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The so-called "signal-to-noise ratio" refers to the ratio of the signal voltage to the noise voltage, and is usually expressed by the symbol S/N. Since the signal voltage is much higher than the noise voltage under normal conditions, the ratio is very large. Therefore, the actual calculation of the signal-to-noise ratio of the camera is usually based on the ratio of the mean square signal voltage to the mean square noise voltage. The number is multiplied by a factor of 20, that is, the unit of the equation (2-3) is expressed in dB.
Signal-to-noise ratio is also a major parameter of gun-type surveillance cameras. When the gun-type surveillance camera captures a brighter scene, the screen displayed by the monitor is usually brighter, and the observer is not easy to see the interference noise in the picture; when the camera takes a darker scene, the screen displayed by the monitor is dim. Observers can easily see the snow-like interference noise in the picture at this time.
The strength of the interference noise (that is, the degree of influence of interference noise on the picture) is directly related to the quality of the signal-to-noise ratio of the gun-type surveillance camera. The higher the signal-to-noise ratio of the gun-type surveillance camera, the influence of interference noise on the picture. The smaller it is.
The signal-to-noise ratio value given by the general gun-type surveillance camera is the value when the AGC (Automatic Gain Control) is turned off, because when the AGC is turned on, the small signal is boosted, so that the noise level is also increased accordingly. The typical value of the SNR of the CCD gun surveillance camera is generally 45dB~55dB. When measuring the signal-to-noise ratio parameter, it should be directly connected to the video output terminal of the gun-type surveillance camera using a video clutter meter.
The minimum illumination is an important parameter to measure the pros and cons of the gun-type surveillance camera. Sometimes the word "lowest" is omitted and it is directly referred to as "illuminance".
The minimum illuminance is the brightness value of the scene when the level of the video signal output by the gun-type surveillance camera is lowered to a certain value when the brightness of the subject is low.
When determining this parameter, the maximum relative aperture of the lens should also be specified. For example, when using the lens of F1.2, when the brightness value of the subject is as low as 0.04LX, the amplitude of the video signal output by the camera is 50% of the maximum amplitude, that is, 350mV (the maximum amplitude of the standard video signal is 700mV). ), the minimum illumination of the gun surveillance camera is 0.04 LUX / F1.2.
If the brightness value of the subject is lower, the amplitude of the video signal output by the gun-type surveillance camera will not reach 350mV, which is reflected on the screen of the monitor, which will be difficult to distinguish the layered, gray image on one screen. .