Vaginal photoplethysmography (VPG, VPP) is a technique using light to measure the amount of blood in the walls of the vagina.
The use of the device is done with the assumption that the more light that is scattered back, and that the photocell senses, the more blood is in the walls of the vagina.
[4][needs update] VPA is defined as the peak-to-trough amplitude of the vaginal pulse wave.
Since VPA does not have a standard unit of measurement, it is difficult for researchers to make between-participant comparisons.
[1] VPG was first introduced in the 1960s by Palti and Bercovici, who affixed a light source and photosensitive cell onto a gynecological speculum and recorded vaginal pulse waves.