It is shown that the emission of quanta in the visible region by the heating unit of a thermoluminescence reader is a major source of spurious signal in observations of low radiation dosage. At a heater temperature of 300 degrees C, about 120 photoelectrons per second are emitted from an S11 photomultiplier cathode for a typical reader construction. A quantum response tail for the cathode, extending into the infra-red, is also discussed as a possible source of with careful optical filtering, and by cooling the photomultiplier tube to a dark-current plateau region, the lower limit of observation of thermoluminescence signals can be extended.