That radiation frees electrons that get trapped in crystal defects, just like dosimetry badges.
Energy absorbed from ionizing radiation (alpha, beta, gamma, cosmic rays) frees electrons to move through the crystal lattice and some are trapped at imperfections in the lattice.
Subsequent heating of the crystal, or stimulation by absorption of light can release some of these trapped electrons with an associated emission of light - thermoluminescence (TL) or optically stimulated luminescence (OSL) respectively.
This is the technology used for dosimetry badges in areas where radiation safety is a concern.
The time over which the badge has been exposed is well known, and the total radiation does controls the final luminescence.
The badges are heated (TL), luminescence recorded, and total dose derived.
Since we know the time period of exposure and total does, we know the average dose per unit time.Now turn the process around; if you know the average dose per unit time, and the total dose from the luminescence, then you know the time period of exposure.This is the fundamental process behind luminescence dating (TL and OSL), as well as electron spin resonance (ESR) dating, which uses a different technique to achieve the same result.OSL and TL dating techniques don't get as much press in the creation/evoluton game, because they cannot be used to measure the billions of years time period characterized by the age of the Earth.But they can be used to measure significant periods nonetheless, especially in the range between about 40,000 to 50,000 years where radiocarbon dating cuts off, and the 1,000,000 years or so required for most radiometric techniques to become reliable.The idea here is that all materials carry extremely low concentrations of radiogenic isotopes, line Uranium, which in turn expose the material to extremely low doses of radiation over a long time.