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Question

How night vision goggles work?
Asked by: John Sunderland

Answer

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 of detection.
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 source.

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


Science Quote

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Steven Weinberg
(1933-)


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