Originally Posted by ATWindsor
What is "very"? SO much that projector contrast is unimportant? What kind of number can you expect?
Well, I'd say that if you're paying big bucks for a projector, which usually indicates a very high contrast ratio, you want to preserve that ratio as much as possible. But no matter the specs of the projector - light reflected on the screen is going to impact its performance.
My point is that if there is very bright parts of the image, or eyes will perceive for instance dark greys as black, because its next to a powerful light.
To some degree, yes, but this is all about minimizing stray light, from sources other than the original projected image, from reaching the screen and therefore our eyes.
But concering the reflected light, what percentage of the light going to the screen reflects to the wall and then back again to the screen?
You won't get a number, because there are too many variables. But if you read any thread here, you'll see the same basic advice. Darker walls are better, non-reflective surfaces around the screen (stuff closer to the front of the room is more important), light-absorbing frame around the screen itself to provide a higher (perceived, in some sense) contrast.
What you will not find is anyone that painted their room, or treated the area near the screen, to be less reflective saying "gosh, that turned out awful and I'm going back to white walls".
I wouldn't say ambient light an reflected light is the same issue, they have a key difference, reflected light is less and less the darker the picture is, the effect is the smallest when its most important that it is small. That is why I am trying to find out how big/small this problem is.
That wasn't the point. Any light hitting the screen other than the projected image is detrimental to the image quality. A candle as "ambient" light and the same amount of reflected light would do the same thing - which is why I made the comparison. Obviously an open window with sunlight streaming in is going to be a bigger light source. It was a point about the results, not the magnitude of the problem.
As to how big a problem it is - again, read any thread here where folks have painted / treated to reduce reflections, and you'll see no regrets.