What is LCoS?

by Evan Powell, July 18, 2003

We've talked a lot about two projection technologies lately - LCD and DLP. Most folks who've been studying projectors for a while are up to speed on the differences between them. However, we haven't given a third dynamic video display technology, LCOS (Liquid Crystal on Silicon), the focus it deserves. LCOS can produce truly beautiful video that is capable of surpassing either LCD or DLP. Thus it merits serious attention.

What is LCOS technology?
Well, you could think of it as a hybrid between LCD and DLP. LCD uses liquid crystals, one for each pixel, on glass panels. Light passes through these LCD panels on the way to the lens and is modulated by the liquid crystals as it passes. Thus it is a "transmissive" technology. On the other hand, DLP uses tiny mirrors, one for each pixel, to reflect light. DLP modulates the image by tilting the mirrors either into or away from the lens path. It is therefore a "reflective" technology.

LCOS combines these two ideas. It is a reflective technology that uses liquid crystals instead of individual mirrors. In LCOS, liquid crystals are applied to a reflective mirror substrate. As the liquid crystals open and close, the light is either reflected from the mirror below, or blocked. This modulates the light and creates the image.

LCOS-based projectors typically use three LCOS chips, one each to modulate light in the red, green, and blue channels. In this it is similar to an LCD projector which uses three LCD panels. Both LCOS and LCD projectors deliver the red, green, and blue components of the light to the screen simultaneously. There is no spinning color wheel used in these projectors as there is in single-chip DLP projectors.

The Advantages of LCOS
LCOS projectors have several key advantages over the more popular technologies. First, due partly to inherent high resolution, and partly to high fill factors (minimal space between pixels) on the chips, visible pixelation on an LCOS machine is nonexistent. Even close up the pixel structure is less visible than you get with the high resolution 1280x720 DLP chip. So the resulting video image can be smooth as silk.

Second, with LCOS the pixel edges tend to be smoother compared to the sharp edges of the micro-mirrors in DLP. This gives them an analog-like response, whereas micro-mirrors add high frequencies that accentuate their digital nature. In practical terms, this gives the LCOS image a smoother, more natural look and feel, while DLP tends to impart a synthetic sharpness to the image that some would describe as harsh. (On the other hand, some people prefer the sharper image that DLP delivers. This is a matter of personal taste.)

Third, LCOS and LCD projectors deliver continuous red, green and blue simultaneously onto the screen. Single-chip DLP's deliver color sequentially, alternating between red, green, and blue one color at a time. Though DLP projectors can be capable of delivering rich, well saturated colors, both LCOS and LCD products tend to be superior in this regard. We believe this is due to the way color is managed sequentially in the DLP machines.

Fourth, the absence of a color wheel means there is no chance of you or anyone you invite into your theater being bothered by rainbow artifacts, eye-strain, or headaches that some people can be susceptible to when viewing single-chip DLP projectors. This is normally not a problem for most users of DLP products. And the more expensive high-end DLP systems have higher speed color wheels that further reduce these side effects as well as the percentage of the population that are bothered by them. But the lack of a color wheel in an LCOS projector eliminates the problem entirely.\

Variations in LCOS designs
Though LCOS is a generic term, there are several different variations. The most popular LCOS implementation so far is that from JVC, which the company calls

D-ILA, for Direct Drive Image Light Amplifier.
Not all LCOS implementations are technically the same, and they should not be thought of as identical. This has practical consequences. For example, those familiar with the recent that highlighted a particular failure mode in LCD are aware that LCD panels may eventually degrade over the long run due to a breakdown of organic compounds used in their construction. JVC has made it clear that there are no organic compounds used in the D-ILA technology and therefore this failure mode does not exist with D-ILA. Thus image reliability of their products over the long run is comparable to or exceeds that of DLP. The same cannot be said for every version of LCOS on the market.

Is LCOS right for you?
This is ultimately a matter of personal taste. For the majority of the readers of this article, the real practical question is whether a leading edge product like the D-ILA-based is preferable to the various DLP products that sell in its same general price range.

There is definitely a market for both technologies in the higher end home theater market. Due to the proliferation of Mustang/HD2 DLP machines from at least half a dozen vendors, many people assume that DLP is the unrivaled king of home theater. We agree that they are popular. We would disagree that they are the best solution for everyone. Videophiles who prefer a natural, well-balanced, elegant image over one with particularly high contrast and sharpness would do well to check out the best LCOS machines on the market today.

(Article edited for length & clarity)