Photometrics: Lux, Lumens and Beam Angle
Lighting has its own vocabulary of measurement, and keeping the units straight is what lets you predict whether a fixture will actually do the job before you ever hang it. Three quantities carry most of the weight. A lumen measures the total amount of visible light a source puts out in every direction — its raw output. A candela measures luminous intensity in one particular direction, which is what a focused beam concentrates. And lux measures illuminance, the light that actually lands on a surface, defined as one lumen spread across one square metre.
The relationship between them is governed by the same inverse-square law that shapes how sound spreads. Illuminance falls off with the square of the distance, so a fixture that delivers 1000 lux at 5 metres gives only a quarter of that — 250 lux — at 10 metres, because the same light is now spread over four times the area. This is why throw distance matters as much as raw output, and why a powerful fixture placed far away can land dimmer on the subject than a modest one placed up close.
Beam angle is what ties it all together. A fixture's beam angle is the cone within which intensity stays above half its centre value, and it sets the trade between coverage and punch: a narrow beam concentrates candelas into a bright, tight pool, while a wide beam spreads the same lumens thinly over a large area. Put the three together — output, distance, and beam angle — and you can predict the diameter and the brightness of the pool a fixture will throw, which is exactly what the Beam / Photometric tool works out for you.
Lumens are output, lux is what lands — distance decides the difference.