Join Date: Oct 2000
Location: Stockholm, Sweden
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Remember that the system is linear. The brightness of the ANSI pattern white sections is reduced for the grey screen and therefore also the light reflected back to the black sections. Contrast ratio remains the same regardless of screen if there is no background light. Only if the screen significantly changes the reflective properties of the room, the suppression of higher order reflections may increase CR for the grey screen.
However, for digital PJ's there is often a need for reducing the maximum brightness. Since the black level is fixed, this will reduce the contrast ratio if the projector peak output is reduced. Using a grey screen will reduce both maximum brightness and black level, keeping CR constant.
Subjectively it seems that absolute black level may be just as important for PQ as CR, at least for some viewers. The grey screen obviously provides benefits here as well.