This PDF file contains the front matter associated with SPIE Proceedings Volume 12618, including the Title Page, Copyright information, Table of Contents, and Conference Committee information

Recently deep neural networks (DNN) has shown the great capability of solving various inverse problems in computational optical imaging. Conventionally, DNN should be trained by a large set of paired or unpaired data. The most critical issue with this paradigm is that the neural network inference has no physical interpretation or limited generalization. In order to resolve these issues, one solution is to incorporate the physics of the problems in hand into the training of DNN, resulting in a novel framework that is called physics-enhanced deep neural networks or, PhysenNet, for short. Here we present a brief review of recent works in this regard with the use cases of phase imaging and ghost imaging. We will show that PhysenNet does not need any data to train. Instead, it learns from the physics of the given problem. Therefore the output of PhysenNet is naturally satisfied with the constrain imposed by the corresponding physical model. We would also like to emphasize that the concept of PhysenNet is quite generic, and can be used to solve many other inverse problems even outside the scope of imaging.

AI-based solution for sensing of dynamical processes with optical microresonators to enable automatic and self-adjusting predictions of the probed parameters is proposed. The sensing chip is characterized by dense packaging of more than one thousand individual spherical microcavities with strong spectral variations grouped on up to 10 sensing areas of different functionalities. Spectral diversity of microresonators supplemented with different deep learning models have been utilized to ensure detection of small entities of biochemical nature in the complex aqueous environment.

Thin films have been widely used in advanced high-technology manufacturing processes such as semiconductors, displays, and batteries. A representative method among non-destructive thickness measurement methods is a spectral reflectometer. An optical layout of the spectral reflectometer is compact with only a few optical components, and a mathematical model for analysis is also relatively simple. However, in order to find a thickness solution based on the model-based analysis, an initial value should be well chosen to cover. In addition, the analysis time takes longer for improving the thickness resolution due to large numbers of comparison. To overcome this practical difficulty, an artificial neural network algorithm with several different conditions such as number of hidden layers and nodes were designed and trained within a thickness range around 100 nm. The training data set and the validation data set were used by theoretically generating an interference spectrum based on the Fresnel equation. In this work, the spectral reflectometer was realized in a wavelength range of 355 nm to 657 nm based on our previous work. For quantitative analysis in this measurement, certified reference materials having a nominal thickness of 10 nm, 30 nm, 50 nm, and 100 nm silicon dioxide thin film was measured, and an uncertainty analysis was performed on the thickness measurement value determined through this. Uncertainty factors include measurement uncertainties related to the calibration of the spectral reflectometer in use, measurement repeatability, and artificial neural network algorithms.

This research means to solve the challenges in measuring deep high-aspect-ratio (HAR) and thin-film structures in 3D integrated circuits. As the semiconductor industry reaches its physical limitations in device scaling, advanced technologies such as advanced lithography and packaging have become crucial in extending Moore's law. This has led to the use of denser nano-to-sub-micron structures in three-dimensional integrated circuits (3D-IC), resulting in smaller, more functional devices. However, measuring these complex and deep HAR and thin-film structures with a large depth range from a few nanometers to a few hundred micrometers using a single optical system is challenging. To address this need, this article presents an AI-guided scatterometry method using numerical aperture control to achieve a large measurement range. The system uses broadband light to generate multi-wavelength reflection responses from the samples. With the help of an electromagnetic simulation tool and an artificial neural network model, the depth resolution can be improved through inverse modeling. The results demonstrate the ability to measure a wide range of samples with depths ranging from nanometers to micrometers scale, including sub-micron HAR openings and ultra-thin films, as long as the measurement bias is controlled within acceptable limits.

Laser powder bed fusion (LPBF) is an emerging technology that is gaining more and more market share in the overall additive manufacturing market. One crucial component of the manufacturing process is a possibly perfect flat and homogeneous powder bed. If the powder recoating runs into problems, e.g., because of recoater defects or missing powder, or if the previously printed layers generate powder bed defects, this requirement cannot be fulfilled. Defects in the powder bed can lead to defects in the final part. Hence, the powder bed for each new layer should be inspected for anomalies. Doing so, the generation of defects, e.g., de-lamination, can be detected in early stages. We present a specialized machine vision system which is capable to detect and partly classify all powder bed anomalies. Machine vision in the building chamber of LPBF machines is challenging. One needs oblique observation because the centered view is blocked by the laser(s). One needs many pixels and optimized imaging to catch all the defects. The field of view is large. In order not to increase the production time of a layer, the imaging and powder bed assessment should work fast. All relevant information about the defects must be captured by the imaging process. The main advantage of our approach is, that we acquire not only one image, but four images, each illuminated with an individual light source at the ceiling of the building chamber. We will show, how these four images can be used to find, distinguish, and classify defects. This includes classical image processing as well as machine learning techniques.

We propose a snapshot uniaxial 3D profilometry system that illuminates a sample with a color structured polarization pattern with axial chromatic aberration. We have previously shown that real-time 3D shape measurement can be performed without the occlusion problem using a RGB polarization camera and a monochromatic polarized pattern. Here we show that it is possible to double the depth range of the system, with no tradeoff in depth resolution, measurement speed, or accuracy, by using color pattern projection and an RGB polarization camera. We present the measurement principles of the system and show the results of a real-time 3D profilometry experiment.

Femtosecond laser processing is useful for drilling micro holes in many kinds of materials. The laser parameters according to the material should be carefully selected, but the procedure is time-consuming. In-process monitoring, which allows observation of the processed structure while real-time feedback of processing parameters, is necessary to reduce processing time. In this study, the laser-excited sound wave is used for controlling the axial position of the object lens in hole drilling.

Carbon fiber-reinforced Plastics (CFRP) are a key factor for achieving a carbon-neutral mobility sector due to their lightweight properties. Despite their near-net-shape manufacturing subsequent precision machining is required. In case of large-scale CFRP components, which are characterized by small quantities and low machining volumes, such processes are difficult to operate economically with conventional systems. Robotic systems offer a suitable solution. Their use in machining, however, requires additional measures to compensate for structural system limitations. In addition, individual deviations of the blanks require time-consuming measuring for referencing the machining system. To enable the use of cost-efficient robotic systems and to eliminate unproductive downtime for measuring, the intended manufacturing of CFRP components by Resin Transfer Molding (RTM) offers a new opportunity. RTM allows for the integration of defined features into the molding tool, which are reproduced inverted on the component and can be referenced by subsequent processes. This is utilized with adapted strategies for time-efficient measuring and high-precision machining. Fundamentals for the corresponding feature design as well as the selection of a suitable measuring device are presented. Focusing on laser-based sensors, the relationship between wavelength and (semi-)transparent surfaces is discussed and validated. Besides the metrological detection, production-specific aspects are also taken into account. Based on the gained knowledge, the main aspects of a proper sensor-feature combination are elaborated and prototypically realized. Finally, an outlook is provided on the challenges involved when implementing a robust algorithm for feature detection.

Fiber reinforced polymers (FRP) are predestined for the construction of large high-speed rotors. However, their anisotropy makes complex models necessary, which must be validated by in-situ measurements. By reading out the far field of diffraction gratings on the surface of a 500 mm diameter FRP disk, we measured its deformation, damage and modal behavior as a function of the rotational velocity. We achieved high spatiotemporal resolution and high measurement resolution at surface velocities up to 275 m/s. To reduce systematic uncertainties and to measure at more complex rotor geometries, we introduce a new sensor setup with in situ calibration.

To achieve high accuracy and precision in optical metrology for advanced semiconductors, it is crucial to identify and compensate for errors from optical components and environmental perturbations. In this study, we investigated the sources of the errors in the interferometric ellipsometer developed for next-generation OCD. The objective lens and beam splitters, the critical optical components of the system, are intensively investigated. The system errors induced by temperature fluctuation, wavelength inaccuracy, and defocus were quantitatively examined. We also proposed methods for compensating individual errors and analyzed the effect of the compensation. As a result of error compensation, the accuracy and precision of the system is improved by 6.9 times and 2.3 times, respectively. Although the investigation was conducted based on our interferometric ellipsometry system, the finding is not limited to this system, as these errors are commonly found in most optical metrology systems. The proposed method for error compensation will be essential strategies for various ellipsometry systems suffering from a low level of accuracy and precision.

There are a growing number of industrial applications for shearography, especially in non-destructive testing of composite materials. Its tolerance to hostile environments is probably the most relevant success factor for industrial applications. The introduction of carrier fringes for phase recovery, replacing temporal phase shift techniques, allows measurements with a single exposure for each loading state, making shearography even more robust. Some defects are better evident when the shearing direction is horizontal. Others are best viewed with a vertical shearing direction. Thus, it is desirable to inspect in both directions. Fortunately, the literature reports some configurations capable of using carrier fringes to perform simultaneous measurements in more than one shearing direction. These systems work well, but since they use a mask with circular holes, they require a high amount of light. An alternative is the use of a pair of oblong slits, which increases the amount of light captured. This paper extends the use of oblong slit pairs for simultaneous measurement in two orthogonal shearing directions. Two pairs of parallel oblong slits are arranged orthogonally on the system mask. Although the Fourier transform of the resulting signal at the camera sensor contains the desired signals as well as a crosstalk between the slit pairs. However, the mask can be designed in such a way that the orthogonal signals are separable. To promote shearing in orthogonal directions, a polymeric lens was sectioned, forming four quadrants of 90°. The quadrants were radially displaced and arranged so that each oblong slit covers only one quadrant of the lens. As a result, four laterally shifted images are formed, two of them with horizontal shearing and the other two with vertical shearing. A proof-of-concept prototype was developed and the first results were evaluated by the authors in the laboratory environment and are presented in this article. The paper also shows details of the resulting Fourier spectrum and the strategy used to separate the orthogonal components and recover the phase using carrier fringes. The performance of the prototype met expectations and validated the concept. Future works involve miniaturization of the prototype and tests in industrial environments.

A dual-fibre single-plane angle-sensing tape that utilizes optical Fibre Segment Interferometry and Range- Resolved Interferometry (RRI) for angle sensing is presented. The sensing tape facilitates the multiplexing of an array of angle sensors along its length and can be retrofitted into small robots and construction equipment. We demonstrate its application on three non-rotational joints of a small five-axis robot, describing the design, construction, measurement principle, and presenting measurement results. Preliminary data shows that the angles measured by the sensing tape exhibited agreement within a range of ±0.005° with the manufacturer-installed angle encoder of the robot.

Spatial light modulators (SLM) offer a broad range of opportunities in optics. Especially liquid crystal on silicon (LCOS) devices are in common use, due to their extreme high spatial resolution and up-scalable production capabilities compared to other technologies. Still, the architectural complexity of these displays causes well known phase errors, such as the inherent backplane curvature, crosstalk between adjacent pixels or spatial varying phase response (SVPR). In our latest work we presented a robust method to characterize the 2D resolved phase response of a SLM by means of a Twyman- Green interferometer and how to calculate a compensation phase mask. Based on the non-linear deviation of the pixel retardation measurements, it was indicated that a pure LC layer thickness variation is not the only contributor to the SVPR. In this work we want to propose a more representative model by taking interfering reflections of different layers, the socalled Etalon effect, into account. The 2D pixel phase response was simulated via Ericksen-Leslie equation and included in the transfer matrix method. The introduced Etalon model is validated by complementary measurements for different center wavelengths. A white-light monochromator was added to the setup to adjust these properties accordingly. First implications of systematic error compensation will be discussed.

We report the development of space qualified three-channel bistatic Laser Detection and Ranging (LADAR) system capable of working up to 2.2 km which includes Master oscillator power amplifier (MOPA) based on High Power Fiber Lasers with specifications of 5W Peak power, 4-millisecond pulse-width at 1550 nm central wavelength. Transmitter-Receiver collimators are aligned in Bistatic configuration within 3.6 arc second accuracy. Doppler shift measurement is estimated using all-fiber optical heterodyne interferometry and thereby highly accurate velocity detection is achieved with 3 sigma accuracy of ±3.108, ±4.063, and ±3.3903 mm/s. The developed LADAR sensor system for velocity measurement is space qualified and has shown nominal performance during all phases of thermal and vibration tests.

Phase-stepping shearography is a useful speckle interferometric technique having wide applicability in various fields. Nyquist criterion has long been considered to be the gold standard in signal processing and imaging. This criterion states that the spatial frequency of image sampling should be more than twice the maximum spatial frequency in the image. As applied to speckle imaging in shearography, this implies that, if a two-dimensional charge-coupled device (CCD) is used for imaging, the pixel separation of the CCD device should be half of the minimum feature size in the image, or half of the speckle size. We examined the applicability of the Nyquist criterion in image sampling in phase-stepping shearography. We used a shearographic system employing a 24-megapixel digital still camera and a Michelson interferometer based shearing arrangement. We generated phase maps from a deformed object, which was a center-loaded thin aluminum plate fixed at the periphery. For a given amount of deformation and image shear, detailed phase maps where generated using the 4+4 phase stepping protocol for a different F-numbers, representing different sampling situations, ranging from the sub-Nyquist domain (undersampling) through the Nyquist domain, to the above-Nyquist (oversampling) domain. From the generated phase maps and quantitative measurements, we show that adherence to the Nyquist criterion is not a strict requirement in digital shearography. Operation in the sub-Nyquist domain is acceptable, even desirable, in many cases. Phase maps of good contrast and quality, with or without filtering, can be generated in the sub-Nyquist domain, which can be useful in strain analysis and non-destructive testing applications.

Fast and non-contacting 3D shape measurements of objects for quality assurance, human machine interaction, or robot handling, e.g., in the industrial sector, have become well established. Recently, we have successfully combined thermography and triangulation to tackle the challenge of measuring the 3D shape of uncooperative materials, i.e., materials with optical properties such as being glossy, transparent, absorbent, or translucent. Therefore, we have developed the principle of thermal 3D measurements, a two-step process consisting of (1) the projection and absorption of projection patterns in the thermal infrared and (2) the stereo recording of heat patterns re-emitted by the object surface. We match image points by evaluating the temporal normalized cross correlation between pixels in both camera image stacks. In order to measure dynamic scenes, the previously achieved measurement times of a few seconds must be reduced by at least one order of magnitude to the range < 0.1 s. For this purpose, we apply established single-shot methods from the visible spectral range to our thermal 3D approach. Instead of temporal sequences of multi-fringe patterns or scanning single fringes, we now project statistical point patterns and record only one thermal stereo image pair. In this paper, we theoretically investigate our approach by using a simulation model for thermal point pattern generation on static measurement objects. We analyze the temporal and spatial behavior of the heat patterns taking the material parameters into account. Finally, we show a first thermal single-shot 3D measurement.

For high-precision measurements through a inspection window, a 3D scanner based on fringe projection profilometry is being researched. The 3D scanner combines a micromirror array projector and two telecentric cameras. The affine camera model is commonly used to calibrate telecentric imaging systems, in which a single magnification factor is introduced and optimized for each lens. However, 3D reconstructions based on this model indicated that reconstruction uncertainties in the peripheral areas of the measuring volume are significanty affected by a possible inspection window. These uncertainties may occur due to the model-based determination of the magnification factor and the reduction in parallelism of the visible rays within the telecentric lens that can occur at larger working distances. To address this issue, a new method for calculating and identifying the influence of the magnification factor on the 3D point scaling for telecentric measuring systems is proposed. First, the measuring system is calibrated using an affine camera model. Then, the reconstructed 3D target points are used to estimate the magnification factor locally and assessing the influence of an inspection window in the optical path. In order to further investigate the influence of the inspection window on the imaging performance of the cameras the focus is estimated locally within the measuring volume. Initial measurements using these methods reveal that scale variations and the reduction of focus can be quantified locally and a model based correction as well as the removal of poorly reconstructed points is feasible.

The PIC (Photogrammetry Inside Cryostat) is a cutting-edge periscope designed to be used during the prototyping, testing and alignment phase of HARMONI, one of the first-generation instruments of the Extremely Large Telescope. The challenge posed by operating the HARMONI instrument at a temperature of 130 K required the development of different non-contact measurement techniques to qualify optical and mechanical parts without touching them. Photogrammetry is a non-contact measurement technique, but it needed to be adapted to be used in a cryostat; this led to the development of the PIC. The periscope consists of a combination of six lenses, two mirrors, and three motorized degrees of freedom (the whole is mounted upside down on the lid of the test cryostat at CRAL in Lyon) and work together with an external camera to capture images at various angles. This is essential for obtaining accurate photogrammetric measurements. The motorized rotation systems allow for precise and controlled movements, and the combination of lenses and mirrors ensure that the images captured by the external camera are of the highest quality. The goal of the PIC is to obtain an accuracy of 25 μm + 5 μm, making it an essential component of the HARMONI tools and a major advancement in cryogenic photogrammetry.

The aim of this paper was to test the suitability of using photogrammetry for metal additively manufactured parts with respect to metrology applications. Another aim was to test the influence of the chosen parameters on the quality indicators of the measurement data and the suitability of using single and stereo camera setups. The parameters of the measurement system whose influence was assessed were the exposure time, number and resolution of images taken. The individual measurements were evaluated in terms of the computational complexity, number of points obtained, part coverage ratio and the standard deviation of the distances of all points from the surface of the reference model. By comparing the results, a matrix of optimal parameters for the given part types was created and further trends describing the influence of individual parameters on the results were observed. The experimental comparison performed shows the possibilities of photogrammetric measurements, gives an idea of the appropriate parameters settings and has also shown that photogrammetry can be a competitor to fringe projection scanning of metal additively manufactured parts. The main drawback of this method is still the computational time and the need for a distinctive surface texture.

Optical microscopy is one the oldest and most widely used techniques for sample inspection in life and material sciences. Microsphere-assisted microscopy (MAM) has emerged as a simple yet efficient approach to boost the spatial resolution. MAM uses a microsphere, placed in the immediate vicinity of the object, to enhance the numerical aperture and improve the resolution. MAM can be used in conjunction with white-light, wide-field, bright-field, dark-field, confocal, fluorescent, second harmonic generation, two-photon, interferometric, and digital holographic microscopies. It can be employed in reflection and transmission modes. There are several open questions and challenges associated with MAM, including the resolution limit, the mechanism behind resolution improvement, and the limited imaging field-of-view. Here, we present the progress in MAM in the past decade along with the associated opportunities and challenges.

Coherence Scanning Interferometry (CSI) is a non-contact technique for three-dimensional (3D) surface characterization with angstrom-scale vertical resolution, but whose lateral resolution is diffraction limited. To enhance the lateral resolution, micro-optical elements like microspheres and fibres can be used [1,2]. This combination allows the design of an apparatus for 3D imaging with nanometre vertical resolution and a lateral resolution better than 100 nm. We demonstrate the use of two-photon polymerization (2PP) for 3D-printing microspheres as a photonic nanojet (PNJ) generating structure. Since 2PP enables true 3D processing of photoresists at the microscale with sub-100 nm resolution, highly complex structures can be printed without compromises to their geometry [3]. We modelled imaging properties of different PNJ generating structures in open-source software using Monte Carlo path tracing and manufactured three versions of them using photoresists with different refractive indices. The components were tested in a custom-made coherence scanning interferometer by imaging a sinusoidal structure with a period of 280 nm. Initial results show that the new PNJ generating structures, in combination with the CSI device, are capable of imaging structures with a lateral resolution better than 100 nm. 3D printing by 2PP allows mass-production of PNJ generating structures with complex shape without requiring any vacuum, harsh chemicals, or cleanroom environment. It also allows the PNJ generating structures to be manufactured straight onto a glass substrate, providing an integrated mounting solution for easy use with conventional optics. This will allow increased working distance and field of view as well as easier handling for the next generation 3D super-resolution imaging systems. [1] I. Kassamakov, S. Lecler, A. Nolvi, A. Leong-Hoi, P. Montgomery, E. Hæggström, Sci. Rep. 7, 3683 (2017). [2] A. Nolvi, I. Laidmäe, G. Maconi, J. Heinämäki, E. Hæggström, I. Kassamakov, Proc. SPIE 10539, 1053912-1 (2018). [3] M. Farsari, B. Chichkov, Nature Photon. 3, 450 (2009).

Nanoscale non-destructive metrology is a key requirement in several steps of the manufacturing process of modern semiconductor devices. In particular, with the introduction of EUV lithography into the high-volume manufacturing, enabling further shrinking of feature sizes, metrology for future technology nodes will become increasingly challenging. Depending on the specific requirements and constraints of the metrology tasks, the choice of the measurement methods is critical, and sometimes limited. Conventional metrology techniques must be constantly adapted to keep up with the device scaling roadmap. To explore new and easily scalable methods for semiconductor wafer metrology, we developed the EUV reflective grazing-incidence nanoscope (REGINE). REGINE is a lensless nanoscope prototype that combines reflectometry, scatterometry and coherent diffraction imaging (CDI) in a single instrument. Being a lensless imaging system, it is compact, cost-effective, and free of lens-induced aberrations. In this work, we will present the REGINE system and the latest results of reflectometry, scatterometry and EUV pellicle transmission experiments.

Paul Montgomery is a senior research scientist with the CNRS at the Engineering science, computer science and imaging laboratory (ICube) in Strasbourg, France. He has over 30 years' experience in developing optical instrumentation for the characterization of materials and biomaterials. He is currently interested in label-free far field nanoscopy, microsphere-assisted interference microscopy, local spectroscopy and the use of environmental chambers for measuring specific parameters. He is a senior member and a member of the Board of Directors of SPIE as well as being a member of IOP and SFO.

The precise production of micro- or micro-structured components of increasingly different materials requires ever more precise and flexibly adjustable geometry measurement methods. Today’s optical metrology offers various innovative approaches for this purpose. A major shortcoming is, however, that not all surfaces and structures to be measured are optically cooperative and return too little light or information to the measurement system for the signal analysis. Therefore, indirect optical geometry measurements are introduced as a new approach: Instead of directly detecting the outer boundary layer of the measuring object, the shape of the object’s imprint in the surrounding medium is examined. For this purpose, the surrounding medium is enriched with fluorescent substances and a confocal microscope scans the space surrounding the measured object. The spanned area above which the fluorescence signal disappears is then determined as the boundary layer between the measurement object and the surrounding medium. As a result, the object geometry is obtained completely independently of the optical response behavior of the object. While first realizations studied measurements in a liquid environment, this work demonstrates for the first time the feasibility of indirect optical geometry measurements in air environments with the aid of fluorescent microparticles. In order to maximize the measurement accuracy, different model-based signal evaluation approaches for determining the interface geometry from the fluorescence signals are investigated and compared, taking both cases (liquid and air environment) into account. Finally, indirect optical measurements are performed on a step geometry, reconstructing the height profiles using the theoretically derived model function.

Additive manufacturing (AM) has brought a new level of innovation to the production of metal parts, allowing for greater design freedom and on-demand fabrication capabilities. However, the measurement of the resulting surface texture has increased difficulties. Whilst optical techniques provide fast measurements, the high surface irregularities, the large range of scales of the features, and the high variations in surface reflectivity largely compromise performance. In practice, conventional systems have an insufficient field-of-view (FOV) or produce an excessive amount of measurement artifacts. We present an optical system optimized to measure AM parts, featuring a sufficiently wide field-of-view to capture the surface scales of interest, and a high numerical aperture (NA). We integrated a commercially available objective (SHR Plan Apo 2×, Nikon) with a custom-designed tube lens in a measuring head, achieving a magnification of 4.2× and an effective NA of 0.26. The system implements the confocal-like HiLo technique, and we experimentally demonstrate successful characterisation of metal AM parts. Furthermore, we report and discuss the design of a 6×/0.45 NA microscope objective, with great promise for increased performance. Additionally, we explore high dynamic range (HDR) approaches to tackle the issue of the high reflectivity variation. Multiple low dynamic range images taken at a range of illumination intensities are combined to reconstruct a single topography map. In all, the combination of the optical system, capable of capturing a large scale of spatial features, with the HDR capability offers a versatile solution that can be used in a variety of scientific and industrial contexts.

Direct laser writing is a popular method for mask-less lithography that already achieved commercial grade. However, it is still challenging to realize homogeneously exposed structures on 3D-shaped substrates. A common source of the variation in exposure is a changing distance of the substrate surface with the photoresist towards the plane of the narrowest waist of the exposure beam. To tackle this issue, we propose a differential confocal probe that employs a spatially modulated pinhole. This phase-modulated lock-in principle enables highly resolved depth sensing without ambiguity about the direction of the deviation from the focal plane. However, the modulation contrast must be high enough to achieve this, which is why the measuring beam passes the pinhole twice. This probe is integrated into the nanopositioning and nanomeasuring machine via a position-based controller. We demonstrate the capability to follow 10◦ inclined substrates.

In this paper the design of a Time-Domain Full-Field OCT (TD-FF-OCT) setup for non-contact volumetric layer thickness measurement is presented and quantified in terms of achievable accuracy and performance. The capabilities of the instrument regarding its measuring accuracy are verified using foil thickness standards of different strength. Afterwards, a technical application of measuring a thin and rough varnish coated PET foil (foil thickness t_foil ≈ 150 μm, rough varnish layer thickness t_varnish ≈ 10 μm) is carried out. Since the device is designed to conduct areal measurements, the thickness can be accurately determined over several measurement points. The results are compared with results achieved by applying an alternative but destructive and more time-consuming measuring method (evaluation of microscopic images of respective foil slices produced by using a microtome). Finally, the achievements are summarized and identified optimization potential is highlighted.

We recently demonstrated that the 3D shape of micro-parts can be measured using LED illumination based on contrast evaluation. The technique is based on imaging the object under test using partially coherent illumination. The limited spatial coherence of LED illumination was utilized to discriminate depth. For a fast depth scan without mechanically moving parts, an electrically tunable lens (ETL) in a 4f optical configuration is used. This approach is efficient, takes less than a second to capture all required images, is eye-safe, and offers a depth of focus of a few millimeters. However, the main limitation of the proposed approach arises from the underlying assumption that a small angle of variation of illumination is required. Such a small illumination aperture affects the axial scan resolution that dominates the measurement uncertainty. In the present paper we propose a method to overcome the main limitation of the above-mentioned profiling technique. The approach combines measurements with multiple illumination directions achieved by illuminating the object simultaneously with several independent light sources. In this way, the full-width half maximum of the contrast envelope is reduced. A tilted metallic plate is used for proof of concept.

Surface characterization is important for the functionality of many applications. For process control applications it is important to measure surface characteristics in-line. Scattering light sensor technique is well suited for in-line measurement due to its robustness against vibrations and the fast measurement speed. But the non-trivial connection between the surface-angle resolved data from the scattering light data and the 3D surface data from other surface measurement system hampers the use of scattering light sensor technique for structured surfaces. To overcome these hindrances we present a method to reconstruct surface profiles from scattering light measurements based on an accurate simulation of the measurement process and a sparse representation of the surface profile incorporating a priori knowledge like the feed rate of a turning process.

Axial scanning is inherent to optical profilometers, which sample a signal and infer the object height as the location of its maximum. Imaging confocal, interferometric or focus variation methods, are common example technologies for acquiring such signal. Of course, in these approaches, the precision and accuracy of any measurement is directly affected by those of the scanning system, usually based on piezo scanners or motorized linear stages. The latter provide an inexpensive solution but are subject to non-linearities between the commanded and executed positioning, yielding pseudo-random errors, which in practice limit the precision and accuracy of the measurement. We propose a computational method to correct such non-linearities on a measurement using data from the very same scan, therefore providing a zero-cost solution to increase the precision of profilometers employing motorized linear stages. The method is based on the reconstruction of two topographies that are axially shifted by a known amount. Assuming that the two topographies are identical, differences between these two topographies reflect only the errors of the positioning system. Our algorithm predicts and corrects the non-linearity errors by iteratively analyzing the differences in the topographies. Here, we explore two methods for obtaining the topography pair: by performing two scans, and by localizing two axially shifted features of the axial response from a single scan. To validate the technique, we measured a variety of samples including step heights and compared the results against the piezo-measured counterparts, obtaining equivalent performance.

Precise, accurate, and fast characterization of overlay between two layers of semiconductors during chip fabrication is an important monitoring and feedback step. Overlay, which is to be measured within the accuracy of a nanometer, is susceptible to many small imperfections in the measurement system. A digital holographic microscope measures the complex field of overlay targets using simple optics followed by computational algorithms for overlay metrology. We improved the precision of overlay measurement by correcting the inhomogeneities and asymmetries caused by the illumination spots, presenting a robust method resulting from simple calibration steps.

With the improvement in microscopy application and industrial production technologies, many products related to optical components have been developed. Because of the commercial application, compact systems and bottom-up technologies represent the golden standard for the manufacturing of optical devices. Moreover, the use of polymeric materials combines two main advantages: reduce the fabrication cost and improve the flexibility in terms of geometry and dimension of the optical components. Recently, innovative approaches have been considered for the production of micro-lens array in a single step and one of the best candidates for the array preparation is based on the self-assembling activated onto a polymeric film. The aforementioned paradigm has been growing rapidly, representing a single step procedure that avoids the fabrication of mold and multi-steps procedures for shaping and curing the polymer material. In this framework, we present an innovative strategy for the self-assembling of polymeric micro-lens array starting from a thin polymeric film. The shaping and curing of the microstructures is guided by the pyro-electric effect activated onto a periodically poled Lithium Niobate crystal. The complete characterization of polymeric arrays of different geometries and dimension is reported as a function of the experimental conditions. Moreover, digital holography is implemented to provide the optical properties of the lenses; Based on this strategy, we performed a series experiments of lens formation controlling completely avoiding the use of electrodes, molds and costly equipment.

The demand for accurate and sub-nanometer precise overlay measurements in semiconductor industry increases with the shrinking feature sizes in integrated circuits. Overlay, the lateral displacement between two layers allows monitoring the chip fabrication process and is part of an import feedback step. In our approach, a digital holographic microscope measures the complex field of the overlay target using simple optics and an additional reference beam. The measured complex valued field allows us to apply computational algorithms to correct for field-position dependent lens aberrations in a computational efficient manner. We present experimental results that show the capability of our computational aberration correction in the visible and near infrared wavelength regimes.

In this article, a reliable method for surface topography is proposed employing a digital holographic microscopy setup. The proposed method is based on extracting the surface profile from the phase map encoded in the hologram signal. For hologram acquisition and reconstruction, we used a Raspberry Pi computer and camera module in our experimental setup, which is a step toward low-cost non-contact surface profilometry. A graphics processing unit (GPU) assisted state space method is used for rapid and reliable phase estimation, providing high robustness against noise. The experimental finding for a standard calibration target corroborates the practical viability of the proposed method.

In this work, we present a novel design and implementation of a lens-free phase imaging system with multi-angle illumination that enhances axial resolution and image quality. The technology, which is based on a common-path shearing interferometer with phase shifting, enables ultra-high sensitivity better than 0.2 nm in optical path difference (OPD), while operating over a wide FoV (>10 mm²) and a large volume (>10 mm³). We show results in several applications, from surface topographies to volumetric structures, including imaging of 10 nm thin transparent topographies and of volumetric laser-written refractive index structures in glass. The high sensitivity and low noise make the proposed technology ideally suited for imaging of low contrast structures on the surface or inside transparent materials, such as defects, impurities, or changes in refractive index.

The standardized material measures of ISO 25178-70 are categorized as either profile or areal surface geometry with regard to their application. This categorization limits many types of material measures to be only applicable to certain types of measuring instruments. To enable comparability and uncertainty estimation for many different types of measuring instruments, we examine the adaption of the either profile or areal assigned standardized material measures of ISO 25178-70 to multiple types of measuring instruments. For this enhancement the structures can be either imaged in different lateral directions, or changed to circular geometries that allow a sampling in different directions. The revised geometries are designed, manufactured, and measured to practically demonstrate the possibilities for a multifunctional calibration of different measuring instrument categories and to illustrate the effect of directionality on the results. Many types of material measures which are assigned to the calibration of profile measuring instruments can be enhanced to also allow a specific calibration of areal surface topography measuring instruments and the additional axes that is introduced hereby.

Exemplified by a stitching procedure that is frequently applied in optical surface metrology and that is based on the maximization of the cross-correlation function, the article shows the dependency of the accuracy of a stitching result on many different interdependent influence factors. Furthermore it is shown that the remaining deviations of the regions that are regarded to be the overlapping ones is no generally valid indication for the accuracy of a stitching result. Thus, in the second part of the article, a method to determine the uncertainty of a stitching result based on a Monte Carlo simulation is presented. The input quantities of the stitching procedure, namely the topographic data of the sub-apertures and the position and orientation data of the positioning system are characterized by a probability density function. Afterwards, it is sampled randomly from these probability density functions and the stitching procedure is applied many times. From the obtained distributions, the result of the stitching procedure along with its uncertainty is derived. The obtained results show metrological compatibility to the true values and furthermore enable the recognition of the cases for which the stitching did not improve but impair the accuracy compared to the accuracy of the positioning system.

At PTB, a modulation transfer function (MTF) measurement setup is developed for the traceable determination of the MTF of lenses and objectives. In recent works the MTF of a reference objective has been determined within the image field in an angular range of up to θ = ±20° to the optical axis. The measurements have shown characteristic deviations that presumably were caused by misalignments of parts of the setup. Thus, the procedure for the alignment of the setup components has been improved and new results with strongly decreased influence of misalignments are obtainable. Furthermore, the investigation of the angular dependency of the MTF for a given spatial frequency requires a datum point as a fixed reference. A common choice of the datum point is the focus point P_f. Different methods to obtain the focus point from the measurement data are investigated and show a varying repeatability in the determination of the focus point P_f. In this contribution the recently improved procedure used for the measurement setup alignment is described. New measurement results are presented and the contributions of misalignments to the MTF results is discussed. In addition, several approaches for the determination of the focus point are presented and rated with respect to their repeatability.

In this paper, we present a novel technique for stabilisation of widely wavelength modulated lasers (>100pm) over long time scales, where modulation depths exceed the spectral width of standard reference features, such as gas absorption lines, by over an order of magnitude. The technique operates by controlling the temporal separation between successive appearances of a gas absorption line on the up and down sweeps of a sinusoidal laser wavelength modulation waveform. The influence of the signal distortions introduced by the laser intensity modulation that are associated with laser diode injection current modulation are also addressed. The technique is applied to a range-resolved interferometric system interrogating a Mach-Zehnder interferometer operating in thermally stable conditions, using an absorption feature from a fibre-coupled gas cell as a reference. Proof-of-principle measurement results achieved using this technique are presented, demonstrating a notional fractional stability of 3.9×10⁻⁷ without further correction.

Biconic surfaces are among the most fundamental freeform surfaces and have important applications in the ophthalmic industry. They are characterized by just four parameters, namely the two radii and two conical constants in the x- and y-direction, and they have even symmetry along x and y. As for many other optical surface types, the manufacturing of these surfaces requires form measurements with a precision of a tenth of a wave. We introduce an approach to measure the form of biconic surfaces using Fizeau interferometry. The approach builds on concepts we have used in the past for the measurement of aspheres and toric surfaces. A combination of sub-aperture data acquisition, fringe order tracking and distance function removal produces a series of sub-aperture deviation maps which can then be combined into a full field deviation map of the entire surface. This final map can be further analyzed for errors in the four design parameters as well as higher-order form errors. In addition, it is possible to determine the orientation of the principal curvatures of the test part with respect to its mount in the instrument. We discuss the details of the measurement procedure, show first measurement examples, and discuss how the method can be validated using toric and aspheric surfaces which can be measured as such but can also be measured as biconics allowing for comparisons.

We show in this paper a digital random target method for measurement of Modulation Transfer Function and Point spread function locally in all the field of view of the instrument, and with a good accuracy. The point is to use a “pixel locking method” to identify point in object space with its conjugate point in the image space in pixel accuracy.

We will present a new approach of linearized focal plane technique (LIFT), formerly developed by ONERA, which results in an improvement of a factor of 16 (4x4) of the spatial resolution. This technology is based on the combination of standard SH technology with phase retrieval algorithms applied on all spots of the microlens array that provides information on high spatial frequencies. We will show some measurements performed on extremely complex wavefronts. This technology presents very promising perspectives for optical and freeform metrology and can advantageously replace, at lower cost and better usability, Fizeau interferometry.

Over the past years a lot of effort is being focused on realizing the vision of fully autonomously driving vehicles. The achievement of this goal strongly depends on the development of sensors that allow the perception of the environment by scanning it with high speed, precision and resolution. The sensors employed in autonomous vehicles typically comprise cameras, Radar and LiDAR Systems. Especially LiDAR and Camera Sensors deliver the necessary high-resolution data, but both suffer from strongly degrading signals in low visibility conditions. To guarantee the safe operation of autonomously driving vehicles the existing sensors need to be improved with respect to these conditions and new sensors need to be developed. In this contribution we present a LiDAR system design that is optimized for the operation in low visibility conditions. On one hand we address the technical details of the system such as choice of laser, detector, deflection unit and signal processing electronics. Besides the technical details of the system, we discuss the physical and technological limitations such as wavelength dependent scattering and absorption and eye safety considerations. We further give an outlook on a sensor fusion approach with a time-gated sensor with high lateral resolution for a better recognition of objects obscured by scattering media.

Laser scanners are widely used for the 3D-measurement of large-scale infrastructure objects. For objects like bridges or special tunnels unmanned aerial vehicles (UAVs) are well suited mobile platforms for inspection. Laser scanners used in these scenarios must be lightweight and low in power consumption to maximize flight time. A laser scanner optimized for UAV-based applications was developed by the Fraunhofer Institute for Physical Measurement Techniques IPM. The system is based on the pulsed time-of-flight measurement technique. A 1550 nm pulse laser featuring a repetition rate of 1 MHz for high point density is used as a light source. The short pulse length of less than 1 ns allows for a precise detection of the reflected signal in the time domain. Beam deflection is done with a rotating 45° mirror as in typical profile scanners. To optimize for detection aperture and weight, a custom mirror was designed. A lightweight scanning motor with a maximum rotation frequency of 120 Hz was chosen. An optical deflection path was developed that allows for a full 360° scan without any shading. The housing is made from aluminium and carbon fibre to reduce weight. The prototype of the laser scanner has a total weight of 2.1 kg and a power consumption of less than 100 W. The laser scanner is eye-safe (laser class 1) which is especially important for UAV-based applications. Test measurements in an indoor facility show a measurement uncertainty (one standard deviation) of approximately 3 mm on a surface at 10 m distance. The system was also mounted on a drone for flights in a tunnel which resulted in dense point clouds with high precision confirming the laboratory tests. Regarding the measurement uncertainty there is still a large potential for improvement by optimizing the full waveform analysis. First tests indicate that a reduction of the uncertainty by one order of magnitude may be possible.

There exist two key aspects to enable autonomous vehicles: Robust sensors to provide all relevant data concerning the vehicle's surroundings as well as algorithms to evaluate this data in real time. Apart from radar and ultrasonic sensors, optical sensors such as lidar and cameras are state-of-the-art in prototype autonomous vehicles. In adverse weather conditions, such as e.g. fog, snow, dust, heavy rain or poorly illuminated scenes, though, those sensors do not perform reliably. Recently, we proposed to use a time-gated-single-pixel-camera to not only significantly reduce the amount of recorded data but additionally filter ballistic object photons, i.e. suppress the effect of noise from the obscuring medium. Apart from generating 3D object information, such a system can operate fast enough to deal with the highly dynamic environment as well as respect eye-safety norms. Moreover, a time-gated-single-pixel-camera offers the ability of image-free detection of all relevant objects within the scene which speeds up data evaluation as well. Here, we want to report on our progress towards realizing such a system. We will demonstrate image-free object detection on simulated data. We realize multi-object detection by generating object heat-maps for the different classes. Additionally, we discuss the difficulties we have to overcome to robustly detect objects in real measured data and shortly present our prototype setup, which we have implemented on a car together with our partners from Fraunhofer Institut für Physikalische Messtechnik and Institut für Autonome Intelligente Systeme, Universität Freiburg.

Digital holography benefits from interferometric amplification, which enhances sensitivity. Coherence-gated digital holography allows for the suppression of noise sources such as multiple scattered photons and other light sources. In addition, digital holography provides access to optical phase information, which is an important metrological parameter for three-dimensional measurements. The aim of this study is to investigate the potential of digital holography as a new sensor concept for the environmental perception of autonomous vehicles under difficult visibility conditions. Our experiments are conducted using a 27-meter-long fog tube and serve in particular to characterize the capacity to effectively filter multiple scattered photons based on their coherence property. From the comparison between holography and time-of-flight (ToF) imaging, it follows that although the ballistic photon filtering works significantly better in ToF, the increase in sensitivity due to the interferometric amplification effect results in holography outperforming ToF. In addition, we combine these results with previous experiments and show the importance of the advantages of digital holography for environmental perception through scattering media.

Holographic 3D imaging through random media has been attracting increasing interest. This paper reviews some of our recent efforts in developing functionally-integrated waveguide illuminators (FIWI). FIWI enables the compact system configuration that is free from strict alignments and bulky optical components required in conventional holographic microscopes, and thereby offers a new dedicated platform for digital holographic microscopy and imaging through random media.

Roughness is one of the most important surface quality parameters in metal sheet processing. Thus contact-free optical methods are highly interesting. Using the innovative approach of spectral speckle correlation (SSC) has the potential to measure the spatial roughness distribution of large surfaces quickly and thus inline. In SSC, speckle patterns are recorded at different wavelengths and correlated with each other to determine the roughness of the sample. We show that the relationship between roughness and correlation coefficient as a function of wavelength difference is valid over a large range. The roughness of larger surfaces can be determined by separate evaluation of sub-images. We present the importance of the sub-image size to get reliable, reproducible measurements relating to existing standards. In this work we show how to measure the roughness parameter Sa for values ranging from 0.59 μm to 7.75 μm with a spatial resolution below 1 mm by SSC. We demonstrate that the theory is valid over a large range of wavelength differences. This is shown using wavelength differences between 0.2 nm and 93 nm in the visible spectrum.

In this paper we report the application of a laser speckle odometer to a mobile industrial robot in a typical factory floor environment. The suitability of typical floor surfaces and features is assessed in terms of the ability to form speckle patterns with sufficient signal to noise for correlation-based processing. All tested surfaces including concrete, rubber tile, dried paint and oil stains, and hazard tapes were found to be suitable. A comparison of the velocimetry sensor output to the industrial robot’s internal SLAM and wheel encoder data is presented with good agreement of < 0.3mm/s at tested speeds of up to 250mm/s. Finally, a comparison of speckle odometry to the robot’s internal SLAM based navigation will be presented using a laser tracker to provide ground-truth measurement data. Both techniques were found to perform similarly, with errors of up to 80mm when traversing a 16m square path of 4m sides. The laser speckle odometry was however found to perform significantly better over the initial sides of the path with a maximum error of < 10mm in comparison to < 47mm for the robot’s internal odometry.

In this contribution a laser-based photometer detecting inelastic scattered light is presented capable of measuring the calorific value of natural gas and the H2 content in combination with an acoustic resonance analyzer to measure gas density. It is shown that the relevant parameters calorific value and gas density can be measured within several seconds with an uncertainty that is required for the gas turbine application using high hydrogen/ natural gas blends. A hydrogen-based economy shows us further applications for this exciting combination of gas sensing principles.

Dynamic speckle technique (DST) is based on speckle formation on the surface of objects illuminated with coherent light. Temporal speckle intensity fluctuations depend on the speed of micro-changes ongoing within the controlled objects. The DST visualizes as a set of 2D activity maps the temporal change of spatial speed distributions. In general, the DST set-up comprises components as a laser source with the required optics, vibration-isolated table, high-resolution camera and computer for data storage and image processing. Such setups are stationary, massive and relatively expensive, which decreases the number of possible DST biological or industrial applications. In this paper we propose a miniature portable device, based on a low-cost laser attached to a smartphone, and checked its efficiency under the field conditions. A strong argument for using a portable set-up is the fact that the absolute values of the speckle intensity are not needed to construct a reliable activity map. We proved this conclusion by numerical simulation of DST in noisy environment. We studied speckle patterns captured with a smartphone’s camera. A personal computer (PC) was used for postprocessing of speckle images. We conducted four sets of experiments. The raw data were recorded on the PC while the smartphone was connected as i) IP-camera and ii) USB-camera. In the third experiment, speckle images were captured and stored in the smartphone’s memory. Data were transferred to the PC after the end of recording. To obtain ground truth activity maps, we repeated the experiment with the same object under laboratory conditions.

This study presents a complete Mueller matrix imaging polarimeter which speeds up the pixel-wise recovery of the Mueller matrix of the scene. Our system allows us to acquire the Mueller matrix in a large field of view using white light in only 4 acquisitions. The camera is based on a division of aperture scheme that allows obtaining four sub-images on the sensor surface, decreasing the typical time of the measurement. The polarization states that analyse the input polarization are optimized to immunize the polarimeter to Gaussian and Poisson noise during the acquisitions. The results show the system can measure the complete Mueller matrix in broadband with errors lower than 10% at each component.

We developed a fast infrared sensor to quantify the film thickness of industrially applied SiO_x or AlO_x plasma coatings on non-flat polymer substrates at production speed. Its underlying physical principle is comparable to infrared reflection absorption spectroscopy (IRRAS) at selected, fixed wavelengths. The coating materials can be measured via their specific phononic absorption bands in the infrared spectral range. In this paper, we demonstrate the functionality of our sensor on actual packaging materials to showcase its aptitude for industrial inline inspection. Furthermore, we discuss a possible extension of our sensor to enable spectroscopic measurements using a tunable Fabry-Pérot filter.

Mid-infrared (mid-IR) hyperspectral imaging is of great practical interest in various fields of science and industry due to its ability to provide spatially resolved label-free chemical information. However, the widespread adoption and advancement of mid-IR hyperspectral imaging are hindered by high costs and the specification lag of mid-IR focal plane arrays. In this contribution, we propose a method to circumvent this problem and thus demonstrate a solution for mid-IR hyperspectral imaging through the use of a single-pixel imaging paradigm involving a mid-IR enhanced digital micro-mirror device (DMD); as a light source, a high-brightness mid-IR supercontinuum source is exploited. The developed mid-IR hyperspectral microscope enables the acquisition of hyperspectral cubes with dimensions of 64x64 pixels and a spectral resolution of 120 nm accomplished within approximately 450 ms. The developed laser-based system provides rapid mid-IR hyperspectral imaging with a programmable scalable field of view and variable spatial resolution. Thus, this solution represents a cost-effective alternative to state-of-the-art mid-infrared imagers.

Additive Manufacturing (AM) to produce parts of optical systems is gaining popularity due to design flexibility and functional integration. However, one of the significant challenges in the field of AM for optical systems is the limited manufacturing accuracy compared to traditional manufacturing methods. This causes an increase in manufacturing and assembly errors, which affects the fit and function of the produced parts and therefore the performance of the optical system. This work aims to reduce the negative impact of manufacturing and assembly errors on optical performance, as exemplified in practice by additively manufacturing housings and lens mounts for a Raman spectroscopy system. For this, the simulated optical system of a Raman spectrometer is used to perform the optical tolerance analysis, such as identifying critical components and sensitivity analysis. The Manufacturability of additive manufacturing is then evaluated by measuring printed standard specimens. According to optical and mechanical tolerance analysis, design and manufacturing problems can be found. Then the structure is optimized using the design flexibility provided by additive manufacturing, and accumulated errors from assembly and manufacturing are reduced by minimizing the number of installed components. Measuring the intensity of the Raman signal revealed that the improved design reduced the accumulated errors in the mechanical structure. In addition, the signal-to-noise ratio is significantly enhanced by 265 % compared to the non-optimized design. Hence, this design optimization based on tolerance analysis is an effective methodology for enhancing the performance of additively manufactured optical systems from the perspective of AM technology development in optical domain.

Fluorescence imaging is a powerful tool to detect the presence of processing agents, e.g., oils, lubricants, and organic coatings on metal surfaces. This imaging technique can be used in production, e.g. to determine the oil coverage on sheet metal to assure the best forming results or for inspecting the cleanliness of components before further processing like gluing, welding, coating, etc. The presented sensor is an evolution of the existing fluorescence laser scanner (F-Scanner) system1 and is optimized for measurement in motion. Adding this additional degree of freedom by mounting the scanner on a robot or gantry enables complete scans of large and complex shaped parts as well as selected areas that require special care, e.g., an increased level of cleanliness. The F-Scanner system has been improved in terms of footprint, weight, durability, and robustness. It can be operated with lightweight industrial robots and even cobots. In situations where ambient light makes fluorescence imaging challenging, a fast Fourier transform (FFT) based technique is used to ensure unperturbed measurements and high-contrast images. This work gives an overview of the advanced developments made and demonstrates the effect of FFT signal processing.

3D DIC is an optical noncoherent full-field optical technique that enables the measurement of shape, displacements (u,v,w) and strains (ε_xx, ε_yy, ε_xy) of a mechanical structure subjected to an external force. During investigations of large engineering structures (e.g., wind turbine blades), a single 3D DIC system is insufficient to deliver the information from the entire object. Hence multi 3D DIC systems should be used and the output information from all systems must be combined into a single coordinate system. This task is relatively simple if each system’s Field-of-Views (FoV) partially overlap. The task gets much more complicated if the FoVs of each system are separated. In such a case, an additional system defining reference coordinate system is required. In this work, we analyse the possibility of using a laser tracker system, an optical ranging method that provides a common reference coordinate system to FoVs distributed in space. Such an approach supporting multi-3D DIC measurements requires specific calibration targets and additional calibration procedures to deliver reliable and accurate results. The work presents the full calibration and data processing path and an experimentally estimated accuracy for the technique.

Measurement and characterization of small defects on large areas are time consuming but can be accelerated using high speed cameras and GPGPUs (general purpose graphic processing units). A height map of a filter glass with a 50 mm diameter scanned with a spatial sampling of 1 μm must present the data of more than 1000 single scans or 2.5 billion measuring points on a screen. Stitching processes of such data amount exceed the capacity of actual available metrology software packages even while operating on high end PCs. Resampling processes may result in a loss of information about surface roughness and small defects. Batch processing of the single scans is possible but make the interpretation of measuring results complicate and time consuming. Task specific data processing strategies overcome the given limitations. Disturbing residual stitching errors in overlapping zones are avoided and small scratches can still be observed in 3d maps of the complete sample. This simplifies the localization and provide information for preventive measures. Similar but adapted processing strategies are available to visualize local fluctuations of the surface textures. Test was done using the optical profiler smartWLI firebolt. The sensor operates on the measuring principal CSI - coherence scanning (white-light) interferometry and uses a 1.3 MP camera with 935 fps / full resolution. Data are processed in real time using a CUDA programmable NVIDIA graphic board which provides a calculation power of up to 82.6 FP32-TFLOPs. Thus, resulting in acquisition performance of up to approximately 1 single scan per second.

Optical triangulation metrology is an essential part of modern industrial quality assurance. Due to their robustness and cost-effectiveness Laser Light Section Sensors have become a widespread solution for geometry inspections. The configuration of a measurement system involves balancing the size of the measurement volume against the accuracy to be achieved. Therefore, in order to provide accurate measurements on a larger scale, it is required to combine several individual measurements of different configurations. This study thus investigates the identification of parameters of a focus-adjustable triangulation sensor. Adaptability of the working distance is achieved by automatic focusing of the camera and repositionability of the laser with a piezo rotation stage and a mirror. The movement of the projected laser plane has to be identified with regard to the tilting angle. This ensures accurate calibration of the measuring system while the working distance being adjustable. Two general approaches are suitable for solving this task: One is based on a simplified identification of the rotational axis of the tilted laser plane. However, this does not take into account the deviations resulting from the offset of the reflection axis on the mirror surface and the rotational axis. This paper extends conventional models by deriving the position of the projected plane from the complete set of all rigid body transformations with regard to the rotation angles. The position of the laser source, the rotation axis and the mirror surface are described in the camera coordinate system. The validity of the extended model is assessed in comparison with a simplified model. Furthermore, the influence of the focusable camera on the calibration of the laser rotation axis is further investigated.

We present digital multi-wavelength holography measurements generated inside a machine tool. The digital holographic sensor HoloTop NX used in the application has a field of view of 12.5 mm × 12.5 mm, each field of view consists of 3008 px × 3008 px. Three separately stabilized diode lasers connected to a fiber switch serve as light sources. The unambiguous measurement range of the holographic sensor defined by the laser wavelengths is approximately 140 μm and the axial reproducibility is ~0.4 μm. The sensor is mounted to a standard 5-axis machine tool Hermle C 32U. An automatically created numerical control (NC) program enables the machine tool to meander the sensor over larger test objects, here 280 mm × 94 mm. For each field of view, a height map with 9 million data points is generated using digital holography. The images are acquired with 2 mm lateral overlap and are then stitched together to achieve a height map of the complete sample with more than 1.4 billion data points. For comparison the shape of the sample was measured with a state-of-the-art coordinate measurement machine (CMM). The datasets from the CMM and the holographic measurement are semi-automatically referenced to each other. The holographic height data are averaged over a rectangular area with radius 50 μm around each of the 360 measurement locations where the tactile CMM measurement data have been acquired. This allows us to evaluate deviations between the holographic and the CMM dataset. It turns out that the root mean square (RMS) error between the two data sets is smaller than 0.4 μm.

Dimensional inspection during manufacturing of high-temperature forging workpieces is still an unsolved research topic. In this work, an approach to reconstruct the hot forging surface using passive stereovision was proposed. The challenges are the irregular thermal radiation of the forging in the visible spectrum, which is affected by scales, heat haze, and an unstable environment. Unlike the approaches presented in the literature, we use the radiated light, without an external light source. High dynamic resolution images were used to image the forging surface with highly varying brightness. The effect of optical filtering on the contrast of the fine surface structure was investigated and evaluated by spectral analysis of the images, with the best results obtained in the green spectrum. A comparison of feature detectors was performed for the specific surface structure of the hot forging. The best results in terms of density of reconstructed points were obtained using the KAZE method. The final verification of the achievable accuracy was performed in a measurement volume of approx. 500×400×350 mm. An 877 °C hot steel pipe sample was measured with a single point standard deviation error of 0.28 mm, which is an order of magnitude higher than under ideal conditions, but acceptable in comparison with manufacturing tolerances. Further developments could be related to the application of dense image matching, further investigation of interfering effects, and application of the proposed methods in the industrial environment.

Polygon mirror (PM)-based laser scanners are the fastest opto-mechanical scanning system. One of their most common applications is in optical micrometers for industrial applications. The aim of the present work is to study experimentally such a system from the point of view of the non-linearity of their scanning function. In this respect, we built upon the developed opto-mechanical analysis and design of PM scanning heads. Thus, testing measurements are performed regarding both the dimension and the position of the object to be measured, which is positioned between the two lenses (i.e., achromatic doublets) of a telescope positioned after the PM and preceding the system’s photodetector (PD). The measuring method is discussed, and its errors are pointed out in obtaining the object’s dimension on the direction of the scanning velocity (of the beam that sweeps the space between the telescope’s lenses perpendicular on their optical axis), using the time interval when the PD receives no laser signal because of the obscuration by the object to be measured. The energy received by the PD with the rotation of the PM is measured for objects of different diameters placed between the lenses of the telescope at different positions with regard the optical axis of the lenses.

The performance of defect detection in composite materials using digital shearography is important for correct decisionmaking in non-destructive testing. In this work, we compared a high-resolution 24-megapixel digital still camera (DSLR) and a conventional medium-resolution 5-megapixel camera to determine the detectability of blind holes in an aerospacegraded carbon-fiber reinforced polymer (CFRP) sample. The hole diameters ranged from 0.2 to 3 mm with a material thickness of 4 mm and the test sample dimensions of 200×200 mm. The sample was heated and observed from the front (defect-free side) by three halogen lamps for 5 minutes in pulsed heating mode. Speckle interferograms were acquired during the heating and cooling phases from both cameras simultaneously using identical shearing interferometers and shearing distances. Phase maps were calculated using the 4+4 temporal phase step algorithm and then unwrapped. Further, defect-induced deformation (DID) phase maps were obtained by polynomial curve fitting. The DID phase maps obtained from the two cameras were compared. Blind holes with diameters up to 1 mm were detected, which are one of the smallest defects detected with shearography and reported in literature. In addition, the DLSR camera was able to detect holes of 0.8 mm in diameter. We observed that nearly comparable detection capabilities were obtained from both cameras, even though the spatial resolution of the second camera (DLSR) was 5 times higher. Possible reasons of this limitation include effects such as fiber-related deformation in CFRP and speckle noise.

At present, the medium power calibration system at the National Institute of Metrology (Thailand), which has a CO₂ laser as a laser source, can be used to calibrate customers’ laser power sensors from 100 mW-10 W. This power range of CO₂ laser offers various applications, such as cutting of wood, paper, fabric, rubber, metals, and ceramics. CO₂ lasers are also used widely as medical lasers for surgeries and cosmetic purposes such as removing mold and facial resurfacing. Originally, the calibration system of the CO₂ laser has a measurement uncertainty of 4.0 % at a 95 % confidence level. This uncertainty level is considered high for medium laser measurement and sensors’ accuracy. One of the reasons is the high power linearity error of the detector, which is taken from manufacturing specifications. Other reasons are overestimation of the uncertainty of the non-uniformity of the detector, and the large uncertainty from the calibration certificate of the standard. Several efforts have been made to reduce the total uncertainty of the system. First, by reducing the uniformity and positioning error of the standard detector and unit under the calibration detector, and second by reducing the power linearity error of the monitor detector of the calibration system. This paper discusses how to experimentally determine the uniformity and positioning error of the detector and the power linearity error of the detector. The new uncertainty of the system is then determined and adjusted from 4.0 % to 1.1 %.

Photoelasticity is a non-destructive optical testing technique that focuses on stress analysis. Traditional methods of demodulating stress fields are limited by various conditions, such as the image acquisition set, material properties, load values, light sources and isoclinics. As an alternative, deep convolutional neural networks (DCNNs) have been used to recover stress fields in automated and predictive methods. In this study, different DCNNs architectures are trained by means of two datasets, each one with 45000 images. First dataset has images with four polarization states (0°, 45°, 90° and 135°). Second dataset has images with 3-channel, each one corresponding to a Stokes parameter (s0, s1, s2). The quality of predicted images is evaluated with quality metrics such as MSE, SSIM, and PSNR. MSE and Adam are used as loss function and optimizer, respectively. Results show that on average, the use of DCNNs with images with four polarization states achieve better quality metrics than DCNNs with Stokes images. These results indicate that it is possible to obtain real-time stress fields using different representations of polarized images in deep networks and opens new opportunities for representing polarized images in deep learning models and extending its applications of stress analysis.

Critical dimension (CD) uniformity has recently become increasing essential with miniaturization in semiconductor production. For high-volume manufacturing, high-speed in-wafer uniformity inspection is inevitable. The conventional optical critical dimension (OCD) spectroscopy has low resolution issue because of the large spot size. Consequently, line-scan hyperspectral imaging (LSHI) is presented by Samsung Electronics in 2022. Compared with the OCD, the LSHI has advantages of high throughput, high spatial and spectral resolution, and large field of view. The LSHI can improve the throughput of OCD significantly and provide high-resolution spatial and spectral information. In this study, the optical design of the refractive imaging spectrometer based on the ruled reflection grating is presented. Most imaging spectrometers, such as Czerny-Turner or Ebert-Fastie setup, are off-axis reflective configuration. However, the performance is limited by serious astigmatism and smaller numerical aperture. The astigmatism leads to low spatial and spectral resolution, and the smaller numerical aperture requires more exposure time, which results in low throughput. Therefore, the refractive type using lens combinations for imaging spectrometers can eliminate aberrations effectively and increase numerical aperture significantly. In this study, the numerical aperture is designed as f/2.4, and the slit length is 14 mm for high throughput. The high spatial and spectral resolution is accomplished by optimizing lens combinations from 400 nm to 800 nm spectral region.

In this work, a specific procedure to measure the thickness of aluminum thin films by interferogram analysis is described. Six interferograms correspond to the image of the aluminum thin film superimposed with interference fringes are obtained from a Michelson-type interferential microscope. A six-frame phase shift algorithm is used to demodulate the optical phase. The measured phase is proportional to the height variation between the thin film wafer and the substrate. Finally, the obtained 3D height map permits us measure the thickness of the sample over an area of 1024 μm × 1280 μm.

Tunnel inspections require the detection of deformations in the tunnel geometry, cracks, delamination, and water inflow. Solutions for an automated detection of deformations, cracks and water inflow already exist and typically comprise mobile laser-scanners and cameras combined with deep learning methods. Delaminations on the other hand are often not visible on the surface and can’t be detected using these methods. The detection of delamination in tunnel linings is therefore up to date performed by manual hammering and acoustic detection. The results are time consuming and labor-intensive inspections, subjective measurements, poor comparisons over epochs and a low degree of digitization. We present a concept of a novel system that aims to replace the manual hammering for acoustic delamination detection using a remote sensing approach. A strong, pulsed laser serves as a hammer and creates a plasma induced shockwave on the concrete surface. If a delamination is present this shockwave excites characteristic, resonant vibrations. A second, narrow-linewidth laser is employed in a customized laser doppler vibrometer setup to remotely detect these vibrations via a coherent measurement technique. In combination with laser scanners and cameras, the laser based remote sensing technique has the potential to help automating the process of tunnel inspections by delivering objective data that can be used in deep learning-based evaluation methods and for building information modeling (BIM) compliant assessment. A first mobile prototype for measurements outside the lab has been developed and is being presented in detail.

In the automotive industry it is necessary to automatically inspect Ball Grid Arrays (BGAs) components. As BGAs are surface-mounted devices that are soldered on a Printed Circuit Board (PCB), they have soldered balls fixed on their bottom. In order for such a device to work properly, it is important that these balls do not create „bridges” between them when they are soldered to the PCB. In order to verify this aspect in the industry, X-ray imaging is utilized for the inspection of BGA components. The aim of this work is to assess the capability of Optical Coherence Tomography (OCT) to perform the inspection of solder bridges, as well as to try to image the integrated circuits located inside a BGA component. A second aim is to compare the performances of OCT and X-ray imaging for such purposes. We utilized an industrial X-ray system and an in-house developed SS-OCT system working at 1310 nm, the latter characterized by an axial resolution of 15 μm. Five BGA components were inspected and the obtained images were utilized to perform the proposed comparison. While using OCT one cannot penetrate the upper layers of the BGAs, proof-of-concept 3D OCT images were obtained from the bottom of the investigated components. Therefore, X-ray imaging remains to be utilized for the inspection of components after they are soldered on the PCB, whereas OCT is only able to inspect smaller unsoldered components. However, OCT proves to be a viable method to cover the existing gap in the process because it can inspect the surface of the solder balls before entering the production area. While the industrial X-ray imaging used for this research is not able to inspect individual BGA components, OCT can perform this task. Therefore, using OCT could prevent supplying defect components to be soldered on the PCB.

A multi-layered structure is being extensively applied in the high-tech devices fabrication in the semiconductor and display industries. For measuring the thin-film thicknesses of the multi-layered structure, various techniques like spectral reflectometry, spectral ellipsometry, and SEM/TEM have been used depending on the application fields. Among them, the spectral reflectometry is being widely used because of the advantages of simple configuration, non-destructive characteristics, and high-speed measurement. In spectral reflectometry, the reliability of the reflectance model is very important, because the higher the agreement of the modeled reflectance to the measured one, the lower the measurement uncertainty of the thin-film thicknesses determined by the reflectance model. In case of the single-layer thin-film sample, the thickness can be verified using a certified reference material, but the multi-layer thicknesses are not easy to be verified unlike single-layer case. In this study, to check the reliability of the multi-layer reflectance model, two different methods were used; (1) the extension of the single-layer model and (2) the multi-layer model based on the transfer matrix. The first one is to sequentially determine the thin-film thicknesses from layer to layer. The second one is to simultaneously determine all the thin-film thicknesses of a multi-layer structure. By applying two methods to double-layered thin-film sample(SiO₂-SiN), the thin-film thicknesses of both layers were determined and compared to each other by considering the measurement uncertainty. The applicability of the theoretical reflectance model can be confirmed according to whether the thin-film thickness measurement results are agreed within the uncertainty.

We study efficiency of intensity-based dynamic speckle method for characterization of dynamic events which occur at variable rate in time within the temporal averaging interval. We checked the ability of the method to describe the speed of evolution by i) numerical simulation at variable speed, ii) processing of speckle patterns obtained from phase distributions fed to a SLM at controllable change of the temporal correlation radius of speckle intensity fluctuations and iii) conducting experiments with a polymer solution drying by using a hot-stage. The numerical and SLM simulation experiments allowed for modification of the used estimates in order to obtain relevant information.

We propose a lens assessment method based on incoherent holography, offering a novel alternative to conventional optical testing methods that rely on coherent illumination and interferometry. The traditional approach involves the complicated optical configurations and it is hard to accurately model the actual user experience. In contrast, our proposed incoherent holographic system, utilizing self-interference with a geometric phase lens, enables the acquisition of the complex hologram of incoherent illumination in a single exposure. By reconstructing the hologram captured from structured light patterns, we can calculate the wavefront aberration and displacement, providing a comprehensive lens assessment solution.

The adaptive Shack-Hartmann wavefront sensor (SHWFS), which integrates deformable mirror (DM) with SHWFS, is effectively used for measuring and characterizing optical elements such as thick lenses, contact lenses, and intraocular lenses (IOLs). The major advantage of combining the DM with SHWFS is the ability of the DM to implement dynamic corrections of any existing wavefront aberrations. However, the measurement dynamic range of the system is limited by the active area of the DM which has a relatively small bandwidth product. This issue can be physically overcome using compound-lens methodology that uses an additional lens or lens system along with the lens under test. The selection criterion of the compound-lens system is based on the focal length of the tested lens, the effective focal length of the tested lens with the compound lens, and the distance between them. Thus, the optimal selection of the compound-lens system and the setup parameters significantly contribute to readjusting the laser beam diameter to properly fit the DM’s active area. Hence, the measurement dynamic range could be successfully extended. To prove the concept, the dynamic range of the used adaptive SHWFS system is first determined. Then, a group of lenses beyond the dynamic range of the system was measured using the compound-lens methodology. The experimental results reveal that the proposed compound-lens methodology can effectively extend the dynamic range of the measurements while keeping the root-mean-square error minimal.

A large number of drugs are sensitive to light and therefore their formulated products may degrade during manufacturing, storage, and administration. This may result in potency loss, altered efficacy, and adverse biological effects. The use of the appropriate containers and packaging material can protect the products from the deleterious effects of light and UV radiation. The sensitivity of a drug to a particular spectral region of light may vary with its chemical structure, photoreactivity, and nature of the dosage form. The rate of a photochemical reaction may be influenced by the intensity/wavelengths of the radiation source and the transmission characteristics of the container, which is a critical issue for pharmaceutical and food industries and its related inspection and sorting activities. This makes the accurate measurements of spectral transmittance of great interest for many industries and vital activities in the medical, pharmaceutical and food sectors. This proposed new instrumentation scheme, including the associated corrections, can be applied for both the spectrally flat samples as well as the spectrally selective samples. Both sets of measurements, for both types of samples, presented in this work with the corresponding uncertainty level, which is spectrally and sample-dependent as shown. The used instrument, and related instrumentation scheme including the detailed equipment description are presented. To demonstrate the new concept, we measured the spectral transmittance of highly opaque samples, in the spectral range from 300 nm to 1000 nm, with and without optical attenuators to show the level of accuracy, repeatability and reproducibility. In this approach, to achieve better signal-to-noise when the sample transmittance approaches 10^-6, additional attenuators can be used in the reference beam path. Measurements of some highly opaque samples has been shown, error sources have been investigated and corrected, with associated uncertainty been lowered. Uncertainty budget presented to show the uncertainty level of these measurements of extremely opaque samples. Level of uncertainty associated with the measurement of low transmittance level (< 0.01) at representative wavelength of “550” nm is evaluated to be “0.80%” (k=2).

The current method for rework inspection of previously defective surface areas on aircraft landing gear components involves manual inspection. Measuring tools such as micrometers and gauges are utilized to obtain positions and dimensions of reworks. This information is required for determining whether re-entry of the component into service is possible. Overall, the manual process is time-consuming, highly dependent on the skill and experience of the inspector, and prone to errors. This paper presents a novel approach to inspect reworks on aircraft landing gear components using a robotic inspection system based on white light interferometry (WLI). The proposed method is aimed at improving accuracy, repeatability as well as efficiency of rework inspection. The robotic system handles the WLI and positions it over the component, allowing for detailed 3D measurements of the surface and reworked area. Considering the typical measuring spot size of a WLI, the overall positioning accuracy of industrial robots is crucial. Measures to address this problem as well as general limitations and the potential of the system for the use case are discussed. An exemplary rework inspection validates the applicability and demonstrates the potential of this approach. Future research and optimizations are discussed which could lead to more widespread adoption of this technology and further advancements in the maintenance of aircraft landing gears.

The maximum slope that a microscope objective can measure is an important parameter characterizing the measurement capability of 3D optical microscopes. It is one of the most important criteria for selection of appropriate optical topography measuring instruments for areal surface texture measurements and the setting as well. In this article, a method is proposed using optically smooth spheres for characterization of the maximum measurable slope by optical topography measuring instruments with different objectives. The material measure and the measurement procedure are described and the method for the calculation of the measured sphere radius, the maximum measurable local slopes and characterization of the homogeneity of the slope transfer function within the FOV of the objective measured by a confocal microscope are presented.

Detection of nanoparticles gets more and more relevant in various fields of application, e.g., semiconductor technology, medicine or biology. A fast and universal detection method of nanoparticles is essential for these leading-edge technologies. Classical imaging methods struggle with the resolution of nanoparticles smaller than the diffraction limit. Established optical near-field methods are complex and time consuming. We demonstrate that using Coherent Fourier Scatterometry can overcome most of these challenges. For this purpose, we study the optical response of gold nanosphere arrangements on silicon wafers. In particular, we examine arrays of particles arranged in geometrical structures such as lines, squares, triangles, and L-shapes. The dimensions of these structures are in the order or even smaller than the Airy diameter. Correlations between the geometry of the particle arrangement and the intensity distributions in the Fourier plane are characterized by applying different analyzing methods. Thus, it is possible to distinguish between different structures and, furthermore, to extract their geometrical orientations. In addition, gold nanostructures of defined shape, but different sample thicknesses top-down fabricated on silicon are also investigated. These nanostructure shapes studied comprise the same geometries as mentioned above as well as spirals with a diameter comparable to the spot size of the used microscope objective.

Both, optical tweezers and Raman spectroscopy are well-established methods in biomedicine. In combination they allow spectroscopic studies on single isolated micro-components. The aim of this work is to investigate micro-objects for noncontact micromanipulation with simultaneous acquisition of the Raman spectrum using a Photonic Jet (PJ) fiber optic. This optic consists of a fiber with a curved fiber-tip through which a low-divergence and highly concentrated Laser beam is achieved that can be focused below the diffraction limit. Based on this, a measurement setup has been designed and implemented which allows the excitation of Raman scattering of selected substances via the fiber optic. These samples could be analyzed and confirmed via the backscatter signal of the fiber in the wavenumber range of the detected Raman spectrum of 1200 cm^-1 – 4000 cm^-1. As for micromanipulation, particles with a size of 3 μm could be shifted transversely in the direction of the propagating Laser radiation and to a small extent (μm regime) laterally to the beam propagation, which corresponds to the behavior of a two-dimensional optical trap. The realized measurement setup provides the basis for extended investigations of combined Raman spectroscopy and manipulation of micro-objects.

Rubber, particularly carbon black-filled rubber used in tires, plays a critical role in a world of mobility. Tires are exposed to various influences, including temperature fluctuations, dynamic mechanical loads, and exposure to solar-origin UV radiation, which can affect their durability. To investigate the effects of UV radiation on tire aging, this study aimed to identify spectral absorption ranges in specifically aged tire samples providing information about their aging condition. The study utilized samples of different rubber compounds and exposed them to UV radiation for varying durations from 48 h up to 1000 h. ATR (attenuated total reflection) spectroscopy was used to analyze the aged samples and evaluate changes in their chemical structure. The results showed consistent aging behavior across all samples, with a decrease in the intensity of the double peak at approximately 2900 cm^-1, corresponding to the C-H bond. Another aging-relevant range from 1500 cm^-1 to 1450 cm^-1, characteristic of C-H groups as well, exhibited a declining absorption trend with UV aging. The impacts on the absorption spectrum resulting from the different rubber mixtures' compositions led to a reduction in the intensity of characteristic peaks. For evaluating the aging state of used tires, the CH related peaks can be utilized.

A phase-only spatial light modulator (SLM) is a device that is commonly used in various optical applications. Generally, SLM offers great advantages such as low power consumption and compact design. However, due to the manufacturing process, the main drawbacks of the SLM are surface non-uniformity and cross-talk between adjacent pixels, which add undesirable phase modulation. As a result, the SLM’s functionality is impacted, leading to image quality degradation, in terms of the signal-to-noise ratio (SNR), of optical reconstruction in holographic projection, for instance. Therefore, the aim of the current study is to measure and compensate for the surface non-uniformities of the SLM and improve its phase modulation. To achieve this, Shack-Hartmann wavefront sensor (SHWFS) is utilized. At first, a flat constant phase pattern is displayed on the SLM, and its surface phase shape is measured using a plane wave illumination. The reflected wavefront from the SLM is measured using SHWFS and then its phase information has been calculated. Hence, the calculated phase values are converted into a phase-only computer-generated hologram (CGH). The calculated CGH is displayed on the phase-only SLM to compensate for the phase errors of the SLM. The reflected wavefront has been measured after displaying the CGH to evaluate the compensation process. The experimental results reveal that the SHWFS provides high accuracy in the measurement of the phase distortion introduced by the surface of SLM. The SHWFS method is simple, robust, offers real-time performance, and is vibration-insensitive when compared with interferometric approaches.

In this work, we will show the procedure to design a null screen to evaluate a convex surface without symmetry of revolution, which is of great interest in developing optical devices. The process to obtain the shape of the surface without symmetry of revolution from images reflected from the null screen by the surface under test will be presented. In addition, we will present the experimental results of the test applied to a free-form surfaces. We will show the advantages of the method compared to others and its drawbacks.

Due to the advancements in the field of optical metrology, it has found its applications in various areas such as biomedical, automotive, semiconductors, aerospace, etc. The popularity of optical techniques for metrology has increased by multiple folds owing to its non-invasive nature with ease of setup, fast data acquisition, and remote sensing ability. Optical techniques include hologram interferometry, speckle photography, speckle interferometry, Moire interferometry, photoelasticity, fringe projection technique, etc. The holographic interferometry technique works by quantifying the optical phase of the object by measuring the change in the interference fringes due to the shape of an object. This technique has a large number of advantages, but a steep object leads to a large number of fringes in the field of view, which are not resolvable as they fail to satisfy the Nyquist criterion. In this work, the fringe projection technique, which is a non-interferometric, non-invasive technique for generating 3D surface information is employed to measure the shape of a phase object like a wedge. Fringe projection is presented as a robust and compact technique for shape measurement of phase objects as it utilizes lesser components and has less complexity compared to the holographic technique.

Active infrared thermography (IRT) has been used to detect delamination on yttria-stabilized zirconia thermal barrier coatings (YSZ-TBC) using various heat sources. Thermal shock tests were performed to induce delamination on 1x1 inch flat samples. Each thermal cycle consisted of heating the samples in a muffle furnace, holding at 1020°C for 5 minutes, and quenching on water at approximately 20°C. Two samples were used, the first was brought to failure (184 cycles) and the second was almost intact to the naked eye (100 cycles). In addition to these samples, a discarded gas turbine blade was cut, and a delaminated fragment was used for comparison. IRT was performed using two 600 W flash lamps, one 500 W halogen lamp, and a long-wave infrared camera with a frame rate of 30 Hz. The acquired signal was first processed by the Thermal Signal Reconstruction (TSR) method using a 4th order polynomial interpolation and its first and second time derivatives. The thermographic data were processed using the Principal Component Analysis (PCA) algorithm. PCA was applied to both raw and pre-processed data. The results were compared using signal-to-noise ratio (SNR) to measure the quality of the algorithms used to detect delaminated areas. The comparisons showed that PCA results are improved when applied to the TSR sequences. However, the selection of the principal component with the highest SNR depends on the heating condition (flash or long pulse) and on the samples analyzed.

The need for rapid, high resolution, accurate metrology of mass produced aspheric and freeform surfaces continues to grow as the application space evolves. Expanding the use cases of standard interferometers, computer generated holograms enable snapshot, megapixel resolution measurements of freeform surfaces that may have large and non-symmetric departure from spherical. CGH metrology at production scales can be realized without specialized engineers or in-house knowledge of setup. In this paper, we highlight the impact of the CGH vendor to provide not only a hologram but a suite of hardware to streamline high volume aspheric and freeform metrology. Alignment becomes straightforward, data acquisition remains standard to the interferometer and data is processed automatically. Early integration of this metrology solution in product planning helps users scale from prototype to manufacture.

Active thermography is a non-destructive testing (NDT) technique with great potential for evaluating the integrity, thickness, homogeneity, and structure of thermal barrier coatings (TBCs), which are widely used in the hot gas path of thermoelectric power plants. In this study, active thermography was used as a non-destructive analysis technique to measure the thickness of the YSZ ceramic layer in thermal barrier coatings subjected to a chemical removal process as a function of exposure time to the agent. The TBC samples analyzed consisted of ZrO2-7 wt% Y2O3 atmospheric plasma sprayed with NiCoCrAlY bond coat in nickel-based superalloy. The ablation process consisted of complete immersion of the sample in an aqueous solution of ammonium bifluoride (0 min, 90 min, 240 min, 330 and 420 min), maintaining a temperature of about 60 °C to about 68 °C and subjecting it to an ultrasonic bath. Thermographic evaluation was performed at room temperature using two 600 W flash lamps, one 500 W halogen lamp, and a long-wave infrared camera with an acquisition rate of 30 Hz. Samples were taken for microstructural analysis and thickness measurement using SEM microscopy and eddy currents. Thermographic information was processed using thermal signal reconstruction algorithms and correlated with microstructural characteristics and thickness as a function of immersion time. The results show differences in both the heating and cooling curves of the samples, which can be correlated to differences in the thickness of the coatings. In this study, active thermography was used as a non-destructive analytical technique to measure the thickness of the YSZ ceramic layer in thermal barrier coatings subjected to chemical removal. ZrO2-7 wt% Y2O3 coatings were immersed in an aqueous solution of ammonium bifluoride. Thermographic information was acquired using a 30 Hz infrared camera and two 600 W flash lamps. The results show differences in their heating and cooling curves, which allow them to be correlated with the coating thickness measured by SEM microscopy and eddy currents.

We have developed an interferometric optical microscope (IOM) in phase shift mode (PSM) compatible with standard liquid cells and which uses non-immersive, standard and commercially available Mirau objectives. A 15mW He-Ne laser is used for the illumination and the induced speckle pattern in canceled with a rotating diffusing glass. The interferograms displaying the interference fringes are shown in air and in water for various illumination conditions. We show a detailed comparison of the topography obtained in air and in water in HOPG with our liquid IOM (L-IOM). The noise level is very similar in air and in water and is below 1nm.

The shrinking depth of focus of high numerical aperture immersion microlithography optics requires a tight wafer flatness budget. Bare wafer surface topography variation is a significant part of the focus budget for microlithography. Thus, as the wafer surface quality becomes increasingly important, the metrology to control the surface quality is increasingly challenged1. Advanced lithographic patterning processes require a detailed map of the free, non-gravitational, wafer shape, to avoid overlay errors caused by depth-of-focus issues2. The semiconductor industry has been using interferometry-based techniques for measuring the free form wafer shape of blank silicon wafers for several years1. In this paper we introduce a new measurement technique, Wave Front Phase Imaging (WFPI), that can measure the free form wafer shape of a silicon wafer by acquiring only the intensity of the reflected light. In the WFPI system, the wafer is held vertically to avoid the effects of gravity during measurements. The wave front phase is then measured by acquiring the 2-dimensional intensity distribution of the reflected, non-coherent, light at two or more distances along the optical path. This method allows for very high data acquisition speed and a high number of data points equal to the number of pixels available in the CMOS imaging sensor used. In the measurements presented in this paper, we acquired 16.3 million data points on the full 300mm blank silicon wafer, generating a lateral resolution of about 65μm per pixel. Blank silicon wafer manufacturers need to acquire such metrics as bow, warp, and flatness among other parameters, to provide with the silicon wafer when sent to the device maker fabs. These metrics are easily derived by generating a free form wafer shape map of both the front and the back surfaces.

In this work, a hologram optical element is proposed for optical-electronic angle measuring devices and sighting systems, which is a combination of four three-dimensional diffraction gratings registered in the volume of a photosensitive medium. The method of measuring the angular displacement and sighting of objects is based on the properties of changing the intensity of the information light signal of volumetric hologram optical elements, in accordance with the modulating effect of the object under study. The sensitivity of the proposed device for the measurement range of ±150 ang. min was 5 ang.s. In sighting mode with a range of 3 ang. min. the sensitivity was 0.1 ang. s.

In this paper we report the results related to an integrated approach for the working test of a titanium (Ti) panel as a component for a multi-mission modular satellite platform. The multifunctional panel here characterized has been made by additive manufacturing. The panel has been conceived integrating its mechanical properties for the structural support, electronic cabling, and for loading all the components and payloads of the satellite platform, with the innovative property of heat dissipation. In fact, this prototype module comprises internal heat pipes; The management of the heating in excess is still a demanding for the implementation of advanced modular and multi-mission satellite platform, in 50-kg microsatellite class, with possible housing of multiple interoperable payloads. In this framework, here we focus on the study and development of technologies and characterization of advanced smart structures through the use of innovative materials. The profile and section of the micro-channels have been designed to favor the liquid evaporation under heating in microgravity condition and thus for different space applications. In the following the characterization and test on the earth is illustrated and discussed for the prototype. We believe that functional panels with these novel characteristics would find application for energy waste recovery also in case of electronic dissipation for module dedicated to space exploration.

Microlenses are of technological importance for a wide range of industrial applications enabling in a single element the demand for small form factors, high resolution, and cost effectiveness. The micro-optical elements composed of a series of lens, with diameters ranging from several micrometers to several millimeters have been successfully employed in imaging, sensing, photovoltaics, solar concentrators and optical communications applications. In the last decades, microlenses have been manufactured using multi-step processes such as photolithography, resist-melting, soft-lithography and hot-embossing. More recently, simple and efficient manufacturing methods have emerged in the fabrication of microlenses, which allow the use of different materials and give greater freedom with regards to the choice of substrates and array patterns, like Drop-on-Demand (DoD) inkjet printing. Here we discuss about the dispensing of liquid crystal by pyro-electrohydrodynamic (pyro-EHD) printing. The pyro-electric effect is here used for the non-contact manipulation of liquid solution in air and at room temperature. The results obtained show the great feasibility in the dispensing process in terms of materials, geometry and optical properties.

The paper proposes the conceptual foundations of robotization of serial engineering equipment designed for emergency rescue and other urgent work. The relevance of this approach to extreme robotics is caused by the frequent failure of prototypes of robotic tools and the lack of sufficient funds for their repair, which ultimately does not allow the full use of the existing fleet of automatic devices for fire and rescue and other urgent work. The system of a mobile remotely controlled complex for robotizing serial equipment intended for emergency rescue and other critical work is considered. The developed system is based on optical methods of control and recognition of objects.

Automated inspections and intelligent image processing optimize quality control processes. Images captured in the industrial inspection system have low contrast and faint color. Many enhancement algorithms have recently been proposed to enhance visibility and restore color. We present a new image enhancement algorithm based on multi-scale block-rooting processing. The proposed method based on the frequency-domain coefficient correction of a set of images followed by their fusion based on the Laplacian pyramid. A new stage is presented in obtaining a local-global estimate of high-contrast images, also used in the general fusion model. The main idea is that enhancing the contrast of an image would create more high-frequency content in the enhanced image than in the original image. The experiment results on the test dataset confirmed the high efficiency of the proposed enhancement method compared to the state-of-the-art techniques for industrial inspection systems.

This research presents a novel polarization interferometer using a polarization camera for accurate and reliable wafer surface profile measurement. The proposed technique employs Fizeau-type polarization interferometry to acquire polarization interference patterns, from which four phase-quadrature interference patterns are obtained. By applying a phase-shifting algorithm, the wafer surface profile can be determined. Experimental testing demonstrates the effectiveness of the proposed system, offering precise and accurate results. Compared to existing techniques, it offers advantages such as high measurement resolution, fast measurement speed, and full-field capability. These features make it a promising solution for improving quality control and manufacturing efficiency in the semiconductor industry, leading to enhanced product performance and increased competitiveness.

Environmental disturbance caused by mechanical vibration, air turbulence and thermal stratification makes a great impact on the accuracy of absolute flatness measurement, especially for the optical elements with the aperture larger than 600mm. Simplification of the measurement procedure is crucial for ensuring high precision and repeatability measurements. In this paper, the absolute flatness measurement approach combing the shift-rotation method and the second derivative method in polar coordinates is proposed to improve the stability and error tolerance of absolute measurement. The second derivative of the azimuth angle is achieved by rotating the tested flat, while that of the radial direction is achieved by translating high-order Zernike fitting. The absolute surface shape can be calculated simply by integration. The experiment procedure is simplified to two rotations and two translations with two involved mirrors. Furthermore, expanding the wavefront error to second-order can reduce the influence of environmental disturbance and make the measurement more robust. This study may provide technical support on the absolute flatness measurement for large aperture.