Scanners

In computing, a scanner is a device that analyzes an image (such as a photograph, printed text, or handwriting) or an object (such as an ornament) and converts it to a digital image. more...

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Most scanners today are variations of the desktop (or flatbed) scanner The flatbed scanner is the most common in offices. Hand-held scanners, where the device is moved by hand, were briefly popular but are now not used due to the difficulty of obtaining a high-quality image. Both these types of scanners use charge-coupled device (CCD) or Contact Image Sensor (CIS) as the image sensor, whereas older drum scanners use a photomultiplier tube as the image sensor.

Another category of scanner is a rotary scanner used for high-speed document scanning. This is another kind of drum scanner, but it uses a CCD array instead of a photomultiplier.

Other types of scanners are planetary scanners, which take photographs of books and documents, and 3D scanners, for producing three-dimensional models of objects.

Drum scanners

Drum scanners, the oldest scanning technology, have a scanning photomultiplier tube, which moves back and forth along a single axis. The image to be scanned is soaked in oil, then wrapped around the drum; this process is known as wet mounting. The drum then rotates in front of the photomultiplier tube. The use of drum scanners has declined significantly as flatbed scanners based on charge-coupled devices have dropped in price; however, drum scanners are still used for certain high-end applications, such as museum-quality archiving of photographs and for print production of high-quality books and magazine advertisements.

They are very expensive and only manufactured by a few companies. Because a photomultiplier tube is much more sensitive to light than a charge-coupled device or CIS and the scanning beam can be focused very narrowly, drum scanners can produce scans superior to those of flatbed scanners, both in resolution and in the gradations of color and brightness. Also, since drum scanners have the advantage in resolution, their use is generally recommended when a scanned image is going to be greatly enlarged.

In fact, in most current graphic arts operations, drum scanners have been replaced by very high quality flatbed scanners. Oil-mounted transparencies are rare.

Hand scanner

Hand scanners are manual devices which are dragged across the surface of the image to be scanned. Scanning documents in this manner requires a steady hand, as an uneven scanning rate would produce distorted images. They typically have a \"start\" button which is held by the user for the duration of the scan, some switches to set the optical resolution, and a roller which generates a clock pulse for synchronisation with the computer. Most hand scanners were monochrome, and produced light from an array of green LEDs to illuminate the image. A typical hand scanner also had a small window through which the document being scanned could be viewed. They were popular during the early 1990s and usually had a proprietary interface module specific to a particular type of home computer, usually an Atari ST or Commodore Amiga.

Read more at Wikipedia.org