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Large Holographic 3D Displays for Tomorrows TVand Monitors Solutions, Challenges, and Prospects

(Invited Paper)

Stephan Reichelt, Ralf H aussler, Norbert Leister, Gerald F utterer, and Armin SchwerdtnerSeeReal Technologies GmbH

Blasewitzer Str. 43, 01307 Dresden, GermanyEmail: sre@seereal.com

Abstract Novel solutions for large real-time holographic 3Ddisplays are presented. The holographic display combines atailored holographic recording scheme with active tracking of the observer. This approach dramatically reduces the demand forthe space-bandwidth product of the hologram and thus allowsthe use of state-of-the-art spatial light modulators and enablesreal-time calculation. The fundamentals and challenges of theholographic display technology are described, its implementation

in prototypes is demonstrated, and the bright prospects for the3D display market are discussed.

I. I NTRODUCTION

Holographic television, commonly regarded as the holy grailof holography [1], is one of the most promising and challeng-ing developments for the future display market. Only hologra-phy provides all depth cues and thus allows the reconstructionof natural-looking 3D scenes. Recently, we have developeda novel approach to real-time display holography overcomingthe challenges of classic holography by combining an overlap-ping sub-hologram technique with a tracked viewing-windowtechnology [2], [3]. For the rst time, this approach enablessolutions for large screen interactive holographic displays.

II . R EAL -TIME HOLOGRAPHIC DISPLAY TECHNOLOGY

Thus far the most serious restriction of video holographyhas been the dynamic representation of the hologram by anelectrically addressed spatial light modulator (SLM) having apixelized structure with limited spatial resolution. The amountof information which can be encoded in the hologram isdirectly related to the spatial resolution and the size of theSLM. This fact is represented by the dimensionless space-bandwidth product SBP = x bx y by with being the max-imum spatial frequency and b the width of the modulatorin x and y direction. Considering conventional holography,different areas of the digital hologram encode the wave eldoriginating from another perspective of the object. To put it theother way around, each pixel of the hologram contributes toeach point of the 3D object. This corresponds to a huge amountof information which even if large SLM with tiny pixelswould be available must still be handled in data processingand computing.

Accordingly, the primary objective of conventional hologra-phy is to reconstruct the 3D object in space which can be seenbinocularly from different view points at different perspectives.In contrast to that, our approach to digital display holography

strongly takes the last optical element of the imaging chaininto account: the human eyes with its limited angular spec-trum acceptance, accommodation depth, resolution, etc. Thefundamental idea is to reconstruct a limited angular spectrumof the wave eld of the 3D object which is adapted in size toabout the humans eye entrance pupil. The designated area, i.e.the virtual viewing window from which an observer can see

the proper holographic reconstruction is located at the Fourierplane of the holographic display and it corresponds to thezero-order extension of the underlying SLM cross grating.

A. The sub-hologram concept

The principle of our holographic approach is depicted inFig. 1. A positive lens (L+) images the light source into theobserver plane and creates the spherical reference wave forhologram illumination. The inherent regular SLM structuregenerates a diffraction pattern in the far eld whose zero-orderextension is the viewing window (VW) where the eye of theobserver is located.

Given small angles, the size of the viewing window is

obtained from the grating equation and trigonometry to

wx,y = d px,y

, (1)

with d being the observer distance, the wavelength and p thepixel pitch of the SLM in x or y direction, respectively. Onlywithin the viewing window the information of the wave eldof the 3D object has to be existent. Vice versa this implies that

L+ SLM

Part of the3D scene

Lightsource VW

p

d

w

SH

Fig. 1. Schematic principle of the sub-hologram concept.

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each point of the scene (which can be considered as a point-like emitter for interference based hologram modeling) is asso-ciated with a locally limited area of the hologram. Size and po-sition of this so-called sub-hologram (SH) is dened via simpleprojection from the edges of the viewing window through thescene point that has to be encoded. By superimposing all sub-holograms, i.e. strictly speaking the angular limited complexamplitudes U n (x, y ) = An (x, y )exp[ i n (x, y )] emanatingfrom each of the n scene points, the entire hologram iscomposed. Since the complex amplitudes have to be calculatedonly within each sub-hologram area, the computational effortis dramatically reduced which enables a real-time hologramcalculation. A benecial side effect of that special holographicrecording scheme is the reduced demand for the temporalcoherence of the light source which must have at least acoherence length of N F , where N F is the Fresnel number of the maximum sub-hologram. Parallax information is deliveredwithin the viewing window, which may be either in full-parallax or single-parallax, depending on the recording schemeof the hologram and the overall optical setup.

B. Holographic displays based on tracked viewing windows

For a static hologram the reconstructed 3D object can beseen from a single viewing window only. Advantageously,dynamic holography offers the additional freedom of temporalor temporal-multiplex operation. By incorporating a trackingsystem which detects the eye positions of one or more viewersvery fast and precisely and repositions the viewing windowaccordingly, a dynamic 3D holographic display can be realizedwhich circumvents all problems involved with the classicapproach to holography. The steering of the viewing windowcan be done in different ways, either by shifting the lightsource and thus shifting the image of the light source, or by

placing an additional element close to the SLM that realizesa variable prism function [4]. 3D content can be visualizedas stationary scenes where the user can look around objectsor xed perspective 3D scenes where all users see the sameperspectives. Depending which mode is chosen the hologramis entirely new synthesized or the positions of the sub-holograms is shifted only.

III. I MPLEMENTATIONS AND PROTOTYPES

Our novel holographic approach has been successfullydemonstrated by prototypes having 20-inch diagonal 1 . Thesecond generation of this direct view holographic display(VISIO 20) comprises a grayscale amplitude-modulating

liquid crystal SLM with a 3

5 megapixel resolution witha pixel pitch of px = 156 m and py = 52m, an oper-ation frequency of 60 Hz with a relatively slow responsetime of 30ms (Fig. 2). The used 1D hologram encoding(here vertical-parallax only) is a common practice to furtherreduce bandwidth requirements and is well-suited for thegiven pixel arrangement and geometry. The optical schemeof the prototype, comprehensively described in [2], is as

1Presented at SID 2007 (Long Beach) and Display 2008 (Tokyo)

Fig. 2. 20-inch direct view prototype (VISIO 20).

follows. Light coming from an RGB-LED backlight is mostlyblocked by a rst LC display which acts as shutter or variablesecondary light source. Only those pixels which are switchedon transmit the light, and thus a variable (secondary) line lightsource is realized having a spatial coherence corresponding tothe pixel opening. Secondary line light sources and arrayedcylindrical lenses (lenticular) are aligned such, that all lightsource images coincide onto the tracked viewing window. Inthe SLM the sum of all complex amplitudes U n (x, y ) isencoded by combining three amplitude-modulating pixel foreach complex value [5]. Two viewing windows deliveringslightly different holographic perspectives of the scene aregenerated by a vertically aligned lenticular beamsplitter and aninterlaced (horizontally multiplexed) hologram. High-precisionuser tracking is realized by a stereo camera incorporatedin the holographic display and advanced eye recognition

algorithms combined with active light source shifting bythe shutter panel. Holographic reconstruction is performedeither in monochrome (optionally R, G, or B with 60 Hz) ortemporally-multiplexed in full-color at a low frame rate of 10 Hz.

IV. C ONCLUSION

In conclusion, a novel approach for real-time holographythat has a strong market potential for displays and TV hasbeen presented. Further development will focus on brightnessand tracking range enhancement as well as full-color recon-struction in real-time.

R EFERENCES

[1] S. A. Benton and V. M. Bove, Holographic Imaging . John Wiley andSons, 2008.

[2] A. Schwerdtner, N. Leister, and R. H aussler, A new approach to electro-holography for TV and projection displays, in SID-Proc. , 2007, 32.3.

[3] A. Schwerdtner, R. H aussler, and N. Leister, A new approach toelectro-holographic displays for large object reconstructions, in OSATechnical Digest on Digital Holography . OSA, 2007, PMA5.

[4] N. R. Smith, D. C. Abeysinghe, J. W. Haus, and J. Heikenfeld, Ag-ile wide-angle beam steering with electrowetting microprisms, Optics Express , vol. 14, pp. 65576563, 2006.

[5] C. B. Burckhardt, A simplication of Lees method of generatingholograms by computer, Appl. Opt. , vol. 9, no. 8, pp. 19491949, 1970.

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