December 2003

LCoS and D-ILA Video Displays: Pros and Cons

Last month I discussed LCD, and before that, DLP. This month we come to a video display technology that perhaps deserves your attention more than either of the others. Why LCoS/D-ILA is not the dominant fixed-pixel display technology is one of the mysteries of the technology market. LCoS/D-ILA devices have clear advantages over other fixed-pixel technologies -- and one drawback. Unfortunately for consumers, LCoS/D-ILA offers the fewest commercial products to choose from, though the ones that exist are impressive.

LCoS stands for Liquid Crystal on Silicon, D-ILA for JVC’s Digital direct drive Image Light Amplifier. There are two names because JVC wants you to think that D-ILA is a unique version of the LCoS technology. But I’m not crazy about JVC’s label, because D-ILA devices do not amplify light in any way -- in fact, you lose some light with the D-ILA process, as you do with any LCoS device. For the purposes of this article, LCoS will stand for D-ILA as well.

Technology

LCD technology relies on a transparent LCD panel: The light source is behind the panel, the projection screen in front, and light passes directly through the panel to get to the display screen. LCoS is different -- the liquid crystal is on top of an opaque silicon chip containing an array of electronic pixel-drive devices. There is a layer of highly reflective material on top of all the control circuits and devices. The liquid-crystal sandwich (liquid crystal between two layers of very thin transparent material with a protective glass or quartz outer surface) mounts on top of the mirrored surface. Light cannot pass through an LCoS chip as it does through a conventional LCD panel. Normally, that would be a serious problem, because the light source and projection screen would have to be in line with each other -- the light source would block the light trying to get to the projection screen.

For example, let’s assume that the LCoS light source is pointed at the ceiling. But you need the image to be on the screen of a rear-projection TV or on the wall. A special 45-degree angled mirror is placed above the light source to reflect light toward the LCoS chip. The light travels through the liquid-crystal layer, reflects off the mirrored surface, and bounces back through the liquid-crystal layer again on its way to the screen. But that pesky 45-degree mirror is in the way. How does the light get through? That’s the trick: the mirror has a specially polarized surface. Light from the LCoS device is polarized at the same 45-degree angle: the light from the LCoS chip can pass right through the 45-degree mirror as if it was not there.

However, the polarized mirror loses some light. There is also some small loss from the light passing twice through the liquid crystal layer, though this is offset by the fact that the two passes help produce better blacks. Some light is attenuated from blacks when the light makes its first pass through the liquid-crystal layer (only when pixels are darkened for black/dark areas of the image, of course). But that same light reflects off the mirrored surface under the liquid-crystal layer, then passes through it a second time, further attenuating the light where there are darker pixels. In essence, LCoS blacks are twice as dark, or half as contaminated with light, as blacks from LCD, DLP, or plasma displays.

Furthermore, because the transistors and supporting components that turn LCoS pixels on and off are below the liquid-crystal layer, they don’t interfere with light passing through the panel, nor do they affect pixel spacing, as in LCD displays. This means that LCoS pixels can be spaced closer together than LCD pixels. As you approach an LCoS image, or increase its projected sized, it will look better than an LCD image because the pixels in the display panel itself are closer together.

Lamp considerations

Because of the slight losses in the light path already discussed, illumination is the major issue with LCoS displays. Typically, their lamps cost as much as LCD lamps ($100-$400 USD).

Black performance

LCoS and D-ILA display devices have produced the best video images I have ever seen from a fixed-pixel display -- only CRT projectors can equal or beat the black levels of a well-designed LCoS product. This will be quite noticeable in night and other darkly lit scenes. Shadows will be far more detailed, and action will be much more easy to follow on a good CRT or LCoS display than on any other currently available technology. Highlight detail in LCoS products is also impressive, perhaps owing to the two passes the light has to make through the LCoS chip.

Contrast ratio

For the same reason lamps are an issue for LCoS displays, contrast ratio is a little more of a challenge. Contrast ratios tend to be in the 600:1 range -- about middling for a display technology. But there are exceptions; one manufacturer of LCoS displays, who makes sets to be sold only under other brand names, claims a 2500:1 contrast ratio, though with no documentation that supports how this is achieved. While a higher contrast ratio is usually better, beautiful images with absolutely no compromise in white/bright levels are easily possible in dark or near-dark home theaters with LCoS consumer products. But the relatively low maximum brightness levels of LCoS displays make them unsuitable for viewing in brightly lit rooms.

Resolution

LCoS consumer displays are the only consumer fixed-pixel displays that can display the highest HDTV resolution, 1920x1080, with no loss of resolution. The largest LCoS panels actually have 2048x1536 pixels. This applies to monochrome LCoS panels. To display color HDTV images you would need three such panels: one each for red, green, and blue light. Single-panel LCoS chips have RGB pixels in groups of three pixels on a single chip. White light is needed to illuminate the color image these LCoS chips produce. However, the resolution of single-panel LCoS chips is limited to 1280x720 -- still fairly impressive for a three-color chip.

Single-chip LCoS displays are now being developed for the consumer market. Three-chip consumer projectors for front-projection and self-contained rear-projection sets are now available in 1920x1080 and 1280x720 progressive resolutions.

Scaling

Like all fixed-pixel devices, an LCoS display needs to scale the various resolutions of video sources to match its own "native" resolution and thus fill the screen. Without scaling, lower-resolution sources would be displayed as smaller images. With scaling, pixels are added to lower-resolution images to fill up the LCoS panel, so that the image remains as large as possible on the video display screen. Of course, this tends to make the scaled-up video sources look a bit soft -- once an image has left the source device, detail can’t be "added" to it. Also, conversion of interlaced sources to progressive-scan images is tricky at best, and requires a lot of processing power.

Other issues

Some have criticized the LCoS technology for using organic compounds that can degrade, and lead to failure of LCoS devices. JVC has written engineering white papers detailing the construction of their D-ILA/LCoS chips, and showing that no organics whatsoever are used in the manufacture of the chips.

JVC has also published documents, available to casual Web searchers, that detail tests from which they extrapolate that the working lives of various materials used in the chips are "over 50 years" or "over 150 years," etc. These materials are then subjected to further abuse that results in findings that components of the chip will last "more than eight years" or "more than 10 years" -- no worse than the materials used in other solid-state display technologies. I have not seen such documentation from other LCoS manufacturers, but that doesn’t mean it doesn’t exist.

 ...Doug Blackburn
db@hometheatersound.com

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