Illuminance

Illumination. The quantity of light or Luminous Flux falling on unit area of a
surface. Illuminance is inversely proportional to the square of the distance
of the surface from the source of light, and proportional to the cosine of the
angle made by the normal to the surface with the direction of the light rays.
Illuminance is measured in lux. One lux is equivalent to one lumen per
square metre.