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.
Two forms of luminescence dating are used by archaeologists to date events in the past: thermoluminescence (TL) or thermally stimulated luminescence (TSL), which measures energy emitted after an object has been exposed to temperatures between 400 and 500°C; and optically stimulated luminescence (OSL), which measures energy emitted after an object has been exposed to daylight.
To put it simply, certain minerals (quartz, feldspar, and calcite), store energy from the sun at a known rate.
Different materials vary considerably in their suitability for the technique, depending on several factors.
Subsequent irradiation, for example if an x-ray is taken, can affect accuracy, as will the "annual dose" of radiation a buried object has received from the surrounding soil.
The emission of light that does not derive energy from the temperature of the emitting body, as in phosphorescence, fluorescence, and bioluminescence.
As a result, there is no upper date limit set by the sensitivity of the method itself, although other factors may limit the method's feasibility.All sediments and soils contain trace amounts of radioactive isotopes of elements such as potassium, uranium, thorium, and rubidium.These slowly decay over time and the ionizing radiation they produce is absorbed by mineral grains in the sediments such as quartz and potassium feldspar.Luminescence dating (including thermoluminescence and optically stimulated luminescence) is a type of dating methodology that measures the amount of light emitted from energy stored in certain rock types and derived soils to obtain an absolute date for a specific event that occurred in the past.The method is a direct dating technique, meaning that the amount of energy emitted is a direct result of the event being measured.