How to investigate the camera's minimum illumination?

Low-light camera refers to a camera that can still take clear images under low light conditions. There is no uniform standard in the technical specifications of the CCTV industry. Therefore, it is impossible to define why the minimum illuminance is a low-light camera. . Moreover, the minimum illuminance value is related to the lens aperture size, electronic sensitivity, infrared switch status and other conditions, so it is necessary to investigate the minimum illumination of the camera under the same test conditions. Understand the low-light camera, you can understand from a misunderstanding of the three parameters.

System integrators often face a variety of choices when choosing a camera. Different manufacturers' different expressions of camera index parameters are often confusing. However, limited to the actual situation, it is often impossible to test the camera's low illuminance effects one by one. How to understand the camera's low illuminance parameter index, thus measuring the advantages and disadvantages of different camera's low illuminance performance, has become a concern for system integrators. Then, on the premise that the camera's propaganda index parameters are authentic, can we find out a way to effectively measure the low-illuminance performance of the camera by analyzing and interpreting the camera's low-illuminance parameter index?

Misunderstanding

Two different cameras, in the same state (color or black and white), A camera's low-light index is 0.1LUX, B camera's low-light index is 0.05LUX, can we determine that the B-camera's low-light effect is better than the A-camera? ? If we do not add judgement, we can easily draw this inaccurate conclusion based on the comparison of the two values. What is the actual situation?

If you do not add any restrictions, say that a camera's low illuminance indicator is 0.1 Lux, this camera can work only if the ambient illuminance is 0.1 Lux. However, under such lighting conditions, it is not known whether the output video of the camera has a viewability and practical significance. For example, the low illumination index of a camera is 0 Lux. Does this mean that the camera can really image in the absence of visible light? The answer is obviously negative. If you exclude infrared light imaging, the camera will take a dark picture. There is no practical significance.

Three parameters

The camera's low illuminance indicator should not be just a simple XXLux value. On any accurate and professional camera parameter data, the camera's low illuminance indicator should at least be expressed as the following format: Color Mode: XXLux@FX (XIRE, AGCXX) Black and White Mode :XXLux@FX(XIRE,AGCXX)

That is, the camera's low illuminance performance is related to at least three parameters, namely, the lens' F value, IRE value, and camera's AGC parameter. With these parameters intact, the previous illuminance values ​​are meaningful and comparable. At the same time, it must be pointed out that if the camera is turned black, it must list the respective low-lightness indicators in the color and black and white states.

So what is the significance of the individual quantities in the low-illumination standard expression and what is the relationship between them?

1, F value

F is the aperture coefficient of the lens, which is inversely related to the amount of light entering. Because the camera must rely on the aperture of the lens to determine the amount of light, thus determining the shooting effect. Therefore, when expressing the camera's low-lightness indicator, there must be an F-condition. For an automatic aperture zoom lens, the lens will generally indicate the F value. Note that this value only represents the aperture value when the focal length is minimum, that is, in the wide-angle state. It is generally believed that the lens advances with the increase in focal length. The amount of light is gradually reduced, so when the lens is in a wide-angle state, the picture is bright, and when it is in a telephoto state, the picture is darkened. This phenomenon is particularly evident at night. Changes in the focal length of the lens drive the change in the amount of light entering the lens and reflect the change in the corresponding F value. However, the F value on the lens only indicates the maximum amount of light entering the lens. Therefore, only the F value marked on the lens is observed. Can judge the lens into the wide-angle lens into the light efficiency, and can not fully reflect the true performance of the lens, the lens focal length extreme corresponding to the F value is usually very different, so some cameras at night when the wide-angle, the picture is not bad, But once in a telescopic state, the picture is very dark. In a really good shot, when the focal length is maximum and minimum, the F value is not much different, for example, from 1.8 to 1.6, the nighttime effect of this lens is relatively better. The F value usually marked in the low illumination index indicates the F value of the lens used during the test in the wide-angle state, that is, when the amount of light entering is the maximum.

2, IRE value

As we all know, the analog video is an irregular waveform with a peak-to-peak voltage of 1 volt. Below the X-axis is the sync level. The amplitude is 0.3 volts, which is 300 millivolts. Above the X-axis is the actual video waveform. This part of the waveform will follow the video brightness as well. The content of the video changes and its amplitude is 0.7 volts, which is 700 millivolts. 700 millivolts is the limit amplitude of the video signal. As the lighting conditions decrease, this value will decrease. If you cover the lens completely, you can see that the amplitude of the video waveform is close to 0 through the oscilloscope. So what is IRE? The IRE is a unit in video measurement. It is specified in the broadcast video level that the luminance level of any video signal during playback cannot exceed 100 IRE, ie, 700 millivolts is equivalent to 100 IRE. That is, 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), ie, 175 mV, the video signal at this time is considered to be insignificant because the effect is reflected on the monitor at this time. The screen is basically dark and cannot be effectively monitored. In view of this, in the low-illuminance indicator of the camera, the IRE item is generally not less than 25, and is often tagged with 30 majority, and a few products are tagged with 50. Obviously, both the video amplitude and the IRE value decrease as the ambient illumination decreases. When examining the camera's low-light performance, the IRE value may be low, but it must be ensured that the displayed video is still meaningful.

3, AGC parameters

Inside the camera there is an amplifier circuit for amplifying the original video signal. The DSP chip in the camera will detect the original video signal sent by the CCD. If 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. The DSP will detect the amplitude of the original video signal in real time, so as to dynamically control the gain value of the amplifier circuit. This is the so-called "AutoGain" function, and the mechanism for determining whether to enable this function is automatic gain control (AGC). When the AGC function is turned on (AGCON), the automatic gain circuit will automatically amplify the video amplitude in the case of poor illumination, making the screen brighter. (It is worth noting that even if the AGC is ON, under the conditions of good lighting, automatic The gain circuit will not work, and the picture will not become brighter. On the contrary, if the AGC function is not turned on (AGCOFF), the automatic gain circuit will not work. Obviously, if the AGC is turned on, the amplifier circuit is in working condition 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, some camera AGCs not only have the ON or OFF function, but also there are several gears that can be used to adjust the maximum gain of the gain circuit, such as the AGC20db, 40db, etc., and should also be used as conditions in the camera's low illumination index. Listed.

About low light camera

CCTV cameras that can work in extremely low light. A closed-circuit television system that can operate in light conditions that are below the normal visual response. Illuminance is expressed in Lux. 0Lux means that you can shoot without light. Generally, most cameras are at 0 Lux or 0.1 Lux. The size of the illumination value depends on the aperture of the lens (F-number: F is the ratio of the lens focal length to the aperture diameter, the smaller the ratio, The larger the aperture, the greater the amount of light passing through. The smaller the value of F, the lower the required illumination. Current cameras range from 0.001Lux to 1Lux, and there are many opinions. When the Lux value is 0.001, it has basically reached the level of starlight illumination.