BRE average sky luminance and illuminance Lvav, Evavh--Algorithm 1.34

The BRE average sky is based on sky brightness measurements made in Berlin but with the constants in the equation chosen to fit horizontal and vertical measured illuminances in south-east England.

a. Luminance

Input
Solar altitude, gs radians
Altitude of sky point, g radians
Angular distance of sky point from sun, q radians
(algorithm 5.13 or using cosq = sing sings + cosg cosgs cos(a-as) )

Equation
The luminance of a sky point at altitude g and angular distance q from the sun is

Source
Littlefair(1,2)

b. Horizontal illuminance

Input
Solar altitude, gs radians

Equation

Illuminance from direct sunlight is excluded

Source
Littlefair(1,2)

References
  1. Littlefair P J The luminance distribution of an average sky Lighting Research and Technology 13 (4) 192-198 (1981)
  2. Littlefair P J Modelling real sky availability with the BRE average sky Commission Internationale de l'Eclairage, 20th session Amsterdam 1983 Commission Internationale de l'Eclairage

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