The ratio of the illuminance at a point on a given vertical plane due to the light received directly from a sky of assumed or known luminance distribution, to that on a horizontal plane due to an unobstructed hemisphere of this sky. Direct sunlight is excluded from both values of illuminance (i.e. CIE Overcast Sky). Usually the ‘given vertical plane’ is the outside of a window wall. The VSC does not include reflected light, either from the ground or from the buildings. The maximum VSC possible for a completely unobstructed vertical window is approx. 40%.
