Solid-state image sensors (also known as solid-state optoelectronic imaging devices) are available in both CCD and CMOS. CCD is an abbreviation for "Charge Coupled Device", and CMOS is an abbreviation for "Complementary Metal Oxide Semiconductor". The CCD was invented by W.B. Boyle and G.E.Smith of Bell Labs, USA in 1970, thus unveiling the prelude of charge transport devices. Since then, people have used this technology to create cameras and digital cameras, pushing the image processing industry to a whole new field. CCD is a photosensitive semiconductor chip used to capture images. It is widely used in scanners, copiers, camcorders and filmless cameras. As a camera, similar to the principle of a film, an optical image (ie, an actual scene) is projected through the lens onto the CCD. However, unlike the film, the CCD has no "exposure" capability, and it does not have the ability to record and store image data. Instead, the image data is sent to an A/D converter, signal processor and storage device without stopping, but Repeat shooting and instant adjustment, the image can be copied indefinitely without degrading the quality, and it is convenient for permanent storage.
CMOS is originally an important chip in a computer system, which can store a large amount of data required for system booting. In the early 1970s, it was discovered that the introduction of CMOS into a semiconductor photodiode can also be used as a sensitized sensor, but it is not developed in terms of resolution, noise, power consumption, and image quality. . With the development of CMOS process technology, high-quality, low-cost CMOS imaging devices can be produced using standard CMOS processes. This type of device is easy to mass-produce, and its low power consumption and low cost are all the dreams of the merchants. Nowadays, both CCD and CMOS coexist, and CCD is still "mainstream" for the time being, but CMOS will replace CCD and become the mainstream of image sensor.
Comparison of information reading methodsThe charge information stored by the CCD optoelectronic imaging device needs to be read one by one in a row-by-row sequence under the control of two-phase or three-phase or four-phase clock drive pulses.
The optical image information of the CMOS optoelectronic imaging device is photoelectrically converted to generate a current or voltage signal. This electrical signal does not need to be read line by line like the CCD, but is directly read from the CMOS transistor switch array, which can increase the image taking. flexibility. The CCD has no such function.
Speed ​​comparisonIt is known from the above that the CCD imaging device needs to output information one bit at a time in the behavior unit under the control of the second, third and fourth phase clock driving pulses, so the speed is slow.
The CMOS imaging device can take out the electrical signal while acquiring the photoelectric image signal, and can simultaneously process the image information of each unit, so the speed is much faster than the CCD imaging device. Since the row and column electrodes of the CMOS imaging device can be driven at a high speed, and the A/D conversion is performed on the same chip, the image signal can be quickly taken out, so that it can operate at a relatively high frame rate. Some CMOSs ​​designed for machine vision claim to be as fast as 1000 frames per second.
Comparison of power and power consumptionSince the pixels of the CCD are composed of MOS capacitors, a two-phase or three-phase or four-phase timing pulse signal having a relatively large voltage (at least 12V) is required to read the charge signal, so that the charge can be efficiently transmitted. Therefore, in addition to having multiple power supplies, the CCD image capture system consumes considerable power from its peripheral circuits. Some CCD imaging systems consume 2 to 5 W of power.
The CMOS optoelectronic imaging device only needs to use a single power supply of 5V or 3V, and the power consumption is very small, only 1/8~1/10 of the CCD. Some CMOS image capturing systems only consume 20~50mW.
Comparison of imaging qualityThe CCD imaging device fabrication technology started early and the technology is mature. The PN junction or silicon dioxide (sio2) isolation layer is used to isolate noise, so the noise is low and the imaging quality is good.
Compared with CCD, the main disadvantages of CMOS are high noise and low sensitivity, because CMOS imaging devices have high integration, the distance between each photoelectric element and circuit is very close, and the optical, electrical and magnetic interference between them is serious, and the noise is on the image. The quality has a great impact and it has not been practical for a long time. Later, the noise problem was reduced with active pixel (AcTIve Pixel) design and noise correction circuitry. In recent years, with the continuous development of CMOS circuit noise reduction technology, it has provided good conditions for the production of high-density and high-quality CMOS imaging devices. Some manufacturers have claimed that the developed technology has no better image quality than CCD.
The sensitivity of CMOS imaging devices is low because the pixel area is used to make lines such as amplifiers. On a fixed chip area, the resolution of the imaging device cannot be made too high in order to maintain a fairly high level of sensitivity, unless a finer manufacturing process is used. (Conversely, a fixed resolution sensor cannot be made too large. small). However, at present, 4096 & TImes; 4096 ultra-high resolution CMOS image sensor has been developed using 0.18 μm manufacturing technology.
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