Resolution comparison between integral-imaging-based hologram synthesis methods using rectangular and hexagonal lens arrays

Hexagonal diffractive optical elements

Diffractive optical elements (DOEs) have widespread applications in optics, ranging from point spread function engineering to holographic display. Conventionally, DOE design relies on Cartesian simulation grids, resulting in square features in the final design. Unfortunately, Cartesian grids provide an anisotropic sampling of the plane, and the resulting square features can be challenging to fabricate with high fidelity using methods such as photolithography. To address these limitations, we explore the use of hexagonal grids as a new grid structure for DOE design and fabrication. In this study, we demonstrate wave propagation simulation using an efficient hexagonal coordinate system and compare simulation accuracy with the standard Cartesian sampling scheme. Additionally, we have implemented algorithms for the inverse DOE design. The resulting hexagonal DOEs, encoded with wavefront information for holograms, are fabricated and experimentally compared to their Cartesian counterparts. Our findings indicate that employing hexagonal grids enhances holographic imaging quality. The exploration of new grid structures holds significant potential for advancing optical technology across various domains, including imaging, microscopy, photography, lighting, and virtual reality.

Lattice light sheets generated with a firmly arranged dielectric regular hexagonal pyramid array

We present a firmly arranged dielectric regular hexagonal pyramid array to generate lattice light sheets with high conversion efficiency and low stray light. Both the size and working distance of the lattice light sheets can be modulated by changing the structural parameters. We experimentally recorded the lattice light sheets illumination, which is consistent with the corresponding simulation. To evaluate the imaging quality, we compared the light field generated with and without structure by using polystyrene fluorescent microspheres. This study provides a potential method for the building of light sheet fluorescence microscopy with high resolution and low phototoxicity.

Crosstalk Reduction in Voxels for a See-Through Holographic Waveguide by Using Integral Imaging with Compensated Elemental Images

The representation of three-dimensional volumetric pixels, voxels, is an important issue for the near-to-eye displays (NEDs) to solve the vergence-accommodation conflict problem. Although the holographic waveguides using holographic optical element (HOE) couplers are promising technologies for NEDs with the ultra-thin structure and high transparency, most of them have presented only a single and fixed depth plane. In this paper, we analyze the imaging characteristics of holographic waveguides, particularly to represent the arbitrary voxels and investigate the voxel duplication problem arising from the non-collimated light from the voxels. In order to prevent the image crosstalk arising from the voxel duplication, we propose an adjustment method for the emission angle profile of voxels by using the integral imaging technique. In the proposed method, the sub-regions of elemental images, which correspond to the duplicated voxels, are masked in order to optimize the emission angle of integrated voxels. In the experimental verification, a see-through integral imaging system, based on the organic light-emitting diode display and a holographic waveguide with the thickness of 5 mm, was constructed. The fabricated HOE in the waveguide showed high diffraction efficiency of 72.8 %, 76.6%, and 72.5 % for 460 nm, 532 nm, and 640 nm lasers, respectively. By applying the masked elemental images, the proposed method resulted in a reduced crosstalk in the observed voxels by 2.35 times. The full-color experimental results of see-through holographic waveguide with integral imaging are provided, whereby the observed 3D images are presented clearly without the ghost images due to the voxel duplication problem.

Performance enhancement of integral imaging based Fresnel hologram capturing by the intermediate view reconstruction

A method aiming at improving the performance of integral imaging (II) based Fresnel hologram is proposed, which is generated by using the intermediate view reconstruction (IVR). The conventional integral holograms are generally generated through Fourier transforming the elemental images (EI) of II into hogels. However, a trade-off between the angular resolution and the spatial resolution of II is inevitable within the generation of integral hologram. The IVR is introduced to enhance the angular spectrum of II-based Fresnel hologram while keeping a compact image size and being free from moving the lenslet array. Multiple elemental image array (EIA) sequences are generated with the IVR and transformed to the corresponding holograms. All the generated hologram sequences shift depending on the relative position of the virtual lens array and are added together to synthesize the Fresnel hologram with a high angular spectrum. The synthesized hologram can reconstruct the 3D image with the combined light fields of all the integral hologram sequences. Finally, both the simulation with multiple objects and experiments of real 3D object are numerically and optically conducted. The high matching results among them confirm this work a better performance over the conventional methods.

3D view image reconstruction in computational integral imaging using scale invariant feature transform and patch matching

This paper realizes a computational integral imaging reconstruction method via scale invariant feature transform (SIFT) and patch matching to improve the visual quality of reconstructed 3D view images. To our knowledge, the 3D view images reconstructed from the elemental images suffer from artifacts, which leads to degradations in the visual quality. To prevent image degradation, in this paper, we use the correct regions obtained from the view images taken directly from the original object or use patch matching to replace the distorted regions. However, the initial matching regions could not meet our requirements owing to the limitations of the equipment and the inevitable shortcomings of the experimental operation. To solve these problems, we adopt SIFT descriptors and perspective transform to get the satisfying correct regions. We present the simulation and experimental results of the 3D view images and the evaluation of the quality of the corresponding images to test the performance of the proposed method. The simulation and experimental results indicate that the proposed method can significantly improve the visual quality of the 3D view images and verify the feasibility and effectiveness of the proposed method.

Holographic display system with adjustable viewing angle based on multi-focus optofluidic lens

In this paper, a holographic display system with adjustable viewing angle is proposed. The system consists of a collimated beam, a spatial light modulator (SLM), a multi-focus optofluidic (MFO) lens and an aperture. The MFO lens with high focal power is produced and it consists of two substrates, one multilayer substrate and two chambers. When the liquids are pulled in/out from the channels, the curvature of the liquid-liquid interface changes due to the surface tension and adsorption between the liquids and the multilayer substrate. The relationship between the parameters of the MFO lens and the holographic display viewing angle is revealed for the first time. Based on the theoretical analysis, the mechanisms of the high focal power and mechanical stability of the proposed MFO lens are also clarified. The experiments show that the focal power of the proposed MFO lens can be varied from -20 D (m^-1) to 4 D (m^-1), respectively. By using the MFO lens, the viewing angle of the holographic display system can be adjusted without moving any components mechanically. Meanwhile the setup of the system is greatly simplified. The experimental results verify the feasibility of the system, and it is expected to bring new ideas to the holographic display with large viewing angle.

Orthographic projection images-based photon-counted integral Fourier holography

Unlike coherent imaging techniques, light field imaging uses incoherent (white light) illumination to generate a digital hologram of three-dimensional (3D) objects in real time. Multiple projections (or elemental images) of a 3D object are captured using a microlens array attached to a digital camera. Orthographic projection images (OPIs) can be synthesized from the recorded elemental images. The synthesized intensity-based OPIs are then multiplied by the corresponding phase functions and combined to form a digital hologram (also known as an integral hologram) of a 3D object under illumination. In this study, we analyze the performance of a synthesized integral hologram under low light imaging (photon-counting) conditions. The feasibility of this technique is verified experimentally by capturing the elemental images and subsequently generating orthographic projection images and by varying photon counts to reconstruct the digital holograms.

3D Imaging Based on Depth Measurement Technologies

Three-dimensional (3D) imaging has attracted more and more interest because of its widespread applications, especially in information and life science. These techniques can be broadly divided into two types: ray-based and wavefront-based 3D imaging. Issues such as imaging quality and system complexity of these techniques limit the applications significantly, and therefore many investigations have focused on 3D imaging from depth measurements. This paper presents an overview of 3D imaging from depth measurements, and provides a summary of the connection between the ray-based and wavefront-based 3D imaging techniques.

Fast and high-resolution light field acquisition using defocus modulation

In the conventional microlens-array-based light field imaging system, there is a trade-off between the angular and spatial resolutions. Light field reconstruction from images captured by focal plane sweeping, such as light field moment imaging (LFMI) and light field reconstruction with back projection (LFBP), can achieve high transverse resolution comparable to the modern camera sensor. However, the acquisition of a series of focal plane sweeping images along the optical axis is time consuming and requires fine alignment. Furthermore, different focal-plane-based light field reconstruction techniques require images with different characteristics. To solve these problems, we present an efficient approach for fast light field acquisition with precise focal plane sweeping capture by defocus modulation, rather than mechanical movement. Also, we verify the validity and the improvement of our system. With the controllable point spread function, we can capture images for light field reconstruction with both LFMI and LFBP. Otherwise, we quantitatively compare the two methods using images captured with the proposed systems.

Analysis of the noise in backprojection light field acquisition and its optimization

Light field reconstruction from images captured by focal plane sweeping can achieve high lateral resolution comparable to the modern camera sensor. This is impossible for the conventional micro-lenslet-based light field capture systems. However, the severe defocus noise and the low depth resolution limit its applications. In this paper, we analyze the defocus noise in the focal-plane-sweeping-based light field reconstruction technique, and propose a method to reduce the defocus noise. Both numerical and experimental results verify the proposed method.

Analysis of the depth of field in hexagonal array integral imaging systems based on modulation transfer function and Strehl ratio

Integral imaging is a technique for displaying three-dimensional images using microlens arrays. In this paper, a method for calculating root mean squared wavefront error and modulation transfer function (MTF) of a defocused integral imaging capture system with hexagonal aperture microlens arrays is introduced. Also, maximum allowable depth of field with Century MTF analyzing and Strehl criterion are obtained.

High-resolution Fourier hologram synthesis from photographic images through computing the light field

We present a technique for synthesizing the Fourier hologram of a three-dimensional scene from its light field. The light field captures the volumetric information of an object, and an important advantage is that it does not require coherent illumination, as in conventional holography. In this work, we show how to obtain a high-resolution digital hologram with the light field obtained from a series of photographic images captured along the optical axis. The method is verified both by simulations and experimentally captured light field.

Real-time 3D display system based on computer-generated integral imaging technique using enhanced ISPP for hexagonal lens array

This paper proposes an open computer language (OpenCL) parallel processing method to generate the elemental image arrays (EIAs) for hexagonal lens array from a three-dimensional (3D) object such as a volume data. Hexagonal lens array has a higher fill factor compared to the rectangular lens array case; however, each pixel of an elemental image should be determined to belong to the single hexagonal lens. Therefore, generation for the entire EIA requires very large computations. The proposed method reduces processing time for the EIAs for a given hexagonal lens array. By using the proposed image space parallel processing (ISPP) method, it can enhance the processing speed that generates the 3D display of real-time interactive integral imaging for hexagonal lens array. In our experiment, we implemented the EIAs for hexagonal lens array in real-time and obtained a good processing time for a large of volume data for multiple cases of lens arrays.