Best Screen Size for Optoma HD20
Actually, the larger the screen, the lower the black level, which is important with the HD20. In our review of that projector, the black level was quite high on a relatively small screen (65 inches wide), and the peak-white level was also higher than ideal, so a larger screen would help the picture at both ends of the brightness range.
How much larger should it be? All else being equal (aspect ratio, screen gain, amount of light coming from the projector, etc.), the measured amount of reflected light is proportional to the area of the screen. I measured a peak-white level of 24 foot-lamberts, and the ideal is 16 in a darkened room, so a screen with an area 1.5 times larger than the one I used would get you to 16fL. That translates to a screen 80 inches wide, which would drop the black level from 0.035fL to 0.023fL.
That's still not great, so enlarging the screen even more would help at the expense of the maximum brightness. To get to a black level of 0.010fL (my threshold for "good" blacks), you'd need a screen 122 inches wide, but your peak-white level would be only 6.8fL, which is quite dim. You could enable the projector's Bright lamp mode to increase the peak-white level, but that would also increase the black level.
In addition, consider the optimum screen size for your seating distance. Assuming a 16:9 aspect ratio, to achieve the minimum SMPTE viewing angle of 30 degrees at 13 feet, you need a screen 84 inches wide; for the recommended THX viewing angle of 36 degrees, the screen needs to be 101 inches wide, which would yield a peak-white level of 10fL and a black level of 0.015fL. That might be okay if you can completely darken the room.
Keep in mind that my measurements were taken on a unity-gain screen. If you use a screen with some gain, which is much more common, these measurements would increase accordingly if you're directly on axis and decrease as you move off axis.
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