Only a part of the sun radiation which penetrates the atmosphere reaches the earth surface directly. Another part of the solar radiation is scattered by the atmosphere and reaches the earth surface indirectly as diffuse radiation.

In areas of a temperate climate, diffuse daylight manifests itself most clearly on a cloudy day in winter. To simulate diffuse daylight the CIE-overcast sky is used, of which the luminance L varies with the vertical angle a. In formula:

TOC o "1-5" h z L = ((1 + 2 cosa)/3) LZenith. (1)

where LZenith is the luminance of the highest part of the sky.

The illuminance in a free field situation (Ehor, ff) is:

Ehor, ff LZenith 7я / 9~ 2.44 LZenith (2)

The daylight factor (df) for a point P inside a building is defined as

df = Ep/ Ehor. ff (3)

where Ep is the illuminance at point P in the building.

The direction of the direct sunlight is defined by two angles, the azimuth (a) and the height (h). The formula for the height of the sun is:

h = arcsin ( sin 9.sin d — cos 9.cos d. cos u) (4)

where ф = the degree of latitude of the place on earth; d = the angle of the declination; u = the hour angle, the angle corresponding with the daily rotation of the earth.

The formula for the azimuth is:

a = arcsin {( cos d. sin u)/cos h} (5)

The azimuth and the height of the sun vary during the day and depend on the place on earth. The computer program Radiance can be adjusted for every location.