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Is There A Laser In Your Future?

Although laser imaging technology has been around for a few years, we haven't seen it in a projection TV application ?until now.

THIS PAST April, Mitsubishi Electronics showed a laser-powered rear-projection TV as part of its 2006 product line show. This demo didn't exactly come out of the blue; Mitsubishi had private meetings at CES 2006 to let selected customers inspect this technological marvel. But the event, held at the Hyatt Huntington Beach Resort in California, was the first public demonstration of a laser projection system for consumer TVs in a long time.

The idea of using lasers to illuminate images isn't new. In theory, it's been possible for a long time — if the lasers are bright enough, and if the lasers and their power supplies can be made small enough.

Problem was, lasers required large power supplies and blue lasers didn't generate sufficient power to be practical. That changed a few years back with the development of blue diode lasers capable of 5 or more watts of power, matching the energy developed in red and green versions.

The Mitsubishi demo used a special type of laser known as a C-SEL, or Cavity Surface Emitting Laser, manufactured by Novalux Corp. Three individual laser emitters (red, green, and blue) are manufactured on a semiconductor surface in precise alignment.

The power ratings of each laser vary, but for a 50-inch microdisplay rear-projection TV the red laser would produce just less than 3 watts at 620 nanometers, the green, 3 watts at 532 nanometers, and the blue, 2.5 watts at 465 nanometers. That would be sufficient to replace a 150-watt UHP short-arc lamp, which might at best last 4,000 to 5,000 hours before reaching half-brightness.

By using laser light, which is coherent (focused) and already polarized, Novalux claims that many optical components in rear- and front-projection systems can be eliminated, such as condensers and light integrators, polarizers and polarizing films, and color wheels and dichroic filters. In theory, laser light could also be used to illuminate LCD flat-panel displays by employing a sequential color scanning system.

Two other potential advantages accrue to lasers: They're instant-on, instant-off devices, and can operate for 50,000 hours or more before reaching half-life. That has obvious appeal to rear-projection TV manufacturers, particularly those companies trying to hold off the onslaught of low-cost plasma and LCD flat-panel TVs.

So it's all positive, right? Not exactly. The price we pay for the tightly focused coherent light from a laser is speckle, a shifting, grain-like optical interference pattern that's the signature of a laser light source. If you've ever been to a laser light show at a planetarium, or seen laser text and images projected, you know what speckle is and how distracting it can be.

To successfully implement a laser light engine, we've got to eliminate as much speckle as possible. It's even more of a problem with rear-projection TVs, as their screens already have a grain-like micro lens structure that creates optical beat frequencies with the ever-shifting speckle from the laser.

Sure enough, the Mitsubishi demo projector had plenty of speckle, particularly when saturated colors such as green were being shown. Several members of the press standing near me at the demo noticed the combination of grain and speckle and commented on how soft the HD images looked, as well as the unearthly shades of red and green solid colors that were seen.

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