The wavelength dependence of the photoplethysmogram and its implication to pulse oximetry

Pulse oximetry essentially uses photoplethysmography to calculate oxygen saturation. Consequently the wavelength dependence of the photoplethysmogram(PPG) is of direct relevance to the performance of pulse oximeters. This paper describes this dependence of the PPG signal in both "reflection" and "transmission" modes from 450 to 1000nm in studies on the fingertip. The results obtained show distinct differences between the two modes. This is of significance to the technique of pulse oximetry suggesting that perhaps other wavelengths should be considered for use, and that use of both "reflection" and "transmission" probes on the same oximeter may lead to inaccurate readings in one of the modes.