How does a TLD measure radiation?

The TLDs contain crystals of the lithium fluoride (LiF) and calcium fluoride (CaF2) type. When a TLD is exposed to ionizing radiation at ambient environment, the radiation interacts with the crystal in the TLD and ionizes some of the crystal atoms, producing free electrons. Some of these free electrons of higher energy are trapped in the crystal. When the TLDs are heated in a reader machine back at the laboratory, heating causes the trapped electrons to release from the crystals and relieve the captured energy as light, hence the name thermoluminescent. The amount of light is counted by the photomultiplier tubes of the reader machine. As the amount of light is proportional to the radiation absorbed by the crystal, the ambient radiation level at the monitoring site over the measurement period can be calculated. The crystals can then be re-used after annealing to remove any residual energy.