Heat or light can eject charges from traps T back into the conduction band.When an electron recombines with a luminescence center L, a photon is emitted.The amount of light emitted during luminescence measurement of the sediment depends upon the total radiation dose to which the crystalline material has been exposed during its burial and is called as natural signal.
Trapping: Upon exposure to nuclear radiation, some bound electrons of the atoms making up a mineral's lattice are detached from their parent nuclei and become freely mobile: they are said to enter the conduction band.
Structural defects in the lattice (vacancies, interstitial atoms, and substitutional impurities) create localized charge deficits, which act as traps T for the conduction electrons.
Better still, unlike radiocarbon dating, the effect luminescence dating measures increases with time.
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.
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.Most electrons recombine or are briefly trapped in very shallow traps, but a few are trapped at deep traps and remain there over geological time-scales (1-1000 Ma).The now charge-deficient ion that contributed the trapped charge becomes a luminescence center L Recombination: Electrons trapped in deep traps T do not readily recombine unless induced to do so by natural "clock-resetting events", or under strictly controlled laboratory conditions.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 refers to a group of methods of determining how long ago mineral grains were last exposed to sunlight or sufficient heating.