Abstract: A selective infra-red detector is used to determine the amount of a selected gas present in a gas sample. A beam of infra-red energy is passed through a gas sample and into the selective infra-red detector. An outer annular portion of the beam is reflected from a paraboloidal mirror onto a first focal point, while an inner annular portion of the beam passes through a chamber containing a fluid which will absorb the spectral wavelengths of the selected gas and is reflected by a spherical mirror onto a second focal point. Separate infra-red energy detectors are positioned near the first and second focal points to measure the infra-red energy. The difference or ratio between the infra-red energy measured by each infra-red energy detector is a measure of the amount of selected gas present in the gas sample.