Lensless Digital Holography Research Articles

High-resolution imaging for in-situ non-destructive testing by quantitative lensless digital holography

Quantitative imaging technologies for in-situ non-destructive testing (NDT) demand high-resolution, wide-field, and stable metrology capabilities. Moreover, live processing and automation are vital for real-time quality control and inspection. Conventional methods use complex optical setups, resulting in large, immobile systems which can solely operate within controlled environmental conditions due to temporal instabilities, rendering them unsuitable for in-situ measurements of micro-to nano-scale physical phenomena. This article delves into the multiphysics application of lensless digital holography, emphasizing its metrological capacity for various in-situ scenarios, while acknowledging and characterizing the differing constraints imposed by various physical phenomena, both transient and steady-state. The digital reconstruction of holograms is computed in real-time, and numerical focusing capabilities allow for instantaneous retrieval of the optical phase at various working distances without the need of complex optical setups, making lensless digital holography well-suited for in-situ quantitative imaging under various types of environments. Current NDT capabilities are demonstrated, including high-resolution and real-time reconstructions, simultaneous measurements for comparative metrology, and practical applications ranging from vibrations and acoustics to thermo-mechanics. Furthermore, methodologies to enhance overall metrology capabilities are exploited, addressing the study of existing physical phenomena, thereby expanding the applicability of holographic techniques across diverse industrial sectors.

Holographic Air‐Quality Monitor (HAM)

We introduce the holographic air‐quality monitor (HAM) system, uniquely tailored for monitoring large particulate matter (PM) over 10 μm in diameter—particles critical for disease transmission and public health but overlooked by most commercial PM sensors. The HAM system utilizes a lensless digital inline holography (DIH) sensor combined with a deep learning model, enabling real‐time detection of PMs with greater than 97% true positive rate at less than 0.6% false positive rate and analysis of PMs by size and morphology at a sampling rate of 26 L/min for a wide range of particle concentrations up to 4000 particles/L. Such throughput not only significantly outperforms traditional imaging‐based sensors but also rivals some lower‐fidelity, nonimaging sensors. Additionally, the HAM system is equipped with additional sensors for smaller PMs and various air quality conditions, ensuring a comprehensive assessment of indoor air quality. The performance of the DIH sensor within the HAM system was evaluated through comparison with brightfield microscopy, showing high concordance in size and morphology measurements. The efficacy of the DIH sensor was also demonstrated in two 2‐h experiments under different environments simulating practical conditions, with one involving distinct PM‐generating events. These tests highlighted the HAM system’s advanced capability to differentiate PM events from background noise and its exceptional sensitivity to irregular, large‐sized PMs of low concentration.

Measurement of Fly Ash Particle Size Distribution Using Lensless Digital In-Line Holography

Abstract We propose the use of a lensless digital in-line holography setup for the measurement of particle size distribution (PSD) of fly ash obtained from wet electrostatic precipitators of different power plants. The fly ash samples were prepared by spreading the ash nonuniformly over a glass slide. A number of holograms corresponding to distinct regions of the glass slide were recorded and reconstructed. The particle sizes in each reconstructed image were estimated using a circular Hough transform–based edge detection algorithm. PSDs in the range of 10 to 90 μm were obtained. A numerical scheme to extend the measurement range below 10 μm is discussed. The performance of the proposed method is compared with the laser particle-size analyzer and conventional hydrometer. The experimental results indicate that the performance of the proposed method is similar to the hydrometer, with lower experimental and time complexity.

Adaptive sparse reconstruction for lensless digital holography via PSF estimation and phase retrieval.

In-line lensless digital holography has great potential in multiple applications; however, reconstructing high-quality images from a single recorded hologram is challenging due to the loss of phase information. Typical reconstruction methods are based on solving a regularized inverse problem and work well under suitable image priors, but they are extremely sensitive to mismatches between the forward model and the actual imaging system. This paper aims to improve the robustness of such algorithms by introducing the adaptive sparse reconstruction method, ASR, which learns a properly constrained point spread function (PSF) directly from data, as opposed to solely relying on physics-based approximations of it. ASR jointly performs holographic reconstruction, PSF estimation, and phase retrieval in an unsupervised way by maximizing the sparsity of the reconstructed images. Like traditional methods, ASR uses the image formation model along with a sparsity prior, which, unlike recent deep learning approaches, allows for unsupervised reconstruction with as little as one sample. Experimental results in synthetic and real data show the advantages of ASR over traditional reconstruction methods, especially in cases where the theoretical PSF does not match that of the actual system.

Resolution and Contrast Enhancement for Lensless Digital Holographic Microscopy and Its Application in Biomedicine

An important imaging technique in biomedicine, the conventional optical microscopy relies on relatively complicated and bulky lens and alignment mechanics. Based on the Gabor holography, the lensless digital holographic microscopy has the advantages of light weight and low cost. It has developed rapidly and received attention in many fields. However, the finite pixel size at the sensor plane limits the spatial resolution. In this study, we first review the principle of lensless digital holography, then go over some methods to improve image contrast and discuss the methods to enhance the image resolution of the lensless holographic image. Moreover, the applications of lensless digital holographic microscopy in biomedicine are reviewed. Finally, we look forward to the future development and prospect of lensless digital holographic technology.

Automated detection of apoptotic versus nonapoptotic cell death using label-free computational microscopy.

Identification of cell death mechanisms, particularly distinguishing between apoptotic versus nonapoptotic pathways, is of paramount importance for a wide range of applications related to cell signaling, interaction with pathogens, therapeutic processes, drug discovery, drug resistance, and even pathogenesis of diseases like cancers and neurogenerative disease among others. Here, we present a novel high-throughput method of identifying apoptotic versus necrotic versus other nonapoptotic cell death processes, based on lensless digital holography. This method relies on identification of the temporal changes in the morphological features of mammalian cells, which are unique to each cell death processes. Different cell death processes were induced by known cytotoxic agents. A deep learning-based approach was used to automatically classify the cell death mechanism (apoptotic vs necrotic vs nonapoptotic) with more than 93% accuracy. This label free approach can provide a low cost (<$250) alternative to some of the currently available high content imaging-based screening tools.

Noniterative sub-pixel shifting super-resolution lensless digital holography.

Lensless digital holography (LDH) is gaining considerable attention lately due to a simple experimental setup, wide field-of-view, and three-dimensional (3D) imaging capability. Since the resolution of LDH is limited by the Nyquist frequency of a detector array, the major drawback of LDH is resolution, and a lot of efforts were made to enhance the resolution of LDH. Here we propose and demonstrate a fast noniterative sub-pixel shifting super-resolution technique that can effectively enhance the resolution of LDH by a factor of two. We provide detailed frequency-domain formulae for our noniterative frequency-domain super-resolution method. The validity of our proposed method is experimentally demonstrated both for scattering and phase objects.

Classification of unlabeled cells using lensless digital holographic images and deep neural networks.

Image-based cell analytic methodologies offer a relatively simple and economical way to analyze and understand cell heterogeneities and developments. Owing to developments in high-resolution image sensors and high-performance computation processors, the emerging lensless digital holography technique enables a simple and cost-effective approach to obtain label-free cell images with a large field of view and microscopic spatial resolution. The holograms of three types of cells, including MCF-10A, EC-109, and MDA-MB-231 cells, were recorded using a lensless digital holography system composed of a laser diode, a sample stage, an image sensor, and a laptop computer. The amplitude images were reconstructed using the angular spectrum method, and the sample to sensor distance was determined using the autofocusing criteria based on the sparsity of image edges and corner points. Four convolutional neural networks (CNNs) were used to classify the cell types based on the recovered holographic images. Classification of two cell types and three cell types achieved an accuracy of higher than 91% by all the networks used. The ResNet and the DenseNet models had similar classification accuracy of 95% or greater, outperforming the GoogLeNet and the CNN-5 models. These experiments demonstrated that the CNNs were effective at classifying two or three types of tumor cells. The lensless holography combined with machine learning holds great promise in the application of stainless cell imaging and classification, such as in cancer diagnosis and cancer biology research, where distinguishing normal cells from cancer cells and recognizing different cancer cell types will be greatly beneficial.

Looking through diffusers by phase correction with lensless digital holography

Optical imaging through diffusive or scattering media has attracted a great deal of attention. Lensless digital holography is used to reconstruct the intensity and phase of an object located behind a diffuser. For this study, we propose a method of reconstructing the object's intensity by compensating the complex amplitude of the diffuser by lensless digital holography. A priori information is necessary to obtain the complex amplitude of the diffuser, and we investigated the image quality of reconstructed images through diffusers. Our method does not use approximations to describe the propagation of the object light and the wavefronts disturbed by diffusers, and thus provides a more rigorous description of lightwave propagation. The image quality of the reconstructed images was dependent on the distance between the diffuser and the image sensor or between the diffuser and the sample. We investigated the image contrast of reconstructed images under different conditions.

Fourier digital holography of real scenes for 360° tabletop holographic displays.

Recently, the tabletop holographic display has been introduced to present a large 3D hologram floating over the table. When the observer looks down at the hologram, the display reconstructs upper perspectives of the object at a 45° angle. This paper presents the full imaging chain for the tabletop holographic display based on capture, processing, and reconstruction of a 360° observable hologram of the real object. Two different imaging methods, which involve lensless Fourier digital holographic recordings and the tabletop holographic display, are introduced. The first method utilizes the conventional capture approach with a side view perspective and numerical tilt correction for 45° angular mismatch between the acquisition and reconstruction systems. The second method presents a modified lensless digital Fourier holography for holographic recording of the upper perspective. Experimental results including numerical and optical reconstructions present various visual aspects of both capture approaches such as viewpoint correction, refocusing, 3D effects, and 3D deformations.

Compressive holography with resolution improvement and lensless adjustable magnification

In lensless digital holography the resolving power is limited by factors such as the finite size of recorded area, pixel dimensions and sampling rate of digital acquisition system. These factors acts as a filter that discards high-frequency components with useful information about the analyzed object. In the present work, we propose a method to overcome these limitations to proving the resolution recovery in digital in-line holography by utilizing adjustable magnification in a compressive sensing framework. Here, we model the sensed field as low-pass filtered and projected on a Fresnel base with adjustable magnification and use sparse regularization in an inverse problem approach to image reconstruction. Our method enhances image reconstruction resolution from a single recorded hologram with magnification independent of parameters such as propagation distance and wavelength of light, which enables to be used in applications like multi-wavelength digital holography. In addition, this technique offers high noise immunity and eliminates typical problems of holographically reconstructed images such as twin image and speckle. Therefore, we demonstrate by simulations and experimentally results that our approach significantly improves the quality of reconstructed images when compared with conventional techniques.

Digital plasmonic holography

We demonstrate digital plasmonic holography for direct in-plane imaging with propagating surface-plasmon waves. Imaging with surface plasmons suffers from the lack of simple in-plane lenses and mirrors. Lens-less digital holography techniques, however, rely on digitally decoding an interference pattern between a reference wave and an object wave. With far-field diffractive optics, this decoding scheme provides a full recording, i.e., a hologram, of the amplitude and phase of the object wave, giving three-dimensional information from a two-dimensional recording. For plasmonics, only a one-dimensional recording is needed, and both the phase and amplitude of the propagating plasmons can be extracted for high-resolution in-plane imaging. Here, we demonstrate lens-less, point-source digital plasmonic holography using two methods to record the plasmonic holograms: a dual-probe near-field scanning optical microscope and lithographically defined circular fluorescent screens. The point-source geometry gives in-plane magnification, allowing for high-resolution imaging with relatively lower-resolution microscope objectives. These results pave the way for a new form of in-plane plasmonic imaging, gathering the full complex wave, without the need for plasmonic mirrors or lenses.

Highly stable lens-less digital holography using cyclic lateral shearing interferometer and residual decollimated beam

Lens-less digital holography for quantitative phase contrast imaging is established with a cyclic lateral shearing interferometer and non-collimated light beam. In a self-reference type configuration, the object and reference beams are generated from the same spherical wavefront after the light beam has passed through the sample plane. The experimental configuration is implemented with simple optical components involving beam splitter and mirrors. This proposed architecture has the advantage that the system does not require any special optical element such as pinhole, gratings, and other customized optical components and hence can be easily realized with significantly simplified alignment procedure. Experimental results for both amplitude and phase objects are presented as a proof of concept.

Three-dimensional vibrometry of the human eardrum with stroboscopic lensless digital holography.

The eardrum or tympanic membrane (TM) transforms acoustic energy at the ear canal into mechanical motions of the ossicles. The acousto-mechanical transformer behavior of the TM is determined by its shape, three-dimensional (3-D) motion, and mechanical properties. We have developed an optoelectronic holographic system to measure the shape and 3-D sound-induced displacements of the TM. The shape of the TM is measured with dual-wavelength holographic contouring using a tunable near IR laser source with a central wavelength of 780 nm. 3-D components of sound-induced displacements of the TM are measured with the method of multiple sensitivity vectors using stroboscopic holographic interferometry. To accurately obtain sensitivity vectors, a new technique is developed and used in which the sensitivity vectors are obtained from the images of a specular sphere that is being illuminated from different directions. Shape and 3-D acoustically induced displacement components of cadaveric human TMs at several excitation frequencies are measured at more than one million points on its surface. A numerical rotation matrix is used to rotate the original Euclidean coordinate of the measuring system in order to obtain in-plane and out-of-plane motion components. Results show that in-plane components of motion are much smaller (<20%) than the out-of-plane motions’ components.

Holographic otoscope using dual-shot-acquisition for the study of eardrum biomechanical displacements

Recently an optoelectronic holography system was deployed in the clinic with the purpose of quantifying the tympanic membrane (TM) displacements of various mammal species, the objective being the understanding of their middle ear biomechanics. The optoelectronic holography system has an in-line configuration where the data gathered is decoded using lensless digital holography with the Fresnel approximation. This paper presents quantitative data obtained from an acoustically excited postmortem chinchilla's TM. To achieve this we used a robust customized windowed unwrapping method to unwrap the noisy optical phase obtained by subtracting phase maps of two recorded holograms and the results were compared with those obtained when using the unwrapping branch-cut algorithm. Additionally, phase maps obtained by the phase-stepping technique were compared applying both unwrapping methods. For in vivo applications particular emphasis is made on post-processing dual-shot-acquisition of holograms as one of various acquisition strategies and algorithms to diminish measurement error due to heartbeat, breathing, and patient's head motion as well as environment induced mechanical disturbances present in a noncontrolled environment, such as in a clinic. By recording only two holograms representing a stationary and deformed state of eardrum, respectively, we can increase the acquisition speed of the camera used to record faster events happening on the TM surface.

Imaging live humans through smoke and flames using far-infrared digital holography

The ability to see behind flames is a key challenge for the industrial field and particularly for the safety field. Development of new technologies to detect live people through smoke and flames in fire scenes is an extremely desirable goal since it can save human lives. The latest technologies, including equipment adopted by fire departments, use infrared bolometers for infrared digital cameras that allow users to see through smoke. However, such detectors are blinded by flame-emitted radiation. Here we show a completely different approach that makes use of lensless digital holography technology in the infrared range for successful imaging through smoke and flames. Notably, we demonstrate that digital holography with a cw laser allows the recording of dynamic human-size targets. In this work, easy detection of live, moving people is achieved through both smoke and flames, thus demonstrating the capability of digital holography at 10.6 μm.

Optical parameters and space–bandwidth product optimization in digital holographic microscopy

This paper considers some of the most important optical parameters that characterize a digital holographic microscope (DHM) and presents their mathematical derivation based on geometrical and diffraction-based models. It supports and justifies the use of the out-of-focus recording of holograms by showing that the field of view can be increased when recording the hologram in front of the in-focus image plane. In this manner a better match between the space-bandwidth product (SBP) of the microscope objective and that of the reconstructed hologram can be obtained. Hence, DHM offers a more cost-efficient way to increase the recorded SBP compared to the application of a high-quality microscope objective (large numerical aperture and low magnification) used in conventional microscopy. Furthermore, an expression for the imaging distance (distance between hologram and image plane), while maintaining the optical resolution and sufficient sampling, is obtained. This expression takes into account all kinds of reference-wave curvature and can easily be transferred to lensless digital holography. In this context it could be demonstrated that an object wave matched reference wave offers a significantly smaller imaging distance and hence the largest recoverable SBP. In addition, a new, to our knowledge, approach, based on the influence of defocus on the modulation transfer function, is used to derive the depth of field (DOF) for a circular aperture (lens-based system) and a rectangular aperture (lensless system), respectively. This investigation leads to the finding that a rectangular aperture offers an increased resolution combined with an increased DOF, when compared to a circular aperture of the same size.

Lensless digital holography with diffuse illumination through a pseudo-random phase mask

Microscopic imaging with a setup consisting of a pseudo-random phase mask, and an open CMOS camera, without an imaging objective, is demonstrated. The pseudo random phase mask acts as a diffuser for an incoming laser beam, scattering a speckle pattern to a CMOS chip, which is recorded once as a reference. A sample which is afterwards inserted somewhere in the optical beam path changes the speckle pattern. A single (non-iterative) image processing step, comparing the modified speckle pattern with the previously recorded one, generates a sharp image of the sample. After a first calibration the method works in real-time and allows quantitative imaging of complex (amplitude and phase) samples in an extended three-dimensional volume. Since no lenses are used, the method is free from lens abberations. Compared to standard inline holography the diffuse sample illumination improves the axial sectioning capability by increasing the effective numerical aperture in the illumination path, and it suppresses the undesired so-called twin images. For demonstration, a high resolution spatial light modulator (SLM) is programmed to act as the pseudo-random phase mask. We show experimental results, imaging microscopic biological samples, e.g. insects, within an extended volume at a distance of 15 cm with a transverse and longitudinal resolution of about 60 μm and 400 μm, respectively.

基于无透镜数字全息成像的活体细胞培养状态无损监测

基于无透镜数字全息成像的活体细胞培养状态无损监测

Non-invasive monitoring of living cell culture by lensless digital holography imaging

Non-invasive monitoring of living cell culture by lensless digital holography imaging