02 May 2014

Laser Projectors. Here's how they work and why they're good - especially for 3D

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Lasers seem ideal for projection: pinpoint sharp and with the purest, most controllable of colours. And yet it's only now that we're seeing them in reality. And they don't work quite the way you might think, either

People have been using lasers to create video displays for quite some time, with the first patents registered in the late 70s in Germany. The field has since been reasonably quiet until the last half-dozen years, probably because a full-colour display – using any of the various available technologies – requires red, green and blue lasers. Red has been reasonably easy for a while, although green mainly relies on DPSS (diode-pumped solid state) devices using frequency-doubled infra-red lasers, and blue, at least as a commercial product in mass production, is not much older than Sony’s blu-ray disc technology. Prior to this, the only type tended to be gas-tube devices of considerable complexity and high power consumption, especially at the sort of power levels required for theatrical-grade projection.

The other problem with historical laser projection has been that it tended to use the perhaps most instinctive approach of scanning the image onto the screen much like a CRT. This creates some interesting problems regarding the high-frequency horizontal scan, which must be produced by a mechanical device. For a PAL standard-definition signal, this implies a mirror producing the horizontal scan moving at something not far from a million revolutions a minute, which is clearly impractical, and workarounds must be applied.

Modern laser projection

Modern  laser projection – characterised specifically as laser-illuminated projection – avoids these issues by using solid-state lasers as an unmodulated light source, relying on established micromirror technology to produce the spatial modulation. The current tech dates back more or less to the consumer electronics show in this very town in 2006, where Novalux showed a prototype display. Barco, Christie and others were all showing upscale theatrical projectors at NAB 2014 where I saw a demonstration by Christie. NEC has an alternative technology involving illuminating a phosphor with blue-emitting lasers, much like a “white” LED, which is suitable for intermediate-sized installations.

Close observers will be raising their hands at this point to ask why all this is actually useful. Xenon and metal halide lamp technology is very well established in projection  and has been for decades, and since the lasers in current projectors are used simply for their ability to emit photons rather than their special abilities as lasers one might reasonably raise the question as to how this all helps. There are also some regulatory hurdles based on the fact that large laser projectors are by their very nature rated high on various international scales for optical hazard, even though their output doesn’t actually consist of a collimated laser beam. The answer to these questions lies in two main regions: first, convenience, and second, 3D.

The convenience benefits are easy to understand. Solid state lasers are fundamentally extremely small LEDs, with an emissive region about the size of a bacterium. This makes for cooling challenges (the Christie laser rack their demonstration was emitting significant, though far from problematic, fan noise). Even so, given the ability to cheaply and easily replace fans, the technology is being touted as significantly longer-lived than traditional approaches. The life of a xenon projector lamp can be 1500 hours. Barco claim that their DP4K-60L laser projector has a life of 30,000 hours to 80% output, meaning that there will be no need to purchase and store expensive spare parts, and also removes the risk of handling fragile and potentially explosive high-pressure lamps. Furthermore, the absolute efficiency of the lasers is higher than the xenon lamp, which was always used for its superb colour rendering more than for its efficiency.

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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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