inverse square law

Definition: The brightness of an area illuminated by a point light source falls rapidly as the light-source is taken away, such that: at e.g. double the original distance, the brightness is a quarter of the original; at triple the distance, the brightness is a ninth, etc. * The illuminance of a surface normal to a point source varies with the square of the distance from the source to the surface. * For ordinary light-sources, the law holds for general purposes, but clearly fails with (a) large sources e.g. fish-fryer or flourescent tubes and (b) highly focused ones e.g. spot-light. * Also known as distance-luminosity relationship.

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