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In this article, we describe a system suitable for the evaluation of residual stress in ceramic coating under industrial conditions. Laser ablation is used for material removal, and digital holography measures the displacements produced by the residual stress release around the processed area. The residual stresses at different depths below the surface of the coating are calculated by finite element method from the displacements, the shapes of the machined surface, and the material parameters. Experimental results are presented.

Zusammenfassung Mit einem Multi-Punkt-Positionsmesssystem ist es möglich, Positionen und Orientierungen mit sehr hoher Genauigkeit bildbasiert zu messen. Das Messsystem besteht aus einem einfachen Kamerasystem, das um ein diffraktives optisches Element (DOE) erweitert wird. Durch die Multi-Punkt-Methode (MPM) werden Messunsicherheiten von deutlich unter 1/100 Pixel erzielt. Um dies in der Praxis anwenden zu können, ist eine entsprechend hochgenaue Kalibrierung nötig. In diesem Beitrag werden verschiedene Kalibrierverfahren und -funktionen vorgestellt und untersucht. Die Auswertung zeigt, dass sich zur Kalibrierung ein allgemeiner Polynomansatz zusammen mit der Nanomess- und Positioniermaschine NPMM-200 am besten eignet. Es werden Standardabweichungen des Restfehlers von 0,31 µm im Objektraum (etwa 6/1000 Pixel im Bildraum) auf einer Fläche von etwa 87 mm × 49 mm 87\hspace{0.1667em}\text{mm}\times 49\hspace{0.1667em}\text{mm} erreicht (Polynomgrad = 9).

A method is proposed for 3D imaging through a highly heterogeneous double-composite random medium made of a thick mildly inhomogeneous medium followed by a thin strongly scattering layer. To realize the immunity to the heterogeneous random medium, a system of common-path phase-shift digital holography is designed in such a manner that the wavefront distortion caused by the first inhomogeneous medium is canceled out by the common-path geometry, and the influence of the random phase introduced by the second scattering layer is removed by the intensity-based recording of the digital hologram on the thin scattering layer. The validity of the method was confirmed by experiments.

Holography was invented 75 years ago by Dennis Gabor. Meanwhile it is much more than a fascinating technique for the recording and reconstruction of 3D images. The paper addresses the outstanding peculiarities and the big variety of applications of holography.

The modal holographic wavefront sensor enables fast measurements of individual aberration modes without the need for time-consuming calculations. To minimize the effect of inter-modal crosstalk on the measurement accuracy, we present sensor optimization and show its performance in closed-loop adaptive optics in simulation.

We propose a time-gated-single-pixel-camera as a promising sensor for image-free object detection for automotive application in adverse weather conditions. By combining the well-known principles of time-gating and single-pixel detection with neural networks, we aim to ultimately detect objects within the scene rapidly and robustly with a low-cost sensor. Here, we evaluate the possible data reduction such a system can provide compared to a conventional time-gated camera.

The modal holographic wavefront sensor enables fast measurement of individual aberration modes without the need for time-consuming calculations. However, the measurement accuracy suffers greatly from intermodal crosstalk, caused when the wavefront contains more aberrations than the one to be measured. In this paper, we present sensor optimization to minimize this effect and show the improvement when using Karhunen–Lòeve instead of Zernike modes as the basis. Finally, we show in simulation that an open-loop adaptive optics system based on the optimized sensor can be used to correct the effect of realistic, dynamic atmospheric turbulence on a wavefront and increase its Strehl ratio significantly.

Digitization and digital transition are frequently used terms in modern communication. Facts as the transition of the classical Data-Internet to the Internet of Things IoT, and Industrial Internet, respectively, are more and more influencing all technologies and thus it is quite appropriate to think about the future of metrology in such a changing environment. In this paper we discuss the state of the art, the challenges, and the perspectives of optical metrology in times of digital transition.

Atmospheric turbulence limits the performance of laser systems operating within the atmosphere. Therefore, adaptive optics systems are designed to measure and correct the effects of turbulence in real-time. A crucial part of such a system is the wavefront sensor. The modal holographic wavefront sensor is a promising alternative to well-established sensor types (e.g. Shack-Hartmann wavefront sensor). It measures the strength of individual aberration modes directly. Since there is no need for complex calculations, bandwidths in the megahertz range are possible. However, different aberration modes present in the laser beam influence each other's measurements. This inter-modal crosstalk has a very significant impact on the performance of the sensor. Careful design of the holographic sensor can reduce this influence. In this paper we show a method to optimize the sensor design for a given turbulence strength. We use a merit function to find the optimal combination of two design parameters: the detector size and the phase bias. This optimization is done on a mode-by-mode basis. We simulate realistic turbulence scenarios and evaluate the performance of the optimized holographic sensor. By considering the expected turbulence strength during the design process, we can increase the measurement accuracy significantly. We also compare two different modal bases and achieve a further improvement in accuracy when using Karhunen-Loeve instead of Zernike modes. We evaluate the efficiency of an open-loop adaptive optics system based on the optimized holographic sensor and show that it can be used to correct the effects of realistic dynamic atmospheric turbulence.

Residual stress levels induced during the process are one of the more relevant characteristics of thermal spray coatings. Therefore, there are different techniques, such as the x-ray diffractometry or the classical micro hole drilling and milling method, to analyse and optimize the thermal spray coating process in order to obtain a desired level of residual stresses on coatings. The state of the art of the new developed non-contact, quasi non-destructive residual stress analysis method is presented. The material removal is based on laser ablation and complex ablation geometries are possible by using a spatial light modulator (SLM) for beam conditioning. 3D deformations on the specimen surface are measured in the nanoscale range by high-resolution digital holographic interferometry. Numerical procedures using the finite differences method are performed in order to calculate the shape geometry and the calibration curves required for the residual stress calculation from the measured 3D displacements, coating-substrate material combination and the ablation geometry. Experimental results on thermal spray coatings are presented and discussed with comparative measurements by the hole-drilling method. The potentials and difficulties of the method are discussed.

Zusammenfassung Mikroelektromechanische Systeme (MEMS) werden heute in einer Vielzahl von Anwendungen eingesetzt. Zur Herstellung von MEMS werden üblicherweise fotolithografische Verfahren eingesetzt. Insbesondere bei sicherheitsrelevanten MEMS, wie z. B. Airbag- oder ESP-Sensoren, ist eine 100 % Inspektion erforderlich. Nach einer optischen Inspektion der MEMS-Strukturen werden diese durch das Bonden eines Silizium-Kappenwafers geschützt. Dieses Bonding oder das anschließende Packaging kann zu einer zusätzlichen Spannung im Waferstapel führen, die die MEMS-Funktion negativ beeinflussen kann. Im Falle eines fehlgeschlagenen elektronischen Tests kann das Problem nicht lokalisiert werden, da der Kappenwafer für die gängigen optischen Oberflächensensoren undurchsichtig ist. In dieser Publikation wird die Herausforderung der optischen Topografiemessung von verkapselten MEMS-Strukturen diskutiert. Dabei werden Defekte wie Verbiegung oder Festklemmen einzelner MEMS-Finger betrachtet. Für diese Messaufgabe wird ein kurzkohärentes Interferenzmikroskop implementiert. Die Wellenlänge liegt im Short-Wave Infrarot (SWIR), da Silizium im Infraroten transparent wird. Die interferometrische Oberflächenmessung der MEMS-Strukturen wird durch den Kappenwafer stark beeinflusst. Hierfür wurden Simulationen durchgeführt, die die systematischen Messabweichungen aufgrund der Kappe zeigen. Eine Möglichkeit zur Korrektur und Reduzierung der systematischen Abweichungen wird ebenfalls beschrieben.

The surface polarization scattering is investigated in terms of the coherence matrix for the electric field scattered from a birefringent material with a random interface between its surface and air. The relationship between the statistical properties of the scattered light at the scattering surface and the micro-structure of the anisotropic media has been explored for the first time to understand the underlying mechanism of the surface scattering phenomena for the electric field with random states of polarization.

This Letter presents a ray phase mapping model (RPM) for fringe projection profilometry (FPP) that avoids calibrating intrinsic parameters. The novelty of the RPM, to the best of our knowledge, is the ability to characterize the imaging system with independent rays for each pixel, and to associate the rays with the projected phase in the illumination field for efficient 3D mapping, which avoids complex imaging-specific modeling about lens layout and distortion. Two loss functions are constructed to flexibly optimize camera ray parameters and mapping coefficients, respectively. As a universal approach, it has the potential to calibrate different types of FPP systems with high accuracy. Experiments on wide-angle lens FPP, telecentric lens FPP, and micro-electromechanical system (MEMS)-based FPP are carried out to verify the feasibility of the proposed method.

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.

Scatter-plate microscopy (SPM) is a lensless imaging technique for high-resolution imaging through scattering media. So far, the method was demonstrated for spatially incoherent illumination and static scattering media. In this publication, we demonstrate that these restrictions are not necessary. We realized imaging with spatially coherent and spatially incoherent illumination. We further demonstrate that SPM is still a valid imaging method for scatter-plates, which change their scattering behaviour (i.e. the phase-shift) at each position on the plate continuously but independently from other positions. Especially we realized imaging through rotating ground glass diffusers.

We propose a method for three-dimensional imaging through a thick phase-fluctuating medium based on phase-shift digital holography with two adjacent point light sources for common-path geometry. Suppression of three-dimensional phase fluctuation was confirmed by numerical simulation and experiments.

Some widely used optical measurement systems require a scan in wavelength or in one spatial dimension to measure the topography in all three dimensions. Novel hyperspectral sensors based on an extended Bayer pattern have a high potential to solve this issue as they can measure three dimensions in a single shot. This paper presents a detailed examination of a hyperspectral sensor including a description of the measurement setup. The evaluated sensor (Ximea MQ022HG-IM-SM5X5-NIR) offers 25 channels based on Fabry–Perot filters. The setup illuminates the sensor with discrete wavelengths under a specified angle of incidence. This allows characterization of the spatial and angular response of every channel of each macropixel of the tested sensor on the illumination. The results of the characterization form the basis for a spectral reconstruction of the signal, which is essential to obtain an accurate spectral image. It turned out that irregularities of the signal response for the individual filters are present across the whole sensor.