The invention relates to a sensor having a filter arrangement, downstream of which there is arranged a detector arrangement, and an evaluating device which is connected to the detector arrangement, the filter arrangement has at least a first filter, the suspect filter, which is configured as a band pass filter allowing the passage of a first predetermined band, the suspect band, at least one second filter, the reference filter(s), which is configured as band pass filters allowing the passage of a second predetermined band(s), the reference band(s), and where the detector arrangement has at least one detector associated with the at least one of the filters. The sensor uses the band pass filters to measure the temperature of an emitting source. The sensor with advantage could be utilized within the IR band, and could advantageously be used to detect CO
2 .