Scanner (computing)
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.
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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.
The earliest scanners, rarely seen today, were drum scanners where the scanning sensor could only move back and forth on a single axis. The paper to be scanned was mounted on a drum which rotated in front of the sensor.
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.
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.
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 samples per inch (SPI), sometimes referred to as dots per inch (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 SPI, while its interpolated resolution can easily be as high as 19200 SPI.
A good description of the factors involved in scanner quality is at Digital Imaging Guy (http://www.michaelpapet.com/shome.htm).
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 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.
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