The camera image quality is a very important link for customers who install surveillance systems. If the camera image quality is not good and is poor, it will affect the monitoring effect. Let's analyze the four main points of the camera image quality!
1.Vertical resolution
The vertical resolution is determined by the number of scan lines of the television system. The PAL system indicates that the effective scanning line for one frame is 625 lines, and an average field has 20 lines of vertical blanking time. Therefore, only 585 lines appear in the actual picture. In addition, since CCTV uses a progressive scan method, the resolution is reduced. The ratio is called the Kyle coefficient and, empirically, a value of 0.7 is used. In this way, the final vertical resolution HV = (625-20x2) x0.7, and about 410 lines is the limit of the vertical resolution. As mentioned above, the vertical resolution is determined by the TV system and does not indicate the performance of the camera.
2.Horizontal resolution
This is one of the important indicators of CCTV surveillance camera performance. The actual CCD determines the clock frequency and sampling frequency. The sampling frequency of the high-resolution CCD is 14.18MHz. According to the Shannon sampling principle (the sampling frequency must be more than twice the upper limit of the continuous signal frequency), the image signal frequency is limited to about 7MHz.
In addition, if the image signal component has a frequency component that exceeds 1/2 of the sampling frequency, a folding phenomenon will occur, and a false signal / moiré image will be generated. In order to prevent this phenomenon, it is necessary to use a high-efficiency band-pass filter with a frequency above 7 MHz. It is actually difficult to make such a filter, so the image signal frequency becomes about 6 MHz and the horizontal resolution reaches 480 lines.
3.Sensitivity
Another important performance of CCTV surveillance cameras is the minimum illumination (sensitivity) of the subject. The so-called minimum subject illuminance is "the necessary subject illuminance required to obtain an image output level at the lowest limit of the identifiable subject", and is expressed in lux (lux).
4.Environmental illumination
It is recommended that engineers conduct more comprehensive tests on the cameras (including lenses) that need to be installed, focusing on vertical and horizontal resolution tests, color reproduction tests, linearity tests, focus tests, facial recognition level tests, impedance matching feature tests, especially For testing under the installation environment illuminance, when the installation environment illuminance cannot be confirmed, refer to the lighting design reference values ​​for offices, public rooms, living rooms, and star-rated restaurants. Because of the large number of video surveillance projects, engineers should purchase special equipment for comprehensive testing of cameras (including lenses).
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