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An infrared
night vision system senses heat radiated by things and produces
a video picture of the heat scene. 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.
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. "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.
Infrared
Night Vision is excellent equipment, especially for police and
military, when there is insufficient visible light. The radiation
is detected and turned into an image on a screen,
hotter objects showing up brighter, enabling the police and military
to acquire thermally significant targets, such as human beings and
automobiles. Infrared
Night Vision serves greatly for firefighters as well; smoke is
transparent to infrared than to visible light, therefore firefighters
use infrared
night vision equipment when working in smoke-filled areas.
Night Vision Gear carries a selection of Infrared
Night Vision, such as infrared
night vision binoculars and infrared
night vision goggles.
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