As video surveillance is more and more widely 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. Because color imaging generally requires higher brightness, and the actual environment may not be able to meet this requirement, the low-illumination technology of the camera (also called night vision technology) appears accordingly. Due to different monitoring requirements, different application technologies have emerged.
Due to reasons such as cost and special materials, the human eye night vision technology that is widely used in the military has not been transplanted into the field of civilian surveillance in large numbers. Therefore, currently there are several technologies used in civilian surveillance products: high-sensitivity materials, digital Slow shutter technology, color-to-black technology, passive infrared imaging technology, etc., because of the different monitoring requirements and application occasions, so different application markets have appeared 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 and processing devices, and signal processing to add some special processing techniques, etc., to improve the reduction of image acquisition under low light conditions, but Due to cost and volume considerations, although this is the best solution, it is still difficult to be widely used in a short time.
At present, for the CCD photosensitive element, there are two main ways to improve the sensitivity. Since the photosensitive area is increased by physical means, it cannot be achieved due to the limitation of the volume of the device, so one of them is to increase the sensitivity area by increasing the photosensitive area by displacing the tiny lens on each photosensitive diode (single pixel) of the device. Works well. This design is like putting the CCD on the glasses, but after 35 years of development, the room for this technology to improve has been quite limited. The second is to obtain a reasonable exposure based on data calculation through a specific signal enhancement circuit, but this usually results in noise due to the uneven sensitivity of the pixels caused by the rapid CCD sensitivity. At this time, the shot graininess will be more obvious. At this time, you have to take measures to balance the contradiction between high sensitivity and high image quality, which will inevitably bring higher costs.
Digital slow shutter technology
Digital slow shutter technology (digitalslowshuttle), in fact, it is not a kind of shutter, but its function is similar to the shutter to a certain extent, the shutter (shuttle) and aperture (IRIS) are both on the camera to control the light through the lens to achieve A component of the light capture effect. It can also be understood that the iris is a hole that light can enter when passing through the lens. The size of the hole is the size of the iris. The larger the hole, the more light passes through under the same circumstances. The shutter is the part that controls the iris switch and controls the iris. Is it always on or is it switched on and off at regular intervals.
We know that according to the persistence of the human eye, in order to ensure that the image seen is continuous, the standard of PAL television signals is 25 frames / second interlaced scanning, that is, every second image passing through our eyes is actually A continuous frame composed of 25 frames can only be scanned once every 1/25 second when shooting a target. Because it is an interlaced scan, a frame can be formed every 2 fields, so every 1 second The PAL system image is 50 fields, and the time of one field is the shutter interval. The shutter must work 50 times per second to ensure that the output image is a 50 field / second PAL system image, so the minimum PAL system The shutter speed is 1/50 second (at this time the aperture is actually always open). In practical applications, because the light in the environment may be very strong, this time you may need to control the amount of light entering, you need to control the shutter speed, the faster the speed At this time, the less time light can enter, the less light will enter. Relatively speaking, the image will appear darker. Conversely, the slower the shutter speed, the brighter the image. When the light intensity is insufficient, even using 1 / 5 The 0 second 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 be scanned to 1 time after 1/25 second, the scanning time is also very short, and its brightness is very weak. If the brightness of this point is saved and superimposed before output, the point can become brighter. Therefore, the technical principle of the digital slow shutter is to superimpose and output multiple images within a corresponding period of time as required to increase 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 conditions of meeting the monitoring requirements, but the range of applications of this technology is actually very narrow, because the point-by-point accumulation The premise is that the brightness of the same point is accumulated at different times, and once the object that is photographed changes or moves, the two pixels at the same time may not accumulate the same pixel, so that the moving object will appear on the overall image. "Dragging" phenomenon, if the object moves too fast and the frame accumulation time is too long, the moving object will even become a ghost image. Therefore, the frame accumulation technique is generally applied to monitor static scenes in low-light environments.
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