An optical scanning device used to convert an image—such as a photograph—to digital form, in order to allow later manipulation and output as part of a page, as a color-separated image, or by itself.
In a flatbed scanner, the original image (either a print or transparency) is placed face-down on a glass platen. A light source illuminates the image line by line from beneath the platen and moves across or down the image. The light is picked up by an array of charge-coupled devices, which analyzes the image pixel by pixel and stores the particular color or grayscale information for each pixel in a digital file, commonly by use of colored filters that pass over the CCD array. (Some newer CCD scanners have multiple arrays of CCDs, each of which is responsible for a particular color.) When one line is complete, the light source moves and images the next one, continuing this process until the entire picture is imaged. Scanners that have multiple CCD arrays are faster than those that have only one array, as the scanner needs to only make one pass (rather than three) to glean all the color information it needs.
After scanning, the image is processed as one large digital file (if the image needs further manipulation) or as separate color-separated films.
Other types of scanners—such as high-end drum scanners—utilize photomultiplier tubes rather than CCD arrays. Although these devices produce much higher-quality scans than flatbed scanners, they are far more expensive. Flatbed scanners—due to the fixed, flat copyboard—also allow scanning images from books and other sources that would be hard to image on a drum scanner. See Scanner and Scanning. See also CCD Scanner.