How does infrared night vision work?
(Duane, Grants Pass, Oregon) An infrared night vision system senses heat radiated by things and produces a video picture of the heat scene. See figure. The gadget that senses the heat is a photocathode, similar to the one in a video camera, except it is sensitive to infrared radiation instead of visible light. An infrared image of nv hummer. Photo courtesy of www.x20.org. To understand photocathodes, consider how light and metals interact. When a photon (a small particle of light) hits a metal surface, it might kick out an electron. I say “might” because each metal needs a certain minimum amount of energy before it emits an electron. Infrared photons, however, have such puny energies they can only knock an electron out of special metals. A heat-sensitive photocathode contains a very thin layer of such a metal coated on an optically flat piece of glass. A lens focuses heat from the scene you want to “see” onto the photocathode glass. The metal layer on the glass is so thin that when hea
