Abstract: A method and apparatus for detecting and quantitatively measuring a molecular species of gaseous material in a sample to be analyzed are provided. Light containing incoherent infrared radiation is collected, collimated and transmitted by a light conditioning means to a primary filtering means. The primary filtering means selectively transmits light having a frequency range in the region of an absorption band for a molecular species to be detected. A secondary filtering means, adapted to receive the filtered light, transmits light at a plurality of discrete frequencies, providing a detectable signal, through the gaseous material. The intensity of the signal changes in proportion to the concentration of the molecular species. Means are provided for measuring and recording the magnitude of the signal intensity change.