The most common desktop scanners resemble copy machines, in that the item being scanned rests on a glass plate while the scanning head moves underneath it. These flatbed scanners are versatile because they can scan flat originals of various sizes, and they can even scan small three-dimensional objects.
Most flatbed scanners require a transparency adapter with a separate light source in order to scan slides, x-rays, and other transparent originals. And unless the user is willing to place each page on the scanner individually, they require an automatic document feeder to handle large numbers of documents.
Over the last few years, personal sheetfed scanners have grown in popularity. These units are more like a fax machine than a copier, because they move the page being scanned past the scanning head, rather than the other way around.
Some sheetfed scanners only scan a single sheet of paper at a time, while others come with built-in document feeders that can scan multiple-page documents unattended.
Sheetfed scanners also tend to be less exact than their flatbed counterparts, because of the difficulty of moving a sheet of paper without introducing distortions. Still, a sheetfed scanner is a good choice for handling paperwork without giving up much desk space.
Some items demand special handling during scanning. Slides, for instance, require a scanner that passes light through the image rather than reflecting off light off it. Because of their small size, slides also need to be scanned on a unit with very high resolution (in other words, a lot of eyes on the scanning head).
To meet these requirements, several manufacturers have developed dedicated scanners that handle only 35mm slides. These devices are usually much more expensive than flatbed or sheetfed scanners, and are much less versatile, but they are still the right choice for those who need to make high-quality scans of more than a few slides.
Before the advent of desktop scanning, most images were loaded into computers through drum scanners. Expensive and difficult to operate, these units were found primarily in color prepress companies. Technicians there would carefully mount originals on a glass cylinder, which would then be rotated at high speeds around a sensor located in the center.
Today, drum scanners remain the best high-end choice, but while their prices have come down significantly, they remain expensive in comparison to flatbed or sheetfed scanners. Their advantage lies in the fact that they typically use photomultiplier tubes (PMTs), which are more sophisticated sensors than the charge-coupled devices (CCDs) and contact image sensors (CISs) used in other kinds of scanners. That, and the fact that the original is rotated past the PMTs again and again at high speed, makes drum scanners important professional tools.