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Tritium illumination is the use of gaseous tritium, a radioactive isotope of hydrogen, to create visible light. Tritium emits electrons through beta decay, and, when they interact with a phosphor material, fluorescent light is created, a process called radioluminescence. As tritium illumination requires no electrical energy, it found wide use in applications such as emergency exit signs and illumination of wristwatches. More recently, many applications using radioactive materials have been replaced with photoluminescent materials. == Design == Tritium lighting is made using glass tubes with a phosphor layer in them and tritium 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). The tritium in a gaseous tritium light source undergoes beta decay, releasing electrons that cause the phosphor layer to fluoresce. During manufacture, a length of borosilicate glass tube that has had the inside surface coated with a phosphor-containing material is filled with the radioactive tritium. The tube is then fused with a carbon dioxide laser at the desired length. Borosilicate is preferred 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. Tritium is not the only material that can be used for self-powered lighting. Other beta particle-emitting radioisotopes can also serve. Radium was used to make self-luminous paint from the early years of the 20th Century until approximately 1970, but it has been replaced by tritium, which is less hazardous.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 phosphors are green, red, blue, yellow, purple, orange, and white. 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 12.32 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.〔https://www.usfa.fema.gov/downloads/pdf/techtalk/techtalk_v1n1_0709.pdf〕 The difference between the signs is how much tritium the manufacturer installs. The light produced by GTLSs varies in colour and size, green are usually the brightest color and white usually the least, sizes can be found ranging from tiny tubes small enough to fit on the hand of a watch to ones the size of a pencil, when it comes to large ones (5mm diameter and up to 100mm long) they are usually only found in green and can surprisingly be not as bright as the standard 22.5mm x 3mm sized tritium, this smaller size is usually the brightest and is used mainly in key chains available commercially. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tritium illumination」の詳細全文を読む スポンサード リンク
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