so that the flashlight can be easily located in the dark. Tritium is used to illuminate the iron sights of some small arms. The reticle on the SA80's optical SUSAT sight as well as the LPS 4x6° TIP2 telescopic sight of a PSL rifle, contains a small amount of tritium for the same effect as an example of tritium use on a rifle sight. The electrons emitted by the radioactive decay of the tritium cause phosphor to glow, thus providing a long-lasting (several years) and non-battery-powered firearms sight that is visible in dim lighting conditions. The tritium glow is not noticeable in bright conditions such as during daylight, however. As a result, some manufacturers have started to integrate fiber optic sights with tritium vials to provide bright, high-contrast firearms sights in both bright and dim conditions. Safety While these devices contain a radioactive substance, it is currently believed that self-powered lighting does not pose a significant health concern. 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 the International Commission on Radiological Protection, but encapsulated tritium lighting devices, typically taking the form of a luminous glass tube embedded in a thick block of clear plastic, prevent the user from being exposed to the tritium at all unless the device is broken apart. Tritium presents no external beta radiation threat when encapsulated in non-hydrogen-permeable containers due to its low penetration depth, which is insufficient to penetrate intact human skin. However, GTLS devices do emit low levels of X-rays due to bremsstrahlung. According to a report by the Organisation for Economic Co-operation and Development, any external radiation from a gaseous tritium light device is solely due to bremsstrahlung, usually in the range of 8–14 keV. The bremsstrahlung dose rate can not be calculated from the properties of tritium alone, as the dose rate and effective energy is dependent on the form of containment. A bare, cylindrical vial GTLS constructed of 0.1 mm thick glass that is 10 mm long and 0.5 mm in diameter will yield a surface dose rate of 100 millirads per hour per curie. If the same vial were instead constructed of 1 mm thick glass and enclosed in a plastic covering that is 2–3 mm thick, the GTLS will yield a surface dose rate of 1 millirad per