Natural lighting or daylighting is provided by direct sunlight, light from a clear sky and daylight from an overcast sky. An overcast sky is one in which the outline of the sun cannot be seen through the cloud cover. Two mathematical models have been used for the luminance distribution of an overcast sky: uniform overcast sky; and CIE standard overcast sky. The CIE standard overcast sky is a model which bears close comparison to the luminance distribution of a real overcast sky. The CIE definition of daylight factor is the ratio of the daylight illuminance at a point on a given plane due to the light received directly and indirectly from a sky of assumed or known luminance distribution. For calculation purposes the daylight factor at a point in a room is divided into three components, each expressed as a percentage of the external illuminance. The component includes Sky component (SC), Externally reflected component (ERC), Internally reflected component (IRC).