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. Recently, self-powered lighting has used gaseous tritium, a radioactive isotope of hydrogen.
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[edit] 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).
[edit] 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, emitting 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 phosphorus 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. As found in self luminous exit signs tritium often comes in three brightness levels providing 10, 15 & 20 years of certified life.
[edit] 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 light source may not be. Some uses of this sort are analog dials in aircraft and in compasses. They were invented in the 1960s as a reliable self-powered light source for NATO.
[edit] 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), devices containing tritium are considered dual-use technology in the U.S.A., and are therefore illegal for export. However, they are widely available in the U.K., most of Europe (some countries like Belgium have outlawed them however), Asia and Australia. Paradoxically, tritium exit signs are quite common within the U.S.A.
[edit] Health concerns
- For more details on this topic, see Tritium.
While they contain a radioactive substance, self-powered lighting does not pose a significant health concern. 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 due to that beta decay radiation from tritium is not very energetic; it is incapable of penetrating through glass containers or even intact human skin.[1] Direct, short-term exposure to small amounts 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, but in very small quantities.
[edit] References
- ^ www.ehso.emory.edu. Nuclide Safety Data Sheet Hydrogen-3. Retrieved on 09 November 2006.
[edit] External links
[edit] See also
| Lighting and Lamps
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|---|---|---|
| Incandescent: | Conventional - Halogen - Parabolic aluminized reflector (PAR) |  |
| Fluorescent: | Compact fluorescent (CFL) - Linear fluorescent - Induction lamp | |
| Gas discharge: | High-intensity discharge (HID) - Mercury-vapor - Metal-halide - Neon - Sodium vapor | |
| Electric arc: | Arc lamp - HMI - Xenon arc - Yablochkov candle | |
| Combustion: | Acetylene/Carbide - Candle - Gas lighting - Kerosene lamp - Limelight - Oil lamp - Safety lamp | |
| Other types: | Sulfur lamp - Light-emitting diode (LED) - Fiber optics - Plasma | |
