Abstract: A device for determining at least one condition parameter relating to a concentration or a partial pressure of an analyte in a gaseous medium, the device including at least one light source configured to emit light; at least one light receiver configured to detect the light; at least one optical sensor unit, and at least one temperature measuring device configured to measure a temperature of the optical sensor unit at least indirectly, wherein at least the light source, the light receiver and the temperature measuring device are arranged within a housing and at least the sensor unit is arranged outside of the housing, wherein at least one optical property of the sensor unit is a function of the condition parameter of the analyte, wherein light rays emitted by the sensor unit due to the sensor unit being irradiated by the light source are detectable by the light receiver.