A medical application of a photoconducto is the plethysmograph(or pulse detector) used by athletes(Fig. 14.35). The amount of light from the LED that is reflected to the photoconductor is inversely proportional to the volume of material the light passes through. An arterial pulse will increase the artery volume, thus increasing the photoconductor's resistance. A constant current source through the photoconductor results in a voltage pulse applied to a rate counter and display. It is important that the spectral responses of the LED and photoconductor be matched for maximum transfer efficiency.