A black light is a type of illumination that emits ultraviolet radiation (UV) and has very little visible light. You may also have heard of a black light being referred to as a UV (ultraviolet) light or a Woodâs lamp. This type of light has very little visibility because most of the observable light is blocked by a dark blue optical filtering material in the glass of the bulb. Thus, a black light bulb will emit mostly ultraviolet light, which is invisible to the human eye. However, when certain materials are illuminated by this UV light, a small fraction of visible light will be emitted onto these substances, giving them a bright, fluorescent glow.