Thermoluminescent dosimeter (TLD) is a passive radiation detection device that is used for personal dose monitoring or to measure patient dose. 

Parts

• plastic holder

• nickel-coated aluminium card with TLD discs

  • the discs are made of a thermoluminescent material, commonly calcium sulphate doped with dysprosium (CaSO₄:Dy) or lithium fluoride (LiF)

    • nearly tissue equivalent, although not at all x-ray energies ¹

  • the discs are 0.8 mm thick and have a 1.35 cm diameter

• three filters against each disc

  • top: aluminium and copper

  • middle: perspex

  • lower: open

When the radiation falls on TLD, the electrons are excited and store energy. After a varied period (can be one month, bimonthly, quarterly or even biannual), the TLD badges are sent for reading. The TLD reader consists of a heater. On getting heated, the excited electrons again come back to the ground state and emit light which is then read by a photomultiplier ². The light output is proportional to the radiation exposure ³. 

TLD has better accuracy than a film dosimeter and do not require an estimate of photon energy. It also has a higher dynamic range and are re-usable ³.

If a radiation worker is wearing a lead apron, the TLD badge can be worn under the lead apron (to reflect body dose) or outside the lead apron (to reflect extremity/thyroid dose) ². TLDs can measure doses from 0.01 mGy to 10 Gy ³.