Personal radiation dose meter (dosimeter). This is a thermoluminescent dosimeter (TLD), a reusable dosimeter that uses lithium fluoride (lower left) to measure the radiation dose. When the lithium fluoride is heated to over 250 degrees Celsius, it gives out an amount of light proportional to the radiation dose received. Dosimeters are worn by people who are exposed to radiation regularly during their work. There are strict limits to the amount of radiation to which those working with radioactive materials can be exposed. Personal dosimeters are used to track exposure levels. This is a dosimeter from the UK's Health Protection Agency. For an assembled dosimeter, see T168/106.