How night vision goggles work?
Asked by: John Sunderland
There are two type of 'nightvision' that are commonly used -- passive and active.
Passive (often called 'starlight vision') nightvision equipment simply amplifies
the available light and will not work in absolute darkness. It is similar in
principle to adjusting the contrast (difference between light and dark) of a
picture. [see the additional answer below]
Active nightvision, like the type found on many new camcorders, actually projects
an near-infrared light source out. The optics in the camera are sensitive to this
spectrum, but it is just out of visual range so you can't see it.
Some military equipment uses ultraviolet radiation instead of near-infrared because
of the potential interference with common devices (like remote controls) and ease
Answered by: Charles Hill, Network Engineer, Deltona, FL
Night vision goggles depend on the photo electric effect. This effect was first
discovered experimentally by Heinrich Hertz in 1887 and explained by Albert
Einstein in 1905 using quantum theory.
Basically when a polished plate is exposed to electromagnetic radiation, it may
emit electrons. These electrons are referred to as photoelectrons. These electrons
are only emitted if the frequency of the incident light exceeds a threshold value
f>f0. The threshold value f0, depends on the particular metal.
The magnitude of the emitted current of electrons depends on the intensity of the
light source and the kinetic energy dependent on the frequency of the light
In the night vision goggles the photoelectric effect is used to amplify the
presence of individual photons. A lens system sends any collected light to a glass
plate coated with a photoelectric material on the back side. The photoelectrons are
accelerated through a potential difference of several hundred volts to a 'channel
plate' containing many fine holes about 10 microns in diameter. This hole has a
conducting surface and an additional potential difference from one side of the
plate to another. When a photoelectron strikes a hole, it ionizes atoms at the
point of impact. This in turn releases several electrons, which are accelerated
farther down the hole to produce even more electrons. This part of the device is
called a photomultiplier. The result of this device is tens of thousands of
electrons leave the hole. These electrons are further accelerated to
strike a fluorescent screen, where the effects can easily be seen by the eyes.
Noise keeps this device from being infinitely sensitive. Photons arriving on the
photoelectric surface are discreet and hence the numbers fluctuate with time. The
smaller the rate of arrival the larger the the fluctuation of any given pixel. This
manifests itself as a fluctuating brightness called 'photon noise'.
To combat this, the arriving photons are integrated for longer periods by using a
fluorescent screen in which the brightness builds up and decays slowly. There are
limits to the integration time. Too long an integration time will cause images to
become 'smeared' across the screen.
Answered by: David Latchman, B.S., University of the West Indies, Trinidad
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