How do night vision goggles work?

Compared to many animals and birds, the ability of humans to see in the darkness is significantly low. But thanks to that amazing brain we possess and the insatiable thirst for progress, we have been able to develop instruments that solve our numerous defects. Night vision goggles are one of the equipment that help us see better in the dark. They are mainly used by the military and law enforcement departments but are also available to civilians.
Night vision goggles work based on the photo electric effect. It is the phenomenon that metals emit electrons when electromagnetic radiation above a particular frequency hit on the metal surface. The emitted electrons are called photoelectrons. It was discovered by Heinrich Hertz in 1887 and explained by Albert Einstein in 1905 for which he was awarded the Nobel Prize in Physics in 1921.
Night vision devices are of two kinds: image intensifiers and thermal images. However, the latter kind is not quite common as the technology is still under development. The night vision goggles mainly use image intensifying technology.
Night vision goggles need a little bit light for working. They employ photoelectric effect to amplify the photons in the visible light. There is a lens system in the device that collects the available light and sends it to a glass plate imbued with photoelectric material. The photons in the light are then accelerated using a potential difference to a plate with many tiny holes. These holes are 10 microns in diameter and have conductive surfaces with a potential difference applied from one side to another side of the plate. When the electrons pass through the holes, they ionize the atoms there and trigger more flow of electrons. They all take place in a part called photo multiplier. Thousands of electrons are produced in this manner and they are all further accelerated to hit a fluorescent screen and produce an amplified image.
The image is intensified about 3000 times of the normal vision. The images appear green in the devices. It is because humans can identify green color better than others in the visible spectrum. It helps to distinguish between the objects and gauge the distance.
Example of Night Vision
Night vision devices were first put to use in the Second World War. The USA used them widely in the Vietnam War to fight the Viet Kong guerrillas. The technology of the device has undergone significant changes since then. Now the night vision devices, especially goggles are used by drivers to aviators.

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