Experiments show that human beings are capable of detecting self-illuminating surfaces in their visual field, and that this capacity is based, to a great extend, on immediate processing by the visual system, rather than on a higher-level analysis. What are the physical parameters associated with this perception? The following factors are discussed; The highest intensity in the scene, absolute intensity value, local and global contrast, comparison with the average intensity, and lightness computation. All these factors are shown to be insufficient for explaining human's ability to identify light sources in their visual field. Finally, a method for accomplishing the source detection task in the so-called Mondrian world is presented.