photometric distance law

law relating the illuminance E on a surface to the luminous intensity I of an illuminating point source whereby:


where θ is the angle between the normal to the surface and the direction of illumination and d is the distance between the source and the surface

NOTE 1: If the surface is perpendicular to the direction of illumination the relationship becomes , which is often referred to as the photometric inverse square law.

NOTE 2: This law strictly only applies for a point source. However, it can also be applied for non-point sources at sufficiently large distances; in this case the degree of approximation at the distance used should be confirmed by measurement.

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