A TLD card, or thermoluminescent dosimeter card, is a device used to measure radiation exposure levels that can be harmful to humans
When radiation hits the TLD card, electrons in the thermoluminescent material are excited and store energy. When the card is heated, the electrons release the energy as light. The intensity of the light is proportional to the amount of radiation.

A TLD card cassette is a holder that holds the TLD card, with the clip to be worn under the apron to estimate the whole body dose.