Several luminous efficacy models have been tested against simultaneous illuminance and irradiance measurements in Helsinki, Finland (60 degrees 11 ＇N, 24 degrees 50＇E). When compared with the measured values, the Perez luminous efficacy model had the lowest relative root mean square difference (RMSD) of 6.7%. All tested models had an RMSD within 2% of the constant model which used the measured yearly average of 110 lm/W as the constant. For all models, the error was smaller for high solar altitudes but greater for the low altitudes. The monthly average luminous efficacy was fairly constant between April and October but it was clearly lower during the winter months when the sun is very low. (C) 2000 Elsevier Science Ltd. All rights reserved.