Illuminance on a surface facing a source equals the source's intensity in that direction divided by distance squared, multiplied by the cosine of the angle of incidence when the surface is tilted. It holds well when the distance is at least about five times the largest dimension of the source. Every point-by-point calculation and every accent lighting estimate rests on it.
Inverse square law says moving the spot from 3 to 6 meters cuts the art from 800 lux to 200, so stick with the closer track.
The five-times rule: treat a source as a point only when you are at least five times its largest dimension away, otherwise the law overestimates close-range illuminance.
Also heard as
- inverse-square rule
Related terms
Candela
The unit for how much light a source throws in one direction, and the SI base unit behind lumens and lux.
Luminous intensity
How much light a source sends out in one specific direction, measured in candelas.
Point-by-point
A lighting calculation that predicts illuminance at each point on a grid from every luminaire's photometric data.
Throw
The distance from a lighting fixture to the surface or subject it lights, which sets beam size and intensity.