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Author Topic: Rear Projection Daryl C. W. O'Shea Film God Posts: 3977 From: Midland Ontario Canada (where Panavision & IMAX lenses come from) Registered: Jun 2002 posted 10-27-2004 02:07 AM                        You'd a be seein' things backwards there.  |  IP: Logged Carl Martin Phenomenal Film Handler Posts: 1424 From: Oakland, CA, USA Registered: Feb 2002 posted 10-27-2004 02:19 AM                        gee frank, you must be a projectionist!  |  IP: Logged Tom Sauter Expert Film Handler Posts: 163 From: Buffalo, NY, USA Registered: Sep 2000 posted 11-02-2004 07:29 AM                        Daryl C.W. O'Shea: quote: Daryl C. W. O'Shea You can't make a translucent (for rear projection) gain screen either. Then why do manufacturer's specify rear projection screens with gains (either perceived or actual?)  |  IP: Logged Daryl C. W. O'Shea Film God Posts: 3977 From: Midland Ontario Canada (where Panavision & IMAX lenses come from) Registered: Jun 2002 posted 11-02-2004 10:51 PM                        Gain is a measure of screen reflectiveness. Since you can't reflect directly off of the viewing surface when using rear projection, any gain value would be based on the screens transmissive properties. It would be quite difficult to accomplish a "gain" equal to a high gain front projection screen.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-03-2004 12:31 AM                     Gain in and of itself is a misnomer to begin with... A screen can never gain light...just distribute it differently. A perfect mirror (also unobtainable) has an infinate gain. However a gain of 1.0 is such that 1 ft. candle produces 1 ft. lambert. Whether the light is reflective or transmissive is a matter of semantics rather than correctness. So if I hit a rear screen with 16 ft candles and on the otherside I read 16fL, then the "gain" is indeed 1.0 Does the meter know the light is transmitting through the screen rather than bouncing off it? Certainly there is some light loss in there somewhere since some of the light hitting from behind the screen is bouncing back but what does it matter? What is important is being able to predict the light quality such that the level is correct and the distribution is good and even. If I put in a higher gain screen such that without increasing the light output I now read 32fL in the theatre but with a pronounced hot-spot...you would not consider that a gain of 2.0? It sure measures like it and not surprisingly, whole screen behaves like a gain screen in that the further off axis one gets the darker the image. I suppose, one could come up with a new terminology for "Transmissive factor" or some such nonsense but then again the existing term of "gain" is already not exactly accurate since nothing is ever gained on a reflective screen either...merely distributed and absorbed. Steve  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 11-03-2004 05:03 PM                     How about "effective gain" (or "equivalent gain")? --jhawk  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 11-08-2004 11:50 AM                        OK, so you hit a piece of material with a focused image. Where the projection beam strikes a front projection screen, it is focused on that first surface and reflected back. But now we hit a rear projection screen; what do you focus on? The first surface which is the rear of the screen is not what the audience sees. There has to be enough specular material that will actually "hold" the image rather than transmit it thru the strata otherwise it simply be transparant. So for us to see the image on the audience side, somehow the focused image has to get thru to the other side and without any of the subsequent screen material defocusing it as it passes thru the remaining screen material, yet ALL of the plastic material has the same "holding" properties as the front surface meaning it will be out of focus except at one point. How can this happen? No matter how thin the strata, any point after the first surface, the material is defocusing the image. If the imaged is focused on the very last molecules of the strata, then doesn't the translucent (not transparant) surface that the bean hits first, doesn't display an out of focus image? Obviously rear screens work, so this is all wrong, but I just don't understand how they can maintain focus thru the entire screen material. For rear screens to work in theory, they would have to exist only in one plane and infinately thin, which of course they can't.  |  IP: Logged

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