Sorry, but there never is a simple answer with this hobby and electronics. But for the sake of this conversation, when using a projector, it's best to strive for a light controlled room / area, because ambient light WILL adversely affect the image. No way to get around that. Just the nature of the beast and your favorite topic, physics.
It a nutshell, really depends on the projector and what it's being used for. I've got a couple at work for conference rooms with over 3000 lumens and you can watch a game just fine with all the overhead florescent lights on... But a movie??? Neh...
In my HT, we usually turn the lights off when watching a movie, and it is VERY dark, with dark satin paint. But when watching a game, or TV or something where I don't really care to get 'the best' out if it, the lights are only dimmed a bit so that folks can come and go into the room without tripping on something.
A grey screen helps in rooms where there is a bit of ambient light, but they are not a magical device and can only do so much. But they do help..
A screen with some gain is also helpful, especially if ambient light is present. A grey or silver screen with some gain is the most beneficial for ambient light. But…always a “but”, screens with high gain (like the High Power) screen, have a very narrow viewing cone, and work best with the projector below the screen. They also tend to have “sparkles” or “hot spots”. The latest trend for better projectors is to just get a good quality neutral gain white screen.
What I advise folks to do with consideration to lumen output, is to first find out the projector's REAL lumen output, calibrated to 6500K. The real number will generally significantly lower than advertised (JVC seams to be the most honest). Then, multiply that by 65% for rapid bulb output reduction (bulbs loose roughly 30% within the first 500 hours of use). About the only way find this info, believe it or not, is forums. Sometimes the professional reviews will have this data, but not always. So you take that lumen output, and divide your screen size (square foot) into it to give you a FL (foot-lambert) figure.
Example:
Output (calibrated to 6500K) – 400
Screen size – 42” X 96” = 28 square feet
400 / 28 = 14.28 FL
Screen gain – 1.2
14.28 X 1.2 = 17.14
OK, so what FL does a person want??? Well, again, that depends. Some folks like a really bright image, some like a dim image. Some folks THINK they like a bright image because their projector has really crappy contrast, so brightness gives the impression of better contrast. But, generally speaking, 12 – 15 FL is about right for movies in a light controlled room. Sports, a bit more tends to work well; 15 – 18 FL or so.
Also, with respect to the projector’s lumen output, that measurement is taken with 100% light control, so keep that in mind.
Ideally, a person would have an idea of what FL they prefer. Then you figure out what screen size you want. With those two things known, you can then whittle down your projector choices, or expect to pony up for a High Power screen to get the FL you want.
