A bioluminescence monitor comprises a photodetector 19 for detecting the amount of light emitted by a sample vessel 11 in a chamber 13. A reflector in the chamber 13 comprises a pair of elongate concave surfaces 16A, 16B which reflect a large proportion of the emitted light towards the photodetector 19. The monitor is calibrated by an LED 20 directed at the photodetector 19 and driven by a pulsed drive signal to provide a light output similar to the light output of an actual sample. The LED 20 is not continuously driven over the calibration period and thus does not heat up significantly enough to effect the level of the light output.