Stephan Reichelt

Depth cues in human visual perception and their realization in 3D displays

Over the last decade, various technologies for visualizing three-dimensional (3D) scenes on displays have been technologically demonstrated and refined, among them such of stereoscopic, multi-view, integral imaging, volumetric, or holographic type. Most of the current approaches utilize the conventional stereoscopic principle. But they all lack of their inherent conflict between vergence and accommodation since scene depth cannot be physically realized but only feigned by displaying two views of different perspective on a flat screen and delivering them to the corresponding left and right eye. This mismatch requires the viewer to override the physiologically coupled oculomotor processes of vergence and eye focus that may cause visual discomfort and fatigue. This paper discusses the depth cues in the human visual perception for both image quality and visual comfort of direct-view 3D displays. We concentrate our analysis especially on near-range depth cues, compare visual performance and depth-range capabilities of stereoscopic and holographic displays, and evaluate potential depth limitations of 3D displays from a physiological point of view.

Full-range, complex spatial light modulator for real-time holography

We demonstrate a full-range complex and transmissive spatial light modulator (SLM) for simultaneous and independent amplitude and phase modulation of an input wave field. Arbitrary scalar complex optical fields are generated by stacking a pixelated liquid crystal display operating in phase-only (2π) modulation with passive polarization-sensitive components. The principle is based on optical combining the light fields of two neighboring phase-only modulating pixels, which were made orthogonally polarized by a structured half-wave plate, then passing through a birefringent plate to laterally shift one of the beams collinear to the other, and finally bringing to interference by a linear polarizer. Complex modulation by the proposed SLM is experimentally verified in monochrome green operation.

Design of spherically corrected, achromatic variable-focus liquid lenses

A design method for correcting chromatic as well as spherical aberrations of variable-focus, multi-chamber liquid lenses is described. By combining suitable optical liquids with appropriate radii of the liquids interfaces, liquid lenses with superior, diffraction-limited resolution over a wide focal tuning range are possible. For an infinite object distance, the analytic thin-lens approximation of an achromatic positive/negative varifocal liquid lens is derived and the obtained results are compared with ray-traced optimized designs which consider finite thicknesses and rigid cover glasses. As a design example, the optical performance of a 4mm-diameter positive/negative f /3.6 achromatic liquid lens is given in detail.

Combined Twyman–Green and Mach–Zehnder interferometer for microlens testing

A new interferometer design for microlens testing is presented. The phase-shifting system combines the advantages of a Twyman-Green and a Mach-Zehnder interferometer and permits full characterization of the aberrations of microlenses as well as radius of curvature and focal length measurements. The Twyman-Green system is applied to surface testing in reflection (single reflection), whereas the Mach-Zehnder system is used for lens testing in transmission (single pass). Both measurements are performed without removal of the test part, allowing for combination of the results without confusion of the actual lens and without an azimuthal orientation error. The interferometer setup is explained, the test procedure is described, and experimental results are given.

Large real-time holographic displays: from prototypes to a consumer product

Large real-time holographic displays with full color are feasible with SeeReals new approach to holography and todays technology. The display provides the information about the 3D scene in a viewing window at each observer eye. A tracking system always locates the viewing windows at the observer eyes. This combination of diffractive and refractive optics leads to a significant reduction of required display resolution and computation effort and enables holographic displays for wide-spread consumer applications. We tested our approach with two 20 inch prototypes that use two alternatives to achieve full color. One prototype uses color filters and interlaced holograms to generate the colors simultaneously. The other prototype generates the colors sequentially. In this paper we review our technology briefly, explain the two alternatives to full color and discuss the next steps toward a consumer product.

Holographic 3-D Displays - Electro-holography within the Grasp of Commercialization

Holography is a diffraction-based coherent imaging technique in which a complex threedimensional object can be reproduced from a flat, two-dimensional screen with a complex transparency representing amplitude and phase values. It is commonly agreed that real-time holography is the ne plus ultra art and science of visualizing fast temporally changing 3-D scenes. The integration of the real-time or electro-holographic principle into display technology is one of the most promising but also challenging developments for the future consumer display and TV market. Only holography allows the reconstruction of naturallooking 3-D scenes, and therefore provides observers with a completely comfortable viewing experience. But to date several challenges have prevented the technology from becoming commercialized. But those obstacles are now starting to be overcome. Recently, we have developed a novel approach to real-time display holography by combining an overlapping sub-hologram technique with a tracked viewing-window technology (Schwerdtner, Leister & Haussler, 2007; Schwerdtner, Haussler & Leister, 2007). For the first time, this enables solutions for large screen interactive holographic displays (Stolle & Haussler, 2008; Reichelt et al., 2008). This chapter presents these novel solutions for large real-time holographic 3-D displays in the context of previous and current approaches to electro-holography. The holographic display developed by us combines a tailored holographic recording scheme with active tracking of the observer. This unique approach dramatically reduces the demand for the space-bandwidth product of the hologram and thus allows the use of state-of-the-art spatial light modulators and enables real-time calculation. The fundamentals and challenges of the holographic display technology are described, its implementation in prototypes is demonstrated, and the bright prospects for the 3-D display market are discussed.

Development of an Implantable Pulse Oximeter

A long-term implantable photoplethysmographic sensor system is proposed. The system employs an elastic cuff which is directly wrapped around an arterial blood vessel. The optically transparent cuff is equipped with light emitting diodes and a photo transistor including the technology of pulse oximetry. The sensor will permit real-time, continuous monitoring of important vital parameters such as arterial blood oxygen saturation and pulse rate over a long-term period in vivo. We emphasize on the specific requirements for design and instrumentation of the implantable sensor and discuss first in vitro data acquired with that new photonics-based sensor.

Capabilities of diffractive optical elements for real-time holographic displays

This paper illustrates one of the various capabilities of static diffractive optical elements (DOE) beneficial to realtime holographic displays. Custom kinoform-type DOE can be used as elements for illumination of the spatial light modulator, i.e. the display where the video hologram is encoded. For an RGB application of diffractive optical elements, particular issues concerning the inherent wavelength-dependence have to be addressed. Multiorder DOE offer a way to compensate for chromatic as well as monochromatic aberrations. We will discuss concepts and performance of multi-order DOE, show their application in holographic displays, describe issues of fabrication and replication, and give experimental results of the multi-order DOE performance.

Computational hologram synthesis and representation on spatial light modulators for real-time 3D holographic imaging

In dynamic computer-generated holography that utilizes spatial light modulators, both hologram synthesis and hologram representation are essential in terms of fast computation and high reconstruction quality. For hologram synthesis, i.e. the computation step, Fresnel transform based or point-source based raytracing methods can be applied. In the encoding step, the complex wave-field has to be optimally represented by the SLM with its given modulation capability. For proper hologram reconstruction that implies a simultaneous and independent amplitude and phase modulation of the input wave-field by the SLM. In this paper, we discuss full complex hologram representation methods on SLMs by considering inherent SLM parameter such as modulation type and bit depth on their reconstruction performance such as diffraction efficiency and SNR. We review the three implementation schemes of Burckhardt amplitude-only representation, phase-only macro-pixel representation, and two-phase interference representation. Besides the optical performance we address their hardware complexity and required computational load. Finally, we experimentally demonstrate holographic reconstructions of different representation schemes as obtained by functional prototypes utilizing SeeReals viewing-window holographic display technology. The proposed hardware implementations enable a fast encoding of complex-valued hologram data and thus will pave the way for commercial real-time holographic 3D imaging in the near future.

25.2: Eye‐Tracking Solutions for Real‐Time Holographic 3‐D Display

The SeeReal electro-holographic display approach — using an observer window — has for the first time made possible large volume holographic reconstruction with real-time calculation. The observer tracking solution as implemented in the first SeeReal prototype is compared to new concepts providing for a larger tracking angle.