"Thermoluminscent" means "light emitted from heat." It is necessary that the material that the TLD "reads" for radiation is transparent to the light it emits. While there are many different materials that can be used for this purpose, there are two that are most frequently used. Calcium fluoride is one. Known as the "window material' in laboratory use, calcium fluoride is transparent enough for a TLD to accurately record gamma radiation levels. The calcium fluoride TLD is mainly used for the environmental detection of radiation exposure.
Lithium fluoride is the other material commonly used for TLDs. In addition to gamma radiation, it is also used to detect ionizing radiation exposure from beta particles and neutrons in nuclear reactors. Lithium fluoride TLDs are usually used for personal radiation detectors, such as those worn by workers on the trunk that give readings of the whole body, or the extremity version that detects radiation exposure levels to the hands.
The amount of light emitted from a TLD crystal is small. To read the output correctly, the TLD is placed in a dark room equipped with a photomultiplier tube, a vacuum container that is extremely sensitive to light. When the chip is heated, the photomultiplier turns the light into an electronic signal which is then amplified. The resulting output is called a "glow curve" and the region below this curve is directly proportional to the amount of radiation absorbed in the chip. When monitoring whole body exposure, a single TLD crystal is placed under each filter in the device worn like a clip-belt on a jacket. When used as a finger monitor, the TLD crystal is placed in a small hollow of a plastic ring and covered with a protective ID label.
All TLDs are versatile, in that they can be used in different radiation environments and provide an accurate response to a range of radiation types. They are also hard-wearing with long usage lives because of their resistance to environmental factors like heat and humidity, although TLDs using lithium fluoride tend to last longer -- up to two years.