Lensless Imaging Technique Research Articles

Subgradient-projection-based stable phase-retrieval algorithm for X-ray ptychography.

X-ray ptychography is a lensless imaging technique that visualizes the nano-structure of a thick specimen which cannot be observed with an electron microscope. It reconstructs a complex-valued refractive index of the specimen from observed diffraction patterns. This reconstruction problem is called phase retrieval (PR). For further improvement in the imaging capability, including expansion of the depth of field, various PR algorithms have been proposed. Since a high-quality PR method is built upon a base PR algorithm such as ePIE, developing a well performing base PR algorithm is important. This paper proposes an improved iterative algorithm named CRISP. It exploits subgradient projection which allows adaptive step size and can be expected to avoid yielding a poor image. The proposed algorithm was compared with ePIE, which is a simple and fast-convergence algorithm, and its modified algorithm, rPIE. The experiments confirmed that the proposed method improved the reconstruction performance for both simulation and real data.

CFZA camera: a high-resolution lensless imaging technique based on compound Fresnel zone aperture.

In lensless imaging using a Fresnel zone aperture (FZA), it is generally believed that the resolution is limited by the outermost ring breadth of the FZA. The limitation has the potential to be broken according to the multi-order property of binary FZAs. In this Letter, we propose to use a high-order component of the FZA as the point spread function (PSF) to develop a high-order transfer function backpropagation (HBP) algorithm to enhance the resolution. The proportion of high-order diffraction energy is low, leading to severe defocus noise in the reconstructed image. To address this issue, we propose a Compound FZA (CFZA), which merges two partial FZAs operating at different orders as the mask to strike a balance between the noise and resolution. Experimental results verify that the CFZA-based camera has a resolution that is double that of a traditional FZA-based camera with an identical outer ring breadth and can be reconstructed with high quality by a single HBP without calibration. Our method offers a cost-effective solution for achieving high-resolution imaging, expanding the potential applications of FZA-based lensless imaging in a variety of areas.

Double-slit holography—a single-shot lensless imaging technique

In this study, we propose a new method for single-shot, high-resolution lensless imaging called double-slit holography. This technique combines the properties of in-line and off-axis holography in one single-shot measurement using the simplest double-slit device: a plate with two apertures. In double-slit holography, a plane wave illuminates the two apertures giving rise to two spherical waves. While diffraction of one spherical wave from a sample positioned behind the first aperture (the object aperture) provides the object wave, the other spherical wave diffracted from the second (reference) aperture provides the reference wave. The resulting interference pattern in the far-field (hologram) combines the properties of an in-line (or Gabor-type) hologram and an off-axis hologram due to the added reference wave from the second aperture. Both the object and reference waves have the same intensity, which ensures high contrast of the hologram. Due to the off-axis scheme, the amplitude and phase distributions of the sample can be directly reconstructed from the hologram, and the twin image can be easily separated. Due to the object wave being the same as in-line holography with a spherical wave, imaging at different magnifications is similarly done by simply adjusting the aperture-to-sample distance. The resolution of the reconstructed object is given by the numerical aperture of the optical setup and the diameter of the reference aperture. It is shown both by theory and simulations that the resolution of the reconstructed object depends on the diameter of the reference wave aperture but does not depend on the diameter of the object aperture. Light optical proof-of-concept experiments are provided. The proposed method can be particularly practical for X-rays, where optical elements such as beam splitters are not available and conventional off-axis holography schemes cannot be realised.

Ptychographic lensless coherent endomicroscopy through a flexible fiber bundle

Conventional fiber-bundle-based endoscopes allow minimally invasive imaging through flexible multi-core fiber (MCF) bundles by placing a miniature lens at the distal tip and using each core as an imaging pixel. In recent years, lensless imaging through MCFs was made possible by correcting the core-to-core phase distortions pre-measured in a calibration procedure. However, temporally varying wavefront distortions, for instance, due to dynamic fiber bending, pose a challenge for such approaches. Here, we demonstrate a coherent lensless imaging technique based on intensity-only measurements insensitive to core-to-core phase distortions. We leverage a ptychographic reconstruction algorithm to retrieve the phase and amplitude profiles of reflective objects placed at a distance from the fiber tip, using as input a set of diffracted intensity patterns reflected from the object when the illumination is scanned over the MCF cores. Our approach thus utilizes an acquisition process equivalent to confocal microendoscopy, only replacing the single detector with a camera.

Dual-domain mean-reverting diffusion model-enhanced temporal compressive coherent diffraction imaging

Temporal compressive coherent diffraction imaging is a lensless imaging technique with the capability to capture fast-moving small objects. However, the accuracy of imaging reconstruction is often hindered by the loss of frequency domain information, a critical factor limiting the quality of the reconstructed images. To improve the quality of these reconstructed images, a method dual-domain mean-reverting diffusion model-enhanced temporal compressive coherent diffraction imaging (DMDTC) has been introduced. DMDTC leverages the mean-reverting diffusion model to acquire prior information in both frequency and spatial domain through sample learning. The frequency domain mean-reverting diffusion model is employed to recover missing information, while hybrid input-output algorithm is carried out to reconstruct the spatial domain image. The spatial domain mean-reverting diffusion model is utilized for denoising and image restoration. DMDTC has demonstrated a significant enhancement in the quality of the reconstructed images. The results indicate that the structural similarity and peak signal-to-noise ratio of images reconstructed by DMDTC surpass those obtained through conventional methods. DMDTC enables high temporal frame rates and high spatial resolution in coherent diffraction imaging.

Semantic representation learning for a mask-modulated lensless camera by contrastive cross-modal transferring.

Lensless computational imaging, a technique that combines optical-modulated measurements with task-specific algorithms, has recently benefited from the application of artificial neural networks. Conventionally, lensless imaging techniques rely on prior knowledge to deal with the ill-posed nature of unstructured measurements, which requires costly supervised approaches. To address this issue, we present a self-supervised learning method that learns semantic representations for the modulated scenes from implicitly provided priors. A contrastive loss function is designed for training the target extractor (measurements) from a source extractor (structured natural scenes) to transfer cross-modal priors in the latent space. The effectiveness of the new extractor was validated by classifying the mask-modulated scenes on unseen datasets and showed the comparable accuracy to the source modality (contrastive language-image pre-trained [CLIP] network). The proposed multimodal representation learning method has the advantages of avoiding costly data annotation, being more adaptive to unseen data, and usability in a variety of downstream vision tasks with unconventional imaging settings.

Lensless imaging via multi-height mask modulation and ptychographical phase retrieval

Multi-height phase retrieval by adding a thin diffuser between the object and the detector has emerged as an effective approach for lensless imaging. However, up to 100 measurements are required to jointly recover the object and diffuser, which makes the data acquisition time-consuming and cumbersome. In this paper, we report a novel multi-height lensless imaging technique. In our scheme, the diffuser is replaced with a random binary amplitude mask and placed upstream of the object to introduce speckle illumination. Multiple diffraction patterns are then recorded by moving the mask axially, while the object and the detector remained stationary. Inspired by the ptychography, the extended ptychographic iterative engine algorithm is adopted to reconstruct the object and the unknown mask simultaneously. The imaging performance is first validated by simulation and then experimentally tested with resolution target, phase plate, and biological sample. Compared with the original method, the proposed method can achieve high-quality reconstruction with as few as ∼20 images, which greatly accelerates the data acquisition and reconstruction.

Three-dimensional tomographic imaging of the magnetization vector field using Fourier transform holography

The authors present a new tomographic technique, based on Fourier transform holography. It is a lensless imaging technique to retrieve the object of interest from its diffraction pattern in a single step of computation. A combination of dual-axis tomographic projections provides a 3D vectorial magnetic image of an 800-$n\phantom{\rule{0}{0ex}}m$-thick Fe/Gd multilayer in a 5-$\ensuremath{\mu}\phantom{\rule{0}{0ex}}m$-diameter field of view with a resolution of 80 $n\phantom{\rule{0}{0ex}}m$. The reconstruction reveals not only wormlike domains with magnetization pointing mostly out of plane, but also the subtle features of magnetic flux closure around the domain walls.

Characterisation of engineered defects in extreme ultraviolet mirror substrates using lab-scale extreme ultraviolet reflection ptychography

Ptychography is a lensless imaging technique that is aberration-free and capable of imaging both the amplitude and the phase of radiation reflected or transmitted from an object using iterative algorithms. Working with extreme ultraviolet (EUV) light, ptychography can provide better resolution than conventional optical microscopy and deeper penetration than scanning electron microscope. As a compact lab-scale EUV light sources, high harmonic generation meets the high coherence requirement of ptychography and gives more flexibilities in both budget and experimental time compared to synchrotrons. The ability to measure phase makes reflection-mode ptychography a good choice for characterising both the surface topography and the internal structural changes in EUV multilayer mirrors. This paper describes the use of reflection-mode ptychography with a lab-scale high harmonic generation based EUV light source to perform quantitative measurement of the amplitude and phase reflection from EUV multilayer mirrors with engineered substrate defects. Using EUV light at 29.6nm from a tabletop high harmonic generation light source, a lateral resolution down to ∼88nm and a phase resolution of 0.08rad (equivalent to topographic height variation of 0.27nm) are achieved. The effect of surface distortion and roughness on EUV reflectivity is compared to topographic properties of the mirror defects measured using both atomic force microscopy and scanning transmission electron microscopy. Modelling of reflection properties from multilayer mirrors is used to predict the potential of a combination of on-resonance, actinic ptychographic imaging at 13.5nm and atomic force microscopy for characterising the changes in multilayered structures.

Fourier Transform Holography: A Lensless Imaging Technique, Its Principles and Applications

Fourier transform holography (FTH) is a lensless imaging technique where the wave scattered by an object is superimposed with the wave scattered by a reference source positioned in the same plane. The distribution of the object is then reconstructed by simply calculating the Fourier transform of the recorded hologram. In this study, we outline the basic principles of FTH and provide an overview of the different types of references and the associated reconstruction algorithms. Current applications of FTH with different waves (light, electron, and X-ray) are presented, and their relationships with other coherent imaging techniques are discussed.

Lensless computational imaging with a hybrid framework of holographic propagation and deep learning.

Lensless imaging has attracted attention as it avoids the bulky optical lens. Lensless holographic imaging is a type of a lensless imaging technique. Recently, deep learning has also shown tremendous potential in lensless holographic imaging. A labeled complex field including real and imaginary components of the samples is usually used as a training dataset. However, obtaining such a holographic dataset is challenging. In this Letter, we propose a lensless computational imaging technique with a hybrid framework of holographic propagation and deep learning. The proposed framework takes recorded holograms as input instead of complex fields, and compares the input and regenerated holograms. Compared to previous supervised learning schemes with a labeled complex field, our method does not require this supervision. Furthermore, we use the generative adversarial network to constrain the proposed framework and tackle the trivial solution. We demonstrate high-quality reconstruction with the proposed framework compared to previous deep learning methods.

Physics-driven deep learning enables temporal compressive coherent diffraction imaging

Coherent diffraction imaging (CDI), as a lensless imaging technique, can achieve a high-resolution image with intensity and phase information from a diffraction pattern. To capture high-speed and high-spatial-resolution scenes, we propose a temporal compressive CDI system. A two-step algorithm using physics-driven deep-learning networks is developed for multi-frame spectra reconstruction and phase retrieval. Experimental results demonstrate that our system can reconstruct up to eight frames from a snapshot measurement. Our results offer the potential to visualize the dynamic process of molecules with large fields of view and high spatial and temporal resolutions.

Analysis of influence of object–detector distance error on the reconstructed object and probe in ptychographic imaging

Ptychography is a lensless imaging technique that is capable of reconstructing an object and illumination simultaneously by scanning the object at several positions with respect to a localized illumination beam. It has emerged as a powerful tool with applications ranging from high resolution imaging to optical metrology. A precise image reconstruction in ptychography is of utmost importance in several applications. An error in the measurement of the object–detector distance was found to affect the reliability of the reconstructed probe and the object. Several axial distance correction algorithms have been proposed to address the issue. However, the additional axial distance correction increases the complexity of the reconstruction algorithm. In this study, a detailed theoretical analysis of the relationship between the reconstructed images and the object–detector distance error is presented. It shows that high resolution object and probe images can still be reconstructed without an axial distance correction algorithm. The theoretical analysis is quantitatively studied by numerical simulations and experimental results.

Three-dimensional phase and intensity reconstruction from coherent modulation imaging measurements.

Coherent modulation imaging is a lensless imaging technique, where a complex-valued image can be recovered from a single diffraction pattern using the iterative algorithm. Although mostly applied in two dimensions, it can be tomographically combined to produce three-dimensional (3D) images. Here we present a 3D reconstruction procedure for the sample's phase and intensity from coherent modulation imaging measurements. Pre-processing methods to remove illumination probe, inherent ambiguities in phase reconstruction results, and intensity fluctuation are given. With the projections extracted by our method, standard tomographic reconstruction frameworks can be used to recover accurate quantitative 3D phase and intensity images. Numerical simulations and optical experiments validate our method.

X-ray Diffraction Imaging of Deformations in Thin Films and Nano-Objects.

The quantification and localization of elastic strains and defects in crystals are necessary to control and predict the functioning of materials. The X-ray imaging of strains has made very impressive progress in recent years. On the one hand, progress in optical elements for focusing X-rays now makes it possible to carry out X-ray diffraction mapping with a resolution in the 50–100 nm range, while lensless imaging techniques reach a typical resolution of 5–10 nm. This continuous evolution is also a consequence of the development of new two-dimensional detectors with hybrid pixels whose dynamics, reading speed and low noise level have revolutionized measurement strategies. In addition, a new accelerator ring concept (HMBA network: hybrid multi-bend achromat lattice) is allowing a very significant increase (a factor of 100) in the brilliance and coherent flux of synchrotron radiation facilities, thanks to the reduction in the horizontal size of the source. This review is intended as a progress report in a rapidly evolving field. The next ten years should allow the emergence of three-dimensional imaging methods of strains that are fast enough to follow, in situ, the evolution of a material under stress or during a transition. Handling massive amounts of data will not be the least of the challenges.

Resolution-enhanced ptychography framework with an equivalent upsampling and precise position.

As a lensless imaging technique, ptychography provides a new way to resolve the conflict between the spatial resolution and the field of view. However, due to the pixel size limit of the sensor, a compromise has to be reached between the spatial resolution and the signal-to-noise ratio. Here, we propose a resolution-enhanced ptychography framework with equivalent upsampling and subpixel accuracy in position to further improve the resolution of ptychography. According to the theory of pixel superresolved techniques, the inherent shift illumination scheme in ptychography can additionally enhance the resolution with the redundant data. An additional layer of pooling is used to simulate the downsampling of a digital record, and the pixel superresolved problem is transformed into an automatic optimization problem. The proposed framework is verified by optical experiments, both in biological samples and the resolution targets. Compared to the traditional algorithm, the spatial lateral resolution is twice as large using the same data set.

Lensless imaging for droplet identification towards visual feedback-based pressure controlled droplet microfluidic platforms

The usability and robustness of droplet microfluidic devices constitute significant challenges to the widespread adoption of droplet microfluidics despite the field’s immense promise. Active droplet manipulation via visual feedback-based pressure driven control has addressed some of the concerns; however, the use of large and costly optical microscopes for acquiring visual feedback limits such systems for many applications. We present a compact and cost-effective (200 [USD]) droplet sensing system that leverages lensless imaging techniques using a single LED and a CMOS sensor. The imaging system can detect nearly all droplet interfaces (0.05 < miss rate, > 0.95 positive predictive value). Its accuracy (mean errors less than 10 [µm]), Field of View (7.9 [mm2]), resolution (2.2 [µm]) and sensing rate (40.1 [Hz]) are similar to existing optical microscope-based droplet identification systems. The lensless imaging system has acceptable performance while having a significantly smaller footprint and cost, which presents tremendous potential to miniaturize and modularize droplet microfluidic systems paving the road for widespread adoption.

Maskless Fourier transform holography.

Fourier transform holography is a lensless imaging technique that retrieves an object's exit-wave function with high fidelity. It has been used to study nanoscale phenomena and spatio-temporal dynamics in solids, with sensitivity to the phase component of electronic and magnetic textures. However, the method requires an invasive and labor-intensive nanopatterning of a holography mask directly onto the sample, which can alter the sample properties, forces a fixed field-of-view, and leads to a low signal-to-noise ratio at high resolution. In this work, we propose using wavefront-shaping diffractive optics to create a structured probe with full control of its phase at the sample plane, circumventing the need for a mask. We demonstrate in silico that the method can image nanostructures and magnetic textures and validate our approach with a visible light-based experiment. The method enables investigation of a plethora of phenomena at the nanoscale including magnetic and electronic phase coexistence in solids, with further uses in soft and biological matter research.

Achieving high spatial resolution in a large field-of-view using lensless x-ray imaging

X-ray ptychography, a powerful scanning lensless imaging technique, has become attractive for nondestructively imaging internal structures at nanoscale. Stage positioning overhead in conventional step-scan ptychography is one of the limiting factors on the imaging throughput. In this work, we demonstrate the use of advanced fly scan ptychography to achieve high-resolution ptychograms of modern integrated circuits on a large field-of-view at millimeter scale. By completely removing stage overheads between scan points, the imaging time for millimeter-size sample can be significantly reduced. Furthermore, we implement the orthogonal probe relaxation technique to overcome the variation of illumination across the large scan area as well as local vibrations. The capability of x-ray ptychography shown here is broadly applicable for various studies, which requires both high spatial resolution and large scan area.

Backscattering X-ray imaging using Fresnel zone aperture

We demonstrate backscattering X-ray imaging using a Fresnel zone aperture (FZA), where the line width of the outermost circle is 100 μm. The method is a lensless imaging technique employing an optical aperture instead of a lens. We successfully reconstructed test objects from four images observed using a series of FZAs without an iterative procedure during the reconstruction process. We also confirmed the successful reconstruction of a series of images by changing the focal plane in a short computer calculation time, indicating that 3D information can be obtained by measuring a single set of four images.