The photoplethysmograph adds new physiological data to existing polygraph information by measuring relative changes in the total blood volume in a body segment and by rapidly detecting any changes in pulse blood volume. The photoplethysmograph uses a photo-sensitive cell to measure infrared light reflected or passed though human tissue. The light is scattered by red blood cells so that the amount of light reaching the photo sensor after passing through the tissue indicates the amount of blood that it had to pass through. Implementation and measurement considerations are discussed, such as the external factors that can affect photoplethysmograph recordings like ambient temperature and the postural location of the monitoring site. Empirical research on the effectiveness of the photoplethysmograph has revealed that this technique provides an independent index of sympathetic arousal and that the underlying physiological principles of the photoplethysmograph are scientifically based and reliable in laboratory research. Figures, references