The difference between sensor CCD and CMOS

The sensor is the core component of the camera. It has functions such as photoelectric conversion, information storage, delay and sequential transmission of electrical signals. It has high integration and low power consumption, so it has subsequently developed rapidly. It is an indispensable key device for image acquisition and digital processing. Currently, there are two types of photosensitive chips commonly used in cameras on the market: CCD and CMOS.

1. CCD (Charge Coupled Device)

   Currently, there are three structures of commonly used CCD cameras: full frame transfer, frame transfer and line transfer.

   1. Interline Transfer

      Among the three structures, row transfer is the most commonly used one, and most mid- and low-end products use this method.

   2. Full frame transfer (Full frame)

      The full frame transfer structure is the simplest, the photosensitive area and the storage area are together, and a 100% fill factor can be obtained. The disadvantage is that in order to avoid image light leakage, external light must be prevented from entering during readout. This can be achieved by using a mechanical shutter, using a strobe, or making the exposure time much longer than the readout time to minimize light leakage.

   3. Frame transfer

      The frame transmission structure is that the photosensitive area and the storage area are completely separated and equal in size. The exposed signal charge is transferred to the storage area at a very fast speed (usually less than 1% of the frame period) and then output line by line. It is obvious that the frame transfer sensor achieves 100% fill factor and during the readout process, the next frame is exposed. The disadvantage is that the chip size is large and the cost is high.

2. CMOS (Complementary Metal Oxide Semiconductor) Sensor

   The principles of the photosensitive part of CMOS sensors and CCD sensors are the same. The difference is that in each pixel unit, in addition to the photosensitive part, there is also an amplifier and readout circuit part. The entire CMOS sensor also integrates an addressing circuit, amplifier and A/D.

1. The first point is the difference in full well capacity: Since each pixel of the CMOS sensor includes a photosensitive diode, amplifier and readout circuit, and the entire sensor also includes an addressing circuit and A/D, the photosensitive area of ​​each pixel is much smaller than the surface area of ​​the pixel itself. Therefore, with the same pixel size, the full well capacity of the CMOS sensor is lower than that of the CCD sensor.
2. The second point is the cost difference: Since CMOS sensors use the CMOS process most commonly used in general semiconductor circuits, peripheral circuits (such as AGC, CDS, timing, or DSP, etc.) can be easily integrated into the sensor chip, thus saving the cost of peripheral chips.
3. The third point is that the CMOS sensor can be randomly addressed, making it very convenient to read out only the interesting part of the queue and improve the frame rate.
4. The fourth point is noise difference: Since each photodiode of the CMOS sensor is equipped with an amplifier, and the amplifier is an analog circuit, it is difficult to make the results obtained by each amplifier consistent. Therefore, compared with a CCD sensor with only one amplifier at the output end of the horizontal register, the noise of the CMOS sensor will increase a lot, affecting the image quality.
5. The fifth point is the difference in power consumption: the power consumption of CMOS sensors is lower than that of CCD sensors.

To sum up, the advantage of CCD is that the image quality is better than that of CMOS, but the disadvantages are also obvious. Its process is too complicated, and there are very few manufacturers that can master its technology, so the price is destined to not be cheap, especially for some large CCD components, the price is even more unacceptable. Therefore, in the current camera market, high-end cameras generally use CCD imagers to demonstrate identity and status. The biggest advantage of cmos imager is that it saves power. With the same battery and the same configuration, cmos can be used 1-2 hours longer than ccd, because it only consumes power when the circuit is connected, while ccd consumes power all the time. It is the so-called "electric tiger". Based on this, we can maximize our strengths and avoid weaknesses, and reasonably choose CCD or CMOS cameras for different applications.