As video monitoring the use of more and more widely in the field of civil, application environment is becoming more and more complex, one of the most important application requirement is 24-hour monitoring. Due to color imaging generally need higher brightness, the practical environment is not necessarily can meet this requirement, so the camera low illumination technology (also called night vision technology) correspondingly appeared. Due to the different monitoring demand, thus appeared different application technology.
Because of cost and special materials, the night vision technology is widely applied in the army in human eyes has not been a large number of transplanted into civil monitoring field, so the current civil monitoring products there are usually several technologies used in the high sensitive materials and digital slow shutter technology transfer, the color black, passive infrared imaging technology, and so on, because the monitoring requirements and applications of different, so in practice in different application market.
High sensitivity materials
Using high sensitive materials, including the use of high sensitivity optical sensing materials, high SNR of signal analysis and processing devices, signal processing to add some special processing technology and so on, in order to improve the reduction effect of low light situations image acquisition, but considering the cost and volume and other reasons, although this is the best solution is still difficult to be applied extensively in a short period of time.
The present for CCD sensor, improve the sensitivity mainly through two ways. Due to the increasing photosensitive area by physical way, restricted by equipment volume can not be achieved, so that one is on the component a photosensitive diode (single pixel) device of tiny lenses disguised increase the way of the photosensitive area to obtain the iso, became effective. This design is like help CCD hang up the glasses, but the CCD after 35 years of development, the technology promotion space is quite limited. Secondly, through the specific signal enhancement circuit, according to data operation to obtain the reasonable exposure, but it is usually because the CCD photosensitive cause uneven photosensitive pixels quickly and form the noise, then shoot the images of the particles will be more apparent. At this time and had to take the way to balance the contradiction between high speed and high quality, which inevitably will bring higher costs and investment.
Digital slow shutter technology
Digital slow shutter technology (digital missile shuttle), in fact it is not a shutter, only its function to some extent is similar to the shutter, aperture and shutter (shuttle) (IRIS) control the light through the lens is a camera, a component to capture light effect. Also can understand it, the aperture is able to enter when the light through the lens of a hole, the size of the hole is the size of the aperture, the bigger the hole, through the more light is the same case, and the shutter is part of the master aperture switch, control the aperture is in has been open or according to certain time interval timer switch.
As we know, according to the persistence of vision characteristic of human eyes, in order to ensure that see image is continuous, standards of PAL TV signals is 25 FPS interlaced scanning, that is to say, every one second after the image of our eyes is actually made up of 25 images continuous images, at the time of shooting target, every 1/25 of a second, a point can be scanned to the time, because is the interlaced scanning, every two matches to form a frame, so every 1 seconds, the image of PAL is 50 games, one time is the interval of the shutter, every second, shutter must work 50 times, in order to ensure that the output image is 50 games/SEC PAL image, is the minimum a shutter speed of 1/50 seconds so PAL (aperture is actually open) at this time, the actual application, because the light in the environment may be very strong, this time may need to control into the light, you need to control the shutter speed, the faster the speed, the less light to enter time, into the less light, relatively speaking, the image will appear darker, conversely the slower shutter speed, the image will be more bright, when the light intensity of illumination is insufficient, even with 1/50 of a second image is still not enough bright, this requires using other technologies. The stacks of alone, according to the theory of optical, although in a very dark environment each point to 1/25 of a second before they can be scanned to 1 times, scanned time is very short, its brightness is very weak, if all the brightness before and after a period of time that point to save and superposition output again, after this point can brighten. So digital technology principle of slow shutter is in accordance with the requirements to appropriate over a period of time after the superposition of multiple image to the output, in order to improve the signal strength. This technology because they do not need any external environment changes, so under the condition of meet the requirements of monitoring can be said to be the ideal solution, but this technique can actually application range is very narrow, because the implementation point accumulation is the premise of the brightness of the same in different time points accumulation, and once change or move to take pictures of objects, both before and after the time together sum may not be the same pixel, so in the whole moving object in the image will appear "ghosting" phenomenon, if the object moves too fast and frame accumulation time is too long, even turn on mobile object. So, the general frame accumulation technology applied in the weak light environment monitoring the stillness of the scene.
Color turn black and white
When the light is not enough to switch images as black and white images, and color to rub off carrier synchronization disturbance, and the AGC increased, to a certain extent, can improve the image quality of low visible light environment, but is only part of this scheme can solve the very special environment, will not solve the weak light environment, but also can improve the image quality is also very limited, this technology is commonly used and other technologies.
In a certain light conditions, by using line switch to image from color to black and white. In color/black and white lines in the process of the transformation of technology evolution, the early used 2 star Sensor Sensor (1 color, 1 black and white) share a set of circuit switch again, now this kind of camera, single CCD (color) design has been adopted or light enough for color video camera during the day, when night or insufficient light source (usually in 1 lux? 3 lux) is using the digital circuit will eliminate the color signal, become black and white images, although this approach can achieve the goal of "low illumination at night, the day has blurred images, color is not natural flaws.
Passive infrared imaging technology
Passive infrared imaging technology is the application premise of light capture devices in addition to the need to be able to collect visible signals, but also to collect the infrared signal, and signal processing to the original infrared signal processing into gray scale signal (say, often said that the black and white). Black and white video camera to be able to achieve this function and the sensitivity is very high. At present all cameras are based on the technology of digital processing to be able to finish the work, but because of the color image and visible light environment, contradictions also begin to emerge. In dealing with color signals, for DSP processing need to separate the video signal into gray signals and chromaticity (or color) for processing respectively, and the infrared signal itself is the human eye is not visible, but after light trap to collect and DSP processing has become the human eye can distinguish the gray scale signal, two gray scale signal superposition (visible and infrared light), will inevitably make the image in gray and chromaticity synthesis synthesis cannot be carried out in accordance with the ideal situation, this will cause the image grayscale and chromaticity distortion, the most typical example is if infrared light is strong, make the whole image graying. At present about the passive infrared imaging in video monitoring, the main solution has the following four:
1, pure color video camera, this solution is to prevent the passive infrared imaging, also is to try to avoid the infrared, the use of Optical Low pass Filter (OPLF: Optical Low Filter, known as a low-pass Filter, figure 1 for the pass rate and the relationship between the wavelength of light, it can be seen that the basic do not absorb and reflect visible light, but completely isolated basic infrared) directly to the infrared blocking, image will be basic is not influenced by the infrared signal. The aim is to avoid using infrared imaging.
2, completely regardless of the infrared signal of image color cases the influence of the mass by using infrared color filter, it is a low cost solution, in terms of passive infrared imaging effect is better, but it is easy to appear the above said in color mode of chromaticity and gray level distortion.
3, only through specific infrared light such as 850 nm, most of the other cannot through infrared light, this is the principle of single filter type induction infrared camera, the main technical basis is adopted different from color video camera color filter, this scheme can to a certain extent, solve the problem of color from the color, and also to be able to use in place of using infrared imaging without visible light. But this plan also has some problems, when sufficient sunshine, infrared signal is very rich, will cause the color image and gray level distortion. But in the use of infrared imaging, because only a very narrow frequency of infrared light can also pass and make the imaging is not sensitive, so this type of cameras in general use in indoor area within 30 meters in diameter.
4, strong in visible when using OPLF, most infrared blocking, to ensure that its true color reduction and grayscale signals, and weak in the visible light when not using OPLF, and use can make the most of the visible light and infrared light can be through the high-pass filter, because rely mainly on infrared imaging, is gray scale signal, so this time usually remove the color composition, only keep gray scale signal, so see is just a gray image which was known as black and white image.
This scheme is, in fact is a combination of the advantages of color video camera under the condition of visible light is good, and the advantages of black and white camera under low illumination, can be said to be solve all-weather, the light changed the environment monitoring of the best solutions. In the natural light the real color reduction high sensitive passive infrared and visible light imaging, the static effect can even 4 times and some of the same technology and join DSS photographic technology all-in-one machine.
While using passive infrared imaging can well solve the above depending on the light and to monitor the contradiction between, but because of infrared and visible light optical difference, there are also some problems need to be solved, the refractive index of a medium glass of two main types of optical waves are different will cause the focal length of the optical components, so it's easy to make the focus problems, but these problems have been in constant use and improve the corresponding solution. With the passive infrared imaging technology unceasingly mature and widely used, greatly improve monitoring efficiency, people's life will be more reliable guarantee.
