Gas sensor with long absorption path length

A gas sensor comprising:

• a light source configured to emit light, and a detector configured to detect light, wherein the light source and the detector are arranged side by side,
• a first reflector, which is concave and arranged to reflect and concentrate light emitted from the light source to a first light spot, on the side of the light source, wherein the gas sensor is configured for detection of a first wavelength portion of the light, characterised in that it comprises
• a second reflector,
• a third reflector, which is concave, and
• a reflector base with a dome shaped surface facing the light source and the detector,
• wherein the first reflector and the third reflector are arranged as reflective surfaces on the dome shaped surface,
• wherein the second reflector and the third reflector are arranged such that the first light spot is on the second reflector and light reflected from the first light spot onto the third reflector is reflected and concentrated by the third reflector to a second light spot on the side of the first light spot,
• wherein the gas sensor is arranged such that, during operation of the gas sensor, the detector is illuminated by light from the light source, which in an optical path from the light source has been reflected at least once in each one of the first reflector, the second reflector, and the third reflector, and
• wherein the gas sensor is arranged such that gas to be sensed may enter into the optical path.