Black Light Poster Basics
Black light posters are powered by fluorescent inks that react to ultraviolet or "black" light. Under normal circumstances, these posters look vibrant and bright, but when exposed to ultraviolet light, they actually glow in the dark. Fluorescence is not unusual---white T-shirts, fabric washed in certain fabric brighteners, body fluids and many other things also glow under black lights.
Ultraviolet Light
Ultraviolet (UV) light is a frequency of light just above the visible spectrum. Normal sunlight contains UV light, as well as visible light. Black lights are designed to emit as much black light as possible. They look purple, since they also emit some purple light. Purple light is at the far end of the visible spectrum, a slightly lower frequency than black light.
Fluorescence
When a molecule of a fluorescent material such as the ink in a black light poster is struck by a photon of ultraviolet light, it absorbs the photon. This raises the energy level of one of its electrons. The electron is unstable at the high energy level, and quickly releases the photon again, this time at a visible frequency. The inks in black light posters usually fluoresce in about the same color they normally reflect. A red black light poster, for example, will usually look red in both daylight and under a UV light.
