Radiance was developed at LBNL California, for predicting the distribution of visible radiation in illuminated spaces. It is internationally recognised as one of the most accurate and extensively validated lighting simulation tools available. The implementation of Radiance within DesignBuilder uses the full 3-D geometric model of the physical environment as seen on the Visualisation screen as input, and applies a "backward ray-tracing" method in which light is followed backwards from the working plane to the source(s) to produce a map of illuminance on the working plane. The calculations allow light to be transmitted through exterior and interior windows, and the shading and reflective effect of local shading devices and component and assembly blocks is included.