[汽车修理知识]Detailed interpretation of car camera

Classification of CCD and COMS

CCD and CMOS can be divided into two categories according to the imaging process: dynamic (fast scan) and static (slow scan); according to different applications, they can be divided into professional level and civilian level.Common video chatCamera(Such as Logitech) and various digital cameras use civilian-grade dynamic and static CCD/CMOS respectively.

CCD imaging principle

The process of CCD imaging is as follows: The silicon semiconductor photosensitive element coated on the surface of the CCD captures photons and generates photo-generated electrons. These electrons are first accumulated in the insulating layer under the CCD, and then exported to the analog-to-digital circuit by the control circuit in a serial manner. In the process, an image is formed through an imaging circuit such as DSP. The biggest difference between fast scan and slow scan lies in the speed of photoelectron export and the circuit system. Fast scan exports electrons at a very fast frequency in order to achieve a video-level refresh rate, but this will result in loss of electrons, increased noise, and incomplete emptying of photogenerated electrons. On the contrary, slow scan is the opposite. Its circuit design focuses on the accumulation of photogenerated electrons. In terms of protection, the derived frequency is not high, but it ensures that the loss and loss of electrons during the transmission process are minimized. Its analog-to-digital converter has extremely high dynamic range and sensitivity, which ensures that the signal conversion process is not distorted, and at the same time, in order to reduce thermal effects The noise generated is generally cooled by a Cooling system.

After reading the above explanation, we can know why professional-grade scientific research cameras are so expensive, starting from the material and area of ​​the CCD photosensitive layer, to the accumulation of photo-generated electrons, to electronic export circuits, transmission circuits, analog-to-digital conversion circuits, and images. The display circuit, the cooling circuit, and the craftsmanship of the professional-grade scientific research camera at each step are different from the civilian-grade, and the cost is tens to hundreds of times. There is only one purpose. The professional camera can collect all the optical signals as completely as possible. Generally speaking, civilian-grade cameras or digital cameras can only reflect less than 50% of the light signal.

Basic indicators for evaluating CCD

Mainly include pixel value, signal-to-noise ratio, cooling temperature, etc.

Signal-to-noise ratio (SNR)

The signal-to-noise ratio truly reflects the detection capability of the camera. In order to improve the performance of the camera, all CCD camera manufacturers try their best to maximize the signal (the number of electrons that can reach the full well) while reducing noise as much as possible.

SNR = full well electronics / noise electronics = dynamic range = maximum gray scale = 2bit number

In the same full-well electronic CCD, reducing the CCD noise can improve the CCD's monitoring ability. Heat or dark current is noise to the CCD, and the noise can basically be eliminated by the deep cooling Peltier in the Cool CCD. When the exposure exceeds 5-10 seconds, the CCD chip will heat up. Without a cooling device, the "hot" or white pixels will cover the image, and snowflakes will be seen everywhere in the image. The -20°C camera can take up to 5 minutes of images, and the -40°C camera can take more than 1 hour.

CCD structure design, digitization method, etc. will affect the generation of noise. By improving the structure and optimizing methods, the generation of noise can also be reduced.

Pixel area

This indicator is an important indicator in the chip. The larger the pixel area, the more sensitive it is to light. Because there are more electrons in the pixel area, more signals can be generated. Large pixels increase sensitivity, and small pixels increase resolution. To improve the image quality, it is necessary to increase the pixels of the CCD. Therefore, when the size of the CCD is fixed, increasing the pixels means reducing the photodiodes in the pixels. We know that the smaller the area of ​​the unit pixel, the lower the photosensitive performance, the lower the signal-to-noise ratio, and the narrower the dynamic range. Therefore, this method cannot increase the resolution indefinitely. Therefore, if you do not increase the CCD area and blindly increase Resolution will only cause deterioration of image quality. However, if you want to maintain the existing image quality while increasing the CCD pixels, you must increase the total area of ​​the CCD while at least maintaining the area of ​​the unit pixel. At present, it is more difficult to process and manufacture larger-size CCDs, and the yield rate is relatively low, so the cost has not been reduced. This contradiction is difficult to overcome for CCDs.

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