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.

Desktop scanner, with the lid raised. An object has been laid on the glass, ready for scanning.

Scan of the jade rhinoceros seen in the photograph above.

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.

Table of contents
1 Physical description
2 Scanner quality
3 Output data
4 Computer connection

Physical description

A desktop scanner is usually composed of a glass pane, under which there is a light which illuminates the pane, and a moving CCD. 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.

Scanner quality

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 dpi. 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. A good scanner (as of 2003) has an optical resolution of 1600 or 3200 dpi, while its interpolated resolution can easily be as high as 19200 dpi.

A good description of the factors involved in scanner quality is at Digital Imaging Guy.

Output data

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).

Computer connection

The amount of data generated by a scanner can be very large: a 600 dpi 9"x11" page-sized image requires 100 megabytes of data to be transferred and stored 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.