The current interest in energy efficient design has led to a demand for methods of predicting daylight availability in buildings. However, basic daylight data, which are the basis of these prediction methods, are not available for most regions of Greece, as well as for many areas worldwide. For that purpose luminous efficacy can be used in order to generate daylight illuminance from solar radiation data, which are much more widely recorded. This paper concentrates on the analysis of global luminous efficacy in Greece under clear sky conditions. The study involves the evaluation of existing models with respect to daylight data recorded in Athens, Greece, as well as the development of new luminous efficacy models for global radiation. The study was based on climatic data provided by the National Observatory of Athens during the years 1996-2000 and tested according to CIE. recommendations. The proposed luminous efficacy models were derived by the analysis, correlation and linear regression of the accepted values of measured data. Statistical analysis techniques were used for the assessment of their performance, as well as for the evaluation of the accuracy in predicting global illumination provided by the proposed and the existing models. The new proposed models can offer engineers and researchers a more reliable and accurate approach for estimating the global illumination levels under clear skies. Nowadays, the knowledge of daylight levels is considered highly important in local architecture, the energy conservation and environmental policies in general, particularly in the case of Mediterranean regions, where clear sky conditions dominate. (C) 2004 Elsevier Ltd. All rights reserved.