In computing, a scanner is a device which analyzes a physical image (such as a photograph, printed text, or handwriting) or an object (such as ornament) and converts it to a digital image.
Most scanners today are variations on 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 impossibility of obtaining a high-quality image. Both these types of scanners use a CCD as the image sensor, whereas a drum scanner uses a photomultiplier tube (PMT) as the image sensor.
The oldest scanner technology is the drum scanner, where the scanning PMT moves back and forth on a single axis. The image to be scanned is wet-mounted on a drum, meaning it is soaked in oil. The drum then rotates in front of the PMT. The use of drum scanners has reduced significantly as CCD-based flatbed scanners have dropped in price; however drum scanners are still used for certain high-end applications. They are very expensive and only a handful of companies manufacture them. As a PMT is much more sensitive to light than a CCD and the scanning beam can be focused very narrowly, drum scanners produce superior scans than flatbed scanners, both in resolution and in the ability to detect small changes in color and brightness. Today, they are mostly used for high-end applications like museum-quality archival of photographs and print production of books and magazines. Because drum scanners have the advantage for resolution, their use is generally recommended when an scanned image is going to be enlarged by a significant amount.
A desktop scanner is usually composed of a glass pane, under which there is a bright light (often xenon) which illuminates the pane, and a moving CCD. Colour scanners typically contain three rows of CCD elements with red, green, and blue filters. Images to be scanned are placed face down on the glass, the light turns on, and the CCD and light move across the pane reading the entire area. An image is therefore visible to the CCD only because of the light it reflects. Transparent images do not work in this way, and require special accessories that illuminate them from the upper side.
Some models are equipped with an "automatic document feed" or "ADF" feature, which allows the user to place a stack of pages into a hopper, from which each page is automatically fed individually into the scanner. The CCD remains still during ADF scanning, while the page is moved through the scanner by rollers at a constant rate. A separate exit hopper collects the pages after they are scanned.
Scanners typically read RGB data from the CCD, process it with some proprietary algorithm to correct for different exposure conditions, and send it to the computer via the device's input/output interface (usually SCSI or USB). Color depth varies depending on the CCD characteristics, but is usually at least 24 bits. High quality models have 48 bits or more color depth.
The other qualifying parameter for a scanner is its resolution, measured in dots per inch (dpi), sometimes more accurately referred to as samples per inch (spi). Instead of using the scanner's true optical resolution, the only meaningful parameter, manufacturers like to refer to the interpolated resolution, which is much higher thanks to software interpolation. As of 2004, a good flatbed scanner has an optical resolution of 1600–3200 dpi, high-end flatbed scanners can scan up to 5400 dpi, and a good drum scaner has an optical resolution of 8000–14,000 dpi. Manufacturers often claim interpolated resolutions as high as 19,200 dpi, but those numbers are meaningless, as the number of possible interpolated pixels is infinite.
A good description of the factors involved in scanner quality is at Digital Imaging Guy.
The final result is a non-compressed RGB image which is typically transferred to a host computer's RAM. Such an image can be processed with a raster graphics program (such as Photoshop or the GIMP) and saved on a storage device (such as a hard disk).
The amount of data generated by a scanner can be very large: a 600 SPI 9"x11" uncompressed 24-bit image consumes about 100 megabytes of data in transfer and storage on the host computer. Recent scanners can generate this volume of data in a matter of seconds. Therefore, a fast connection is required.
Early scanners had parallel connections that could not go faster than 70 kilobytes/second. Professional models adopted the SCSI-II connection, which was much faster (a few megabytes per second) albeit expensive, and frequently requiring a dedicated expansion card to be put inside the host computer.
Recent economic models come equipped with USB connections. In its first version, USB was capable of roughly 1 megabyte per second. Recent models use USB 2.0 connections that can transfer about 40 megabytes per second, eliminating the bottleneck.
Two main interface standards exist in the market. TWAIN is generally used for low-end and home-use equipment. ISIS, which still uses SCSI-II for performance reasons, is used by large, departmental scale, machines.