Rendering of 3D scenes in analytical polygon-based computer holography with texture mapping.

A computer-generated hologram (CGH) is a technique that generates an object light field by superimposing elementary holograms. Unlike traditional holography, this technique does not require the generation of an additional reference light to interfere with the calculated object light field. Texture mapping is a method that enhances the realism of 3D scenes. A fast method is presented that allows users to render holograms of 3D scenes consisting of triangular meshes with texture mapping. All calculations are performed with analytical expressions to ensure that the holograms generated by this method are fast and can reconstruct three-dimensional scenes with high quality. Using this method, a hologram of a three-dimensional scene consisting of thousands of triangles is generated. Our algorithm generates the same reconstruction results as those of Kim et al. [Appl. Opt.47, D117 (2008)APOPAI0003-693510.1364/AO.47.00D117], but significantly reduces the computation time (the computation time of our algorithm is only one-third of that of Kim et al.'s algorithm). The results show that the proposed method is computationally efficient as compared to a previous work. The proposed method is verified by simulations and optical experiments.

Adaptive layer-based computer-generated holograms.

We propose a novel, to the best of our knowledge, and fast adaptive layer-based (ALB) method for generating a computer-generated hologram (CGH) with accurate depth information. A complex three-dimensional (3D) object is adaptively divided into layers along the depth direction according to its own non-uniformly distributed depth coordinates, which reduces the depth error caused by the conventional layer-based method. Each adaptive layer generates a single-layer hologram using the angular spectrum method for diffraction, and the final hologram of a complex three-dimensional object is obtained by superimposing all the adaptive layer holograms. A hologram derived with the proposed method is referred to as an adaptive layer-based hologram (ALBH). Our demonstration shows that the desired reconstruction can be achieved with 52 adaptive layers in 8.7 s, whereas the conventional method requires 397 layers in 74.9 s.

Comparison of adaptive optical scanning holography based on new evaluation methods.

Adaptive Optical Scanning Holography (AOSH) represents a powerful technique that employs an adaptive approach to selectively omit certain lines within holograms, guided by the utilization of Normalized-Mean-Error (NME) as a predictive measure. This approach effectively diminishes scanning time and conserves the storage space required for data preservation. However, there exists alternative methods superior to NME in terms of evaluating the model's efficacy. This paper introduces two novel methods, namely Normalized-Root-Mean-Square-Error (NRMSE) and Normalized-Mean-Square-Error (NMSE), into the AOSH system, leading to the development of NRMSE-AOSH and NMSE-AOSH. These new systems aim to further minimize duration of holographic recording. Through a comparative analysis of hologram lines between the two newly proposed AOSH systems and the original AOSH, we demonstrate that both NRMSE-AOSH and NMSE-AOSH effectively reduce the number of hologram lines while maintaining the hologram's informational content. Among the three methods, our two new methods exhibit better performance compared with the original method.

Efficient rendering by parallelogram-approximation for full analytical polygon-based computer-generated holography using planar texture mapping.

We have developed a full analytical method with texture mapping for polygon-based computer-generated holography. A parallel planar projection mapping for holographic rendering along with affine transformation and self-similar segmentation is derived. Based on this method, we further propose a parallelogram-approximation to reduce the number of polygons used in the polygon-based technique. We demonstrate that the overall method can reduce the computational effort by 50% as compared to an existing method without sacrificing the reconstruction quality based on high precision rendering of complex textures. Numerical and optical reconstructions have shown the effectiveness of the overall scheme.

Automatic elimination of phase aberrations in digital holography based on Gaussian 1σ- criterion and histogram segmentation.

We propose a numerical and automatic quadratic phase aberration elimination method in digital holography for phase-contrast imaging. A histogram segmentation method based on Gaussian 1σ-criterion is used to obtain the accurate coefficients of quadratic aberrations using the weighted least-squares algorithm. This method needs no manual intervention for specimen-free zone or prior parameters of optical components. We also propose a maximum-minimum-average-standard deviation (MMASD) metric to quantitatively evaluate the effectiveness of quadratic aberration elimination. Simulation and experimental results are demonstrated to verify the efficacy of our proposed method over the traditional least-squares algorithm.

Asymmetric cryptosystem based on optical scanning cryptography and elliptic curve algorithm.

We propose an asymmetric cryptosystem based on optical scanning cryptography (OSC) and elliptic curve cryptography (ECC) algorithm. In the encryption stage of OSC, an object is encrypted to cosine and sine holograms by two pupil functions calculated via ECC algorithm from sender's biometric image, which is sender's private key. With the ECC algorithm, these holograms are encrypted to ciphertext, which is sent to the receiver. In the stage of decryption, the encrypted holograms can be decrypted by receiver's biometric private key which is different from the sender's private key. The approach is an asymmetric cryptosystem which solves the problem of the management and dispatch of keys in OSC and has more security strength than the conventional OSC. The feasibility of the proposed method has been convincingly verified by numerical and experiment results.

Off-axis optical scanning holography [Invited].

Optical scanning holography (OSH) involves the principles of optical scanning and heterodyning. The use of heterodyning leads to phase-preserving, which is the basic idea of holography. While heterodyning has numerous advantages, it requires complicated and expensive electronic processing. We investigate an off-axis approach to OSH, thereby eliminating the use of heterodyning for phase retrieval. We develop optical scanning theory for holographic imaging and show that by properly designing the scanning beam, we can perform coherent and incoherent holographic recording. Simulation results are provided to verify the proposed idea.

Polygon-based computer-generated holography: a review of fundamentals and recent progress [Invited].

In this review paper, we first provide comprehensive tutorials on two classical methods of polygon-based computer-generated holography: the traditional method (also called the fast-Fourier-transform-based method) and the analytical method. Indeed, other modern polygon-based methods build on the idea of the two methods. We will then present some selective methods with recent developments and progress and compare their computational reconstructions in terms of calculation speed and image quality, among other things. Finally, we discuss and propose a fast analytical method called the fast 3D affine transformation method, and based on the method, we present a numerical reconstruction of a computer-generated hologram (CGH) of a 3D surface consisting of 49,272 processed polygons of the face of a real person without the use of graphic processing units; to the best of our knowledge, this represents a state-of-the-art numerical result in polygon-based computed-generated holography. Finally, we also show optical reconstructions of such a CGH and another CGH of the Stanford bunny of 59,996 polygons with 31,724 processed polygons after back-face culling. We hope that this paper will bring out some of the essence of polygon-based computer-generated holography and provide some insights for future research.

Near-Infrared Self-Powered Linearly Polarized Photodetection and Digital Incoherent Holography Using WSe2/ReSe2 van der Waals Heterostructure.

Polarization-sensitive photodetection has attracted considerable attention as an emerging technology for future optoelectronic applications such as three-dimensional (3D) imaging, quantum optics, and encryption. However, traditional photodetectors based on Si or III-V InGaAs semiconductors cannot directly detect polarized light without additional optical components. Herein, we demonstrate a self-powered linear-polarization-sensitive near-infrared (NIR) photodetector using a two-dimensional WSe2/ReSe2 van der Waals heterostructure. The WSe2/ReSe2 heterojunction photodiode with semivertical geometry exhibits excellent performance: an ideality factor of 1.67, a broad spectral photoresponse of 405-980 nm with a significant photovoltaic effect, outstanding linearity with a linear dynamic range wider than 100 dB, and rapid photoswitching behavior with a cutoff frequency up to 100 kHz. Strongly polarized excitonic transitions around the band edge in ReSe2 lead to significant 980 nm NIR linear-polarization-dependent photocurrent. This linear polarization sensitivity remains stable even after exposure to air for longer than five months. Furthermore, by leveraging the NIR (980 nm)-selective linear polarization detection of this photodiode under photovoltaic operation, we demonstrate digital incoherent holographic 3D imaging.

Optical scanning holography with a polarization directed flat lens.

Recently, an optical scanning holographic system with a polarization directed flat lens was proposed to realize coaxial scanning holography (CSH). The advantage of CSH is its small form factor and the stability. However, the diffraction efficiency of the polarization directed flat lens cannot be 100%, and thus there is always zeroth order light in the scanning beam. The imperfect diffraction property of the polarization directed flat lens results in an incomplete scanning Fresnel zone plate. Consequently, the reconstructed image is blurred and noisy. In this paper, we compared different methods, including the back propagation, the phase correlation, and inverse filtering, for the hologram reconstruction. It is demonstrated that inverse filtering is the only method that can retrieve the high-frequency component of the hologram. However, additional noise also arises with the use of inverse filtering. Therefore, the imaging performance of CSH by using a polarization directed flat lens is inherently worse than that of conventional OSH.

Optical scanning Fourier ptychographic microscopy.

We propose a lower-cost and practical active scanning optical scanning Fourier ptychographic microscopy (OSFPM). Featured is a simple setup of Galvo mirrors capable of scanning large-sized objects. The active scanning laser beam is projected onto the sample in a circular pattern to form multiple lower-resolution images. With multiple lower-resolution images, a higher-resolution image is subsequently reconstructed. The OSFPM is able to more precisely control the overlap of the incident light illumination as compared to that in conventional LED-based or other laser-based scanning FPM systems. The proposed microscope is also suitable for applications where a larger size of the object needs to be imaged with efficient illumination.

Recording of a curved digital hologram for orthoscopic real image reconstruction.

Based on the concept of optical scanning holography, a holographic system for recording a curved digital hologram was proposed and demonstrated. In the system, an interference beam without the object information is first generated and then used to two-dimensionally scan a three-dimensional object along a cylindrical path. As a result, a complex curved hologram of a real object is digitally holographically recorded for the first time, to the best of our knowledge. The method of digital reconstruction and the properties of the curved digital hologram are then discussed.

Accurate phase-shift estimation for fringe-pattern profilometry.

Fringe-pattern profilometry (FPP) has been widely used for phase reconstruction. It involves the use of phase shifting for phase retrieval. Phase-shift errors can affect the accuracy of phase reconstruction, and limited studies have been dedicated to studying phase-shift errors due to experimental, human, or environmental factors. We propose a simple and yet accurate phase-shift estimation method. Our study shows that the method is able to accurately estimate the actual phase shifts used in the FPP technique. The proposed method can find its applications in FPP and other phase shifting-based three-dimensional imaging techniques.

Digital holography and 3D imaging: introduction to the Applied Optics and Journal of the Optical Society of America A joint feature issue.

The OSA Topical Meeting on Digital Holography and 3D Imaging (DH) was held June 25-June 28, 2018, in Orlando, Florida, USA. Feature issues based on the DH meeting series have been released by Applied Optics (AO) since 2007. This year, AO and the Journal of the Optical Society of America A (JOSA A) jointly decided to have one such feature issue in each journal. This feature issue includes thirty-eight papers in AO and nine in JOSA A, and covers a large range of topics, reflecting the rapidly expanding techniques and applications of digital holography and 3D imaging. The upcoming DH Conference (DH 2019) will be held May 19-May 23 in Bordeaux, France.

Digital holography and 3D imaging: introduction to the Applied Optics and Journal of the Optical Society of America A joint feature issue.

The OSA Topical Meeting on Digital Holography and 3D Imaging (DH) was held June 25-28, 2018, in Orlando, Florida, USA. Feature issues based on the DH meeting series have been released by Applied Optics (AO) since 2007. This year, AO and the Journal of the Optical Society of America A (JOSA A) jointly decided to have one such feature issue in each journal. This feature issue includes thirty-eight papers in AO and nine in JOSA A, and covers a large range of topics, reflecting the rapidly expanding techniques and applications of digital holography and 3D imaging. The upcoming DH Conference (DH 2019) will be held May 19-23 in Bordeaux, France.

Fast generation of full analytical polygon-based computer-generated holograms.

A fast calculation method to obtain the full-analytical frequency spectrum of a spatial triangle based on the three-dimensional (3D) affine transformation is presented. Computer-generated holograms (CGHs) of an object can then be generated rapidly using the angular spectrum for propagation. The derivation process in the theory, which has more preciseness, indicates a difference from previous methods based on affine transformations ([Appl. Opt.47, 1567 (2008)Appl. Opt.52, A290 (2013)]). The proposed method to achieve 3D transformation from an arbitrary triangle to a primitive triangle includes two steps: 3D rotation and 2D affine transformation. The overall transform matrix is given by the product of a rotation matrix and a 2D affine matrix. A modified back-face culling is also introduced based on exterior normal for correct occlusion relation. Several complex 3D objects are implemented successfully using the proposed method in numerical simulations and optical experiments. The resulting computation time demonstrates that the efficiency of the proposed method is enhanced as compared to that of previous works.

How do plants see the world? - UV imaging with a TiO2 nanowire array by artificial photosynthesis.

The concept of plant vision refers to the fact that plants are receptive to their visual environment, although the mechanism involved is quite distinct from the human visual system. The mechanism in plants is not well understood and has yet to be fully investigated. In this work, we have exploited the properties of TiO2 nanowires as a UV sensor to simulate the phenomenon of photosynthesis in order to come one step closer to understanding how plants see the world. To the best of our knowledge, this study is the first approach to emulate and depict plant vision. We have emulated the visual map perceived by plants with a single-pixel imaging system combined with a mechanical scanner. The image acquisition has been demonstrated for several electrolyte environments, in both transmissive and reflective configurations, in order to explore the different conditions in which plants perceive light.

Evaluation of finite difference and FFT-based solutions of the transport of intensity equation.

A finite difference method is proposed for solving the transport of intensity equation. Simulation results show that although slower than fast Fourier transform (FFT)-based methods, finite difference methods are able to reconstruct the phase with better accuracy due to relaxed assumptions for solving the transport of intensity equation relative to FFT methods. Finite difference methods are also more flexible than FFT methods in dealing with different boundary conditions.

Phase retrieval based on transport of intensity and digital holography.

We propose a technique in which intensity images are reconstructed from a digital hologram to provide inputs for the transport-of-intensity equation for unwrapped phase recovery. By doing this, we avoid shifting of the sample or the camera in the experiment, a method commonly employed while using the method of transport-of-intensity equation for phase retrieval. Computer simulations as well as experimental results have been demonstrated to verify the effectiveness of the proposed idea. The underlying numerical technique can also be viewed as an alternative to existing phase-unwrapping algorithms.

Optical cryptography with biometrics for multi-depth objects.

We propose an optical cryptosystem for encrypting images of multi-depth objects based on the combination of optical heterodyne technique and fingerprint keys. Optical heterodyning requires two optical beams to be mixed. For encryption, each optical beam is modulated by an optical mask containing either the fingerprint of the person who is sending, or receiving the image. The pair of optical masks are taken as the encryption keys. Subsequently, the two beams are used to scan over a multi-depth 3-D object to obtain an encrypted hologram. During the decryption process, each sectional image of the 3-D object is recovered by convolving its encrypted hologram (through numerical computation) with the encrypted hologram of a pinhole image that is positioned at the same depth as the sectional image. Our proposed method has three major advantages. First, the lost-key situation can be avoided with the use of fingerprints as the encryption keys. Second, the method can be applied to encrypt 3-D images for subsequent decrypted sectional images. Third, since optical heterodyning scanning is employed to encrypt a 3-D object, the optical system is incoherent, resulting in negligible amount of speckle noise upon decryption. To the best of our knowledge, this is the first time optical cryptography of 3-D object images has been demonstrated in an incoherent optical system with biometric keys.