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Luminous efficacy relates two important quantities-daylight and solar radiation. Its use enables calculations of daylight availability and passive solar gain in buildings to be integrated to a large extent. Luminous efficacy also enables daylight data to be generated where directly measured illuminances are not available. This paper reviews measured and calculated luminous efficacies obtained by authors in various parts of the world. For direct sunlight luminous efficacy increases noticeably with solar altitude (70-105 lm/W). For diffuse skylight average luminous efficacies range from around 130 lm/W (clear) down to around 110 lm/W for overcast skies. Global efficacies are around 105 lm/W for clear skies and for average conditions. In each case'measured luminous efficacies are not constant but can vary over a substantial range depending, for example, on solar altitude, atmospheric aerosol and water vapour content, and type and amount of cloud. These effects are examined in detail and their influence on luminous efficacy discussed. Finally an Appendix describes the experimental measurement of luminous efficacy and outlines the types of experimental error that can occur.
Paul Littlefair (Sun,) studied this question.