Accurate assessment of the daylighting resource is essential for a proper design of naturally lit architectural spaces. Many regions of the world, including those of South America, do not yet possess recorded data of exterior illuminance, predictive analytical models being the only available tool for such assessment. Several models were studied comparatively before developing the one presented in this paper. It was designed aiming at the inclusion of variables that precisely define the local atmospheric conditions and the albedo, thus allowing for its generalised applicability, in conjuction with the solar position as defined by latitude, Julian day and solar time.