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We explore the principles, implementation details, and performance characteristics of a lensless multi-spectral digital holographic sensor and demonstrate its potential for quality assurance in semiconductor manufacturing. The method is based on capturing multi-spectral digital holograms, which are subsequently utilized to evaluate the shape of a reflective test object. It allows for a compact setup satisfying high demands regarding robustness against mechanical vibrations and thus overcomes limitations associated with conventional optical inspection setups associated with lens-based white light interferometry. Additionally, the tunable laser source enhances the versatility of the system and enables adaptation to various sample characteristics. Experimental results based on a wafer test specimen demonstrate the effectiveness of the method. The axial resolution of the sensor is ± 2.5 nm, corresponding to 1σ. [ABSTRACT FROM AUTHOR]

The editorial discusses the recent advancements in Digital Holographic Microscopy (DHM) and its applications in various fields such as industrial inspection, biomedical imaging, and semiconductor manufacturing. DHM combines digital holography and optical microscopy to provide label-free and quantitative phase imaging, allowing for detailed analysis of samples. The research topic emphasizes cutting-edge techniques like lensless multispectral digital holography and lensless digital holography for in-situ non-destructive testing, showcasing their high-resolution capabilities and real-time metrology. The editorial aims to serve as a valuable resource for students, researchers, engineers, and industry professionals interested in quantitative imaging and non-destructive testing in industrial applications. [Extracted from the article]

Quality inspection is an integral part of the production process and often part of the quality management agreements between manufacturer and customer. Especially when it comes to safety-relevant parts, i.e. in the automobile or medical industry, often a 100% quality inspection is mandatory. Here, we present a solution comprised of a digital holographic measurement system, as well as fast algorithms for geometric evaluation and surface defect detection that paves the way for the inspection of metallic micro cups in less than a second. By use of a telecentric lens instead of standard microscope objective, we compensate scaling effects and wave field curvature, which distort reconstruction in digital holographic microscopy. Due to limited depth of focus of the microscope objective, depth information from different object layers are then stitched together to yield 3D data of its geometry. The resulting point cloud data is automatically decomposed into simple geometric shapes in order to analyse geometric deviations. Amplitude as well as phase distribution images are then analysed for surface defects. Our approach is demonstrated by inspecting cold formed micro cups. Defects larger than 2 μm lateral resolution and 5 μm depth can be detected.

Studying polymer fiber's mechanical properties leads to an increased understanding of the behavior of polymeric structures. This will support the design of new synthetic fibers with improved properties. Thus, there is great interest in the exact knowledge of these properties. In this work, phase-shifting digital holographic interferometry is used to investigate the variation of the optical properties of drawn polyvinylidene fluoride (PVDF) fiber. The study shows that the refractive index for light which is polarized in parallel direction to the fiber axis increases as the draw ratio increases. This implicates that the molecules become more oriented in this direction. However, the decrease in the values of the refractive index for light polarized perpendicular to the drawing direction reflects the reduced orientation of the molecules in this direction. This has been confirmed by studying the variation of the molecular orientation versus birefringence of the drawn PVDF fiber. Holograms and their relationship with the optomechanical parameters obtained are given for illustration. POLYM. ENG. SCI., 51:1218-1225, 2011. [C] 2011 Society of Plastics Engineers, INTRODUCTION One property of polymeric materials is that the molecular unit, the polymer chain, can be highly anisotropic, i.e., the chain can be fully extended or curled up in an [...]

We present an experimental configuration for phase retrieval from a set of intensity measurements. The key component is a spatial light modulator located in the Fourier domain of an imaging system. It performs a linear filter operation that is associated to the process of propagation in the image plane. In contrast to the state of the art, no mechanical adjustment is required during the recording process, thus reducing the measurement time considerably. The method is experimentally demonstrated by investigating a wave field scattered by a diffuser, and the results are verified by comparing them to those obtained from standard interferometry. OCIS codes: 100.5070, 120.6650, 050.1960, 290.5880.

Flash-profilometry is a novel measurement approach based on the fullfield lensless acquisition of spectral holograms. It is based on spectral sampling of the mutual coherence function and the subsequent calculation of its propagation along the optical axis several times the depth-of-field. Numerical propagation of the entire coherence function, rather than solely the complex amplitude, allows to digitally reproduce a complete scanning white-light interferometric (WLI) measurement. Hence, the corresponding 3D surface profiling system presented here achieves precision in the low nanometer range along an axial measurement range of several hundred micrometers. Due to the lensless setup, it is compact, immune against dispersion effects and lightweight. Additionally, because of the spectral sampling approach, it is faster than conventional coherence scanning WLI and robust against mechanical distortions, such as vibrations and rigid body movements. Flash-profilometry is therefore suitable for a wide range of applications, such as surface metrology, optical inspection, and material science and appears to be particularly suitable for a direct integration into production environments.

We present an experimental configuration for optical inspection of single vision soft contact lenses based on an active adaptive wavefront sensor. At first, the soft lenses were immersed in a saline filled wet cell to prevent surface deformation during measurements. Thereafter, refractive powers and aberrations were accurately measured before and after correcting illumination laser beam aberrations and wet cell-induced aberrations. The results reveal that there is a significant difference between the measured aberrations and refractive powers before and after aberration compensation. Accordingly, the proposed system is recommended as an optical inspection tool for precise assessment of commercially available contact lenses.

We present a fast shape measurement of micro-parts based on depth discrimination in imaging with LED illumination. It is based on a 4f-setup with an electrically adjusted tunable lens at the common Fourier plane. Using such a configuration, the opportunity to implement a fast depth scan by means of a tunable lens without the requirement of mechanically moving parts and depth discrimination using the limited spatial coherence of LED illumination is investigated. The technique allows the use of limited spatially partially coherent illumination which can be easily adapted to the test object by selecting the geometrical parameters of the system accordingly. Using this approach, we demonstrate the approach by measuring the 3D form of a tilted optically rough surface and a cold-formed micro-cup. The approach is robust, fast since required images are captured in less than a second, and eye-safe and offers an extended depth of focus in the range of few millimetres. Using a step height standard, we determine a height error of ±1.75 μm (1σ). This value may be further decreased by lowering the spatial coherence length of the illumination or by increasing the numerical aperture of the imaging system.