CIE standard overcast sky definition

CIE standard overcast sky

completely overcast sky for which the ratio of its luminance L_? in the direction at an angle ? above the horizon to its luminance L_z at the zenith is given by the relation L_? = L_z (1 + 2 sin ?)/3
Found on http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=845-09-90

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