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