1. An infrared (IR) lamp directs waves of light through a tube filled with a sample of air toward an optical filter in front of an IR light detector. 2. The IR light detector measures the amount of IR light that passes through the optical filter. 3. The IR wavelength must match the absorption band of the target gas. For example 4.2 micron band for CO2. 4. As the IR light passes through the length of the tube, the target gas molecules absorb the specific band of IR light while letting other wavelengths of light pass through.