Basis for selection of high-speed ball for video surveillance system

1) High-speed dome color: the camera has black and white and color. Generally, the horizontal resolution of the black and white camera is higher than that of the color camera, and the black and white camera is more sensitive than the color camera. However, the color image is easy to distinguish the color of the clothing and the scene, which is convenient for obtaining and distinguishing the real-time information on the spot in time. One of the main construction goals of a safe city is to combat and prevent crime, so the camera ’s chroma signal recognition function is the most important, including the identification of the microscopic chroma signal and other parts of the criminal suspect or criminal ’s outline, clothing, and expression. In order to meet the monitoring of the environment with weak light, in order to meet the monitoring needs of safe cities throughout the day, you must choose a camera with color to black and white function.

2) The definition of high-speed ball: there are two types of horizontal and vertical definition. The definition in the vertical direction is limited by the TV system, which has a maximum limit. Since the TV signals in China are all in the PAL system, the vertical resolution in the PAL system is 400 lines.

Therefore, the resolution of the camera is generally expressed in horizontal resolution. The horizontal definition represents the measurement of the horizontal detail definition of the television image by the human eye, and is expressed by the television line TVL. At present, the horizontal resolution of high-speed dome cameras on the market is mostly around 480TV lines. The higher the resolution of the camera, the better it is to capture the details of objects. For example, in the current bank monitoring system, the use of high-definition cameras can be more clear To monitor the denomination, also in the monitoring system of safe cities, high-definition cameras can capture the details of each picture very clearly, so high-definition cameras are important parameters for the selection of safe city construction equipment.

3) Illumination of high-speed ball: The luminous flux received per unit illuminated area is called illuminance. Lux (lux) is the illuminance when the beam of nominal brightness (lumens) strikes the lm2 area evenly. The sensitivity of the camera is expressed by the lowest illuminance. This is the camera takes the specific test card as the uptake standard. When the lens aperture is 0.4, adjust the illuminance of the light source, and use the oscilloscope to measure the amplitude of the video signal at the output of the oscilloscope to 10% of the rated value The measured illuminance of the test card is the minimum illuminance of the camera. So in fact the illumination of the subject is about 10 times the minimum illumination to get a clearer image.

According to the current situation of urban street lights in China, many main roads can reach more than 20LX, but most street lights are below 10LX, and even some road sections do not have street lights at all, and these road sections are often the commonplace of public security incidents. The low-illumination function presents a severe test. Normally, the illumination of half moonlight is only 0.2LX, which means that imaging in this environment and full real-time images must require the camera to reach 0.02LX. Many manufacturers now also use frame accumulation technology to reduce the illuminance of the camera on low-illumination technology, that is to say, many cameras need to reach 0.02LX, using the frame superposition technology. In this case, the image collected by the front end is not In real-time, users will have some animation effects during the monitoring process, but the images of the animation effects will have a tailing phenomenon when monitoring moving objects, and the purpose of video surveillance in Ping An China is to fight crime, often crimes are in the dark In the place where the trailing image appears, it brings a lot of inconvenience to real-time monitoring and investigation and forensics.

4) Selection of high-speed dome lens: The optical magnification of high-speed smart dome cameras on the market is 22 times, and its maximum focal length is concentrated around 80mm, which means that its effective monitoring distance can be calculated according to the focal length calculation formula F = v (image field High) * L (object distance) / V (field of view height) calculation. For example, to monitor an object with a field height of 2M, the height of the target surface (that is, the height of the image field) of the 1/4 "CCD camera (most high-speed smart dome cameras are currently 1/4" CCD) is 2.4mm, L = 80 * 2 / 2.4 = 66.7 meters.

Now some manufacturers on the market have launched high-speed dome cameras with 30x optical zoom. The maximum focal length is around 100mm, and the effective monitoring distance is L = 100 * 2 / 2.4 = 83 meters, which effectively increases the monitoring distance. Indirectly saves the investment of the system, which is also a problem for users to consider, so try to choose a large optical multiple when choosing a camera.

5) Wide dynamics of high-speed dome: In some cases where the contrast between light and dark is too large, the general camera is limited by the photosensitive characteristics of the CCD, and the captured image often appears too bright in the background or too dark in the foreground. In view of this situation, the wide dynamic technology came into being, which solved this problem well. Prior to this, traditional cameras generally used backlight compensation to adapt to situations with high light contrast.

Objects in the field of view of a conventional camera need to look at objects in the doorway or out of the window when the background brightness is high, usually using the central backlight compensation (BLC) mode, which mainly depends on increasing the brightness of the central part of the field of view and reducing the surrounding area of ​​the field of view Part of the brightness to achieve the purpose of seeing the objects in the central position.

In some applications, the field of view may contain a very bright area, and the contained subject is surrounded by the bright field, the picture is dim and no layers. At this time, because the signal level detected by AGC is not low, the gain of the amplifier is very low, and the brightness of the main body of the picture cannot be improved. When backlight compensation is introduced, the camera only detects the sub-region of the entire field of view. The average signal level of the area determines the operating point of the AGC circuit. Because the average level of the sub-region is very low, the AGC amplifier will have a higher gain, which will increase the amplitude of the output signal, thereby making the main picture on the monitor clear, greatly reducing the subjective brightness difference between the background picture and the main picture. The visibility of the field of view is improved. Although backlight compensation improves the brightness of the subject, the image quality will be more or less deteriorated.

In the monitoring of safe cities, especially in the case of weak street lights or no light at night, it is difficult to see the license plates of vehicles that turn on the high beam lights. This is because the ratio of the brightness of the lights to the license plates exceeds the human eye's resolution. Dynamic range, the dynamic range that the human eye can distinguish is generally 1000: 1, and the dynamic range of ordinary cameras is 3: 1, so ordinary cameras can not achieve the license plate of the driving vehicle at night. Under normal circumstances, the illuminance of the glare surface of the car headlight is about 1000LX, and the full moon night light is about 4LX. Considering the improvement of the headlight itself to the surrounding illuminance, when the illuminance of the license plate surface reaches above 8LX, the dynamic range drops to 1000: 8 = 125 times At present, the dynamic range of the third-generation wide dynamic camera has reached 128 times, which can fully meet the requirements.

The core technology of WDR is two exposures at the same time, one fast and one slow, and then the synthesis makes it possible to see the bright and dark objects on the screen at the same time. The quality of the slow shutter control directly affects the effect of the image. When purchasing a wide dynamic camera, it depends on the color contrast of the image and whether the manufacturer has the development capability of the wide dynamic camera DSP.