Friday, 24 November 2017
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Why is night vision green?

Night vision technology has a variety of uses in both the civilian and military sector. It can be integrated in cameras, goggles and scopes, providing us with the ability to see in the dark. When people think about night vision goggles, the first thing that comes to mind is the green color of the visual field. We have seen it in lots of movies and video games but most of us don’t know why do we see everything in this particular color.

Invention of night vision

The first generation of night vision devices was developed by German and US military in the mid-1930s and was used by tanks and infantry during WWII. These devices required a large power supply to be carried by soldiers and they worked on image intensification technology.

With the advancement of technology in following decades, night vision devices became more and more sophisticated. They switched to image intensification technology and were small enough to be worn on the head. These devices were first used by US military during Vietnam war.

Today these second generation devices are widely available for civilian use, however both military and police forces have upgraded their technology. Despite the technological advances, the night vision goggles based on image intensification technology still work on the same principle.

Technology behind green vision

There are three types of technology used in night vision goggles, scopes, and cameras. First one is thermal imaging – it detects the temperature difference between the background and the foreground. The second type of technology is active illumination technology that couples imaging intensification with a source of illumination in the near infrared band. The third one, image intensification, holds the answer as to why night vision is green. Image intensification technology provides us with that famous bright green light in night vision goggles.

night vision shades

It maximizes the amount of light received from natural sources such as starlight or moonlight and “amplifies” it in night vision devices so that we get as clear image as possible. Unfortunately, this means that night vision goggles with this technology can’t work in complete darkness, since it actually doesn’t amplify the light, rather it illuminates it up to a level at which the human eye can detect it. When photons hit the lens at the front of night vision goggles, they are still carrying the light of all colors. But when they are turned into electrons they lose that information and incoming color light is turned into black and white. But why is night vision green then?

The first reason is that image intensification screen inside the device is made of phosphor. This substance is used because of its luminance effect and when struck by electrons which don’t carry color information it glows bright green. Green phosphor is used because the human eye is most sensitive to the green color pallet.

Chromaticity
Chromaticity diagram

 

To better understand how color sensitivity works, we need to look at chromaticity diagram. Chromaticity describes the quality of a color determined by its dominant wavelength and purity, while the  chromaticity diagram shows us the range of colors human eye can see, mapped from red to blue to green. Based on the chromaticity diagram above it is clear that the color green is the most prominent, which means we can see more shades of green color than any other. It is also easier to look at the green light for longer periods of time, that is also why early computer screens were usually green.

As we can read in this article, there are a few reasons for night visions green color, but the main one is our eyes sensitivity and ability to distinguish between different shades of green. We hope this article gives you a good insight on what’s going on inside the device and the reason behind green night vision.

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