Currently commonly used security surveillance products, high-definition cameras usually have such technologies: high-sensitivity materials, digital slow shutter technology, color-to-black technology, passive infrared imaging technology, and so on. Because the environment used is different, so the requirements are different. Let's analyze the technical characteristics.
High sensitivity material
The use of high-sensitivity materials, including the use of high-sensitivity light-sensing materials, ultra-high signal-to-noise ratio signal analysis processing devices, signal processing adds some special processing techniques, etc., in order to improve the reduction of image acquisition in low light conditions, but Due to cost and consideration of volume, this is the best solution but it is still difficult to be widely used in a short time.
At present, for CCD photosensitive elements, the sensitivity is mainly improved by two methods. Since the photosensitive area is increased by physical means, it cannot be realized by the limitation of the volume of the device. Therefore, the sensitivity of the microlens in each of the photodiodes (single pixel) of the device is increased in a distorted manner to obtain a sensitivity. Effective. This design is like hanging glasses for CCD, but after 35 years of development of CCD, the space for this technology has been limited. The second is to obtain a reasonable exposure according to the data operation through a specific signal enhancement circuit, but this usually causes noise due to uneven sensitization of the pixel caused by the rapid sensitization of the CCD, and the graininess of the image taken at this time is more obvious. At this time, it is necessary to adopt a method to balance the contradiction between high sensitivity and high image quality, which inevitably leads to higher cost investment.
Digital slow shutter technology
Digital slow shutter technology (digitalslowshuttle), in fact, it is not a kind of shutter, but its function is somewhat similar to the shutter. Shutter and aperture (IRIS) are all controlled by the camera through the lens. A component of light capture effects. It can also be understood that the aperture is a hole that can enter when the light passes through the lens. The size of the aperture is the size of the aperture. The larger the aperture, the more light passes through the same situation, and the shutter is the part that masters the aperture switch, and controls the aperture. Whether it is always on or at a certain time interval.
We know that according to the visual persistence characteristics of the human eye, in order to ensure that the images seen are continuous, the standard of the PAL television signal is 25 frames per second interlaced scanning, that is, the image that passes through our eyes every second is actually A continuous screen consisting of 25 screens, one point can be scanned once every 1/25 second when shooting a target. Because it is interlaced, every 2 fields can form a frame, so every 1 second The PAL image is 50 fields. The time of one field is the interval of the shutter. Every second, the shutter must work 50 times to ensure that the output image is a PAL image of 50 fields/second, so the minimum PAL system. The shutter speed is 1/50 second (the aperture is actually always on). In practical applications, because the light in the environment may be strong, you may need to control the amount of light at this time. You need to control the shutter speed. The faster the speed. When the light can enter, the less time there is, the less the amount of light entering. Relatively speaking, the image will appear darker. On the contrary, the slower the shutter speed, the brighter the image will be. Even when the light illumination is insufficient, even 1/ 5 The 0 second image is still not bright enough, which requires the use of other techniques.
According to the optical theory, light can be superimposed. Although it takes 1/25 of a second to scan one point in a very dark environment, the time of scanning is very short, and the brightness is very weak. This point can be brightened when the brightness of the point is saved and superimposed and then output. Therefore, the technical principle of the digital slow shutter is to superimpose and output multiple images in a corresponding period of time according to requirements, thereby improving the signal strength.
This technology can be said to be the most ideal solution because it does not require any changes to the external environment, but the range in which this technology can be applied is very narrow, because point-by-point accumulation is achieved. The premise is that the brightness of the same point is accumulated at different times, and once the object being shot changes or moves, the two pixels together may not be the same pixel point, so that moving the object on the overall image will appear. The phenomenon of smear, if the object moves too fast and the frame accumulation time is too long, the moving object may even become a ghost. Therefore, frame accumulation techniques are generally applied to monitoring still scenes in low light environments.
Color to black technology
When the light is not enough, the image is switched to black and white image, the color carrier and color burst interference are removed, and the AGC is increased, which can improve the image quality in low visible light environment to a certain extent, but this solution can only solve Some very special environments can't solve the low-light environment, and the image quality that can be improved is very limited. Generally, this technology should be used in conjunction with other technologies.
In certain light source conditions, the image is switched from color to black and white by means of line switching. In the process of color/black-and-white line conversion technology evolution, two sensor elements (one color, one black and white) were used to switch between a group of circuits. At present, such cameras have adopted a single CCD (color) design. In the daytime or when the light source is sufficient, it is a color camera. When the night falls or the light source is insufficient (generally in 1LUX-3LUX), the digital signal is used to eliminate the color signal and become a black and white image. This method can achieve "low illumination" at night. The purpose is to have the disadvantages of blurred images and unnatural colors during the day.
Passive infrared imaging
The application of passive infrared imaging technology is that the optical capture device must be able to acquire infrared signals in addition to the visible light signals, and the signal processing can process the original infrared signals into grayscale signals. Black and white cameras can achieve this function and The sensitivity is very high. At present, all cameras based on digital processing technology can also do this work, but due to the color imaging in the visible light environment, contradictions have begun to appear. When processing color signals, because the DSP processing needs to separate the video signal into gray signal and chrominance (or color difference) signals for processing separately, and the infrared signal itself is invisible to the human eye, but is collected in the light trap. After DSP processing, it has become a gray signal that can be recognized by the human eye. The superposition of two gray signals (visible light and infrared light) will inevitably make the image not in the ideal situation when performing grayscale and chroma synthesis. Synthesizing, which will cause grayscale and chromatic distortion of the image, the most typical example is that if the infrared light is too strong, the entire image will be grayed out. At present, there are four main schemes for passive infrared imaging in video surveillance:
1, pure color camera, this scheme is to prevent passive infrared imaging, that is, to avoid infrared light, it uses optical low-pass filter (OPLF: OpticalLowFilter, commonly known as low-pass filter, Figure 1 for its light The relationship between the rate and the wavelength can be seen that it does not absorb and reflect visible light, but basically completely isolates the infrared light. It directly blocks the infrared light, so that the image is basically immune to the infrared signal. The purpose of this is to avoid using infrared imaging.
2, completely regardless of the impact of the infrared signal on the color of the image using a large number of filters through the infrared, this is a low-cost solution, the effect of passive infrared imaging is also better, but it is easy to appear above Chroma and grayscale distortion issues in color mode.
3, only a specific infrared light such as 850nm pass, most of the other infrared light can not pass, this is the principle of single filter induction infrared camera, the main technical basis is the use of color filter technology different from color cameras.
Such a solution can solve the problem of color cast color to some extent, and can also be used in the case where infrared imaging is used without visible light. However, there are some problems with this scheme. When the daylight is sufficient, the infrared signal is very rich, which causes image color and grayscale distortion. In the case of infrared imaging, because the infrared light of a very narrow frequency can pass through and the imaging is not sensitive, such cameras are generally used in an area within 30 meters of the indoor diameter.
4. Use OPLF when the visible light is strong, block most of the infrared light, ensure the color reproduction and the trueness of the gray signal, and do not use OPLF when the visible light is weak, but use most of the visible light and infrared. The high-pass filter that light can pass, because it mainly relies on infrared light imaging, is a gray-scale signal, so this time, the color components are generally removed, only the gray-scale signal is retained, so only the grayscale image is what we said. Black and white image.
This scheme actually combines the advantages of color cameras in better visible light conditions and the advantages of black and white cameras in low illumination. It can be said that it is the best solution for monitoring in an environment where all weather and light changes greatly. The true color reproduction under natural light and the highly sensitive passive infrared imaging without visible light can match the static effects of some integrated machines that use the same technology and add 4 times DSS sensitization technology.
Although the use of passive infrared imaging can better solve the contradiction between no visible light and monitoring, but because of the optical difference between infrared and visible light, there are some problems to be solved. The optical main medium glass has different refractive indices for the two kinds of light waves. It will cause different focal lengths of optical components, so it is easy to cause problems with focusing, but these problems have been solved in the continuous use and improvement. With the continuous maturity and wide application of passive infrared imaging technology, the monitoring performance is greatly improved, and people's lives will be more reliable.
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