Question: what are the two types of scanners? 

Answer: Most scanners today are variations of the desktop (or flatbed) scanner. 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 a charge-coupled device (CCD) as the image sensor, whereas a drum scanner uses a photomultiplier tube as the image sensor.\n\nOther types of scanners are planetary scanners, which take photographs of books and documents, and laser range scanners, for producing three-dimensional models of objects.\n\nDrum scanners:\n--------------\nDrum 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 dry 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, desktop publishing, and print production of books and magazines. They are very expensive. Only few companies manufacture them. Due to the fact that a photomultiplier tube is much more sensitive to light than a charge-coupled device 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 for resolution, their use is generally recommended when a scanned image is going to be greatly enlarged.
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