Originally Posted by B.D
this is my first time posting but have read a lot of the posts on this site, that said, I still am a little confused with the gray vs. white screen. I understand that gray is better for low light rooms for contrast, but wouldnt it also be better for dark rooms? I guess my question would be why use the white screen in any condition? since gray gives better contrast, better blacks.
Ellebob and Domimic are absolutely correct about projection light striking the screen for the first time. A neutral gray will reduce all light of all colors equally and proportionally in all regards and will maintain the same CR when calibrated to return the exact same image in terms of returning brightness measured in FL (foot Lamberts) or lumens per sq foot.
The simple example I will make is what I use at home in my media room. My screen is 50% gray so in terms of a 1.0 gain white screen (unity gain) my screen is a .5 gain. In my case to get the returning FL I want for my given size screen if it was white I would need 1000 lumens but because I have a .5 gain gray I hit mine with 2000 lumens. In a pitch black room with no light coming in and every item in that room painted flat black or draped in black velvet, the two images would be exactly the same.
Very few rooms are that perfect though. And projected light hits the screen and displays the image to the whole room. You only see the small amount of that light that goes into the opening of your eye and falls on your optic nerve. If you are the only person in the room just what goes in your two eyes is needed it is nice it goes all over in case you move around of have 500 other people watching like in a movie theater. But even then the rest is wasted. What happens to it is the question. If it strikes a piece of black velvet it is 99.9% sucked into the velvet and converted to heat. If it strikes a 50% neutral gray wall 50% is converted to heat and 50% bounces back into the room. If it hits a white wall almost all of it returns to the room. Some of that bouncing around in the room will find its way back to the screen and is now ambient light no different than a window or a light bulb turned on in the room. That light striking the screen goes into direct competition with the projected light and diminishes “waters down” the CR the projector is producing.
Another way to think of it is that projectors do not project black. What they do when a area on the screen calls for black is to try and project no light. A very dark color projects almost nothing with a slight bit of color. If you measure the lumens hitting the screen where it is trying to be black it will measure very little so you can see if another area on the screen is white and lighting up the room the ambient level could be high and it takes very little of it to dilute a black that is no light. The deepest black a screen can produce will be what the screen looks like with whatever ambient is in the room. It is not just black that gets diluted by ambient light adding brightness to all colors changes them from what they should be it is just the dark colors we notice going away the fastest.
That’s the short answer as others have posted and now there is a bit more to the long answer. It is the term you will hear a lot but seldom see explained. It is also harder to grasp maybe, but is very important.
It is called perception of contrast and how our eyes work in seeing and processing contrast. Millions of years of evolution of how humans see gave us the ability to see on the darkest of moonless nights and also out in the brightest summer sun lit days. If you took a light meter out and measured the light levels where our eyes work you would find the level increases something like a million times. The way we can do that without burning out our eyes that our eyes adjust instantly as the iris changes size getting smaller as things get brighter. When we watch a projected image nature takes over and when the screen is very bright like watching a sporting event our iris adjust and get smaller limiting the amount of light coming in. The level gets adjusted on the overall brightness in our field of vision and areas that try to be black are perceived as blacker because the overall light coming back from the black area on the screen is cut back along with the brightness and we see the black as darker than it is if we took a light meter to the screen and measured it. One think to always remember is our eyes are amazing devices, but they are the worlds worst light meters because they are always changing trying to give us the best image possible.
Some people like me have dark gray screens and dark light absorbing walls and ceilings making our rooms well suited for a white screen but still use gray because no room is perfect first, and second we like to turn some task lights on so when watching sports or regular TV we feel like we are in more of a living room or sports bar lighting than pitch black like a movie theater. We can still kill the task lights for a movie experience if we want and the gray screen hurts nothing and helps with reflected ambient. When I want to have a sports viewing with a bunch of people coming and going and eating and drinking I do two things. I turn up the ambient task lighting so the room isn’t pitch black, and I also select a preset on my projector that makes it brighter. The brighter setting on already bright content forces perception of contrast and the dark screen and walls work to preserve contrast.
The only drawback to such a setup is I need a much brighter projector than I would in a dedicated lights out theater. Brighter projectors we used to call light cannons are sometimes not the cream of the crop in terms of all the advanced features and sometimes are more like business projectors that favor brightness over accuracy. There is a trend though going brighter and there are quality projectors that are very bright also. And of course screen size plays a part.
In my above example I neglected to explain that with my .5 gain gray screen that absorbs half the projected light coming to it. It also absorbs half the ambient light coming to it. So in the case of my room with a black ceiling and 50% gray walls rebound ambient light leaves the screen and any that hits my ceiling gets mostly absorbed maybe 90% of it. 10% goes into the room and some part of that back to the screen what does hit the screen half again is absorbed. With my walls it come off the screen and the walls trap half and the other half lights up the room most of that bounces around the room getting halved each time and the little bit that returns to the screen is only half the effect on PQ reduction. The room and the screen is a light trap.
Welcome to the forum and a very good first post. Don’t be a stranger.