Prescribing for Daylight: Can We Account for the Disparate Measures Within A Unified Framework?

The potential for a building design to provide daylight for general illumination was, until very recently, evaluated using only the daylight factor, i.e. a ratio of internal to external illumination under a single standardised overcast sky. Other known effects of daylight, such as the occurrence of visual discomfort which is more likely to occur during non-overcast conditions, were assessed or estimated by other means, often relying more on the skill of the experienced lighting designer than by use of a repeatable set procedure. In the last few decades there has been a gradual increase in awareness of the non-visual effects of daylight/light received by the eye Webb (2006). The quality and nature of the internal daylit environment is believed to have a significant effect on human health in addition to general well-being and worker productivity. Demonstrating compliance with various guidelines at the design stage is an ever increasing concern. For daylight this is invariably carried out nowadays using simulation rather than scale models. After many decades of reliance on the daylight factor as the sole quantitative daylight metric, there has been an explosion of activity in daylight modelling research which has delivered numerous new techniques, approaches and metrics. This paper describes various end-user requirements - both current and emerging - for daylight modelling and discusses how these might be accommodated within a single modelling framework.