How low-light night vision devices work and the difference between green and white images

Low-light night vision device refers to the use of natural night skylight such as weak moonlight, starlight, atmospheric glow, galaxy light, etc. as illumination at night, with the help of an image intensifier (also called an image intensifier tube) to amplify and amplify the weak photons reflected back from the target. An instrument that converts into visible images to enable nighttime observation.

The core component of low-light night vision device-image intensifier

Night vision devices in the traditional sense refer to night vision devices that use image intensifiers as the core component. According to the level of the image intensifier, night vision devices can be divided into 1st generation night vision devices, 2nd generation night vision devices, 3rd generation night vision devices, etc. What can really be called low-light night vision devices are night vision devices of the second generation or above. The first-generation night vision device must still rely on infrared auxiliary lights to achieve night observation. It is an active night vision device. The low-light night vision device we usually call does not require an active light source and is a passive night vision device.

The picture below compares the effects of 1st generation night vision goggles and 2nd generation night vision goggles. The target detection capability of the first-generation night vision device is obviously much lower.

As the core component of low-light night vision equipment, the image intensifier plays a vital role. The image intensifier is mainly composed of three parts, namely the cathode surface, the microchannel plate, and the fluorescent screen. The three parts are closely arranged inside the image intensifier, which is completely vacuum. The purpose of the vacuum is first to prevent air from interfering with the electrons, and also to prevent components from being oxidized by air.

The main difference between the 2nd generation night vision goggles and the 3rd generation night vision goggles is the type of cathode side of the image intensifier. Usually the cathode surface of the 2nd generation night vision device is S25, and the cathode surface of the 3rd generation night vision device is (GaAs) gallium arsenide. The two cathode surfaces have different photosensitive abilities, and the third-generation night vision device is better.

How low-light night vision devices work

When ambient light at night enters the night vision device through the objective lens, the first thing it encounters is something called a photocathode. Photons shine up, electrons fly out, and light is converted into electricity. Under the influence of the electric field, the electrons continue to move forward. The second layer is called a microchannel plate. There are many small channels evenly distributed on it. Each channel is more than ten micrometers long. The channels are more than ten micrometers apart and the channels are inclined at 8 degrees. This is a magical place. After electrons enter, they will collide with the channel. Each collision will knock out more electrons. New electrons will continue to collide forward. The effect is that one electron goes in and tens of thousands of electrons rush out. , so the signal is greatly enhanced. The electron army continues to rush to the third layer under the action of the electric field, and the third layer is the fluorescent screen. As long as the electrons are hit, they will emit fluorescence, and the electricity will turn back into light, and we can see the enhanced image.

Low light night vision device green imaging

In the night environment, light of different colors will enter the night vision device, but the color is not distinguished, only the intensity is distinguished. Most of the coating materials on the fluorescent screen use zinc sulfide phosphor, which emits green light under the action of this phosphor, so the images seen by most low-light night vision devices are green. But some places are very bright green, and some places are very dark green. This is the same as early computer monitors.

Low light night vision white and color imaging

The fluorescent screen is a glass plate coated with fluorescent agent (phosphor powder), located in the image intensifier tube. The fluorescent agent emits light after being hit by electrons. It is a device that converts electric current amplified tens of millions of times into visible light. The fluorescent screens have different colors, green, white and bluish (so-called white phosphorus tubes, but in fact white phosphorus tubes are bluish). It should be noted that the disadvantage of the white phosphorus tube night vision device is that it will reduce the brightness of the image. The advantage is that it is not so easy to cause glasses fatigue.

Color night vision devices first use filters to filter out the three colors of light, red, green and blue, and then enhance them respectively. The fluorescent screen is also divided into many small grids, and each grid is coated with a different color coating. Using masks, different colors of light enter the grids of corresponding colors. This is more difficult, and in the process of filtering the three-color light, the intensity of the light will also be lost, so color night vision devices are currently rare on the market.

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