BRE average sky luminance and illuminance Lvav, EvavhAlgorithm
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 southeast 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(aas)
)
 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
 Littlefair P J The luminance distribution of an average sky Lighting
Research and Technology 13 (4) 192198 (1981)
 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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