The intrinsic thermoluminescence (TL) efficiency of a TL detector relates the absorbed dose in the detector material to the light yield observed upon evaluation. Knowledge of the TL efficiency is of interest when performing numerical simulations of detector response, where only absorbed dose can be predicted. Here, the experimental determination of TL efficiency for calcium sulphate (CaSO(4):Tm) and lithium borate (Li(2)B(4)O(7):Cu) is reported. These materials are widely used in Panasonic dosemeter badges. The results of the study are in agreement with predictions from track structure theory and microdosimetry, relating an enhanced light yield at low X-ray energies to supralinear behaviour of the TL phosphor.