Fast digital hologram generation and adaptive force measurement in liquid-crystal-display-based holographic tweezers

Computer-generated holograms in conjunction with spatial light modulators (SLMs) offer a way to dynamically generate holograms that are adapted to specific tasks. To use the full dynamic capability of the SLM, the hologram computation should be very fast. We present a method that uses the highly parallel architecture of a consumer graphics board to compute analytical holograms in video real time. A precice characterization of the SLM (Holoeye LC-R-2500) and the adaption of its settings to our near-infrared application is necessary to guarantee an efficient hologram reconstruction. The benefits of a fast computation of adapted holograms and the application of an efficient SLM are demonstrated by measuring the trapping forces of holographic tweezers.     

Superresolved imaging in digital holography by superposition of tilted wavefronts

A technique based on superresolution by digital holographic microscopic imaging is presented. We used a two dimensional (2-D) vertical-cavity self-emitting laser (VCSEL) array as spherical-wave illumination sources. The method is defined in terms of an incoherent superposition of tilted wavefronts. The tilted spherical wave originating from the 2-D VCSEL elements illuminates the target in transmission mode to obtain a hologram in a Mach-Zehnder interferometer configuration. Superresolved images of the input object above the common lens diffraction limit are generated by sequential recording of the individual holograms and numerical reconstruction of the image with the extended spatial frequency range. We have experimentally tested the approach for a microscope objective with an exact 2-D reconstruction image of the input object. The proposed approach has implementation advantages for applications in biological imaging or the microelectronic industry in which structured targets are being inspected.     

Implementation of a spatially multiplexed pixelated three-dimensional display by use of a holographic optical element array

A pixelated holographic stereogram is proposed and experimentally studied for the emulation of a spatially multiplexed composite three-dimensional (3-D) pixel display. With this approach, pixelated holograms are utilized to compose spatially multiplexed images. Each composite pixel in the holographic optical element array has a diffraction pattern that scatters light into predefined spatial directions. Under reconstruction, each pixel generates different intensities along a range of viewing angles. When the composite holographic pixel array is assembled, it has the capability to deliver 3-D effects. The technique, together with a novel recording scheme that is designed to synthesize a computerized 3-D display system based on this concept, is described in some detail.     

Achromatized transmission-type holographic screen for a multiview stereoscopic image system

The main drawback of the use of transmission-type holographic screens is poor color reproduction caused by their high spectral dispersion. For overcoming this drawback, a long, narrow diffusing slit is used as an object when recording the screen. The necessary size and position of the slit relative to the photoplate and to the recording and reconstruction beams are determined by the phase relations of the beams. By use of the slit, holographic screens of 30 cm x 40 cm are recorded with a diverging reference beam and are used to display a multiview full-color stereoscopic image. The images displayed on the screen show no sign of color separation except near the edges of the screen. The image brightness on the screen is high enough that it can be watched in a normally illuminated room.     

Distortion compensation and elimination in holographic reconstruction

We discuss the quantitative location of objects from holographic images when the reconstruction wavelength differs from the recording wavelength. The holographic image equations are interpreted in a way that clarifies the meaning of stereo pairs of holographic images and indicates how backprojection methods can be used in holography to locate objects. Alternative methods involving the production of distortion-free regions in the holographic image field during reconstruction, the use of self-calibrating objects in the object field during recording, and triangulation can be used to locate objects.     

Holographic display with tilted spatial light modulator

In this paper, we analyze a holographic display system utilizing a phase-only spatial light modulator (SLM) based on liquid crystal on silicon (LCoS). An LCoS SLM works in reflection, and, in some applications, it is convenient to use with an inclined illumination. Even with a highly inclined illumination, the holographic display is capable of good-quality image generation. We show that the key to obtain high-quality reconstructions is the tilt-dependent calibration and algorithms. Typically, an LCoS SLM is illuminated with a plane wave with normal wave vector. We use inclined illumination, which requires development of new algorithms and display characterization. In this paper we introduce two algorithms. The first one is designed to process a digital hologram captured in CCD normal configuration, so it can be displayed in SLM tilted geometry, while the second one is capable of synthetic hologram generation for tilted SLM configuration. The inclined geometry asymmetrically changes the field of view of a holographic display. The presented theoretical analysis of the aliasing effect provides a formula for the field of view as a function of SLM tilt. The incidence angle affects SLM performance. Both elements of SLM calibration, i.e., pixel phase response and wavefront aberrations, strongly depend on SLM tilt angle. The effect is discussed in this paper. All of the discussions are accompanied with experimental results.     

Computer-generated holograms of a real three-dimensional object based on stereoscopic video images

A novel method of using stereoscopic video images to synthesize the computer-generated hologram (CGH) patterns of a real 3D object is proposed. Stereoscopic video images of a real 3D object are captured by a 3D camera system. Disparity maps between the captured stereo image pairs are estimated and from these estimated maps the depth data for each pixel of the object can be extracted on a frame basis. By using these depth data and original color images, hologram patterns of a real object can be computationally generated. In experiments, stereoscopic video images of a real 3D object, a wooden rhinoceros doll, are captured by using the Wasol 3D adapter system and its depth data are extracted from them. Then, CGH patterns of 1280 pixels x 1024 pixels are generated with these depth-annotated images of the wooden rhinoceros doll, and the CGH patterns are experimentally displayed via a holographic display system.     

Pulsed holography for hypervelocity impact diagnostics

The development of pulsed holography has two principal objectives. The first objective is to quantify the three dimensional characteristics of hypervelocity impact events, and the second is to provide a diagnostic with the ability to capture high fidelity information for the validation of sophisticated three-dimensional hydrocodes. The holographic image-capturing subsystem uses a Q-switched, seeded, frequency-doubled Nd-YAG laser which produces 5 ns, 750 mJ, coherent pulses at 532 nm. Holographic images have been captured of the back-surface debris bubble from 4 km/s perforating impacts and crater ejecta from 2 km/s non-perforating impacts. A prototype holographic reconstruction and image analysis subsystem has been assembled that provides the ability to measure both the spatial distribution of particles and the morphology of individual particles produced in a hypervelocity impact event. The demonstrated image resolution of this system is 20 μm; however, higher resolutions are possible with magnification optics.     

A holographic waveguide based eye tracking device

We demonstrated the feasibility of using a holographic waveguide imager for eye tracking. A holographic waveguide placed in front of the eye was used to capture images of the anterior segment of the eye and to guide the images to a camera distant from the eye. The pupil centre (PC) and corneal reflection (CR) of the eye was used to compute eye position. A custom-built model eye was used to validate the prototype eye tracker. A linear relationship between the angular eye position and the PC/CR vector was found over 60 horizontal degrees and 40 vertical degrees. The tracking accuracy and precision were 0.72 degree and 0.50 degree over these tracking ranges. These results confirmed that holographic waveguide could be a viable platform for developing compact, wearable, see-through eye trackers that can continuously monitor eye movements during real life tasks and thus can facilitate diagnosis of oculomotor disorders.     

Computer-generated holograms with optimum bandwidths obtained with twisted-nematic liquid-crystal displays

We discuss a computer-generated hologram for encoding arbitrary complex modulation based on a commercial twisted-nematic liquid-crystal display. This hologram is implemented with the constrained complex modulation provided by the display in a phase-mostly configuration. The hologram structure and transmittance are determined to obtain on-axis signal reconstruction, maximum bandwidth, optimum efficiency, and high signal-to-noise ratio. We employed the proposed holographic code for the experimental synthesis of first-order Bessel beams.     

Hologram reconstruction by use of optical wavelet transform

High-speed in-line holography is used to visualize the trajectories of glass fibers being drawn out in a turbulent flame. To improve the signal-to-noise ratio, the images are not observed by a conventional reconstruction setup, but the holographic plate is placed directly on the input plane of a wavelet-transform optical system. This processing system is based on a VanderLugt correlator with inclusion of an electrically addressed spatial light modulator. The shape of the matched filters is deduced by successive rotation and dilatation operations of wavelet functions in the Fourier domain. We estimate the three-dimensional location of a fiber element and its orientation by searching for the daughter wavelet that yields the maximum intensity on the output plane of the correlator, which also contains the reconstructed image. The results are compared with those obtained by conventional optical reconstruction. The signal-to-noise ratios of the images observed on the output plane are improved. Moreover, it is shown that the axis coordinate accuracy is improved to Dz = +/-50 microm, instead of +/-0.5 mm for holographic reconstruction.     

Speckle-reduced three-dimensional volume holographic display by use of integral imaging

We propose a method to implement a speckle-reduced coherent three-dimensional (3D) display system by a combination of integral imaging and photorefractive volume holographic storage. The 3D real object is imaged through the microlens array and stored in the photorefractive crystal. During the reconstruction process a phase conjugate reading beam is used to minimize aberration, and a rotating diffuser located on the imaging plane of the lens array is employed to reduce the speckle noise. The speckle-reduced 3D image with a wide viewing angle can be reconstructed by use of the proposed system. Experimental results are presented and optical parameters of the proposed system are discussed in detail.     

Digital holography of particle fields: reconstruction by use of complex amplitude

Digital holography appears to be a strong contender as the next-generation technology for holographic diagnostics of particle fields and holographic particle image velocimetry for flow field measurement. With the digital holographic approach, holograms are directly recorded by a digital camera and reconstructed numerically. This not only eliminates wet chemical processing and mechanical scanning, but also enables the use of complex amplitude information inaccessible by optical reconstruction, thereby allowing flexible reconstruction algorithms to achieve optimization of specific information. However, owing to the inherently low pixel resolution of solid-state imaging sensors, digital holography gives poor depth resolution for images, a problem that severely impairs the usefulness of digital holography especially in densely populated particle fields. This paper describes a technique that significantly improves particle axial-location accuracy by exploring the reconstructed complex amplitude information, compared with other numerical reconstruction schemes that merely mimic traditional optical reconstruction. This novel method allows accurate extraction of particle locations from forward-scattering particle holograms even at high particle loadings.     

Properties of DMDs for holographic displays

Digital micromirror device’s (DMD) properties as being a display device for holographic displays are investigated. High speed, a large separation between reconstructed image and reconstruction beam, two symmetric diffraction patterns, and low intensity (0,0)th-order beam at a blazed grating condition are the desired properties for the displays. The blazed grating condition of a DMD can reconstruct images with higher diffraction efficiency than the line grating condition. DMD’s high speed enables to present colors and gray levels to the reconstructed image. However, reconstructed images from a gray-level computer-generated hologram (CGH) and its binary form hologram reveal no noticeable difference between them, except the background noise in the image from the CGH.     

Single-shot full resolution region-of-interest (ROI) reconstruction in image plane digital holographic microscopy

We describe a numerical processing technique that allows single-shot region-of-interest (ROI) reconstruction in image plane digital holographic microscopy with full pixel resolution. The ROI reconstruction is modelled as an optimization problem where the cost function to be minimized consists of an L2-norm squared data fitting term and a modified Huber penalty term that are minimized alternately in an adaptive fashion. The technique can provide full pixel resolution complex-valued images of the selected ROI which is not possible to achieve with the commonly used Fourier transform method. The technique can facilitate holographic reconstruction of individual cells of interest from a large field-of-view digital holographic microscopy data. The complementary phase information in addition to the usual absorption information already available in the form of bright field microscopy can make the methodology attractive to the biomedical user community.     

Aberration compensation of holographic particle images using digital holographic microscopy

Characterisation of small and large-scale vortices in turbulent flows demands a system with high spatial resolution. The measurement of high spatial resolution, three-dimensional vector displacements in fluid mechanics using holography, is usually hampered by aberration. Aberration poses some problems in particle image identification due to low fidelity of real image reconstruction. Phase mismatch between the recording and the reconstruction waves was identified as the main source of aberration in this study. This paper demonstrates how aberration compensation can be achieved by cross-correlating the complex amplitude of an aberrated reconstructed object with the phase conjugate of a known reference object in the plane of the hologram (frequency space). Results favourably show significant increase in Strehl ratio and suppression of background noise that are more pronounced for particle images of 10 and 5 microns. It is clear from the work conducted that wavefront aberration measurement and compensation of holographic microscopic objects are now possible with the use of a variant digital holographic microscope.     

Point-spread function synthesis in scanning holographic microscopy

Scanning holographic microscopy is a two-pupil synthesis method allowing the capture of single-sideband in-line holograms of noncoherent (e.g., fluorescent) three-dimensional specimens in a single two-dimensional scan. The flexibility offered by the two-pupil method in synthesizing unusual point-spread functions is discussed. We illustrate and compare three examples of holographic recording, using computer simulations. The first example is the classical hologram in which each object point is encoded as a spherical wave. The second example uses pupils with spherical phase distributions having opposite curvatures, leading to reconstructed images with a resolution limit that is half that of the objective. In the third example, axicon pupils are used to obtain axially sectioned images.     

Underwater digital holography for studies of marine plankton

Conventional and digital holographies are proving to be increasingly important for studies of marine zooplankton and other underwater biological applications. This paper reports on the use of a subsea digital holographic camera (eHoloCam) for the analysis and identification of marine organisms and other subsea particles. Unlike recording on a photographic film, a digital hologram (e-hologram) is recorded on an electronic sensor and reconstructed numerically in a computer by simulating the propagation of the optical field in space. By comparison with other imaging techniques, an e-hologram has several advantages such as three-dimensional spatial reconstruction, non-intrusive and non-destructive interrogation of the recording sampling volume and the ability to record holographic videos. The basis of much work in optics lies in Maxwell's electromagnetic theory and holography is no exception: we report here on two of the numerical reconstruction algorithms we have used to reconstruct holograms obtained using eHoloCam and how their starting point lies in Maxwell's equations. Derivation of the angular spectrum algorithm for plane waves is provided as an exact method for the in-line numerical reconstruction of digital holograms. The Fresnel numerical reconstruction algorithm is derived from the angular spectrum method. In-line holograms are numerically processed before and after reconstruction to remove periodic noise from captured images and to increase image contrast. The ability of the Fresnel integration reconstruction algorithm to extend the reconstructed volume beyond the recording sensor dimensions is also shown with a 50% extension of the reconstruction area. Finally, we present some images obtained from recent deployments of eHoloCam in the North Sea and Faeroes Channel.     

Method for fringe enhancement in holographic interferometry for measurement of in-plane displacements

Theoretical background along with experimental results are given for a simple method for in-plane fringe enhancement in dual-beam illumination holographic interferometry. In this method, the fringes representing in-plane displacements arise as a moirépattern between two interferograms. To distinguish the in-plane displacement, a sequence of images is recorded while the reference mirror is continuously tilted at random. The in-plane fringes arethen found as the maximum contrast of the out-of-plane fringes in the image sequence. The resulting fringe quality is close to the quality of the out-of-plane fringes.     

面向髋关节穿刺手术的实用化增强现实导航方法

目的 旨在利用成熟的技术路线和功能整合,实现面向髋关节穿刺手术的无创伤实用化骨骼增强现实(AR)导航。方法 采用基于神经网络的自动分割方法对现有临床CT影像数据进行骨骼图像分割,并进行三维体积重建;利用虚拟现实开发手段,实现手术导航功能。结果 实现了面向AR的CT扫描方法及配准装置设计,开发了基于CT影像的实用化骨骼组织三维自动重建功能。成功实现了与真实人体配准的骨组织AR预览。完成了从临床CT影像采集到形成真实人体配准的实时骨骼显示的全过程验证。结论 为无创骨骼导航提供了一种新的技术路线和方法,证明了在无创条件下快速实现虚拟骨骼与真实皮肤配准显示的技术可行性。为临床骨科手术提供了新的思路和参考,并可为其他领域的外科手术导航研究提供技术支持和参考。