The Luminescence of Black Light Black Light. What is it? It is a portion of the Ultra-Violet Spectrum that is invisible to our eyes. We can not distinguish it. However, when this radiation impinges on certain materials visible light is emitted and this is known as "fluorescence." Fluorescence is visible to the human eye, in that it makes an object appear to "glow in the dark." There are several sources of ultra-violet light. These sources are: the sun, carbon arcs, mercury arcs, and black lights. In most cases, the production of ultra-violet light creates a reasonable amount of heat. Many materials exhibit the peculiar characteristic of gi