The assessment of interior daylight conditions, under the current Slovak standards, is based on the Daylight Factor concept. It means that the worst exterior daylight conditions are reflected in calculations, represented by the CIE overcast sky. Since this evaluation criterion was adopted, several decades have passed. Because the overcast sky is not the only sky type observable in nature, the development of a new criterion, based on photometric variables, e.g. illuminance in lux, became necessary. This article presents a study which investigates the relation between relative and absolute illuminance, in specified points on the working plane illuminated by the standard overcast and clear sky.