Front Projection 101

Basic Anatomy

At the heart of all video projectors is an imaging device. One such device is the liquid crystal display (LCD), or its relative LCoS (liquid crystal on silicon), which also comes in two other varieties—D-ILA (Direct-drive Image Light Amplification, a JVC technology) and SXRD (Silicon X-tal Reflective Device, from Sony). Another common imaging device is the DLP (Digital Light Processing) chip from Texas Instruments. LCD is a “transmissive” technique, in which light shines though the LCD as it does through film. The DLP chip, LCoS, D-ILA, and SXRD are reflective devices, although the latter three are otherwise similar to LCD in operation. All use transistors to operate liquid crystal cells, which are either opaque or translucent depending on their electrical state. LCDs have drive transistors next to their crystal cells, and LCoS devices have them behind the cells, enabling closer packing of these imaging elements.

DLP chips are a completely different technology. These devices are festooned with hundreds of thousands of “digital micromirror devices” (DMDs), actually tiny mirrors on stalks, like miniature sunflowers rooted in a silicon substrate, each of which can be turned several degrees. (Think of Boy Scouts signaling with handheld mirrors.) The duration of the angle held by each DMD determines how bright it appears. Video processing circuitry converts incoming video signals into control signals to operate imaging chips. Like a newspaper photograph made up of thousands of dots, the video image is the total composite of all the DMDs on the chip. A bright lamp reflects off the chip, through a rotating color wheel, and then through a lens onto a screen.

A revival of an idea that was toyed with during color experiments in the early days of TV, color wheels are the most primitive part of DLP technology—sequentially “gating” color bursts to the screen. Cheaper DLP projectors use four-segment color wheels running at relatively low speed, which cause color breakups, rainbow artifacts, or other departures from fidelity visible to some viewers. Some people are susceptible to headaches or nausea induced by color wheels. Manufacturers continue to experiment with colorwheel designs—six- and eight-segment wheels, for example— as well as with higher rotational speeds. Just as higher frame rates look more believable, higher-speed color wheels are said to generate more realistic color, and fewer complaints.

The combination of lamp, imaging device, mirrors, and lenses is called a projector’s “light engine.” Well-designed projectors don’t let light leak out of the housing. Projector lamps get extremely hot and require fans and air vents to prevent housing meltdown. The cooling system, in turn, generates acoustic noise that must be taken into account when planning where the projector will be mounted. A noisy projector installed in an adjacent room and shining through a hole in the wall is no problem, but would be annoying, indeed, sitting on a coffee table in front of you. Some projectors are very quiet.

Cool It

All projectors generate heat and need external ventilation. Don’t jam your projector into a tight spot on a bookshelf because that’s where it looks best. Doing so risks shortening the projector’s lifespan or possibly causing a fire. Keep your projector out in the open air.

Through the Looking Glass

The lens is a major consideration and major cost in making a projector. Like cameras, inexpensive projectors tend to use polycarbonate lenses, and better ones use opticalquality glass. Better ones have larger lenses with lower light loss and a wider range of adjustment. Some manufacturers make more than one lens for individual models, asking customers to specify whether they want a “short-throw” lens or its “long-throw” equivalent. Cheap lenses often lose sharpness toward the edge of the image, an artifact usually noted only by experts.

Pixel Count and Aspect Ratio

Digital images are composed of discrete picture elements (“pixels”), and digital imaging devices—monitors, image sensors, and projectors—are specified by the number of pixels available to generate an image. A pixel count specification might say “1280x720,” meaning 1280 horizontal pixels by 720 vertical pixels. The column count often corresponds to a projector’s “native scan rate”; a 1280x720 projector is optimized for 720p high-definition video. Video projectors with a 480-column count are likely optimized for 480i/480p outputs from DVD players. An 854x480 pixel count is a common spec among entry-level projectors; 1280x720 is common in higher price niches. JVC’s $225,000 DLA-QX1G professional video projector boasts an astounding 2048x1536 pixel count. Despite the advent of widescreen high-definition TV and the film industry’s decades-long adherence to widescreen formats, many projectors have a native aspect ratio (ratio of width to height) of 4:3, the same as the squarish standard of legacy video. Projecting a 16:9 image from a 4:3 imaging device means that much of its light-generating capability and vertical resolution goes unused. Likewise, a superwidescreen film shot in 2.35:1 won’t use the full pixel count of a 16:9 video projector. HDTV’s 16:9 aspect ratio (1.78:1) is very close to the film standard of 1.85:1.