Foot-lamberts: how bright a cinema screen should be

A xenon lamp loses output steadily over its life, and a laser module more slowly. A screen gets dirtier. A port glass collects dust. None of it happens overnight, which is why nobody notices a room that has slid from 14 fL to 9 — until the audience does, in the form of a vaguely flat and murky picture they cannot name.

3D is the worst case. Glasses and filters absorb a large share of the light, and a projector that is already dim in 2D can be genuinely hard to watch in 3D. It is one of the quieter reasons 3D lost audiences.

A spot meter pointed at screen centre with a white test pattern is the whole procedure, and a quarterly reading per screen is enough to catch drift. The fix is usually unglamorous — a lamp change, a clean port glass, or a laser power adjustment — and the reward is a picture that matches what the audience paid for.