A gas sensor (1) is described comprising a light source (2), and a detector (4), a first reflector (7), which is concave and arranged to reflect and concentrate light emitted from the light source (2) to a first light spot (31), and an interference filter (5). The gas sensor comprises a second reflector (8), a third reflector (9), which is concave, and a reflector base (37) with a dome shaped surface (17) with the first and third reflectors facing the light source (2) and the detector (4). During operation of the gas sensor (1), the detector (4) is illuminated by light from the light source (2), which in an optical path from the light source (2) has been reflected at least once in each one of the first reflector (7), the second reflector (8), and the third reflector (9). The gas sensor (1) is configured for detection of a first wavelength portion of the light.