Photo plethysmography  (PPG) is based on the determination of the optical properties of a selected skin area. For this purpose non-visible infrared light is emitted into the skin. Depending on the blood volume in the skin more or less light is absorbed. Consequently,  the reflected light corresponds with the variation of the blood volume. Therefore, by means of the measurement of the reflected light, blood volume changes can be determined whereby the optical properties of tissue and blood are used.

For pulse wave analysis in acral regions, like fingers or toes,  photo-plethysmography (PPG) can be used (in addition to impedance plethysmography).  For the measurement optical probes must be applied which are very small and easy to handle. Therefore the photometric method is much more handy for measurements in acral regions. Additionally, this method allows you to determine blood volume changes in the small vessels of the skin.

Photo-plethysmography can be considered as a completion of impedance plethysmography, because the photometric method has a limited penetration depth so that only the vessels of the skin can be analysed. Consequently, examinations with photo-plethysmography can only be carried out on selected measuring segments.

Because of its simplicity, photo-plethysmography (PPG) is very suitable for screening analysis of the acral blood circulation.