Infra-red night vision
Q: 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 heat photons strike its front surface, they propel
electrons from its back surface. It turns a pattern of heat into a corresponding pattern of
electrons.
A high voltage flings the electrons at accelerating speeds against a phosphor-coated anode layer
located very close. The electrons pick up so much energy in flight that they make the phosphor
glow when they hit. This turns the pattern of electrons into a pattern of light. That's the picture
you see.
One photon striking the photocathode can release dozens of electrons from the phosphor, which
intensifies the image. Most modern systems, however, do much more. They send the electrons
through narrow channels with highly-charged walls. The electrons bounce through these walls
and knock out hundreds, then thousands, even millions of other electrons. The original photon
can release more than one million electrons. With such a system you can "see" heat from the
stars.
Further Surfing:
How things work, sniperscopes, by Louis A. Bloomfield
How night vision works, Sierra Pacific Innovations
Technology to aid night driving, USATODAY.com
An animated infrared Earth image taken by the GOES satellite. |
