Vaginal photoplethysmography (VPG, VPP) is a technique using light to measure the amount of blood in the walls of the vagina.
There is an overall poor correlation (r = 0.26) between women's self-reported levels of desire and their VPG readings.
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.
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.