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.

Input

Angles a, b and c from figure above radians

Equation

Note

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

Source

Walsh(1)

References

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