1. An apparatus comprising: a. a camera; b. an electrical signal generator; c. an electrode, coupled to the signal generator, that stimulates emission of light from the finger when the finger is at a position relative to the camera; d. a light source that illuminates the finger at the position; and e. a circuit that activates the light source and the camera to obtain a first image of the finger at the position, and activates the signal generator and the camera to obtain a second image of the emission of light from the finger at the position.