Angles Of View
Steven T. Barlow is Product Group Manager for Large Screen Display Products at BARCO, Inc. in Kennesaw, Georgia. He has spent the last thirteen years managing the development and sales of Barco projectors. He has written numerous articles explaining the technologies which enable various projection devices and is an acknowledged expert on large screen displays. Mr. Barlow is interviewed here on the subject of
Projecting the Future - the Liquid Crystal DisplayDa-Lite: Much of your history with your company has been involved with three-gun CRT projectors. Yet over the past several years Liquid Crystal Display technology has begun to compete vigorously against the traditional CRT devices. What qualities do LCDs have which make them special?
Barlow: Beginning in about 1987 I participated in many discussions about alternate projection technologies which were then just coming into the rumor mill. From all of the various approaches we looked at, my company felt that the Active Matrix Liquid Crystal Display (AMLCD) was the most viable because that’s where the big investments were being made on development throughout the technical community. We concluded that AMLCDs were not only technically viable but commercially viable as well.
Da-Lite: Is it fair to say that the market which drove and continues to drive that development was screens for notebook computers?
Barlow: I think that’s a big part of the driver but I also think that in a general way people look at the classical CRT and say, this is awkward, this is a vacuum tube; we don’t use vacuum tubes anymore.’ So they’re looking for a better way to make a display. They want a lower cost, more reliable, higher resolution device that isn’t based on vacuum tubes. Paradoxically, of course, CRTs have just gotten better and better. Whether we’re talking about your TV set, your computer monitor, or a video projector, compare today’s model with the best that could be done five years ago and you’ll see just how spectacular the improvements have really been. CRT technology still has a lot of life left in it.
The LCD, however, is what people have found to be the most attractive alternative. There are others. Gas plasma or electroluminescent displays, to name two. But clearly the LCD technology is by far the most well researched and most actively developed.
Barlow: I think that the primary reason why has to do with what you put in versus what you get out. The amount of money needed to manufacture an LCD panel, speaking in broad terms, versus the quality of the display that you can get is very attractive. The downside to that is that the initial cost, the entry cost of LCD manufacturing is extremely high which prohibits all but a select few companies from really participating. Those that have overcome the initial problems relating to yield are now very successful at producing high volumes. This results in a large number of panels which can create high quality displays which have theoretically zero maintenance over a very long useful life.
Da-Lite: So when you were looking to produce high brightness, large scale, projection systems that incorporate non-CRT technology, you choose LCDs, right?
Barlow: Yes. Really, we’ve taken special derivatives of those LCD panels which nevertheless are part of a very reliable core technology, and built around them all of the other components necessary to make a high performance LCD projector.
Da-Lite: How does it work?
Barlow: An LCD projector can be considered as an electronic slide projector. You have a light source, you have a slide’, you have a lens, you have a picture on the wall. The question is, when we substitute an LCD panel for the slide, how does this new slide really work? We know that with a slide projector, we have light and it goes through the slide and it images that light onto the screen.
The LCD is a little bit more complicated than that because it deals with the polarization of light and with the liquid crystal’s characteristic of rotating the polarization of light.
The light which emanates from the lamp is not polarized, so it must undergo processing to be useful. To start with, we very carefully position a metal-halide arc lamp at the exact focal point of a parabolic reflector. This serves as the first step in the light collection process and results in a directed beam.
A problem with this beam is that it’s circular and we’re using rectangular LCDs. Another problem is that the light that comes out of this lamp is wide spectrum: it contains ultra-violet and infrared as well as visible components. So we have to insert a filter in the beam’s path which eliminates the UV and the IR. Thereafter we end up getting a bundle of cool light’ which, however, is still circular.
The next thing that we do is to feed it through a very special optical integrator that’s designed to change the shape of that bundle from circular to rectangular. The effect of this device is that we achieve a high degree of luminance uniformity.
Da-Lite: Is this different than CRT projectors?
Barlow: Yes. For example, if we measure 100 units of brightness at the center of a CRT image, we find only about 25 to 30 units in the corners. Whereas in our LCD projector typically what you’ll see is 75-80% of the center brightness at the corners. This is a dramatic improvement and provides considerable presence’ to LCD imagery which CRTs simply can’t achieve.
The next optical element in our projector polarizes the beam so that the light coming out of it has only one orientation. Now it’s ready to be passed through the LCD panel which is made up of a number of pixels each of which contains liquid crystal material. Depending on the electric charge given to that material, it will either rotate, partially rotate, or not rotate the polarization of the light passing through it.
The light then passes through a post polarizer, sometimes called the Analyzer which has an orientation exactly opposite to the first polarizer. Depending on how much rotation occurred in the LCD, the Analyzer fully transmits, partially transmits, or blocks the light from each pixel on the LCD panel. Finally the light is imaged on your screen with the projection lens.
Da-Lite: Actually the process you’ve just described occurs in triplicate. One each for red, green, and blue?
Barlow: That’s right. After we extract the white light from the lamp and render its beam rectangular, we have to split it up into its spectral components. We do this by using dichroic filters which as their name implies act on two colors. Thus they will either pass or block certain spectra of visible light. It takes two dichroics to separate out the R, the G, and the B. And, after each spectrally pure beam is acted upon by its dedicated polarizers and LCD panel, we use another two dichroics to recombine them back into a single beam of white light.
Da-Lite: After all of these manipulations, how much brightness have you left versus the amount you started with?
Barlow: We use lamps which put out 50,000 lumens to get 1,000-2,000 lumens up on the screen. And our collection efficiency is the best in the industry.
Da-Lite: Other than increasing the lamp wattage, are there other things you can do to increase brightness?
Barlow: There are technical limits that are in the LCD panel. A particularly important quantity is what’s called the Aperture Ratio. This is the ratio of the active area of the panel to the total area.
So if you looked at an LCD very closely you’ll find that there are a series of pixels or little windows which are arranged in a series of rows and columns. When driving the LCD what the electronics typically do is to say, Hello, Column 1-Row 3, here’s your data.’ And then the liquid crystal inside the pixel gets a certain amount of charge that says, for example, OK, you’re on 50 per cent.’
Since we can measure the height and width of a pixel we can know its area and since we know the total number of pixels in a display, let’s say it’s VGA: 640 by 480 or 307,200, we can figure out the total size of the display’s active area. The total area, of course, is given by the size of the panel itself: 3-inch, 6-inch, 10-inch or whatever.
Da-Lite: What goes on in the areas of the panel which surround each pixel?
Barlow: They are filled with a series of electrical connections and contacts which make up a matrix of wires which don’t have any value for the projected image. We don’t want those areas to give us light because that light would be meaningless. So all those areas are typically painted black which also enhances the contrast of the display.
Now when people look at an LCD projector, sometimes they say, I don’t like it; it’s too pixellated; it looks like chicken wire.’ What they’re really saying, of course, is that they don’t like their picture broken up into these pieces. Well, I don’t like it either.
Da-Lite: Is there a way to get rid of that?
Barlow: Yes. If we take the same LCD panel, same size, same number of pixels, but this time make the pixels themselves much bigger, then, as you can imagine, the gaps between the pixels become much smaller. Very simply, it’s like having a wall filled with windows. If it’s got x number of small windows only a certain amount of total light will pass through them. If I keep the same number of windows, but I make them twice as big, I’ll get twice as much light through. So aperture ratio has a lot to do with how much light a panel can transmit and it also has a lot to do with how much perceived space there is between pixels. If you look at two projectors, side-by-side, and one has an aspect ratio of 35% and the other of 50%, not only will the 50% machine be brighter, its image will look better because it’s less pixellated. As the distance between pixels shrinks, ultimately approaching zero, then what we would have is a perfectly homogenous and continuous image.
Da-Lite: So is it fair to say that the higher the aspect ratio the closer a viewer can be to the screen without detecting the pixels?
Barlow: Yes. Advances in panel design, particularly in the panels made with polysilicon, have enabled manufacturers to come out with projectors which produce double and even triple the brightness of their predecessors but which are still the same size and the same price. How do they do that? By having made big breakthroughs in the aperture ratios.
Da-Lite: What other problems are being worked on?
Barlow: The biggest problem with LCD projectors is that they’re not flexible. They’re easy to set up, they’re bright and they’re convenient. But at 640 by 480 pixels the only input signals they can take is VGA, Mac, or Video. But typically if you want to display XGA (1024 by 768), for instance, it just doesn’t work.
At BARCO we have developed an advanced Pixel Map Processor which, through processes we call pixel decimation or pixel interpolation, proportionately reduces or increases the pixel count of the signal from the outside world to fit the resolution of the panels inside the projector. With that sort of technology installed, LCD projectors can now be as flexible as CRTs while remaining more reliable, brighter and easier to set up. And that’s the way they ought to be.