Holographic Laser Projection Technology

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holographic projectionHolographic Laser Projection Technology Conventional lamp-based imaging projection technology is unable to simultaneously satisfy the demands from consumer-electronics manufacturers for projectors that are small, low in cost, con-sume little power, and offer a robust implementation, all while providing high-quality images. A number of LED- or laser-based microprojector technologies are now being developed to address these issues. Here, one manufacturer details its approach – a unique holographic laser projection technology that offers advantages over imaging and scanned-beam display technologies.by Edward BuckleyTHE CONVENTIONAL imaging projec- also apply because – as in the CE space – scanned-beam display technologies, has thetor is a near-ubiquitous device in today’sconventional projection systems are generallyunique ability to simultaneously fulfill the keyoffices and is also commonly found in cine-unsuitable for integration. For example, in OEM requirements outlined above.mas and homes. Despite the capabilities ofaddition to the high brightness and contrast ratios such projection systems, technical limitationsrequired for HUD applications1 and extremelyHolographic Laser Projection in terms of miniaturization and power effi-tight space constraints for rear-projectionLBO’s technology represents a revolutionary ciency currently prevent the widespread adop-instrument cluster displays,2 the entire projec-approach to the projection and display of infor-tion of projection subsystems into consumer-tion subsystem must be robust, fault tolerant,mation. Unlike other commercially availableelectronics (CE) and automotive applications.and optically efficient while maintaining wideprojection technologies, this projection engineThe need for small, power-efficient projec-operating and storage temperature ranges.exploits the physical process of two-dimen-tors in the CE space is aptly demonstrated byAs a result, a number of LED- or laser-sional diffraction to form video images.the advent of mobile TV. It is clear that thebased microprojector technologies are nowA typical imaging projection system workshigh-resolution content available is incompati-being developed to address these issues and by displaying a desired image Fxy on a micro-ble with a typical cell-phone display of 2.5-in.to overcome the fact that conventional lamp-display, which is usually sequentially illumi-diagonal and, as consumers and manufacturersbased imaging projection technology is unablenated by red, green, and blue light to formalike have long since realized, such a displayto simultaneously satisfy the key requirementscolor. In this case, the microdisplay simplyformat is inconvenient for sharing contentof small physical size, low cost, low poweracts to selectively block (or amplitude modu-with multiple viewers. These restrictionsconsumption, and a robust implementation. late) the incident light; after passing throughcould be solved by employing a battery-In addition, it is clear from discussions withmagnification optics, the projected image Fxypowered projection accessory or embedded automotive and CE customers that there areappears. Conversely, holographic laser pro-projection device to display the content.several additional, and perhaps more restric-jection forms the image Fxy by illuminating aApplications such as reconfigurable instrument tive, requirements that also must be fulfilleddiffraction (or hologram) pattern huv by laserclusters and head-up displays (HUDs) in the for the commercialization and acceptance oflight with a wavelength of λ. If the hologramautomotive sector are also creating demand for such miniature projection displays:pattern is represented by a display elementminiature projection systems. Whilst there are with pixel size ∆, then the image F• High resolution xy formed inclear benefits in using projectors for boththe focal plane of the lens is related to the• High brightnessapplications, similar barriers to integrationpixellated hologram pattern h• Low speckleuv by the discreteFourier transform F [·] and is written as• Eye safeEdward Buckley is VP of Business Develop-• Large depth of focus and wide projection angle.Fxy = F [huv](1)ment at Light Blue Optics, 4775 CentennialBlvd., Suite 103, Colorado Springs, CO 80919,Light Blue Optics (LBO) has been develop-as shown in Fig. 1.USA; 719/623-1208, e-mail: [email protected] a unique holographic laser projection tech-The key task in a holographic projectionlightblueoptics.com.nology since 2004 that, unlike imaging andsystem is to compute the hologram huv; a 22Information Display 12/080362-0972/12/2008-022$1.00 + .00 © SID 2008rial in existence that can independently andwhich is noise-free, as illustrated in Fig. 2.continuously modulate both amplitude AuvUniquely, the key to this holographic laser-and phase ϕuv where huv = Auvexpjϕuv. Even projection technology lies not in the opticalif such a material became available, the resultdesign but in the algorithms used to calculatecontains amplitude components that wouldthe phase hologram huv from the desiredabsorb incident light and reduce system effi-image Fxy. LBO has developed and patentedciency. A much better approach is to restrictproprietary algorithms for the purposes of the hologram huv to a set of phase-only valuescalculating N sets of holograms per videoϕuv such that huv = expjϕuv. As a result, whenframe, both efficiently and in real time, as firstthe hologram patterns are displayed on ademonstrated in 2004.4 Crucially, such algo-phase-modulating microdisplay and subse-rithms can be efficiently implemented in aquently illuminated, no light is blocked.custom silicon chip.LBO’s system uses a custom-manufacturedA practical realization of a holographicferroelectric-liquid-crystal–on–siliconlaser projector is rather simple and is shown in(LCoS) microdisplay from Displaytech, Inc.,the schematic of Fig. 3. A desired image isto display the hologram patterns, whichconverted into sets of holograms by LBO’srequires that the hologram phase ϕuv is quan-proprietary algorithms and displayed on atized to a set of binary values. This proce-phase-modulating microdisplay that is time-dure inevitably introduces quantization noisesequentially illuminated by red, green, andinto the resultant image Fxy, which must beblue laser light, respectively. The subsequentmitigated in order to maintain high imagediffraction pattern passes through a lens pairFig. 1: The relationship between hologramquality. Thus, the microdisplay is used toL1 and L2, which is chosen to provide an ultra-huv and image Fxy present at the back focaldisplay N independent holograms per videowide projection angle in excess of 100°. As aplane of a lens of focal length f, when illumi-frame within a temporal bandwidth of the eyeresult of the phase-modulating microdisplay,nated by coherent monochromatic light ofof 40 msec, each of which produces a sub-the incident light is steered into the desiredwavelength λ.frame Fxy exhibiting statistically independentimage pixels – without blocking – and, due toquantization noise.3 If the intensity of the ithFourier optics, the image remains in focus atreasonable first guess might be to calculatedisplayed image is I = F 2xy , then the time-all distances from the lens L2.the inverse Fourier transform of the image Fxyaveraged percept over N sub-frames isto obtain the desired result. However, theAdvantages of Holographic Laser Projection Nresult of this calculation would be fully com-12VF(i=∑)xyxy(2)Low Speckle Contrast: One of the hugeplex, and there is no liquid-crystal (LC) mate-N i=1advantages of LBO’s technology is the abilityHologramSubframeFrameN12h(i)uv = exp jϕuvFxy = F [huv]V =F∑xyNxyi=1Fig. 2: The relationship between hologram huv, sub-frame Fxy, and frame Vxy in LBO’s holographic projection technology.Information Display 12/0823holographic projectionIF MWITH2nd Cmethods have been demonstrated to remove 11-13A demonstration of the efficacy of LBO’s speckle, but each require that the operation is combined speckle reduction techniques isperformed multiple times per laser dwell period. shown in Fig. 4; note that laser speckle isThis is straightforward in LBO’s systemsubstantially reduced in the projected image,because the laser modulation frequency is lowwithout significant loss of focal depth or and all pixels are formed simultaneously. Inresolution.addition, the intermediate image plane formedHigh Brightness and Efficiency: It hasbetween L1 and L2 in Fig. 3 makes it simple topreviously been shown15 that, due to theembed a speckle-reduction mechanism in thephase-modulating approach to image forma-projector optics. The high laser modulationtion, a holographic display can project signifi-frequency and lack of an image plane makecantly brighter images than imaging andtime-varying speckle reduction techniques dif-scanned-beam systems when displaying videoficult to implement in a scanned-beam system,and photo content. In addition, because the as experts in the field have noted.14 Theseimage pixels are formed using an expandedrestrictions dictate that speckle in a scanned-beam which has an extremely wide projectionbeam projector can only be acceptably sup-angle, it is possible to make a holographicpressed by using potentially costly custom laser projection system much brighter than adiffusing screens, thereby limiting the utilityscanned-beam display for the equivalent laserof such systems in CE applications.safety classification.Fig. 3: A schematic diagram of LBO’s pro-jection technology.to substantially reduce laser speckle, a phe-nomenon that makes the image “sparkle” dueto scattering of coherent light from an opti-cally rough projection surface and subsequentinterference at the retina. The ability toreduce speckle is important because, not onlydo users find the artifact very unpleasant, italso severely impacts the perceived imagequality and effective resolution.Although there have been several demon-(a)(b)strations of speckle reduction in the literature,5-10thus far only LBO has demonstrated the possi-bility for speckle reduction within the projec-tion optics of a miniature laser projector.As has been shown previously,3 several methods can be combined in the LBO projector in order to reduce speckle. The first results from the method of image generation and display used in the LBO system. Since N phase-inde-pendent sub-frames per video frame are shown within the eye’s integration period, then the eye acts to add N independent speckle patterns on an intensity basis, and the contrast of the low-frequency components of the speckle in the field Vxy falls as N1/2. Evidently, some speckle reduc-tion is inherent in LBO’s multiple holograms(c)(d)per video frame approach to image display.Additional methods can be combined to Fig. 4: A sample image from LBO’s projector (a) without speckle removal and (b) using a com-further reduce the speckle contrast because Nbination of speckle reduction techniques. The close-ups (c) and (d) clearly show substantialcannot be increased indefinitely. Several speckle reduction.24Information Display 12/08The same combination of laser and phase-in fact, each pixel on the microdisplay con-6 L. Wang, T. Tschudi, T. Halldorsson, and modulating hologram provides a highlytributes to every pixel in the image, making P. R. Petursson, “Speckle reduction in laserpower-efficient method of projection since,the system tolerant to microdisplay defects.projection systems using diffractive opticalunlike imaging displays, no light is blocked inIt is generally accepted that microdisplay-elements,” Appl. Opt. 37, 1770–1775 (1998).the system. Furthermore, the phase-modulating based technologies, whether LCoS or DLP-7J. I. Trisnadi, “Speckle contrast reduction innature of the microdisplay means that it is not based, have the capability to offer higherlaser projection displays,” Technical Reportnecessary to continuously illuminate the micro-image qualities than scanned-beam systems.17(2003).display; the lasers are modulated in accordance Scanned-beam systems tend to suffer from8J. I. Trisnadi, “Hadamard speckle contrastwith the frame brightness, thereby only utiliz-poor image quality due to the unacceptablyreduction,” Opt. Lett. 29, No. 1, 11–13 ing the power required to illuminate “on” pixels high speckle contrast ratios,18,19 scanning (2004).in the same way as a scanned-beam system.artifacts,20 and image distortion, making the9A. Mooradian, S. Antikichev, B. Cantos, Although LBO’s projection technology use of such a technology unfeasible in appli-G. Carey, M. Jansen, S. Hallstein, W. Hitchens,carries the overhead of hologram computa-cations where high image quality is required.D. Lee, J. M. Pelaprat, R. Nabiev, G. Niven,tion, the overall system efficiency is expectedIt is well known that laser sources can pro-A. Shchegrov, A. Umbrasas, and J. Watson,to be comparable to that of scanned-beam vide images with extremely wide color gamuts, “High power extended vertical cavity surfacesystems. This is principally due to the lowdue to their narrow spectral bandwidth; theemitting diode lasers and arrays and theirfrequency at which the lasers are modulated inHelmoltz-Kohlrausch effect can also increase applications,” Micro-Optics ConferenceLBO’s system; since gray scale is formed byperceived brightness due to the psychophysical(Tokyo) (2005).the hologram and because the phase-modulat-effects of highly saturated primaries. RGB LED 10B. Dingel, S. Kawata, and S. Minami,ing microdisplay directs a fixed proportion ofsystems exhibit an acceptable color space for“Speckle reduction with virtual incoherentlight onto the image, it is only necessary toCE applications, although systems that uselaser illumination using a modified fibermodulate the laser sources with respect towhite LEDs have significantly reducedarray,” Optik 94, 132–136 (1993).average video-frame brightness. This neces-gamuts due to the strong absorption of the11S. Lowenthal and D. Joyeux, “Specklesitates a laser modulation frequency on themicrodisplay color filters at red wavelengths.removal by a slowly moving diffuser associ-order of 1 kHz, allowing efficient digital ated with a motionless diffuser,” J. Opt. Soc.modulation schemes to be used for each color.Conclusion Am. 61, 847–851 (1971).This is significantly more efficient than theLBO’s holographic laser projection technology 12Y. Imai and Y. Ohtsuka, “Laser specklemethod typically employed by scanned-beamrepresents a revolutionary approach to the pro-reduction by ultrasonic modulation,” Opt.projectors, where the pixel-by-pixel method jection and display of information, exploiting theCommun. 27, 18–22 (1978).of image formation requires the lasers to bephysical process of two-dimensional diffraction 13L. Wang, T. Tschudi, T. Halldorsson, and switched at high currents and frequencies ofto form video images. Such an approach simul-P. Petursson, “Speckle reduction in laser pro-tens of MHz using complex and power-taneously provides many compelling benefits jections with ultrasonic waves,” Opt. Eng. 39,hungry circuitry. Furthermore, limitations incompared to competing LED- and laser-based No. 6, 1659–1664 (2000).green-laser switching speed also necessitateminiature projection systems, and the ability of 14J. W. Goodman, Speckle Phenomena in Optics: that the laser is inefficiently analog modulated this technology to satisfy the stringent require-Theory and Applications (2008), p. 223.above and below threshold to render gray scale.ments outlined by CE and automotive customers15E. Buckley, A. Corbett, P. Surman, and High Resolution, High Image Quality, andwill allow LBO to bring this truly disruptive I. Sexton, “Multi-viewer autostereoscopic Wide Color Gamut: In imaging systems, it isdisplay technology to market in 2009.display with dynamically addressable holo-difficult to achieve high resolutions whilegraphic backlight,” SID Symposium Digest 39,maintaining an acceptable form factor becauseReferences340–344 (2008).field breakdown, diffractive effects, and 1M. Moell, “Color HUD for Automotive16E. Buckley, A. Cable, T. Wilkinson, and étendue-matching considerations set the mini-Applications,” Technical Report, Troy, MI.N. Lawrence, “Viewing angle enhancementmum pixel size of the microdisplay.16 The2E. Buckley, “Color holographic laser projec-for two- and three-dimensional holographicresolution of scanned-beam systems, on thetion technology for heads-up and instrumentdisplays using random super resolution phaseother hand, is principally limited by thecluster displays,” Proc. 14th Annual Vehiclemasks,” Appl. Opt. 45, No. 28, 7334–7341achievable laser modulation frequency.Displays Symposium (2007).(2006).In LBO’s system, the resolution of the3E. Buckley, “Holographic laser projection 17Insight Media LLC, “Laser Projection image is decoupled from that of the micro-technology,” SID Symposium Digest 39,Systems 2007. display and is controlled largely by the holo-1074–1078 (2008).18M. Handschy, “Moves toward mobile pro-gram computation algorithm; this allows the4A. J. Cable, E. Buckley, P. Mash, N. A. Lawrence, jectors raise issue of panel choice,” Displayresolution to be fully variable up to a maxi-T. D. Wilkinson, and W. A. Crossland, “Real-Devices, 6–8 (2007).mum of WVGA using just a 7 mm × 7 mmtime binary hologram generation for high-19M. Schmitt and U. Steegmuller, “Greenactive-area microdisplay. Furthermore, thequality video projection applications,” SIDlaser meets mobile projection requirements,”holographic system does not have a 1:1 corre-Symposium Digest 35, 1431–1433 (2004).Optics and Laser Europe, 17–19 (2008).spondence between microdisplay pixels and5D. Gabor, “Laser speckle and its elimination,” 20Insight Media LLC, “Large Display Report,” projected image pixels as imaging systems do;IBM J. Res. Dev., 509–514 (1970).pp. 71–72 (June 2008). IInformation Display 12/0825