By Zencalcy Editorial TeamLast reviewed About us
Illuminance is the amount of visible light landing on a surface, measured in lux. It falls off with the square of distance from a small source, which is why a lamp that lights a desk evenly is useless across a room.
Lux from luminous flux, area or intensity and distance.
E = Φ / A E = I / r²
E is illuminance in lux, Φ luminous flux in lumens, A area in square metres, I luminous intensity in candela and r distance in metres. The inverse-square form assumes a point source and normal incidence.
| Symbol | Meaning |
|---|---|
| E | Illuminance, in lux (lm/m²). |
| Φ | Luminous flux, in lumens. |
| A | Area receiving the light, in m². |
| I | Luminous intensity, in candela. |
| r | Distance from the source, in metres. |
E = 1000 / 10 = 100 lux, similar to a bright living room. A 100 cd source at 2 m gives 25 lux, falling to 6.25 lux at 4 m.
Continue with the Law of Reflection Calculator, Combined Lens Power Calculator, or the Lens Maker's Equation Calculator — or browse more optics and light tools and all physics calculators.
Lumens measure total light output; lux measures light per unit area at a surface. The same bulb gives different lux depending on how its light is spread.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Free calculator · No signup
Content updated: September 2026