System integrators often face multiple choices when purchasing cameras. Different manufacturers' different expressions of camera index parameters are often confusing, limited to the actual situation, and it is often impossible to test the low-light effects of cameras one by one. How to understand the low-illumination parameter index of the camera, so as to measure the pros and cons of the low-illuminance performance of different cameras, has become a concern of system integrators. So, under the premise that the camera's publicity index parameters are true and credible, can we find an effective way to measure the camera's low illumination performance through the analysis and interpretation of the camera's low illumination parameter index?
The low illumination index of the camera should not be just a simple XX Lux value. On any accurate and professional camera parameter information, the low illumination index of the camera should be expressed in at least the following format: color mode: XX Lux @FX (X IRE, AGC XX) Black and white mode: XX Lux @FX (X IRE, AGC XX)
In other words, the low-illumination performance of the camera is related to at least three parameters, that is, the F value, IRE value of the lens, and the AGC parameter of the camera. On 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 must be listed.
So what is the meaning of each quantity in the low-light 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. Because the camera must rely on the aperture of the lens to determine the amount of incoming light, which determines the shooting effect. Therefore, when expressing the low illumination index of the camera, there must be an F value condition. For an auto-iris zoom lens, the F value is generally marked on the lens. Note that this value only represents the aperture value at the minimum focal length, 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 the wide-angle state, the picture is relatively bright, and when it is in the telephoto state, the picture becomes dark, which is especially noticeable at night. The change of the lens focal length drives the change of the lens light input, which is reflected in the change of the corresponding F value. However, the F value on the lens only indicates the maximum light input of this lens, so only look at the F value marked on the lens, only Can judge the light-intake efficiency of the lens in the wide-angle state, but can not fully reflect the true performance of the lens. The F values ​​of the two focal lengths of the lens are usually very different, so some lenses are good at wide angle at night. But once in the telephoto state, the picture is very dark. For a really good lens, at the maximum and minimum focal lengths, the F value is not much different. For example, from 1.8 to 1.6, the night effect of this lens is relatively better. And the F value usually marked in the low illumination index indicates the F value of the lens used in the test in the wide-angle state, that is, when the amount of light entering is the maximum.
IRE value
As we all know, analog video is an irregular waveform with a peak-to-peak value of 1 volt. The X-axis below is the synchronization level, and the amplitude is 0.3 volts or 300 millivolts. The X-axis above is the actual video waveform. The content of the video changes and its amplitude is 0.7 volts or 700 millivolts. 700 millivolts is the limit amplitude of the video signal. As the lighting conditions decrease, this value will decrease accordingly. If the lens is completely covered, you can see on the oscilloscope that the amplitude of the video waveform is close to zero. So what is IRE? IRE is a unit of video measurement. The broadcast-level video level specifies that the brightness level of any video signal during playback cannot exceed 100IRE, that is, 700 millivolts is equivalent to 100IRE, that is, the IRE value will indicate The amplitude of the video signal. Generally speaking, when the video signal is lower than 250IRE (a quarter of 700 millivolts), that is, 175 millivolts, the video signal at this time is considered to be meaningless because of the effect 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 item is generally not less than 25, and it is often marked with 30, and a few products are marked with 50. Obviously, when the environmental illuminance decreases, both the video amplitude and the IRE value decrease accordingly. When examining the low-illumination performance of the camera, the IRE value may be very low, but it must be ensured that the displayed video is still meaningful.
AGC parameters
There is an amplifier circuit inside the camera for amplifying the original video signal. The DSP chip in the camera will detect the original video signal sent by the CCD. If the amplitude of the original video signal is too small, the DSP will drive this amplifier circuit to amplify the original signal, and then output the amplified video signal. DSP will detect the amplitude of the original video signal in real time to dynamically control the gain value of the amplifier circuit. This is the so-called "Auto Gain" function, and the mechanism to determine whether to enable this function is automatic gain control (AGC). When the AGC function is turned on (AGC ON), the automatic gain circuit will automatically enlarge the video amplitude to brighten the picture in the case of poor lighting (It is worth noting that even if AGC is ON, under good lighting conditions, The automatic gain circuit will not work, and the picture will not become brighter.) Conversely, if the AGC function is not turned on (AGC OFF), the automatic gain circuit will not work. Obviously, if the AGC is turned on, the amplifier circuit is in working state when testing the low-illumination performance of the camera. It should be noted that the amplifier circuit will amplify the noise in the video signal together, so noise will appear on the screen.
Of course, the AGC of some cameras not only has ON or OFF functions, but also has several gears that can be used to adjust the maximum magnification of the gain circuit, such as AGC 20db, 40db, etc., should also be used as a condition in the low illumination index of the camera And list.
Nantong Boxin Electronic Technology Co., Ltd. , https://www.bosencontrols.com