In the field of video surveillance, the camera's low illumination performance is an important performance indicator that determines the camera's shooting in low light conditions. Therefore, low illumination indicators have always been an important reference for measuring camera performance and quality. Just after knowing its importance, how to correctly and effectively understand and use it?
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System integrators often face a variety of choices when purchasing cameras. Different manufacturers' different expressions of camera index parameters are often confusing, and limited to the actual situation, it is often impossible to test the camera's low illumination effect one by one. How to understand the low illumination parameter of the camera to measure the low illumination performance of different cameras has become a concern of system integrators. Then, under the premise that the camera's propaganda index parameters are authentic, can we find a way to effectively measure the low illumination performance of the camera by analyzing and interpreting the camera's low illumination parameter?
a misunderstanding
2 different cameras, in the same state (color or black and white), the low illumination index of the A camera is 0.1LUX, and the low illumination index of the B camera is 0.05LUX. Can it be concluded that the low illumination effect of the B camera is better than that of the A camera? If we do not add judgment, it is easy to draw this inaccurate conclusion based on the comparison of two values. What is the actual situation?
If there is no restriction, it is said that the camera's low illumination index is 0.1Lux, and only if the ambient illumination is 0.1Lux, the camera can work. However, under such lighting conditions, it is not known whether the output video of the camera has visibility and practical significance. For example, the low illumination index of a camera is 0Lux. Does this mean that the camera can actually image in the absence of visible light? The answer is obviously no, such as eliminating infrared light imaging, the image taken by the camera is dark. , no practical significance.
Three parameters
The camera's low illumination indicator should not be just a simple XX Lux value. On any accurate professional camera parameter data, the camera low illumination indicator should be at least expressed as follows: Color mode: XX Lux @FX(X IRE , AGC XX) Black and white mode: XX Lux @FX(X IRE ,AGC XX)
That is to say, the low illumination performance of the camera is related to at least three parameters, that is, the F value of the lens, the IRE value, and the AGC parameter of the camera. Under the premise that these parameters are complete, the previous illuminance values ​​are meaningful and comparable. At the same time, it must be pointed out that if the camera is color-to-black, the respective low-illuminance indicators in color and black-and-white states must be listed.
So what is the meaning of each quantity in the low-illumination standard expression and what is the relationship between them?
F value
F is the aperture coefficient of the lens, which is inversely proportional to the amount of light entering. Since the camera must rely on the aperture of the lens to determine the amount of light entering, it determines the shooting effect. Therefore, when expressing the low illumination indicator of the camera, it is necessary to have an F value condition. For an auto-iris zoom lens, the F value is generally indicated on the lens. Note that this value only represents the aperture value when the focal length is the minimum, that is, the wide-angle state. It is generally believed that as the focal length increases, the lens advances. The amount of light is gradually reduced, so when the lens is in a wide-angle state, the picture is brighter, and when it is in the telephoto state, the picture is darkened, which is especially noticeable at night. The change of the focal length of the lens, which changes the amount of light entering the lens, reflects the change of the corresponding F value, but the F value on the lens only indicates the maximum amount of light entering the lens, so only look at the F value indicated on the lens, only It can judge the light efficiency of the lens in the wide-angle state, but can not fully reflect the true performance of the lens. The F value corresponding to the two focal lengths of the lens usually differs greatly, so some lenses are good at night at wide angle. But once in the teleport, the picture is very dark. A really good lens, when the focal length is the largest and the smallest, the F value is not much different, such as from 1.8 to 1.6, the night effect of this lens is relatively better. The F value generally indicated in the low illumination index indicates the F value of the lens used in the test at the wide angle state, that is, when the amount of incoming light is maximum.
IRE value
As we all know, the analog video is an irregular waveform with a peak-to-peak value of 1 volt. The X-axis is below the synchronous level. The amplitude is 0.3 volts or 300 millivolts. Above the X-axis is the actual video waveform. This part of the waveform will follow the video brightness. The video content changes to an amplitude of 0.7 volts or 700 millivolts.
700 millivolts is the maximum amplitude of the video signal, which will decrease as the lighting conditions decrease. If you completely cover the lens, you can see that the amplitude of the video waveform is close to zero on the oscilloscope. So what is IRE? IRE is a unit in video measurement. It is specified in the broadcast-level video level that the brightness level of any video signal during playback cannot exceed 100 IRE, that is, 700 millivolts is equivalent to 100 IRE, that is, Said, the IRE value will indicate the amplitude of the video signal. In general, when the video signal is lower than 250 IRE (a quarter of 700 mV), that is, 175 mV, it is considered that the video signal at this time is meaningless because it is reflected on the monitor at this time. The picture is basically dark and cannot be effectively monitored. In view of this, in the low illumination index of the camera, the IRE is generally not lower than 25, often marked with more than 30, and a few products marked 50. Obviously, when the ambient illumination is reduced, both the video amplitude and the IRE value decrease. When looking at the camera's low illumination performance, the IRE value may be low, but it must be guaranteed that the displayed video is still meaningful.
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