A brief analysis of the day and night and color and black and white conversion of surveillance cameras

As video surveillance is increasingly used in the civilian field, the application environment is becoming more and more complex. One of the most important application requirements is 24-hour uninterrupted monitoring. Since color imaging generally requires higher brightness, and the actual environment may not be able to meet this requirement, the camera low-light technology (also called night vision technology) has emerged accordingly. Due to different monitoring requirements, different application technologies have emerged.

Due to the cost and special materials, the human night vision technology currently widely used in the military has not been transplanted to the civilian monitoring field. Therefore, the following technologies are usually used in civilian monitoring products : high-sensitivity materials, digital slow shutter technology, color-to-black technology, passive infrared imaging technology, etc. Due to the different monitoring requirements and application occasions, different application markets have emerged in actual applications.

Highly sensitive materials

Use highly sensitive materials, including highly sensitive light-sensitive materials, ultra-high signal-to-noise ratio signal analysis and processing devices, and add some special processing technologies to signal processing, etc., to improve the restoration effect of image acquisition under low light conditions. However, due to cost and volume considerations, although this is the best solution , it is still difficult to be widely used in a short period of time.

At present, there are two main methods to improve the sensitivity of CCD photosensitive elements. Since it is impossible to increase the photosensitive area by physical means due to the size of the device, one way is to install a tiny lens on each photodiode (single pixel) of the element to increase the photosensitive area in disguise to obtain an increase in sensitivity, which becomes effective. This design is like putting glasses on the CCD, but after 35 years of development, the room for improvement of this technology is quite limited. The second is to obtain reasonable exposure based on data calculation through a specific signal enhancement circuit, but this usually causes uneven pixel sensitivity due to the rapid photosensitivity of the CCD, resulting in noise. At this time, the graininess of the captured picture will be more obvious. At this time, we have to take measures to balance the contradiction between high sensitivity and high image quality, which will inevitably bring higher cost investment.

Digital slow shutter technology

Digital slow shutter technology (digital slow shutter) is not actually a shutter, but its function is similar to that of a shutter to some extent. Shutter (shuttle) and aperture (iris) are both components on the camera that control light passing through the lens to achieve the light capture effect. It can also be understood that the aperture is a hole that light can enter when passing through the lens. The size of the hole is the size of the aperture. The larger the hole, the more light can pass under the same conditions. The shutter is the part that controls the aperture switch, controlling whether the aperture is always open or switched on and off at a certain time interval.

We know that according to the visual persistence characteristics of the human eye, in order to ensure that the image seen is continuous, the PAL TV signal standard It is 25 frames per second interlaced scanning, that is to say, the image passing by our eyes every second is actually a continuous picture composed of 25 frames. When shooting the target, a point can only be scanned once every 1/25 second. Because it is interlaced scanning, every 2 fields can constitute a frame, so every 1 second, the PAL image is 50 fields, and the time of 1 field is the shutter interval. Every second, the shutter must work 50 times to ensure that the output image is a PAL image of 50 fields per second, so the minimum shutter speed of the PAL system is 1/50 second (at this time the aperture is actually always open). In actual applications, because the light in the environment may be very strong, it may be necessary to control the amount of light entering at this time, so it is necessary to control the shutter speed. The faster the speed, the less time the light can enter, and the less the amount of light entering. Relatively speaking, the image will appear darker. Conversely, the slower the shutter speed, the brighter the image will be. When the light illumination is insufficient, even if 1/50 second is used, the image is still not bright enough, which requires the use of other technologies. According to optical theory, light can be superimposed. Although in a very dark environment, each point can only be scanned once after 1/25 second, the scanning time is also very short and its brightness is very weak. If the brightness of the point in the previous and next period of time is saved and superimposed before output, the point can be brightened. Therefore, the technical principle of digital slow shutter is to superimpose multiple images in the corresponding period of time according to requirements and then output them, so as to improve the signal strength.

Because this technology does not require any changes to the external environment, it can be said to be the most ideal solution under the condition of meeting the monitoring requirements. However, the scope of application of this technology is actually very narrow, because the premise of point-by-point accumulation is the accumulation of brightness at the same point at different times. Once the photographed object changes or moves, the two accumulated times before and after may not be the same pixel point, so the moving object will appear "smear" on the overall image. If the object moves too fast and the frame accumulation time is too long, the moving object will even become a ghost. Therefore, frame accumulation technology is generally used to monitor static scenes in low-light environments.

Color to black and white technology

When the light is insufficient, the image is switched to black and white, the color carrier and color synchronization interference are removed, and the AGC is increased. This can improve the image quality in low visible light environments to a certain extent. However, this solution can only solve some very special environments and cannot solve low-light environments. The image quality that can be improved is also very limited. Generally, this technology must be used in conjunction with other technologies.

Under certain light conditions, the image is converted from color to black and white by line switching. In the early evolution of color/black and white line conversion technology, two photosensitive elements (1 color, 1 black and white) were used to share a set of circuits for switching. At present, this type of camera has adopted a single CCD (color) design. It is a color camera during the day or when the light source is sufficient. When night falls or the light source is insufficient (usually 1LUX?3LUX), the color signal is eliminated by digital circuits to become a black and white image. Although this method can achieve the purpose of "low illumination" at night, it has the disadvantages of blurred images and unnatural colors during the day.

Passive infrared imaging technology

The application premise of passive infrared imaging technology is that the light capture device must be able to collect infrared signals in addition to visible light signals, and the signal processing can process the original infrared signal into a grayscale signal (commonly known as a black and white signal). Black and white cameras can achieve this function and have very high sensitivity. At present, all cameras based on digital processing technology can also complete this task, but due to the color imaging in visible light environment, contradictions have begun to appear. When processing color signals, because DSP processing requires the video signal to be separated into grayscale signals and chrominance (or color difference) signals for separate processing, and the infrared signal itself is invisible to the human eye, but after being collected by the light capture device and processed by DSP, it has become a grayscale signal that can be recognized by the human eye. The superposition of the two grayscale signals (visible light and infrared light) will inevitably make the image unable to be synthesized according to the ideal situation when synthesizing grayscale and chrominance, which will cause the grayscale and chrominance of the image to be distorted. The most typical example is that if the infrared light is too strong, the entire image will be gray. At present, there are four main solutions for passive infrared imaging in video surveillance:

1. Pure color camera: This solution is to prevent passive infrared imaging, that is, to prevent infrared rays from entering. It uses an optical low-pass filter (OPLF: Optical Low Filter, commonly known as a low-pass filter. Figure 1 shows the relationship between its light transmittance and wavelength. It can be seen that it basically does not absorb and reflect visible light, but basically completely isolates infrared light) to directly block infrared rays, so that the image will be basically unaffected by infrared signals. The purpose of this is to avoid using infrared imaging.

2. Completely ignore the impact of infrared signals on the color of the image and use a color filter that can pass a large amount of infrared light. This is a low-cost solution that works well in passive infrared imaging, but it is easy to have the chromaticity and grayscale distortion problems mentioned above in color mode.

3. Only specific infrared light, such as 850nm, is allowed to pass, while most other infrared light cannot pass. This is the principle of single-filter sensing infrared camera. Its main technical basis is the use of color filter technology that is different from that of color cameras.

This solution can solve the color cast problem to a certain extent, and can also be used in situations where infrared imaging is used without visible light. However, this solution also has some problems. When there is sufficient sunlight, the infrared signal is very rich, which will cause image color and grayscale distortion. When using infrared imaging, because only infrared light with a very narrow frequency can pass through, the imaging is not sensitive, so this type of camera is generally used in areas within a diameter of 30 meters indoors.

4. When visible light is strong, OPLF is used to block most of the infrared light to ensure the color restoration and authenticity of the grayscale signal. When visible light is weak, OPLF is not used, but a high-pass filter that allows most visible light and infrared light to pass is used. Because imaging mainly relies on infrared light, which is a grayscale signal, the color component is generally removed at this time and only the grayscale signal is retained, so what you see is only a grayscale image, which is what we call a black and white image.

This solution actually combines the advantages of color cameras in good visible light conditions and black and white cameras in low illumination conditions. It can be said to be the best solution for all-weather monitoring in environments with large light changes. The true color reproduction under natural light and the highly sensitive passive infrared imaging under no visible light, its static effect can even be comparable to some all-in-one cameras that use the same technology and add 4x DSS photosensitivity technology.

Although the use of passive infrared imaging can better solve the contradiction between the absence of visible light and monitoring, there are also some problems that need to be solved due to the optical differences between infrared and visible light. The different refractive indices of the main optical medium glass for the two light waves will cause different focal lengths of the optical components, so it is easy to cause focusing problems. However, these problems have been solved accordingly through continuous use and improvement. With the continuous maturity and widespread application of passive infrared imaging technology, the monitoring efficiency has been greatly improved, and people's lives will be more reliably protected.