Four factors affecting the quality of infrared surveillance cameras
I believe that many engineering friends will ask such a question when installing a surveillance camera : Is the user of the picture quality of this surveillance satisfied, and how to solve the dissatisfaction? As a manufacturer, how to meet the needs of users? These problems are real and need to be solved urgently, so let's discuss the major factors that affect the quality of the monitoring picture first.
Another important feature of surveillance cameras is the illumination (sensitivity) of the subject. The so-called illuminance of the subject is "the illuminance of the subject required to obtain an image output level that can recognize the limits of the subject" and is expressed in lux (lux).
This is one of the important indicators of 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 is 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 bandpass filter with a frequency above 7MHz. 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.
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 illumination, when the installation environment illumination cannot be confirmed, refer to the lighting design reference values for offices, public rooms, living rooms, and restaurants. Because of the large number of video surveillance projects, engineers should purchase special equipment for comprehensive testing of cameras (including lenses).
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 the progressive scanning method is used, the resolution is reduced. The ratio is called the Kyle coefficient and, empirically, a value of 0.7 is used. In this way, the vertical resolution HV = ( 625-20x2 ) x 0.7 , about 410 lines is the limit of vertical resolution. As mentioned above, the vertical resolution is determined by the TV system and does not indicate the performance of the camera.
After understanding the reasons that affect the picture quality, no matter whether you are purchasing a home surveillance camera, an outdoor infrared night vision waterproof camera, or a simple surveillance, you will make a wise choice and find the product that suits you.
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