Luminous paint

Luminous (or luminescent) paint exhibits luminescence. In other words, it gives off visible light through fluorescence, phosphorescence, or radioluminescence.

Fluorescent paint

Fluorescent paint reacts to long-wave ultraviolet (UV) radiation, commonly known as black light. Through the mechanism of fluorescence, UV-sensitive pigments present in the paint absorb black light and give off visible light in return.

There are two basic kinds of fluorescent paint: visible and invisible. Visible fluorescent paint can appear any bright color under white light, and glows brilliantly under black light. Invisible fluorescent paint appears white or clear under white light, but glows a particular color—depending on the pigment used—under black light.

This type of paint has extensive applications in the entertainment industry, and it can be used to create black-light effects such as invisible images, dual images, day–night transitions, and 3-D effects.

Phosphorescent paint

Phosphorescent paint is commonly called "glow-in-the-dark" paint. It is made from phosphors such as silver-activated zinc sulfide or, more recently, doped strontium aluminate, and typically glows a pale green to greenish blue color. The mechanism for producing light is similar to that of fluorescent paint, but the emission of visible light persists for some time after it has been exposed to light. Phosphorescent paints have a sustained glow which lasts for some minutes or hours after exposure to light, but will eventually fade over time.

This type of paint has been used to mark escape paths in aircraft and for decorative use, such as "stars" applied to walls and ceilings. It is also increasingly used as an alternative to radioluminescent paint.

Radioluminescent paint

Radioluminescent paint contains a radioactive isotope combined with a radioluminescent substance. The isotopes selected are typically strong emitters of fast electrons (beta radiation), preferred since this radiation will not penetrate an enclosure. Radioluminescent paints will glow without exposure to light until the radioactive isotope has decayed (or the paint itself degrades), which may be many years. They are therefore sometimes referred to as "self-luminous".

Radioluminescent paint was invented in 1908 and originally incorporated radium-226. The toxicity of radium was not initially understood, and radium-based paint saw widespread use in, for example, watches and aircraft instruments. During the 1920s and 1930s, the harmful effects of this paint became increasingly clear. A notorious case involved the "Radium Girls", a group of women who painted watchfaces and later suffered adverse health effects from ingestion. It is now recognised that radium paint requires great care in application, maintenance and disposal to avoid creation of a hazardous condition.

In the second half of the 20th century, radium was progressively replaced by safer radioisotopes such as promethium-147 and later tritium. Because of safety concerns and tighter regulation, consumer products such as clocks and watches now increasingly use phosphorescent rather than radioluminescent substances. Radioluminescent paint may still be preferred in specialist applications, such as diving watches. [ [http://www.hse.gov.uk/radiation/rpnews/rpa21.htm#a11 Hazards from luminised timepieces in watch/clock repair] , UK Health and Safety Executive]

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