Computed by dividing the amount of indoor illumance at a point on a workplane by the outdoor illumance under overcast conditions.

Interior horizontal daylight illuminance expressed as a percentage of the horizontal daylight illuminance available to an unobstructed site.

The ratio of daylight illumination at a given point on a given plane due to the light received directly or indirectly from a sky of assumed or known luminance distribution, to the illumination on a horizontal plane due to an unobstructed hemisphere of this sky, expressed as a percentage. Direct sunlight is excluded for both values of illumination. The daylight factor is the sum of the sky component, the external reflected component, and the internal reflected component. The interior plane is usually horizontal. If the sky condition is the C.I.E. standard overcast condition, then the DF will remain constant, regardless of absolute exterior illuminance. If used in conjunction with other than standard overcast conditions, the sky conditions should be specified. The term is also informally applied to the ratio of horizontal interior to exterior illuminance in the fenestration plane; under clear sky conditions, the DF remains constant only if the fenestration is completely diffusing (such as an ideal opalescent glass).