What is a photometer arrangement?

A photometer arrangement is the physical set-up used to measure a fixture’s light. The fixture is mounted on a rotating stage so it can be turned to each test angle, and a photometer sits at a fixed distance, in line with the beam, reading the light that arrives. In this arrangement intensity at each angle is found by turning the fixture about its photometric centre, not by moving the photometer. The photometer reads illuminance, and intensity in candela follows from the inverse-square law: intensity = illuminance × distance squared.

Three things make the reading valid. The test distance must be long compared with the luminous area of the fixture (a common rule of thumb is at least five to ten times its largest luminous dimension; the test standard or laboratory procedure sets the actual figure) so it acts as a point source, and it must be constant and accurately known, measured from the photometric centre of the fixture to the face of the photometer. The photometer itself must be calibrated, traceable to a national standard and corrected to the response of the eye. The beam axis must be aimed at the photometer, and the photometer must face the fixture. And stray light must be kept out: a dark room with matt black surfaces and baffles stops light reflected from the surroundings from adding to the direct beam. The fixture is also run at the rated current for the brightness step being tested and allowed to settle, so lamp or LED output is stable, before readings are taken.