Quantitative estimation of clear sky light in Madrid

We have measured over a six-month period, from June 1993 to January 1994, the illuminance and luminance in the Ciudad Universitaria of Madrid on clear days. These measurements have allowed us to find the average levels of those magnitudes. We have established simplified models concerning absolute value of global and diffuse horizontal illuminance, and zenithal luminance in clear days. This treatment could be extended to other periods and geographical places with similar conditions.