Full-color Holograms Research Articles

Multiplane full-color phase-only holograms generation based on iterative algorithm with a modified amplitude constraint

Depth-division multiplexing (DDM) is distinguished for generating full-color holograms. However, this method is confronted with such issues as poor quality and severe crosstalk when reconstructing multiplane color images. In this paper, we propose an improved iterative method based on Gerchberg-Saxton (GS) algorithm to generate multiplane full-color phase-only holograms (POHs), and the information of red (R), green (G), and blue (B) channels of images is recorded in three POHs, respectively. In the iterative process, we introduce an improved hologram plane constraint that consists of modified amplitudes given a static weighting factor, and display the reconstruction results for three-layer images, which ultimately improve the quality of the reconstructed images and effectively avoid the crosstalk among different color channels. And compared with GS algorithm, our method can effectively balance the reconstruction quality of three-color channels of each image. The feasibility of the method is verified by numerical simulations and optical experiments.

Time-scheduled exposure method for full-color high diffraction efficiency and uniformity of a photopolymer

A full-color reflection holographic optical element (HOE) with uniform diffraction efficiency, achieved via time-scheduled exposure, is proposed in which 660, 532, and 473 nm wavelengths are used for recording. To enhance the diffraction efficiency, the exposure energy of three wavelengths should be controlled to react uniformly in the photopolymer. The photopolymer’s optical characteristics present that the absorptance of the three wavelengths differs, moreover, the reactivity of the photopolymer to exposures is variant at the three wavelengths. The high-efficiency full-color HOE is implemented by determining the pre-exposure times of the three lasers for a set beam intensity. In order to verify the proposed method, we conduct repetitive optical experiments and analyze the results. Finally, a full-color reflection hologram is manufactured with an average efficiency of 61.6 % (standard deviation, 0.5) when the beam intensity of the three lasers is 0.4 mW/cm2. The reconstructed images of the full-color hologram show good uniformity and color reproduction.

See-through aerial display using a dihedral corner reflector array and hologram mirrors.

In this study, we reveal a novel aerial display system, to the best of our knowledge, that combines volume hologram mirrors with a dihedral corner reflector array (DCRA). The suggested aerial display has a see-through capability, which allows for a new design of aerial displays by combining with other types of displays. Furthermore, the virtual image, which frequently disrupts the observation of aerial images, can be suppressed by the Bragg condition of the volume hologram. The color dispersion in the holograms is efficiently compensated by employing features of the DCRA. The findings of preliminary experiments are demonstrated using a DCRA device and full-color hologram mirrors.

Review of Organic Photorefractive Materials and Their Use for Updateable 3D Display.

Photorefractive materials are capable of reversibly changing their index of refraction upon illumination. That property allows them to dynamically record holograms, which is a key function for developing an updateable holographic 3D display. The transition from inorganic photorefractive crystals to organic polymers meant that large display screens could be made. However, one essential figure of merit that needed to be worked out first was the sensitivity of the material that enables to record bright images in a short amount of time. In this review article, we describe how polymer engineering was able to overcome the problem of the material sensitivity. We highlight the importance of understanding the energy levels of the different species in order to optimize the efficiency and recording speed. We then discuss different photorefractive compounds and the reason for their particular figures of merit. Finally, we consider the technical choices taken to obtain an updateable 3D display using photorefractive polymer. By leveraging the unique properties of this holographic recording material, full color holograms were demonstrated, as well as refreshing rate of 100 hogels/second.

Max-depth-range technique for faster full-color hologram generation.

In this paper, a max-depth-range method is proposed to determine the optimum length of depth range for faster generation of full-color holograms. For each color channel, objects are divided by a fixed length to create a temporary depth range, and the wavefront recording plane (WRP) is placed in the middle of all layers within the temporary depth range. The proposed method is used to calculate full-color holograms significantly faster than a conventional multiple-WRP method but with almost the same reconstructed image quality. The feasibility of the proposed method was confirmed using numerical and optical experiments for various scenes containing multiple real objects.

New LEDs improve the quality of illumination of full-color holograms recorded with red 660 nm, green 532 nm and blue 440 nm lasers

This paper discusses the main requirements in today's full-color holograms illumination and evaluates the last generation of LEDs, the actual best light source to render properly the colors of the holograms and in particular those recorded with red 660 nm, green 532 nm and blue 440 nm lasers. This paper presents also the first prototype of lamp designed especially for this kind of holograms.

The Zerotrope, a Dynamic Holographic Display: Design and Implementation

This paper presents the Zerotrope, an improved version of the classic phenakistiscope and zoetrope devices. This device is used to create a new 360-degree dynamic 3D display by the inclusion of a single ultra-realistic full-color hologram. The Zerotrope is built with a zero-degree transplane hologram mounted on a disk rotating at a constant speed. When a stroboscopic lamp synchronized with the rotation illuminates this hologram, the recorded characters, arranged radially around the center of the disk, are animated as in a stop-motion movie. The operation of the Zerotrope is successful and shows the effect of a 3D display without the need for special viewing aids.

Fantatrope, a moving hologram display: design and implementation.

This paper presents the Fantatrope, an improvement of the pre-cinematographic devices such as the zoetrope, which creates a comfortable 3D display with the addition of ultra-realistic full-color holograms. The Fantatrope is built with a set of holograms of 3D-printed figurines mounted on a cylinder rotating at constant speed. A stroboscopic RGB LED lamp synchronized with the rotation successively illuminates the different frames and the recorded character is animated like in a stop-motion movie. All principal functions of this new device can be adjusted and this paper evaluates its performances. The operation of the Fantatrope is successfully demonstrated and shows the effect of a true 3D display without the need for special glasses or other viewing aids.

Optical Reconstruction of Full-color Optical Scanning Holography Images using an Iterative Direct Binary Search Algorithm

We introduce a process for optically reconstructing full-color holographic images recorded by optical scanning holography. A complex RGB-color hologram was recorded and converted into a binary hologram using a direct binary search (DBS) algorithm. The generated binary hologram was then optically reconstructed using a spatial light modulator. The discrepancies between the reconstructed object sizes and colors due to chromatic aberration were corrected by adjusting the reconstruction parameters in the DBS algorithm. To the best of our knowledge, this represents the first optical reconstruction of a full-color hologram recorded by optical scanning holography.

Highly efficient holograms based on c-Si metasurfaces in the visible range.

This paper reports on the first hologram in transmission mode based on a c-Si metasurface in the visible range. The hologram shows high fidelity and high efficiency, with measured transmission and diffraction efficiencies of ~65% and ~40%, respectively. Although originally designed to achieve full phase control in the range [0-2π] at 532 nm, these holograms have also performed well at 444.9 nm and 635 nm. The high tolerance to both fabrication and wavelength variations demonstrate that holograms based on c-Si metasurfaces are quite attractive for diffractive optics applications, and particularly for full-color holograms.

Single-Layer Metasurface with Controllable Multiwavelength Functions.

In this paper, we report dispersion-engineered metasurfaces with distinct functionalities controlled by wavelength. Unlike previous approaches based on spatial multiplexing or vertical stacking of metasurfaces, we utilize a single phase profile with wavelength dependence encoded in the phase shifters' dispersion. We designed and fabricated a multiwavelength achromatic metalens (MAM) with achromatic focusing for blue (B), green (G), yellow (Y), and red (R) light and two wavelength-controlled beam generators (WCBG): one focuses light with orbital angular momentum (OAM) states ( l = 0,1,2) corresponding to three primary colors; the other produces ordinary focal spots ( l = 0) for red and green light, while generating a vortex beam ( l = 1) in the blue. A full color (RGB) hologram is also demonstrated in simulation. Our approach opens a path to applications ranging from near-eye displays and holography to compact multiwavelength beam generation.

Slim coherent backlight unit for holographic display using full color holographic optical elements.

The coherent backlight unit (BLU) using a holographic optical element (HOE) for full-color flat-panel holographic display is proposed. The HOE BLU consists of two reflection type HOEs that change the optical beam path and shape by diffraction. The illumination area of backlight is 150 mm x 90 mm and the thickness is 10 mm, which is slim compared to other conventional coherent backlight units for holographic display systems. This backlight unit exhibits a total efficiency of 8.0% at red (660 nm), 7.7% at green (532 nm), and 3.2% at blue (460 nm) using optimized recording conditions for each wavelength. As a result, we could get a bright full color hologram image.

Holographic near-eye displays for virtual and augmented reality

We present novel designs for virtual and augmented reality near-eye displays based on phase-only holographic projection. Our approach is built on the principles of Fresnel holography and double phase amplitude encoding with additional hardware, phase correction factors, and spatial light modulator encodings to achieve full color, high contrast and low noise holograms with high resolution and true per-pixel focal control. We provide a GPU-accelerated implementation of all holographic computation that integrates with the standard graphics pipeline and enables real-time (≥90 Hz) calculation directly or through eye tracked approximations. A unified focus, aberration correction, and vision correction model, along with a user calibration process, accounts for any optical defects between the light source and retina. We use this optical correction ability not only to fix minor aberrations but to enable truly compact, eyeglasses-like displays with wide fields of view (80°) that would be inaccessible through conventional means. All functionality is evaluated across a series of hardware prototypes; we discuss remaining challenges to incorporate all features into a single device.

Molecular design of azo-carbazole monolithic dyes for updatable full-color holograms

For the use of updatable full-color holography in practical applications, azo-carbazole (ACzE) monolithic dyes dispersed in poly(methyl methacrylate) (PMMA) films with high diffraction efficiency, write/delete capabilities and transparency are of extreme importance. We synthesized seven types of novel ACzE monolithic dyes and examined the influence of substituents introduced in the para position of the phenyl ring and the distinct strength of the substituents (NO2: NACzE, CN: CACzE, CH3CO: AACzE, F: FACzE, CH3O: MACzE, OH: HACzE and NH2: AmACzE) on the phenyl ring of the ACzEs. Spectroscopic and holographic optical properties, including absorption coefficient α, wavelength dependence of diffraction efficiency η, response time τ, the thickness of the recorded gratings d, the Bragg angle Δϑ and the Q-factor, required for the optimization of updatable full-color holograms were investigated. On the basis of the spectroscopic and holographic optical results, the appropriate writing/reading beams were determined as follows: 561/>600 nm for NACzE, 532/>560 nm for CACzE (and AACzE), 491/>530 nm for MACzE (and HACzE). According to the spectroscopic and holographic optical results, we propose here an insight designed to allow updatable full-color holography using ACzEs, and demonstrate a multi-color hologram in MACzE/PMMA. A holographic system that uses low-cost materials to capture and rewrite full-colour 3D images has been developed by Japanese researchers. Naoto Tsutsumi from the Kyoto Institute of Technology and co-workers recently developed a red-coloured hologram based on an azo-carbazole dye that can be aligned, similar to a liquid crystal, using polarized light. Now, to tune the colour output, the team has modified phenyl rings on the original dye with a series of small, electron-donating and -withdrawing chemical groups. Spectroscopic and optical trials revealed that the colours from the functionalized dyes depend on the laser wavelengths used to write and read the hologram. The researchers exploited this behaviour with a system that combines red, green and blue holograms together in an updateable, full-colour video display. We report novel azo-carbazoles that can be applied to updatable full-color holography. We introduced a molecular design concept of updatable full-color holographic systems based on spectroscopic and optical characteristics. Taking advantage of characteristics of the designed azo-carbazoles, updatable red, yellow and green holograms can be recorded, which enabled us monochromic hologram as well as full-color hologram in the future.

Full-color hologram using spatial multiplexing of dielectric metasurface.

In this Letter, we demonstrate theoretically a full-color hologram using spatial multiplexing of dielectric metasurface for three primary colors, capable of reconstructing arbitrary RGB images. The discrete phase maps for the red, green, and blue components of the target image are extracted through a classical Gerchberg-Saxton algorithm and reside in the corresponding subcells of each pixel. Silicon nanobars supporting narrow spectral response at the wavelengths of the three primary colors are employed as the basic meta-atoms to imprint the Pancharatnam-Berry phase while maintaining minimum crosstalk between different colors. The reconstructed holographic images agree well with the target images making it promising for colorful display.

Ultrarealistic imaging: the future of display holography

Ultrarealistic imaging is the science of producing images that faithfully recreate the light field surrounding an object, such that the unaided eye of a human observer cannot distinguish the difference between the original and the image. Recent technology improvements are now set to transform the fields of both analog and digital display holography, permitting both techniques to operate in the ultrarealistic regime. In particular, ultrarealistic analog holograms have now heralded the serious use of holography in such areas as museum display and cultural heritage protection. These full-color holograms are characterized by a substantially lower noise and a greater spectral fidelity. New recording systems, based on recent diode-pumped solid-state and semiconductor lasers combined with recording materials and processing, have been behind these improvements. Progress in illumination technology, however, has also led to a major reduction in display noise and to a significant increase in the clear image depth and brightness of holograms. Recent progress in one-step direct-write digital holography (DWDH) is now also opening the way to the creation of a new type of ultrarealistic display: the high virtual volume display. This is a large format full-parallax DWDH reflection hologram having a fundamentally larger clear image depth.

Full Color Holographic Endoscopy

The ability to produce color holograms from the human tissue represents a major medical advance, specifically in the areas of diagnosis and teaching. This has been achieved at Glyndwr University. In corporation with partners at Gooch & Housego, Moor Instruments, Vivid Components and peninsula medical school, Exeter, UK, for the first time, we have produced full color holograms of human cell samples in which the cell boundary and the nuclei inside the cells could be clearly focused at different depths – something impossible with a two-dimensional photographic image. This was the main objective set by the peninsula medical school at Exeter, UK. Achieving this objective means that clinically useful images essentially indistinguishable from the object human cells could be routinely recorded. This could potentially be done at the tip of a holo-endoscopic probe inside the body. Optimised recording exposure and development processes for the holograms were defined for bulk exposures. This included the optimisation of in-house recording emulsions for coating evaluation onto polymer substrates (rather than glass plates), a key step for large volume commercial exploitation. At Glyndwr University, we also developed a new version of our in-house holographic (world-leading resolution) emulsion.

Special Section Guest Editorial: Digital Holography and Holographic Displays

Holography is now more than 60 years old since the time when Dennis Gabor published his first paper in 1948, introducing the new coherent imaging technique. Holography had to wait for its first practical applications a little over 10 years until the laser was invented and laser light could be used for recording the first holograms. This happened almost simultaneously in both the USA and the former USSR. Since that time, interest in holography has been growing at an increasing rate, and has given rise to numerous applications of holographic techniques in science, industry, medicine, and art. During the first decades mainly analog holograms were recorded, using continuous-wave and pulsed lasers. Display holograms, holographic optical elements (HOE), holographic interferometry, and holographic nondestructive testing (HNDT) used different types of recording materials, often combined with wet processing. It was a time-consuming technique and required special expertise to obtain good results. When computers became faster and their storage capacity increased, very simple computer-generated holograms (CGHs) were introduced to show the potential of generating holograms and HOEs without having to record a real object. Very soon computer programs were introduced for the evaluation of interference fringes in metrology and nondestructive testing. Since the beginning of the new millennium, there has been a dramatic progress in computer technology in regard to computer storage capacity, speed, and graphics, which has made applications of CGHs and holographic displays an important part of current holographic techniques. HNDT and speckle interferometry are now only performed with the help of computers, images being recorded on electronic holographic recording devices. For example, CGHs are also used as null elements in optical testing. Today computers and electronic recording devices are used in different ways. The possibility of recording very high spatial frequencies employing electronic devices is, however, still very limited. For ultrahigh-resolution holographic recording, spatial frequencies of up to 10,000 line pairs/mm are required. For example, to display a full-parallax hologram in real time on a 40-in. wide screen with 30-deg horizontal and 10deg vertical field of view, a bandwidth of 10 Hz is required. The ultimate goal is to computer generate the interference fringe structure which is necessary to generate a holographic image for display. For a large-sized hologram, it is not possible to generate the necessary data fast enough to get a refreshment rate of 25 to 30 Hz. If we compare this with the analogue technique, using a pulsed laser to record a very large hologram of a complex object or scene, the interference structure is created and stored in a high-resolution photographic material in 20 ns, provided the pulsed laser energy is sufficient. Holographic microscopy using in-line holography combined with charge-coupled device (CCD) recording devices is common practice today. Some CCDs have sufficient spatial frequency resolution and can be used for holographic particle studies in real time, using fast Fourier transform programs. CGHs are also used to generate small holographic optical variable devices for important security document applications. Today a combination of techniques is used in order to obtain various types of CGHs. A technique for recording large, color display holograms is to print them pixel by pixel on a high-resolution holographic material. Each pixel hologram, or hogel, is computer generated and displayed on a LCD or liquid crystal on silicon (LCOS) device. Using this technique to obtain a large one-square-meter color display hologram, the printing of the entire hologram can take many hours, depending of the RGB laser’s pulse repetition rate. Using this type of hologram printing technique, both completely computer-generated three-dimensional (3D) images can be recorded, as well as a combination of a large number of photographically recorded images, such as those captured by a digital camera moving along a horizontal rail, for example. The two-dimensional images are then transformed and used to create the printed hologram. For holographic displays, HOEs can be used as a stereo projection screen generating autostereoscopic moving 3D images. There is a lot of research going on in these fields, aiming to create 3D images in real time for potential 3D television techniques. In the future, when computers become even faster, and when nanotechnology-based electronic display devices have sufficient spatial frequency resolution to display full parallax real-time color holograms directly, electronic ultrarealistic 3D images could become a reality. This special section contains state-of-the-art papers on the applications mentioned above. The papers are ordered according to the date of acceptance. I would like to thank all the authors for providing the papers for this issue, as well as the peer reviewers for their comments and meticulous work, the SPIE staff, and finally Ron Diggers, the Optical Engineering editor, for giving me the opportunity to edit this special section.

Method for removing longitudinal chromatism in full color holographic projection system

Hologram, as a type of diffractive optical element, is sensitive to wavelength in the process of optoelectronic reconstruction. Due to the different wavelengths of three prime colors used in full color holographic projections, there are chromatisms which badly spoil the reconstructed image. The chromatisms are composed of transverse and longitudinal chromatisms. For a computer generated hologram, transverse chromatism can be compensated by resampling the object information. However, it becomes more complex for longitudinal chromatism. This paper analyzes how the image is reconstructed from the phase-type hologram in a holographic projection system and the causations of longitudinal chromatism. To remove it, this paper proposes loading a specially designed phase distribution on a phase-type hologram. The advantage of this method is that it can be achieved by computer calculation and without adding any hardware such as achromatic optical element. A time-sharing system for a full-color hologram projection is developed in this paper. Comparisons have been made between the reconstructed images with and without chromatism. The experimental result shows that the method is effective in removing longitudinal chromatism and the quality of the reconstructed image is improved.

Real-time image plane full-color and full-parallax holographic video display system

For practical holographic video display, it is important to generate holographic fringes as fast as possible. It is also important to display the full-color image with a simple display system. In our previous study, a full-color hologram was realized as a rainbow hologram, which discards the vertical parallax to reduce computation complexity. Its color reproduction, however, is not enough. Since this hologram uses the horizontal slit, proper color reproduction can be obtained only from a narrow viewing area in the vertical direction. In contrast, the image hologram provided a good contrast image, although it had only a single color. Also, the image hologram has full-parallax information. In this work, we investigate the image hologram for fast calculation and the display optics for full-color holograms with full parallax. Since image points are located close to the hologram, the required computation area of the image hologram becomes much smaller than that of the entire hologram. Therefore, vastly improved computational speed is obtained. The full-color hologram is displayed on a holographic video display system, which uses part of the original optics and liquid crystal on silicon (LCoS) panels of the conventional video projector to separate and combine color components. From the experimental results, we obtain a full-parallax reconstructed image and better color reproduction compared with the rainbow hologram.