The invention relates to a measurement apparatus for measuring the concentration of a gaseous substance. The apparatus comprises a light source, a light sensor, and a housing comprising at least one first housing member having a low thermal conductivity. A light path is formed from said light source to said light sensor, wherein the light path passes through a measurement region within said housing. The light source is configured to emit light with a spectral distribution such that said light is absorbed by said gaseous substance. Said light sensor is configured to receive the light emitted bythe light source after it has passed through the measurement region. The first housing member comprises a thermal shielding region facing said measurement region on its one side and said light sensoron its other side, and is configured to permit the passage of light.