CCD technology is responsible for having made scanning a desktop application and has been in use for a number of years in devices such as fax machines and digital cameras. A CCD consists of many photo-sensitive elements, arranged in a grid in the case of a video or digital camera, or in a long, thin line in the case of desktop scanners; the more photo-sensitive elements per unit length, the higher its resolution.
A charge-coupled device is a solid state electronic device that converts light into an electric charge. The light reflected from the object being scanned is directed into the CCD array via a system of mirrors and lenses. The CCD acts as a photometer, converting the measured reflectance into an analogue voltage, which can then be sampled and changed to discrete digital values by an analogue-to-digital converter (ADC).
A desktop scanner claiming a horizontal optical resolution of 300dpi and a maximum document width of 8in will have 8 x 300, that’s 2400 usable elements on the CCD. The CCD itself is usually around four inches wide, so an optical system in the scanning head focuses the light down to the correct size.