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optical scanner


A device that can read text or illustrations printed on paper and translate the information into a form the computer can use. A scanner works by digitizing an image -- dividing it into a grid of boxes and representing each box with either a zero or a one, depending on whether the box is filled in. (For color and gray scaling, the same principle applies, but each box is then represented by up to 24 bits.) The resulting matrix of bits, called a bit map, can then be stored in a file, displayed on a screen, and manipulated by programs.

Optical scanners do not distinguish text from illustrations; they represent all images as bit maps. Therefore, you cannot directly edit text that has been scanned. To edit text read by an optical scanner, you need an optical character recognition (OCR ) system to translate the image into ASCII characters. Most optical scanners sold today come with OCR packages.

Scanners differ from one another in the following respects:

  • scanning technology: Most scanners use charge-coupled device (CCD) arrays, which consist of tightly packed rows of light receptors that can detect variations in light intensity and frequency. The quality of the CCD array is probably the single most important factor affecting the quality of the scanner. Industry-strength drum scanners use a different technology that relies on a photomultiplier tube (PMT), but this type of scanner is much more expensive than the more common CCD -based scanners.
  • resolution: The denser the bit map, the higher the resolution. Typically, scanners support resolutions of from 72 to 600 dpi.
  • bit depth: The number of bits used to represent each pixel. The greater the bit depth, the more colors or grayscales can be represented. For example, a 24-bit color scanner can represent 2 to the 24th power (16.7 million) colors. Note, however, that a large color range is useless if the CCD arrays are capable of detecting only a small number of distinct colors.
  • size and shape: Some scanners are small hand-held devices that you move across the paper. These hand-held scanners are often called half-page scanners because they can only scan 2 to 5 inches at a time. Hand-held scanners are adequate for small pictures and photos, but they are difficult to use if you need to scan an entire page of text or graphics.
  • Larger scanners include machines into which you can feed sheets of paper. These are called sheet-fed scanners. Sheet-fed scanners are excellent for loose sheets of paper, but they are unable to handle bound documents.

    A second type of large scanner, called a flatbed scanner, is like a photocopy machine. It consists of a board on which you lay books, magazines, and other documents that you want to scan.

    Overhead scanners (also called copyboard scanners) look somewhat like overhead projectors. You place documents face-up on a scanning bed, and a small overhead tower moves across the page.







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