I once had a "discoloration" of the plastic projection screen in my rear projection. It looked normal with the set off. However, it manifested in blotchy brightness in different areas of the screen when the TV was on--some areas brighter than others. It was still under the extended warranty so it was replaced for free--a $900 savings, $600 just for the part. When the tech was replacing it I asked him what could cause that. He said that it could be caused by sunlight. Ever since then, I have closed the blinds on the window across from the TV when I leave for the day. I don't know for sure if sun caused my damaged screen (particularly because there was no pattern to it), but the tech said he'd seen them damaged by sun before. If you think of it as a piece of plastic and think about the fact that just about every substance known to man bleaches after a year in the sun, it's not hard to imagine that the sun's rays could damage the screen. Remember the frisbee you left in the backyard for the summer?