Angles Of View
Keith Blackey is Vice President of Marketing for Light Valve Projectors at the AmPro Corporation in Mountain View, California. He has been directly involved with the design and development of light valve projectors for the past 25 years. A co-founder of one of the technology’s progenitors, Greyhawk Systems, Inc. Mr. Blackey can be reached at firstname.lastname@example.org. He is interviewed here on the subject of
The Light Valve - Through a Glass, BrightlyDa-Lite: Let’s start with an historical perspective. How long have people been working with light valves?
Blackey: There are actually two trunks to the light valve tree. In the early 70's work was started on reflective light valve projectors by Hughes Electronics and, more or less simultaneously, AT&T Bell Labs. I was a part of the latter group.
Da-Lite: And what is meant by the phrase reflective light valve”?
Blackey: The reflector was a mirror; and in front of the mirror was a film of liquid crystal that was blocking the mirror. So if you were to shine light at the mirror, there was no reflection because the liquid crystal film blocked it from reaching the mirror. The only way to get the light to pass through the liquid crystal was to open up the Venetian blinds (so to speak).
Da-Lite: How did you do that?
Blackey: The way we got the crystals to open or rotate in those days was to put a small electrical charge across the molecules of the crystals and, because their film was extremely viscous (almost like molasses), they wouldn’t move until we drew on them with a hot laser beam. And whichever molecules were touched by this hot laser beam would warm up enough to be snapped open because of the electric charge and now light could at those points pass through and be reflected back out again by the mirror beneath them. So whatever design the laser beam wrote into the liquid crystal film was immediately reflected.
Da-Lite: Did the laser scan like the raster in a CRT or was it calligraphic, writing just in vectors?
Blackey: We worked with both kinds actually. In each case, however, the liquid crystals had some storage ability in the sense that as the beam moved on, the crystals it had warmed would re-freeze opened. Thus you’d never have to go back and refresh them.
So those early projectors were extremely high resolution because the laser had a very fine tip but they couldn’t refresh.
Da-Lite: Given that limitation, what was the market for these devices like?
Blackey: It was larger than you might suppose. People used them for displaying things like satellite imagery or other, relatively permanent pictures. Incidentally, when I say you couldn’t refresh them I really mean that you couldn’t get them to rewrite in less than about 30 seconds.
But to give you some idea of the available resolution, we had projectors that could display 10,000 pixels by 10,000 pixels.
Da-Lite: Can light valves do that today?
Blackey: Conceivably, yes. The difference in today’s projectors is that they no longer employ lasers. Instead they use CRTs to write the image.
Da-Lite: : And what brought about that change?
Blackey: Well, it was a change for my team only. Hughes, the other trunk of the tree that I mentioned, had always used CRTs. But when my group failed at getting the laser to write fast enough to get an acceptable refresh rate, we converted over to CRTs as well.
Da-Lite: So how does the modern light valve projector work?
Blackey: First off, I’d like to say that in my opinion light valve projectors are not a projection technique. There are only two projection techniques on the market. There’s CRT. And there’s LCD.
All light valves are really just turbo-chargers. We can put a light valve on the face of a CRT and take that image and make it ten to twenty times brighter. But the light valve is not a display technique in itself, it’s a turbo-charger for CRTs.
Da-Lite: OK, but how exactly does it do this turbo-charging?
Blackey: The principle is still pretty much unchanged. There’s a mirror with a film of non-structured liquid crystal spread across it like peanut butter on bread. Today, though, the liquid crystals are much less viscous than peanut butter; in fact they’re watery. And now what twists them is solely an electronic charge.
You’ve got a CRT with an image on it and in front of that CRT you put a photoelectric film which maps the optical pattern of the image into an electronic pattern. This electronic pattern is then traced into the liquid crystal film.
Da-Lite: Where does the brightness come from?
Blackey: If I went up and shined a flashlight onto the face of that liquid crystal film, the image written into it by the photoconductor would reflect back onto to me.
If I put a brighter flashlight on the image, it would be brighter. And technically there is no limit to how bright the flashlight can be.
Da-Lite: So your "flashlight," your light source shines on the light valve from the front, correct?
Blackey: Exactly. This is the great virtue of light valve projectors. If I want a brighter picture, I simply exchange the flashlight for a more powerful one. If I want a better picture, I simply change the CRT. The result, of course, is that the light valve uncouples, for the first time in history, resolution from brightness.
Always before there has been some inescapable trade off between brightness and resolution. With the light valve no such price need be paid.
Da-Lite: Are there really no limits to either?
Blackey: There’s a limit on LCD projectors and there’s a limit on DMD projectors, but if somebody said to me that the current standard is no longer going to be 2500 ANSI lumens and a resolution of 1024 by 1280, it’s instead going to be 5000 lumens and 2400 by 3200, I could have such a projector by Infocomm '97. It would depend entirely on market demand.
Da-Lite: If you have that much independent control over resolution and brightness, what about uniformity?
Blackey: The typical fall-off in a CRT projector can be as much as 70%, center to edge. Mine is 20% and sometimes only 10% to 15%. Part of that improvement comes from the back side - it’s only a 3½” CRT, after all. And part comes from the front side because I can illuminate the light valve much more evenly by some simple collimation optics placed in front of the lamp.
Da-Lite: And now what about contrast?
Blackey: Good question. In order to control contrast some tricky things need to happen. The first is that the raw light from the lamp in a light valve projector needs to get polarized. Now, when this polarized light reaches the liquid crystal light valve one of two things will happen. Those portions of the incoming light which reach the mirror will be reflected back with their polarization unchanged. Those that fall onto the surface of the liquid crystal and not onto the mirror will have their polarity rotated.
Da-Lite: So this enables you to distinguish between image light (that which has reached the mirror) and non-image light (that which reaches just the surface of the film)?
Blackey: Yes. The only way for a light valve to exhibit good contrast is to project just the light that has reached the mirror. The other light, distinguishable by its rotated polarity, is not allowed to escape and is thrown away.
Da-Lite: Some light valves, it is said, can have a contrast ratio as high as 1000:1. Can that be true?
Da-Lite: What about color? How do you manage that?
Blackey: We take a single white light source, polarize it, and then, by using dichroic mirrors which pass light of certain wavelengths and reflect others, we split up the light into three bundles, red, green, and blue. Each of these is allowed to fall onto its own light valve (hence in our projectors there are actually three light valves) before being recombined by the optical system in such a way as it exits the projector through a single lens.
Da-Lite: That’s pretty much how many of the LCD devices do it too, isn’t it?
Blackey: Yes, just exactly like they do, except that we need to throw away less light.
Da-Lite: Are there any deficiencies, any limitations to the light valve projector?
Blackey: I really don’t think so. Cost is an issue today, but it need not always be. A liquid crystal light valve is like any semiconductor device. You begin with a substrate onto which you lay down thin films and if you compared the AmPro facility in Mountain View with the Intel facility in Sunnyvale you wouldn’t see a lot of difference in the way we build these things. Now when you are only building fifty a month, like any semiconductor, they’re extremely expensive. But when you’re building a million a month, the price goes way, way down.
Da-Lite: Aside from manufacturing costs, is there no other limitation?
Blackey: I’m biased of course, but I see that there’s a place for a light valve projector in every living room in the world. After all, don’t we want in our houses the same thing we see in somebody’s boardroom? We want a very bright projector that doesn’t take up much room, that’s pretty quiet, and that’s easy to operate. All that is very achievable with the light valve.