The thermo Luminescent Dosimeter (TLD) is a type of Passive Dosimeter which is used to measure exposure from Ionising Radiation. It is one of a number of methods used in the area of Dosimetry. The TLD consists of a crystal (e.g. CaSO4) which gives off light when heated, the light being proportional to the degree of exposure seen by the TLD. The crystal is usually placed in a holder which contains filters which can be used to differentiate between skin doses and penetrating doses of ionising radiation. The TLD is normally worn on the trunk of the body but can also be worn on the extremities (e.g. for measuring doses to the fingers).