Materials of geological origin will have absorbed considerable quantities of radiation since their formation, so any human-caused exposure to heat or light will reset the luminescence clock considerably more recently than that since only the energy stored since the event will be recorded.
The way you measure energy stored in an object that you expect has been exposed to heat or light in the past is to stimulate that object again and measure the amount of energy released.
It is useful to geologists and archaeologists who want to know when such an event occurred.
Alternate names sometimes used are optically stimulated luminescence dating (OSL dating) and photoluminescence dating (PL dating).
Thermoluminescence was first clearly described in a paper presented to the Royal Society (of Britain) in 1663, by Robert Boyle, who described the effect in a diamond which had been warmed to body temperature.
The possibility of making use of TL stored in a mineral or pottery sample was first proposed by chemist Farrington Daniels in the 1950s. The potential of using thermoluminescence to date buried soils developed on colluvial and fluvial sediments from Utah and Colorado, U.
These slowly decay over time and the ionizing radiation they produce is absorbed by other constituents of the soil sediments such as quartz and feldspar.
The resulting radiation damage within these minerals remains as structurally unstable electron traps within the mineral grains.
TL dating is a matter of comparing the energy stored in a crystal to what "ought" to be there, thereby coming up with a date-of-last-heated.
In the same way, more or less, OSL (optically stimulated luminescence) dating measures the last time an object was exposed to sunlight.
The accuracy obtainable under optimum circumstances is about 5%.