Abstract
This paper presents some formulae for the calculation of the luminance distribution of a clear sky and of the resultant horizontal illuminance. Both the luminance distribution of a clear sky and the resultant horizontal illuminance are mainly affected by the extraterrestrial sunlight illuminance, the altitude of the sun, and the transparency of the atmosphere. A formula for the luminance distribution of a clear sky has been derived through theoretical consideration upon these factors. The formula predicts the luminance in absolute values as proposed to the ClE formula for clear sky, which provides relative pattern of luminance distribution1). Obviously the horizontal illuminance from an unobstructed clear sky can be calculated from the luminance distribution of the sky by means of a fundamental formula for the calculation of illuminance. However, from a different point of view a relatively simple expression has been formulated for the calculation of the horizontal illuminance, by applying the factors which were used to develop the formula for the luminance distribution, but in a different way.
