Night vision goggles are immensely useful to hunters, soldiers and law enforcement agencies, but despite being used since World War II, there are still many people who don’t know how these devices function. Thanks to military, action and spy movies almost everyone knows what night vision goggles do, but how it is done remains a mystery to many.
Night Vision Principles [Source – NiviTech]
What happens when light particles, called photons, enter the goggles is that they hit the photocathode . The photocathode is light sensitive and immediately converts these photons into subatomic particles called electrons. Using a photomultiplier, the goggles then amplify these electrons. This means that there are much more electrons leaving the photomultiplier than what enters it. The final step in the process is a phosphor screen which causes the electrons to emit tiny flashes of light as they hit the phosphor. Since the whole process results in much more photons than what there was originally the result is that the original scene appears might brighter on the screen.
The process also explains why the night vision goggles displays the original screen in green colors. The reason is that photons that are converted to electrons lose all the color information, effectively turning the scene black and white. Since human eyes are actually more sensitive towards green light this color was deliberately chosen for the night vision goggles to display. The other advantage of using the green color is that it is more comfortable to look at for extended periods of time as black and white cause eye fatigue sooner.
Of course, the above described process produces a lower resolution and blurrier version of the original scene. Other drawbacks to first generation devices include a high-pitched noise which occurs when the goggles are switched on. The unity usually continues to glow green for a short time after it has been turned off as well. Second generation night vision goggles, which are more advanced, but also comes with a heftier price tag, forces the electrons through a so called micro channel plate for multiplication.
The MCP is made up of millions of tiny glass tubes which releases thousands more electrons when the original electrons pass through them. The end result is much more electrons reaching the phosphor screen and a better image. For even better, albeit more expensive results, third and fourth generation night vision goggles make use of chemical layers, consisting of gallium arsenide, to cover the phosphor screen which produces a brighter image and improves the contrast.
Despite the geometric distortion first generation night vision goggles are some of the most popular devices amongst hunters due to the lower price. Thanks to the increased price of second generation devices they are primarily in use by law enforcement agencies. Third and fourth generation devices are mostly confined to professional or military use only because of the high cost.
Since some light is required for these scopes to operate they come with infrared illuminators which produces infra-red light. This light is invisible to the naked eye, but is picked up by the goggles and allows for use in situations where there is total darkness.