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)

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