Global luminous efficacies on vertical surfaces for all sky types

Luminous efficacies are determined at Madrid for North, South, East, West f acing surfaces in two ways: by taking into account all the global illuminan ce and irradiance values available, and by considering data for each of thr ee sky categories as defined from values of the sky clearness index epsilon ' and the sky brighteness index Delta. Both methods are compared, and for epsilon ' < 1.23 (overcast skies) the second method is found to be more acc uarate than the first. (C) 1999 Elsevier Science Ltd. All rights reserved.