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G6
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and Bulbs
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Intermits
1 C t 1 I H I
The
LIGHT on your
SCREEN
By CHARLES E. SHULTZ
Member SMPE . . . Honorary Life Member LOCAL 365, IATSE & MPMO
Does Length of Throw Affect Screen Brightness?
WILLIAM E. GOEWEY of
Elmhurst, 111., writes, "Congratulations on your column. There is one question that arouses my curiosity every time anyone brings up the subject. I have been involved in more arguments on it than anything else. The subject is the length of throw versus screen brightness.
"I am not one of these fellows that think that the screen brightness decreases with
say that if that is true, then a candle will be just as bright at 5 feet as it is at one foot. I can readily see that the statement will hold true only with focused light.
"Now here is my question. If a projection throw of 100 feet on a screen size of 15x30 feet, gives 10 foot-candles, and a projection throw of 200 feet on the same screen size gives 10 foot-candles, what is the limit on how far this will work? If you can get a lens of long enough focal length, will it work for 500 feet, or 2,000 feet, or five miles? Why cannot we invent a focal length of a lens of such a length
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DIAMETER
10O IT. THROW 20' 7" SCMEN i-1 £11 SYSTEM.
3.5V DIAMETER
-T
2.00 IT. THROW 20' 7' SCREEN f;2.2 SYSTEM!
16" E.E.
T
-4
I'U DIAMETER
4r00 IT. THROW 10' T SCBJEEN SYSTEM Relation of lens diameters and focal length (see text).
the length of throw if the screen is the same size in both cases and the light source the same. I think where most people get the misconception is trying to think of this as unfocused light. If you tell a novice that the length of throw of a projector has no effect upon the screen brightness he probably will not believe you. He probably will
that we can project a light (theoretically) to infinity?
"In the above case we doubled the distance of the projection throw and merely by changing the lens to a longer focal length to get the same size screen image, we had the same screen intensity. If this will work at 200 feet with no loss of in