The ratio of the illuminance at a point on a given plane within an interior due to the light received directly and indirectly 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).
E = illuminance on unobstructed plane
e = illuminance at point in interior
Daylight Factor = e/E (often expressed as a percentage)
