Sky factor for unglazed vertical rectangular opening SF -- Algorithm 2.31

The sky factor is used as a measure of daylight obstruction in legal cases, such as those concerned with Rights of Light. The formula is derived from that giving the illuminance from a rectangular source on a perpendicular plane. It gives the sky factor on a horizontal surface from a vertical opening, at a point in line with the lower corner of the opening.


Angles a, b and c from figure above radians



When the opening is H high, W wide and a perpendicular distance D away from the reference point,




1. Walsh J W T The science of daylight (London: Macdonald) (1961)

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