Comprehensive night vision monitoring

    Security experts know that 24/7 surveillance is the most basic surveillance requirement, but in many cases, it is very difficult to truly achieve 24/7 surveillance; the biggest obstacle is the inability to image at night, especially in a dark environment. So, does night vision become a blind spot for our surveillance? The answer is no. In this issue, we will try to analyze night imaging technology.

　　The imaging principle of video surveillance is to convert the information contained in light into image signals that can be judged by the human eye. Therefore, light is a "necessity" for video surveillance. If the ambient light is low or no light, the video surveillance system will become a decoration. In this regard, the current industry mainly uses 4 imaging technologies to achieve night vision monitoring functions.

　　Natural light source low illumination technology (starlight level)

　　When the illumination is low, the camera receives the near-infrared light domain of 0.75-1000μm in the natural light environment by switching the infrared filter, and by improving the video processing algorithm, the camera can read the image information carried by the received near-infrared light domain. A photoresistor is placed at the front end of this type of camera. When the ambient illumination reaches the switching illumination value set by the camera, the camera automatically switches the infrared filter; then the algorithm is relied on to process the low-illumination video. The most typical low-illumination picture processing algorithm is the automatic gain function, which can automatically increase the picture brightness as the ambient illumination decreases. However, at present, to achieve a good low-illumination function, low-illumination sensors and processing chips are the two core components. From the perspective of sensor imaging principle, the photosensitivity of CCD is twice that of CMOS for sensors of the same specifications. Therefore, many low-illumination cameras today, especially intelligent traffic cameras, are mainly based on CCD sensors. However, CMOS technology has been greatly improved, and there are also many cameras that use CMOS sensors to achieve low-illumination functions. At present, low-illumination cameras with CMOS as sensors can achieve 0.001Lux. In addition, the selection of lens is also critical. The luminous flux of general lenses is generally F=1.4-360, and the use of low-light lenses or improved lens technology can also achieve greater light flux, thereby achieving greater night vision effects. However, the disadvantage of such products is that there are not many good low-light cameras at present, and even if the low-light performance is good, the ambient illumination must not be lower than 0Lux to form an image.

　　In low-light applications, if a spherical lens is used, it is easy to cause inaccurate focusing between the near-infrared light domain and the sensor target surface. This problem can be improved by using aspherical lenses.

　　Infrared thermal imaging monitoring

　　All objects in nature, as long as their temperature is higher than the absolute temperature (-273℃), have irregular movement of molecules and atoms, and their surfaces continuously radiate infrared rays that are invisible to human eyes. Its working principle is: receiving infrared rays emitted by objects, displaying the temperature distribution on the surface of the measured object through colored pictures, thus forming a readable image; its core is the thermal imager, which is a sensor that can detect extremely small temperature differences and convert the temperature difference into real-time video images for display. However, only the thermal outlines of people and objects can be seen, and the true appearance of the objects cannot be seen clearly. Infrared thermal imagers can penetrate rain and fog, and can still display images even in a lightless environment. However, the core of thermal imagers lies in their sensors, but due to technical limitations, there are not many well-made products, and the price is high. In addition, their images cannot reflect the color of the monitored scene, detailed appearance characteristics and other information, and are not suitable for conventional security applications; moreover, infrared thermal imagers are also difficult to penetrate glass for imaging, and are also blocked by transparent objects, which also affects their further application.

　　Visible light supplementary lighting monitoring

　　That is, adding visible light sources to the monitoring scene so that the monitoring equipment can capture clear and colorful images. However, this application method is clumsy and requires adding enough light sources to the monitoring scene to ensure the quality of the color image. The energy consumption and cost expenditure are very high. This method is rarely used in other scenarios except for urban road monitoring, where street lamps can be used as light sources.

　　Laser/infrared fill light monitoring

　　This is to install the fill light source on the camera as a part of the camera. This is the most widely used method for night monitoring. There are two types of fill light technology: laser and infrared.

　　Laser fill light: It is to equip the camera with a laser light source. The laser light adopts the technology of uniform light spot enhancement and automatic light spot focusing. It has the advantages of strong brightness, more uniform picture, less power consumption and longer service life. However, the laser lights used in security are almost all products of laser manufacturers, and the matching is relatively passive.

　　Infrared fill light technology: The infrared lamps on the market mainly use infrared emitting diodes, which are composed of infrared light-emitting diode matrices; infrared LEDs with emission wavelengths of 850nm and 940nm are the mainstream applications. The most worrying thing about infrared fill light technology is the life of infrared lamps. The main solution at present is to make the infrared emitting diodes into PN junctions with materials with high infrared radiation efficiency (commonly gallium arsenide GaAs), and inject current into the PN junctions with forward bias to excite infrared light, which can achieve low or even no red burst and long life applications. The second concern of infrared application is to prevent overexposure. If the infrared lamp cannot be adjusted, then at the same power, it will cause the close-range image to be overexposed and the long-distance illumination to be insufficient. To solve this problem, many infrared cameras now support SMART IR technology, that is, intelligent dimming technology. The camera will automatically calculate whether the brightness and saturation of the image screen with the assistance of infrared lamps are reasonable. If it is overexposed or insufficient, the infrared lamp's luminous power will be reduced or increased accordingly, so as to obtain a suitable image effect for monitoring. The implementation of this technology also effectively controls energy consumption and reduces energy consumption support. Nowadays, in order to achieve better infrared applications, many security manufacturers generally use good infrared anti-reflection glass, which can achieve a certain degree of night concealment effect while reducing infrared loss. If better infrared lamps and anti-reflection glass are selected, infrared cameras will achieve excellent red-blast-eliminating stealth applications.

　　Through artificial fill light technology, although the picture loses color information, the details of the image can be clearly identified one by one, and active fill light application can be realized in a light-free environment.

　　Follow-up application

　　The fill light source must be intelligently followed by the camera lens zoom and pan/tilt to display a good real-time picture effect. When the pan/tilt rotates, the monitoring depth of field will change. At this time, the comfort of the picture can be achieved by intelligently adjusting the power of the fill light device. This technology is already relatively mature. However, if the lens zooms, the follow-up performance of the fill light device is still being improved. Among them, infrared fill light is generally not used in high-zoom cameras due to its low power, so the difference in follow-up is not obvious; and laser cameras, due to strong beams and long distances, when the lens zooms quickly, there will be a delay of 1-5 seconds. For example, when the lens quickly changes from high zoom to low zoom, the picture will first show dark areas that are not irradiated by the laser and bright spots at the laser. The dark areas will disappear after the lens stabilizes; the follow-up of the laser is relatively lagging.

　　Applications at different monitoring distances

　　In actual applications, infrared cameras generally have an effective monitoring distance of less than 30 meters, and medium-distance applications are within 100 meters. In addition, if infrared monitoring of more than 80 meters is required, a very strong infrared light is required. In terms of effective monitoring distance, infrared fill light technology is mainly suitable for monitoring scenes with shorter distances, such as shopping malls, stores, office buildings, corridors, etc.

　　As for laser fill light technology, 50-150 meters is the short-distance monitoring range, and 0.5-2 kilometers is the more common medium and long-distance application. If the laser power is enhanced, longer-distance monitoring applications can also be achieved. It is suitable for forests, border defenses, coastal defenses, factories and mines and other places.

　　Chip Core Balance

　　Chips also play a vital role in DSP image processing of low-light, infrared/laser cameras. For example, the low-light cameras of China Electronics Xingfa and Hikvision mainly rely on DSP chips to ensure the improvement of low-light image quality, which is also widely used in laser and infrared cameras. For example, before switching between day and night modes, gamma correction is required to ensure image accuracy and balance 3D noise reduction and automatic gain effects; after switching, the infrared screen brightness is automatically calculated, which is to intelligently adjust the uniformity of the picture and the strength of the infrared/laser light. In addition, the image quality needs to be calculated to match the image with the changes in the lens and the gimbal in real time.