A comparison of luminous efficacy models with illuminance and irradiance measurements

Several luminous efficacy models have been tested against simultaneous illu minance and irradiance measurements in Helsinki, Finland (60 degrees 11 'N, 24 degrees 50'E). When compared with the measured values, the Perez lumino us 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 wh ich used the measured yearly average of 110 lm/W as the constant. For all m odels, the error was smaller for high solar altitudes but greater for the l ow altitudes. The monthly average luminous efficacy was fairly constant bet ween April and October but it was clearly lower during the winter months wh en the sun is very low. (C) 2000 Elsevier Science Ltd. All rights reserved.