DF is the ratio of interior illuminance to the exterior illuminance under the same overcast sky conditions, e.g. the CIE overcast sky distribution. DF can be converted to illuminance by multiplying the daylight factor with the external horizontal illuminance at ground level and dividing the result with 100. Usually DF is calculated with an external horizontal illuminance of 10,000 lux, so DF=1% corresponds to 100 lux. DF is often used to asses the ‘worst case’ daylight level in a design. DF can be calculated for a point in the room, as an average value for the entire room or as contour/isolines illustrating the DF distribution throughout the room.