# Irradiance

### Definition

Irradiance is the **power of light per unit area**.

Irradiance follows an **inverse-square law**: irradiance is **directly proportional** to the inverse square of the distance from the source.

### Formulae

Variable Key

is the irradiance of a surface, in watts per square metre ( ). is the power received by the surface, in watts ( ). is the surface area, in square metres ( ). is the distance of the surface from the light source, in metres ( ). is a constant, which ensures direct proportionality.

Warning

This formula only works if the direction of the light source is **normal** to the surface!

Since

Variable Key

is the irradiance of a surface at a distance of from the light source. is a distance from the light source. is the irradiance of a surface at a distance of from the light source. is another distance from the light source.