AVS Special Member
Join Date: May 2008
Location: San Francisco - East Bay area
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3 what? cd/m2 or fL?
If your reading is 3 fL, that's REALLY dark. If it is 3 cd/m2 it is unbelievably dark. Assuming 3 fL, you probably won't be able to measure
Is your iris opening settable manually? If so, see how high the readings are if you fully open the iris. Opening and closing the iris won't/shouldn't affect anything about calibration except the peak white level and the black level. But if you start at 3 fL for 100% white, you probably won't be able to measure grayscale steps below 30% or so and 30% might even be dicey re. measurement accuracy (depends on the meter). Measuring the screen then the projector directly (same 100% white pattern for both measurements) allows you to measure light from the projector but apply a correction factor that will account (more or less) for any shift the screen causes even though you are measuring brighter light directly from the projector.
Acoustically transparent screens typically do NOT have the gain the manufacturers claim. If the manufacturer says the gain is 1.0-1.2, it's very likely that the gain you measure at home will be more like 0.8 which is not surprising since "transparent" screens typically "leak" enough light that if you put a solid screen behind the "transparent" screen, you can see enough of the image to make out what the image is on the second/solid screen. A lot of people have too much screen or not enough light from the projector or both. The current SMPTE (Society of Motion Picture Technicians and Engineers) is 16 fL for 100% white with a tolerance of 12-20 fL. But keep in mind that if you start with 16 fL and that's the brightest your projector/screen combo can get with a new lamp, when the lamp is near the end of life, you'll have only 8 fL or so.
"Movies is magic..." Van Dyke Parks
THX Certified Professional Video Calibration
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Widescreen Review -- Home Theater & Sound