Excerpt from Photometry of Projectors at the National Bureau of Standards
The photometer is calibrated by using a standard lamp of known luminous intensity in a specified direction oriented in that direction at a known distance from the photosensor.
Projectors are mounted at one end of a photometric range. There are two ranges for the photometric measurement of projectors at the National Bureau of Standards. On the shorter range, the distance between the projector and the photosensor can be varied to a maximum of 30 meters. On the longer range, this distance is a fixed 279 meters.
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