Eye-safety analysis of phase-only holographic projection systems Edward Buckley (SID Senior Member) Abstract — Recently, laser-safety analyses have been presented for scanned-beam and LCOS imaging projectors. A third class of projection technology, based on the properties of phase-only diffraction, is differentiated from scanned-beam and LCOS counterparts in its ability to provide a significantly higher effective luminous flux for video style images. In this paper, this desirable property is recognized by the definition of the “video lumen” and a detailed design for a hypothetical holographic projector and corresponding laser safety analysis is presented. As in the case of conventional amplitude-modu- lating LCOS projectors, a holographic projector is capable of delivering several hundred white lumens in Class 2; an appropriately specified holographic projector can also provide several tens of video lumens while maintaining a Class 1 classification. Keywords — Holographic, laser, display, safety. DOI # 10.1889/JSID19.11.723 1 Introduction Laser projectors have received much interest recently and, by 2010, architectures falling into three broad categories had been demonstrated and were commercially avail- able. 1–3 All three are targeted at “pico-projection” applica- tions, which are loosely characterized by battery operation at luminous-flux output values of less than 50 lm, and each technological approach has sought to fulfill the theoretical advantages of smaller form factors, a long depth of field, polarization independence, and potentially higher efficien- cies associated with laser sources. The first, and most mature, architecture employs lasers as illumination sources for a small amplitude-modulating liquid-crystal–on–silicon (LCOS) panel, the resultant field being magnified by conventional relay projection optics. 4 Scanned-beam architectures, on the other hand, employ a rapidly moving silicon micromirror to mechanically deflect a rapidly modulated laser spot across the image. 5 A third and radically different approach utilizes a fast LCOS panel to show sets of phase-only hologram patterns, which are sub- sequently demagnified by projection optics, 6 thereby form- ing images by diffraction rather than projection. As the commercial adoption of laser-based projection technology increases, the issue of laser safety will become important as consumers and OEMS alike seek to under- stand potential safety issues and brightness limitations imposed by the current regulations. The widespread misun- derstanding of safety issues for the burgeoning class of laser- based pico-projectors recently prompted a number of studies to address the question of brightness roadmaps for LCOS and scanned-beam systems with reference to the IEC 60825-1 laser-safety standard. 7,8 These works provided a detailed method for deriving the maximum luminous-flux output for scanned-beam and LCOS projection for Class 1 and Class 2 operation. In this paper, the previous analyses are extended by providing a laser-safety analysis for diffraction-based phase- only holographic projection and the concomitant maximum achievable D65 white-balanced luminous-flux values for Class 1 and Class 2 operation. 2 Principle of operation A holographic display employs a phase-modulating element in combination with a coherent light source to form images by diffraction, rather than projection. This approach removes the need for polarizers and provides high optical efficiency by forming images by routing, rather than blocking, the incident light source. A Fraunhofer (or far-field) holographic display is based on the result that, when a hologram h(u, v) is illumi- nated by coherent collimated light of wavelength λ, the complex field F(x, y) formed in the back focal plane of the lens of focal length f due to Fraunhofer diffraction from the pattern h(u, v) is the two-dimensional spatial Fourier trans- form of the hologram pattern 9 : (1) If the continuous hologram pattern is then replaced by a P × P pixel LCOS microdisplay with pixel size ∆, then the image P × P pixel image F xy formed (or replayed) in the focal plane of the lens is related to the pixellated hologram pattern h uv by the discrete Fourier transform F[⋅], and is written as (2) and it is a standard result 10 that the diffraction angle θ re- sulting from a hologram pattern of pixel size ∆ is Fxy huv j f ux vy dudv (,) ( , )exp ( ) . = + R S T U V W -• • -• • z z 2 p l F Fh h j P ux vy xy uv uv v P u P = = + R S T U V W = - = - Â Â exp ( ) 2 0 1 0 1 p Received 05-05-11; accepted 07-05-11. The author is with Pixtronix, 300 Burtt Rd., Andover, MA 01810 USA; telephone 1+719-271-3939, e-mail: ebuckley@pixtronix.com. © Copyright 2011 Society for Information Display 1071-0922/11/1911-0723$1.00. Journal of the SID 19/11, 2011 723