An infrared detector includes a radiation source configured to emit electromagnetic radiation. The detector includes a source monitoring apparatus configured to generate output signals (62) related to electrical resistance through the radiation source. The output signals are used to account for radiation source temperature in measurements by the detector. The output signals may be used (72) in the provision of power to the radiation source to maintain the temperature of the detector at a beneficial level. The detector is configured to monitor a level of a gaseous molecular species within a flow of breathable gas.