Making the switch from task illumination to ambient illumination standards: Principles and practicalities,including energy implications

Abstract

It has been proposed that general lighting practice should switch from specifying indoor illumination standards in terms of task illuminance to ambient illumination specified in terms of mean room surface exitance. The reasons for this proposal are reexamined and the energy implications are assessed. To enable comparison of equivalent levels of illumination, it was first necessary to establish mean room surface exitance levels that equate to task illuminance levels as being typical for similar applications. A simulation was developed using AGi32 lighting design software to determine lighting power density levels for providing equivalent illumination by three luminaire luminous flux distributions for three room shapes with three surface reflectance combinations. As anticipated, the downlight distribution was found to be the most efficient way of satisfying task illuminance criteria, but also it was found that uplighting achieves similar lighting power density levels for the equivalent mean room surface exitance criteria if room surface reflectances are reasonably high, indicating that the switch could be made without departing from the lighting power density limiting values currently proposed for efficient use of lighting energy. However, to base lighting standards on low lighting power density levels would have the effect of severely restricting options in lighting practice. Accordingly, a range of lighting power density levels is proposed for lighting standards that would eliminate inefficient use of lighting energy while permitting reasonable levels of design flexibility in lighting practice.