Pinhole Array Research Articles

P‐36: Design of Simulation Tools for Light‐field Near‐eye Displays with a Pinhole Array

We have developed a group of simulation tools to analyze the display performance of light‐field near‐eye displays based on a pinhole array. The characteristics of human visual system have been considered to show the accommodation, the display resolution, the eye box and the tolerance of the given specifications. Therefore, the developed tools provide a useful reference during the design, optimization and manufacture process.

2500-Channel single-shot areal profilometer using hyperspectral interferometry with a pinhole array

Surface profilometry techniques such as coherent scanning interferometry or focus variation require long scan times and are thus vulnerable to environmental disturbance. Hyperspectral interferometry (HSI) overcomes the problem by recording all the spatial and spectral information necessary to reconstruct a 2D surface height map in a single shot. In this paper, we present a new HSI system that uses a pinhole array to provide the necessary gaps for the spectral information. It is capable of measuring 2500 independent points, twice the previous maximum number, with a maximum unambiguous depth range of ∼825 µm and a larger maximum surface tilt angle of 33.3 mrad. The use of phase information allows height to be measured to a precision of ∼6 nm, an order of magnitude improvement on previous HSI systems.

Crosstalk-free integral imaging 3D display using pinhole array

We propose a crosstalk-free integral imaging three-dimensional (3D) display using a pinhole array. It consists of a back light unit, a pinhole array and a liquid crystal panel. The pinhole array, which is attached to the back light unit, is located between the back light unit and the liquid crystal panel. Three imaging models of the integral imaging 3D display with different pinhole arrays are analyzed to reveal the relationship between the aperture width of the pinhole array and the crosstalk. The optimal value of the aperture width of the pinhole array is also obtained to eliminate the crosstalk. A prototype using the optimal pinhole array is developed, and the experiment results agree well with the theory.

Research on exposure-time uniformity of pulse-dilation framing camera

Framing cameras are important ultrafast diagnostic tools and used to study spatiotemporal evolution of plasmas through pinhole array images in inertial confinement fusion (ICF) experiments. The exposure-time (ET) and its uniformity are main factors for analyzing effect of plasma evolution. A 10-ps pulse-dilation framing camera typically has a non-uniform ET between array images, which limits its plasma diagnostic in ICF. Therefore, in order to obtain deeper insights into this drawback, ET uniformity of pulse-dilation framing camera is studied. The factors influencing ET uniformity are theoretically analyzed and camera system is built for experimental studies. Results show that the transmission of dilation pulse along the photocathode (PC) causes accelerating potential changes, in which result inconsistent ET at different positions of the PC. When gradient of dilation pulse is 2 V/ps, length of PC is 56 mm and acceleration energy is 3k eV, the ET is changed from 14.8ps(pulse dilation magnification is 6.22×) to 12.6ps(7.30×) in the process of transmission along the PC and the non-uniform rate is about 8%, that consistent with theoretical predictions. Furthermore, the research have important theoretical and practical significance for further understanding plasma evolution in ICF.

Development of an auto-interchangeable multi-pinhole array for confocal laser scanning microscopy systems using precision laser processing

Confocal laser scanning microscopy systems rely on a pinhole placed at the confocal point to act as a spatial filter. The diameter of the pinhole critically affects both the brightness and resolution of the imaging. Furthermore, different imaging conditions, such as sample type, depth of the sample, wavelength of the signal, and its inherent emission or reflection strength, require varying balances between the brightness and optical resolution for optimal observability. However, exchanging the pinhole, and its subsequent alignment, is often a time consuming challenge when implemented in customized systems. These systems are important since they are often used in studies for which commonly available ones lack sufficient capabilities. As such, we have developed an auto-positioning interchangeable pinhole array, manufactured via a patented laser machining process and designed for convenient implementation in confocal laser scanning microscopy systems and we demonstrate its effectiveness and versatility.

Three-dimensional display with optimized view distribution

A three-dimensional (3-D) display with optimized view distribution is proposed. The proposed 3-D display consists of a two-dimensional (2-D) display panel, a pixel mask, and a lenticular lens. The 2-D display panel is used to provide parallax images. The pixel mask has a designed pinhole array and is located close to the 2-D display panel. This pinhole array can modulate the location and size of the pixels on the 2-D display panel to provide an optimized view distribution. Finally, the lenticular lens projects the pixels in different spatial directions to form 3-D images. Different from conventional 3-D displays, the proposed 3-D display can concentrate views into the center area and form a better parallax continuity. A prototype of the proposed 3-D display is developed. Experiment results agree well with the theory.

Noniterative spatially partially coherent diffractive imaging using pinhole array mask

We propose and experimentally demonstrate a noniterative diffractive imaging method for reconstructing the complex-valued transmission function of an object illuminated by spatially partially coherent light from the far-field diffraction pattern. Our method is based on a pinhole array mask, which is specially designed such that the correlation function in the mask plane can be obtained directly by inverse Fourier transforming the diffraction pattern. Compared to the traditional iterative diffractive imaging methods using spatially partially coherent illumination, our method is noniterative and robust to the degradation of the spatial coherence of the illumination. In addition to diffractive imaging, the proposed method can also be applied to spatial coherence property characterization, e.g., free-space optical communication and optical coherence singularity measurement.

Maskless lithography based on digital micromirror device (DMD) and double sided microlens and spatial filter array

A new type of maskless lithography system based on digital mirror device (DMD) is proposed, constructed, and experimentally demonstrated. It includes a pin-hole array sandwiched by two microlens arrays on each side, known as double-sided microlens/spatial-filter array (D-MSFA), and aligned with a DMD. Ultraviolet (UV) light reflected by DMD is first collected by the first microlens array, filtered through the pin-hole array, and then re-focused by the second microlens array into a UV spot array. Along with an obliquely scanning method, this D-MSFA/DMD-based maskless lithography system can perform not only 2D but also 3D UV patterning. Experimental testing successfully generates complicated patterns with a minimum line-width of 3.36 μm. Direct 3D patterning and 3D microfabrication are also experimentally demonstrated on a photoresist layer. Excellent profile accuracy and surface structure qualities are observed with great potentials for future 2D and 3D microfabrication in a maskless manner.

Spatially varying aberration calibration using a pair of matched periodic pinhole array masks.

For advanced imaging systems, e.g., projection systems for optical lithography, spatially varying aberration calibration is of utmost importance to achieve uniform imaging performance over the entire field-of-view (FOV). Here we present an efficient, accurate, and robust spatially varying aberration calibration method using a pair of 2-dimensional periodic pinhole array masks: the first mask in the object plane and the second mask in the image plane. Our method divides the entire FOV of the imaging system into partially overlapping subregions by using a measurement system consisting of an additional imaging system and a camera sensor. Each subregion, which covers several mask periods, is imaged onto a distinct camera pixel by the measurement system. Our method measures "Airy disc"-like patterns simultaneously in all subregions by scanning the second mask relative to the first mask over one mask period. The number of subregions is equal to the number of camera pixels, and the sampling number of the measured patterns is equal to the scanning step number. The aberrations can be retrieved from the patterns measured in through-focus planes using an iterative optimization algorithm. In this paper, we performed experimental validation on a realistic lithography machine and demonstrate that our method is capable of retrieving the coefficients of 37 aberration terms, expressed as Zernike polynomials, with a sensitivity at nanometer scale.

Augmented Reality Lightfield Display for a Smart Window Using an Active Pinhole Array

An augmented reality (AR) lightfield display can provide three-dimensional (3D) images at the locations of real objects. However, a conventional AR lightfield display has a few disadvantages in that its 3D resolution is poor and it is bulky in form owing to the use of an optical combiner or relay optics. In this paper, we propose a novel method to realize an AR lightfield display using a transparent display and an active pinhole array (APA) to enhance both the 3D resolution and visibility of real objects. An experimental demonstration verifies that the proposed method is appropriate for smart window applications.

Multi-probe ptychographic iterative engine method

As a lensfree imaging technique, ptychographic iterative engine (PIE) can provide quantitative amplitude and phase distributions of samples in an extremely large field of view but without aberrations. However, relying on the single pinhole scanning, PIE is time-consuming in both diffraction recording and sample reconstruction, thus limiting its potentiality in dynamic imaging applications. In order to reduce the diffraction captures and increase its processing efficiency, we design the multi-probe PIE, in which a pinhole array is adopted instead of traditionally used single pinhole, thus diffractions at different positions can be captured simultaneously. Moreover, the multi-probe PIE reconstruction algorithm is also implemented to further accelerate the sample reconstruction speed. Proved by experiments, less than 10 s is required for diffraction recording for a large field of view of ∼150 mm2 and combining with the modified multi-probe PIE reconstruction algorithm, high-resolution quantitative sample information can be extracted accurately with much faster speed compared to traditional PIE reconstruction algorithm. It is believed the proposed multi-probe PIE can be future applied in dynamic imaging.

Talbot interferometry for imaging two-dimensional electron density distribution over discharge plasma with higher sensitivity.

The basic properties of a Talbot interferometer implementing pinhole arrays were experimentally and numerically investigated for the improvement of measurement sensitivity of laser wavefront sensors utilized for electron density imaging over discharge plasmas. A numerical simulation using a plane wave decomposition method indicated that the pinhole arrays with a pitch of 300 μm and a pinhole diameter of 150 μm were most suitable for the measurement of the millimetre-scale discharge plasmas, in consideration of the spatial resolution and measurement accuracy. The plane wave decomposition simulation expected that the measurement sensitivity of the 8th-Talbot-length interferometer could be improved by a factor of 4 compared with the previously developed Shack-Hartmann type laser wavefront sensors, which was experimentally verified by the self-image behavior of the pinhole arrays. The Talbot interferometric system was successfully used for electron density imaging over the vacuum arcs generated between a 3-mm gap. The electron density image observed by the Talbot interferometers was in excellent agreement with that visualized by the previously developed Shack-Hartmann sensors. The practical notification for the pinhole array fabrication was also presented.

Optimizing neutron imaging line of sight locations for maximizing sampling of the cold fuel density in inertial confinement fusion implosions at the National Ignition Facility.

Neutron imaging provides a ready measurement of the shape of the "hot spot" core of an inertial confinement fusion implosion. The 14-MeV neutrons emitted by deuterium-tritium reactions are imaged at the National Ignition Facility using a pinhole array onto a scintillator, and the images are recorded on a camera. By changing the gate time of the camera, lower energy neutrons, downscattered by the cold fuel surrounding the hot spot, are recorded. The cold fuel density can be reconstructed using the two images. The kinematics of the scattering coupled with the scattering cross sections restrict the angular extent of the cold fuel sampled, with the backside of the implosion not being sampled at all. This work demonstrates the limited region of the cold fuel measured by the current line of sight (40%). At completion of the three planned lines of sight, 79% of the cold fuel will be sampled.

High-speed ptychographic imaging based on multiple-beam illumination.

Ptychography is a lensless phase imaging technique that obtains an image by scanning a specimen at several points with respect to a localized illumination beam. For larger specimens, it takes a longer time to complete scanning and hence higher stability is required in the setup which is often not guaranteed. An alternative technique is proposed here that reduces the sequential scanning time for such applications. A pinhole array is used to generate multiple tiny spatially separated beams to scan an object simultaneously at various points. The resulting diffraction patterns are recorded and processed in the Fresnel regime to obtain the images. Unlike other ptychographic methods using multiple beams, the proposed method does not require the use of a multimode ptychography algorithm or autocorrelation filtering of the diffraction patterns. The effectiveness of the method is studied through simulations and experiments. In contrast to conventional single-beam ptychography, the proposed method has the ability to achieve a larger field of view while leaving the number of scanned positions unchanged.

Coded pinhole lens imaging

This paper presents a coded pinhole lens imaging system consisting of a pinhole array aperture, followed by a thin convex lens, mounted onto a standard DSLR camera body. The combination of pinhole and lens incurs two questions: (1) Is the new camera based on pinhole or lens imaging principle? (2) Can the lens improve the imaging quality? The study reveals that the camera is based on pinhole imaging, but the lens can improve its optical resolution from 0.44 mm (pinhole size) to 0.042 mm, leading to a significant improvement in imaging quality. The numerous pinholes on the aperture also improve the camera’s light throughput ability over single pinhole camera. The camera could be used for applications where a large depth of field is required while the illumination condition is poor.

Note: A Laue crystal imager for high energy quasi-monochromatic x-ray.

A newly designed transmission type x-ray Laue imager for tens of keV hard x-rays is reported. Compared with the traditional reflection type x-ray crystal imager, the transmission geometry produces a much better image quality for high energy hard x-rays. This system was assessed via a calibration experiment performed at the SPring8 synchrotron radiation facility. With a Ta x-ray fluorescer, the mono-energetic x-ray at 70 keV from the synchrotron radiation was converted to an isotropically emitted Ta K-shell source at 57.5 keV and 65 keV. A tungsten pinhole array was employed as the test object, and clear images of the pinholes with a magnification of ∼5 were acquired. These images exhibited superior quality in the dispersion plane. As an extension of this work, a slit-free full-spectral Laue imager is proposed for high resolution hard x-ray imaging.

Optical sectioning in multifoci Raman hyperspectral imaging

AbstractIn this study, we compared the depth discrimination and speed performance of multifoci Raman hyperspectral imaging with the reference standard of a single laser point confocal Raman mapping. A liquid crystal spatial light modulator was employed for the generation of multifoci laser beams, and a digital micromirror device was used as a software‐configurable reflective pinhole array. The patterns of the laser foci and pinhole array can be rapidly changed without requiring any hardware alterations. Confocal patterns with different distance‐to‐size ratios were tested and compared. After optimization of the laser‐foci pattern, we demonstrated the feasibility of multifoci Raman hyperspectral microscopy for recording depth‐resolved molecular maps of biological cells (Acanthamoeba castellanii trophozoites). Micrometric depth discrimination and short acquisition times (20 min for single plane confocal image) were achieved.

Design and Fabrication of a Three-Dimensional Artificial Compound Eye Using Two-Photon Polymerization.

Microlens arrays have been widely used in the fields of micro-optics because of the advantages of their high diffraction efficiency, high fill factor, and wide operating band. However, the microlens array still has problems with its smaller field of view (FOV) and lower utilization of light energy. In this paper, a 3D compound eye system consisting of a microlens array and a pinhole array was designed according to the optical principle of insect compound eye. The artificial compound eye structure was processed in two-photon polymerization processing technology. Ray tracing and optical system simulation of the designed artificial compound eye structure were performed. The results showed that the artificial compound eye structure had a wider FOV and higher light energy utilization than a conventional 2D microlens array. This thesis may lay a theoretical foundation for the structural optimization design of microlens arrays.

4H-Silicon Carbide Wafer Surface after Chlorine Trifluoride Gas Etching

In order to develop the high etching rate reactor for silicon carbide, the 50-mm-diameter C-face 4H-silicon carbide wafer was etched using the chlorine trifluoride gas at 500 °C. By the deep etching, the concentric-circle-shaped valleys were formed at the positions corresponding to the radii of the pin-hole arrays of the gas distributor, as predicted by the calculation. The etching rate profile of 4H-silicon carbide was concluded to have a relationship with the local chlorine trifluoride gas supply . The wafer bow was small, even the wafer was very thin, about 160 μm thick.

Fabrication of pinhole/micro‐lens array for improving resolution of integral imaging 3‐D display

AbstractIn this paper, we present a new fabrication of micro‐lens array (MLA) with pinhole array—pinhole/micro‐lens array (P/MLA) for integral imaging 3‐D display (II), which combine lithography and ink‐jet printing. A black circular groove array (BCGA) is used as pinhole array, and laser 3‐D microscope and a homemade setup have been used for the characterization of P/MLA. The results show that high‐precision P/MLA can be obtained using BCGA as templates. By controlling the driving voltage at different steps, the distance between nozzle and substrate, as well as the number of liquid droplets, P/MLA with smooth morphology, different sizes, good repeatability of geometry parameters, great uniformity of focusing, and good converging performance can be achieved. For demonstration, P/MLA with curvature, focal length, numerical aperture, and F‐number of 815.8 μm, 1.60 mm, 0.1311, and 3.8 are applied for the reconstruction in II, exhibiting good reconstruction performance with high resolution, and BCGA reduces the influence of stray light on II and improves the quality of the reconstructed image.