Holographic Laser Projection Technology


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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. from Light Blue Optics (LBS)

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Holographic Laser Projection Technology

  1. 1. holographic projection Holographic 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 Buckley T HE CONVENTIONAL imaging projec- tor is a near-ubiquitous device in today’s also apply because – as in the CE space – conventional projection systems are generally scanned-beam display technologies, has the unique ability to simultaneously fulfill the key offices and is also commonly found in cine- unsuitable for integration. For example, in OEM requirements outlined above. mas and homes. Despite the capabilities of addition to the high brightness and contrast ratios such projection systems, technical limitations required for HUD applications1 and extremely Holographic Laser Projection in terms of miniaturization and power effi- tight space constraints for rear-projection LBO’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 available electronics (CE) and automotive applications. and optically efficient while maintaining wide projection technologies, this projection engine The 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 by As a result, a number of LED- or laser- sional diffraction to form video images. the advent of mobile TV. It is clear that the based microprojector technologies are now A typical imaging projection system works high-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 manufacturers based imaging projection technology is unable nated by red, green, and blue light to form alike have long since realized, such a display to simultaneously satisfy the key requirements color. In this case, the microdisplay simply format is inconvenient for sharing content of small physical size, low cost, low power acts to selectively block (or amplitude modu- with multiple viewers. These restrictions consumption, and a robust implementation. late) the incident light; after passing through could be solved by employing a battery- In addition, it is clear from discussions with magnification optics, the projected image Fxy powered projection accessory or embedded automotive and CE customers that there are appears. Conversely, holographic laser pro- projection device to display the content. several additional, and perhaps more restric- jection forms the image Fxy by illuminating a Applications such as reconfigurable instrument tive, requirements that also must be fulfilled diffraction (or hologram) pattern huv by laser clusters and head-up displays (HUDs) in the for the commercialization and acceptance of light with a wavelength of λ. If the hologram automotive sector are also creating demand for such miniature projection displays: pattern is represented by a display element miniature projection systems. Whilst there are with pixel size ∆, then the image Fxy formed in • High resolution clear benefits in using projectors for both the focal plane of the lens is related to the • High brightness applications, similar barriers to integration pixellated hologram pattern huv by the discrete • Low speckle Fourier transform F [·] and is written as • Eye safe Edward Buckley is VP of Business Develop- • Large depth of focus and wide projection angle. Fxy = F [huv] (1) ment at Light Blue Optics, 4775 Centennial Blvd., Suite 103, Colorado Springs, CO 80919, Light Blue Optics (LBO) has been develop- as shown in Fig. 1. USA; 719/623-1208, e-mail: edward@ ing a unique holographic laser projection tech- The key task in a holographic projection lightblueoptics.com. nology since 2004 that, unlike imaging and system is to compute the hologram huv; a 22 Information Display 12/08 0362-0972/12/2008-022$1.00 + .00 © SID 2008
  2. 2. rial in existence that can independently and which is noise-free, as illustrated in Fig. 2. continuously modulate both amplitude Auv Uniquely, the key to this holographic laser- and phase ϕuv where huv = Auvexpjϕuv. Even projection technology lies not in the optical if such a material became available, the result design but in the algorithms used to calculate contains amplitude components that would the phase hologram huv from the desired absorb incident light and reduce system effi- image Fxy. LBO has developed and patented ciency. A much better approach is to restrict proprietary algorithms for the purposes of the hologram huv to a set of phase-only values calculating N sets of holograms per video ϕuv such that huv = expjϕuv. As a result, when frame, both efficiently and in real time, as first the hologram patterns are displayed on a demonstrated in 2004.4 Crucially, such algo- phase-modulating microdisplay and subse- rithms can be efficiently implemented in a quently illuminated, no light is blocked. custom silicon chip. LBO’s system uses a custom-manufactured A practical realization of a holographic ferroelectric-liquid-crystal–on–silicon laser projector is rather simple and is shown in (LCoS) microdisplay from Displaytech, Inc., the schematic of Fig. 3. A desired image is to display the hologram patterns, which converted into sets of holograms by LBO’s requires that the hologram phase ϕuv is quan- proprietary algorithms and displayed on a tized to a set of binary values. This proce- phase-modulating microdisplay that is time- dure inevitably introduces quantization noise sequentially illuminated by red, green, and into the resultant image Fxy, which must be blue laser light, respectively. The subsequent mitigated in order to maintain high image diffraction pattern passes through a lens pair Fig. 1: The relationship between hologram quality. Thus, the microdisplay is used to L1 and L2, which is chosen to provide an ultra- huv and image Fxy present at the back focal display N independent holograms per video wide projection angle in excess of 100°. As a plane of a lens of focal length f, when illumi- frame within a temporal bandwidth of the eye result of the phase-modulating microdisplay, nated by coherent monochromatic light of of 40 msec, each of which produces a sub- the incident light is steered into the desired wavelength λ . frame Fxy exhibiting statistically independent image pixels – without blocking – and, due to quantization noise.3 If the intensity of the ith Fourier optics, the image remains in focus at reasonable first guess might be to calculate displayed image is I = Fxy 2, then the time- all distances from the lens L2. the inverse Fourier transform of the image Fxy averaged percept over N sub-frames is to obtain the desired result. However, the Advantages of Holographic Laser Projection 1 N ( 2 result of this calculation would be fully com- Vxy = ∑ Fxyi ) N i =1 (2) Low Speckle Contrast: One of the huge plex, and there is no liquid-crystal (LC) mate- advantages of LBO’s technology is the ability Hologram Subframe Frame 1 N ( 2 huv = exp jϕuv Fxy = F [huv] Vxy = ∑ Fxyi ) N i =1 Fig. 2: The relationship between hologram huv, sub-frame Fxy, and frame Vxy in LBO’s holographic projection technology. Information Display 12/08 23
  3. 3. holographic projection methods 11-13 have been demonstrated to remove A demonstration of the efficacy of LBO’s speckle, but each require that the operation is combined speckle reduction techniques is performed multiple times per laser dwell period. shown in Fig. 4; note that laser speckle is This is straightforward in LBO’s system substantially reduced in the projected image, because the laser modulation frequency is low without significant loss of focal depth or and all pixels are formed simultaneously. In resolution. addition, the intermediate image plane formed High Brightness and Efficiency: It has between L1 and L2 in Fig. 3 makes it simple to previously been shown15 that, due to the embed a speckle-reduction mechanism in the phase-modulating approach to image forma- projector optics. The high laser modulation tion, a holographic display can project signifi- frequency and lack of an image plane make cantly brighter images than imaging and time-varying speckle reduction techniques dif- scanned-beam systems when displaying video ficult to implement in a scanned-beam system, and photo content. In addition, because the as experts in the field have noted.14 These image pixels are formed using an expanded restrictions dictate that speckle in a scanned- beam which has an extremely wide projection beam projector can only be acceptably sup- angle, it is possible to make a holographic pressed by using potentially costly custom laser projection system much brighter than a diffusing screens, thereby limiting the utility scanned-beam display for the equivalent laser of 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” due to scattering of coherent light from an opti- cally rough projection surface and subsequent interference at the retina. The ability to reduce speckle is important because, not only do users find the artifact very unpleasant, it also severely impacts the perceived image quality and effective resolution. Although there have been several demon- (a) (b) strations of speckle reduction in the literature,5-10 thus 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 N bination of speckle reduction techniques. The close-ups (c) and (d) clearly show substantial cannot be increased indefinitely. Several speckle reduction. 24 Information Display 12/08
  4. 4. 6 The same combination of laser and phase- in fact, each pixel on the microdisplay con- L. Wang, T. Tschudi, T. Halldorsson, and modulating hologram provides a highly tributes to every pixel in the image, making P. R. Petursson, “Speckle reduction in laser power-efficient method of projection since, the system tolerant to microdisplay defects. projection systems using diffractive optical unlike imaging displays, no light is blocked in It is generally accepted that microdisplay- elements,” Appl. Opt. 37, 1770–1775 (1998). 7 the system. Furthermore, the phase-modulating based technologies, whether LCoS or DLP- J. I. Trisnadi, “Speckle contrast reduction in nature of the microdisplay means that it is not based, have the capability to offer higher laser projection displays,” Technical Report necessary to continuously illuminate the micro- image qualities than scanned-beam systems.17 (2003). 8 display; the lasers are modulated in accordance Scanned-beam systems tend to suffer from J. I. Trisnadi, “Hadamard speckle contrast with the frame brightness, thereby only utiliz- poor image quality due to the unacceptably reduction,” 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 the 9 A. 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 expected It 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 surface systems. This is principally due to the low due to their narrow spectral bandwidth; the emitting diode lasers and arrays and their frequency at which the lasers are modulated in Helmoltz-Kohlrausch effect can also increase applications,” Micro-Optics Conference LBO’s system; since gray scale is formed by perceived brightness due to the psychophysical (Tokyo) (2005). 10 the hologram and because the phase-modulat- effects of highly saturated primaries. RGB LED B. Dingel, S. Kawata, and S. Minami, ing microdisplay directs a fixed proportion of systems exhibit an acceptable color space for “Speckle reduction with virtual incoherent light onto the image, it is only necessary to CE applications, although systems that use laser illumination using a modified fiber modulate the laser sources with respect to white LEDs have significantly reduced array,” Optik 94, 132–136 (1993). 11 average video-frame brightness. This neces- gamuts due to the strong absorption of the S. Lowenthal and D. Joyeux, “Speckle sitates a laser modulation frequency on the microdisplay 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). 12 This is significantly more efficient than the LBO’s holographic laser projection technology Y. Imai and Y. Ohtsuka, “Laser speckle method typically employed by scanned-beam represents a revolutionary approach to the pro- reduction by ultrasonic modulation,” Opt. projectors, where the pixel-by-pixel method jection and display of information, exploiting the Commun. 27, 18–22 (1978). 13 of image formation requires the lasers to be physical process of two-dimensional diffraction L. Wang, T. Tschudi, T. Halldorsson, and switched at high currents and frequencies of to 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 in compared to competing LED- and laser-based No. 6, 1659–1664 (2000). 14 green-laser switching speed also necessitate miniature projection systems, and the ability of J. 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. 15 above and below threshold to render gray scale. ments outlined by CE and automotive customers E. Buckley, A. Corbett, P. Surman, and High Resolution, High Image Quality, and will allow LBO to bring this truly disruptive I. Sexton, “Multi-viewer autostereoscopic Wide Color Gamut: In imaging systems, it is display technology to market in 2009. display with dynamically addressable holo- difficult to achieve high resolutions while graphic backlight,” SID Symposium Digest 39, maintaining an acceptable form factor because References 340–344 (2008). 1 16 field breakdown, diffractive effects, and M. Moell, “Color HUD for Automotive E. Buckley, A. Cable, T. Wilkinson, and étendue-matching considerations set the mini- Applications,” Technical Report, Troy, MI. N. Lawrence, “Viewing angle enhancement 2 mum pixel size of the microdisplay.16 The E. Buckley, “Color holographic laser projec- for two- and three-dimensional holographic resolution of scanned-beam systems, on the tion technology for heads-up and instrument displays using random super resolution phase other hand, is principally limited by the cluster displays,” Proc. 14th Annual Vehicle masks,” Appl. Opt. 45, No. 28, 7334–7341 achievable laser modulation frequency. Displays Symposium (2007). (2006). 3 17 In LBO’s system, the resolution of the E. Buckley, “Holographic laser projection Insight Media LLC, “Laser Projection image is decoupled from that of the micro- technology,” SID Symposium Digest 39, Systems 2007. 18 display and is controlled largely by the holo- 1074–1078 (2008). M. Handschy, “Moves toward mobile pro- 4 gram computation algorithm; this allows the A. J. Cable, E. Buckley, P. Mash, N. A. Lawrence, jectors raise issue of panel choice,” Display resolution 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 mm time binary hologram generation for high- 19 M. Schmitt and U. Steegmuller, “Green active-area microdisplay. Furthermore, the quality video projection applications,” SID laser 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). 5 20 spondence between microdisplay pixels and D. Gabor, “Laser speckle and its elimination,” Insight Media LLC, “Large Display Report,” projected image pixels as imaging systems do; IBM J. Res. Dev., 509–514 (1970). pp. 71–72 (June 2008). I Information Display 12/08 25