A Critical Review of the Preetham Skylight Model

The Preetham skylight model is currently one of the most widely used analytic models of skylight luminance in computer graphics. Despite its widespread use, very little work has been carried out to verify the results generated by the model, both in terms of the luminance patterns it generates, and in terms of numerical reliability and stability. In this paper, we show where the model exhibits problematic behaviour, and compare the computed luminance values with the CIE 2003 Standard General Sky, and our own measurements of real, cloudless skies.