The implementation of Complex Fenestration Systems (CFS) in the day-to-day life by practitioners, such as lighting designers, architects and façade makers is not an easy task. However, computer design tools can reinforce their usage through the determination of the luminous properties of a building interior prior to its construction or refurbishment. This paper presents a methodology based upon computer daylighting simulations carried out with RADIANCE in which the mkillum procedure was improved in order to render the light distribution due to diffuse daylight and sunlight on the inner surface of a fenestration system by the way of BTDF data. For the sake of standardization, the BTDF data accepted as input may be formatted according to the course of IEA Task 31 “Daylighting in Buildings” (e.g. following Tregenza format).