Tritium radioluminescence 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, light is emitted through the process of phosphorescence. The overall process of using a radioactive material to excite a phosphor and ultimately generate light is called radioluminescence. As tritium illumination requires no electrical energy, it has found wide use in applications such as emergency exit signs, illumination of wristwatches, and portable yet very reliable sources of low intensity light which won't degrade human night vision. Gun sights for night use and small lights (which need to be more reliable than battery powered lights, yet not interfere with night vision or be bright enough to easily give away one's location) used mostly by military personnel fall under the latter application. History Tritium was found to be an ideal energy source for self-luminous compounds in 1953 and the idea was patented by Edward Shapiro on 29 October 1953 in the US (2749251 – Source of Luminosity). 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 phosphoresce. During manufacture, a length of borosilicate glass tube that has had the internal surface coated with a phosphor-containing material is filled with radioactive tritium. The tube is then sealed at the desired length using a carbon dioxide laser. 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. Radium was used to make self-luminous paint from the early years of the 20th century until approximately 1970. Promethium briefly replaced radium as a radiation source. Tritium is the only radiation source used in radioluminescent light sources today due to its low radiological toxicity and commercial availability. Various preparations of the phosphor compound can be used to produce different colors of light. For example, doping zinc sulfide phosphor with different metals can change the