A number of years ago, Texas Instruments was embroiled in controversy for underwriting a research project that concluded that the image on some LCD front projectors turns yellow with time. The research results raised more questions than they answered, as they did not release important details such as naming which projectors were used in the tests.
Well, I was in Chicago last week and got a chance to visit the city’s wonderful Museum of Science and Industry. In addition to getting to ride down into a coal mine (very cool), I also came across a gallery devoted to imaging technology. So far, so good. And they had a video loop playing on two large projection screens. I could sit in one vantage point and see both at the same time. Here is a photo that I took of the two screens.
The large image on the left was from a Dukane 8801 LCD projector (I photographed the projector nameplate to record the information). The smaller image on the right was from another projector a bit further down the gallery. If you look at the right hand image, you can see that it’s a black and white x-ray image of a human skull in profile. The larger image on the left clearly has a yellow region taking up most of the center of the screen. This yellow coloring is not part of the original image.
What’s going on here? Here’s how I understand it. An LCD projector uses three LCD microdisplay panels as imagers. The light from the lamp is split in three beams: red, green, and blue. The three beams are recombined to create the color image; if all three color lights are used, you get a white image. The light has to be polarized before (and after) it passes through the imagers in order for the LCDs to work. These polarizers typically are made from plastic films. Ultraviolet light causes many plastics to degrade. Because the light beam is split in three, the blue beam gets a much larger share of the ultraviolet light from the lamp than do the red or green beams. As a result, the blue polarizers get damaged, and fail to cause the light to be transmitted through the LCD imager. With the blue beam blocked, the red and green beams combined to make yellow light. So what you see in this image is the result of damage to the center portion of the blue imager’s polarizer films, though they continue to work properly around the edge; presumably this is because the edges have not received as much UV exposure as the center portion has.
Is this an inherent defect in all LCD projectors? No. Many designs have addressed this problem by using more expensive inorganic (not plastic) polarizers for the blue beam, and some use them all three light beams. So if you’re looking for an LCD front projector for a home theater, you would do well to get one with inorganic polarizers.