Claas Falldorf

Hypercentric imaging is a powerful imaging modality that allows for the inspection of a specimen and its sides in the same recording. Typically, hypercentric imaging involves special lens objectives, which are bulky and limit the use of interferometric methods. To overcome this, we introduce lensless hypercentric imaging utilizing digital holography. We show, that the same hologram can be used to render various digital image perspectives, depending on the size and the position of a virtual aperture. We describe the perspective imaging model in detail and present experimental results, which proof the main concept and exemplarily show the dependence of the image perspective from the position of the aperture. To the best of our knowledge, this is the first time that lensless hypercentric imaging was realized. The results combine the benefits of lensless digital holography with the hypercentric imaging modality, thereby opening a path to a new class of inspection systems.

Computer generated volume holograms (CGVHs) are optical elements designed to manipulate light for desired optical functionalities. Two-photon polymerization (2PP) lithography offers a straightforward fabrication method for such CGVHs. However, non-uniform height profiles at the air-polymer interface introduce unwanted optical path differences (OPDs) for 2PP-fabricated CGVHs that disturb their optical performance. We address this effect by printing a several µm-thick cover layer with a uniform surface on top of the fabricated structures to minimize the undesired OPD caused by height profile variations at the grating-cover interface to a negligible level. The approach is successfully tested on simple volume gratings, where refractive index changes are achieved by varying the laser power used to print the volume grating. Confocal microscopy and focused ion beam-scanning electron microscopy determine the top surface height variations of ≲400nm, which are concealed beneath the uniform top cover layer. The optical performance of the buried diffractive structures is confirmed by computational shear interferometry (CoSI), demonstrating a dynamic refractive index modulation of 2 × 10⁻³ as volume gratings are polymerized with different powers within the structure. This approach offers a new and simple way to design and fabricate complex CGVHs and isolate their optical functionality from the surrounding environment.

We present a novel approach for securing valuable documents using a complementary approach based on the encryption of computer-generated holograms (CGHs). The proposed approach utilizes the well-known iterative Fourier transform algorithm (IFTA) to generate a phase-only CGH for valuable digital and/or physical documents. The generated CGH is then secured by binary phase randomization, which is implemented using the symmetric encryption technique, exclusive OR (XOR). The reconstruction process for the calculated secured CGHs varied slightly depending on whether the documents were digital or physical. For digital documents, reconstruction was performed using a symmetric decryption key followed by an inverse Fourier transform (IFFT). On the other hand, the reconstruction of the physical document involved two additional processes: printing and scanning. To evaluate the quality of the digital reconstruction, the speckle signal-to-noise ratio (SSNR) was estimated for both printed grayscale and binary CGHs. The security analysis of the XOR-encrypted CGH was quantitatively evaluated to ensure the level of protection against various cryptographic attacks such as plaintext and brute-force attacks. The results revealed that the combination of phase CGHs and the XOR encryption/decryption provides robust cryptographic protection for valuable documents, benefiting document security and anti-counterfeiting.

We present a novel fast simulation approach to simulate the influence of refractive freeform microstructures on a wave field. The FRISP (Finite Refractive Index Selective Propagation) method combines the Rayleigh-Sommerfeld diffraction integral with a thin element approximation and provides a comprehensive framework for understanding the optical properties of these microstructures. The main advantage of this method is its reduced complexity, which leads to a remarkable reduction in computation time by more than two orders of magnitude compared to finite-difference time-domain (FDTD) methods. This efficiency facilitates the iterative optimization of refractive microstructures and thus represents a practical tool to improve this type of microstructures. The verification of the FRISP method is realized by comparing the focal position and spot size of refractive microstructures. For this purpose, we compare FDTD, Mie theory and experimental data on microspheres with the predictions of FRISP. This comparison demonstrates the robustness and reliability of the approach, emphasizes its validity and demonstrates it as a valuable tool for the design and analysis of microstructures.

Four invited papers have been accepted for publication in this research topic. The Guest Editors would like to thank the authors for their valuable contributions, the anonymous reviewers for their insightful feedback, and the editorial team for ensuring the success of this research topic. The Guest Editors hope that this collection will serve as an important resource for students, researchers, engineers and industry professionals and will advance the field of quantitative imaging and nondestructive testing in industrial applications.All the published articles are listed below: Summary: Monitoring lipid accumulation in microalgal cultures is a promising approach for sustainable nutraceuticals and biodiesel production. This paper introduces a rapid, non-destructive method for quantifying lipid content in the microalga Phaeodactylum tricornutum using digital holographic microscopy (DHM). The approach leverages the DHM's refocusing capability to analyze recorded hologram sequences, enabling the evaluation of lipid droplet volumes within living cells. Over one thousand lipid droplets are automatically analyzed from a hundred recorded holograms per sample. The method has been validated through correlative quantitative phase contrast and fluorescence imaging, and its versatility is demonstrated by applying it to larger calibrated spherical refractive particles [1].Summary: This article explores the use of lensless digital holography for in-situ non-destructive testing (NDT), providing high-resolution, wide-field and stable metrology suitable for real-time quality control. Traditional methods rely on bulky, complex systems that are unsuitable for in-situ measurements due to environmental constraints and temporal instability. Lensless digital holography, with real-time digital reconstruction and numerical focusing, provides accurate phase retrieval without complex setups, making it ideal for a variety of in-situ scenarios. The current capabilities of the technology are being demonstrated in applications ranging from vibration to thermo-mechanics, with metrology enhancement methods further extending its use in various industrial sectors [2].Summary: Automated cell counting method has the potential to enhance blood inspection, offering valuable pathological insights for disease diagnosis and treatment. In this article, the authors a novel approach for counting and analyzing rat blood cells (RBCs) flowing in a microfluidic device using digital holographic microscopy enabled by a Sobel operator. Authors employed Dice coefficients for automatic threshold selection, facilitating the efficient counting of RBCs that enables the rapid extraction of key parameters such as size, concentration, and dry mass in a label-free manner. Additionally, the proposed technique was shown to apply to various other types of cells, including COS7 and Siha cells [3].Summary: This paper presents a lensless multispectral digital holographic sensor, Flash-WLI, designed for quality assurance in semiconductor manufacturing. By capturing digital holograms at different wavelengths, the sensor accurately evaluates the shape of reflective objects, providing a compact, robust alternative to traditional white light interferometry (WLI), which suffers from mechanical vibrations and bulky setups. Experimental results on a wafer sample confirm the sensor's high accuracy, with an axial resolution of ±2.5 nm (1σ). The consistency of these measurements with the Keyence VKX-3000 WLI model reinforces the reliability of Flash-WLI and positions it as a powerful tool for microstructure analysis in semiconductor manufacturing [4].

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.

Today’s 3D dynamic holographic display techniques suffer from severe limitations due to an available number of pixels that is several orders of magnitude lower than required by conventional approaches. We introduce a solution to this problem by introducing the concept of functional pixels. This concept is based on pixels that individually spatially modulate the amplitude and phase of incident light with a polynomial function, rather than just a constant phase or amplitude. We show that even in the simple case of a linear modulation of the phase, the pixel count can be drastically reduced up to 3 orders of magnitude while preserving most of the image details. This scheme can be easily implemented with already existing technology, such as micro mirror arrays that provide tip, tilt and piston movement. Even though the individual pixels need to be technologically more advanced, the comparably small number of such pixels required to form a display may pave the way towards true holographic dynamic 3D displays.

Terahertz (THz) radiation has shown enormous potential for non-destructive inspection in many contexts. Here, we present a method for imaging defects in chocolate bars that can be extended to many other materials. Our method requires only a continuous wave (CW) monochromatic source and detector at relatively low frequencies (280 GHz) corresponding to a relatively long wavelength of 1.1 mm. These components are used to construct a common-path configuration enabling the capturing of several images of THz radiation diffracted by the test object at different axial depths. The captured diffraction-rich images are used to constrain the associated phase retrieval problem enabling full access to the wave field, i.e., real amplitude and phase distributions. This allows full-field diffraction-limited phase-contrast imaging. Thus, we experimentally demonstrate the possibility of identifying contaminant particles with dimensions comparable to the wavelength.