Optical Measurement Systems for Industrial Inspection XIII

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 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.

In order to measure the thickness of the thin film, a spectral reflectometer was installed with a white-light source emitting from 355 nm to 657 nm in wavelength. To analyze the interference spectrum obtained through this, an artificial neural network algorithm with several different conditions such as number of hidden layers and nodes were designed and trained instead of the conventional model-based analysis. For quantitative analysis, a certified reference material with a nominal thin film thickness of 100 nm was measured, and an uncertainty analysis was performed on the thickness measurement value determined through this.

To overcome the challenges in measuring deep high-aspect-ratio (HAR) and thin-film structures in 3D integrated circuits, this article proposes 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 a few nanometers to a few hundred micrometers, 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 AM technology. One crucial component of the manufacturing process is a perfect flat and homogeneous powder bed. Hence, the powder bed for each new layer should be inspected for anomalies. We present a specialized machine vision system which is capable to detect and classify all powder bed anomalies. The main advantage of our approach is, that we acquire four images, each illuminated with an individual angle of incidence. 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.

Laser drilling is one of key applications for material processing of glass, semiconductor, and metal. Sound waves are generated at a focusing point of the laser beam on a target material. The excited sound waves are observed with a microphone. The axial position of the objective lens was controlled by the acquired excited sound, and the results of hole drilling according to changes in surface shape caused by ablation during processing are reported. The processing is also observed by an optical imaging and the optimization of the laser processing is estimated from both the optical and acoustic methods.

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 cross-talk 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.

The use of small robotics in manufacturing is desirable to allow increased automation, however such robots do not use complex high resolution angle encoders and suffer from limited angular resolution, gear backlash/wear limiting the positioning performance and safety can compromised through dirt ingress. A robust, high-resolution alternative in the form of an angle sensing fibre or tape to measure the actual pose is one possible solution. A single-angle fixed-plane sensing tape could be retrofitted to industrial robot arms and even be used in harsh environments, such as for the automation of construction equipment such as excavators providing enhanced angle feedback. A full shape sensing fibre (pitch, roll and yaw) could be used where traditional rotary encoders would not be easily applied such as in snake like continuum robots [1]. Here we present a dual-fibre single-plane angle-sensing tape using Fibre segment interferometry (FSI) [2] to measure the differential strain between the two segments of fibre located on either side of a metal substrate. The angle between the two ends of the segments in the plane of the fibre-pair, can then be determined from this differential phase assuming a fixed lateral separation of the fibres [3]. The use of a metal substrate ensures a fixed lateral separation of the fibres and provides the necessary resistance to out-of-plane twisting of the fibre pair needed to ensure the measurement accuracy [3]. The fibre segments, co-located in both fibres, are defined using weakly reflecting (<1%) Fibre Bragg gratings, each of which forms an interferometer with a fibre-tip reference. Interrogation of the multiple interferometers needed to measure segment strains is made possible using Range-resolved interferometry (RRI) [4] allowing multiplexing of angle sensors along the tape. The design, and construction of the tape, measurement principle and results for an installation with three joints on a low-cost robot arm will be presented, along with the performance characterisation using a rotary stage. [1] M. Wang, D. Palmer, X. Dong, D. Alatorre, D. Axinte, and A. Norton, “Design and Development of a Slender Dual-Structure Continuum Robot for In-situ Aeroengine Repair.”, 2018, IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Madrid, Spain, 2018, pp. 5648-5653, https://doi.org/10.1109/IROS.2018.8594142 [2] T. Kissinger, R. Correia, T.O.H. Charrett, S.W. James, & R.P. Tatam. “Fibre Segment Interferometry for Dynamic Strain Measurements.”, 2016, Journal of Lightwave Technology, 34(19), pp. 4620–4626. https://doi.org/10.1109/JLT.2016.2530940 [3] P. M. Blanchard et al., “Two-dimensional bend sensing with a single, multi-core optical fibre”, 2000, Smart Mater. Struct., vol. 9, no. 2, pp. 132–140. https://doi.org/10.1088/0964-1726/9/2/302 [4] T. Kissinger, T.O.H. Charrett, & R.P. Tatam, “Range-resolved interferometric signal processing using sinusoidal optical frequency modulation”, 2015, Optics Express, 23(7), pp. 9415–9431. https://doi.org/10.1364/OE.23.009415

LADAR systems are in big demand for various space agencies and industrial applications for velocity measurements. Space missions corresponding to soft landing on interplanetary surface require absolute velocity measurements which require fiber lasers. We report development of three channel LADAR system consisting of robust and compact high power fiber laser systems in Master Oscillator Power Amplifier (MOPA) configuration developed using pulsed pumping and all fiber heterodyne interferometry setup for velocity detection. This system consists of ultra-narrow linewidth external cavity seed laser with 2 KHz linewidth following two pre-amplifier stages and final power amplifier stage with output power of 5.2W, 5.5W and 5.1W peak powers, 4ms pulse width at 192 ms repetition rate in three different channels. The lasers in three different channels reflect through target and reflected signal is mixed with 4 mW local oscillator in 2x2 coupler spliced with Balanced Detector and as a result Doppler shift is measured. Thereby using All-Fiber Optical Heterodyne Interferometry, highly accurate velocity detection is achieved with 3 sigma accuracy of ±3.108, ±4.063 and ±3.3903 mm/s in three different channels respectively. This system is passed through qualification tests including EMI/EMC, Thermal Soak as well as TVAC for long cold and hot cycle (+10°C to +50°C), vibration (20grms sine waveform), radiation (10KRad dose) and shock test (600grms) for space applications. We report that the developed state of the art LADAR system in aligned Bi-static configuration (Tx-Rx aligned parallel to each other within 3arcsecond accuracy) for velocity measurement has passed all qualification tests with nominal performance during all phases of test and is capable of working upto 2.2 km target distance.

We examined the applicability of the Nyquist criterion for phase-stepping shearography. We used a shearographic system employing 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. Phase maps were generated for various F-numbers for the deformed object, representing the undersampling, the Nyquist, and the oversampling situations, according to the Nyquist criterion. From our results, we show that it is not essential to satisfy the Nyquist criterion for image sampling in shearography. Operation in the sub-Nyquist domain is acceptable, and even desirable, in many cases.

By advancing the “scanning from heating” approach, we have managed to three-dimensionally measure the surface shape of uncooperative (e.g., black, glossy, transparent, or translucent) objects and materials. Sequentially projecting thermal fringes has enabled us to perform 3D measurements within one second. However, many applications require measurement times that are even an order of magnitude shorter. In this contribution, we present theoretical investigations on single-shot thermal 3D measurements, particularly with respect to the design of the projected point pattern. Based on the results, we propose the specific setup of a projection optics for a single-shot LWIR 3D sensor.

A telecentric fringe projection system is typically calibrated by optimizing the magnification factor of a single lens for the entire measuring volume. However, this can result in uncertainty in the 3D reconstruction when the geometry is located near the edge of the measuring volume. 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. 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. 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.

HARMONI is one of the first-light instrument of the ELT and is almost fully enclosed in a cryostat. During the prototyping, testing, and alignment phase, we need to perform many measurements in this cryogenic environment. The challenge lies in conducting metrology on these components while they are within the test cryostat, maintained at a temperature of 130 K. This will be used to determine how the components respond to temperature changes and ensure overall proper alignment. The Photogrammetry Inside Cryostat (PIC) is a high-end periscope that enables photogrammetric measurements inside the test cryostat, providing a reliable and effective way to align and control the different parts of HARMONI. This new solution represents a significant advancement in cryogenic photogrammetry and is crucial for ensuring the success of HARMONI on the ELT.

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).

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.

In indirect optical geometry measurement, it is not the outer boundary layer of a measuring object that is examined, but the shape of the surrounding medium. For this purpose, a confocal fluorescence microscope is used to scan the surrounding space of the object, which contains fluorescent microparticles. The area in which the fluorescence signal disappears defines the geometry of the measured object. This indirect measurement principle is completely independently of the optical properties of the body. First experiments prove the feasibility of the principle and its measurement possibilities using the example of a step geometry.

We report an optical system for measuring the surface texture of additively manufactured (AM) metal parts. The system implements the confocal-like HiLo technique and features a large field-of-view and high numerical aperture, providing high-quality measurements. Furthermore, we propose high dynamic range algorithms to increase the measurement robustness, tackling the high variation in surface reflectivity of typical AM parts. The HDR algorithm combines low dynamic range images to produce a single topography map, further reducing the amount of measurement artifacts. The results demonstrate robust measurement of metal AM surfaces, making the approach an attractive solution for industrial and scientific applications.

Direct laser writing is a popular method for maskless lithography that already achieved commercial grade. However, it is still challenging to realize homogenously exposed structures on 3D-shaped substrates. A common source of the varia-tion 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, but only if the modulation contrast is high enough. In order to raise this, the measurement beam is passing the pinhole twice. This probe is integrated into the nano-positioning and nano-measurement machine via a position-based controller. We demonstrate the capability to follow 5° inclined sub-strates.

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 ~150 µm, rough varnish layer thickness ~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.

Here, we propose a method to overcome the main limitation of the 3D form profiling technique which is based on imaging with partially coherent illumination. The approach is based on combining 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.

The scattered light technique is well suited for in-line measurement due to its robustness to vibration and high measurement speed. However, the non-trivial relationship between the surface angle resolved data from scattered light sensors and the spatially resolved surface topography hinders the interpretation of the data and the comparison with other surface measurement systems. To bridge this gap, we present a method to reconstruct surface profiles from scattered light measurements based on an accurate simulation of the measurement process and a sparse representation of the surface profile incorporating a priori knowledge such as the feed rate of a turning process.

We propose a computational solution to correct the non-linearity errors of motorized linear stages used in optical profilometers. Our algorithm reconstructs two topographies axially shifted by an amount, and iteratively calculates a calibration curve accounting for the scanner non-linearities. We explore two methods for obtaining the topography pair, one based on a double axial scan, and one based on extracting two topographies from a single axial scan. Interestingly, the method only requires data from the scan(s) itself to correct the linearity errors. We evaluate the method on different samples, including step height standards, yielding results comparable to piezo-based measurements.

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 to correct for hardware imperfections for overlay metrology. We show higher accuracy and precision of overlay measurement by correcting the inhomogeneities and asymmetries caused by the illumination spots, presenting a robust method resulting from simple calibration steps.

We propose a contact-free approach 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 results show that the self-assembling guided by the pyroelectric effect is a good strategy for the direct fabrication of optical components of different geometry and for the functionalization of compact systems.

Measuring overlay (OV) between two layers of semiconductor devices is a crucial step during electronic chip fabrication. We present a dark-field digital holographic microscope (DHM) with non-isoplanatic lens aberration correction to improve the accuracy of overlay metrology aiming for sub-nm precision. Our DHM setup simplifies the optics of the overlay metrology tool by using a two-element imaging lens. This approach increases the aberration levels, and significant non-isoplanatic aberrations do appear. Therefore, these so called 4d-aberrations need to be corrected via computational imaging algorithms. The non-isoplanatic aberration calibration is measured using a nano-hole grid. Each nano-hole, with a width and depth of 100 nm, acts as a point-scatterer by diffracting a spherical wave.

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 noncontact 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.

Lens less Fourier Transform Digital holography is ideal for the real time study and visualization of dynamic phenomena. This technique coupled with thermal stressing can be used for identification and quantification of phase objects occluded by semi-opaque objects. We have developed such a system which measures the spatio-temporal change in optical path length because of thermal stressing. The proposed technique is validated by recording a time series of holograms using glass plates of different thicknesses, which are subjected to a constant thermal stress. Using the proposed technique, thicknesses were measured with 0.01mm accuracy.

Performance characteristics for interferometers that measure surface topography include the ability to resolve closely spaced surface features, referred to as topographic spatial resolution. Within well-defined limits, scalar diffraction theory and classical Fourier optics provide a software model for prediction of the resolution and spatial frequency response for interference phase-based measurements of surface topography. Analytical solutions and adaptive sampling allow for rapid simulation of both the nominal linear transfer function and an estimate of intrinsic residual nonlinearities.

In this contribution, we present a technique for the determination of optical aberrations, which is based on measurements of the point spread function and a Bayesian optimization of rigorous simulations. The measuring system is a UV-microscope in a reflected light configuration with a 200x magnification, unpolarized light, and an illumination and imaging NA of 0.44 and 0.55, respectively. The PSF is measured by imaging a small quadratic chrome dot (side length ≈ 180 nm) on a glass substrate. We investigate the impact of different adjustment states, different dot locations and different optical microscopes.

Dimensional optical microscopy allows for the rapid inspection of devices at the cost of limited accuracy. Introducing a model-based approach that includes diffraction effects allows for increased accuracies. The model needs to be efficient and accurate to evaluate the measurements in an acceptable time frame. We present an overview of the illumination model and different incidence-pupil sampling techniques. Furthermore, we will demonstrate strategies for efficiently calculating the near-field scattering response from structures using the finite element method. Using these aspects, we demonstrate a significant increase in the accuracy of dimensional estimates for a range of structures.

Within the framework of the European TracOptic project we developed the UFO (Universal Fourier Optics) model, which simulates virtual CSI measurements of surface topography treating the surface as a two-dimensional phase object. The scattered light field is multiplied by a two-dimensional transfer function in the Fourier domain prior to surface reconstruction. Parameters affecting the final results are the central wavelength and the spectral bandwidth of light as well as the numerical aperture of the objective lens and the chosen evaluation wavelength. We introduce the UFO model and discuss its capability to predict systematic measurement errors.

I will present a photonic sensor that can be used for remote sensing of many biomedical parameters simultaneously and continuously. The technology is based upon illuminating a surface with a laser and then using an imaging camera to perform temporal and spatial tracking of secondary speckle patterns in order to have nano metric accurate estimation of the movement of the back reflecting surface. The capability of sensing those movements in nano-metric precision allows connecting the movement with remote bio-sensing and with medical diagnosis capabilities. The proposed technology was already applied for remote and continuous estimation of vital bio-signs (such as heart beats, respiration, blood pulse pressure and intra ocular pressure), for molecular sensing of chemicals in the blood stream (such as for estimation of alcohol, glucose and lactate concentrations in blood stream, blood coagulation and oximetry) as well as for sensing of hemodynamic characteristics such as blood flow related to brain activity. The sensor can be used for early diagnosis of diseases such as otitis, melanoma and breast cancer and lately it was tested in large scale clinical trials and provided highly efficient medical diagnosis capabilities for cardiopulmonary diseases. The capability of the sensor was also tested and verified in providing remote high-quality characterization of brain activity.

Dimensional inspection during manufacturing of high-temperature forging workpieces is still an unsolved research topic. Conditions are difficult due to the thermal radiation in the visible spectrum, heat haze, and an unstable environment. Passive stereovision may overcome these problems and enable fast measurement of hot components. The paper investigates the effect of optical filtering for highlighting hot surface texture, the performance of various feature detection methods, and the stereovision system accuracy. The developed algorithms with KAZE feature detection method achieved 0.28 mm standard deviation error for a single point in a measurement volume of approx. 500×400×350 mm.

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 [V.-F. Duma, M.-A. Duma, Applied Sciences 12(11), 5592, 2022]. 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 decision-making in non-destructive testing. We tested the performance of a high-resolution (DLSR) and a medium-resolution camera in a digital shearography setup for non-destructive testing of blind holes in carbon-fibre reinforced polymer (CFRP) under thermal loading. Defect-induced deformation (DID) phase maps obtained from two cameras were compared. We observed comparable detection capabilities from the two cameras, even though the spatial resolution of the DLSR was 4 times higher.

Off-axis single wavelength digital holography (DH) can acquire height information in a single shot acquisition. However, for single wavelength DH the maximum depth range is limited due to 2π phase wrapping to half a wavelength. Dual wavelength digital holography (DWDH) enables the measurement of the height of objects without generating 2π phase ambiguity over a range larger than a wavelength. Single-shot DWDH can be enabled by multiplexing the two wavelengths in two different directions on the detector. To increase the available bandwidth and hence improve the image resolution the zero-order needs to be removed. In this work we demonstrate both with theory and in experiments an iterative zero-order suppression method that removes the zero order in single-shot DWDH. We utilize the spatial frequency multiplexing technique to achieve the single-shot dual-wavelength digital holography. Then we propose an iterative algorithm to approach the autocorrelation terms from the measured hologram. Simulation and experimental results show that this algorithm can effectively suppress zero order and improve the quality of reconstruction results. We believe that this method has great application potential in 3D detection and imaging.

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 over-estimation 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 %. The improvement of the uncertainty of the measurement reflects the quality of the national measurement system in Thailand which will disseminate the standard to the customers in the country, especially in the industrial sector. With less uncertainty, consumers can achieve a better quality of products, reducing cost, and material waste.

Digital photoelasticity is used for stress analysis in materials. In this paper deep convolutional neural networks (DCNNs) have recovered stress fields. We compare different polarization representations in DCNNs, including polarized images and Stokes parameters. The performance was evaluated with quality metrics such as MSE, SSIM, and PSNR. Results show that using polarized images in different states results in better quality metrics than using Stokes images. This offers new opportunities for real-time stress field analysis.

For high-volume semiconductor manufacturing, the high-speed in-wafer uniformity inspection is inevitable. Compared with the conventional OCD spectroscopy, the LSHI has advantages of high throughput, high spatial and spectral resolution, and large field of view. In this study, the optical design of the refractive imaging spectrometer based on the ruled reflection grating is presented. Most imaging spectrometers are off-axis reflective configuration, such as Czerny-Turner. However, the performance is limited by serious astigmatism, and the numerical aperture is relatively small. The refractive type using lens combinations for imaging spectrometers can eliminate aberrations effectively and increase numerical aperture significantly.

The technique proposed in this paper provides inspection of functional surfaces quality of components by nondestructive testing, with high resolution and increased sensitivity. The control is done in real time and instantaneously on all inspected surface. Component geometry accuracy is one of parameters which influences, function accuracy. Moiré topography is full-field optical technique in which the shape of object surfaces is measured by means of moiré interferometry. The technique has found various applications in diverse fields, from biomedical to industrial and scientific applications. In many industrial metrology applications, contactless and non-destructive shape measurement is a desirable tool for, quality control and contour mapping. This method of optical scanning presented in this paper, is used for precision measurement deformation in shape or absolute forms in comparison with a reference component form, of optical or mechanical components, on surfaces that are of the order of few mm2 and more. The optical device used allows a significant dimensional surface magnification of up to 1000 times the area inspected for micro-surfaces, which allows easy processing and reaches an exceptional nanometric imprecision of measurements. According to the principle of measurement, the high resolution and the high sensitivity of the optical system can vary according to the size of the defects to be measured.

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.

A crucial part of tunnel inspections is the detection of delamination. Up to date this task is performed by manual hammering and acoustic detection. We present a new concept and measurement results 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. This shockwave excites characteristic, resonant vibrations if a delamination is present. A second, narrow-linewidth laser is used to remotely detect these vibrations via a coherent measurement technique.

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.

To check the reliability of a multi-layer reflectance model in spectral reflectometry, two different methods for multi-layer thickness measurement were tested on a double-layered thin-film sample (SiO2-SiN) with nominal thickness of 100 nm. The first method represents the sequential determination of the thin-film thicknesses through the extension of the single-layer model, and the second method is to simultaneously determine the thin-film thicknesses using the transfer matrix method. The thin-film thicknesses were determined by both methods and compared to check the reliability and applicability of the multi-layer reflectance model.

The need for thermal control of wafers is becoming increasingly important for semiconductor manufacturing. We demonstrate an ultrafast silicon surface temperature measurement and calibration method. When p-polarized light is incident with an angle of incidence greater than the Brewster angle on a solid silicon, the reflectance drops as the temperature rises. When liquefied, the reflectance increases, allowing precise determination of the melting point. We were able to accurately measure the rapid surface temperature change (~1400 K) of a silicon wafer over 6.2 ns, induced by 6.7-ns-long (FWHM), 532-nm laser pulses, with 0.5-ns temporal resolution and 14-K temperature resolution.

Technical review Transparent optical flats have been extensively used in the production process of semiconductor industries. For improving the performance of semiconductor chips, the surface shape of the optical flat should be profiled with nanoscale uncertainty. Among the measurement techniques, a phase-calculation formula using a wavelength-scanning interferometer has been widely used because of its high resolution and noncontact measurement. In this technique, the phase difference between a target wavefront and a reference wavefront is changed linearly during wavelength scanning. However, the measurement accuracy is degraded by not only linear and nonlinear phase-modulation errors, but also coupling errors between the higher harmonic components and the phase-modulation errors. Phase-modulation errors can occur because of environmental uncertainties such as temperature fluctuation and floor vibration. Design methods of the phase-calculation formulas have been proposed by several researchers based on the averaging method of successive samples, Fourier description of the sampling functions, and an analytical expansion. In this study, a new 15-sample phase-calculation formula was developed that can suppress the coupling errors caused by harmonics up to the fourth order and nonlinear phase-modulation errors. The 15-sample formula was derived using linear equations consisting of the total number of interferograms and the sampling amplitudes. The characteristics of the proposed formula were visualized on the frequency domain and complex plane. In addition, the error-suppression capability was confirmed by using the theory of the root mean square (RMS) error. Finally, the surface of the transparent optical flat was profiled by using the wavelength-scanning Fizeau interferometer and the new 15-sample formula. Summary Fizeau interferometer with wavelength scanning has been widely used for surface profiling of transparent optical flat. However, measurement uncertainty can be degraded by not only linear and nonlinear phase-modulation errors, but also coupling errors between higher harmonic components and phase-modulation errors. In this study, a new 15-sample formula that can suppress the nonlinear phase-modulation errors, and the coupling errors was proposed. The characteristics of the proposed formula were visualized on the frequency domain and complex plane. In addition, error-compensation capability of the formula was confirmed by comparing it with other conventional formulas using the numerical analysis of RMS error.

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 ability of the method to describe the speed 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 thermo-stage. The numerical and SLM simulation experiments allowed for modification of the used estimates in order to obtain relevant information.

Immersive augmented reality and virtual reality display devices demand compact form factors with high image quality. To decrease the system's volume, next-generation optics including the pancake lens have been proposed. However, reducing the optical path is associated with unwanted aberration terms. Here, we propose the lens assessment method based on incoherent holography. Conventional optical testing methods are based on coherent illumination and interferometry. The traditional approach requires complicated optical configurations and hard to model the actual user experience. Our proposed incoherent holography system is based on self-interference using a geometric phase lens and can acquire complex amplitude of incoherent illumination with a single exposure. This also can measure the light field profile of the display and imaging system with numerical analysis using a diffraction equation. Moreover, the compact optical configuration is realized by utilizing the polarization image sensor and enables measuring the personalized display device. The experimental result demonstrates the simple convex lens phase profile compared with Zemax opticstudio simulation.

In this paper, a new phase-extraction method based on deep learning with generated training dataset and a revised training model is proposed. The training dataset, pairs of two fringes and one phase, is generated considering the similarity of the real phases and interference fringes. The Unet model is used as a training model, and some parameters and layers are modified to improve the extraction performance. The phase map obtained by the proposed method demonstrates high accuracy and minor errors. The obtained phase is analyzed and assessed in various ways and shows superior results compared to conventional phase-shifting techniques.

Adaptive Shack-Hartmann wavefront sensor is effectively used for characterizing optical elements due to dynamic aberrations compensation. However, the dynamic range of the system is limited by the deformable mirror (DM) active area. This issue was overcome using compound-lens methodology that utilizes additional lens(es) along with the lens under test to readjust the laser beam diameter to properly fit the DM’s active area. This depends on tested lens’s focal length, the effective focal length of both tested lens and compound lens(es), and the distance between them. The results reveal that the dynamic range of the measurements was extended by ca. 40%.

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 reflectance of great interest for many industries and vital activities in the medical, pharmaceutical and food sectors. In measuring spectral transmittance of the samples below 0.05, there always be low signal-to-noise issues. Moreover, higher measurement uncertainty likely to be encountered. Making measurements with optical attenuators, as well as comprising the nonlinearity, baseline and stray light corrections resolves SNR issue as well as reduces the associated uncertainty. By using this method, the spectrophotometer's full range can be utilized with high level of accuracy meets the critical requirements in many relevant industries. In this work, as per our scheme, the used attenuators are spectrally flat (i.e. have a uniformly flat response as a function of wavelength). If needed, multiple reference beam attenuators can be used together to produce a specific level of attenuation in the reference beam for having the optimal measurement condition to achieve the best accuracy and lowest uncertainty. This 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 250 nm to 950 nm, with and without optical attenuators to show the level of accuracy, repeatability and reproducibility. 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.

Coherent Fourier scatterometry (CFS) is a non-destructive, scattering-based optical metrology tool where the effect due to multiple incident angles is studied simultaneously using a focused spot that is scanned across the sample. We present the versatility of the coherent Fourier scatterometry using different scanning and detection modalities to enhance edge contrast and provide quantitative topological information on low-contrast samples. The increased edge contrast detection is demonstrated for low contrast scatterers by measuring structures of 60 nm depth etched onto a fused Silica glass substrate. The quantitative phase information is provided by the adaptation of the synthetic optical holography (SOH) technique to the CFS setup with the addition of a slow moving reference mirror.

For the precise determination of the spectrally resolved reflectance from a scattering and absorbing sample, we modified a detection unit to render the measuring system almost insensitive considering changes in the distance between sample and detection unit. This makes the measurement significantly more robust and suitable for industrial use. The key of the improvement was using the angular space and defining a certain captured angular range; which is held constant. This results in an almost constant measured reflectance signal over a distance change between the sample and the detection unit of several centimeters. Very low scattering coefficients and absorbing coefficients can limit the usability of this system.

Wyse Light, an innovative startup based in the French Rhone-Alpes region, has been developing a simple, cost-effective, and patented table-top deflectometer capable of measuring the absolute shape of reflective 3D surfaces to better than lambda/20 (20-30 nm) accuracy RMS. The overall goal is to match the performance of established optical metrology techniques with a fully-automated deflectrometry-based instrument, bypassing the need of fine tuning and alignment. To lift the fundamental shape ambiguity typically associated with deflectometry, measurements are performed with varying camera positions along the optical axis (‘coaxial’ deflectometry). Bundle adjustment yields a single shape coherent with all the redundant data. With its integrated self-calibration setup, this quantitative deflectometer offers multiple new advantages: - measurement of any shape (including freeforms) without the need of any null or Computer Generated Holograms. - typical accuracy of lambda/20 on the low frequency absolute shape approaching interferometric performance. - robustness against parasitic vibrations and turbulence. - cost-effectiveness. In this poster, we will present the conceptual basis of this deflectometry-based metrology instrument, the recent pre-series characterization and performance on spherical concave and convex surfaces, and also on an 'extreme' freeform

A robust instrument for optical metrology has been designed in order to characterize surface shape and flatness or transmitted wavefront of flat components such as filters, crystals, thin substrates or mirrors. The sensing technology has been selected for its insensitivity to vibrations and consists in a Hartmann Shack wavefront sensor upgraded with a new reconstruction algorithm enabling high resolution phase sampling. Achromaticity is being simulated and measured at several wavelengths within the spectral range of the sensor, demonstrating the compatibility of the technique with at-wavelength testing. Capability of measuring samples on a wide range of testing diameters while maintaining sampling resolution is shown. Last robustness to reflection from back sample surface is explained and illustrated with results obtained on thin plane parallel samples.

We will present an innovative method for the measurement of Parallel Optics or optics with parallel surfaces, often planes such as windows, filters, mirrors. This new and patented approach solves issues related to the metrology of this type of samples, mainly associated to the signal reflected by the back surface of the substrate. Instead, and with no extra hardware or specific optical add-on making the testing more complicated and expensive, our implementation allows for a straightforward characterization of Parallel Optics with no manipulation or preparation of the sample of any kind. We will present the concept as well as its implementation and results obtained on samples as compared with other historic reference techniques.

This paper proposes a new approach for the inspection of reworks on aircraft landing gear components using a robotic inspection system based on white light interferometry. The current manual inspection process is slow and prone to errors. The new method aims to improve accuracy, repeatability, and efficiency. The robotic system handles the white light interferometer for detailed 3D measurement. The accuracy of industrial robots is crucial considering the small size and limited range of white light interferometer measurements. The limitations and potential of the system are discussed, and the results of a rework inspection are presented to validate its applicability.

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.

We adapt the method of Coherent Fourier Scatterometry for the detection of geometrically patterned arrangements of gold nanoparticles and top-down fabricated nanostructures on silicon wafers. Our assemblies consist of different numbers of nanoparticles in different orientations to each other and gold nanostructures. We examine individual particles as well as linear-, square-, triangular- and L-shaped arrangements and spirals. The metallic nanostructures have different thicknesses down to 60 nm. Correlations between the particle geometry and the intensity distribution of the Fourier plane are observed, which allows to distinguish the different geometrical arrangements and to determine their geometrical orientations.

The aim of this work is to investigate micro-objects for non-contact micromanipulation with simultaneous acquisition of the Raman spectrum using a Photonic Jet (PJ) fiber optic. Based on this, a measurement setup was designed and implemented which allows the excitation of Raman scattering by the fiber and the analysis of selected samples via the backscatter signal of the fiber. As for micromanipulation, particles with a size of 3 μm could be shifted transversely and laterally in the direction of the propagating laser radiation. 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 our mobile world. Tires experience various stresses, 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 using UV radiation, providing information about their aging condition. The study utilized samples of different rubber compounds and exposed them to UV radiation for varying durations. A Bruker ALPHA II system 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.

Phase-only spatial light modulator (SLM) modulates light’s phase based on displayed pattern. Drawbacks of SLM are surface non-uniformity and cross-talk between adjacent pixels. Therefore, this study aims at measuring surface non-uniformities of the SLM and estimate cross-talk effect in holographic projection using Shack-Hartmann wavefront sensor (SHWFS). Constant phase pattern is displayed on the SLM and its surface phase shape is measured using plane-wave illumination. Consequently, blazed grating is displayed on the SLM to investigate cross-talk effect, at positions where phase jumps between 0 and 2π for two neighbor pixels exist. The results reveal that SHWFS provides accurate phase distortion measurement.

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 biconical and 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 phase object such as 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) was used to detect delaminated areas on Yttria-stabilized Zirconia Thermal Barrier Coatings (YSZ-TBC). A flat sample subjected to thermal shock tests and a curved section of a discarded blade from a gas turbine were used to detect delaminations. IRT was performed using two flash lamps and a halogen lamp. Principal Component Analysis was applied to raw and to previously processed by Thermal Signal Reconstruction data. Signal-to-Noise ratio was used to compare the capacity of delamination detection and the best principal component was found to be dependent on heating form (flash or long pulse) and source.

The need for rapid, high resolution, accurate metrology of mass produced aspheric and freeform surfaces continues to grow as the application space evolves. CGH metrology at production scales can be realized without specialized engineers or in-house knowledge of setup. 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. Early integration of this metrology solution in product planning helps users scale from prototype to manufacture.

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. ZrO2–7 wt.% Y2O3 top coatings were submerged in an aqueous solution of ammonium bifluoride. Thermographic information was acquired with an infrared camera with rate of 30 Hz and two 600 W flash lamps. The results show differences in their heating and cooling curves, which allows them to be correlated with the coatings thickness measured using SEM microscopy and Eddy currents.

Interferometric optical microscopy (IOM) is a powerful 3D metrological technique which enables for nanometer range vertical resolution (lateral resolution is diffraction limited as any other far-field microscopy). It is widely used in many industrial application, including MEMS. Several nanometrology application requires liquids operation, such as in-situ corrosion or biology. Unfortunately, IOM is not well suited for liquids environment, as no immersive interferometric objectives are commercially available. We have setup a liquid IOM (L-IOM) in the phase shift mode (PSM) with standard Mirau objectives that are compatible with our home-made liquid cell. The illumination is done by a 10mW He-Ne laser. To get rid of speckle effects, a rotating diffusion disk is used. The fringe visibility in liquid is not reduced compared to air operation. We used the L-IOM to measure the free vibration of a silicon cantilever in water. The noise level is 1.4 nm, which enable to measure the biological activity of bacteria adsorbed on the cantilever.

Wave Front Phase Imaging (WFPI) is a new technique for measuring the free shape of a silicon wafer. The wafer held vertically to avoid the effects of gravity while measured the wave front phase of the non-coherent light reflected off the surface of the silicon wafer. The wave front phase is measured using a new method that only records the intensity of the light at two or more distances along the optical path allowing for very high speed and a high number of data points. 16.3 million data points was acquired in 12 seconds on a full 300mm wafer providing lateral resolution of about 65µm. The flatness of the silicon wafers used to manufacture integrated circuits (IC) is controlled to tight tolerances to help ensure that the full wafer is sufficiently flat for lithographic processing. Advanced lithographic patterning processes require a detailed map of the free, non-gravitational wafer shape, to avoid overlay errors caused by depth-of-focus issues1. We present WFPI as a new technique with high resolution and high data count acquired at very high-speed using a system where the wafer is held vertically and free from the effects of gravity. A large variety of new materials are being introduced to the Back end of Lines (BEOL) processes to ensure innovative architecture for new applications. The standard in-line control plan for the BEOL layer deposition steps is based on film thickness and global stress measurements, which can be performed on blanket wafers to check the process equipment performance. However, the challenge remains to ensure high performance metrology control for process equipment during high volume manufacturing. With the product tolerance getting tighter and tighter and architecture more and more complex, there is an increasing demand for knowledge of the wafer shape3. For the semiconductor industry to uphold Moore’s Law, one of the key challenges are the ever-tightening overlay requirements. For the latest immersion scanners that perform at the sub-2 nm overlay level, the overlay budget becomes more and more determined by process-induced overlay errors from above mentioned fab steps. All these processing steps can introduce stress, or stress changes, in the thin films on top of the silicon wafers, that again can result in significant wafer distortions4. Measurements of the wafer geometry before and after fab processes can be used to determine the residual stress induced during such processes5. A 300mm blank silicon test wafer, polished on both sides, were used with the following specifications described in Table 1 below and compared to the Semi M1 standards for a silicon wafer. According to Semi standard MF1390 [12], one can calculate the deflection of a silicon wafer, assuming a chuck holding the wafer at the center point only, using the equation: Deflection (in µm) = S=((3×10^8 )kgdD^4)/(32Et^2 )= (K∙D^4)/t^2 (1) Where D is the nominal wafer diameter in mm, t is the nominal wafer thickness in µm, and K is the constant of proportionality equal to 7.83×10-3µm3/mm4. The proportionality constant K includes the geometrical constant (k = 0.5854), the gravitational constant (g = 980cm/s2), the density of silicon (d = 2.329g/cm3) and Young’s modulus (E ≈ 1.6×1012 dyne/cm2) [12]. Using equation 1, one get’s that a 300mm silicon wafer with 775µm thickness will deflect 105.6µm PV if held at the center point using a mini chuck. According to Semi M1 [8], production ready silicon wafer can have a diameter of 300mm ± 0.20mm, a center thickness of 775µm ± 25µm and a Total Thickness Variations (TTV) of 5µm giving natural variations in deflection ranging between 97.61µm and 114.58µm. These variations caused by gravity needs to be removed. According to Semi standards [12] there is for all practical matters just one good solution for removing gravity from the measurement to generate the true free shape; hold the silicon wafer vertically. The working principle of WFPI is based on registering the intensity distribution at two different optical planes (measurement planes). The intensity distribution is recorded using a conventional imaging sensor acquiring two images with the exact same field of view. The wave front phase is defined as the surface perpendicular to the direction of propagation of the light rays. The sensor assumes geometrical propagation of light, and in this regime the light can be considered as a collection of light rays which bends according to Snell’s law and reflects on a surface keeping its angle with respect to the surface normal. The reflected beam will carry the wave front phase, which value is proportional to the surface height map. In our case, we are using a collimated red (λ = 650nm) light beam that reflects onto the surface, the reflection angle of each ray is exactly two times the angle of the surface normal [6]. The reflected light beam, which carries the wave front phase information of the sample surface profile is de-magnified by a telecentric lens. For image acquisition, two paired imaging sensors were used, each one placed at a different optical plane allowing the pair of images to be acquired at the same time at different optical paths from the sample being measured with the exact same field of view. With this setup, a single image snapshot can collect wafer topography data of the entire wafer with the same number of pixels as being present in the imaging sensor [6]. The newly invented wave front phase sensor works by acquiring two images, I_1 and I_2, around a conjugated plane of the reflective sample, in this case a silicon wafer, with symmetric positions (in front of and behind the reflective sample) with a total distance between the images equal to z (see Fig. 1). A collimated light beam, with an assumed flat phase map, is generated from an incoherent light source using a light-emitting diode (LED). In principle, any type of light can be used, but the use of incoherent light has the advantage of not producing speckle, which worsens the performance of the WFPI. The light beam passes through lens L3 and is then reflected on the beam splitter changing direction 90°, and then passing through lens L2 and then lastly lens L1 to generate the collimated light beam with the same size, or larger, than the sample being measured, to hit the silicon wafer. The collimated light is then reflected from the reflective sample, which modifies the shape of the collimated beam proportional to the optical behavior of the reflective sample surface, thus generating a distorted outgoing phase map proportional to the wafer surface. The imaging plane P of the reflected light is then being translated into a conjugated plane, P′, after the distorted phase map passes through lens L1 and lens L2 and then through the beam splitter without changing direction, to then being recorded by one or more digital image sensors which acquires the two required intensity images, I1 and I2. These intensity images are acquired around the conjugated plane P′ in equidistant planes along the z-axis. Choosing focal lengths of lens L1 and lens L2 appropriately allows for adjusting the sample size to the image sensor size17.

3D HOLOGRAM OPTICAL ELEMENT FOR ANGLE MEASURING DEVICES AND SIGHTING SYSTEMS Tyurin A. V., Zhukov S. A., Akhmerov A. Yu. Odessa National University named after I. I. Mechnikov, 2 Dvoryanska Str., Odessa, 65082, Ukraine e-mail: oleksandr.akhmerov@onu.edu.ua ANNOTATION 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. EXPERIMENTAL SETUP The manufacture of a hologram optical element (HOE), which is a combination of four three-dimensional diffraction gratings registered in the volume of a photosensitive medium, was carried out on the setup shown in Fig. 1. As a bulk photosensitive medium, we used additively colored alkali halide crystals and chalcogenide glassy semiconductors, the technology for recording three-dimensional diffraction gratings on which we described in [1, 2]. CONCLUSIONS Thus, as follows from the results obtained, the use of the angular selective properties of a combination of three-dimensional diffraction gratings registered in the volume of a photosensitive medium makes it possible to create fundamentally new optoelectronic angle measuring devices based on them, which are distinguished by their simple design, high measurement accuracy (due to the use of two parallel channels) and wide functional opportunities. The measurement does not depend on fluctuations in the power of laser radiation (this is automatically taken into account in the calculations), allows you to unambiguously determine the direction of the angular displacement of the object and allows simple visual observation, which distinguishes it favorably from other currently existing methods. LITERATURE 1. Belous V.M., Mandel V.E., Popov A.Yu., Tyurin A.V. Mechanisms of high-temperature holographic recording in As-S materials. // Solid state physics. – 1996. – V.38, No. 2. – P. 379–390. 2. Belous V.M., Mandel V.E., Popov A.Yu., Tyurin A.V. Mechanisms of holographic recording based on photothermal transformation of color centers in additively colored alkali halide crystals. // Optics and spectroscopy. – 1999. –V.87, No. 2. – S. 305–310.

In recent years, heat dissipation has been manly considered a problem in our everyday life. We are still increasing the amount and type of electronic equipment used, increasing the total amount of energy and power dissipation involved. Finding a way to remove and/or transform the heating in excess could represent a strong innovation with several technological consequences. The present work is focused on the characterization and test of an advanced multifunctional panel made by additive manufacturing functionalized with internal heat pipes. We report on the thermal simulation, thermography measurements and experimental results discussing its application for energy waste recovery

In the frame of space mission development, the measurement and control of temperature is an important problem for thermal vacuum and thermal balance tests. Replacing a substantial amount of thermocouples by a non-contact thermographic detector is highly desirable. Our work investigates the feasibility of extending the temperature measurement range of commercial infrared thermographic system to temperatures down to -100 °C for thermal vacuum test (TVAC). It simplifies test integration, and multiplies the number of measurement channels. A test configuration has been built to measure simultaneously infrared thermographic signals of samples cooled down to -100 °C, with different emissivity, using a commercial infrared camera. Management of parasite infrared radiation, estimation measurement uncertainty (absolute and relative) were realised and compared numerical expectation. The demonstration showed that a temperature error of 3 K @ -100°C for high emissivity objects is achievable.

Microlenses are of technological importance for a wide range of industrial applications, enabling the demand for small form factors, high resolution, and cost-effectiveness in a single element. In recent decades, microlenses have been manufactured using multi-step processes and cost-effective and cumbersome technologies. Herein, we discuss dispensing liquid crystals by pyro-electrohydrodynamic (pyro-EHD) printing. The pyro-electric effect is utilized as a bottom-up technology for the non-contact manipulation of liquid solution in air and at room temperature

In this study, a moiré-based method for warpage measurement of wafers under operate at high rotational speed is proposed. The proposed technique combines the characteristics and advantages of moiré shift theory, digital moiré method, projection moiré method and stroboscope technique, and has full-field wafer warpage measurement capability. The experimental results demonstrate that the proposed system can accurately measure the warpage information of the sapphire wafer to be tested. Compared with the existing warpage measurement technologies, the proposed method has the advantages of high resolution, full-field measurement and high stability.

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 proposes a polarization interferometer for measuring the surface profile of polished wafers. The technique is based on Fizeau-type polarization interferometry and employs a polarization camera, which enables full-field measurement of the wafer surface profile. Polarization interference patterns are acquired from a Fizeau interferometer using the camera, which outputs four phase-quadrature interference patterns. The surface profile of the wafer can be determined by applying a phase-shifting algorithm to the four phase-quadrature interference patterns. This innovative approach offers high accuracy and reliability in measuring wafer surface profiles and has potential applications in modern semiconductor manufacturing.

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.

In this study, a novel heterodyne speckle interferometer based on an asymmetric optical path design is proposed for both in-plane (IP) and out-of-plane (OP) displacement measurement. The optical arrangement allows for measuring IP and OP displacement through frequency separation and simultaneous equations, while only requiring four laser beams to converge on two detection points on the measured surface, making this a simple yet effective design. By combining heterodyne interferometry, speckle interferometry and asymmetric optical path design techniques, the proposed system can perform precision displacement measurement, while having the advantages of high resolution, long-range non-contact measurement and a relatively simple configuration.

Conventional coherence scanning interferometry (CSI) surface reconstruction algorithms mainly encompass envelope detection method, frequency domain analysis, and fringe correlation method. These three types of methods typically only utilize the one-dimensional interference signal at individual pixels to determine the surface height on a pixel-by-pixel basis. Here, we propose a surface reconstruction algorithm grounded in the linear 3D imaging theory of CSI. The 3D interference fringe stack generated through vertical scanning process can be considered as the convolution of the foil model of surface and the 3D point spread function (PSF) of CSI system according to this theoretical model. This algorithm employs a priori knowledge of the 3D PSF to perform deconvolution on the 3D interference fringe stack to obtain the filtered foil model of surface, and subsequently extracting the surface height from the foil model. This method may boost the instrument response to the high spatial frequency of a surface and can simultaneously compensate the aberration of the optical system.

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.

Although stitching is frequently applied in optical surface metrology, so far no method has been proposed to evaluate the uncertainty of a stitching result. Since the accuracy of a stitching result depends on many different interdependent influence factors, in this contribution a method to determine the uncertainty of a stitching result based on a Monte Carlo simulation is proposed.

One important cause for limited traceability in optical metrology is the presence of systematic measurement errors caused by the interaction of the sensor and the measured object. These effects are complex and influenced by many factors, hence, they may differ significantly even among similar measurement systems. This also implies, that it is usually necessary to model the whole measurement chain including the relevant characteristics of the measured surface. We are currently developing a model of a chromatic confocal point sensor dedicated to simulate object-dependent systematic measurement errors and estimating task-specific measurement uncertainties. The simulations already cover all relevant fundamental aspects of the system, some important details are currently being developed. We recently introduced realistic reflection characteristics based on methods originating in physically based rendering. We show how to phenomenologically describe the light-object-interaction using bidirectional reflectance distribution functions and how the principle of Monte Carlo Ray Tracing can be adopted for this use case. We can already show the general influence of surface curvature and slope and can qualitatively predict systematic effects. However, simulations using the current model still show clear deviations from measurement results. While some effects are caused by non-ideal characteristics of the real system, others are likely caused by the approximations within our model. Therefore, further investigations and model developments are pursued.

A reliable tool for simulations of confocal microscopes shall be developed to enable improved model-based dimensional metrology. To simulate measurements on rough surfaces the BEM simulation tool SpeckleSim, provided by the ITO of the University of Stuttgart, is combined with a Fourier optics based image formation. SpeckleSim, which calculates the light-structure interaction by solving Maxwells equations, is compared with the well-known FEM based solver JCMsuite. As an example, a rectangular shaped line is used as an object. Due to different boundary conditions the results show expected small deviations, which require further investigations. First results and the general concept will be presented.

Coherence scanning interferometry (CSI) is a widely used optical method for surface topography measurement of industrial and biomedical surfaces. Approximate methods use physics-based approaches to model a CSI instrument with minimal computational effort. A crucial aspect of CSI modelling is defining the transfer function for the imaging properties of the instrument, to predict the interference fringes created by the instrument. Approximate methods, such as elementary Fourier optics, universal Fourier optics and foil models, use scalar diffraction theory and the imaging properties of the optical system to model surface topography measurement. In this paper, the surface topographies obtained by these three methods are compared.

At PTB, a multi camera 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 +/-20 deg to the optical axis. Characteristic deviations observed in the measurements have led to a revision of the alignment procedure of the setup. Furthermore, approaches for the determination of a datum point for the investigation of the angular dependency of the MTF are discussed.

A novel technique for the long-term stabilisation of widely wavelength modulated diode lasers used in sinusoidal pseudo-heterodyne interferometry systems is presented. The technique works by controlling the temporal separation of successive appearances of a gas absorption line feature during the sinusoidally swept waveform, while compensating for the signal distortions due to laser intensity modulation that is typically associated with laser diode injection current modulation. The technique is applied to a range-resolved interferometric system interrogating a Mach-Zehnder interferometer in controlled conditions, and results are presented from proof-of-principle measurements achieved using this technique.

Biconic surfaces are among the most fundamental freeform surfaces. They are characterized by just four parameters, namely the two radii in the x- and y-direction and the two conical constants Kx and Ky. We introduce an approach to measure the form of biconic surfaces using Fizeau interferometry. 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. This final map can be further analyzed for errors in the four design parameters as well as higher-order form errors.

We present a method to measure Modulation transfert function (MTF) of an optical device with digital random target Current know method using this way to compute MTF must average the calculus accross a part of the Field of View of the device We use the digital target with the possiblity to display a sequence of random target and we use the local intensity correlation identify pixels of the target and pixels of the sensor : this permit the calculation of the MTF in local region of the Field of view (center or border for example) by also and direct measurement of the Point Spread Function and primary aberration and distortion with very low noise

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.

A lot of effort is focused on realizing the vision of fully autonomously driving vehicles. One of the main challenges is the development of sensors that scan the environment with high speed, precision and resolution even under harsh weather conditions. Especially LiDAR and Camera Sensors deliver the necessary high-resolution data, but both suffer from strongly degrading signals in low visibility conditions. We present a LiDAR system design that is optimized for the operation in low visibility conditions, and we discuss the physical and technological limitations. We further give an outlook on a sensor fusion approach with a time-gated sensor with high lateral resolution for better recognition of obscured objects.

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. A pulsed time-of-flight laser scanner optimized for UAV-based applications was developed by the Fraunhofer Institute for Physical Measurement Techniques IPM. The prototype of the laser scanner has a total weight of 2.1 kg, a power consumption of less than 100 W, a measurement uncertainty (one standard deviation) of approximately 3 mm, and is eye-safe (laser class 1). 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.

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. Optical sensors such as lidar and cameras are state-of-the-art in prototype autonomous vehicles. In adverse weather conditions, 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. Moreover, this system offers the ability of image-free object detection which speeds up data evaluation as well. Here, we want to report on our progress towards realising such a system.

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. First, we will introduce an ultra-compact Planar Lightwave Circuit-based Digital Holographic Microscope (PLC-DHM) using a point light source emitted from the end of an optical waveguide, which utilizes thermo-optic effects to achieve fast and precise phase shifting simultaneously at multiple wavelengths so that color 3D movie imaging of moving targets can be realized. Next, we explain our most recent design and fabrication principle that can extend the advantages of the PLC-DHM to a common-path digital holographic microscope, and then demonstrate microscopic 3D imaging through a highly heterogeneous double-composite random medium.

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.

Mobile industrial robots are increasingly important for multiple areas of manufacturing and automation; offering the ability to fully utilize robot resources or in the manufacturing of large objects. Current navigation approaches predominately rely on a combination of laser scanner, inertial measurement unit (IMU) and wheel encoder data together with simultaneous localization and mapping (SLAM). However, this approach may have insufficient accuracy or be unreliable due to environmental conditions such as featureless areas or constantly changing areas where mapping becomes unreliable. Laser speckle odometry is an optical approach measuring the real robot motion using a ground-facing camera and laser illumination and image correlation-based processing capable of providing complementary data to overcome these issues. Here 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 and 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.

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 set-ups 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 setup 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.

Polarimetry focus in the measurement of polarization of a scene of interest. Mueller matrix imaging allows to measure the Mueller matrix that gathers polarization-related optical interactions comprising the depolarization, the scattering and the retardance properties of a sample that modified the input Stokes vector and it is measured acquiring 16 images. Our system allows to acquire the Mueller matrix pixel-wisely in the complete visible band using only 4 acquisitions, improving the duration in time of the measurement and progress towards real-time polarimetric imaging. The camera is based on a division of aperture scheme that allows obtaining four different sub-images on the CMOS sensor. The results are demonstrated to have errors lower than 10% over the complete components of the Mueller matrix .

Thin films of SiOx or AlOx drastically improve the properties of modern plastic products like food or medical packaging. At a thickness ranging from few nanometers to 50 nm, inline quality inspection is mandatory. The coatings can be addressed via their infrared absorption bands. We developed a miniaturized sensor to quantify the film thickness on food containers at production speed. Its optical principle is comparable to infrared reflection absorption spectroscopy (IRRAS). Additionally, stoichiometric inspection using a compact tunable Fabry-Pérot filter instead of a bulky Fourier-transform infrared spectrometer will also be discussed.

Mid-infrared (mid-IR) microscopy provides spatially resolved chemical information through the spectroscopic evaluation of characteristic absorption features for each pixel. Since molecular information is accessible under ambient conditions in a contactless, label-free and non-destructive way, mid IR microscopy has evolved as an essential tool in scientific and industrial laboratories. Although this technique brings valuable benefits in chemical analysis, conventional mid-IR microscopy based on Fourier-transform IR (FTIR) spectroscopy struggles with the limited spectral brightness of thermal sources leading to a trade-off between signal-to-noise ratio, spatial resolution and acquisition time. The modern counterpart – tunable IR laser-based microscopy – on the other hand is typically associated with extensive costs, which are also driven by the application of high-end mid-IR cameras. To overcome these limitations, we demonstrate an alternative approach for hyperspectral microscopy that, to the best of our knowledge, is the first to exploit a single-pixel (SPI) imaging approach for microscopy in the mid-IR spectral range. SPI applies a spatial light modulator, such as a digital micromirror device (DMD) with frame rates of up to 40 kHz, to mask an image with a time-varying pattern. The masked image is then collectively projected onto a single-pixel detector for synchronized intensity measurements, which allows for the reconstruction of the image. Especially for the mid-IR spectral range, this concept brings decisive advantages: image acquisition in the ms time regime, use of inexpensive single pixel detector with excellent detectivity and exploiting the multiplex advantage – known from FTIR spectroscopy – in the spatial domain. Additionally, in the presented SPI microscope, we utilize the dispersion by diffraction of mid-IR wavelengths at the DMD micromirrors (in the range of 10 µm) and apply a single mid-IR modified DMD as a hyperspectral imaging tool [1]. Diffraction limited 64×64 images are acquired and reconstructed in 450 ms and 162 ms per wavelength, respectively, thus, drastically improving the the sample throughput in mid-IR chemical and biomedical imaging. [1] Ebner, A., Gattinger, P., Zorin, I. et al. Diffraction-limited hyperspectral mid-infrared single-pixel microscopy. Sci Rep 13, 281 (2023). https://doi.org/10.1038/s41598-022-26718-6

Additive Manufacturing (AM) for producing 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. This present work aims to reduce the negative impact of manufacturing and assembly errors from AM on optical performance. For this, a tolerance sensitivity and tolerance range analysis for optical systems is conducted. Then accumulated errors from assembly and manufacturing are reduced by design optimization, and the signal-to-noise ratio is significantly enhanced by 265 % compared to the non-optimized design.

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) system 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 the 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.

Conventional ultrasonic systems are commonly used in the industry for the non-destructive evaluation (NDE) investigation of carbon fiber reinforced polymer (CFRP). These systems are widespread and very efficient, but they require contact with the structure to be analyzed, which induces long measurement times. Moreover, major problems arise when the shape of the element to be investigated is complex (peak, valley, small radius of curvature…). During the last decade, multiples optical metrology techniques have been proposed and studied, as alternative or complement. Techniques such as thermography, shearography, or CT-scan have now achieved good maturity. Additionally, laser ultrasonic systems can be used and the recent developments show promising results. All these techniques allow detection to be carried out in a faster, cheaper and just as effective manner. Furthermore, the combination of all these measurement techniques could widen the range of detectable defects and the field of applications. They could also allow an increase in inspection rates, and therefore production speed. In this paper, we present the comparison of the result obtain on multiple CFRP samples specially manufactured with different type of defect such as insert, impact damage, or porosity with all these techniques. We discuss the complementarity of multiple measurements for industrial NDT. We also show the current development of an independent laboratory, with its complete range of CND techniques and capable of managing complex geometries and in a robotic way for future industrial NDE.

The existing nondestructive testing (NDT) technology is difficult to accurately detect the corrosion in steel structures. A novel non-contact terahertz time-domain spectroscopy (THz-TDS) is therefore proposed to measure the corrosion thickness under different coatings, which provides a method to quantify the corrosion degree of steel structures. The experiments show that the delay time difference corresponding to the transmission signal amplitudes exhibits a linear relation with the refractive index, and the refractive indexes are 2.80 (corrosion products), 1.94 (epoxy resin), 2.18 (rubber) and 2.04 (cement paste), respectively. The reflected signal can accurately measure the corrosion layer with thickness greater than 40 μm. The THz-TDS shows the ability to estimate the corrosion of coated steel structures, and to accurately measure the corrosion thickness with an accuracy of more than 90% irrespective of the surface coating materials. It proves the applicability and accuracy of THz-TDS for NDT corrosion thickness of coated steel structures.

3D DIC is an optical noncoherent full-field optical technique that enables the measurement of shape, displacements (u,v,w) and strains (εxx, εyy, εxy) 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 may be insufficient to deliver the information from the entire object. Hence multi 3D DIC systems must be used. In such cases, the output information from all systems must be combined into a single coordinate system. This task is relatively simple if the Field-of-Views (FoV) of each system 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 analyze and compare two optical methods that provide a common reference coordinate system to FoVs distributed in space and provided by multi-3D DIC measurements, namely laser tracker and photogrammetry. Both differ significantly in equipment requirements and require specific calibration targets and additional calibration procedures. The work presents the full calibration and data processing path, measurement uncertainty budget, and estimated accuracy for both techniques. Experimental results of selected engineering structure displacement measurements show the usefulness of both techniques in industrial testing conditions.

The demand for faster surface profilometry is growing for the production of 3-D devices. We demonstrate a new frequency comb-based high-speed, large-area time-of-flight (TOF) detection using electro-optic sampling of space-to-wavelength-encoded optical pulses. By scanning the spatially dispersed line-beam of a broadband comb-signal with a Galvano scanner, rapid and large-FOV surface imaging is possible. After the line-beam is electro-optically sampled, a line-scan camera detects the output spectrum, which contains TOF information. The best axial resolution was 330-pm and 10-nm for 1560-nm and 780-nm combs, respectively. Imaging 6.4-mmX2.4-mm region with 1024X882 pixels and 25-nm axial resolution took only 3.5-ms for 1560-nm system.

Surface structures that affect the imaging properties of optical components are often very small. The measurement of scratches with a lateral width around 1 µm and a height of a few nanometres require high-resolution measuring devices. If areas of several cm² in size must be completely measured and evaluated with the required resolution further challenges arise in terms of measurement speed, data reduction and visualization of small defects. The lecture explains in detail how these challenges have been solved.

Laser Light Section Sensors have become a widespread solution for geometry inspections. In an adaptive triangulation sensor, adaptability of the working distance is achieved by automatic focusing of the camera and repositionability of the laser with a piezo rotation axis and a mirror. This paper presents an extended calibration model of the laser plane, where the position of the projected plane is derived 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.

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. 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 comparison the shape of the sample was measured with a state-of-the-art coordinate measurement machine (CMM). It turns out that the root mean square (RMS) error between the two data sets is smaller than 0.4 µm.