The invention relates to a sensor having a filter arrangement, downstream of which there is arranged a detector arrangement, and an evaluating device connected to the detector arrangement. The filter arrangement has at least a first filter, the suspect filter, and at least one second filter, the reference filter(s). The first filter is configured as a band pass filter allowing the passage of a first predetermined band, the suspect band. The at least one second filter is configured as a band pass filter allowing the passage of a second predetermined band(s), the reference band(s). The detector arrangement has at least one detector associated with at least one of the filters. The band passes reference filters are distributed above and below the band pass of the suspect filter. The sensor with advantage could be utilized within the IR band, and could advantageously be used to detect CO2.