A penile plethysmograph is a device placed around a male's penis for the purpose of measuring sexual arousal. The plethysmograph is capable of measuring minute changes in the circumference in the penis, and these changes in circumference are presumed to be correlated with sexual arousal. A typical penile plethysmograph examination involves exposing the patient to various visual and/or auditory depictions of sexual behavior and measuring his response to each.
It is used frequently with sex offenders, who if on parole or probation cannot refuse it (or they would be sent to prison). The penile response reveals what ages and genders the testee finds sexually stimulating.
A roughly equivalent procedure for women, vaginal photoplethysmography, measures blood through the walls of the vagina, which researchers claim increases during sexual arousal.
These devices have no use outside the criminal justice system.