The inverse square law relates Illuminance Ep on the Working Plane (point illuminance) to light emission from a light source (Luminous Intensity I) at a distance r:

here ε is the angle of incidence of the light, i.e. the angle formed between the incident ray of light and the vertical to the illuminated plane.

According to the inverse square law, illuminance on a surface decreases inversely proportionally to the square of the distance between the light source and the illuminated surface, i.e. if the distance between the light source and the surface is doubled,illuminance is reduced to a quarter.

Strictly speaking, however, this law applies only to point sources of light and receptors, and adequate accuracy is not achieved below a certain limit distance. This distance depends on:

  • tolerated measuring error

  • maximum size of the light source

  • luminous intensity distribution of the light source

  • luminance distribution on the surface of the light source in the direction of vision

  • the size of the light aperture on the head of the illuminance photometer

  • the sensitivity curve of the photometer head

In Illuminance Photometry especially, it is important to observe the inverse square law. In this context illuminance is measured and the inverse square law applied to obtain luminous intensity and thus establish the luminous intensity distribution of a luminaire.