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Why is night vision green?

Last Updated: 02.06.20

 

If you have ever watched a movie in which night vision was used, or you have ever used a device that takes advantage of this technology to help people see in the dark, you have most probably noticed that the image shown through such devices is always green. The two come together so well that the moment someone mentions night vision binoculars to you, that instantly brings in front of your eyes that green image you are so familiar with.

The advantages of seeing what our eyes can’t see in the dark are tremendous. For military applications, as well as civilian operations, night vision technology offers tremendous benefits. It can help locate survivors of natural cataclysms, and it can pinpoint the location of an enemy. As a layman, you are, however, interested in learning why the image of this type of vision is green. Let’s make it all clear.

When was this technology invented?

You might be surprised to learn that this technology has been around for a while, being used extensively in World War II. Both the US military and the Reich, worked on this technology before and during the war, with the purpose of using it on the battlefield. Both the infantry and the tank divisions used it to identify the location of their enemies.

However, the night vision technology used more than half a century ago was nothing like the elegant portable night vision goggles we’re practically identifying it with. The devices used during those times were bulky and needed manpower to be transported and used in the field.

The idea of making this technology portable, as well as the possibilities, did not occur on a practical level until the Vietnam War when the US military began to use the first devices that could be worn by a soldier on his head.

The main principle that night vision goggles use remains the same: it is the use of whatever light is emitted by the surrounding environment, coupled with image intensification technologies. No matter how advanced night vision is today, you should remember that the same original idea that was used during the aforementioned conflicts continues to stay at its foundation.

So, what’s the idea with the green image?

To understand how the green vision occurs, we need to learn a bit more about all the technologies that are involved. These technologies are employed on goggles, as well as scopes and night vision cameras. They are essential for how the green image occurs, and we will approach them as follows.

Thermal imaging

This technology is capable of telling the heat signature of an object against the surrounding environment. In other words, it is capable of telling when the temperature of the target is different from the rest of the objects around.

Active illumination

While thermal imaging sounds something that is used in military-grade operations, illumination is pretty straightforward. The image intensification used by goggles, or other optical devices, might need more light input to process and render the image before your eyes when you look through them.

The illumination used is very close to the infrared band. By adding more light to the environment, the devices create better conditions for the night vision device to harness it and transform it into an image.

Image intensification

We have already mentioned it a few times, and now it’s the moment to reveal that this is the technology responsible for the green image you see through your night vision monocular, goggles, scope, or any other similar device, designed to offer you the ability to see in the dark.

 

How does image intensification create the image you see?

What happens when we look through any vision device is that the image intensification technology employed on the device takes the light emitted by the celestial bodies and multiplies it. That is why we can get a pretty good image of what surrounds us.

As you can guess by now, it is not possible for your goggles, monocular, or binoculars to work in pitch dark, but, the thing is, most of the time, you won’t need your device to perform in such a manner. Absolute darkness is difficult to achieve, so situations that would meet such conditions are extremely rare.

What about the green vision?

To get our explanation farther here is some more information that will help you understand how night vision devices work. When the photons from the light around hit the lens, they have all the colors. The thing is that the photons are turned into electrons, and then all the information which might relate to colors perceived by our eyes/brain turn only into black and white.

The explanation is that the image intensification element is made from phosphorus. As you may well know, phosphorus is green, and its luminance capability manages to capture the light from the surrounding environment and present it to you. Therefore, the image you will see is green, as well.

 

But why is phosphorus used?

It is not by accident that these devices use phosphorus. It all has to do with something called chromatic sensitivity. This ability is related to how sensitive the human eye is toward different colors. From the entire spectrum, the color green is the absolute winner, and that is why night vision goggles and other similar products are made with phosphorus.

To go even deeper, this chromatic sensitivity of the human eye toward the green color means that we perceive more shades of this color than of any other. What happens when our eyes can tell the differences between different values of green is that we can perceive shapes and objects.

Another advantage identified by specialists in this field is that we can tolerate looking at the green color for more extended periods of time than at any other color. The computer screens are green for this reason, as well.

And now you know why the night vision showed through various devices designed to help you see in the dark is green, and how that is beneficial for the human eye.

 

How the eyes work and how they perceive light and color

To understand exactly why night vision goggles show a green image, it is important to understand how the brain perceives color. Vision in human beings involves the eyes and the brain, but the brain is the one that tells you what you see. The eyes are very complex, but it is very simple to understand how this process works.

The first thing you need to know is that light is not actually made of colors, but comes in wavelengths. When these waves get in touch with a certain material that has certain properties, they are reflected from the material and enter the human eye. After this happens, they are interpreted by the brain and we perceive them as colors.

There are multiple waves of light but the eye is not capable of seeing them all, it can only perceive a part of them called the visible light. Visible light is only a part of these waves, the others include gamma rays, radio waves, microwaves, infrared waves, ultraviolet rays, and X-rays.

When the waves present in the visible light get to the brain, it transforms them into colors. Violet is made of high-frequency waves, red is made of low-frequency waves and green is in the middle, made of mid-frequency waves.

The eye has a natural lens that gathers the rays of light onto the back of the eye called the retina. The retina is formed of cells that gather information and, through the optic nerve, send the information to the brain. There are two types of light-detecting cells: cones and rods. 

The cones are responsible for our vision during the day and they are of three types, one type that reacts to green, one type that reacts to red, and one type that reacts to blue. When it is dark outside, the ones who take over are the rods, that are a lot more sensitive than the cones and respond only to light without detecting color. This is why the image we see is black and white.

 

Is green compulsory to see in the dark or is it a choice?

Why do night vision goggles show a green image? People think it is because this is the only way that you can manufacture night vision goggles, but this is not true, it is actually a choice of the manufacturers and to better understand why they make this choice, we first have to go back to our ancestors.

Thousands of years ago, people used to live in the forest and the color green was all around them. To survive, they had to be able to make the distinction between which plants were good for them and which ones were poisonous. Because many of them were the same when it comes to shape, they had to make this distinction by analyzing their color. 

This is how the brain developed a capacity to better distinguish the shades of green, so seeing green is a lot more relaxing for the brain than any other color simply because our ancestor’s brains were really used to it. 

The ability to differentiate among many shades of a certain color is called chromatic sensitivity, and the highest chromatic sensitivity that humans have is to green. This is also the reason why many hospitals have green walls to relax their patients.

To help you better understand why green is the best color for night vision, we should mention that during low light conditions, when the rods are active, the eyes are very sensitive to violet, blue, and ultraviolet. This means that if the image we see is violet, we experience eye discomfort.

The wavelength that is in the middle of the light spectrum is of 555 nanometers, which is actually the wavelength of the color green, meaning our eyes experience the least discomfort during low light conditions when the image we see is green.

So, the fact that green is the most relaxing color and the brain distinguishes between the shades of green, better than any other color, made manufacturers use green. Also, the image is a lot clearer especially because of this, because you will be able to detect every shade and the image will be very similar to what you see during the day.

The magnificent technology that the image intensifier uses

Earlier in this article, we have mentioned that night vision goggles use image intensification to help people see through the dark. This technology is able to detect light even when it is present at low levels and it amplifies it. The system is actually very simple and it is very interesting, as well.

Researchers have managed to intensify an image by using a tube that is used for light to travel through. This tube has some kind of filters, if you want to call them that, which have the role of amplifying the light step by step and we are going to take you through this process.

The image intensifier is composed of a lens that the light goes through when it enters the night vision goggles, a photocathode, a microchannel plate, and a phosphor screen. This may all seem confusing but it will make sense in a moment.

So, when the light enters the night vision goggles it is in the form of photons. The problem is that the amount of light is really low, so scientists have managed to amplify it using a photocathode and a microchannel plate. 

The photons go through the lens which amplifies the image and from there they go through the photocathode which has the capacity of transforming photons into electrons. After the electrons are released, they go through the microchannel plate that has the ability to multiply the electrons. The more electrons, the more light.

At the end of this journey, the electrons hit a phosphor screen which is covered with green phosphor. Phosphor normally has a yellowish color, but scientists have used green because the eyes are more receptive to green and also because it is really easy to produce. 

So, when the electrons hit the phosphor screen they are converted back into photons. These photons enter a fiber optic inverter which has the role of twisting the image and then the light goes through an eyepiece lens and there you go, you can see in the dark. Pretty cool, right? 

 

Are night vision goggles better than infrared vision in terms of image quality?

There has been a lot of debate among people who want to use night vision devices regarding which one is better, infrared or green vision. Comparing the two types of devices does not involve comparing whether the infrared image is better than the green-colored image, but analyzing the conditions that each of these technologies is influenced by.

For the ones who have not read anything about infrared technology, you should know that devices that use infrared technology do not show a red image, but a black and white one. Infrared waves are not perceptible by the human eye, this is one of the reasons why the image portrayed by this technology is not very detailed and looks a little bit awkward.

To summarize the benefits of using either one of them, we can tell you that infrared technology is influenced by heat so it can be used regardless of the amount of light that is present. Also, this technology is better for identifying targets when they are camouflaged and it can be counteracted by using cooling methods. 

Green night vision, however, is dependent on the light conditions and in complete darkness, it does not work because it has no light that it can amplify. It can be counteracted by using very bright light and it is not so effective in detecting targets that are camouflaged because they can not detect heat.

In terms of image, however, there are some differences. If you compare two images of the same environment, one taken by using infrared technology and the other by using night vision technology, you will be able to see that the green image has a lot more details and it resembles an image taken during the day.

It will be a lot more pleasant to watch footage taken using night vision technology because the image is much more relaxing for your eyes. Furthermore, the experience will seem much more natural as it resembles more the way you look at things during the day and in your natural environment than the image shown by the infrared technology does.

However, now that you understand how night vision works and why the image that this technology makes is green, you will be more informed and you will better understand why using night vision goggles is so relaxing compared to gadgets that use infrared technology. 

Even though the science behind it seems very simple, the ones who invented it made a huge discovery that has helped people all around the world to detect danger or engage in different nighttime activities, like observing the activity of wild animals.

 

 

Ioana Moldovan

Ioana’s professional experience in the optics field has helped her understand the value of passing her knowledge forward. Her curious personality helps her gather useful information for her readers and her goal is to make technical information fun and accessible to everyone.

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