The imaging principle of video surveillance is to convert the information contained in light into an image signal 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, then the security video surveillance system will become a decoration. In this regard, the current industry mainly adopts four imaging technologies to realize the night vision monitoring function.
When the illumination is low, the camera can receive the near-infrared light field from 0.75 to 1000m in the natural lighting environment by switching the infrared filter, and by improving the video processing algorithm, the camera can read the received image information carried in the near-infrared light field. . A photoresistor is placed at the front end of such cameras. When the ambient illumination reaches the switching illumination value set by the camera, the camera automatically switches the infrared filter; and then relies on algorithms to process low-illumination video, the most typical low-illumination image processing algorithm. It is the automatic gain function, which can automatically increase the brightness of the picture as the ambient illumination decreases.
But at present, to achieve a good low-light function, the low-light sensor and the processing chip are the two core components. From the sensor imaging principle, the sensitivity of CCD is twice that of CMOS for sensors of the same specification. Cameras, especially intelligent traffic cameras, are dominated by CCD sensors. However, CMOS technology has been greatly improved now, and there are many cameras that use CMOS sensors to achieve low-light functions. At present, low-light cameras using CMOS sensors can achieve 0.001Lux.
In addition, the selection of the lens is also critical. The luminous flux of general-purpose lenses is generally F=1.4-360, and the use of low-illumination 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 low-light cameras that have been done well at present, and even if the low-light performance is good, it still needs the ambient illumination to not be lower than 0Lux to image.
In low-illumination applications, if spherical lens lenses are used, inaccurate focusing between the near-infrared light field and the sensor target surface is likely to occur. 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 ℃), there are irregular motions of molecules and atoms, and their surfaces continuously radiate infrared rays that are invisible to the human eye. Its working principle is: receiving the infrared rays emitted by the object, and 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 kind of thermal imaging camera that can detect extremely small temperature differences. The sensor converts the temperature difference into a real-time video image and displays it. But you can only see the thermal outline of people and objects, and cannot see the true face of objects. Infrared thermal imagers can penetrate rain and fog, and can still image even in a dark environment. However, the core of the thermal imager lies in its sensor, but due to technical limitations, there are not many well-made products, and the price is high. In addition, its image cannot reflect the color of the monitoring scene, detailed appearance characteristics and other information, which is not suitable for conventional security. Moreover, it is difficult for infrared thermal imagers to penetrate glass for imaging, and it is also blocked by transparent objects, which also affects its further application.
Visible light supplementary light monitoring
That is to add a visible light source to the monitoring scene, so that the monitoring equipment can capture clear, full-color color images. However, this application method is very clumsy. It needs to add enough light sources to the monitoring scene to ensure the quality of color images. The energy consumption and cost expenditure are very large. Except for the street light source that can be used for urban road monitoring, this method is rarely used in other scenarios.
Laser/infrared supplementary light monitoring
This is to install the fill light source on the camera as a part of the camera. This is currently the most widely used way of night monitoring. There are two types of fill light technology: laser and infrared.
Laser fill light: It is equipped with a laser light source on the camera. The laser light adopts the uniform spot enhancement technology and the spot automatic focusing technology; it has the advantages of strong luminosity, 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 supplementary light technology: Infrared lamps with infrared emitting diodes are mainly used in the market, and the illuminants are composed of infrared light emitting diodes matrix; 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 the infrared lamp. At present, the main solution is to make the infrared emitting diode into a PN junction from a material with high infrared radiation efficiency (commonly used GaAs), and add a forward bias voltage to inject current into the PN junction to excite infrared light, which can achieve low red explosion. Even no red explosion, long life applications.
The second concern of infrared applications is to prevent overexposure. If the infrared lamp cannot be adjusted, then under the same power, it will cause the adverse effects of overexposure and insufficient long-distance illumination for close-up images. To solve this problem, Many infrared cameras now support SMARTIR technology, that is, intelligent dimming technology. The camera will automatically calculate whether the brightness and saturation of the image are reasonable with the help of infrared lights. If it is overexposed or the brightness is insufficient, it will be reduced or enhanced accordingly. The luminous power of the infrared lamp can be obtained to obtain a picture effect suitable for monitoring; and the realization of this technology also effectively controls the energy consumption and reduces the 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 concealment at night while reducing infrared damage; if better infrared lamps and anti-reflection glass are used , the infrared camera will realize the excellent elimination of red burst stealth applications.
Through the artificial light supplement technology, although the color information of the picture is lost, the details of the image can be clearly identified one by one, and the application of active light supplement can be realized in the environment without light source.
Post time: Jul-08-2022