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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 offices. 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 charge-coupled device (CCD) or Contact Image Sensor (CIS) as the image sensor, whereas older drum scanners use a photomultiplier tube as the image sensor.
Another category of scanner is a rotary scanner used for high-speed document scanning. This is another kind of drum scanner, but it uses a CCD array instead of a photomultiplier.
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 capture image information with photomultiplier tubes (PMT) rather than the charged coupled device (CCD) arrays found in flatbed scanners and inexpensive film scanners. Reflective and transmissive originals are mounted to an acrylic cylinder, the scanner drum, which rotates at high speed while it passes the in front of precision optics that deliver image information to the PMTs. The Most modern color drum scanners use 3 matched PMTs, which read red, blue and green light respectively. Light from the original artwork is split into separate red blue and green beams in the optical bench of the scanner.
One of the unique features of drum scanners is the ability to control sample area and aperture size independently. The sample size is the area that that scanners encoder reads to create an individual pixel. The aperture is the actual opening that allows light into the optical bench of the scanner. The ability to control aperture and sample size separately is particularly useful for smoothing film grain when scanning black and white and color negative originals.
While drum scanners are capable of scanning both reflective and transmissive artwork a good quality flatbed scanner can produce excellent scans from reflective artwork. As a result, drum scanners are rarely used to scan prints now that high quality inexpensive flatbed scanners are readily available. Film, however, is where drum scanners continue to be the tool of choice for high-end applications. Because film can be wet mounted to the scanner drum and because of the exceptional sensitivity of the PMTs, drum scanners are capable of capturing very subtle details in film originals.
Currently only a few companies continue to manufacture drum scanners. While prices of both new and used units have come down over the last decade they still require a considerable monetary investment when compared to CCD flatbed scanners and film scanners. However, drum scanners remain in demand due to their capacity to produce scans which are superior in resolution, color gradation and value structure. Also, since drum scanners are capable of resolutions up to 12,000 ppi, their use is generally recommended when a scanned image is going to be enlarged.
In most current graphic arts operations, very high quality flatbed scanners have replaced drum scanners, being both less expensive and faster. However, drum scanners continue to be used in high-end applications, such as museum-quality archiving of photographs and print production of high-quality books and magazine advertisements. In addition, due to the greater availability of pre-owned units many fine art photographers are acquiring drum scanners, which has created a new niche market for the machines.
A flatbed scanner is usually composed of a glass pane (or platen), under which there is a bright light (often xenon or cold cathode fluorescent) which illuminates the pane, and a moving optical array, whether CCD or CIS. Colour scanners typically contain three rows (arrays) of sensors with red, green, and blue filters. Images to be scanned are placed face down on the glass and the sensor array 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.
Scanners typically read red-green-blue color (RGB) data from the array. This data is then processed with some proprietary algorithm to correct for different exposure conditions and sent 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 scanning array 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 higher the resolution, the larger the file. In most cases, there is a trade-off between manageable file size and level of detail. Resolutions higher than 1200dpi are overkill for colour printers and monitors.
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.
The scanned result is a non-compressed RGB image which can be transferred to a computer's memory. Some scanner compress and clean up the image using embedded firmware. Once on the computer, 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, TIFF, 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. This process is called Optical Character Recognition (OCR).
The amount of data generated by a scanner can be very large: a 600 DPI 9"x11" (slightly larger than 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, originally used for low-end and home-use equipment, is now widely used for large-volume scanning.
- 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 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 Applied Science Fiction (which was subsequently purchased by Kodak, and renamed as Kodak's Austin Development Center), while Canon uses its own FARE (Film Automatic Retouching and Enhancement) system.
The scanning or digitization of paper documents for storage is quite different from the scanning of pictures for reproduction though it uses some of the same technology. While document scanning can be done on general-purpose office scanners, in major operations it is performed on dedicated, specialized scanners, manufactured by companies like Canon, Fujitsu, Kodak, and others.
Document scanners have document feeders, generally larger than those found on copiers or all-purpose scanners. They scan at lower resolution than other scanners, usually in the range 150dpi to 300dpi, since higher resolution is usually not needed and makes files much larger to store.
A lot of scans can be made at high speed, traditionally in grayscale but now in color as well. Many are capable of duplex (two-sided) scanning at or near full speed (20ppm (pages per minute) to 150ppm). Sophisticated document scanners have either firmware of software that “cleans up” scans as they are produced, eliminating accidental marks and sharpening type. They also usually compress the scan on the fly.
Many document scans are converted using OCR technology into searchable files. Most scanners use ISIS or Twain device drivers to scan documents into TIFF format so that the scanned pages can be fed into a document management system that will handle the archiving and retrieval of the scanned pages.
The biggest issues with document scanning are preparation and indexing. Preparation involves taking the papers to be scanned and making sure that they are in order, unfolded, without staples or anything else that might jam the scanner. This is a manual task and can be time consuming. Indexing involves associating keywords with files so they can be found later. This process can be automated in some cases, but may involve manual labour.
A specialized form of document scanning is book scanning. Technical difficulties arise from the books usually being bound and sometimes fragile and irreplaceable, but some manufacturers have developed specialized machinery to deal with this. Often special robotics are used to turn the pages automatically.
- Barcode reader
- Film scanner
- Optical character recognition
Hand scanners are manual devices which are dragged across the surface of the image to be scanned. Scanning documents in this manner requires a steady hand, as an uneven scanning rate would produce distorted images. They typically have a "start" button which is held by the user for the duration of the scan, some switches to set the optical resolution, and a roller which generates a clock pulse for synchronisation with the computer. Most hand scanners were monochrome, and produced light from an array of green LEDs to illuminate the image. A typical hand scanner also had a small window through which the document being scanned could be viewed. They were popular during the early 1990s and usually had a proprietary interface module specific to a particular type of home computer, usually an Atari ST or Commodore Amiga.
Quality criteria and testing procedures for book scanners
- "Is Drum Scanning Really Alive and Well?" from Digital Output by Jim Rich
- "Can a Fine-Art Large-Format Photographer Find Happiness With a $30,000 Scanner?" by Bill Glickman
Backends and standards
- Scanner Access Now Easy (SANE) Project
- TWAIN Working Group
- Scanner at the Open Directory Project