We have all already witnessed the scene. A war movie hero enters dangerous territory when everything suddenly goes dark, and the next thing you see is his view through a pair of glowing green night vision goggles. When the lights flip that switch, you know it’s about to go out down…but why does it always appear green? Night vision would apparently be more effective if it offered the full color spectrum. However, if they did that, they wouldn’t be night vision goggles. The technology works because it amplifies the part of the visual light spectrum that human eyes are most sensitive to, and that happens to be the color green.
If you paid attention in school, you may remember the rainbow acronym ROYGBIV (red, orange, yellow, green, blue, indigo, violet), which represents the color spectrum of visible light. It is no coincidence that G is here in the middle; green is where our vision peaks. In fact, humans can perceive more shades of green than any other color, which some scientists attribute to the abundance of green on our planet. However, this does not explain why most non-primate mammals, such as dogs and cats, cannot see green. Regardless, understanding the particular position of green in the visual light spectrum is essential to understanding how night vision goggles work. They target the natural strength of the human eye by displaying green, but the question now is how do these glasses do that?
How Night Vision Technology Really Works
At their core, night vision goggles work by amplifying the available light. Some systems can also detect near infrared light, but they do not convert infrared light to visible light. Infrared is right next to visible light on the electromagnetic spectrum, close enough that scientists have even been able to give mice infrared vision (which they also perceive as green). The procedure could one day be applied to humans, but until then we have to rely on night vision goggles and cameras, most of which are based on a technology called image enhancement.
Image enhancement devices are equipped with a high-voltage, battery-powered tube called an image intensifier, which collects all available light, including visible light and some near-infrared wavelengths, captured by the device’s lens. The light hits a photocathode which emits electrons in response. The electrons then pass through a perforated glass plate called a microchannel plate, where they are hit with thousands of volts of electricity. The process multiplies the electrons thousands of times before they reach the end of the image intensifier. At the end of the tube is a screen covered with phosphors, which release photons when the mass of electrons collides with it. These phosphors are what you ultimately see glowing green through night vision goggles, because green is easier for the human eye to distinguish in low light conditions.
It’s a terribly complex process, so here’s the gist: a small amount of visible light is collected, converted to electrons, multiplied thousands of times, and finally converted back to visible light. The fact that night vision lenses continue to get thinner while doing all of this is a breathtaking technological feat.
Could you make night vision goggles in other colors?
Most night vision devices use the same image enhancement technology that makes everything green. However, there is another, less common, night vision technology that allows you to see in the dark with other colors. This is called thermal imaging and is based on the fact that infrared light is emitted by objects as heat. If you’ve ever seen the Predator movies, you’ll be familiar with this technology. The mask the titular monster wears is a thermal imaging device, and its view is exactly like what you’d see with a typical pair of night vision goggles built with this technology.
Thermal imaging devices have special lenses designed to collect infrared light, as well as built-in infrared detector arrays. The lens focuses the infrared radiation and the detector array scans it and records the radiant heat of anything in its field of view. It then generates a heat map, technically known as a thermogram, and transmits it to a circuit board with a chip that converts the data into a visual display.
Thermal imaging can be used to see in the dark, and images appear in different colors depending on the amount of heat they give off. However, this does not create as sharp an image as alternative image enhancement technology, which is why the vast majority of night vision devices do and will continue to display green images.
