Image scanner

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. Most scanners today are variations of the desktop (or flatbed) scanner The flatbed scanner is the most common in homes. 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 a charge-coupled device (CCD) as the image sensor, whereas a drum scanner uses a photomultiplier tube as the image sensor.

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, the oldest scanning technology, have a scanning photomultiplier tube, which moves back and forth along a single axis. The image to be scanned is soaked in oil, then wrapped around the drum; this process is known as wet mounting. The drum then rotates in front of the photomultiplier tube. The use of drum scanners has declined significantly as flatbed scanners based on charge-coupled devices have dropped in price; however, drum scanners are still used for certain high-end applications, such as museum-quality archiving of photographs, desktop publishing, and print production of books and magazines. They are very expensive and only manufacture by a few companies. Because a photomultiplier tube is much more sensitive to light than a charge-coupled device and the scanning beam can be focused very narrowly, drum scanners can produce scans superior to those of flatbed scanners, both in resolution and in the gradations of color and brightness. Also, since drum scanners have the advantage for resolution, their use is generally recommended when a scanned image is going to be greatly enlarged.

Physical description

A desktop scanner is usually composed of a glass pane, under which there is a bright light (often xenon or cold cathode fluorescent) which illuminates the pane, and a moving charge-coupled device. Colour scanners typically contain three rows of charge-coupled device elements with red, green, and blue filters. Images to be scanned are placed face down on the glass and the charge-coupled device and light source move across the pane reading the entire area. An image is therefore visible to the charge-coupled device 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 charge-coupled device remains still during automatic document feed 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 red-green-blue color (RGB) data from the charge-coupled device, 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, or LPT in machines pre-dating the USB standard). Color depth varies depending on the charge-coupled device 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 pixels per inch (ppi), 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 ppi, high-end flatbed scanners can scan up to 5400 ppi, and a good drum scanner has an optical resolution of 8000–14,000 ppi. Manufacturers often claim interpolated resolutions as high as 19,200 ppi; but such numbers carry little meaningful value, because the number of possible interpolated pixels is unlimited. The third important parameter for a scanner is its density range. A high density range means that the scanner is able to reproduce shadow details and brightness details in one scan.

Output data

The scanned result is a non-compressed RGB image which can be transferred to a computer's memory. The 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).

In common use, scanned pictures are stored on a computer's hard disk, normally in image formats such as JPEG, Bitmap, and PNG. Some scanners can also be used to capture editable text, so long as the text can be read by the computer in a discernable font.

Computer connection

The amount of data generated by a scanner can be very large: a 600 DPI 9"x11" (slightly larger A4 paper) uncompressed 24-bit image consumes about 100 megabytes of uncompressed 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 desirable.

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.

FireWire is replacing SCSI as the standard in production (high volume) document scanners.

Recent models come equipped with USB connections. In its first version, USB 1.1 was capable of 1.5 megabytes per second. Recent models use USB 2.0 connections that can transfer up to 60 megabytes per second, eliminating the bottleneck.

Two main interface standards exist in the market for PCs running Windows or Macs:

TWAIN is generally used for low-end and home-use equipment.
ISIS, created by Pixel Translations, which still uses SCSI-II for performance reasons, is used by large, departmental scale, machines.

SANE (Scanner Access Now Easy) is a free/open source API for accessing scanners. Originally developed for Unix and Linux operating systems, it has been ported to OS/2, Mac OS X, and Microsoft Windows. Unlike TWAIN, SANE does not handle the user interface. This allows batch scans and transparent network access without any special support from the device driver.

Infrared Cleaning

Infrared cleaning is a technique to remove dust and scratches from film. Most modern scanners incorporate this feature. Infrared cleaning works by scanning the film with infrared light. From this, it is possible to detect dust and scratches that cut off the infrared light and they can then be automatically removed based on their position, size, shape and surroundings.

Scanner manufacturers usually have their own name attached to this technique. For example, Epson, Nikon, Microtek and others use Digital ICE developed by Kodak, while Canon uses its own [http://www.canon.com/technology/scan/02.html FARE (Film Automatic Retouching and Enhancement) system.

External links

ScanTips.com
Digital Imaging Guy - description of the factors involved in scanner quality
Is Drum Scanning Really Alive and Well? from Digital Output
Photo Scanner Technology Explained by Ken Rockwell
ICG - Drum Scanner Manufacturer
about drum scanners
^* - Scanner Specialists and Pioneers
twain.org
sane project

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