Luminous paint

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Fluorescent paint used in contemporary art
The scale model of Minas Morgul in The Lord of the Rings: The Return of the King was sprayed with phosphorescent paint

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

Fluorescent paint[edit]

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 bio-luminescence 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 or absorbed by complex natural surfaces, dulling the effect. Furthermore, the complex pigments will degrade quickly in sunlight.

Phosphorescent paint[edit]

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.

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[edit]

Main article: Radioluminescence

Radioluminescent paint contains a radioactive isotope (radionuclide) 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 phosphor degrades), which may be many years. They are therefore sometimes referred to as "self-luminous".

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.[1]

Radium[edit]

A 1950s radium clock, exposed to ultraviolet light to increase luminescence
Main article: Radium dials

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.

Radium paint used silver-doped zinc sulfide phosphor, usually doped with 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, despite the long half-life of the Ra-226 isotope (1600 years); clocks and other devices available from antique shops and other sources therefore are not luminous anymore, though 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.

Promethium[edit]

Promethium(III) chloride being used as a light source for signals in a heat button

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 Pm-147 half-life is however only 2.62 years, it is therefore not too suitable for long-life applications.

Promethium-based paint was used to illuminate Apollo Lunar Module electrical switch tips and painted on control panels of the Lunar Roving Vehicle.[2]

Tritium[edit]

Radioluminescent 1.8-curie (67 GBq) 6-by-0.2-inch (152.4 mm × 5.1 mm) tritium vials are simply tritium gas-filled, thin glass vials with inner surfaces coated with a phosphor. The "gaseous tritium light source" vial shown here is brand new.
Main article: Tritium illumination

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.

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