Working principle of CCD photosensitive element

CCD can be divided into linear CCD and area CCD in function. Linear CCD usually divides the internal electrodes of CCD into arrays, and each array is called a phase and applies the same clock pulse. The required number of phases is determined by the internal structure of CCD chip, and CCDs with different structures can meet the requirements of different occasions. Linear CCD is divided into single channel and dual channel, and its photosensitive area is MOS capacitor or photodiode structure, so the manufacturing process is relatively simple. It is composed of photosensitive area array and shift register scanning circuit, which is characterized by fast information processing speed, simple peripheral circuit and easy real-time control, but the amount of information obtained is small and it can't handle complex images. The structure of area CCD is much more complicated. It is composed of many photosensitive areas arranged in a square array and connected into a device in a certain form, which can obtain a lot of information and process complex images. In the field of digital cameras, the application of CCD is more colorful. Generally, a Bayer filter is added to the CCD for color digital cameras. Every four pixels form a unit, one for red, one for blue and two for green (because human eyes are sensitive to green). Therefore, each pixel receives a photosensitive signal, but the color resolution is not as good as the photosensitive resolution.

The 3CCD system composed of three ccds and a beam splitting prism can better divide colors. The beam-splitting prism can analyze the incident light into three colors: red, blue and green, and three CCDs are responsible for the image presentation of one color. All professional digital cameras and some semi-professional digital cameras adopt 3CCD technology. At present, the ultra-high resolution CCD chip is still relatively expensive, and the price of a high resolution still camera equipped with 3CD often exceeds the budget of many professional photographers. Therefore, some high-end cameras use rotating color filters to give consideration to high resolution and faithful color presentation. This multi-imaging camera can only be used to shoot static objects. At the same time, frozen CCD was widely used in astronomical photography and various night vision equipment in the early 1968+0990' s, and large observatories also continuously developed high-resolution CCD to take pictures of celestial bodies with extremely high resolution.

CCD has a wonderful application in astronomy, which can make a fixed telescope work like a tracking telescope. The method is to make the direction of charge reading and moving on CCD consistent with the running direction of celestial bodies, and the speed is also synchronous. Using CCD to guide stars can not only effectively correct the tracking error, but also make the telescope record a larger field of view than before.

Generally, most CCD can sense infrared rays, so infrared images, night vision devices, zero illumination (or near zero illumination) cameras and so on are derived. In order to reduce infrared interference, astronomical CCD is often cooled by liquid nitrogen or semiconductor, because objects at room temperature will have infrared blackbody radiation effect. The sensitivity of CCD to infrared rays has another effect. If all kinds of digital cameras or video recorders equipped with CCD are not equipped with infrared filters, it is easy to capture the infrared rays emitted by the remote controller. Lowering the temperature can reduce the dark current on the capacitor array, improve the sensitivity of CCD in low illumination, and even improve the sensitivity (signal-to-noise ratio) to ultraviolet and visible light.

Temperature noise, dark current and cosmic radiation will all affect the pixels on the CCD surface. Astronomers use the opening and closing of the shutter to expose the CCD for many times and take its average value to reduce the interference effect. In order to remove the background noise, the average value of the image signal is taken when the shutter is closed, that is, the "dark frame". Then open the shutter, subtract the black box value after obtaining the image, and then filter out the system noise (dark spots and bright spots, etc.). ) get clearer details.

The cooled CCD camera used in astrophotography must be fixed in the imaging position with a connecting ring to prevent the influence of external light or vibration; At the same time, because most imaging platforms are naturally huge, astronomers use the technology of "automatic guiding stars" to take pictures of faint celestial bodies such as galaxies and nebulae. Most automatic star guidance systems use extra off-axis CCD to monitor the deviation of any image, however, some systems connect the primary mirror to a CCD camera to shoot. Using an optical device to add the starlight in the main mirror to another CCD star guide device in the camera can quickly detect the tiny error of tracking celestial bodies and automatically adjust the driving motor to correct the error without additional star guide.