Actually 5000 lumens is still not perfectly ideal. The argument goes like the following:
Suppose you start with a room ambient at 10.76 lux. At a gain of 1, if the black level illuminating the screen is equal to the ambient, we have 1 ft-candle illuminance or 1 ft-Lambert luminance leaving the screen.
This is not high ambient. If you want an idea of how bright it is, get a candle and hold a piece of paper 1 ft away in a black room. The average nighttime home interior is about 50 lux.
For a 500:1 contrast ratio, the white level is 500x the black level or an illuminance of 500 ft-Lamberts. Since you don't want all this bouncing around the room, you use a gain=.1 screen to get a luminance of .1*500=50ft-Lamberts.
Since a gain of .1 looks pretty dark compared to most backgrounds, you will always see it as black. For a 30 sq. ft. screen, one gets a required illuminance of 500*30 =15000 lumens!!
One can work backwards to find how dim a room needs to be to get a good contrast ratio for a given projector. 3000 lumens/30 sq. ft. = 100 ft-C white illuminance. 100/500=.2 ft-C black illuminance or about 2 lux maximum ambient.
Thus for 30 sq. ft. and a contrast ratio of 500.
3000 lumens => 2 lux
1500 lumens => 1 lux
750 lumens => .5 lux
Hence the need for controlled lighting for most projectors.
Note that ft-candles is the illuminance or light hitting the screen and ft-Lamberts is the luminance or light coming from the screen.
To get really confused check out:
http://energy.arce.ukans.edu/book/light/chap1.htm#UNITS
It has a nice diagram
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Ken Elliott
[This message has been edited by kelliot (edited 07-28-2001).]
