A detector system configured to monitor a level of a gaseous molecular species within a flow of breathable gas, the detector comprising: a source configured to emit electromagnetic radiation; source optics configured to direct electromagnetic radiation emitted by the source across a flow path of a flow of breathable gas that communicates with an airway of a subject; a radiation sensor assembly configured to receive electromagnetic radiation that has been emitted by the source and has passed through the flow path, the sensor assembly being further configured to generate output signals that convey information related to a parameter of the received electromagnetic radiation; a source monitoring apparatus configured to generate output signals that convey information related to electrical resistance through the source ; and a processor configured to determine a level of a gaseous molecular species within the flow of breathable gas in the flow path (i) based on the output signals generated by the radiation sensor assembly and (ii) further based on the output signals generated by the source monitoring apparatus to account for the electrical resistance through the source , characterized in that the processor is further configured to estimate irradiance of the radiation source based on the output signals generated by the source monitoring apparatus , to determine measured irradiance received at the radiation sensor assembly based on the output signals generated by the radiation sensor assembly , and to compare the estimated irradiance with the measured irradiance to determine a difference, still further wherein responsive to a determination that the difference between the estimated irradiance and the measured irradiance breaches a threshold, the processor generates a notification to initiate a recalibration of the radiation sensor assembly . a source configured to emit electromagnetic radiation; source optics configured to direct electromagnetic radiation emitted by the source across a flow path of a flow of breathable gas that communicates with an airway of a subject; a radiation sensor assembly configured to receive electromagnetic radiation that has been emitted by the source and has passed through the flow path, the sensor assembly being further configured to generate output signals that convey information related to a parameter of the received electromagnetic radiation; a source monitoring apparatus configured to generate output signals that convey information related to electrical resistance through the source ; and a processor configured to determine a level of a gaseous molecular species within the flow of breathable gas in the flow path (i) based on the output signals generated by the radiation sensor assembly and (ii) further based on the output signals generated by the source monitoring apparatus to account for the electrical resistance through the source , characterized in that the processor is further configured to estimate irradiance of the radiation source based on the output signals generated by the source monitoring apparatus , to determine measured irradiance received at the radiation sensor assembly based on the output signals generated by the radiation sensor assembly , and to compare the estimated irradiance with the measured irradiance to determine a difference, still further wherein responsive to a determination that the difference between the estimated irradiance and the measured irradiance breaches a threshold, the processor generates a notification to initiate a recalibration of the radiation sensor assembly . <EOS>