is a poster printed with inks which fluoresce under black light. The inks used contain phosphors which cause them to glow when exposed to the ultra violet light emitted from black lights.
A black light, also referred to as a UV light, is a lamp that emits electromagnetic radiation almost exclusively in the soft near ultraviolet range. Only a very small fraction of visible radiation is emitted, as it escapes filtering, and it is seen as violet/blue light. Black light sources may be made from specially designed fluorescent lamps, mercury vapor lamps, light-emitting diodes, or incandescent lamps. In most black lights either the glass envelope of the lamp itself or the lamp enclosure has an optical filter designed to reduce the emission of visible light and pass desired parts of the ultraviolet spectrum.