Traditional light source modelling is concerned with specific types of light sources, the two most common of which are point sources and daylight. Little attempt has been made, however, to relate different types of sources to each other. For example, how may the lighting from an overcast sky be compared to that from a lamp? Having a theoretical framework to compare different types of light sources is important for computer vision, in particular for understanding shading and shadow cues. A vision system needs to take account of the light source in order to interpret these cues. In this paper, we present a framework for comparing types of light sources which is based on a dimensional analysis of the set of light rays in a free space. Specifically, we introduce a 4-D light source hypercube in which the different types of sources may be embedded and compared. We also present a novel definition for light sources which generalizes the standard definition of a source as an emitter.