Laser Doppler Imaging (LDI) is used to measure superficial blood flow in the skin. A laser Doppler perfusion imager functions by projecting an infrared laser beam over the skin. When light interacts with moving blood cells a small portion of it is shifted, detected, and converted into an electrical signal. LDI provides a direct measure of female sexual response that does not require genital contact; signals are gathered at a depth of two to three millimetres below the skin’s surface.
Two studies have suggested that LDI is a valid measure of female sexual arousal. Waxman and Pukall showed that LDI has discriminant validity; that is, it can differentiate sexual response from neutral, positive, and negative mood induced states.
Compared to vaginal photoplethysmography (VPG), LDI is advantageous because it does not require genital manipulation or contact. Also, LDI provides a direct measure of vasocongestion and has an absolute unit of measurement, consisting of flux or units of blood flow. The disadvantages of LDI are that it cannot provide a continuous measure of sexual response and the laser Doppler perfusion imager is much more costly that other methods of genital sexual arousal assessment, such as VPG