This is the real reason night vision goggles are green

The Tweet: “Night vision goggles are green because the human eye can tell difference between more shades of green than any other color.”

There are three popular types of night vision including thermal imaging and near infrared illumination. But this tweet refers to one of the more popular forms; low light imaging using image intensifiers. This form of night vision gives the distinctive bright green hue and is common to head-mounted night vision goggles used by the military.

Night vision goggles take what little light there is available and amplify it. The green hue can be explained by the image intensifier screen within the goggles. The image intensifier screen is made up of phosphor, which when electrons are concentrated onto it glows bright green. A green phosphor is used because the human eye can distinguish between more shades of green than any other colour. So by amplifying the light available, and converting the light into shades of green we are able to distinguish between subtle differences in shade at very low light levels. However, the goggles would never work in complete darkness as there would be no light to amplify.

A CIE 1931 colour/space chromaticity diagram. The outer curved boundary is the spectral locus with wavelengths shown in nanometers. Image: Wikicommons

Chromaticity describes the quality of colour that is distinct to brightness. The diagram above shows the range of colours the human eye sees, mapped from red to green to blue, with the visible light electromagnetic spectrum running around the perimeter. This shows that colour does not run in a linear line from red to blue but operates in a three dimensional space. This is because the human eye has three colour sensors, red, green and blue (RGB), that respond to different ranges of wavelengths.

Colour can be divided into two parts: brightness and chromaticity. For example, white is a brighter colour than grey, but both are considered to have the same chromaticity. On the above graph green is most prominent, meaning we can see in a greater range of green hues than other colours.

However, unlike thermal imaging and near infrared night vision, it is possible to seriously damage the user’s vision. These goggles cannot control for blooming caused by bright light sources that are amplified to an even greater extent.

Image: STML/ Flickr

This post was first published on The Untweetable Truth (10/12/2014)

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