CCD is the core component of the scanner. CCD is called a photoelectric coupling device. It can output different currents according to the intensity of the irradiated light to complete the photoelectric conversion process. The scanner generally uses a linear CCD. Each line of the linear CCD is generally composed of 3000 to 10000 light detection points, and there are three lines of R, G, and B. The more light detection points in a certain unit length, the higher the optical resolution of the scanner. Many professional scanners use 8000-point professional CCD. Compared with the CCD of ordinary scanners, most of the CCDs of professional scanners use highly insulating silicon dioxide materials, thus ensuring the consistency of signal transmission. Due to cost constraints, common scanners generally use P-N junction CCDs with general insulation. When this type of CCD performs a photoelectric conversion, the error of the output data is large. It can be said that the price of CCD directly affects the price of scanners. At present, only Kodak manufacturers produce professional CCDs in the world, but there are many manufacturers of ordinary CCDs in recent years. This is also the reason why the prices of ordinary household and commercial scanners have fallen repeatedly. In addition, professional CCDs are larger in size, and larger CCDs help to further improve the consistency and sensitivity of each photosensitive point, and the signal-to-noise ratio for color is also higher. The so-called signal-to-noise ratio, in simple terms, is the percentage of useful and unwanted signals. CCD converts light intensity signals into analog electrical signals. At present, the conversion of 8bit and 10bit is relatively mature, but when converting between 12bit and 16bit, the CCD must have a signal-to-noise ratio (S / N) of 12bit and 16bit or more. The 48-bit scanner requires the use of a CCD with a signal-to-noise ratio above 16bit. This type of CCD is currently very expensive. In addition, the signal-to-noise ratio decreases with increasing temperature. On average, for every 10°C increase in temperature, the signal-to-noise ratio decreases by one. Some professional scanners, such as MICROTEK's CCD, are equipped with a special constant temperature system to ensure that the temperature of the CCD does not change.
The signal from the CCD needs to be connected to the A / D converter on the motherboard through the cable connecting the CCD and the scanner motherboard for digital-to-analog conversion. Because the signal from the CCD is an analog signal, it is susceptible to external interference. The design of high-end scanners has taken this into full consideration. The shielding layer made of metal wire outside the cable reduces the degree of signal interference to the lowest. When scanning, we can see the flat cable inside the scanner through the glass. The cables of the home and commercial scanners are bare and not equipped with a shield.