It uses the infrared spectrum of light. Infrared is just outside the visible spectrum of light that your eyes can see. Within the visiable spectrum are different frequencies of light that correspond to different colors. The camera actually projects a flood of infrared light invisible to your eyes. The reflected infrared light is then picked up through the camera lens and converted into the visable spectrum. Due to the fact that the light observed is outside the visable color spectrum then converted into the visable color specturm, colors as your eyes know them are not preserved. That's why "night vision" is always appears green.

Answered by Santosh Kuma , an ibibo Master, at 5:51 PM on February 03, 2009