Lensless In-line Holographic Microscope Research Articles

A sub-wavelength Si LED integrated in a CMOS platform

A nanoscale on-chip light source with high intensity is desired for various applications in integrated photonics systems. However, it is challenging to realize such an emitter using materials and fabrication processes compatible with the standard integrated circuit technology. In this letter, we report an electrically driven Si light-emitting diode with sub-wavelength emission area fabricated in an open-foundry microelectronics complementary metal-oxide-semiconductor platform. The light-emitting diode emission spectrum is centered around 1100 nm and the emission area is smaller than 0.14 μm2 (~varnothing 400 nm). This light-emitting diode has high spatial intensity of >50 mW/cm2 which is comparable with state-of-the-art Si-based emitters with much larger emission areas. Due to sub-wavelength confinement, the emission exhibits a high degree of spatial coherence, which is demonstrated by incorporating the light-emitting diode into a compact lensless in-line holographic microscope. This centimeter-scale, all-silicon microscope utilizes a single emitter to simultaneously illuminate ~9.5 million pixels of a complementary metal-oxide-semiconductor imager.

Accurate and fast registration algorithm for multi-height lensless in-line on-chip holographic microscopy

Lensless in-line on-chip holographic microscope has cost-effective and compact designs that can offer sub-micron resolution over a large field-of-view. In multi-height lensless on-chip holographic microscopy, the in-line hologram series are recorded at different positions along the optical axis, thus, there inevitably exists several kinds of error sources including shifts, scales, and rotations. Therefore, digitally registering and aligning these in-line holograms is of great concern for iterative phase-retrieved recovery. Here, we proposed an accurate and fast registration method, which consists of three major steps. First, Fourier-transform-based phase correlation method is used to roughly correct the spatial translation through locating the correlation peak. Second, we proposed a morphology-based method to accurately search for the matching feature points to obtain the accurate scaling factors, where samples themselves are used as markers without any labeling. Third, the phase correlation method is used for precisely registration again. Experimental results verified the validity and effectiveness of the proposed registration algorithm. And by the comparative experiments with the existing popular registration algorithms, our proposed registration algorithm can register and align the multi-height in-line holograms with greater precision and speed.

Wide field-of-view line-scanning lensless in-line holographic microscope

In a traditional lensless in-line holographic microscope (LIHM), the field of view (FOV) is generally limited by the size of the imaging sensor, which is usually several millimeters wide. We propose a scanning LIHM system using a line-scan camera to achieve wide field-of-view holographic imaging. In addition, the iterative holographic reconstruction method is applied to obtain the sample image with enhanced contrast and resolution using two hologram measurements, which are acquired by slightly shifting the sample-to-sensor distance. The capability of the proposed scanning LIHM system was demonstrated by experiments with a U.S. Air Force target and a biological sample of leaf vein. The reconstructed sample image had a FOV of 2.87 cm × 4.70 cm (2048 × 3357 pixels), which was much larger than the typical FOV of several square millimeters in a traditional LIHM system.

Digital inline holographic reconstruction with learned sparsifying transform

We propose a digital inline holographic reconstruction method based on learned sparsifying transform. An iterative algorithm which includes the steps of updating background, phase, and image, as well as a step of sparse coding, is adopted to optimize the designed penalized least squares object function with regularization based on a sparsifying transform learned from sample images. A lensless inline holographic microscope (LIHM) was built and used to image a U.S. air force target and a pumpkin stem sample. The proposed method was applied to reconstruct the sample images. Compared with conventional holographic reconstruction method or the L1-norm-based penalized least squares reconstruction method, the imaging results show that the proposed method could better suppress the twin-image disturbance, staircase edges and block artifacts, thus enhance the reconstructed image quality.

Image enhancement in lensless inline holographic microscope by inter-modality learning with denoising convolutional neural network

Compared to traditional optical microscope, lensless inline holographic microscope (LIHM) is more compact and low-cost. However, its resolution and imaging contrast are generally inferior mainly because of the twin-image background. In this paper we propose a deep learning-based approach to reduce the noise and enhance the imaging quality in LIHM by inter-modality learning from the traditional microscope images. By exploiting the denoising model in the learning processing, our network can be trained with a dataset synthesized from the direct-reconstructed images of LIHM and the high-resolution ground truth images obtained with a microscope. In the imaging process, other direct-reconstructed images of LIHM can then be enhanced by the trained denoising network. The image enhancement capability of our method was demonstrated by experiments with a U.S. Air force (USAF) target and a pumpkin stem sample. The results show that both the resolution and imaging contrast were significantly improved compared with traditional reconstruction methods in LIHM.

Pixel super-resolution lensless in-line holographic microscope with hologram segmentation

We propose a resolution enhancement method for a lensless in-line holographic microscope (LIHM) by combining the hologram segmentation and pixel super-resolution (PSR) techniques. Our method is suitable for imaging specific target objects in samples, where the in-line hologram is disturbed by other objects in the samples. The resolution-enhancement capability of our method was proved by numerical simulations and imaging experiments while using a standard resolution target in a two-layer setup. We also applied our LIHM system to image the sample of living algae Euglena gracilis in water solution for further demonstration.

Color lensless in-line holographic microscope with sunlight illumination for weakly-scattered amplitude objects

We propose a method for acquiring color images with a lensless in-line holographic microscope (LIHM) using sunlight illumination, which is suitable for weakly scattered amplitude objects. In the color LIHM, the sample is illuminated by sunlight, and the in-line hologram is recorded by a color CMOS imaging sensor located behind the sample. For weakly-scattered amplitude object, we show that the hologram can be described as the convolution of object transmission and a point spread function (PSF) that depends on the spectral distribution of the light. The captured color hologram is first separated into the red, green, and blue components, and then the sub-holograms of each color are used to reconstruct the corresponding color components of the sample by a deconvolution process. We proved that the deconvolution process was able to improve the imaging resolution, which was deteriorated because of insufficient temporal coherence of the light. The resolution enhancement capability of our color LIHM was demonstrated by numerical simulations and imaging experiments with the U. S. Air Force target as the sample. We also imaged a stained root of the herb Saposhnikovia divaricata to further demonstrate the capability of our method for color imaging applications. Our proposed color LIHM method provides a way to realize color holographic imaging with white light sources and thus reduces the cost and complexity of the lensless color microscope.

Differential holographic reconstruction for lensless in-line holographic microscope with ultra-broadband light source illumination

Abstract We propose a differential holographic reconstruction (DHR) method for lensless in-line holographic microscope (LIHM) with ultra-broadband light source illumination. Using the DHR method, the deterioration of imaging resolution and contrast because of insufficient temporal coherence can be noticeably compensated. In our LIHM setup, the sample was illuminated by an ultra-broadband light source without any spectral filters, and the in-line hologram was recorded by a CMOS imaging sensor located behind the sample. The enhanced-resolution sample image was achieved by subtracting two low-resolution sample images, which were obtained by back-propagating the hologram numerically to the sample plane and another plane with slightly different back-propagation distance. The DHR method was studied theoretically, and the resolution enhancement capability was verified by both numerical simulations and imaging experiments. The U.S. air force target and a lily anther as bio-sample were used in the imaging experiments. The results showed that the proposed DHR method achieved better resolution with ultra-broadband light sources such as sunlight and incandescent light sources, which could reduce the cost for compact imaging systems.

Multilayer pixel super-resolution lensless in-line holographic microscope with random sample movement

We report a multilayer lensless in-line holographic microscope (LIHM) with improved imaging resolution by using the pixel super-resolution technique and random sample movement. In our imaging system, a laser beam illuminated the sample and a CMOS imaging sensor located behind the sample recorded the in-line hologram for image reconstruction. During the imaging process, the sample was moved by hand randomly and the in-line holograms were acquired sequentially. Then the sample image was reconstructed from an enhanced-resolution hologram obtained from multiple low-resolution in-line holograms by applying the pixel super-resolution (PSR) technique. We studied the resolution enhancement effects by using the U.S. Air Force (USAF) target as the sample in numerical simulation and experiment. We also showed that multilayer pixel super-resolution images can be obtained by imaging a triple-layer sample made with the filamentous algae on the middle layer and microspheres with diameter of 2 μm on the top and bottom layers. Our pixel super-resolution LIHM provides a compact and low-cost solution for microscopic imaging and is promising for many biomedical applications.

Resolution enhancement method for lensless in-line holographic microscope with spatially-extended light source.

We propose a resolution enhancement method for lensless in-line holographic microscope (LIHM) with spatially-extended light source, where the resolution is normally deteriorated by the insufficient spatial coherence of the illumination. In our LIHM setup, a light-emitting diode (LED), which was a spatially-extended light source, directly illuminated the sample, and the in-line hologram were recorded by a CMOS imaging sensor located behind the sample. In our holographic reconstruction process, the in-line hologram was first deconvoled with a properly resized image of the LED illumination area, and then back-propagated with scalar diffraction formula to reconstruct the sample image. We studied the hologram forming process and showed that the additional deconvolution process besides normal scalar diffraction reconstruction in LIHM can effectively enhance the imaging resolution. The resolution enhancements capability was calibrated by numerical simulations and imaging experiments with the U.S. air force target as the sample. We also used our LIHM to image the wing of a green lacewing to further demonstrate the capability of our methods for practical imaging applications. Our methods provide a way for LIHM to achieve satisfactory resolution with less stringent requirement for spatial coherence of the source and could reduce the cost for compact imaging system.

Enhanced resolution for amplitude object in lensless inline holographic microscope with grating illumination

In a compact digital lensless inline holographic microscope (LIHM), where the sample-to-sensor distance is short, the imaging resolution is often limited by sensor pixel size instead of the system numerical aperture. We propose to solve this problem by applying data interpolation with an iterative holographic reconstruction method while using grating illumination in the LIHM system. In the system setup, the Talbot self-image of a Ronchi grating was used to illuminate the sample, and the inline hologram was recorded by a CMOS imaging sensor located behind the sample. The hologram was then upsampled by data interpolation before the reconstruction process. In the iterative holographic reconstruction, the sample support was defined by the bright areas of the grating illumination pattern and was used as constraint. A wide-field image can also be obtained by shifting the grating illumination pattern. Furthermore, we assumed that the sample was amplitude object, i.e., no obvious phase change was caused by the sample, which provided additional constraint to refine the interpolated data values. Besides improved resolution, the iterative holographic reconstruction also helped to reduce the twin-image background. We demonstrated the effectiveness of our method with simulation and imaging experiment by using the USAF target and polystyrene microspheres with 1 μm diameter as the sample.

Enhanced resolution in lensless in-line holographic microscope by data interpolation and iterative reconstruction

In a compact lensless in-line holographic microscope, the imaging resolution is generally limited by the sensor pixel size because of the short sample-to-sensor distance. To overcome this problem, we propose to use data interpolation based on iteration with only two intensity measurements to enhance the resolution in holographic reconstruction. We did numerical simulations using the U.S. air force target as the sample and showed that data interpolation in the acquired in-line hologram can be used to enhance the reconstruction resolution. The imaging resolution and contrast can be further improved by combining data interpolation with iterative holographic reconstruction using only two hologram measurements acquired by slightly changing the sample-to-sensor distance while recording the in-line holograms. The two in-line hologram intensity measurements were used as a priori constraint in the iteration process according to the Gerchberg–Saxton algorithm for phase retrieval. The iterative reconstruction results showed that the iteration between the sample plane and the sensor planes can refine the interpolated data and thus further improve the resolution as well as the imaging contrast. Besides numerical simulation, we also experimentally demonstrated the enhancement of imaging resolution and contrast by imaging the U.S. air force target and a microscope slide of filamentous algae.

Lensless in-line holographic microscope with Talbot grating illumination

We have developed a wide field-of-view lensless in-line holographic microscope (LIHM) capable of acquiring microscopic images with a compact design. In our imaging system, a Ronchi grating was illuminated by a collimated laser beam to generate a Talbot self-imaging grating illumination on the sample, and the in-line holograms were recorded by a CMOS imaging sensor behind the sample. An iterative reconstruction algorithm was then applied to reconstruct the sample image while eliminating the twin-image background that appears in traditional in-line holography. In the algorithm, the dark areas of the illumination grating were used as a known constraint to define the sample support that led to convergence of the iteration. The whole-sample image can be acquired by laterally shifting the grating. We demonstrated the performance of our iteration algorithm and imaging system by successfully acquiring images of polystyrene microspheres with 5 μm diameter and the wing of a green lacewing.

Lensless 3D Digital Holographic Microscopic Imaging at Vacuum UV Wavelength

We demonstrate imaging of microscopic objects using a lensless in-line 3D digital holographic microscope at vacuum UV wavelength of 193 nm. The absence of microscope objective and a separate reference beam simplifies the setup and reduces the number of UV components required. We also discuss the space-bandwidth considerations in the design of such a system. Experimental results using human blood cells and latex beads are presented.