Three models for the luminous efficacy of direct solar radiation on a horiz
ontal surface, valid for all sky conditions, have been developed in the pre
sent work, and two of them are proposed. In all the models a function for t
he direct luminous efficacy for all sky conditions relative to the direct l
uminous efficacy for clear skies is obtained, using the function for the di
rect luminous efficacy for clear skies given in a previous work, and the br
ightness index as a new independent variable. One of the two models is obta
ined by fitting the values of the relative luminous efficacy as a function
of the brightness index. The other proposed model is developed by obtaining
in the first place models for direct illuminance and direct irradiance, bo
th relative to the corresponding clear sky models, and then finding the rat
io between the relative direct illuminance and irradiance models. Statistic
al assessment of the direct illuminance values estimated with the proposed
models gives similar values for the MBE and the RMSE. However, the model ob
tained from illuminance and irradiance models is mathematically coherent. (
C) 2001 Elsevier Science Ltd. All rights reserved.