Fringe analysis: single-shot or two-frames? Quantitative phase imaging answers.

Accurately measuring sound pressure is not an easy task because every microphone has its own mechanical and electrical characteristics. Moreover, the existence of a measuring instrument inside the field causes reflection and diffraction which deform the wavefront of sound to be measured. Ideally, a sensing device should not have any characteristic nor exist inside a measuring region. Although it may sound unrealistic, optical measurement methods are able to realize such ideal situation. Optical devices can be placed outside the sound field, and some of the sensing techniques, which decode information of sound from the phase of light, are able to cancel optical and electrical characteristics. Thus, optical sound measurement methods have possibility of achieving higher accuracy than ordinary sound measurement in principle. However, they have two main drawbacks that have prevented their applications in acoustics: (1) point-wise information cannot be obtained directly because observed signal is spatially integrated along the optical path; and (2) increasing signal-to-noise ratio is difficult because optical measurement of less than a nanometer order is typically required. To overcome the above difficulties, we have proposed several signal processing methods. Here, those methods are introduced with the physical principle of optical sound measurement.

This study deals with an experimental methodology developed in order to identify the elastic properties of super- conducting ring-shaped windings, constituents of the main coil winding of a magnetic resonance imaging magnet (MRI). Mechanical tensile tests were conducted on real scale specimens associated to an optical full-field displacement measurement technique (stereo image correlation). Strain fields were then obtained from the measured displacement fields by numerical dif- ferentiation. Finally, the four in-plane orthotropic stiffnesses of the windings were determined using the Virtual Fields Method (VFM). Experimental set-up also allows detecting a possible occurrence of a delamination thanks to in-plane displacement fields.

Optical full-field measurement techniques have been widely studied in academia and applied to many actual fields of automated inspection, reverse engineering, cosmetic surgery, and so on. With the advent of color CCD cameras and DMD (Digital Micromirror Device) based color DLP (Digital Light Processing) projectors, their major red, green, and blue channels have been used as a carrier to code fringe patterns. Since three fringe patterns can be simultaneously projected and captured at one shot, the acquisition time reduces to 1/3 of the value by the gray fringe pattern projection. This chapter will introduce two kinds of applications of red, green, and blue as a carrier: 1) modulation and demodulation method of coding sinusoidal fringe patterns into RGB channels of a composite color image; and 2) modulation and demodulation method of coding sinusoidal and binary fringe patterns into RGB channels of multiple composite color images. Experiments on testing the two kinds of applications were carried out by measuring the shape of objects’ surface. The results confirm that red, green, and blue channels can be used as a carrier to reduce the acquisition time.

Differential phase contrast (DPC) is a non-interferometric quantitative phase imaging method achieved by using an asymmetric imaging procedure. We report a pupil modulation differential phase contrast (PMDPC) imaging method by filtering a sample's Fourier domain with half-circle pupils. A phase gradient image is captured with each half-circle pupil, and a quantitative high resolution phase image is obtained after a deconvolution process with a minimum of two phase gradient images. Here, we introduce PMDPC quantitative phase image reconstruction algorithm and realize it experimentally in a 4f system with an SLM placed at the pupil plane. In our current experimental setup with the numerical aperture of 0.36, we obtain a quantitative phase image with a resolution of 1.73μm after computationally removing system aberrations and refocusing. We also extend the depth of field digitally by 20 times to ±50μm with a resolution of 1.76μm.

The paper presents an experimental application of a method leading to the identification of the elastic and damping material properties of isotropic vibrating plates. The theory assumes that the searched para meters can be extracted from curvature and deflection fields measured on the whole surface of the plate at two particular instants of the vibrating motion. The experimental application consists in an original excitation fixture, a particular adaptation of an optical full-field measurement technique, a data preprocessing giving the curvature and deflection fields and finally in the identification process using the Virtual Fields Method (VFM). The principle of the deflectom etry technique used for the measurements is presented. First results of identification on an acrylic plate are presented and compared to reference values. The used theoretical principle only assumes the global equilibrium of the observed part of the coupon. Therefore the identification remains insensible to the parasitic effects of the boundary conditions. This is confirmed by the results of the last presented experimentation. Keywords: vibration, identification, Virtual Fields Method, damping, deflectometry, slope fields, grid method, image processing.

Revealing the structural and functional organization of biological samples at submicrometer scales requires high-resolution imaging techniques that provide real-time, quantitative information. However, resolving and analyzing unlabeled biological structures at the subcellular level from a single-shot image remain challenging. We introduce a novel incoherent quantitative phase imaging technique based on lateral shearing digital holographic microscopy, with shearing distances exceeding the coherence area of the employed illumination—a regime not previously achieved and exploited. This strategy consequently ensures artifact-free, high-accuracy, and high-resolution quantitative phase reconstructions. This regime extends single-shot lateral shearing digital holographic microscopy toward incoherent illumination, thus increasing the space–time bandwidth product of the method. The practical common-path geometry also provides enhanced vibration resistance, which is essential for consistent time-lapse measurements. We verify the scalability and effectiveness of the method by investigating samples via multiple condenser–objective pairs, providing a wide range of lateral resolutions and fields of view, thereby ensuring its applicability for various microscope settings. We experimentally verify long-term phase stability and unprecedented phase-reconstruction accuracy. Finally, we use our method to investigate biological samples, including cheek cells, diatoms, and yeast cells, highlighting its potential for dynamic label-free analysis at the organelle level. These results establish our approach as a powerful tool for quantitative phase imaging, enabling high-precision investigations across a wide range of scientific applications.

Quantitative phase imaging (QPI) through multi-core fibers (MCFs) has been an emerging in vivo label-free endoscopic imaging modality with minimal invasiveness. However, the computational demands of conventional iterative phase retrieval algorithms have limited their real-time imaging potential. We demonstrate a learning-based MCF phase imaging method that significantly reduced the phase reconstruction time to 5.5 ms, enabling video-rate imaging at 181 fps. Moreover, we introduce an innovative optical system that automatically generated the first, to the best of our knowledge, open-source dataset tailored for MCF phase imaging, comprising 50,176 paired speckles and phase images. Our trained deep neural network (DNN) demonstrates a robust phase reconstruction performance in experiments with a mean fidelity of up to 99.8%. Such an efficient fiber phase imaging approach can broaden the applications of QPI in hard-to-reach areas.

Quantitative phase imaging (QPI) has emerged as a powerful tool in label-free bioimaging, in situ microstructure characterization for advanced manufacturing, and high-speed imaging of material property changes. Among various QPI methods, quadri-wave lateral shearing interferometry (QWLSI) stands out for its unique advantages in compactness, robustness, and high temporal resolution, making it an ideal choice for a wide range of applications. The compact design of QWLSI allows for easy integration with existing microscopy systems, while its robustness is manifested in the ability to maintain precise interferometric sensitivity even in high-vibration environments. Moreover, QWLSI also enables single-shot measurements that facilitate the capture of fast dynamic processes. This paper provides an in-depth exploration of the technical aspects of QWLSI, focusing on the evolution of its optical system and the primary algorithms used in wavefront reconstruction. The review also showcases significant applications of QWLSI, with a particular emphasis on its contributions to biomedical imaging. By discussing the advantages, limitations, and potential future developments of QWLSI, this paper aims to provide a comprehensive overview of this powerful QPI technique and its impact on various research fields.

White light diffraction phase microscopy with slightly off-axis blind two-step phase-shifting

In this doctoral thesis, the basic understanding of low-temperature combustion processes and alternative tailor-made fuels is improved with optical measurement methods. Therefore, different optical measurement techniques have been applied using different fuels in a high-pressure injection chamber. In the high-speed measurements different techniques are working simultaneously to detect characteristic values of mixture formation and combustion like liquid and gaseous penetration length as well as flame lift-off length: Mie scattering, shadowgraphy and OH*-radiation. With the help of fundamental spectroscopic laser measurement techniques based on laser induced fluorescence, nitrogen oxide can be quantitatively detected in the jet central region for the first time. Several steps are required for the quantification: correction of the absorption through the flame, correction of the different temperatures in the flame, correction of fluctuations in laser energy and determination of an absolute calibration factor. Quantitative nitrogen oxide concentrations were measured at different times and at two different distances from the injector nozzle hole. The measurement of the quantitative nitrogen oxide concentrations is also possible early in the flame and can be conducted directly next to the lift-off length. It was also used with two alternative fuels. The quantitative nitric oxide measurement shows non-negligible nitrogen oxide concentrations in the middle of the burning jet, in contrast to the prevailing classical diesel combustion model where nitrogen oxides are only formed at the edge of the diffusion zone of the flame. Furthermore, it is shown that due to the different flow states in the combustion nitrogen oxides are affected. To support the measurement, the application of a 0-d constant pressure simulation with a detailed flame chemistry and a prompt-NOx mechanism was performed. As a result, the detection of prompt-NO formation in the flame is possible. To compare with the experiments a 3-d-CFD-LES-simulation was conducted.

In this work, a new micro-extensometric technique to study the local heterogeneities of strain fields in metallic alloys is introduced. It is based on the optical full-field measurement method called the grid method adapted to the micrometric scale. In the first part of the paper, the making of a periodic grating at the surface of the sample by direct interferometric photolithography is explained. The optimization of the grids in terms of phase noise is then addressed.

In this contribution we present a novel one-stop-shop solution providing comprehensive, robust and automatic singleframe fringe pattern analysis for quantitative phase imaging. It is based on the modified variational image decomposition (mVID) algorithm and the Hilbert spiral transform. The VID concept is applied to tailor input data for efficient Hilbert spiral transform (HST). It returns the fringe-signal which is in quadrature to the input VID-filtered zero-mean-value fringe pattern. Both fringe-signals form the 2D complex analytic fringe pattern with phase and amplitude clearly defined by angle and modulus. Additional means for mVID-based compensation of characteristic phase errors are to be provided. The performance of the proposed novel mVID-HST technique is tested on simulated and experimental data. Its versatility and data-driven nature is emphasized processing off-axis, slightly off-axis and on-axis holograms.

As photomask critical dimensions extend well into the submicron range, optical measurement techniques are approaching the end of their useful life. While offering advantages of being nondestructive, relatively fast, and very precise, optical measurement tools may have accuracy problems due to diffraction. Linearity between measured and actual values is typically lost due to interference, resonance, and shadowing effects even before the diffraction limit is reached. In addition, standards for submicron features do not exist for optical tools. These limitations have given rise to using a Scanning Electron Microscope (SEM) for reticle dimension measurement. Another complication is that no NIST standard exists for chromium photomask dimensions. A CD SEM must therefore be calibrated to some other trusted standard or measurement method. A Stylus NanoProfilometer (SNP) has better inherent resolution than a SEM and was chosen as the standard measurement instrument for both CD SEM and optical tool metrology. This paper describes the cross-calibration of the Leica LWM-250 (white light) optical metrology tool and the KLA-Tencor 8100XP-R CD SEM, with reference to a Surface Interface SNP9000. The primary motivation was to define the usable ranges of both SEM and optical CD measurement systems, allowing for flexible implementation into a photomask manufacturing environment.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Dielectric metasurfaces have recently attracted much attention due to their ability to efficiently control light propagation, that is, its amplitude, phase and polarization, on micro- and macro- levels. Such metasurfaces also naturally allow for integration in compact optical setups due to their small sizes. An important issue of energy losses is also addressed by employing all-dielectric platform. Advancements in nanofabrication have enabled realization of metasurface-based high-resolution wavefront engineering devices, which have been employed in different imaging applications, for example, as metalenses, polarization filters, metaholograms, etc. Quantitative phase imaging is a powerful tool for the optical inspection of transparent samples in connection with biological and technical applications. By measuring thickness and/or refractive index profiles, phase imaging facilitates, for example, the observation of dynamic events in unstained cells. As such, it has become indispensable in biological imaging, wavefront correction and metrology. However, conventional cameras and photodetectors are inherently not sensitive to the phase of an incident lightwave. As a consequence, direct phase detection is challenging. Only by multiple intensity measurements or application of specialized interferometric schemes, the phase profile of an object can be in principle reconstructed. In this paper, we report on the ability of metasurfaces to contribute to quantitative phase imaging by independent control of the phase profiles in two orthogonal polarization states of an incident beam.

Digital holographic microscopy (DHM) is an important technique that may be used for quantitative phase imaging of unstained biological cell samples. Since the DHM technology is not commonly used in clinics or bioscience research labs, at present there is no well-accepted focusing criterion for unstained samples that users can follow while recording image plane digital holograms of cells. The usual sharpness metrics that are useful for auto-focusing of stained cells do not work well for unstained cells as there is no amplitude contrast. In this work, we report a practical method for estimating the best focus plane for unstained cells in the digital hologram domain. The method is based on an interesting observation that for the best focus plane the fringe pattern associated with individual unstained cells predominantly shows phase modulation effect in the form of bending of fringes and minimal amplitude modulation. This criterion when applied to unstained red blood cells shows that the central dip in the doughnut-like phase profile of cells is maximal in this plane. The proposed methodology is helpful for standardizing the usage of DHM technology across different users and application developmentefforts. LAY DESCRIPTION: Digital holographic microscopy (DHM) is slowly but steadily becoming an important microscopy modality and gaining acceptability for basic bio-science research as well as clinical usage. One of the important features of DHM is that it allows users to perform quantitative imaging of unstained transparent cells. Instead of using dyes or fluorescent labelling, DHM systems use quantitative phase as a contrast mechanism which depends on the natural refractive index variation within the cell samples. Since minimal wet lab processing is required in order to image cell samples with a DHM, cells can be imaged in their natural state. While DHM is gaining popularity among users, the imaging protocols across the labs or users need to be standardized in order to make sure that the same quantitative phase parameters are used for tasks such as quantitative phased based cellclassification. One of the important operational tasks for any microscopy work is to focus the sample under study. While focusing comes naturally to users of brightfield microscopes based on image contrast, the focusing is not straightforward when samples are unstained so that they do not offer any amplitude contrast. When performing quantitative phase imaging, defocus can actually change the phase profile of the cell due to near-zone (Fresnel) diffraction effects. So unless a standardized focusing methodology is used, it will be difficult for multiple DHM users (potentially at different sites) to agree on quantitative results out of their phase images. DHM literature has prior works which perform numerical focusing of recovered complex wave-field in the hologram plane to find the best focus plane. However such methods are not user friendly and do not allow user the same focusing experience as in a brightfield microscope. The numerical focusing is therefore a reasonably good method for an optics researcher but not necessarily so for a microscopy technician looking at cell samples with a DHM system in a clinicalsetting. The present work provides a simple focusing criterion for unstained samples that works directly in the hologram domain. The technique is based on an interesting observation that the when an unstained cell sample is in the best-focus plane, its corresponding hologram (or fringe pattern) predominantly shows phase modulation manifested by bending of fringes at the location of the cell. This criterion can be converted into a simple numerical method as we have used to find the best-focus plane using a stack of through focus holograms. We believe that the technique can be used manually by visually observing the holograms or can be converted to an auto-focus algorithm for a motorized DHMsystem.