Luminous paint or luminescent paint is a kind of paint that gives off visible light and that glows in the dark. There are several ways this can be done:
- It is possible to convert invisible UV light into visible light. Typically, blue light, and the near-blue part of UV light is changed to a bigger wavelength, such as blue-green, yellow or red. This effect is known as fluorescence. At the same intensity, tones of red or yellow appear brighter to the human eye. This kind of paint is used for high-visibility clothing, for example.
- It is possible to store the energy of the light, and to emit this energy in the form of visible light later on. The effect is known as phosphorescence. Emergency exits are marked with this kind of paint.
- A radioactive substance can be used; the energy of this radioactive decay can be converted into visible light. Today, the radiation of these substances is shielded easily, since the substances used only emit alpha radiation, or low-energy beta radiation. At the beginning of the 20th century, radium isotopes were used, which led to a number of cases of radiation poisoning; one of the most prominent cases is that of the Radium girls.