With the increasing use of video surveillance in the civilian realm and the increasingly complex application environment, one of the most important application requirements is 24 hours of continuous monitoring. Because color imaging generally requires higher brightness, and the actual environment may not be able to meet this requirement, so the camera low illumination technology (also known as night vision technology) appears accordingly. Due to different monitoring requirements, different application technologies have emerged.
Due to the cost and special materials, human eye night vision technology, which is widely used in the military, has not been transplanted into the civilian monitoring field. Therefore, currently there are several technologies used in civilian monitoring products: high-sensitivity materials and digital Slow shutter technology, color-to-black technology, passive infrared imaging technology, and so on, because of the different monitoring requirements and application scenarios, so there are different application markets in practical applications.
High-sensitivity materials use high-sensitivity materials, including the use of high-sensitivity light-sensing materials, ultra-high signal-to-noise ratio signal analysis processing devices, and signal processing to add some special processing techniques, etc., to improve the restoration of image acquisition under low light conditions. Effect, but due to cost and consideration of volume and other reasons, although this is the best solution, it is still difficult to be widely used in a short time.
At present, for CCD photosensitive elements, there are mainly two methods for increasing the sensitivity. Due to the physical increase in light-sensing area, limited by the volume of the device can not be achieved, so one of them is to increase the sensitivity of the device in each device on the photodiode (single pixel) micro lens device to increase the photosensitive area in a way to increase sensitivity, become Effective. This design is like putting spectacles on the CCD, but after 35 years of development, the space for this technology has been quite limited. Second, it is through a specific signal enhancement circuit, according to data calculations to obtain a reasonable exposure, but this is usually due to CCD fast exposure caused by uneven pixel sensitization and the formation of noise, then the grain of the shot will be more obvious. At this point, they have to take measures to balance the conflict between high-sensitivity and high image quality, which inevitably leads to higher costs.
Digital slow shutter technology digitalslowshuttle, in fact, it is not a shutter, but its function is somewhat similar to the shutter, the shutter and iris (IRIS) are the control light on the camera Through the lens, reach a part of the light capture effect. It can also be understood that the aperture is a hole that the light can enter when the lens passes through the lens. The size of the aperture is the size of the aperture. The larger the aperture, the more light passes in the same situation, and the shutter is the part that controls the aperture switch and controls the aperture. Whether it is always on or it is on and off at regular intervals.
We know that according to the persistence characteristics of the human eye, in order to ensure that the images seen are continuous, the standard for PAL TV signals is 25 frames per second interlaced scanning, that is, the image that passes through our eyes every second is actually With a continuous screen consisting of 25 screens, one point can be scanned once every 1/25 second when shooting the target. Because it is an interlaced scanning, only one frame can be formed for every 2 fields, so every 1 second. PAL system image is 50 fields, 1 field time is the shutter interval, every second, the shutter must work 50 times in order to ensure that the output image is PAL image of 50 fields / second, so the lowest PAL system The shutter speed is 1/50 second (at this time the aperture is actually open), in practical applications, because the light in the environment may be very strong, this time may need to control the amount of light, you need to control the shutter speed, the faster the speed At this point, the less time the light can enter, the less light will enter. Relatively speaking, the image will appear dark. On the contrary, the slower the shutter speed, the brighter the image will be. When the light intensity is insufficient, The use of 1/50 second images is still not bright enough, which requires the use of other technologies. According to the optical theory, light can be superimposed, although in a very dark environment, each point can be scanned to 1/25 second and the scanned time is very short. Its brightness is very weak. When the brightness of this point is saved and superimposed and then output, this point can be brightened. Therefore, the technical principle of the digital slow shutter is to superimpose multiple images corresponding to a certain period of time and then output them in order to increase the signal strength.
This technology does not require any changes to the external environment, so it can be said to be the ideal solution under the conditions of meeting the monitoring requirements, but the scope of this technology can actually be applied is very narrow, because of the realization of cumulative point by point The premise is that the brightness of the same point is accumulated at different times, and once the shooting object changes or moves, the two pixels may not be in the same pixel at the same time. This will cause the moving object to appear on the whole image. "Smear" phenomenon, if the object moves too fast and the frame accumulation time is too long, the moving object may even become a virtual image. Therefore, the frame accumulation technique is generally applied to monitor a stationary scene in a low-light environment.
The color-to-black-and-white technology switches the image to a black-and-white image when light is insufficient, removes color carriers and burst interference, and increases the AGC to a certain extent to improve image quality in a low visible light environment, but this solution What can be solved is only a part of a very special environment that cannot solve the low-light environment, and the image quality that can be improved is also very limited. Generally, this technology must be used in conjunction with other technologies.
In a certain light source condition, the image is switched from color to black and white using line switching. In the course of the technological evolution of color/black and white line conversion, two sensor elements (one color, one black and white) were used in the early stages to share a set of circuits for switching. Currently, such cameras have adopted a single CCD (color) design. When the light source is sufficient during the day or when the light source is sufficient, when the night falls or the light source is insufficient (usually at 1LUX?3LUX), the digital signal is eliminated by using a digital circuit to become a black and white image, although this method can achieve "low illumination" at night. The purpose is that during the day there are blurred images and unnatural colors.
Passive infrared imaging technology The application of passive infrared imaging technology is based on the premise that the light capturing device must be able to collect infrared signals in addition to the visible light signal, and the signal processing can process the original infrared signal into grayscale signals. Black and white signal). Both black and white cameras are capable of this function and have very high sensitivity. All cameras based on digital processing technology can do this work, but because of the color imaging in the visible light environment, contradictions have begun to appear. When dealing with color signals, because the DSP processing needs to separate the video signal into a gray signal and a chroma (or color difference) signal to be processed separately, and the infrared signal itself is invisible to the human eye, but is captured in the light trap. DSP processing has become a gray signal that can be recognized by the human eye. The superimposition of two gray signals (visible light and infrared light) will inevitably make the image unable to follow ideal conditions when performing grayscale and chroma synthesis. To synthesize, this will cause the grayscale and chroma of the image to be distorted. 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 solutions for passive infrared imaging in video surveillance:
1. Pure color cameras, this solution is to prevent passive infrared imaging, which is to avoid the ingress of infrared light, which uses an optical low-pass filter (OPLF: Optical Low Filter, commonly known as low-pass filter, Figure 1 is its optical communication The relation between the rate and the wavelength can be seen as that it does not substantially absorb and reflect visible light, but substantially completely isolates the infrared light. It directly blocks the infrared ray, so that the image will be substantially unaffected by the infrared signal. The purpose of this is to avoid using infrared imaging.
2, completely regardless of the impact of infrared signals on the color of the image using a large number of infrared filters, which is a low-cost solution, the effect of passive infrared imaging is also relatively good, but it is easy to appear above said Chromaticity and grayscale distortion issues in color mode.
3, only for certain infrared light such as 850nm through, most of the other infrared light can not pass, this is the principle of single filter infrared sensor, the main technical basis is to use a different color camera color filter, the light The relationship between the pass rate and the light wavelength is shown in FIGS. 1 and 2 .
Such a solution can solve the problem of color cast to a certain extent, and can also be used where infrared imaging is used without visible light. However, there are also some problems with this solution. When the sunlight is sufficient, the infrared signal is very rich, causing image color and grayscale distortion. When using infrared imaging, imaging is not sensitive because only very narrow frequency infrared light can pass through, so this kind of camera is generally used in indoor areas within 30 meters in diameter.
4, when the visible light is strong when using OPLF, most of the infrared light block, to ensure its color reduction and grayscale signal is true, and when the visible light is weak, do not use OPLF, and can make the vast majority of visible light and infrared Light can pass through the high-pass filter, because it mainly relies on infrared imaging, is a gray signal, so this time will generally remove the color components, only to retain the gray signal, so see just the gray image that we say Black and white image.
This scheme actually combines the advantages of the color camera in the case of visible light and the advantages of the black and white camera in low illumination. It can be said that it is currently the best solution to solve the monitoring under all-weather, light-changing environments. The true color reproduction under natural light and the high-sensitivity passive infrared imaging without visible light make the static effect even comparable to some integrated machines that use the same technology and incorporate 4x DSS.
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 also some problems that need to be solved, the optical main medium glass has different refractive indexes for two kinds of light waves. It will cause different focal lengths of the optical components, so it is easy to focus problems, but these problems have been solved in the continuous use and improvement. With the constant maturity and widespread application of passive infrared imaging technology, the monitoring performance has been greatly improved, and people's lives will be guaranteed more reliably.
Due to the cost and special materials, human eye night vision technology, which is widely used in the military, has not been transplanted into the civilian monitoring field. Therefore, currently there are several technologies used in civilian monitoring products: high-sensitivity materials and digital Slow shutter technology, color-to-black technology, passive infrared imaging technology, and so on, because of the different monitoring requirements and application scenarios, so there are different application markets in practical applications.
High-sensitivity materials use high-sensitivity materials, including the use of high-sensitivity light-sensing materials, ultra-high signal-to-noise ratio signal analysis processing devices, and signal processing to add some special processing techniques, etc., to improve the restoration of image acquisition under low light conditions. Effect, but due to cost and consideration of volume and other reasons, although this is the best solution, it is still difficult to be widely used in a short time.
At present, for CCD photosensitive elements, there are mainly two methods for increasing the sensitivity. Due to the physical increase in light-sensing area, limited by the volume of the device can not be achieved, so one of them is to increase the sensitivity of the device in each device on the photodiode (single pixel) micro lens device to increase the photosensitive area in a way to increase sensitivity, become Effective. This design is like putting spectacles on the CCD, but after 35 years of development, the space for this technology has been quite limited. Second, it is through a specific signal enhancement circuit, according to data calculations to obtain a reasonable exposure, but this is usually due to CCD fast exposure caused by uneven pixel sensitization and the formation of noise, then the grain of the shot will be more obvious. At this point, they have to take measures to balance the conflict between high-sensitivity and high image quality, which inevitably leads to higher costs.
Digital slow shutter technology digitalslowshuttle, in fact, it is not a shutter, but its function is somewhat similar to the shutter, the shutter and iris (IRIS) are the control light on the camera Through the lens, reach a part of the light capture effect. It can also be understood that the aperture is a hole that the light can enter when the lens passes through the lens. The size of the aperture is the size of the aperture. The larger the aperture, the more light passes in the same situation, and the shutter is the part that controls the aperture switch and controls the aperture. Whether it is always on or it is on and off at regular intervals.
We know that according to the persistence characteristics of the human eye, in order to ensure that the images seen are continuous, the standard for PAL TV signals is 25 frames per second interlaced scanning, that is, the image that passes through our eyes every second is actually With a continuous screen consisting of 25 screens, one point can be scanned once every 1/25 second when shooting the target. Because it is an interlaced scanning, only one frame can be formed for every 2 fields, so every 1 second. PAL system image is 50 fields, 1 field time is the shutter interval, every second, the shutter must work 50 times in order to ensure that the output image is PAL image of 50 fields / second, so the lowest PAL system The shutter speed is 1/50 second (at this time the aperture is actually open), in practical applications, because the light in the environment may be very strong, this time may need to control the amount of light, you need to control the shutter speed, the faster the speed At this point, the less time the light can enter, the less light will enter. Relatively speaking, the image will appear dark. On the contrary, the slower the shutter speed, the brighter the image will be. When the light intensity is insufficient, The use of 1/50 second images is still not bright enough, which requires the use of other technologies. According to the optical theory, light can be superimposed, although in a very dark environment, each point can be scanned to 1/25 second and the scanned time is very short. Its brightness is very weak. When the brightness of this point is saved and superimposed and then output, this point can be brightened. Therefore, the technical principle of the digital slow shutter is to superimpose multiple images corresponding to a certain period of time and then output them in order to increase the signal strength.
This technology does not require any changes to the external environment, so it can be said to be the ideal solution under the conditions of meeting the monitoring requirements, but the scope of this technology can actually be applied is very narrow, because of the realization of cumulative point by point The premise is that the brightness of the same point is accumulated at different times, and once the shooting object changes or moves, the two pixels may not be in the same pixel at the same time. This will cause the moving object to appear on the whole image. "Smear" phenomenon, if the object moves too fast and the frame accumulation time is too long, the moving object may even become a virtual image. Therefore, the frame accumulation technique is generally applied to monitor a stationary scene in a low-light environment.
The color-to-black-and-white technology switches the image to a black-and-white image when light is insufficient, removes color carriers and burst interference, and increases the AGC to a certain extent to improve image quality in a low visible light environment, but this solution What can be solved is only a part of a very special environment that cannot solve the low-light environment, and the image quality that can be improved is also very limited. Generally, this technology must be used in conjunction with other technologies.
In a certain light source condition, the image is switched from color to black and white using line switching. In the course of the technological evolution of color/black and white line conversion, two sensor elements (one color, one black and white) were used in the early stages to share a set of circuits for switching. Currently, such cameras have adopted a single CCD (color) design. When the light source is sufficient during the day or when the light source is sufficient, when the night falls or the light source is insufficient (usually at 1LUX?3LUX), the digital signal is eliminated by using a digital circuit to become a black and white image, although this method can achieve "low illumination" at night. The purpose is that during the day there are blurred images and unnatural colors.
Passive infrared imaging technology The application of passive infrared imaging technology is based on the premise that the light capturing device must be able to collect infrared signals in addition to the visible light signal, and the signal processing can process the original infrared signal into grayscale signals. Black and white signal). Both black and white cameras are capable of this function and have very high sensitivity. All cameras based on digital processing technology can do this work, but because of the color imaging in the visible light environment, contradictions have begun to appear. When dealing with color signals, because the DSP processing needs to separate the video signal into a gray signal and a chroma (or color difference) signal to be processed separately, and the infrared signal itself is invisible to the human eye, but is captured in the light trap. DSP processing has become a gray signal that can be recognized by the human eye. The superimposition of two gray signals (visible light and infrared light) will inevitably make the image unable to follow ideal conditions when performing grayscale and chroma synthesis. To synthesize, this will cause the grayscale and chroma of the image to be distorted. 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 solutions for passive infrared imaging in video surveillance:
1. Pure color cameras, this solution is to prevent passive infrared imaging, which is to avoid the ingress of infrared light, which uses an optical low-pass filter (OPLF: Optical Low Filter, commonly known as low-pass filter, Figure 1 is its optical communication The relation between the rate and the wavelength can be seen as that it does not substantially absorb and reflect visible light, but substantially completely isolates the infrared light. It directly blocks the infrared ray, so that the image will be substantially unaffected by the infrared signal. The purpose of this is to avoid using infrared imaging.
2, completely regardless of the impact of infrared signals on the color of the image using a large number of infrared filters, which is a low-cost solution, the effect of passive infrared imaging is also relatively good, but it is easy to appear above said Chromaticity and grayscale distortion issues in color mode.
3, only for certain infrared light such as 850nm through, most of the other infrared light can not pass, this is the principle of single filter infrared sensor, the main technical basis is to use a different color camera color filter, the light The relationship between the pass rate and the light wavelength is shown in FIGS. 1 and 2 .
Such a solution can solve the problem of color cast to a certain extent, and can also be used where infrared imaging is used without visible light. However, there are also some problems with this solution. When the sunlight is sufficient, the infrared signal is very rich, causing image color and grayscale distortion. When using infrared imaging, imaging is not sensitive because only very narrow frequency infrared light can pass through, so this kind of camera is generally used in indoor areas within 30 meters in diameter.
4, when the visible light is strong when using OPLF, most of the infrared light block, to ensure its color reduction and grayscale signal is true, and when the visible light is weak, do not use OPLF, and can make the vast majority of visible light and infrared Light can pass through the high-pass filter, because it mainly relies on infrared imaging, is a gray signal, so this time will generally remove the color components, only to retain the gray signal, so see just the gray image that we say Black and white image.
This scheme actually combines the advantages of the color camera in the case of visible light and the advantages of the black and white camera in low illumination. It can be said that it is currently the best solution to solve the monitoring under all-weather, light-changing environments. The true color reproduction under natural light and the high-sensitivity passive infrared imaging without visible light make the static effect even comparable to some integrated machines that use the same technology and incorporate 4x DSS.
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 also some problems that need to be solved, the optical main medium glass has different refractive indexes for two kinds of light waves. It will cause different focal lengths of the optical components, so it is easy to focus problems, but these problems have been solved in the continuous use and improvement. With the constant maturity and widespread application of passive infrared imaging technology, the monitoring performance has been greatly improved, and people's lives will be guaranteed more reliably.
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