Evaluation of the daylight conditions at early stages of an urban project

Abstract The exterior geometry of an urban project is defined at its early stages. The interior design and architectural details are not yet defined and indicators such as the “Daylight Autonomy” (DA) or the “Useful Daylight Indicator” (UDI) cannot be used. In this article, a link is established between both these indicators and the outside light sources luminances. The concepts of “sufficient, excessive and useful luminances” are introduced. In an unobstructed context, the DA and UDI inside a typical office room in Paris are estimated on the basis of the sufficient and useful luminances of the light sources visible from the window with differences inferior to 10% up to mid-depth. Considering an urban context, the differences reach 25% at mid-depth and the DA and UDI inside the room therefore cannot be estimated on the basis of these indicators. Meanwhile, they allow identifying the directions which are the most appropriate for daylighting, and therefore allow a better consideration of this aspect at early stages of an urban project.