You may already be familiar with LCD and DLP displays.  Another dynamic video display technology, LCOS (Liquid Crystal on Silicon) is appearing in some very nice televisions. LCOS can produce truly beautiful video that is capable of surpassing either LCD or DLP.

What is LCOS technology?

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 devices typically use three LCOS chips, one each to modulate light in the red, green, and blue channels. Both LCOS and LCD devices deliver the red, green, and blue components of the light to the screen simultaneously. There is no spinning color wheel used as there is in single-chip DLP devices.

LCOS technology is usually very high resolution, and typically higher in price than most LCD and DLP products. Generally LCOS machines begin to appear in the SXGA (1365x1024) resolution class and higher. So by definition they are not cheap.

Nor are LCOS products particularly compact as compared to portable LCD and DLP units.

Many well-informed videophiles seeking the most elegant home theater solutions opt for products using LCOS technology because of its unique blend of performance characteristics that neither LCD nor DLP offer.

The Advantages of LCOS

LCOS products 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 Mustang 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 products deliver continuous red, green and blue simultaneously onto the screen. Single-chip DLPs deliver color sequentially, alternating between red, green, and blue one color at a time. 

The Limitations of LCOS

The primary weakness of LCOS technology is contrast. Currently most LCOS products are rated in the range of 500:1 to 800:1. So they do not have the contrast performance that most DLP products are able to achieve. The use of the new high contrast screen materials helps offset this weakness to some degree. And if there is indirect ambient light in the viewing space, the differences in contrast become much less of an issue.

Variations in LCOS designs

Though LCOS is a generic term, there are several different variations. Not all LCOS implementations are technically the same, and they should not be thought of as identical.