Luminance Distribution in Overcast Sky
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3. Luminance distribution in overcast sky

In earlier studies, the luminance of the entire overcast sky was taken as uniform. This type of sky is termed as uniform sky and under this condition the illuminance at a point on a horizontal surface due to an unobstructed hemisphere of overcast sky can be given by:

(1)

where,

L = sky luminance, Cd/m2

In recent years, the standardised CIE sky was widely adopted, which represents the diffuse light from a completely overcast sky when the In recent represents ground is free of snow. The luminance distribution of the CIE sky is not uniform. The relative luminance Lq depends on the angle of elevation q measured with respect to the horizon, and is given by : (See Figure 5)

(2)

where,

Lz = the luminance of the sky at the zenith

 

This equation shows that :

(a) Zenith (q = 90o) is the brightest region

(b) The luminance decreases to 1/3 of that of the zenith towards the horizon

(c) The luminance is independent of the position of the sun, and therefore the orientation of the windows has no effect on the illumination of the room.

Figure 5 The CIE Overcast Sky