4.6: LAWS OF ILLUMINATION

If we consider parallel surfaces which cut the pyramid at different distances from the source, the areas of these surfaces are proportional to the square of these distances, and, therefore, the amount of light which falls on one unit of the area of these surfaces is inversely proportional to the square of the distance from the source. This relationship is referred to as the law of inverse squares.

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161251 https://www.engineeringenotes.com/illumination-engineering/laws/laws-ofillumination-illumination-engineering/37229

According to this law the illumination at any point on a surface is proportional to the cosine of the angle between the normal at that point and the direction of luminous flux.