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Luminous paint or luminescent paint is paint that exhibits luminescence. In other words, it gives off visible light through fluorescence, phosphorescence, or radioluminescence. There are three types of luminous paints.
Fluorescent paints offer a wide range of pigments and chroma which also 'glow' when exposed to the long-wave "ultraviolet" frequencies (UV). These UV frequencies are found in sunlight and some artificial lights, but they—and their glowing-paint applications—are popularly known as black light and 'black-light effects', respectively.
In fluorescence the visible light component—sometimes known as "white light"—tends to be reflected and perceived normally, as colour; while the UV component of light is modified, 'stepped down' energetically into longer wavelengths, producing additional visible light frequencies, which are then emitted alongside the reflected white light. Human eyes perceive these changes as the unusual 'glow' of fluorescence.
The fluorescent type of luminescence is significantly different from the natural bioluminescence of bacteria, insects and fish such as the case of the firefly, etc. Bio-luminescence involves no reflection at all, but living generation of light (via the chemistry of Luciferin).
There are both visible and invisible fluorescent paints. The visible appear under white light to be any bright color, turning peculiarly brilliant under black lights. Invisible fluorescent paints appear transparent or pale under daytime lighting, but will glow under UV light in a limited range of colors. Since these can seem to 'disappear', they can be used to create a variety of clever effects.
Both types of fluorescent painting benefit when used within a contrasting ambiance of clean, matte-black backgrounds and borders. Such a "black out" effect will minimize other awareness, so cultivating the peculiar luminescence of UV fluorescence. Both types of paints have extensive application where artistic lighting effects are desired, particularly in "black box" entertainments and environments such as theaters, bars, shrines, etc. Out-of-doors, however, UV wavelengths are rapidly scattered in space[clarification needed] (waves are known to bounce off surfaces, outdoors the bounce is imperceptible) or absorbed by complex natural surfaces, dulling the effect.[clarification needed] (Humans only see reflected waves of photons, which are light; black matte is ultimate absorption at infinite angles thus enhancing any escaped wave of light. Thus, dim becomes bright.) Furthermore, the complex pigments will degrade quickly in sunlight.
Phosphorescent paint is commonly called "glow-in-the-dark" paint. It is made from phosphors such as silver-activated zinc sulfide or 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 long after it has been exposed to light. Phosphorescent paints have a sustained glow which lasts for up to 12 hours after exposure to light, fading 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 an alternative to radioluminescent paint. Kenner's Lightning Bug Glo-Juice was a popular non-toxic paint product in 1968, marketed at children, alongside other glow-in-the-dark toys and novelties. Phosphorescent paint is typically used as body paint, on children's walls and outdoors.
When applied as a paint or a more sophisticated coating (e.g. a thermal barrier coating), phosphorescence can be used for temperature detection or degradation measurements known as phosphor thermometry.
Radioluminescent paint is a self-luminous paint that consists of a small amount of a radioactive isotope (radionuclide) mixed with a radioluminescent phosphor chemical. The radioisotope continually decays, emitting radiation particles which strike molecules of the phosphor, exciting them to emit visible light. The isotopes selected are typically strong emitters of 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 phosphor degrades), which may be many years.
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.
Radioluminescent paint was invented in 1908 by Sabin Arnold von Sochocky and originally incorporated radium-226. Radium paint was widely used for 40 years on the faces of watches, compasses, and aircraft instruments, so they could be read in the dark. Radium is a radiological hazard, emitting gamma rays that can penetrate a glass watch dial and into human tissue. 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. In 1928, Dr von Sochocky himself died of aplastic anemia as a result of radiation exposure. Radium was banned from this use decades ago by international law, but the thousands of legacy radium dials still owned by the public can be a dangerous source of radioactive contamination.
Radium paint used zinc sulfide phosphor, usually trace metal doped with an activator, such as copper (for green light), silver (blue-green), and more rarely copper-magnesium (for yellow-orange light). The phosphor degrades relatively fast and the dials lose luminosity in several years to a few decades; clocks and other devices available from antique shops and other sources therefore are not luminous any more. However, due to the long 1600 year half-life of the Ra-226 isotope they are still radioactive and can be identified with a Geiger counter.
The dials can be renovated by application of a very thin layer of fresh phosphor, without the radium content (with the original material still acting as the energy source); the phosphor layer has to be thin due to the light self-absorption in the material.
In the second half of the 20th century, radium was progressively replaced with promethium-147. Promethium is only a relatively low-energy beta-emitter, which, unlike alpha emitters, does not degrade the phosphor lattice and the luminosity of the material does not degrade so fast. Promethium-based paints are significantly safer than radium; the half-life of 147Pm however, is only 2.62 years, it is therefore not too suitable for long-life applications.
The latest generation of the radioluminescent materials is based on tritium, a radioactive isotope of hydrogen with half-life of 12.32 years that emits very low-energy beta radiation. The devices are similar to a fluorescent tube in construction, as they consist of a hermetically sealed (usually borosilicate-glass) tube, coated inside with a phosphor, and filled with tritium. They are known under many names – e.g. gaseous tritium light source (GTLS), traser, betalight.
Tritium light sources are most often seen as "permanent" illumination for the hands of wristwatches intended for diving, nighttime, or tactical use. They are additionally used in glowing novelty keychains, in self-illuminated exit signs, and formerly in fishing lures. They are favored by the military for applications where a power source may not be available, such as for instrument dials in aircraft, compasses, lights for map reading, and sights for weapons.
Tritium lights are also found in some old rotary dial telephones, though due to their age they no longer produce a useful amount of light.
- Hazards from luminised timepieces in watch/clock repair, UK Health and Safety Executive
- "Radium paint takes its inventor's life; Dr. Sabin A. von Sochocky Ill a Long Time, Poisoned by Watch Dial Luminant. 13 Blood Transfusions. Death Due to Aplastic Anemia-- Women Workers Who Were Stricken Sued Company". The New York Times. 15 November 1928.
- "Apollo Experience Report – Protection Against Radiation" (PDF). NASA. Retrieved 9 December 2011.