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 capture image information with photomultiplier tubes (PMT) rather than the charged coupled device (CCD) arrays found in flatbed scanners and inexpensive film scanners. Reflective and transmissive originals are mounted to an acrylic cylinder, the scanner drum, which rotates at high speed while it passes the in front of precision optics that deliver image information to the PMTs. The Most modern color drum scanners use 3 matched PMTs, which read red, blue and green light respectively. Light from the original artwork is split into separate red blue and green beams in the optical bench of the scanner.

One of the unique features of drum scanners is the ability to control sample area and aperture size independently. The sample size is the area that that scanners encoder reads to create an individual pixel. The aperture is the actual opening that allows light into the optical bench of the scanner. The ability to control aperture and sample size separately is particularly useful for smoothing film grain when scanning black and white and color negative originals.

While drum scanners are capable of scanning both reflective and transmissive artwork a good quality flatbed scanner can produce excellent scans from reflective artwork. As a result, drum scanners are rarely used to scan prints now that high quality inexpensive flatbed scanners are readily available. Film, however, is where drum scanners continue to be the tool of choice for high-end applications. Because film can be wet mounted to the scanner drum and because of the exceptional sensitivity of the PMTs, drum scanners are capable of capturing very subtle details in film originals.

Currently only a few companies continue to manufacture drum scanners. While prices of both new and used units have come down over the last decade they still require a considerable monetary investment when compared to CCD flatbed scanners and film scanners. However, drum scanners remain in demand due to their capacity to produce scans which are superior in resolution, color gradation and value structure. Also, since drum scanners are capable resoulutions of up to 12,000 ppi, their use is generally recommended when a scanned image is going to be enlarged.

Read more at Wikipedia.org