Tritium radioluminescence
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. Nite watches), gun sights, and certain emergency and tactical equipment.
Early self-powered lighting used radium paint, which posed a radiation threat to workers who handled it as well as users of the devices incorporating it. More recently, self-powered lighting has utilized tritium.
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).
The 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 melted closed under pressure 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 phosphorous compound can be used to produce different colors of light. Some of the colors that have been manufactured in addition to the common phosphorous green are red, blue, yellow, purple, and orange.
The types of GTLS used on 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. Tritium is the expensive component involved, so manufacturers are tempted to cut back on it. It is an unstable isotope with a half-life of about twelve and a half years, so tritium lighting loses half its brightness every twelve and a half years. 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 3 three brightness levels providing 10, 15 & 20 years of certified life.
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.
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, Asia and Australia. Paradoxically, tritium exit signs are quite common within the U.S.A., especially in older buildings. Alternative technologies, e.g. lightpanels based on light-emitting capacitor technology, have appeared recently.
Health concerns
While they contain a radioactive substance, self-powered lighting does not pose a health concern. Tritium is only mildly radioactive; it emits beta particles which cannot pass through the glass of the tube, and each tube segment contains only a minute quantity of the gas. 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.