A sensor (1) for detection of gas, in particular for detection of CO2. The sensor (1) has a contact face (2) which is directed towards a measuring site. The sensor (1) includes at least one radiation source (3), a measurement volume (4) for receiving the gas to be measured, and at least a first detector (5) for detection of radiation transmitted from the source (3) to the first detector (5) through the measurement volume (4). The sensor has a path (6) of the radiation between radiation source (3) and first detector (5). The radiation propagates along the path in a non-imaging way.