Thermo
luminescent
Dosimeter
Used to measure the amount of radiation someone has received. The TLD is worn on the belt. There are two different kinds in use that I know of: calcium flouride and lithium flouride. On nuclear submarines, the CaF TLD's are worn by the nuc's, and the LiF TLD's are worn by coners. So in a way, they are really coner identification badges, or a mark of inferiority.
They have a little crystal inside (made of CaF or LiF), mounted on an electrode, and wrapped with wire. The crystal absorbs gamma radiation. Later, to read the TLD, a current is applied, which causes the crystal to emit light proportional to the amount of radiation which was received by the wearer, which is sent to a photomultiplier tube that somehow interprets this and charts the result on graph paper.