The CIE Standard General Sky defines 15 sky types ranging from fully overcast (type 1) to clear (type 15), parameterised by two indicatrix functions for luminance gradation and scattering.
In GroIMP, CIELighting wraps PerezSunSky and selects Perez coefficients that match the chosen CIE type. Solar position is computed from geographic coordinates via the Goudriaan algorithm.
| Type | Description |
|---|
| 1 | CIE Standard Overcast Sky — diffuse, no sun |
| 2 | Overcast, with slight brightening towards sun |
| 3 | Overcast, moderately graded, slight solar effect |
| 4 | Overcast, moderately graded, towards sun brightening |
| 5 | Sky of uniform luminance |
| 6 | Partly cloudy, without gradation, slight solar effect |
| 7 | Partly cloudy, without gradation, brightening towards sun |
| 8 | Partly cloudy, with gradation, slight solar effect |
| 9 | Partly cloudy, with gradation, strong solar effect |
| 10 | White-blue sky, slight solar corona |
| 11 | CIE Standard Clear Sky — low luminance turbidity |
| 12 | Clear sky, low luminance turbidity, polluted atmosphere |
| 13 | CIE Standard Clear Sky |
| 14 | Clear sky, high luminance turbidity, polluted atmosphere |
| 15 | White-blue turbid sky |