Simple holographic projection in color

Automotive Augmented Reality Head-Up Displays

As the next generation of in-vehicle intelligent platforms, the augmented reality heads-up display (AR-HUD) has a huge information interaction capacity, can provide drivers with auxiliary driving information, avoid the distractions caused by the lower head during the driving process, and greatly improve driving safety. However, AR-HUD systems still face great challenges in the realization of multi-plane full-color display, and they cannot truly achieve the integration of virtual information and real road conditions. To overcome these problems, many new devices and materials have been applied to AR-HUDs, and many novel systems have been developed. This study first reviews some key metrics of HUDs, investigates the structures of various picture generation units (PGUs), and finally focuses on the development status of AR-HUDs, analyzes the advantages and disadvantages of existing technologies, and points out the future research directions for AR-HUDs.

Comparison of double-phase hologram and binary amplitude encoding: holographic projection and vortex beam generation

Utilizing computer-generated holograms is a promising technique because these holograms can theoretically generate arbitrary waves with high light efficiency. In phase-only spatial light modulators, encoding complex amplitudes into phase-only holograms is a significant issue, and double-phase holograms have been a popular encoding technique. However, they reduce the light efficiency. In this study, our complex amplitude encoding, called binary amplitude encoding (BAE), and conventional methods including double-phase hologram, iterative algorithm, and error diffusion methods were compared in terms of the fidelity of reproduced light waves and light efficiency, considering the applications of lensless zoomable holographic projection and vortex beam generation. This study also proposes a noise reduction method for BAE holograms that is effective when the holograms have different aspect ratios. BAE is a non-iterative method, which allows holograms to be obtained more than 2 orders of magnitude faster than iterative holograms; BAE has about 3 times higher light efficiency with comparable image quality compared to double-phase holograms.

Pixel-level phase filters for off-axis shifting of sinc envelope in holographic projection

Off-axis projection is a common practice for reconstructions of Fourier holograms displayed on liquid crystal on silicon (LCoS) spatial light modulators (SLMs), as it spatially separates the image from the undiffracted light. Binary gratings encoded within the holograms enable maximum angular separation. However, as a result, two mirror images of equal intensities are present in the reconstruction. To introduce asymmetry to the intensity distribution and suppress one of those images, we propose a phase mask with a subpixel pattern. Presented results prove the potential of in-built SLM phase-mask layers for optimizing efficiency of the off-axis holographic projection.

High-brightness holographic projection

We propose a holographic projection system that achieves high image quality, brightness, and light efficiency. Using a novel, to the best of our knowledge, light-efficiency loss function, we are able to concentrate more light on the projection region and improve display brightness compared with conventional projectors. Leveraging emerging artificial intelligence-driven computer-generated holography and camera-in-the-loop calibration techniques, we learn a holographic wave propagation model using experimentally captured holographic images and demonstrate state-of-the-art light reallocation performance with high image quality.

Compact reconstruction of a Fourier hologram for a 3D object by scaling compensation

The Fourier holographic projection method is compact and computationally fast. However, since the magnification of the displayed image increases with the diffraction distance, this method cannot be used directly to display multi-plane three-dimensional (3D) scenes. We propose a holographic 3D projection method of Fourier holograms by scaling compensation to offset the magnification during optical reconstruction. To achieve a compact system, the proposed method is also used to reconstruct 3D virtual images with Fourier holograms. Different from traditional Fourier holographic displays, images are reconstructed behind a spatial light modulator (SLM) so that the observation position can be placed close to the SLM. The effectiveness of the method and the flexibility of combining it with other methods are confirmed by simulations and experiments. Therefore, our method could have potential applications in the augmented reality (AR) and virtual reality (VR) fields.

Practical method for dynamic color holographic display

A practical method for dynamic color holographic display by using a computer-generated hologram (CGH) with a high space-bandwidth product is proposed, and a dynamic color holographic display system is designed by a space-division method. First, three primary color CGHs of different frames from a color movie are fabricated on holographic recording material by a self-made CGH microfilming system. Secondly, the CGH is fixed on an X-Y moving stage, which is controlled by the system in order to bring the CGH to the appointed position. Thirdly, three primary color lasers are used to reconstruct the CGH. The switch of the lasers is controlled by the system synchronous with the X-Y moving stage. The color video with high quality can be obtained after filtering the three primary color reconstructed wavefronts. The experimental results demonstrate that the proposed dynamic color holographic display method is effective. It has practical application value in high-quality CGH display.

Aberration analysis of a projection-type CGH display with an expanded FOV based on the HOE screen

This paper proposed a holographic optical element as a see-through screen for the computer-generated hologram projection system with 3D images. The proposed holographic screen consisted of a linear grating and a lens phase. The linear grating is used to redirect the information light and guide information into the observer's eye and achieve the see-through function. The lens phase is used to magnify the field of view of the holographic projection system. The aberration caused by the screen was analyzed in this paper and the aberration can be pre-corrected in the hologram calculation algorithm. Finally, the proposed system achieved 20.3 by 14.3 degrees field of view at 532 nm laser based on the spatial light modulator with 6.4 µm pixels.

Review of computer-generated hologram algorithms for color dynamic holographic three-dimensional display

Holographic three-dimensional display is an important display technique because it can provide all depth information of a real or virtual scene without any special eyewear. In recent years, with the development of computer and optoelectronic technology, computer-generated holograms have attracted extensive attention and developed as the most promising method to realize holographic display. However, some bottlenecks still restrict the development of computer-generated holograms, such as heavy computation burden, low image quality, and the complicated system of color holographic display. To overcome these problems, numerous algorithms have been investigated with the aim of color dynamic holographic three-dimensional display. In this review, we will explain the essence of various computer-generated hologram algorithms and provide some insights for future research.

Method of color holographic display with speckle noise suppression

In this paper, a method of color holographic display with speckle noise suppression is proposed. Firstly, the intensity information of the object is extracted according to the red, green and blue (RGB) channels. The band-limited phase is calculated and used as the initial phase for each color channel. Secondly, the double-step Fresnel diffraction algorithm is used to calculate the computer-generated holograms (CGHs), and a filter plane that dynamically adjusts the position of the filter in the optical path is designed. Then, a divergent spherical phase factor is added to the CGHs. Finally, the time average method is used to further reduce the speckle noise. When the CGHs of the RGB channels are loaded on the digital micromirror device and illuminated by the RGB lights emitting in a temporal sequence, the color reconstructed images with speckle noise suppression can be displayed. The validity of the proposed method is verified.

Optimized Fresnel phase hologram for ringing artifacts removal in lensless holographic projection

Ringing artifacts are the main noise sources in holographic projection when the quadratic phase is introduced to suppress speckle noise. In this study, the mechanisms of ringing artifacts on reconstructed images are theoretically analyzed, which illustrates the ringing artifacts are related to the bandwidth properties of the reconstructed wave field. Based on the frequency analysis, a band-limited iterative algorithm is proposed to optimize the phase hologram in the Fresnel domain. The proposed method can effectively suppress the ringing artifacts as well as the speckle noise of the Fresnel hologram by optimizing the phase distribution with bandwidth constraint. Numerical simulations and optical experiments have been performed to validate the proposed method for providing quality reconstructions in lensless holographic projection.

Suppression of spurious image duplicates in Fourier holograms by pixel apodization of a spatial light modulator

Holographic projectors and near-eye displays are a promising technology with truly three-dimensional, natural viewing and excellent energetic efficiency. Spatial light modulators with periodic pixel matrices cause image duplicates, which distract the viewer and waste energy of the playback beam. We present the engineering of the far field intensity envelope, which suppresses higher-order image duplicates in the simplest possible optical setup by physically changing the shape of modulator pixels with attached apodizing masks. Numerical and experimental results show the limited number of perceived duplicates and better uniformity in off-axis projections for the price of compromised energetic efficiency due to amplitude masks.

Single-shot digital multiplexed holography for the measurement of deep shapes

This work develops a single-shot holographic profilometer that enables shape characterization of discontinuous deep surfaces. This is achieved by combining hologram frequency multiplexing and an illumination technique of complex amplitude in multi-incidence angle profilometer. Object illumination is carried out from seven directions simultaneously, where the radial angular coordinates of illumination plane waves obey the geometric series. It is shown that: (i) the illumination pattern provides the required frequency separation of all object wavefronts in transverse frequency space, which is necessary for hologram demultiplexing, and (ii) numerical generation of longitudinal scanning function (LSF) is possible, which has large measurement range, high axial resolution, and small side lobes. Low side lobes of LSF and the developed multiplexed field dependent aberration compensation method are essential to minimize the negative influence of speckle noise of single-shot capture on the measurement result. The utility of the proposed method is demonstrated with experimental measurement of heights of two step-like objects.

Adaptive weighted Gerchberg-Saxton algorithm for generation of phase-only hologram with artifacts suppression

In the conventional weighted Gerchberg-Saxton (GS) algorithm, the feedback is used to accelerate the convergence. However, it will lead to the iteration divergence. To solve this issue, an adaptive weighted GS algorithm is proposed in this paper. By replacing the conventional feedback with our designed feedback, the convergence can be ensured in the proposed method. Compared with the traditional GS iteration method, the proposed method improves the peak signal-noise ratio of the reconstructed image with 4.8 dB on average. Moreover, an approximate quadratic phase is proposed to suppress the artifacts in optical reconstruction. Therefore, a high-quality image can be reconstructed without the artifacts in our designed Argument Reality device. Both numerical simulations and optical experiments have validated the effectiveness of the proposed method.

Toward the next-generation VR/AR optics: a review of holographic near-eye displays from a human-centric perspective

Wearable near-eye displays for virtual and augmented reality (VR/AR) have seen enormous growth in recent years. While researchers are exploiting a plethora of techniques to create life-like three-dimensional (3D) objects, there is a lack of awareness of the role of human perception in guiding the hardware development. An ultimate VR/AR headset must integrate the display, sensors, and processors in a compact enclosure that people can comfortably wear for a long time while allowing a superior immersion experience and user-friendly human-computer interaction. Compared with other 3D displays, the holographic display has unique advantages in providing natural depth cues and correcting eye aberrations. Therefore, it holds great promise to be the enabling technology for next-generation VR/AR devices. In this review, we survey the recent progress in holographic near-eye displays from the human-centric perspective.

Three-dimensional visualization of brain tumor progression based accurate segmentation via comparative holographic projection

We propose a new optical method based on comparative holographic projection for visual comparison between two abnormal follow-up magnetic resonance (MR) exams of glioblastoma patients to effectively visualize and assess tumor progression. First, the brain tissue and tumor areas are segmented from the MR exams using the fast marching method (FMM). The FMM approach is implemented on a computed pixel weight matrix based on an automated selection of a set of initialized target points. Thereafter, the associated phase holograms are calculated for the segmented structures based on an adaptive iterative Fourier transform algorithm (AIFTA). Within this approach, a spatial multiplexing is applied to reduce the speckle noise. Furthermore, hologram modulation is performed to represent two different reconstruction schemes. In both schemes, all calculated holograms are superimposed into a single two-dimensional (2D) hologram which is then displayed on a reflective phase-only spatial light modulator (SLM) for optical reconstruction. The optical reconstruction of the first scheme displays a 3D map of the tumor allowing to visualize the volume of the tumor after treatment and at the progression. Whereas, the second scheme displays the follow-up exams in a side-by-side mode highlighting tumor areas, so the assessment of each case can be fast achieved. The proposed system can be used as a valuable tool for interpretation and assessment of the tumor progression with respect to the treatment method providing an improvement in diagnosis and treatment planning.

Error tracking-control-reduction algorithm for designing diffractive optical element with high image reconstruction quality

The use of the diffractive optical element (DOE) can often significantly reduce the size and enhance the performance of the optical system, but it is mostly prevented by low diffraction efficiency and serious speckle noise due to the quantization error. In this paper, an error tracking-control-reduction (ETCR) algorithm is proposed to suppress the quantization error, which adjusts the accumulative action, controls the current state and predicts the trend of the error. The simulation and experiment results verify that the ETCR algorithm has high diffraction efficiency which can be comparable with the Gerchberg-Saxton (GS) and Modified GS algorithms. Furthermore, the root-mean-square error (RMSE) of the proposed algorithm is significantly lower than that of the GS and MGS algorithms. Based on the proposed method, a 2-level DOE has been designed and fabricated to generate several grey images with only 0.05 RMSE.

Astigmatism correction and quality optimization of computer-generated holograms for holographic waveguide displays

In this manuscript, the astigmatism of the waveguide combiner with a pair of symmetry HOEs was analyzed. The light field can be predicted by the modified convolution formulation of Fresnel diffraction when the information of light passes through the astigmatism causing element. Then the astigmatism can be corrected. The theory was experimentally proved by the system with a phase-only SLM and a diffraction planar waveguide. Furthermore, the image quality of astigmatism corrected phase-type CGHs can be improved via the iteration process. Since the coherence of light source was employed, the temporal averaging method was utilized to avoid speckle noise.

Pre-compensation of an image blur in holographic projection display using light emitting diode light source

Holographic projection displays suffer from image blur when reconstructed from an incoherent light source like a light emitting diode. In this paper, we propose a method that enhances the reconstruction sharpness by pre-compensating the target image. The image blur caused by the incoherent nature of the light emitting diode is analyzed and the corresponding spatially varying point spread function is obtained. The pre-compensation is then performed using an iterative optimization algorithm. Finally, the hologram of the pre-compensated target image is loaded onto a spatial light modulator to obtain optically reconstructed image with reduced blur. The numerically simulated results and optically reconstructed results are in good agreement, showing feasibility of the proposed method.

Fast hologram generation using intermediate angular-spectrum method for high-quality compact on-axis holographic display

In the angular-spectrum method-based computer-generated hologram, the zero-padding method is used to convert circular convolution into linear convolution. However, it will increase the calculation time and memory usage significantly. Therefore, a fast and simple method is proposed to solve the issue of the numerical convolution in the process of hologram generation by using the intermediate angular-spectrum method in this paper. Through replacing numerical Fourier transform by optical Fourier transform in the hologram generation, the calculation speed is approximately 6 times faster than that of the zero-padding method. And due to the scaling factors introduced by the Fourier lens and without the cropping operation, the reconstruction quality of the proposed method is improved significantly compared with the zero-padding method. Moreover, the optical reconstruction system is more compact than the 4-f filter system in the on-axis holographic reconstruction. Both numerical simulations and optical experiments have validated the effectiveness of the proposed method.

Aberration-corrected holographic projection on a two-dimensionally highly tilted spatial light modulator

Image projection by holographic allows efficient and compact optical setups; nevertheless, the limited throw angle and 1:1 image aspect ratio are impractical. We present the method to increase the diffractive angle of a spatial light modulator in one and two directions by introducing the highly 2-dimensionally tilted illuminating beam. The inevitable image aberrations, such as astigmatism, for off-axis imaging are corrected by proper modifications of the phase patterns on the modulator. Experimental results show the image aspect ratio of 2.4:1 suitable for human vision, with sustained image contrast and noise level. Study of the experimental diffractive efficiency is also presented.

Non-invasive correction of thermally induced wavefront aberrations of spatial light modulator in holographic projection

Holographic phase-only projection technique utilizing spatial light modulators (SLM) as an active element may be intended to work in environments in which the temperature changes with time, such as outdoors or automotive applications. In order to achieve maximum possible resolution, the thermally induced aberrations of the SLM must be corrected in real time. We present the results of non-invasive determination of the SLM aberrations in holographic image projection, caused mostly by thermal strains of SLM, with the use of one fixed camera detector and iterative phase retrieval. We show real-time wavefront correction leading to experimentally proven, diffraction-limited image quality in a wide range of SLM temperatures.

Holographic zoom micro-projection system based on three spatial light modulators

In this paper, we propose a holographic zoom micro-projection system based on three spatial light modulators (SLMs). Three color lasers, three filters, and three solid lenses form the system's collimated light sources. Three beam splitters and a prism are used in the system for beam deflection. The SLMs are used as the micro-displays in order to realize phase modulation. A liquid lens, which consists of a circular hole in the center of the middle substrate and several holes around the center, is developed in the system and it. A receiving screen is located behind the liquid lens. When the voltage applied to the liquid lens is changed, the focal length changes accordingly due to electrowetting effect. Three color holograms are loaded on the SLMs, respectively. When three color lasers are used to illuminate the corresponding holograms, the position and size of each color reproduction image can be adjusted by changing the focal length of the liquid lens and holograms loaded onto the SLMs. Therefore, three color images can be reconstructed together perfectly. The proposed system can realize function of zoom micro-projection without chromatic aberration. The experimental results verify its feasibility.

Full-color computer-generated hologram using wavelet transform and color space conversion

In this study, we presented the acceleration of full-color computer-generated holograms (CGHs) bi using WAvelet ShrinkAge-Based superpositIon (WASABI). The WASABI method uses a wavelet transform. Furthermore, the light wave superposition is calculated by using 3%, 5%, and 10% of the light wave components in wavelet space. The WASABI method is implemented for generating full-color CGHs and is used to further combine the color space conversion from the RGB color space to the YCbCr color space. We report that the WASABI method is 10-33 times faster than the conventional look-up table method and 2-7 times faster than the depth layer method based on fast Fourier transform. Further, the WASABI method in the YCbCr color space is approximately 1.5 times faster than that in the RGB color space.

Miniature holographic projector with cloud computing capability

A fully functional miniaturized projection head below 5  cm³ is presented, using computer-generated holograms dynamically displayed on a liquid-crystal spatial light modulator. Spatial division of the modulator is used for color projection without color breakup, and specially designed, anti-reflection coated prisms ensure simple light paths with small losses. Real-time calculations are performed on a remote server with on-the-fly compression of holographic fringes. Cloud computing allows 1 W of local electrical power usage and apparent image brightness equivalent to 15-500 lm/W efficiency, depending on the displayed content. The properties of the projector allow future applications in handheld displays.

Holographic colour prints for enhanced optical security by combined phase and amplitude control

Conventional optical security devices provide authentication by manipulating a specific property of light to produce a distinctive optical signature. For instance, microscopic colour prints modulate the amplitude, whereas holograms typically modulate the phase of light. However, their relatively simple structure and behaviour is easily imitated. We designed a pixel that overlays a structural colour element onto a phase plate to control both the phase and amplitude of light, and arrayed these pixels into monolithic prints that exhibit complex behaviour. Our fabricated prints appear as colour images under white light, while projecting up to three different holograms under red, green, or blue laser illumination. These holographic colour prints are readily verified but challenging to emulate, and can provide enhanced security in anti-counterfeiting applications. As the prints encode information only in the surface relief of a single polymeric material, nanoscale 3D printing of customised masters may enable their mass-manufacture by nanoimprint lithography.

Here, the authors fabricate a device that integrates multiple computer-generated holograms into a single colour print. Under white light, a colour image is seen, whereas illumination with a red, green, or blue beam from a handheld laser pointer projects three different holograms onto a distant screen.

Color holographic display using single chip LCOS

An algorithm for calculating a phase-only computer-generated hologram (CGH) for 3D color display using single chip liquid crystal on silicon (LCOS) is proposed. As an evolution of the G-S iteration, the proposed algorithm was obtained by replacing the Fresnel diffraction of single wavelength in the G-S algorithm with a color Fresnel diffraction process. Moreover, combined with "Ping-Pong" iteration, this algorithm was used to calculate 3D color holograms. Finally, a hologram that is capable of projecting a color 3D image was generated. Numerical simulation was performed, and an optical reconstruction system based on single spatial light modulation (SLM) was built to prove this method. Both numerical and optical reconstruction results indicated that the proposed method realized 3D color holographic display using single chip LCOS.

Interactive Holographic Display Based on Finger Gestures

In this paper, we demonstrate an interactive, finger-sensitive system which enables an observer to intuitively handle electro-holographic images in real time. In this system, a motion sensor detects finger gestures (swiping and pinching) and translates them into the rotation and enlargement/reduction of the holographic image, respectively. By parallelising the hologram calculation using a graphics processing unit, we realised the interactive handling of the holographic image in real time. In a demonstration of the system, we used a Leap Motion sensor and a phase modulation-type spatial light modulator with 1,920 × 1,080 pixels and a pixel pitch of 8.0 µm × 8.0 µm. The constructed interactive finger-sensitive system was able to rotate a holographic image composed of 4,096 point light sources using a swiping gesture and enlarge or reduce it using a pinching gesture in real time. The average calculation speed was 27.6 ms per hologram. Finally, we extended the constructed system to a full-colour reconstruction system that generates a more realistic three-dimensional image. The extended system successfully allowed the handling of a full-colour holographic image composed of 1,709 point light sources with a calculation speed of 22.6 ms per hologram.

Digital hologram transformations for RGB color holographic display with independent image magnification and translation in 3D

A new framework for in-plane transformations of digital holograms (DHs) is proposed, which provides improved control over basic geometrical features of holographic images reconstructed optically in full color. The method is based on a Fourier hologram equivalent of the adaptive affine transformation technique [Opt. Express18, 8806 (2010)OPEXFF1094-408710.1364/OE.18.008806]. The solution includes four elementary geometrical transformations that can be performed independently on a full-color 3D image reconstructed from an RGB hologram: (i) transverse magnification; (ii) axial translation with minimized distortion; (iii) transverse translation; and (iv) viewing angle rotation. The independent character of transformations (i) and (ii) constitutes the main result of the work and plays a double role: (1) it simplifies synchronization of color components of the RGB image in the presence of mismatch between capture and display parameters; (2) provides improved control over position and size of the projected image, particularly the axial position, which opens new possibilities for efficient animation of holographic content. The approximate character of the operations (i) and (ii) is examined both analytically and experimentally using an RGB circular holographic display system. Additionally, a complex animation built from a single wide-aperture RGB Fourier hologram is presented to demonstrate full capabilities of the developed toolset.

Single SLM full-color holographic 3-D display based on sampling and selective frequency-filtering methods

A single SLM (spatial light modulator) full-color holographic 3-D display based on sampling and selective frequency-filtering methods is proposed. Spatially-sampled R(red), G(green) and B(blue)-holograms can provide periodic 3 × 3 arrays of their frequency spectrums. Thus, by allocating three groups of three spectrums to each color hologram, and selectively filtering out those spectrums with their own spectrum filtering masks (SFMs), frequency-filtered R, G and B-holograms can be obtained. These holograms are synthesized into a single color-multiplexed hologram, and optically reconstructed into a color distortion-free full-color 3-D object on the 4-f lens system, where color-dispersion due to the pixelated structure of the SLM can be removed with the optical versions of SFMs. Fourier-optical analysis and experiments with 3-D color objects in motion confirm the feasibility of the proposed system in the practical application.

Full-color holographic display with increased-viewing-angle [Invited]

Among the important features of holographic displays are the wide viewing angles and the full color of the reconstructed images. The present work focuses on achievement of both features. We propose an increased-viewing-angle full-color holographic display using two tiled phase-only spatial light modulators (SLMs), a 4f concave mirrors system, and a temporal-spatial multiplexing method. The 4f optical system consists of two concave mirrors and serves to increase the viewing angle. A temporal-spatial multiplexing synchronization control (TSMSC) method is developed to achieve a full-color image and to remove the color crosstalk of the image. We calculate RGB phase-only holograms of a computer-generated color pyramid by using a slice-based Fresnel diffraction algorithm. The experimental results indicate that the proposed display system is feasible to reconstruct a full-color holographic 3D image with a viewing angle of 12.8°, which is about 3.8 times wider than the viewing angle formed by a single SLM.

Speckle reduced lensless holographic projection from phase-only computer-generated hologram

This paper presents a method for the implementation of speckle reduced lensless holographic projection based on phase-only computer-generated hologram (CGH). The CGH is calculated from the image by double-step Fresnel diffraction. A virtual convergence light is imposed to the image to ensure the focusing of its wavefront to the virtual plane, which is established between the image and the hologram plane. The speckle noise is reduced due to the reconstruction of the complex amplitude of the image via a lensless optical filtering system. Both simulation and optical experiments are carried out to confirm the feasibility of the proposed method. Furthermore, the size of the projected image can reach to the maximum diffraction bandwidth of the spatial light modulator (SLM) at a given distance. The method is effective for improving the image quality as well as the image size at the same time in compact lensless holographic projection system.

Projection-type see-through holographic three-dimensional display

Owing to the limited spatio-temporal resolution of display devices, dynamic holographic three-dimensional displays suffer from a critical trade-off between the display size and the visual angle. Here we show a projection-type holographic three-dimensional display, in which a digitally designed holographic optical element and a digital holographic projection technique are combined to increase both factors at the same time. In the experiment, the enlarged holographic image, which is twice as large as the original display device, projected on the screen of the digitally designed holographic optical element was concentrated at the target observation area so as to increase the visual angle, which is six times as large as that for a general holographic display. Because the display size and the visual angle can be designed independently, the proposed system will accelerate the adoption of holographic three-dimensional displays in industrial applications, such as digital signage, in-car head-up displays, smart-glasses and head-mounted displays.

The design of holographic displays usually involves a trade-off between size and viewing angle. Here, the authors combine holographic projection with a digitally designed holographic optical element so that display size and the visual angle can be designed independently.

Simple calculation of a computer-generated hologram for lensless holographic 3D projection using a nonuniform sampled wavefront recording plane

In this paper, we present a method for calculation of a computer-generated hologram (CGH) from a 3D object. A virtual wavefront recording plane (WRP) which is close to the 3D object is established. This WRP is nonuniformly sampled according to the depth map of the 3D object. The generation of CGH only involves two nonuniform fast Fourier transform (NUFFT) and two fast Fourier transform (FFT) operations, the whole computational procedure is greatly simplified by diffraction calculation from a 2D planar image instead of 3D object voxels. Numerical simulations and optical experiments are carried out to confirm the feasibility of our proposed method. The CGH calculated with our method is capable to project zoomable 3D objects without lens.

Adaptive, spatially-varying aberration correction for real-time holographic projectors

A method of generating an aberration- and distortion-free wide-angle holographically projected image in real time is presented. The target projector is first calibrated using an automated adaptive-optical mechanism. The calibration parameters are then fed into the hologram generation program, which applies a novel piece-wise aberration correction algorithm. The method is found to offer hologram generation times up to three orders of magnitude faster than the standard method. A projection of an aberration- and distortion-free image with a field of view of 90x45 degrees is demonstrated. The implementation on a mid-range GPU achieves high resolution at a frame rate up to 12fps. The presented methods are automated and can be performed on any holographic projector.

Color holographic display with white light LED source and single phase only SLM

This work presents color holographic display, which is based on a single phase only spatial light modulator (SLM). In the display entire area of the SLM is illuminated by an on-axis white light beam generated by a single large LED. The holographic display fully utilizes SLM bandwidth and has capability of full-color, full frame rate imaging of outstanding quality. This is achieved through: (i) optimal use of the source coherence volume, (ii) application of the single white light LED source, (iii) a development of a novel concept of color multiplexing technique with color filter mask in Fourier plane of the SLM, (iv) and a complex coding with improved diffraction efficiency. Within experimental part of the paper we show single color, full-color holographic 2D and 3D images generated for reconstruction depth exceeding 10 cm.

Speckle-suppressed phase-only holographic three-dimensional display based on double-constraint Gerchberg-Saxton algorithm

The Gerchberg-Saxton (GS) algorithm is widely used to calculate the phase-only computer-generated hologram (CGH) for holographic three-dimensional (3D) display. However, speckle noise exists in the reconstruction of the CGH due to the uncontrolled phase distribution. In this paper, we propose a method to suppress the speckle noise by simultaneously reconstructing the desired amplitude and phase distribution. The phase-only CGH is calculated by using a double-constraint GS algorithm, in which both the desired amplitude and phase information are constrained in the image plane in each iteration. The calculated phase-only CGH can reconstruct the 3D object on multiple planes with a desired amplitude distribution and uniform phase distribution. Thus the speckle noise caused by the phase fluctuation between adjacent pixels is suppressed. Both simulations and experiments are presented to demonstrate the effective speckle noise suppression by our algorithm.

Multi-wavelength holography with a single spatial light modulator for ultracold atom experiments

We demonstrate a method to independently and arbitrarily tailor the spatial profile of light of multiple wavelengths and we show possible applications to ultracold atoms experiments. A single spatial light modulator is programmed to create a pattern containing multiple spatially separated structures in the Fourier plane when illuminated with a single wavelength. When the modulator is illuminated with overlapped laser beams of different wavelengths, the position of the structures is wavelength-dependent. Hence, by designing their separations appropriately, a desired overlap of different structures at different wavelengths is obtained. We employ regional phase calculation algorithms and demonstrate several possible experimental scenarios by generating light patterns with 670 nm, 780 nm and 1064 nm laser light which are accurate to the level of a few percent. This technique is easily integrated into cold atom experiments, requiring little optical access.

How to use a phase-only spatial light modulator as a color display

We demonstrate that a parallel aligned liquid crystal on silicon (PA-LCOS) spatial light modulator (SLM) without any attached color mask can be used as a full color display with white light illumination. The method is based on the wavelength dependence of the (voltage controlled) birefringence of the liquid crystal pixels. Modern SLMs offer a wide range over which the birefringence can be modulated, leading (in combination with a linear polarizer) to several intensity modulation periods of a reflected light wave as a function of the applied voltage. Because of dispersion, the oscillation period strongly depends on the wavelength. Thus each voltage applied to an SLM pixel corresponds to another reflected color spectrum. For SLMs with a sufficiently broad tuning range, one obtains a color palette (i.e., a "color lookup-table"), which allows one to display color images. An advantage over standard liquid crystal displays (LCDs), which use color masks in front of the individual pixels, is that the light efficiency and the display resolution are increased by a factor of three.

Colour hologram projection with an SLM by exploiting its full phase modulation range

We demonstrate independent and simultaneous manipulation of light beams of different wavelengths by a single hologram, which is displayed on a phase-only liquid crystal spatial light modulator (SLM). The method uses the high dynamic phase modulation range of modern SLMs, which can shift the phase of each pixel in a range between 0 up to 10π, depending on the readout wavelength. The extended phase range offers additional degrees of freedom for hologram encoding. Knowing the phase modulation properties of the SLM (i.e. the so-called lookup table) in the entire exploited wavelength range, an exhaustive search algorithm allows to combine different independently calculated 2π-holograms into a multi-level hologram with a phase range extending over several multiples of 2π. The combined multi-level hologram then reconstructs the original diffractive patterns with only small phase errors at preselected wavelengths, thus projecting the desired image fields almost without any crosstalk. We demonstrate this feature by displaying a static hologram at an SLM which is read out with an incoherent red-green-blue (RGB) beam, projecting a color image at a camera chip. This is done for both, a Fourier setup which needs a lens for image focusing, and in a "lensless" Fresnel setup, which also avoids the appearance of a focused zero-order spot in the image center. The experimentally obtained efficiency of a two-colour combination is on the order of 83% for each wavelength, with a crosstalk level between the two colour channels below 2%, whereas a three-colour combination still reaches an efficiency of about 60% and a crosstalk level below 5%.

Multiplexing encoding method for full-color dynamic 3D holographic display

The multiplexing encoding method is proposed and demonstrated for reconstructing colorful images accurately by using single phase-only spatial light modulator (SLM). It will encode the light waves at different wavelengths into one pure-phase hologram at the same time based on the analytic formulas. The three-dimensional (3D) images can be reconstructed clearly when the light waves at different wavelengths are incident into the encoding hologram. Numerical simulations and optical experiments for 2D and 3D colorful images are performed. The results show that the colorful reconstructed images with high quality are achieved successfully. The proposed multiplexing method is a simple and fast encoding approach and the size of the system is small and compact. It is expected to be used for realizing full-color 3D holographic display in future.

Minimized speckle noise in lens-less holographic projection by pixel separation

Images displayed by holographic methods on phase-only light modulators inevitably suffer from speckle noise. It is partly caused by multiple uncontrolled interferences between laser light rays forming adjacent pixels of the image while having a random phase state. In this work the experimental proof of concept of an almost speckle-less projection method is presented, which assumes introducing a spatial separation of the image pixels, thus eliminating the spurious interferences. A single displayed sub-frame consists of separated light spots of very low intensity error. The sub-frames with different sampling offsets are then displayed sequentially to produce a non-fragmented color final image.

Lensless zoomable holographic projection using scaled Fresnel diffraction

Projectors require a zoom function. This function is generally realized using a zoom lens module composed of many lenses and mechanical parts; however, using a zoom lens module increases the system size and cost, and requires manual operation of the module. Holographic projection is an attractive technique because it inherently requires no lenses, reconstructs images with high contrast and reconstructs color images with one spatial light modulator. In this paper, we demonstrate a lensless zoomable holographic projection. Without using a zoom lens module, this holographic projection realizes the zoom function using a numerical method, called scaled Fresnel diffraction which can calculate diffraction at different sampling rates on a projected image and hologram.