This PDF file contains the front matter associated with SPIE Proceedings Volume 11782, including the Title Page, Copyright information, and Table of Contents.

This paper discusses an approach for simultaneous tomography based on three-color digital holography and recording monochrome holograms. The numerical processing of the monochrome holograms to yield the multiple views is described. The process for 3D reconstruction from aberration-free view of the object are presented. Experimental results establish the proof of principle.

Optical instruments have long played a role in manufacturing, and strong arguments favor accelerated adoption of fast, non-contact measurements of surfaces, shapes and positions as an enabler for industry 4.0. High-precision techniques such as optical interferometry have advanced considerably and have found applications ranging from semiconductor wafer lithography to automotive engine production. Even though there are clear benefits, there are obstacles to the more widespread adoption of optical techniques for dimensional measurements. Many of these obstacles are technical--such as vibration sensitivity and metrological traceability; but others reflect the cultural gaps between academia, makers of optical instruments, standards organizations and end users. In this talk, I propose that understanding these cultural differences can assist in advancing optical methods for the most critical needs of data-driven manufacturing.

The concept of process signature uses the relationship between a material load and the resulting modification remaining in the workpiece to better understand and optimize manufacturing processes. The metrological recording of the loads occurring during the machining process in the form of mechanical deformations is the basic prerequisite for this approach. An appropriate characterization method is speckle photography, which is already applied for in-plane deformation measurements in various manufacturing processes. A shortcoming of this fast and robust measurement technique based on image correlation techniques is that deformations in the direction of the measurement system are not detected and that they increase the error of measurement for in-plane deformations. Therefore, this work investigates a method that infers local out-of-plane motions of the workpiece surface from the decorrelation of speckle patterns and thus is able to reconstruct three-dimensional deformation fields. The implementation of the evaluation method in existing sub-pixel interpolation algorithms enables a fast reconstruction of 3D deformation fields, so that the desirable in-process capability remains given. Using a deep rolling process, first measurements show that dynamic 3D-deformations below the tool can be detected, which confirms the suitability of the speckle photography not only for the 2D- but also the 3D-analysis of deformations in manufacturing processes.

This paper presents a novel quality monitoring method for additive manufactured surfaces combining machine learning and light scattering. The proposed method aims to monitor undesired topographical modifications of additive manufactured surfaces by detecting changes in a scattering pattern using an autoencoder, which is an unsupervised machine learning model, trained with datasets directly measured from reference surfaces with desired surface topographies. Given the unsupervised learning nature of the autoencoder, training with datasets acquired from surfaces with deviations is not necessary, which makes the proposed method appealing, as there is no need to retrieve defective surface samples to train the autoencoder. More importantly, the autoencoder can be updated when datasets from a new type of surface with desired but different topographies are available. As scattering patterns related to new topographies are relatively easy to obtain by experiment, we demonstrate that our autoencoder can be retrained with new scattering patterns and learn to address a wider variety of surfaces, showing superior performance with respect to machine learning solutions adopting a static model, trained only once on the initially available information. Experiments performed on laser powder bed fusion surfaces show that the proposed method is effective. The relatively simple and low-cost setup of the measurement system also makes the proposed method appealing for implementation on commercial additive manufacturing machines.

In-line inspection during the manufacturing process to evaluate topography related parameters has increasingly become important. However, it imposes strong requirements and the in-line inspection systems. Two of the most established optical metrology techniques that can cope well with different surface materials are white light interference microscopy and confocal microscopy. However, the measurement process is very time-consuming. Another technique, which offers single-shot capability and hence fast data acquisition is structured illumination, but restricted to the inspection of rough scattering surfaces only. Multiple-wavelength digital holography (MWDH) offers the possibility to overcome the aforementioned shortcomings. It can be applied on rough scattering and specular reflective surfaces in a lensless lightweight manner. However, the stability and exact knowledge of the wavelengths employed is crucial for the successful application of MWDH. In this paper, we describe a method to ensure wavelength stability for MWDH. Two VCSELs are mounted on a common heat sink with a distance of a few mm only in order to eliminate the influence of wavelength drift between the two VCSELs caused by different temperature. Moreover, the temperature is stabilized to 20C via a Peltier Element. In addition, the light-emitting surface of the two VCSEL is directly bonded to two corresponding single-mode fibers. Further fiber couplers and a fiber combiner are applied to result in a lightweight, highly robust and flexible setup. Information multiplexing via different angles of the two wavelength corresponding reference waves is introduced to enable single-shot data acquisition. In that manner, disturbances caused by changing environmental conditions as encountered in sequential acquisition are eliminated. The advantages of the VCSEL based MWDH system are demonstrated in comparison to structured illumination on non-cooperative materials (scattering and/or specular samples, samples with different colour properties).

Digital multi-wavelength holography is an emerging technology for very precise and fast 3D measurement. Here, we present a novel digital holographic system that uses a 65-Megapixel camera to achieve high resolution measurements on an 18 × 14 mm² field of view resulting in a lateral sampling of ~2 μm in x- and y-direction. Using three single frequency lasers for illumination in a temporal phase shifting scheme, we achieve data acquisition times below 150 ms for full 65- Megapixel 3D-measurements. The choice of the three lasers enables an unambiguous axial measurement range of 400 μm. On a calibrated height standard with a 20 μm step repeatability of <0.01 μm (1 standard deviation) is demonstrated. More challenging and of high interest for industrial applications are measurement samples that consist of surfaces with varying surface roughness, reflectivity or material. These kinds of samples require a sensor with a high dynamic range and pose several geometrical optical challenges: Light from differently reflecting or scattering surfaces travels through the optical system on different paths. Without compensation, this results in small, yet non-neglectable errors in the measured height values. We have applied approaches well described for single-point interferometers to the full-field imaging system used in the presented optical setup. Without a-priori knowledge about surface quality of the sample, we can compensate for these errors. Thus, the presented digital holographic sensor is able to achieve repeatability of ~0.1 μm (1 standard deviation) for height features consisting of rough and specular surfaces.

The electronics industry is creating complex miniaturized devices with steadily higher power density. The increase of maximum operating temperatures affects the thermo-mechanical load and imposes greater requirements on the quality of electronic packages. Fast and reliable methods for inspecting the quality of electronic components can help to improve production quality and to reduce waste and environmental burden. We present a compact optical sensor based on Electronic Speckle Pattern Interferometry (ESPI) that provides a possibility to carry out such control in a fast, precise and non-contact manner and can be integrated directly in a production line. Analysing thermo-mechanical deformations of objects under study, the system is capable of identifying common defects in electronic modules, such as die attachment delamination.

High speed imaging is applied for drop test on composite samples. The effect of optical blur on image correlation accuracy is described. Blur magnitude is tuned by decreasing the exposure time (at constant sensor sensitivity) for the same drop test series on flyspecked composite plates, while focus, aperture and 1:1 magnification are kept constant. Frame rate (80- 20kfps) and lighting are changed while keeping the same image contrast. Results show correlation quality is already altered with subpixel blur. This study enables to define blur criterion in high-speed imaging and completes study about sampling rate effect in time resolved image correlation by high-speed imaging.

For digital image speckle correlation (DISC), a novel approach is introduced where an ensemble average over multiple different speckle patterns is calculated. As a result, the measurement uncertainty of the displacement is reduced by an order of magnitude without deteriorating the spatial resolution. This enables precise surface displacement field measurements in the micrometer range with a measurement uncertainty lower than 100 nm at a spatial resolution below 20 µm. By using a digital micromirror device (DMD) for illumination modulation, measuring rates in the range of 25 Hz are possible while each measurement is based on 80 images.

A limiting factor for Laser-Doppler vibrometer (LDV) measurements is the laser speckle effect, which occurs when measuring on rough surfaces. This effect limits the minimally detectable vibration amplitudes and increases the noise level. In order to reduce the impact of the effect, signal diversity is used in the state of the art. In a previous publication, we demonstrated a newly developed algorithm that combines raw signals, improving the signal quality of an algorithm from the literature. In this article, we examine its real-time capability and evaluate the application range of our algorithm. In addition, we briefly address an extension of the setup to allow the measurement of the 3D velocity vector.

In this work we explore the use of infrared deflectometry in the case of dynamic vibration measurements. This work also aims to provide a comparison of the performances and implementations of visible and infrared deflectometry with measurements conducted on a laboratory test case, a cantilever aluminium beam under impact excitation. The results obtained show that both visible and infrared deflectometry enable measurements with different time and space resolutions, and that infrared deflectometry can be effectively used to perform full-field vibration measurements on an unprepared surface. It could be simply summarized that both techniques have interesting features, and what is earned with one of these techniques is lost on the other. In fact, even if the cameras used for visible and infrared deflectometry techniques have different technical features that directly influence reachable time and space scales, a key advantage is that established methods and algorithms for images post-processing are identical between visible and infrared deflectometry. This practically broadens the range of materials and surfaces that can be tested using the deflectometry technique. The extension of this technique to curved structures is also under consideration, which should provide another additional support to a greater use of deflectometry. With the increasing performance of visible and infrared high-speed cameras combined with their decreasing cost, the measurement of space- and time-resolved experimental data using the non-contact optical deflectometry techniques opens interesting perspectives.

This paper presents the development of a versatile, accurate and efficient speckle simulation tool for the design of laser-based displacement sensors, capable of handling objective as well as subjective speckles. The simulation tool integrates the statistical nature of speckles with the deterministic properties of ray-tracing simulations, providing a reliable estimation of the performance of laser and/or speckle-based sensors in the design phase, even for more complex optical assemblies. It enables the calculation of several simulation outputs in order to determine the best performing system configuration for a given requirement and measurement principle. To validate the simulation results, they are compared against the experimental data of four designed laser-speckle based sensor setups for measuring in- and out-of-plane displacement of a target as well as against analytical relations for describing speckle pattern translation for simple geometries. With resulting simulation errors of less than 2 µm rms for in-plane (output: correlation peak shift) and 2.6 µm for out-of-plane displacements (output: center of gravity shift) for an integrated laser sensor geometry, the good accuracy of the speckle simulation tool is demonstrated.

A new optical configuration of digital holography that employed two multi-reflection reference mirrors is proposed. The number of reflectance orders from the reference mirrors was doubled from the previous research of the digital holography it allows recording more interference fringes without the expansion of the camera’s sensor size. The proposed double multi-reflection reference mirrors can improve the measurement range to twice better than single multireflection reference mirrors e without any degradation of without any degradation of the spatial resolution. The new system can also help to avoid the low fringe contrast of the interference pattern of the high reflection order.

Over the past two decades, laser beam melting has emerged as the leading metal additive manufacturing process to produce small and medium-sized structures. Due to the complex physical phenomena involved in the laser- material interaction, instabilities in the melt pool morphology affect the final quality of the structure and remain difficult to predict by simulation. Several monitoring approaches, based on the radiation of the melt process or on a secondary illumination source, have been developed to measure its length, width and height. Nevertheless, the final morphology of the part is influenced by the volumetric forces as well as by the capillary forces applied to the melt. Thus, the shape of the melt surface is of primary interest to control the stability of the melt. Due to its intrinsic heterogeneity, its motion on the powder bed and its own dynamics, the measurement of the melt pool surface shape requires a full-field ”one-shot” acquisition with a short exposure time of a few microseconds. In this paper, we propose multi-wavelength digital holography for the in-situ investigation of the melt pool.

Digital holography (DH) has been demonstrated as a very powerful tool for micro-plastics (MPs) imaging and recognition, thanks to its unique capabilities such as label-free 3D imaging, flexible focusing and high-throughput. Moreover, the use of machine learning approaches has permitted to surpass main processing limitations in classifying MPs. In particular, the quantitative phase signature provided by DH permits to identify the unique fingerprint for MPs that is crucial to improve the accuracy in features based classification task. In this paper, we investigate new optical, morphological and texture features that can be calculated from phase images of MPs only.

Optical measurement techniques are of immense importance for research in engineering, industrial production and biomedical applications. In this paper, an experimental study to measure temperature, temperature distribution, and temperature fluctuations in diffusion flames has been conducted using a volume phase holographic grating (VPHG) based digital holographic interferometric (DHI) system. VPHG made on Dichromated gelatin provides high diffraction efficiency and minimizes stray light and coherent speckle noise. The results obtained from VPHG based DHI system show a good agreement with temperature measured by thermocouple. The proposed system is compact, robust, requires lesser number of optical components, simple to align and easy to implementation.

We demonstrate a virtual optical instrument for surface measurement. The virtual instrument is fully powered by physical models derived from first principles and provides deterministic measurement results for surfaces with complex topography as obtained using a real instrument. The primary function of the virtual instrument is task-specific uncertainty evaluation; it can also be used to predict the instrument response and measurement result, in order to assess the feasibility of an instrument for a specific surface, find optimal instrument settings, improve the understanding of the measurement process and test new instrument configurations.

To experimentally determine the transfer behavior and the resolution limits of areal surface topography measuring instruments, numerous approaches have been suggested with the two most common ones being based on either rectangular (type ASG material measure) or sinusoidal (chirp material measure) test geometries, imaging various spatial frequencies. Each of the methods carries individual advantages and disadvantages. Here, we describe the design of a varied chirp material measure which provides sufficient amplitudes throughout a broad range of spatial frequencies. For its evaluation, two different approaches based on either geometrical fitting or a direct analysis in the frequency domain are described and compared with each other. The material measure provides a steady spectrum in the spatial frequency domain that can be evaluated with both approaches. Their advantages and disadvantages are composed to derive information for a reliable implementation that leads to a standardized evaluation routine with small uncertainty. The examination of the two methods results in an evaluation with a high statistical reliability that allows an unambiguous determination of the spatial transfer behavior.

The accuracy of 3D additive manufacturing techniques such as direct laser writing (DLW) depends on a variety of parameters of the manufacturing process. This is why the robustness of the DLW process with changing manufacturing parameters is of particular interest. We investigate this robustness by systematically varying manufacturing parameters within a typical range and examine whether the structures change afterwards when they are exposed to stress factors like temperature and humidity. The metrological characteristics are determined by the relative change, just as well as the deviation between actual and nominal parameters of different surface topographies of various material measures. With the absolute values, it can be shown that a change of the manufacturing parameters in the DLW process does not result in a significant change of the metrological characteristics. The manufacturing process itself is therefore very robust. Even when a non-optimal manufacturing parameter set is chosen, the resulting samples feature stable topographic features. When the structures are exposed to external influences such as temperature and humidity, no significant relative change within the metrological characteristics can be observed.

Digital holographic microscopy allows access to the complex electric field, enabling computational propagation of light. This enables computational corrections for lens aberrations, which remove the requirement for antireflective coatings on the lens and enable imaging over a wide spectral range. This makes digital holographic microscopy an interesting candidate for overlay metrology on semiconductor wafers. We show that a single holographic image contains all data that is required for computing the overlay, by using a source with a limited coherence length and incoherently adding multiple measurements on a camera. As an additional benefit we show that such parallel acquisition improves the reproducibility of the experiment by eliminating noise sources that are common to the two measurements that typically constitute a single overlay measurement.

Confocal microscopy is a working horse of optical profilometry since decades. It is a pointwise measurement method, where the whole sample must be scanned in all three dimensions. The high lateral resolution thereby outstrips its lowered scanning speed compared to widefield based principles. Furthermore, for a single 3D surface, even single-digit nanometre depth-resolution has been shown. However, albeit such high axial resolution, the accuracy may suffer from sample or optics induced wavefront distortions that differ from point to point. The acquired signal then experiences a shift that leads to a wrong acquired depth. Here we model this error through a low NA scalar model. We further present a method to compensate this error significantly by enhancing the principle of differential confocal microscopy. Theoretical results show the possibility for ideal compensation of the error caused by such in-stationary aberrations in confocal depth measurements.

Imaging confocal microscopy (ICM) and focus variation (FV) are two of the most used technologies for 3D surface metrology. Both methods rely on the depth of focus of the microscope objective, which depends on its numerical aperture and wavelength of the light source to compute an optical section. In this paper we study how several methods of structured illumination microscopy affect the metrological characteristics of an areal optical profiler. We study the effect of the projection of different structured patterns, the sectioning algorithms, and the use of high and low frequency components onto the optically sectioned image. We characterized their performance in terms of system noise, instrument transfer function and metrological characteristics such as roughness parameters and step height values.

In optical metrology various approaches have been made in order to push the physical limitations of lateral resolution in microscopic and interferometric devices. Microsphere-assisted interferometry enables the measurement of structures well below Abbe’s resolution limit. In order to give an approach for analyzing the obtained measurement data, this study shows the transfer behavior in the three-dimensional spatial frequency domain. With the construction of an Ewald sphere further insight into the role of microspheres in the imaging process can be obtained. For improved analysis, selective illumination is applied to the select those parts of the field of view where the microsphere is located. Further improvement is achieved by appropriate windowing of the measurement data.

The large spot size of a few mm2 with spectrometers and a few thousand µm2 with ellipsometers means that classical spectroscopic characterization is limited to that of bulk materials. In the development of more recent heterogeneous materials in which there exists local variations between materials on a microscopic scale, a much smaller spot size is required for optical characterization. Several new techniques have been developed for performing local spectroscopy, such as by color camera microscopy, hyperspectral imaging microscopy, scattering type scanning near field optical microscopy (s-SNOM) or spectroscopic optical coherence tomography (s-OCT). Concerning the latter, the related technique of coherence scanning interferometry (CSI) also allows local spectroscopy by applying Fourier Transform processing to the local polychromatic interference fringe signal. This technique offers the advantages of not requiring an external spectrometer since an interferometer is incorporated in the microscope objective, but challenges remain in order to correctly adjust and calibrate the system. In this paper we present some of our latest results of using CSI to accurately measure the local spectra at a microscopic scale with a spot size a little larger than that defined by the diffraction limit, of around 1 µm. Results demonstrate measurements of local reflectance spectra at the surface of a heterogeneous sample and on small structures buried within or under a transparent layer. Other theory has been developed to allow the measurement of local transparent layer thickness and refractive index. As well as performing local point measurements, we show how with a single scan over the optical axis, 2D cartographic maps can be made of reflectance spectra together with the topographic height map of the same area. Any nanometric height errors present due to phase on reflection errors linked to the presence of complex refractive indices can then be corrected using the spectroscopic information.

As the requirements concerning lateral resolution, robustness and reliability continuously increase, appropri- ate modeling of coherence scanning interferometry (CSI) as an important method prerequisite of virtual CSI instruments gains in importance. We recently published the so-called double foil model that is based on the three-dimensional (3D) spatial frequency representation of interference signals. The model is consistent with Kirchhoff’s diffraction theory applied to surface reflection and scattering. Scattered light contributions belong- ing to certain plane wave components of incident light are superimposed incoherently. For an instrument of given numerical aperture the maximum lateral resolution provided by the diffraction limit and the capability of measuring steep surface slopes are closely related to the evaluation wavelength, i.e. the wavelength, at which the interference phase is analyzed. In this contribution we extend the model in order to describe the complete measuring process including the depth scan. Our approach introduces 3D representations of both, the surface under investigation as well as the reference mirror as thin foils in cartesian coordinates. Interference is shown to occur after Fourier transformation with respect to the axial coordinate z in the hybrid xyqz coordinate system, where the surface under investigation is treated as a phase object. Consequently, an axial shift of the measurement object or the reference mirror results in different phase shifts of the monochromatic interference patterns depending on the angle of incidence and the scattering angle. Our study combines theoretical considerations and simulations with exemplary experimental results. Conclusions are drawn with respect to signal filtering and analysis aiming at high topography fidelity of CSI systems.

The era of big data and cloud computing services has driven the demand for higher capacity and more compact semiconductor devices. As a result, semiconductor devices are moving from 2-D to 3-D. Most notably, threedimensional (3D) NAND flash memory is the most successful 3D semiconductor device today. 3D NAND overcomes the spatial limitation of conventional planar NAND by stacking memory cells vertically. Since hundreds of vertically stacked semiconductor materials become the channel length in the final product, accurate thickness characterization is critical. In this paper, we propose a non-destructive multilayer thickness characterization method using optical measurements and machine learning. For a silicon oxide/nitride multilayer stack of <200 layers, we could predict the thickness of each layer with an average root-mean-square error (RMSE) of 1.6 Å . In addition, we could successfully classify normal and outlier devices using simulated data. We expect this method to be highly suitable for semiconductor fabrication processes.

With the increasing application of thick composites in marine, wind energy and aerospace industries, the inspection of thick composites becomes more and more challenging when considering the variety of thick structures (e.g., laminate, sandwich, honeycomb structures). Shearography is a full-field and non-contact optical non-destructive testing (NDT) method which is normally used to inspect composite laminates up to 10 mm while for the thick composite laminates (e.g., with the thickness of more than 50 mm), its performance is not clear yet. In shearography NDT, a defect-induced anomaly is revealed from fringe or phase maps obtained by comparing two states of deformation of the specimen to be inspected. Thermal loading is widely used to deform the specimen due to its advantages of convenience for on-site inspection and cost-effectiveness. The objective of this study is to improve the defect detection capabilities of shearography when used to inspect thick composites. For that, spatial modulated thermal excitations are investigated. A thick composite model has been built in Abaqus to assist the shearography inspection. Various kinds of spatially modulated heating including local heating and global heating are explored for thick composite inspection with shearography in order to evaluate the corresponding efficacies in defect detection. We will present both experimental and numerical results on spatial modulated thermal loading. Defect-induced shearographic responses subjected to local and global thermal excitations will be discussed in this paper, including the influence of short-time heating and long-time heating on thick composite inspection. Current results indicate that long-time heating is more favorable when inspecting deep defects in thick composites, and with local heating it is possible to increase the defect-induced signal when compared with global heating.

To measure the dynamic behavior of rotating fiber reinforced polymer structures, an optical diffraction grating based sensor has been developed to measure in-plane strain and out-of-plane tilts on the rotor surface with high spatio-temporal resolution. A novel modal analysis approach based on the combination of consecutive measurements with a known excitation and varying time delays was used to extract fully featured modal parameters from a rotating composite disk. The results of the diffraction grating based approach were compared to the results of a commercial triangulation sensor and a piezoresistive strain gauge and showed excellent SNR at surface speeds up to 280 m/s.

Here we propose the use of the pyro-electric effect for the easy fabrication of polymer optical micro-structures and the perspective of case use as smart sensors.

The last decades have seen a large development of functional coatings, which often leads to high-tech assemblies. In order to ensure the durability and functionality of such systems, there is a specific need for characterisation at a low scale since the properties of thin materials may differ from those of bulk ones. The nature of coatings also often asks for caution when used in different operating conditions. This work deals with the adaptation of a classical interferometry technique to the expansion characterisation in a specific material configuration: thin films made of potential conditionssensitive materials. Despite being known for ages, the Michelson interferometer has scarcely been used for characterisation of properties in materials science1. Following a variation of a multi-reflection interferometer proposed back in 19822, we propose an adaptation of the Michelson interferometer that allows quantifying the coefficients of expansion of thin polymeric materials, inducing some technical challenges which needed to be coped. After we tackle the experimental issues inherent to those innovative improvements, the validation of the technique is performed by measuring accurately the expansions under several experimental conditions of various specimens made of five different well-known thermoplastics and a polysaccharide used extensively in food packaging. The accuracy of the measurements is estimated. The reliability of the device developed in the laboratory applied to the characterisation of the thin films is discussed as well as the advantages and limitations of the technique. Through the material examples used, we point out the benefits of this technique in materials science characterisation, notably by reporting interesting behaviours of specimens barely explored in the literature so far.

Traditional approach to stray light characterization is intrinsically limited. While the stray light level in an optical instrument can be measured, it is not possible to derive from experimental measurements the origin of the different features. Consequently, when unexpectedly high stray light is present, it is extremely difficult to find how to improve the system. In this paper, we introduce a new method where a pulsed laser and an ultra-fast sensor is used. As different stray light contributors have different optical path lengths, they reach the detector at different times and resolving them temporally allows to measure them separately. Their origin can be retrieved by using the optical path length as a mean of identification. We present the conceptual study and the experimental proof of concept of this new method. We were able to characterize individually the different stray light components in an imaging system and determine their origin. We show how the measurements allow to reverse engineer the instrument properties and even verify sub-system requirements.

This study forms a part of the research in using nanoparticles (NPs) to increase the intensity of light scattering signal in the optical fibres. Increasing the intensity of the backscattered light signal in the optical fibres shows the potential to increase the signal-to-noise ratio in order to improve the sensitivity of the backscatter reflectometry. Doping NPs into the optical fibres can greatly increase the scattered light. However, it is not easy to manufacture NP-doped optical fibres to test different designs. To overcome this problem, in our former work we used the method of dropping refractive index matching liquid containing gold NPs at the optical fibres end tips to investigate the intensity change of the scattered light from the interfaces. In this paper, some new initial experimental results for the scattered light between the optical fibre end tips are shown. Gold NPs have been mixed into the optical adhesive (Norland) and is then dropped and cured at the optical fibre end tips. A backscatter reflectometer (LUNA ODiSI-B) was used in the experiment to measure the intensity of scattered light distribution between the optical fibre end tips. We investigated 4 cases of light scattering between the optical fibre end tips: (i) the backscattered light intensity distribution in the case of the air gap between the optical fibre end tips; (ii) the backscattered light intensity distribution with optical adhesive between the optical fibre end tips; (iii) the backscattered light intensity distribution with optical adhesive containing gold NPs (gold nanopowder (<100 nm), Sigma Aldrich) between the optical fibre end tips before curing process and (iv) the backscattered light intensity distribution with optical adhesive containing gold NPs between the optical fibre end tips after the curing process. Our initial findings are that the scattered light by gold NPs at the optical fibre interfaces can be detected by the backscatter reflectometer. By obtaining the differential signal between the distributed light scattering by cured optical adhesive containing gold NPs and only optical adhesive between the optical fibre end tips, the light scattered by the gold NPs has be determined.

LiDAR technology is increasingly being used as an area-based 3D measurement method. In addition to high aspirations in terms of accuracy, speed and resolution, manufacturers of lidar cameras are competing to reduce size, weight and power consumption. As one of the most compact high-resolution systems, the Intel RealSense L515 has undergone extensive testing according to VDI/VDE Guideline 2634. In addition, tests were conducted with glossy or partially transparent surfaces (acrylic glass, carbon fiber material and aluminum) as well as with human skin. The latter shows the applicability for human-machine interactions. Both laboratory conditions and the influence of natural light were used as environmental conditions. A comparison of the results is given by the Intel RealSense D415 stereo camera system.

Exploiting the optical memory effect and the equivalency between the spatial and the ensemble cross-correlation of speckle patterns, we developed a new single-pixel camera concept. It allows lensless microscopic spectrally and depth resolved imaging with spatial light modulators displaying pseudorandom scattering arrays. Since the image retrieval is based on correlation calculations, the concept proves to be quite robust to noise.

The 3D measurement of the interior of hollow technical bodies is still a challenge for state-of-the-art measurement systems. This is especially true for complex industrial free-form objects inside confined spaces. Fringe projection enables accurate and fast 3D measurements of object surfaces. To fulfill the task of geometric inspection of confined spaces, we have developed an endoscopic and flexible fringe projection system. The fringe patterns are generated with a RGB LED projector. Fibre-coupling of the structured light is achieved by using a microscope lens and bundle of coherent image fibres. A compact sensor head can be achieved by using a micro-objective for projecting the fringes and a Chip-on-Tip camera to capture the images. A micro lens with a diameter of 1.7 mm was used as the projector lens and a 2 megapixel 1/6” chip was used as the image sensor. By synchronizing the projector with the camera, the system is capable of capturing up to 10 grayscale patterns per second. The measurement volumes result to approximately 20 x 13 x 4 mm³. Typically, the measurement time is in the range of 1 - 3 s, depending on the number of projected images. Measurements on a 50 µm step standard confirmed that a measurement uncertainty of less than 29.4 ± 3.2 µm is achieved with this system. Mounted on a carrier system, the presented fringe projection system offers the possibility to enter confined areas and perform high-precision 3D measurements.

Precision cylindrical surfaces are very important elements in mechanical engineering applications involving sealing and guiding for precise movements. It is always important to measure the geometry of these parts to ensure smooth operation. There are several devices on the market that use tactile methods to measure the roundness of cylindrical sections. Although the metrological performance is very good, there is always a risk of scratching the measured part. Non-contact optical methods are very attractive alternatives. Deflectometry is one of those methods with great potential to accomplish this task. It is based on the indirect measurement by analyzing the distortions observed in the image of a structured pattern reflected by the measured surface. However, the application of deflectometry in precision measurement is restricted to flat or near flat surfaces. This paper presents principles and configurations for extending deflectometry to measure external and internal high quality cylindrical surfaces. In both cases the central element is a 45° conical mirror, capable of optically transforming cylindrical surfaces into planar images. An on-axis configuration allows the observation of a sequence of phase-shifted images of appropriate structured patterns reflected on the measured cylindrical surface. The patterns are formed by radial lines with sinusoidal profiles, which are rotated to promote the phase shift. An integration algorithm, suitable for periodic signals, reconstructs the geometry of cylindrical sections. As a result, it is possible to reconstruct the profile of cylindrical sections with great precision, and to determine the roundness parameters. However, it is not possible to determine the absolute radius of the measured section. The paper presents calibration considerations and the first measurements results of sections of precision pin gages. Comparisons with tactile measurements showed deviations of up to 0.07 µm in high polished cylindrical surfaces. The paper also discusses the main limitations and needed improvements. There is much work to be done before the concepts developed can be the basis of a new commercial system. However, there is great potential for the emergence of a new family of optical measurement systems.

Triangulation based optical measuring systems are an important element of industrial quality assurance. Due to their robustness and cost-effectiveness Laser Light Section Sensors have become a widespread solution for Geometry measurements. In order to reconstruct the scene, it is necessary to identify the corresponding laser line, which is distorted due to the geometrical properties of the specimen, in the camera image. In Order to achieve the highest precision possible, the line segmentation has to be performed at sub-pixel accuracy. Furthermore, in an industrial environment, interfering light sources may be present. A distinction between these influences and the laser light ensures a robust measurement. The projected Laser Line of a triangulation sensor is usually formed by a Powell lens from a point source, which results in a uniformly distributed intensity. Another option to achieve highly uniform intensity distributions is by means of a lenticular lens. A side effect of these optics is that the fine-structure of the projected line is formed by a chain of equidistant dots. Across the laser line the intensity distribution can be considered as a Gaussian profile. Challenges to the segmentation are from the fine, dotted structure of the line. Although conventional methods, such as centroid based algorithms can be applied, with the drawback of imprecise peak detection. In order to insure both segmentation accuracy und robustness, this paper introduces a novel segmentation method based on wavelet-transformation. In a first step the periodic fine-structure of the line is utilized for a definite identification of the line with distinction from scattered light. In a second step a gaussian wavelet is used to achieve sub-pixel accuracy in peak detection. The developed method is compared to conventional peak detection methods.

Acousto-optical tunable filters (AOTFs) based on interaction of light and ultrasound in uniaxial birefringent crystals are widely used in imaging spectroscopy applications due to random spectral access, narrow controllable bandwidth, compactness, overal ease-of-use and image trasmittance capability. The spectral transmission of AOTF-based spectral imagers is usually characterized for paraxial light beam, but AOTFs inherently have non- uniform spatio-spectral transmission, so the central wavelength of the transmission window varies with the angle of incident light. We demonstrate that the spatio-spectral characteristics of acousto-optic (AO) interaction may be described either by the dependency of ultrasound frequency on the incident light angle for a given wavelength or by the dependency of wavelength on the incident angle for a given sound frequency. These dependencies are derived from the phase matching condition and are determined by the refractive indices, sound phase velocities and the AO diffraction geometry. We experimentally estimate the appearance of this specific spatio-spectral structure on the images acquired by AOTF-based imagers based on both collimating and confocal (telecentric) schemes and show that the variations of central wavelength and spectral bandwidth are noticeable for collimating setup even in the commonly used small field of view. The results of the study may be applied in design of AOTF- based spectral imagers and image processing algorithms.

Background: The universal pressure to reduce costs is particularly sharp with respect to those manufacturing operations that are not considered to directly contribute to the final product. This often results in defect inspections having an aggressive level of inspection area reduction, especially where it is necessary to identify the extent of excursions or perform engineering experiments. Inspection swath skipping is one of the most convenient ways by which wafer inspections can have their inspection area reduced and consequently inspection times shortened. This presents some challenges for those charged with interpreting the subsequent results. Objective: The recovery of the underlying defect distribution from wafer inspections that use swath skipping, in order to determine the true condition of the wafers and allow the accurate estimate of their upper yield potential, for process control purposes. Approach: A simulated wafer containing three distinct binomial distributions is examined in order to demonstrate the impact of reduced inspection coverage upon the observed defect distribution. Two of the distributions are localized to represent the behavior of defect signatures or systematic distributions. Results: Using the ratio of defect bin counts to identify the most likely defect density to have produced the observed counts, is demonstrated as an effective means to identify distinct density components. These components once scaled can be used to reconstruct the original, underlying defect distribution. Conclusions: The presented method is capable of handling wafers affected by highly localized defect signatures or systematic distributions. Thus allowing dramatic reductions in inspection times without a loss of process control. This extends beyond the capabilities of conventional models to restore distributions with fidelity and should be of benefit to operators of Bright-filed inspection tools and the tool manufacturers.

In the scope of three-dimensional (3D) measuring methods, structured light profilometry (SLP) and shape from focus (SFF) methods are based on distinct optical setups with their own advantages, such as good reliability for SLP and extended depth of field for SFF. This article proposes to adapt an SFF method on a device configured for SLP which constitute a first step towards the fusion of SFF and SFP methods. The proposed SFF method remains valid although the optical axes of the projector and camera are not aligned with the SFF translation direction. This configuration is unconventional for SFF; therefore, each point of the scene is no more static on the captured images during the translation process. To overcomes this phenomenon, each local area of the scene along the set of captured images is tracked using calibration data. A large part of the calibration steps applied to the proposed SFF method are similar to the ones used for SLP, which are based on homography calculation. Furthermore, during the measurement process, the patterns projected for the proposed SFF method to measure the focusing are identical to the ones projected for SLP. To demonstrate the validity of the method, experimental results are provided with depth profile comparison between SLP and proposed approach of SFF.

Fringe projection is a measurement technique able to produce high-density data measurements of Lambertian surfaces using a camera and projector. Evaluating the uncertainty of a fringe projection system is challenging because of the highdensity and the interdependence of data involved in both characterising the instrument parameters and conducting the measurement. Fringe projection systems undergo a characterisation process to define the parameters that triangulate the surface points from camera images. The accuracy of the parameters defined during the characterisation process will partly define the accuracy of the final measurement. The characterisation process is often called “calibration” – yet does not provide a value of uncertainty of the parameter values found during characterisation. This paper outlines a method to allow the evaluation of uncertainty in a fringe projection system caused by imperfect characterisation procedures before applying this method to a flat plane artefact and a sphere plate artefact.

Optical triangulation systems based on fringe projection profilometry have emerged in recent years as a complement to traditional tactile devices. Due to the good scalability of the measurement approach, a highly compact novel sensor for maintenance and inspection in narrow spaces is realized by applying optical fiber bundles. Especially in the field of high-resolution and rapid maintenance in industrial environments, numerous applications arise. Endoscopic 3D measurements of gearing geometries are of particular technical relevance for detecting and quantifying damage. The measurement performance depends to a considerable extent on the technical surface to be inspected. Polished surfaces are particularly problematic due to specular reflections, but can still be partially reconstructed by using HDR imaging. However, if multiple reflections occur due to the specimen geometry and sensor arrangement in such a way that the optical path of each corresponding camera pixel can no longer be reconstructed unambiguously, a measurement is no longer feasible. In this study, the effects of surface roughness, sensor arrangement, and triangulation angle on measurement error are systematically investigated to describe possible application limits and provide guidance on sensor operation.

Hydrodynamic tunnel is an effective mean for studying wing flow process in aerodynamics and hydrodynamics. It allows to study flow characteristics in controlled conditions and to model the conditions that could not be studied in real flight, such as aerodynamic characteristics at critical angles of attack, in icing conditions etc. Techniques for flow visualisation such as coloured jets or small particles allow to have a qualitative data about flow behaviour, being the valuable means for understanding flow behaviour. But it is more important to have quantitative characteristics of the flow allowing to predict the process evolution and to develop safety measures and recommendations. The presented study addresses to developing a system for optical 3D measurements in hydrodynamic tunnel basing on photogrammetric techniques. To provide accurate measurements in condition of two optical media interfaces (air-glass and air-liquid) the accurate model of image formation accounting refraction is developed. The developed photogrammetric system includes several high speed cameras (from 2 to 4 cameras) mounted in a fixed position relatively the working space and a structured light projector. Original technique is applied for the system calibration. Two metrics has been used as a measure of the accuracy of the calibration: the first one being based on the test field points measurements, and the second one using points-to points distance for the surfaces of a reference object. The key contributions of this paper are: (1) accurate model of image formation in case of several media interfaces (2) a technique for photogrammetric system calibration for 3D measurements in hydrodynamic tunnel (3) experimental evaluation of calibration accuracy for multi-media 3D measurements. The performed experimental evaluation of the developed photogrammetric system has proved high accuracy of system calibration and optical 3D measurements in multi-media optical environment. The developed technique for photogrammetric system calibration and 3D measurements demonstrated applicability for the task of 3D flow analysis in hydrodynamic tunnel.

In the development of a high-precision vertical Fizeau interferometer with 300-mm aperture, uncertainty evaluation is a significant method to assess the accuracy of interferometer. In this paper, we reported an uncertainty evaluation method. Then, the uncertainty evaluation of a 300-mm-aperture vertical Fizeau interferometer was carried out based on the method. The uncertainties of interference cavity measurement and absolute testing (e.g., three-flat method) were used as indicators of uncertainty evaluation. In addition, the gravity-induced deformation of three-flat method can be compensated accurately by finite element method modeling. Finally, we obtained the uncertainties of interference cavity measurement and three-flat method. With 300-mm aperture, the expanded uncertainty of interference cavity measurement is 3.7×10^-3 λ, and the expanded uncertainty of three-flat method is 1.47×10^-2 λ.

Whether the center thickness and air gaps of the inner lens can be measured during lens assembly depends on the configuration of the entire lens assembly. The corrector lens of a conventional remote sensing catadioptric telescope generates divergent optical information because of its negative diopter lens constructions. Consequently, the center thickness of the lenses and center air gaps between lens interfaces cannot be easily measured. This can be solved by equipping a positive diopter lens on the image side during remote sensing catadioptric telescope lens optimization and by measuring the center thickness and air gap of the lens from the image side through low–coherence interferometry. The results of this study indicate that the optical signals and air gap interfaces of four lens elements can be clearly identified using low–coherence interferometry and that the center thickness and air gap interfaces of lenses can be calculated accurately.

In this paper, a Fizeau interferometer with double interference cavity is proposed to solve the influence of the environmental factors in high precision phase measurement. The proposed method adds a reference mirror, combined with a low-coherence source, to construct two interference cavities, which have the synchronous phase change. One of them has an adjustable spatial carrier frequency, which is used to calculate the relative deformation phase during the test, while the other has the same null fringe with the standard Fizeau interferometer. The measured phase can be retrieved by using the least square method with the calculated deformation phase and null fringe patterns. In addition, the visibility of the fringe of the two interference cavities are analyzed. Simulation and experiment demonstrate that the proposed method can realize the dynamic measurement of the mirror under the influence of the time-varying environment, and has reliable measurement accuracy.

A two-step iterative algorithm (TSIA) immune to tilt shifts is proposed for phase extraction in phase-shifting interferometry (PSI). The TSIA constructs a model of the least-squares iteration of the phase distribution and the tilt shifts based on parametric decoupling. Finally, the phase distribution is extracted via the least-squares method. The experimental results show that the PTI has high accuracy and fast iterative convergence speed for the conditions of the large amplitude of tilt shifts, closed fringes, nonuniform background and modulation.

Parallel plates are commonly used as optical transmission elements. Their main optical parameters include the optical homogeneity, optical thickness, and surface shape. If the optical parameters are not consistent, the transmissive wavefront will be changed, and the performance of the optical system will be reduced. Herein, a weighted multi-step phase-shifting algorithm is developed. The algorithm can measure the optical parameters with high accuracy based on wavelength tuning interferometry, and extracts the phase at the target frequency by weighting the sampling values. Both the simulated and experimental results show that the algorithm can suppress high-order harmonics and is insensitive to the phase-shifting errors caused by nonlinear wavelength tuning and coupling errors between phase-shifting and harmonic errors. Moreover, the algorithm requires fewer interferograms, improves the fringe contrast, and has a high detection efficiency. It is suitable for high-precision parallel plate optical parameter measurements.

The pursuit of thinner and lighter smartphones drives the demand for advanced packaging now. The current output of advanced packaging wafers are close to 40% of the total global wafer production. Advanced packaging technology needs to combine the circuit, cooling and other technologies to ensure that the electronic parts have the best performance and reliability. PI can meet the requirements of high heat resistance, high mechanical, high insulation, high frequency stability, low dielectric constant, dielectric loss, and low thermal expansion coefficient. PI becomes the core material for advanced packaging. However, PI uniformity, film thickness, etc. will affect the characteristics of the product, so the requirements for uniformity, thickness, etc. also tend to be strict, and various coating accuracy requirements need to be monitored with different monitoring systems. In the monitoring of uniformity, it is more difficult to monitor the surface morphology due to the transparent material. For measuring surface topography, the information on topography comes from analyzing the reflected signal, and the reflectivity of the material is affected by the ratio of the refractive indices of the two media. In this paper, when measuring the topography of a known sample, it is necessary to consider the refractive index ratio of PI and the underlying medium. Through the refractive index changes of different wavelengths, select a suitable wavelength to measure the topography.

Using digital holography in camera-based interferometers, 3D surface topography can be measured extremely quickly and with sub-wavelength precision. Using spatial phase-shifting, a single camera image is sufficient to reconstruct complexvalued wavefronts for multiple wavelengths. Recently, measurements on moving objects were demonstrated using setups with 1× magnification. Increasing the lateral resolution by implementing larger magnification in a microscopic setup would open up new application fields, but the larger numerical apertures (NA) of microscope objectives make the acquisition even more sensitive to motion. In this work, we show the first microscopic setup, measuring objects moving at several mm/s using two-wavelength holography. Despite the large NA of 0.42 of the 10×-objective in our setup, measurements can be acquired at 75 mm/s and beyond. Using two lasers emitting slightly different wavelengths (637.76 nm and 632.87 nm), a maximum height difference of 41.3 μm can be detected unambiguously. One single image covers a lineshaped measurement area of 3.7 mm × 0.2 mm with a lateral pixel pitch of 0.47 μm. In order to inspect larger objects, single frames can be stitched together, permitting an infinite measurement area in the direction of motion. Gap-free stitched measurements are limited to 75 mm/s due to the framerate of the camera. Measurements of the groove depth averaged over several pixels on a groove standard show a repeatability exceeding 10 nm at 35 mm/s and 20 nm at 75 mm/s.

Accounting for nonlinear elasticity of modern materials becomes very important due to their rising operation at high dynamic loads. Generation of strain solitary waves (solitons for brevity) is one of the processes of interest, however details of a transformation of an initial impact into the soliton are not completely clear yet. In this paper we demonstrate the advantages of a combination of classical and digital holographic recording for investigation of the early stages of soliton formation. While classical realization of holographic interferometry allowed for visualizing sharp phase gradients representing, in particular, shock waves, digital recording supplied quantitative data on parameters of smoother disturbances evolving in the course of soliton formation. The applied holographic techniques allowed us to monitor the entire process of soliton formation, to visualize intermediate wave patterns and to obtain quantitative data on the resulting soliton.

Hybrid manufacturing processes, high level of automation, short product service life and decreasing vertical range of manufacture in production request for increasing flexibility and speed of quality control. With HoloCut we previously introduced the world’s first wireless digital-holographic sensor system prototype for fast and precise measurements inside a machine tool. With the experience gained so far, we now present an improved, even more compact sensor system, for the use on various multi-axis systems such as coordinate measuring machines (CMM), robots and machine tools and show first results with different handling systems. Besides improved mechanical stability, a size and weight reduction resulted from a new design approach: The arrangement of components around a central "core" made it possible to create a very compact design with a diameter of 125 mm, a height of ~180 mm and a weight of ~2 kg. The system features a 12.5 × 12.5 mm² measuring field with a lateral sampling of 4 μm. An NVIDIA Xavier embedded system enables pre-evaluations of the recorded measurement data in order to allow re-recording them, even before the complete data transmission (up to 160 MB with 2 Hz measuring rate) and evaluation. This is especially important for the use in vibration-prone environments such as multi-axis systems. Various handling systems such as a HERMLE C32U machine tool, an undamped LEITZ Reference HP 15.9.7 CMM and a UNIVERSAL ROBOT UR16e are examined with regard to vibrations. In future work, the behavior of the system under higher vibration amplitudes will be characterized.

Optical non-contact surface texture measurement is of great value in terms of high speed and non-invasiveness. The purpose of this study is to obtain accurate surface roughness information using an optical non-contact method. We propose a new method to accurately acquire the surface roughness parameter with the scattered light intensity distribution. The roughness parameter Rq was determined using the generalized Harvey-Shack (GHS) theory. Furthermore, the roughness parameter R▵q was estimated from the argument ”a” of the K-correlation model. The results using the proposed non-contact method were in good agreement with the conventional contact method in most cases.

Nano surfaces offer exciting opportunities to implement novel technologies. With their help, surfaces can be created that are very suitable as a reflection-absorbing layer. In order to be able to determine the degree of reflection and to compare it with the current methods of anti-reflection coating, a measuring system was developed which can measure the degree of reflection hemispherically. Furthermore, a simulation model was developed with which the behavior of the nano-surfaces can be examined with different parameters.

Industrial inspection of processes by capture of speckle patterns often requires detection of a small activity area buried in a background. This work presents analysis of sensitivity of the dynamic speckle method by processing simulated and experimental correlated in time 8 bit encoded speckle patterns. Simulation of the patterns was done for an exponentially decreasing temporal correlation function of intensity fluctuations by Fresnel propagation of a monochromatic wave reflected from a delta-correlated in time phase screen and captured at different diameters and focal distances of the optical sensor objective lens. For the experiment, we used a 3D printed flat object with hollow sections that was covered with a transparent film and a droplet of a polymer solution and monitored the process of their drying. Both normalized and non-normalized processing algorithms were used.

The removal of erroneous points is an important pre-processing step in the analysis of point clouds, to ensure a high quality evaluation of the measured objects. However, modern optical 3D scanning technologies generate point clouds that can contain hundreds of millions of points. Existing algorithms for the removal of erroneous points can face difficulties, due to the amount of memory that is required to process these point clouds. We present a new method that is based on the well known Local Outlier Factor algorithm. It adapts the calculation of the factor in minor ways to reduce its runtime. More importantly, we employ a new processing strategy that significantly reduces the overall memory consumption of the algorithm. This enables the detection and removal of outliers even in very large point clouds. In order to show the effectiveness of our new method, we evaluate the processing on multiple, differently sized point sets and demonstrate the configurable memory consumption of our new technique.

The article describes forming the device's software and hardware components for automated analysis of the shape of objects located on the cutting table in industrial robotic systems. A video camera with a resolution of 800x600 pixels is used as a data recording device. When performing an analysis object on the cutting table, a single image is formed. The image stitching process uses algorithms: preprocessing, simplification, and highlighting of key features. The converted data is combined into a single data field that is analyzed at multiple levels. The analysis is carried out based on the accumulated data and the basis of the full image with the subsequent transition to small (original) parts. The second step is to transform the image. On the resulting image highlights the border of the shape of the object, which is located on the cutting table. At the final stage, the shape is analyzed and its boundaries are clarified. Further, a cloud of data is formed, transmitted to the computing PC unit of the cutting table in the form of a complex contour of the object. The application of this approach makes it possible to automate the process of binding the zero point, minimize the number of unused residues, and form a field contour into which an object can be placed for its cutting. As test data, we used the results of applying the blocks of the proposed approach to analyze an object's shape located on the table's working table for waterjet cutting. The working field of the machine is 1.5x1.5 m. The camera resolution was 800x600 pixels in RGB format.

Additive manufacturing has deeply simplified the production of prototypes by speeding up the realization of complex 3- dimensional objects. A well-designed mechanical part requires perfect knowledge of internal stresses and mechanical behavior of the used material. In this study, we propose a new application for the optical-flow method in the field of extensometry. This method has already proven its capabilities for measuring the displacements and speeds of moving objects in video sequences for many motion tracking applications. The orientation of the fused filament deposited strands influences strongly the behavior of the FDM-manufactured part. In order to study the behavior of a mechanical part as a function of the arrangement of the deposited strands, we produced, using FDM, tensile test specimens by superposing several homogeneous layers. Each of the layers is characterized by a fixed strands orientation. These specimens are then tested on a tensile test machine. Due to the different arrangements of the strands according to the layers, the deformations on the surface of the sample can be complex. A conventional extensometer is then no longer usable. Accordingly, we propose to apply the optical flow method to a video of the test sample surface recorded during a tensile test. We present the experimental setup of the measurement method, the implemented algorithm based on a Mathematica optic flow builtin function and some illustration results. The results show that this technique can be used on real parts subjected to stress in order to reveal weakened areas.

The improved autocollimator for measuring angular and shift deformations of the large constructions as mirrors of the rotatable radiotelescopes are analyzed. Two types of the measuring scheme for autocollimator are researched. The first type of the autocollimator scheme uses three cube-corner reflectors which are set in 3 points of the counter reflector of the radiotelescope. This type of the autocollimator measures the angular and line shifts in a large range. The counter reflector is set on the SEMS moving platform and the range of its line and angular shifts is 3 arc degree and 100 mm respectively. The second type of autocollimator scheme uses a tetrahedral reflector. This autocollimator is used for the measuring pitch and yaw angular deviations and line shifts of the section boards of the main mirror of the radiotelescope. The range of section board line and angular shifts is 30 arc minutes and 10 mm respectively. The changing one measuring scheme to another is made by switching the radiation marks of the autocollimator. The technical characteristics of the suggested autocollimator and algorithms of measuring line and angular shifts are discussed.

Diffuse reflectance infrared spectroscopy has gained traction in many industrial applications in the recent years due to the emergence of new generation of low cost handheld spectrometers that did not exist a decade ago. Real-time monitoring puts a limit on the sample preparation process especially with inhomogeneous samples in the food industry, like grains, hay, wheat and corn. The heterogeneity of the samples and the pseudo-random spatial arrangement of the grains in front of the optical interface, leads to prediction errors. The spatial variations depend also on the spot size of the diffuse-reflected scattered light from the sample that is collected by the spectrometer. A larger spot size leads to simultaneous averaging of a larger amount of spectrospatial information from different locations on the sample, leading to better repeatability and better prediction accuracy. Up to date, the Microelectromechanical (MEMS) based spectrometers reported in the literature have limited optical spot size, usually smaller than 3 mm in diameter. We report MEMS based FTIR spectral sensors with optical spot sizes of 6 mm, 10 mm and 20 mm working across the spectral range of 1350 nm to 2500 nm. The core spectral engine comprises monolithic MEMS chip, micro-optics for light coupling and a single photodetector in a tiny package. The optical head combines several miniaturized filament- based lamps and reflective optics for illumination. The sensors are compared and the 10-mm sensor gives an optimal performance with a Signal to Noise Ratio (SNR) of 4000:1 and spectrospatial photometric repeatability down to 0.02 absorbance units.

This paper discusses an intelligent sensing solution based on the phenomenon of the whispering gallery modes in the optical microcavities realized within an affordable instrument configuration featuring simultaneous excitation of multiple microresonators by a single frequency laser along with parallel collection of their signal. Supplemented with a machine learning engine for complex signal interpretation the sensor demonstrates the accuracy of 10^-6 for refractive index prediction and more than 98% for protein concentration classification.

Laserpoint has developed and is starting market introduction of the fastest commercially available High Speed Laser Energy Sensor (named “Blink HS”, patent pending). The advantage of this sensor over pyroelectric sensors and photodiodes for laser measurement is the combination of fast response time (sub-ns), broadband (0.2 to 11 microns), high energy saturation (10 mJ/pulse), pulse duration from femtosecond to microsecond with frequencies up to 1 MHz. In a custom solution the response of the sensor was reported up to 2 MHz. The Blink HS Sensor has been fully validated during benchmark test at Fraunhofer-Institute for Solar Energy Systems; the average power lasers emission was inferred by energy/pulse (of every pulse) and frequency detection, in a wide range of power, wavelength, and pulse duration, and is compliant with specified calibration uncertainty. Here, we focus on two key laser characteristics, i.e. turn on transient effect and pulse to pulse stability of medium power lasers. Some lasers cannot output pulse trains with constant pulse energies when gated. Especially the first pulse of the pulse train might have a considerably higher energy. Blink HS Sensor allowed us to capture and quantify metrologically the pulses just after the gate signal was turned from low to high. The pulse to pulse stability is a paramount figure of merit of high-quality lasers, in order to allow precise and reproducible micromachining processes, laser metrology, etc. Sensor operation in “pulses parameters mode” allows the detection and recording of the energy of 32,768 consecutive pulses, e.g. at 10 kHz a snapshot of 3.28 s emission can be observed. Peak power, energy/pulse and instantaneous frequency are computed and recorded for each pulse; those data, along with quantization error as low as 0.35 microjoule allows the detailed characterization of high stability laser sources.

The simultaneous determination of multiple physical or chemical parameters can be very advantageous in many sensor applications. In some cases, it is unavoidable because the parameters of interest display cross sensitivities or depend on multiple quantities varying simultaneously. One notable example is the determination of oxygen partial pressure via luminescence quenching. The measuring principle is based on the measurement of the luminescence of a specific molecule, whose intensity and decay time are reduced due to collisions with oxygen molecules. Since both the luminescence and the quenching phenomena are strongly temperature-dependent, this type of sensor needs continuous monitoring of the temperature. This is typically achieved by adding temperature sensors and employing a multi-parametric model (Stern–Volmer equation), whose parameters are all temperature- dependent. As a result, the incorrect measurement of the temperature of the indicator is a major source of error. In this work a new approach based on multi-task learning (MTL) artificial neural networks (ANN) was successfully implemented to achieve robust sensing for industrial applications. These were integrated in a sensor that not only does not need the separate detection of temperature but even exploits the intrinsic cross-interferences of the sensing principle to predict simultaneously oxygen partial pressure and temperature. A detailed analysis of the robustness of the method was performed to demonstrate its potential for industrial applications. This type of sensor could in the future significantly simplify the design of the sensor and at the same time increase its performance.