Aging Projector Lamps
According to video guru Joe Kane, neither. The light output of projector lamps does decrease over time, but the amount and rate at which this occurs is very lamp-specific. Also, the color of gray doesn't change much, and the brightness and contrast settings apply to the electronics, not the lamp's inherent characteristics, so there should be no need to tweak these controls or recalibrate the grayscale over time.
To compensate for the decreasing light output, Kane recommends using a smallish screen and closing down the projector's iris as much as possible. Then, as the lamp ages, open the iris to maintain a consistent light level over time.
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