The clear sky index is an appropriate and thus an established quantity for the description of the transmission properties of the Earth’s atmosphere.

It is defined by

k* = Ig / Idear (3)

where the current global radiation Ig is related to the expected clear sky radiation Idear. In addition a model is needed, which describes the atmospheric conditions, a so called clear sky model, see sec. 2.2.

[Beyer et al. 96] introduced the relation

k* = 1 — n (4)

into the Heliosat method. This relation can be used without a previous calibration with ground data, but it is necessary that the clear sky model is optimally adapted to the regarded area. By Fontoynont et al. [2] relation (4) was improved in order to detail the case of complete cloudiness and to limit the clear sky index to meaningful values.