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 age of the pottery, in principle, may then be determined by the relation Age = Accumulated dose / Dose per year Although conceptually straightforward, TL has proven to to be far from simple in practice.
Whilst it advisable to protect the sample from direct sunlight there is no need to sample at night and the orientation of the specimen is not important. It is an absolute dating method, and does not depend on comparison with similar objects (as does obsidian hydration dating, for example). The thermoluminescence technique is the only physical means of determining the absolute age of pottery presently available.An age determination technique in which the amount of light energy released in a pottery sample during heating gives a measure of the time elapsed since the material was last heated to a critical temperature.The older a piece of pottery, the more light produced.