V is for Vaginal Photoplethysmograph

A vaginal photoplethysmograph is basically a light bulb tampon. 

Once inserted, the light is shined on to the vaginal walls and the device can detect blood flow by measuring how much light is reflected back.

The University of Amsterdam VPA/VBV Probe

The University of Amsterdam VPA/VBV Probe

Vaginal photoplethysmographs measure both vaginal blood volume (VBV) and vaginal pulse amplitude (VPA).  Increases in both these measures (especially VPA) are thought to be fairly reliable indicators of female sexual arousal.

See: Penile Plethysmograph