Most of scanners are built with the help of  two types of sensor technologies: CCD and CIS

CCD (Charged Coupled Device) is the key component inside  scanner for light-sensing. It contains an array of photosensitive cells that react to incoming light based on its properties, such as intensity and color. During the scanning process, CCD will first capture the incoming light (via an optical lens) and then generate various analog signals to "reflect" the incoming light properties. CCD uses prisms, lenses, and other optical components to direct reflected light into the sensor. Most scanners sold on the market today, utilize CCD as their light-sensing device. This type of scanner provides better color saturation.

CIS ( Contact Image Sensor) or CMOS (Complementary Metallic Oxide Semiconductor sensors) have the same function as a CCD, except it does not require the use of an optical lens, and is smaller in size. CIS uses a single row of sensors placed extremely close to the image. CCD scanners generally produce better image quality than CIS scanners, but CIS scanners are more compact and less expensive.

A/D Converter, short for Analog / Digital Converter, is a component inside your scanner that converts the analog signals generated by the CCD or CIS into digital signals. The conversion from analog signals to digital signals is crucial for your computer.

Transparency adapters (TA):  Part of flatbed scanners are equipped with  transparency adapters , which let you scan 35mm slides and other "see through" items and automatic document feeders.