Trapping:  When thermoluminescent materials are exposed to ionising radiation  some of the electrons which are freed by the ionisation are given sufficient energy to move through the crystal: 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. Most electrons recombine or are briefly trapped in very shallow traps, but a few are trapped at deep traps and remain there over geological timescales (1-1000 Ma) if the material is kept at room temperature. The now charge-deficient ion which contributed the trapped charge becomes a luminescence centre 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. Heat or light can eject charges (trapped electrons) from traps T back into the conduction band and then they recombine with luminescence centers L and give up their surplus energy by emitting visible light. This phenomenon forms the basis of thermoluminescence and optical stimulated luminescence.

In order to understand the value of the information contained in the TL observation, it is helpful to understand something about the process that produces the light emission. The crystal is rendered capable of emitting TL by exposure to ionising radiation, which redistributes electrical charges within the material. Some of charged particles become confined at trap sites, while others are localised at luminescence centres . During the TL measurement, light is produced in a two stage process, which is illustrated in figure 3. In the first stage, the charges held at trap sites are released by the action of heat and become mobile. In the second stage, these charges are attracted to oppositely charged particles at the luminescence centres, combine with them, and release energy in the form of light. At the end of the measurement, the crystal contains fewer centres with unpaired charges, and thus returns to a situation similar to the one it was in prior to radiation exposure.

In practice, the systematic observation of TL involves the continuous measurement of the varying intensity of light output as the material is progressively heated. During the measurement, the heat input is controlled to produce a constant rate of temperature increase, so that the temperature rise is proportional to the time that has elapsed since the start of the heating. As a result, the graph of TL intensity plotted against time also represents the output of TL as a function of temperature and this graph is referred to as a glow curve and it is a unique product of the TL measurement. . 

Glow curve information may be used to detect and separate different TL processes which contribute to the total light intensity, and allows the properties of the component signals to be determined. This knowledge, in turn, informs the sampling strategy for optimising the TL signal most suited for dating purposes. Additionally, the glow curve provides the means, firstly, to distinguish between thermally stable and unstable luminescences, and secondly, to detect the likely condition of sedimentary TL immediately after deposition. 

The glow curve owes its significance to the fact that the temperature of the crystal defines the amount of thermal energy which is available for initiating the first stage of the TL process. It requires more thermal energy to release charge from a deep trap than from a shallow trap, so that the TL associated with a deeper type of trap appears at a higher temperature. Thus, the temperature scan which constitutes the TL glow curve represents a scan through the various types of trap present in the crystal. 
 

Figure 1 shows the variation of TL intensity against temperature observed during the heating of a sample of calcite. As seen in this example, the glow curve is formed from a number of peaks having different heights, widths and shapes. The TL belonging to each peak is generated by a particular structural element within the crystal, by a mechanism which is described below. The term TL signal is used to refer to the luminescence produced by a given structure. The positions and shapes of the TL peaks are related to the characteristics of these structures, which are in turn typical of the crystal containing them.

In addition to a characteristic glow curve, the light emitted       during TL also possesses a particular colour. This colour can  be expressed in terms of the way that the light's intensity varies  with its wavelength. The graph of light intensity against  wavelength is called the emission spectrum . During the TL  measurement, in addition to the variations of light intensity,  there are also alterations in the colour of the light. This is  illustrated in figure 2, which shows how the emission spectrum  of calcite changes its shape as the temperature increases.  By contrast, the colour of the light emitted is determined, in the second stage of the TL process, by the type of luminescence centre where the charges combine. Since the form of the glow curve is related to the charge traps, and the emission spectrum is determined by the luminescence centres, the TL measurement provides information about both stages of the luminescence process simultaneously.