Photometrics: Lux, Lumens and Beam Angle

Illuminance follows the inverse-square law, the same relation that describes sound spreading from a point source. It falls with the square of the distance: a fixture that delivers 1000 lux at 5 metres delivers a quarter of that, 250 lux, at 10 metres, because the same flux covers four times the area. The law holds when the distance is large compared with the size of the source. Throw distance therefore matters as much as output, and a powerful fixture placed far away can light the subject less than a modest one placed close.

The beam angle is the cone within which intensity stays above half its maximum value. It sets the balance between coverage and intensity: a narrow beam concentrates candelas in a small, bright pool, while a wide beam spreads the same lumens over a larger area. On the beam axis, the illuminance in lux equals the intensity in candelas divided by the square of the distance in metres. Output, distance and beam angle together give the diameter and the illuminance of the pool, which the Beam / Photometric tool calculates.