Why is the image seen by the low-light night vision device green?
Definition of low-light night vision device
The low-light night vision device refers to the use of the weak moonlight, starlight, atmospheric glow, galaxy light and other natural night sky light at night as illumination, and the weak photons reflected by the target are amplified by means of an image intensifier (also called an image intensifier tube) An instrument that converts to visible images for night observation.
How to define low-light night vision device?
The core component of the low-light night vision device-image intensifier
Night vision devices in the traditional sense refer to night vision devices that use an image intensifier 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 a low-light night vision device is a night vision device above the 2nd generation level. The first-generation night vision device still has to rely on infrared auxiliary lights to achieve night observation, and it is an active night vision device. We usually say that the low-light night vision device itself does not need an active light source, it is a passive night vision device.
The picture below is a comparison of the effect of the 1st generation night vision device and the 2nd generation night vision device. The detection ability of the first-generation night vision device for the target is obviously much lower.
Comparison of imaging effects of different night vision devices
As the core component of the low-light night vision device, 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 avoid the air from interfering with the electrons, and also to prevent the components from being oxidized by the air.
Green imaging principle of low-light night vision device
The main difference between the 2nd generation night vision device and the 3rd generation night vision device is the type of cathode surface of the image intensifier. Usually the cathode side of the 2nd generation night vision device is S25, and the cathode side of the 3rd generation night vision device is (GaAs) gallium arsenide. The two cathode surfaces have different photosensitive capabilities, and the third-generation night vision device is better.
Working principle of low-light night vision device
When the ambient light at night enters the night vision device through the objective lens of the night vision device, the first thing it encounters is a thing called a photocathode. The photon shines on it, and the electrons fly out, so that the light is converted into electricity. Under the action of the electric field, the electrons continue to move forward. The second layer is called a micro-channel plate, with many small channels evenly distributed on it, each channel is more than ten microns long, the distance between the channels is more than ten microns, and the channels are inclined at 8 degrees. This is a magical place. After electrons enter, they will collide with the channel. Each collision can knock out more electrons. New electrons continue to collide forward. The effect is that one electron enters and tens of thousands of electrons rush out. , so the signal is greatly enhanced. The army of electrons continues to rush to the third layer under the action of the electric field, and the third layer is a fluorescent screen. As long as the electrons hit it, it will emit fluorescence, and the electricity will turn back to light, and we can see the enhanced image.
Why does the low-light night vision device see green?
"The image seen by the low-light night vision device is green" determining factor - fluorescent screen
Lights of different colors in the night environment 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 phosphors, which emit 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. It's the same thing with early computer monitors.
Different effects presented by low-light night vision devices
Low-light green tube night vision device picture effect
Low Light Night Vision Effect Image Other Colors - White and Color
The fluorescent screen is a glass plate coated with fluorescent agent (phosphor powder), which is located in the image intensifier tube. The fluorescent agent will emit light after being hit by electrons. It is a device that converts the current magnified tens of millions of times into visible light. Fluorescent screens have different colors, green, white and bluish (the so-called white phosphorous tube, but in fact the white phosphorous tube is bluish). It should be noted that the disadvantage of the white phosphorous tube night vision device is that it will reduce the brightness of the image, and the advantage is that it is not so easy to make the glasses tired.
Low-light night vision device presents white imaging effect
The color night vision device uses filters to filter out the three colors of red, green and blue light, and then enhances them separately. The fluorescent screen is also divided into many small grids, and each grid is coated with a different color coating. Using the mask, different colors of light enter the grid of the corresponding color. 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 rarely seen. The so-called color night vision devices or full-color night vision devices currently on the market are not digital night vision devices with image intensifier tubes but also digital night vision devices with CCD or CMOS detectors.