Lux at Distance Calculator
See how far a light's brightness reaches — the lux on a surface at any distance, and the distance at which it hits the level you want.
Illuminance at distance
Inverse-square law: E = I / d². Move twice as far away and the light drops to a quarter. This assumes the surface faces the source and the source is small compared with the distance.
Why light fades with the square of distance
Light leaving a small source spreads out as it moves away. At twice the distance the same beam has to cover four times the area, so the amount landing on any given patch drops to a quarter. That is the inverse-square law, and it is why a lamp that floods a nearby desk barely touches the far wall.
Written as E = I ÷ d², it lets you predict the illuminance anywhere along the beam from a single intensity figure, or work backwards to find how close a fitting must sit to deliver the lux a task needs. It is the same law that governs how quickly a torch beam, a stage light, or a security floodlight loses its punch as the target moves away.
Frequently Asked Questions
What is the inverse-square law for light?
The illuminance a point source casts on a surface falls with the square of the distance: E = I ÷ d², where E is in lux, I is the intensity in candela, and d is the distance in metres. Because the same light spreads over an area that grows with distance squared, moving twice as far away leaves only a quarter of the light.
How do I calculate lux from candela and distance?
Divide the candela by the distance squared. A 1,000-candela source at 2 metres gives 1000 ÷ (2 × 2) = 250 lux on a surface facing it. At 4 metres it would be 1000 ÷ 16 = 62.5 lux.
At what distance does a light reach a target brightness?
Rearrange the law to d = √(I ÷ E). For 1,000 candela and a 250-lux target, d = √(1000 ÷ 250) = 2 metres. This calculator does that for you when you enter a target lux.
Does the surface angle matter?
Yes. The inverse-square figure assumes the surface faces the source square-on. If the surface is tilted, the light spreads over a larger footprint and the illuminance drops by the cosine of the tilt angle — the cosine law. For a steeply angled surface the real lux is lower than this estimate.
When does the inverse-square law break down?
It assumes a small (point-like) source relative to the distance. Very close to a large panel or a long strip the falloff is gentler, and the simple formula overstates how fast the light fades. Once you are a few source-widths away it becomes accurate again.
Educational estimate for a point source hitting a surface square-on. Tilted surfaces and large, close sources behave differently.