The density range is a very important performance parameter for the scanner. The density range is also called pixel depth, which represents the range of bright light and dark tone that the scanner can distinguish. Generally, the density range of the roller scanner is greater than 3.5, while the density range of the plane scanner is generally between 2.4 and 3.5.
The scanner is a high-tech product integrating light, machine, and electricity. It is an important tool for inputting various forms of image information into computers, and it is the third generation of computer input equipment after keyboard and mouse. Scanners have stronger functions than keyboards and mice. With the help of document camera scanner, from the most original pictures, photos, films to all kinds of documents and materials can be input into computers by scanners, thus realizing the processing, management, use, storage, and output of information in image form. In combination with optical character recognition software OCR(Optic Character Recognize), scanned documents can also be converted into the text form of computers.
The core components of the scanner are optical reading device and analog-to-digital (A/D) converter. There are two common optical reading devices: CCD and CIS.
① CCD（Charge Coupled Device）
The Chinese name of CCD is a charge-coupled device, which is similar to common semiconductor integrated circuits. It integrates thousands of photodiodes on a silicon single crystal. These photodiodes are divided into three columns and covered by red, green, and blue color filters respectively, thus realizing color scanning. Photoelectric triode can generate current when irradiated by light and output after amplification. After years of development, the scanner technology using CCD has become quite mature and is the main photosensitive element used by mainstream scanners in the market. The advantage of CCD is that the image scanned by it is of high quality, has a certain depth of field, and can scan uneven objects. The temperature coefficient is relatively low. For normal work, the change in ambient temperature can be ignored.
② CIS（Contact Image Sensor）
The Chinese name of CIS is a contact image sensing device. It uses contact-type photosensitive elements (photosensitive sensors) to carry out photosensitive. 1 mm ~ 2 mm below the scanning platform, 300 ~ 600 red, green, and blue three-color LED (light-emitting diode) sensors are tightly arranged together to generate a white light source. It replaces the CCD array, lens, cathode fluorescent lamp, cold cathode ray tube, and other complex mechanisms in the CCD scanner and turns the optical, mechanical and electrical integration of the CCD scanner into the mechanical and electrical integration of the CIS scanner. Scanners made by CIS technology have the advantages of small volume, lightweight and low production cost.