Principle and application of ultra-long-range infrared laser night vision system

The most basic ultra-long-distance infrared laser night vision system consists of a high-power semiconductor laser LD, a drive controller, an optical beam expansion collimator lens, a camera and its telephoto lens, a transmission system and a monitor. The high-power semiconductor laser LD, driven and controlled by a large current, emits infrared light that is invisible to the naked eye to illuminate the target object being photographed. However, due to the thin beam and high brightness of the laser, it is necessary to expand the infrared beam to illuminate the target scene within the monitored range through an optical beam expansion collimator lens according to the distance and range of the monitored long-distance target. After being reflected by the object, the infrared light enters the telephoto lens of the camera and forms an image on the photosensitive surface. At this time, what we see is the image formed by the reflection of infrared light, not the image formed by the reflection of visible light, that is, at this time, the ultra-low illumination camera can capture images that are invisible to the naked eye in a dark environment. This image is then sent to the monitoring center through the transmission system for recording and display.

Several technical issues and solutions for infrared light transmission

Since it is an ultra-long distance infrared light wave transmission of more than 1 km, there are several technical issues that should be paid attention to.

1. The quality of transmission in the air is greatly affected by the weather.

2. In any atmospheric transmission link, there are several factors that need to be considered (i.e., the reasons that cause the transmission signal to attenuate).

Absorption by molecules in the atmosphere

The signal loss of the atmospheric transmission system of LED and LD light is mainly caused by the absorption of the transmission medium - the atmosphere. Because when the light beam passes through the gas, there will always be a certain degree of molecular absorption. Moreover, the air absorbs certain wavelengths of light very strongly, and these wavelengths cannot be used for signal transmission at all. The band where the attenuation caused by atmospheric absorption is still acceptable is called the atmospheric transmission window. The data of this atmospheric transmission window band can be found in various literatures, so all LED and LD systems must work at the wavelength within this atmospheric transmission window.

Airborne particle absorption

Particles in the air, such as dust and smoke, are another factor that causes optical signal absorption. Obviously, the atmosphere always contains some such particles to a greater or lesser extent, especially near water bodies, where the content of such particles is sometimes very high. In such places, it is generally always best to install optical equipment higher from the ground to improve the effect of optical transmission.

Absorption and scattering of fog

Fog is also a factor that causes severe infrared absorption, and fog can also cause light to scatter forward and backward. Therefore, in foggy areas, the working hours of the optical transceiver must be selected according to the local climate, because the system will not work properly when foggy.

The impact of atmospheric turbulence

The atmosphere has a certain degree of turbulence, which not only causes signal loss, but also introduces noise into the signal. For example, wind can cause atmospheric turbulence, which in turn causes the refractive index of the air on the signal path to change. This phenomenon is similar to the phenomenon of heat waves and mirages in areas exposed to the sun. This effect ultimately refracts the infrared beam to other directions that cannot be determined, so that the camera cannot capture the target to be monitored.

It can be seen that since infrared light wave transmission technology is transmitted in the air medium, its transmission quality is greatly affected by the weather. Generally, sunny days have the least impact on transmission quality, while rain, snow and fog have a greater impact on transmission quality. According to tests, the empirical value of the attenuation of infrared light wave transmission affected by weather is: 5~15dB/Km on sunny days; 20~50dB/Km on rainy days; 50~150dB/Km on snowy days; 50~300dB/Km on foggy days. At present, to solve this problem, higher power LD tubes, more advanced optical devices and multiple beams are generally used.

In fact, for the ultra-long-range infrared laser night vision system, it is mainly affected by the absorption and scattering of the above-mentioned particles and fog in the air. The synchronous pulse range gating technology can better solve this problem.

Synchronous pulse distance gating technology

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Range-Gate technology actually has two key points: one is pulse laser beam irradiation technology, that is, the emitted laser is controllable pulsed; the other is to have an enhanced CCD that can be turned on and off at high speed, and its switching speed is several hundred ns, and it must be strictly synchronized with the laser beam pulse. In this way, the observer can select a specific observation distance, so that the interference of strong scattered light and reflected light generated by water droplets, fog, snow, sand and dust at other distances can be easily eliminated. Because the CCD is closed most of the time and does not receive these interference lights, they will not be displayed on the CCD.

Laser safety affects its use

To solve the problem of weather affecting the quality of light wave transmission, the power of laser diode is often increased. However, lasers exceeding a certain power level may have an impact on human eyes, so the human body may be harmed by the energy released by the laser system. Therefore, in order to increase the night vision distance, the laser power cannot be increased indefinitely.

Selection of several main components of ultra-long-range infrared laser night vision system

The wavelengths of visible light that the human eye can see are 0.78μm and 0.38μm from long to short, and the colors are arranged in the order of red, orange, yellow, green, cyan, blue, and purple. Light with a shorter wavelength than purple light is called ultraviolet light, and light with a longer wavelength than red light is called infrared light. Using an infrared light source, infrared light that is invisible to the naked eye can be emitted to illuminate the object being photographed. After being reflected by the object, the infrared light enters the lens and forms an image on the CCD. At this time, images that are invisible to the naked eye in a dark environment can be captured.

At present, the infrared light sources widely used in the market include LED infrared lamps, micro-array LED infrared lamps, halogen filter infrared lamps, etc. However, these infrared lamps have certain limitations in terms of irradiation distance, power consumption, efficiency, etc., and are not suitable for ultra-long-distance night vision monitoring over 1km. Therefore, this system must use near-infrared high-power semiconductor laser light source illumination. We use domestic semiconductor laser diodes LD, whose wavelength is 808nm and power ranges from 1W to 10W according to the required distance.

Camera selection

As we all know, there are two types of solid-state cameras: CMOS and CCD. Like CCD, CMOS image sensor chips also respond to infrared non-visible light waves, but their sensitivity in the range of 890-980nm is much higher than that of CCD image sensor chips, and the gradient of attenuation with increasing wavelength is also slower. With the rapid development of CMOS image chips, their noise signals are further reduced, and starlight-level CMOS cameras will also be available. Low power consumption, high integration, and small size are only possible with CMOS image sensors, so CMOS cameras can be made as small as shirt buttons or suit buttons. With the corresponding more miniaturized infrared light sources and the introduction of high-efficiency batteries, the third eye will be everywhere. Wearing a pair of night vision glasses and a hat equipped with an infrared light source and a CMOS ultra-micro camera, the night will be as bright as the day. Obviously, this will change the face of our entire social life.

Since we use domestic semiconductor laser diodes LD with a wavelength of 808nm, we choose CCD cameras. But it is worth noting that 1/4"CCD cannot be used for infrared night vision at an effective distance of more than 15m, because the luminous flux of 1/4"CCD is only 50% of that of 1/3"CCD. The larger the size of CCD, the greater the luminous flux it receives; the smaller the size of CCD, the less luminous flux it receives. Therefore, 1/2"CCD is often used for ultra-long-distance night vision cameras.

Generally, night vision cameras require that the minimum illumination of the CCD should not exceed 0.02LUX when no infrared light is added, but some camera manufacturers or sellers falsely report the minimum illumination, which greatly reduces the effective distance of night vision, so specific testing is required. Moonlight-level and starlight-level enhanced-sensitivity cameras can work in very dark conditions, but some places with small reflection coefficients still cannot meet the requirements, such as deserts, green land, forest areas, etc. In this case, it is necessary to use a low-light night vision camera that is directly coupled by a high-performance imaging intensifier and a CCIR-format black and white CCD through a fiber panel and a light cone.

Lens selection

The camera lens is the key equipment of the infrared night vision monitoring system. Its quality (index) directly affects the imaging effect of the whole system. Therefore, whether the lens selection is appropriate is related to both the system quality and the project cost. With ordinary optical lenses, the infrared light reflected back from the object cannot be effectively focused on the CCD target surface, and the infrared night vision effect will be greatly reduced. Therefore, it is best to use an infrared lens.

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When choosing a lens, you should generally pay attention to the following points:

The imaging size of the lens should be consistent with the size of the camera CCD target surface, that is, choose a lens of 1/2 or more size. According to the distance of the camera's monitored target, select the focal length of the lens (the calculation formula can be found in the third section of the first chapter of "TV Monitoring Technology" compiled by me). After the focal length of the lens is determined, the field of view is determined by the camera target surface.

The resolution and transmittance of the lens must meet the requirements. Appropriate aperture or light flux In addition to the camera lens, it is also necessary to select a suitable laser beam expansion collimator lens according to the size and distance of the observed scene, so that the laser beam can illuminate the scene to be monitored, so that the reflected light of the monitored scene can be received by the CCD camera.

Transmission system selection

The transmission of general surveillance video images usually adopts the following four methods:

1. Network transmission.

2. Wireless transmission.

3. Coaxial cable transmission.

4. The advantages and disadvantages of these four methods are well known to the engineers and technicians in the industry, so I will not introduce them here. Generally, the monitoring center is close, and coaxial cable transmission is often used. As for other transmission methods, it depends on the local conditions at that time.

Monitor selection

There are two criteria for selecting monitors:

Black and white and color should match the camera

The definition of the selected monitor should be higher (preferably one level higher) than the definition index of the selected camera. It should be noted that you should not think that if the definition index of the camera is 400 lines, then it is enough to select a monitor with a definition of 400 lines. If so, the definition of the displayed image will be only about 300 lines. Because the so-called 400-line definition means that when the camera is used to capture the standard test card, the video signal output amplitude at 400 lines on the test card is about 40% of the video signal amplitude at 100 lines, and the definition of the monitor is also defined in this way. Therefore, when they are matched, the video signal output amplitude at 400 lines will be only 16%, and the 40% position will be reduced to about 300 lines. Therefore, in order to fully display the definition performance of the camera, a monitor with a higher definition should be selected. Although the price is more expensive, it can fully display the advantages and indicator characteristics of the system.

Choice of protective cover

The protective cover also affects the effect of the infrared lamp. During the transmission process, the infrared light has different transmittance and reflectivity when passing through different media. Different window glasses, especially automatic defrosting coated glass, have different attenuation of infrared light. Because the application occasions of ultra-long-distance night vision systems are relatively special, the product quality and protection requirements of the protective cover are relatively high. Therefore, when choosing a protective cover, comprehensive considerations should be made.

Application of ultra-long-range infrared laser night vision system

The ultra-long-range infrared laser night vision system can be widely used in: border night vision monitoring, coastal defense night vision monitoring, maritime long-distance monitoring (such as port night monitoring of offshore ships, etc.), flood control long-distance monitoring, forest fire lookout monitoring, urban environment detection remote monitoring, oil field night monitoring, seawall long-distance monitoring, public security night monitoring, prison wall night monitoring, railway train airport all-weather monitoring and other places that require long-distance night vision monitoring.

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Taking the application of the ultra-long-range military laser infrared night vision system developed by Beijing Borida Optoelectronics Technology Co., Ltd. in border defense, coastal defense, military facilities and vehicle-mounted night vision reconnaissance and command systems as an example, we can illustrate its wide application and importance.

Border and coastal defense night vision monitoring

Ultra-long-range infrared laser night vision system can be used for night vision monitoring of border defense and coastal defense. The system is designed for day and night monitoring and is mainly composed of infrared laser lighting, color and black and white cameras, and image signal output devices.

Generally, borders and coastal defense lines are long. If one night vision device can monitor an area with a radius of 3 kilometers, multiple night vision devices are needed and controlled by the control center. Continuous monitoring during the day and night can be achieved. Fully automatic pan/tilt and automatic zoom technology can enable night vision devices to observe the situation at any point in the monitoring area. According to the distance that users need to monitor, there are two arrangements for multiple night vision devices.

The video surveillance equipment is installed on a pan/tilt platform that can rotate horizontally and tilt up and down. It is controlled by a computer in the control center and can be set according to different regional conditions. When the monitoring area is not very large, a fixed monitoring method can be used, that is, the camera is fixed to shoot the monitoring area. If the monitoring area is large, an automatic cycle scanning method is used.

Night vision surveillance of military facilities

Monitoring of military facilities, military agencies, military bases and other places often only monitors a specific area. Therefore, it is generally adopted to place fixed-focus night vision goggles at specific locations. Moreover, according to the number of monitored objects, multiple night vision goggles can be installed, or zoom night vision goggles can be used.

Vehicle-mounted night vision reconnaissance and command

The ultra-long-range infrared laser night vision system can be used as a vehicle-mounted night vision system. It consists of three parts: front-end image signal acquisition, intermediate transmission of video signals and control signals, and image control center.

The vehicle-mounted night vision system can be installed on border patrol vehicles, armed police command vehicles, emergency rescue vehicles, engineering rescue vehicles, public security law enforcement vehicles, etc. to become a mobile night vision system. Based on the special laser infrared night vision system, the system adds a video and control system, with a more complete structure and more powerful functions. Its pan/tilt can be controlled by a computer in real time through a decoder, allowing it to rotate 360 ​​degrees horizontally and pitch to achieve continuous monitoring and recording day and night. Automatic zoom technology can monitor the situation at any distance within the customized distance. Its imaging system has a de-tailing function to ensure clear images during movement. GPS locators can be used to locate vehicles, which is convenient for monitoring and commanding the command center. In addition, the monitoring center is equipped with a digital hard disk recorder, which can realize the switching of images, recording and control of front-end equipment.

Conclusion

Ultra-long-distance infrared laser night vision system must use infrared laser light source. At present, none of the infrared light sources for monitoring over several hundred meters can compare with infrared laser light source, and only remote monitoring can achieve the purpose of not alarming the monitored object. This is the need of security monitoring in modern society, and it can be developed and widely used, which should attract the attention of various security monitoring companies. Research shows that ultra-long-distance infrared laser night vision system is feasible, and its key technology can also be solved. Only with infrared laser night vision system, our infrared night vision products of various distances can be considered complete.