Light spreads as it travels, so illuminance falls off with the square of the distance from the source. Double the distance and the light drops to a quarter.
What the terms mean
| Symbol | Meaning |
|---|---|
| E | Illuminance, in lux |
| I | Luminous intensity, in candela (cd) |
| d | Distance, in meters |
Worked example
A 1,000 cd source gives 1,000 ÷ 2² = 250 lux at 2 m, but only 1,000 ÷ 4² = 62.5 lux at 4 m.
See the Luminous Intensity Formula and Lux at Distance Calculator.
The inverse-square law in lighting
Light spreads as it travels, so illuminance falls off with the square of the distance from the source: lux = candela ÷ distance². Double the distance and the light landing on a surface drops to a quarter, not a half; triple it and it’s a ninth. This is why a fixture that’s plenty bright up close can be inadequate just a few metres farther away.
The law applies cleanly to point-like sources (spots, downlights) measured on-axis. For large area sources or very close distances the falloff is gentler, and for a long line or broad panel the relationship changes. Still, the inverse-square rule is the everyday tool for predicting brightness at a distance and choosing mounting heights — raising a fixture even modestly noticeably reduces the light reaching the floor.