Explainer: the Inverse Square Law

The Inverse square law: a law stating that the intensity of an effect such as illumination or gravitational force changes in inverse proportion to the square of the distance from the source.- Oxford languages

The inverse square law describes the intensity of light at different distances from a light source. Every light source is different, but the intensity changes in the same way. The intensity of light is inversely proportional to the square of the distance between light source and measuring point.

The inverse square law is always playing it’s part within lighting design; the truth is that we probably don’t notice because the lighting software works it all out for us. Every time we change the working plane or adjust the beam angle this is the law that the programme will be putting in to practice, to find out the lux or lumen/m2.

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How the ancient Greeks saw it:

From the tip of the pyramid (this being the light source):

  • First plane is 1m away from the source and the size of the square is 1m x 1m = 1m2. This is the point we measure luminance, and publish data, in candelas.
  • Second plane is 2m away from the source, the square is 2m x 2m = 4m2 so the luminance is 1/4 of the original source.
  • Third plane is 3m down, the square is 3m x 3m = 9m2 making the luminance 1/9 of the original source

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