CIE standard clear sky
cloudless sky for which the relative luminance distribution is given by the luminance gradation function,
Note 1 to entry: For additional information, see ISO 15469/CIE S 011,
Note 2 to entry: This entry was numbered 845-09-91 in IEC 60050-845:1987.
Note 3 to entry: This entry was numbered 17-164 in CIE S 017:2011.