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Self-powered lighting
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Self-powered lighting is a generic term describing devices that emit light continuously without an external power source. Self-powered lighting is most frequently used on wristwatches (i.e. Night watches), gun sights, and certain emergency and tactical equipment.
Early self-powered lighting used radium paint, which posed serious health risks to the workers who processed and applied it as well as to the users of devices incorporating it.
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Encyclopedia
Self-powered lighting is a generic term describing devices that emit light continuously without an external power source. Self-powered lighting is most frequently used on wristwatches (i.e. Night watches), gun sights, and certain emergency and tactical equipment.
Early self-powered lighting used radium paint, which posed serious health risks to the workers who processed and applied it as well as to the users of devices incorporating it. Gaseous tritium, a radioactive isotope of hydrogen, has also been used in self-powered lighting applications, such as emergency exit signs. More recently, many applications using radioactive materials have been replaced with photoluminescent materials.
Tritium lighting
Tritium lighting is made using glass tubes with a phosphor layer in them and tritium (a hydrogen isotope) gas inside the tube. Such a tube is known as a "gaseous tritium light source" (GTLS), or beta light, (since the tritium undergoes beta decay).
Self-luminous microspheres
A patent application was filed by the US Patent Office in 2007 for sand-grain-sized tritium containers or glass or polymer "microspheres" that can be applied in self-powered lighting paint.
The patent application states that the plurality of individual containment microspheres minimizes the escape of radioactive gas in the event of any physical damage to an assembly of such microspheres. It also states that the radioactive gas has a relatively large contact surface with the phosphor particles, thus causing a relatively efficient light emission from the surface of the particles.
Physics behind the light
The tritium in a gaseous tritium light source undergoes beta decay, releasing electrons which cause the phosphor layer to fluoresce.
During manufacture, a length of borosilicate glass tube which has had the inside surface coated with a phosphor-containing compound is filled with the radioactive tritium. The tube is then fused with a CO2 laser at the desired length. Borosilicate is preferred because it is a type of glass noted for its strength and resistance to breakage. In the tube, the tritium gives off a steady stream of electrons due to beta decay. These particles excite the phosphor, causing it to emit a low, steady glow. One could use any beta particle-emitting substance, but in practice tritium is preferred because it is not very hazardous.
Various preparations of the phosphor compound can be used to produce different colors of light. Some of the colors that have been manufactured in addition to the common phosphorus green are red, blue, yellow, purple, and orange.
The types of GTLS used in watches give off a small amount of light—not enough to be seen in daylight, but enough to be visible in the dark from a distance of several meters. The average such GTLS has a useful life of 10–20 years. As the tritium component of the lighting is often more expensive than the rest of the watch itself, manufacturers try to use as little as possible. Being an unstable isotope with a half-life of about 12.36 years, tritium loses half its brightness in that period. The more tritium that is initially placed in the tube, the brighter it is to begin with, and the longer its useful life. Tritium exit signs usually come in three brightness levels guaranteed for 10, 15, or 20 year useful life expectancies. The difference between the signs is how much tritium the manufacturer installs.
Uses of self-powered lighting
These 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 and in self-illuminated exit signs. They are also favored by the military for critical applications where illumination of the glow-in-the-dark sort is desired but a power source may not be available. Some uses of this sort are analog dials in aircraft, in compasses, and sights for weapons. They were invented in the 1960s as a reliable self-powered light source for NATO.
Tritium lights are also found in many old rotary dial telephones, though due to their age they no longer produce any useful amount of light.
Small arms sights
Tritium is used to make the sights of some small arms illuminate at night. Most night sights are used on semi-automatic handguns. The reticule on the SA80's optical SUSAT sight (Sight Unit Small Arms Trilux) contains a small amount of tritium for the same effect as an example of tritium use on a rifle sight.
Legal issues
Because tritium in particular is an integral part of thermonuclear devices (though in quantities several thousand times larger than that in a keychain), consumer and safety devices containing tritium for use in the United States are subject to certain possession, resale, disposal, and use restrictions. Devices such as self-luminous exit signs, gauges, wrist watches, etc., which contain small amounts of tritium are under the jurisdiction of the U.S. Nuclear Regulatory Commission, and are subject to possession, distribution, import and export regulations found in 10 CFR Parts, 30, 32 and 110. They are also subject to regulations for possession, use and disposal in certain states. They are readily sold and used in the US and are widely available in the U.K. Some countries, Belgium for example, have outlawed them. Tritium lighting is legal in most of Asia and Australia.
Health concerns
While these devices contain a radioactive substance, it is currently believed that self-powered lighting does not pose a significant health concern. However, a 2007 report by the UK government's Health Protection Agency Advisory Group on Ionizing Radiation declared the health risks of Tritium exposure to be double that previously set by Sweden's International Commission on Radiological Protection.
Tritium presents no external radiation threat when encapsulated in non-hydrogen-permeable containers; its only danger is its constitution into the human body from direct contact, which may in turn cause long-term, low-dose radiation damage. This is because beta decay radiation from tritium is not very energetic; it is incapable of penetrating through glass containers or even intact human skin.
Direct, short-term exposure to small amounts of Tritium is relatively harmless. If a tritium tube should break, one should leave the area and allow the gas to diffuse into the air. Tritium exists naturally in the environment, but in very small quantities.
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