The CCD inside a flatbed scanner is one of the most critical components for determining overall image quality. As the scanner's lens can be thought of as the "eye" of the scanner, the CCD is the scanner's retina. The CCD is an optical element that is light sensitive and captures the information brought in through the lens. That information is then sent from the CCD to the scanner's A/D (Analog to Digital) converter for processing the analog information into digital 0's and 1's.
The CCD in a flatbed scanner contains thousands of photosites (also referred to as pixels or elements) along its horizontal axis. The number of photosites, and the maximum horizontal scanning area of the scanner itself, together determine the maximum optical resolution in dots per inch (dpi). A typical CCD contains 2,700 photosites, 150 of which are reserved for CCD adjustments. So if a scanner has a CCD with 2,550 useable photosites and scans a page of 8.5" width, the maximum horizontal resolution of the scanner will be 300 dpi . This figure is determined by dividing the useable photosite number by the maximum horizontal size of the page (i.e. 2,550/8.5=317.6 dpi).
Some flatbed scanners use two or more lenses with the same CCD. This is so the added lenses can focus on smaller areas with more detail. The optical resolution of a scanner using a second lens increases when the entire focus is placed on a smaller area. If we take the same example above but add a second lens that is designed to focus on a specific area 4.25" width, the effective resolution with the second lens becomes 600 dpi (2,700/4.25=635.3 average to 600dpi). Scanners with multiple lenses are more expensive than single-lens scanners.
The CIS is found in small handheld and sheetfed scanners. The CIS works much differently than a CCD. The CIS consists of a tube that contains a built-in light source (typically an LED array) as well as a series of pixel sensors that picks up the information (e.g. paper document) as it travels across the CIS. Each pixel captured by the CIS is a broken piece of the image that is then put back together by the scanner's hardware. The CIS does not use a separate lens to capture information like the CCD, but instead uses a rod lens array built inside the CIS which then transfers information to the main contact sensor. Because all of the components involved with the CIS system are built within one horizontal tube, the CIS is a more compact system than one that uses a CCD.
Side by Side Comparison
Since the CCD is an optical sensor, it captures much more information than the CIS rod lens array at the initial stage of the scan. When the CCD's image is transferred into pixels by the A/D converter, the result is a smoother and more detailed image.
Current CIS designs are limited to optical resolutions of 300 dpi. Additionally the CIS signal to noise ratio is poorer than a CCDs.
At this time, color CIS units are more expensive than CCDs because they are not as widely produced. CIS units do have their advantages however. Besides their compact size, they consume less current than CCDs and also require no special alignment like a CCD and lens do. Because the CIS simplifies the assembly process of the scanner itself, they have been incorporated into some entry-level flatbed scanners.