric paths from 160 m to 3.2 km long in city conditions.