Abstract: The percentage gas concentration within a sample as measured in a conventional optical bench is derived from the composite signal of the reference beam and sample beam transmitted through a sample cell in the optical bench. The primary source, which generates the reference beam and the secondary source which generates the sample beam are driven ninety degrees out of phase. The composite signal derived from the detector in the optical bench is digitized and then integrated according to the quadrature of the primary component of the composite signal derived from the reference beam, and is also integrated according to the quadrature of the secondary component of the composite signal derived from the sample beam. What is obtained is the integral of the absolute value of the secondary component and the integral of the absolute value of the primary component of the composite signal over one cycle of the reference beam and sample beam.