An apparatus for detecting a gas having distinct infrared radiation absorption characteristics. The apparatus includes a spectral source/bolometer for conducting an electrical current and for producing an infrared radiation. The source/bolometer is disposed along an axis and has a temperature and a characteristic resistance, and the characteristic resistance is a predetermined function of the temperature. A return reflector is disposed along the axis beyond the gas such that at least a portion of the infrared radiation passing through the gas is reflected back through the gas to the source/bolometer. The apparatus also includes a driver/detector for driving a current through the source/bolometer, for determining the characteristic resistance, and for detecting the gas from a variation of the characteristic resistance.