8.5.1 CIE Standard Sky Models
The CIE has constructed a set of “standard sky conditions” to provide guidance regarding the distribution of sky “brightness” for “energy conscious window design, daylight calculation methods and computer programs as well as for visual comfort and glare evaluations” [15, p. 359], see also . According to Kittler, Perez, and Darula , CIE developed a “standard overcast sky” in 1955, and 20 years later established a CIE standard clear sky . In 1997, the analysis of a wide range of measurements from sky luminance scanners and mapping instrumentation by a CIE technical committee resulted in proposed algorithms for computing relative (to the zenith) spatial distributions of luminance from the sky dome for intermediate conditions between the clear sky and overcast sky standard conditions. The proposed algorithms resulted in 15 standard sky conditions corresponding to different combinations of direct solar radiation, circumsolar solar radiation, and diffuse solar radiation relative magnitudes, including both the clear sky and overcast sky standard distributions.