This PDF file contains the front matter associated with SPIE Proceedings Volume 11490, including the Title Page, Copyright information and Table of Contents.

Welcome to Interferometry XX. We thank everyone who helped make this conference happen: especially our 30-member program committee representing 14 countries, all our authors and presenters, and the SPIE staff. Our video intro highlights the winners of the 2018 Fringe Art Competition. Please submit your entries to our virtual “2020 Fringe Art Competition” via email to fringeseverywhere@gmail.com by midnight, Thursday August 27, 2020 (Pacific time - PDT). Ballots will be emailed to authors, entrants and those requesting to be on the list with instructions. Please send an email if you want to be added to the list. May you continue to see fringes everywhere.

The use of the Principal Components Analysis (PCA) for recovering the modulating phase, given a sequence of Phase-Shifted Interferograms (PSI), is a very important contribution to the field. However, its verbatim translation from statistics to PSI has limited the view to consider only constant background illuminations. Here, we show that the Singular Value Decomposition (SVD), used in PCA, actually separates the background illumination (constant or not) and the phase modulation terms. We show that the modulating phase can be correctly recovered if the phase-shifts sample full periods uniformly, independently of the spatial distribution of the number of fringes.

In this work, we introduce the windowed generalized phase-shifting algorithms (WG-PSAs) using static and dynamic weighting functions/widows. These algorithms are derived from a weighted least square fitted to the monochromatic temporal fringe, thereby, the selection of the window plays an important role due to the fact that it shall reduce the influence of those intensities jeopardizing the phase estimation. In order to make the best selection, we propose to employ an adaptive/dynamic window which has the ability to detect the fringe patterns that jeopardize the phase retrieval. This window is computed iteratively by analyzing the error between the measured and fitted intensities. Furthermore, we provide the analysis of our scheme using the frequency transfer function (FTF) formalism for phaseshifting algorithms. Finally, we executed numerical experiments with synthetic data in which we compare the performance of the dynamic window versus several static ones from the state-of-the-art; although our scheme is more computationally expensive due to the iterative procedure, it works better than the traditional generalized PSAs with a window included or not.

In this work, we deduce explicit conditions to design phase shifting algorithms (PSAs) in order to suppress errors in phase measurement, the double-frequency ripple distortion and the spurious piston. These errors arise when non-uniform phase-shifting interferograms are processed with conventional PSAs which assume uniform phase shifts. By modeling the non-uniform phase shifts as a polynomial of the unperturbed phase shift value !0, we show that the conditions for eliminating those errors are associated to the m-th derivative of the PSA's frequency transfer function (FTF). Thus, we propose an approach to design robust algorithms based on the FTF formalism, and we present a ready-to-apply eight-frame PSA. Finally, our conclusions are supported by computer simulations.

For three decades, my work in optics has encompassed a wide variety of research projects and commercial products that all relied on the fringes produced from optical interference to measure different physical parameters. This paper describes some of the most unique experiments and novel commercial products including: measurement of air velocity in wind tunnels, residual stress in metals, space telescope performance in vacuum chambers and surface shape of turbine blades with a hand-held instrument. Some of these ideas achieved commercial success; all were collaborations with other scientists, including the late Chandra Vikram.

In structured-light systems, the lens distortions of the camera and the projector reduce the measurement accuracy when calibrated as a standard stereo-vision system. The conventional compensation via distortion coefficients reduces the error, but still leaves a significant residual. Recently, we proposed a hybrid calibration procedure that leverages the standard calibration approach to improve measurement accuracy. This hybrid procedure consisted of building a pixel-wise phase-to-coordinate mapping based on adjusted 3D data obtained from the standard stereo-vision method. Here, we show experimentally that the measurement accuracy can be significantly improved, even using the linear pinhole model and linear mapping functions. We then move to consider the nonlinear model to improve the measurement accuracy further. Encouraging results show that this new calibration method increases the measurement accuracy without requiring elaborate calibration procedures or sophisticated ancillary equipment.

This work presents a stable noise-robust numerical integration technique derived from a gradient representation of the Q-Forbes polynomials for surfaces with axial symmetry. This modal-integration technique uses an orthogonalization process through the Householder reflections to obtain a numerically orthogonal set for the surface slopes that is used to reconstruct the surface shape. It is shown that for typical Deflectometry measurements, the resulting random component of the uncertainty after numerical integration has a root mean square error well below 1nm.

Directed energy systems developed to reside on airborne platforms can experience a multitude of sources of degradation that reduce optical system performance. Three major sources of these degradations, or aberrations, can be attributed to platform jitter, atmospheric turbulence, and aero-optical disturbances. Aero-optics is the term used to describe the aberrations induced from the aero-dynamical environment surrounding an aircraft as it travels through the air at high speeds and the resulting varying air pressures induced. The Air Force Research Laboratory (AFRL) Laser Division has recently brought online the Aero-Effects Laboratory (AEL) capable of performing various aerooptical experimental tests. The AFRL AEL currently has a supersonic wind tunnel with a test section that allows large optical access for various article testing and system simulations. In this document, the current status and performance of the AEL Optical Metrology System and some initial results from experiments currently in development will be presented. Data acquisition systems include the use of pressure sensors, pitot probes, Schlieren imaging, and wavefront sensors. In this manuscript, the AEL Optical Metrology System has been redesigned to include off-axis parabolic mirrors to significantly increase the optical quality of the system.

The unsupervised variational image decomposition (uVID) algorithm developed in our group allows for automatic, accurate and robust preprocessing of diverse fringe patterns. Classical VID was initially used for image denoising. Its tailoring for fringe pattern preprocessing was justified by clear advantage over other methods (e.g. Wiener or Gauss filters) in maintaining sharp edges and details of the image. Historically first fringe pattern dedicated three-component variational image decomposition model assumed the use of the shearlet algorithm to separate the information component (fringes) and noise and the Chambolle projection algorithm to separate the fringes and background. We noticed that this model is computationally complicated and the result strongly depends on the values of the algorithm’s internal parameters, to be set manually. The uVID automatically introduces the parameters and stopping criterion for Chambolle’s iterative projection algorithm. Nevertheless, determining the stopping criterion in each iteration is a severely time-consuming process, which is particularly important given the fact that in many cases thousands of iterations have to be calculated in order to obtain a satisfactory fringe pattern decomposition result. Therefore, the idea of using machine learning algorithms to classify fringe patterns according to the required number of Chambolle projection iterations has emerged. Thus, it is no longer required to determine the value of the stopping criterion in every iteration, but only in the area of the predetermined number of iterations. We showed that the calculation time is reduced on average by half by employing the machine-learning based acceleration. This way we made a progress in developing uVID algorithm features for real-time studies of dynamic phenomena, i.e., biological cell development investigated by fringe-based bio-interferometry methods.

In an airborne directed energy system, air density variations around an aircraft distort a beam’s wavefront, resulting in degraded performance after propagation. Adaptive Optics (AO) can be used to correct for these aero-optical aberrations in the wavefront to give increased performance. However, in some conditions state-of-the-art AO systems have insufficient spatial and temporal bandwidth to accurately maintain high system performance. Predictive AO control is a promising method to mitigate these problems and therefore improve beam quality. This manuscript builds on previous research with novel Digital Holographic (DH) Wavefront Sensor (WFS) data measuring aero-optical disturbances from the wind tunnel at the Air Force Research Laboratory (AFRL) Aero-Effects Laboratory (AEL) to present a Dynamic Mode Decomposition (DMD) based predictive modeling tool. DMD is a light-weight, equationfree, dimensionality reduction algorithm that can be used to isolate spatio-temporal patterns in a data set into physically meaningful modes. These can be used to predict future states from a given wavefront. This manuscript demonstrates application of this method to data acquired through AFRL as well as discussing the underlying mechanisms.

Phase-measuring deflectometry is an optical inspection technique for reflective surfaces. It enables absolute, quantitative surface measurements, given a calibrated measurement setup. Two general calibration approaches can be found in literature: First, the stepwise approach uses a calibration pattern and determines internal camera parameters and external geometrical parameters in separate, consecutive steps. Second, the holistic approach optimizes all parameters collectively, based on deflectometric measurements of a calibration mirror. Whereas both approaches have been compared regarding the accuracy of subsequent surface measurements, the present contribution focuses on experimental examination of their reproducibility. In experiment E1, we assess the parameter variability by repeating both calibration procedures ten times. In an additional experiment E2, we repeat all calibration measurements related to a mirror/pattern position ten times in a row before rearranging the mirror/pattern, in order to examine the purely noise-related parameter variability. Finally, we calculate the coordinate variability of a set of world points projected onto the image planes of the calibrated cameras. The measured variability is consistently higher in E1 than in E2 (average ratio: 3.2). Unexpectedly, in both experiments, the external parameter variability also turns out to be higher for the holistic approach compared to stepwise calibration (average ratio: 2.3). This is of importance, since the holistic approach is known from literature to be more accurate than the stepwise approach, regarding their respective application to surface measurements. The image coordinate variability is comparable for both calibration approaches with an average of 0.84 and 0.21 camera pixels for E1 and E2, respectively.

The calibration of optical surface topography measuring instruments ensures the comparability of the resulting surface topographies. Such comparability is also desirable for self-built optical microscopes in order to optimize these instruments with regard to their systematic and stochastic measurement uncertainty. However, there is not yet a fixed set of standards in the field of interference microscopy as in the field of stylus instruments that describes methods for their calibration, verification, adjustment and performance specification [1] [2]. The associated standard for optical measuring instruments is still in preparation in the corresponding ISO working group. There is the guideline according to VDI/VDE 2655 Sheet 1.1, for the calibration of interference microscopes for surface texture measurement. This guideline is essentially a transfer of methods that origin from the calibration of stylus instruments [1]. Thus, it describes a profile-like calibration and does specifically consider areal properties of interference microscopes. The field of areal calibration is still work in progress in the international standardization. The ISO 25178-600 that was recently published defines the basic metrological characteristics of areal surface topography measuring instruments [3]. In our study, we describe the optimization and performance specification of a self-built white light interference microscope based on methods that were developed for instrument calibration. For the performance specification we use the metrological characteristics of ISO 25178-600 that are imaged with the help of a previously developed Universal Calibration Artefact for areal calibration [4]. The results of the metrological characteristics are compared with a commercially available white light interferometer (WLI). In order to improve the self-built interference microscope, the measurements are repeated with different configurations and a final setup is chosen that allows a specification of the performance of the self-built interference microscope for measuring areal surface texture.

Fringe projection (FP) is one of the most widely used method to precisely measure the 3D geometry of complicated surfaces. However, when the object is in motion, it is difficult to estimate the motion or displacement that happens between the frames. The most commonly used method for estimating the in-plane displacement is Digital Image Correlation (DIC). It has been widely used in the area of experimental mechanics. As the traditional DIC method uses a single camera, it is difficult to recover the 3D shape of the object. In this paper, we propose a novel method called digital height correlation method for measuring surface profile deformation. The height data obtained from FP can be used to estimate the deformation. The proposed method involves measuring the 3D profile of an object, which has rich surface topographic variations using a high-accuracy fringe projection method. Then, the 3D profiles obtained from the before and after displacement state are correlated to extract the displacement map. Specifically, the out-of-plane displacement is extracted by applying the correlation technique to the height map obtained from the 3D profile of the object. The accuracy and efficacy of the digital height correlation method are validated using rigid-body translation tests and tensile tests of a rubber specimen. Experiments conducted will demonstrate the success of the proposed method.

Surface form metrology for freeform optical components is a significant challenge that impacts the entire process chain. One of the main aims is that the metrology technique provides sufficiently low measurement uncertainty within a reasonable measurement time. We developed a non-contact, non-part-specific cascade optical coherence tomography (COCT) technique within this metrology context. C-OCT utilizes a primary and a secondary interferometer in a cascade that transforms a difficult-to-measure optical path difference (OPD), which encodes the freeform sag in this case, into a more readily measured OPD in the secondary interferometer. To enable high-speed measurements, we developed the secondary interferometer based on a rotating optical cube. A custom telecentric objective lens and a custom pupil relay enable optical scanning over the sample surface. Experimental results validate the C-OCT technique with consecutive single-point measurements showing precision of ±26 nm (~λ/24 at the He-Ne wavelength) and a preliminary flat surface measurement demonstrating 22 nm RMS (~λ/28) over a central 20 mm diameter region. Developments on the system are underway towards surface measurements on freeform optical components.

This paper presents two autofocusing methods for 3D shape measurement. We tried two systems: 1) for a single camera and dual projector system, we only used two projectors for 3D reconstruction, and thus camera calibration is not necessary; 2) for a single camera and single projector system. Because the projector depth of field is large for high-quality fringe pattern generation for 3D shape measurement, we attached an electrically tunable lens (ETL) to the camera. For the first system, we determined the focal plane from the rough measurement with an initial input signal to ETL, and 3D reconstruction was realized through the matching points on two projectors. For the second system, we modeled camera parameters as continuous function of the input signal to ETL that is pre-calibrated; and implemented a traditional passive autofocusing algorithm. Experimental results demonstrated the success for both methods to potentially achieve autofocusing capability for 3D shape measurement systems.

New instrumentation and bioimaging devices are needed for viewing, understanding, and modeling key metabolic processes in living organisms such as within and among microbial cells and microbial communities in terrestrial environments, and/or multicellular plant tissues. Unique features of digital holography can be exploited to produce a new type of microscope that will be useful to biologists in studying, understanding. and modeling dynamic cellular activity in living biological systems. Digital holographic microscopy enables performing the functions of large expensive optics and precision translating devices with software operations that include machine learning and artificial intelligence to extract information more efficiently. The paper describes a project to develop an affordable, low footprint digital holographic microscope than can optically freeze a 4D space and enable microscopic study of dynamic events in a large volume at many instants of time. Thanks to recent advances in electronic imaging systems, computers, cameras, and memory, DHM holograms can now be recorded on small, lightweight, CCD or CMOS arrays at very high rates, processed in real time, and offer the many advantages associated with digital media. By transferring much of the overall optical process into electronics and software, DHM stores all optical information, transmitted and reflected from a biological volume of interest, in a hologram, enabling dynamic, microscopic imaging of the entire volume with a relatively small instrument, a capability that cannot be achieved with conventional imaging devices. The paper summarizes ongoing experiments and possibilities to analyze the hologram, directly, with machine learning techniques, avoiding the computationally costly step of reconstructing images.

Moiré fringes may be encountered in digital holography when the local fringe period is comparable to the pixel separation of the recording CCD array. In this work, we demonstrate the relation between the transverse misalignment of a lens used in an inline digital holographic system using CW illumination with the position of Moiré pattern on the recording CCD plane. A test object such as a one-dimensional opaque block is used along with a single lens imaging system, and the CCD plane is assumed to be in the far-field of the image plane. Our preliminary results show that small transverse misalignments can be tracked by monitoring the location of the Moiré pattern. It is proposed that Moiré patterns can be used to detect and correct optical misalignments in more complex optical systems.

Shearography is an optical measurement technique based on the interference between two laterally displaced images of the region of interest. Due to its quasi-equal-path configuration, shearography is robust enough for applications outside the laboratory. In fact, shearography has been successfully used for nondestructive testing in composite materials mainly in the aerospace, boating and, more recently, in the oil and gas industries. There are several demands in the oil and gas industry for nondestructive testing on subsea metallic elements coated with composite materials to prevent or mitigate the effects of corrosion. The present work deals with the underwater shearography for nondestructive testing in metal parts coated with composite materials. The paper begins with considerations about the propagation of light in underwater environments and a discussion on the elements that disturb the measurements with shearography such as changes in wavelength, absorption, dispersion, distortions caused by refraction in the air-glass-water interfaces among others. The different loading possibilities to reveal defects in the tested elements are analyzed: by pressure loading, thermal loading by radiation and thermal loading by induction. Exploratory experiments carried out inside water tanks involving the three loading techniques are presented and discussed. The results are compared and the combinations that led to the best results are emphasized. Finally, future works that involve a fully immersed system for underwater shearogaphy are briefly announced.

Interferometers for the measurement of topography rely on imaging systems to map surface points to a camera. The response of an interferometer to variations in surface height depends on the filtering properties of the imaging system. Here we provide a simple and practical model of an imaging interferometer using classical Fourier optics, including the effects of partial coherence. The model is useful for understanding basic properties such as lateral resolution and error sources related to measurement principles that make assumptions regarding the fidelity of the instrument response over a range of surface spatial frequencies and light source wavelengths.

We describe a new, simple wavefront sensing method that uses a single measurement of a defocused star and a neural network to determine low-order wavefront components. The neural net is trained on computed diffracted star image data at 640 nm to output annular Zernike terms for an obscured circular aperture over a discrete range of all values. In the context of an actual star, the neural-net also provides the Fried’s parameter as an estimation of atmospheric turbulence. It is shown that the neural-net can produce a robust, high accuracy solution of the wavefront based on a single measurement. The method can also be used to simultaneously determine both on-axis and fielddependent wavefront performance from a single measurement of stars throughout the field. The prototype system can run at a rate of about 1 Hz with Python interpreted code, but higher speeds, up to video rates, are possible with compilation, proper hardware and optimization. This technique is particularly useful for low-order active-optics control and for optical alignment. A key advantage of this new method is that it only requires a single camera making it a simple cost-effective solution that can take advantage of an existing camera that may already be in an optical system. Results for this method are compared to high-precision interferometric data taken with a 4D Technology, PhaseCam interferometer and with an Innovations Foresight StarWave Shack Hartmann sensor from ALCOR SYSTEM under well-controlled conditions to validate performance. We also look at how the system has been implemented to use starlight for aligning multiple mirror telescopes in the presence of atmospheric seeing.

As the industry standard for interferometer frequency response specifications, ITF has historically been calculated using expensive phase targets to capture interference-based phase information. Recent work in transfer function characterization of interferometers has produced a more accessible technique with the ability to predict a system’s ITF utilizing a modified amplitude edge response. This technique does not require a phase measurement and demonstrates correlation between the ITF and an amplitude edge response for coherent systems, even in the presence of significant defocus. Defocused instruments exhibit modulation dropout in their transfer functions, now shown to be related to the Talbot Effect, exhibited here by imaging both phase and amplitude single frequency, square wave targets. This presentation outlines the modified amplitude edge response technique, utilization, and results for focused and defocused system characterization.

Recently, a technique for calibrating the modulation transfer function (MTF) of a broad variety of metrology instrumentation has been demonstrated. This technique is based on test samples structured as one-dimensional binary pseudo-random (BPR) sequences and two-dimensional BPR arrays (BPRAs). The inherent power spectral density of BPR gratings (sequences) and arrays has a deterministic white-noise-like character that allows direct determination of the MTF with uniform sensitivity over the entire spatial frequency range and field-of-view of an instrument. As such, the BPR samples satisfy the characteristics of a test standard: functionality, ease of specification and fabrication, reproducibility, and low sensitivity to manufacturing error. Here we discuss our recent developments directed to the optimization of the sample design, fabrication, application, and data processing procedures, suitable for thorough characterization of large aperture optical interferometers. Compared with the previous coded-aperture based design, the improved, ‘highly randomized’ BPRA pattern of the new test standard provides better accuracy and reliability of instrument MTF and aberration characterization, and enables operation optimization of large aperture optical interferometers. We describe the pattern generation algorithm and tests to verify the compliance to desired BPRA topography. The data acquisition and analysis procedures for different applications of the technique are also discussed.

Betelgeuse has experienced a sudden shift in its brightness and dimmed mysteriously. This is likely caused by a hot blob of plasma ejected from Betelgeuse and then cooled to obscuring dust. If true, it is a remarkable opportunity to directly witness the formation of dust around a red supergiant star. Today's optical telescope facilities are not optimized for monitoring the Betelgeuse surface, so in this work, we propose a low-cost optical interferometer. The facility will consist of 12 x 4 inch optical telescopes mounted to the surface of a large radio dish for model-independent aperture synthesis imaging; polarization-maintaining single-mode fibers will carry the coherent beams from the individual optical telescopes to an all-in-one beam combiner. A fast steering mirror assisted fiber injection system guides the flux into fibers. A metrology system senses vibration-induced piston errors in optical fibers, and these errors are corrected using fast-steering delay lines. We will present the design.

Quantitative phase imaging has emerged as a powerful label-free technique to understand and monitor biological samples. This technique can measure optical phase changes that can be used to estimate dry mass cells, density, and to monitor cell growth. One approach of this technique consists of placing a phase filter at the Fourier plane of a 4-f imaging system. Several studies have demonstrated the use of nonlinear optical materials as phase filters in commonpath configuration to modulate a phase filter as required in phase-contrast imaging (Zernike configuration). Furthermore, quantitative phase measurements have also been reported using liquid crystal materials as a phase filter. This work exploits the nonlinear response of a recently synthesized azobenzene liquid crystal material to obtain multiple interferograms of arbitrary phase shift. This material is of interest because of the extremely high optical nonlinearity (n₂=2.1X10^-1 cm²/W), will potentially enable applications where low-intensity illumination is required. A random phaseshifting algorithm for common-path configuration is employed to measure the phase object using less than twenty interferograms. Quantitative phase imaging includes not only biological applications such as in vitro fertilization, estimation of solution concentration, and live stem cell study; it also includes industrial applications that require a robust system that is able to measure in harsh environments such as mechanical vibrations.

A fiber optic refractive index sensor based on Fabry-Perot interferometer formed by two Chirped Fiber Bragg Gratings on a seven-core fiber is successfully demonstrated. A small part of the fiber cladding is etched to expose the outer 6 cores to the ambient environment. While optical modes supported by the outer 6-cores are affected by both temperature and refractive index changes of the surrounding liquid, the optical mode in the central core is affected by the temperature changes only. Because only a small part of the cladding is removed, the sensor maintains excellent mechanical strength and stability.

Plane-parallel optical elements, such as optical resonators, laser rods or wave plates, are widely used in many optical setups. Both planar surfaces of the element have to be manufactured with high precision in order to assure proper quality of the element and thus these surfaces have to be accurately measured. However, the interferometric measurement process, usually used for the surface form topography characterization, suffers from multiple interference patterns. This increases the total measurement error. In this paper the frequency-tuned interferometric method, that overcomes this problem, is demonstrated and compared to commonly used techniques using immersion liquid. Using a tunable laser source together with a frequency separation of the phase information from both surfaces, fringes from the first and second surface can be distinguished. Also, the interference between both surfaces can be used to obtain the wedge and thickness values of the specimen.

Phase-shifting (PS) is a well-established technique for phase retrieval in interferometry that requires a series of intensity measurements with known or unknown phase-steps. The objective of this work is determinate a tunable PS algorithm that minimize the error in the reconstructed phase when the images have additive random noise and the phase-step was determined with certain error. Simulations to show the performance of the algorithm will be presented.

We present the development of a single shot interferometric system based on polarization and diffractive elements recovered from spare computers. The use of salvaged lenses, beam splitter cubes and linear polarizers lower the cost of our system in comparison with those already reported. Our system is also capable of dynamic phase measurements with an accuracy comparable to similar implementations with high end manufactured optics. We report here our results obtained from four simultaneous interference patterns. The presented interferometer can be easily implemented for various applications in single shot polarizing phase shifting interferometry.

We present the analysis of the response of human cervix epidermoid carcinoma HeLa cells to photodynamic treatment with protoporphyrin IX (PpIX) synthesized in living cells under accumulation of 5-aminolevulinic acid (5-ALA). PpIX synthesis was estimated by means of confocal uorescent microscopy basing on its uorescence intensity in the red wavelength range. The cell death dynamics and pathways caused by intracellular generation of reactive oxygen species under laser irradiation of photosensitized cells were examined using digital holographic tomography. These processes were studied both at early stages of cells death on living specimens and at later stages on paraformaldehyde-fixed specimens. Statistical analysis of changes in cellular morphology monitored using holographic tomography allowed us to conclude on cell death pathways, observed at different irradiation doses. Data obtained by holographic tomography have been validated by a standard AO/EB test for cell membrane integrity conducted using the confocal fluorescence microscope.