scanner

In computing, a scanner is a device that analyzes images, 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, but this type of scanner is considerably more expensive relative to other types of scanners.
Another category of scanner are digital camera scanners which are based on the concept of reprographic cameras. Due to the increasing resolution and new features such as anti-shake, digital cameras become an attractive alternative to regular scanners. While still containing disadvantages compared to traditional scanners, digital cameras offer unmatched advantages in speed and portability.
Drum
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 object being scanned in front of precision optics that deliver image information to the PMTs. 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 the scanner 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 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.

The first scanned image
The first image scanner ever developed was a drum scanner. It was built in 1957 at the US National Bureau of Standards by a team led by Russell Kirsch. The first image ever scanned on this machine was a 5 cm square photograph of Kirsch's then-three-month-old son, Walden. The black and white image had a resolution of 176 pixels on a side.[1]

[edit] Flatbed
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. Color 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, an opaque cover is lowered over it to exclude ambient light, 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.

[edit] Hand
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 computer, usually an Atari ST or Commodore Amiga.

[edit] Quality
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.
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.

[edit] Computer connection
Scanning the document is only one part of the process. For the scanned image to be useful, it must be transferred to a computer. 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, making a fast connection desirable.
There are four common connections used by scanners:
• Parallel - Connecting through a parallel port is the slowest transfer method available. Early scanners had parallel port connections that could not transfer data faster than 70 kilobytes/second.
• Small Computer System Interface (SCSI), which is supported by most computers only via an additional SCSI interface. Some SCSI scanners are supplied together with a dedicated SCSI card for a PC, although any SCSI controller can be used. During the evolution of the SCSI standard speeds increased, with backwards compatibility; a SCSI connection can transfer data at the highest speed which both the controller and the device support. SCSI has been largely replaced by USB and Firewire, one or both of which are directly supported by most computers, and which are easier to set up than SCSI.
• Universal Serial Bus (USB) scanners can transfer data fast, and are easier to use and cheaper than SCSI devices. The early USB 1.1 standard could transfer data at only 1.5 megabytes per second, but the later USB 2.0 standard can theoretically transfer up to 60 megabytes per second (although everyday rates are much lower), resulting in faster operation.
• FireWire is a somewhat faster interface than USB. FireWire is ideal for scanning high-resolution images which require the transfer of much data.
A computer needs software, called a driver, that knows how to communicate with the scanner. Most scanners use a common language, TWAIN. The TWAIN driver, originally used for low-end and home-use equipment and now widely used for large-volume scanning, acts as an interpreter between any application that supports the TWAIN standard and the scanner. This means that the application does not need to know the specific details of the scanner in order to access it directly. For example, you can choose to acquire an image from the scanner from within Adobe Photoshop because Photoshop supports the TWAIN standard.
Other scanner drivers that can be also used are:
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.
In addition to the driver, most scanners come with other software. Typically, a scanning utility and some type of image editing application are included. A lot of scanners include OCR software. OCR allows you to scan in words from a document and convert them into computer-based text. It uses an averaging process to determine what the shape of a character is and match it to the correct letter or number.

อุปกรณ output

A computer printer, or more commonly a printer, produces a hard copy (permanent human-readable text and/or graphics) of documents stored in electronic form, usually on physical print media such as paper transparencies]]. Many printers are primarily used as computer peripherals, and are attached by a printer cable to a computer which serves as a document source. Other printers, commonly known as network printers, have built-in network interfaces (typically wireless or Ethernet), and can serve as a hardcopy device for any user on the network.
In addition, many modern printers can directly interface to electronic media such as memory sticks or memory cards, or to image capture devices such as digital cameras, scanners; some printers are combined with a scanners and/or fax machines in a single unit.
A printer which is combined with a scanner can essentially function as a photocopier.
Printers are designed for low-volume, short-turnaround print jobs; requiring virtually no setup time to achieve a hard copy of a given document. However, printers are generally slow devices (30 pages per minute is considered fast; and many consumer printers are far slower than that), and the cost-per-page is relatively high.
In contrast, the printing press (which serves much the same function), is designed and optimized for high-volume print jobs such as newspaper print runs--printing presses are capable of hundreds of pages per minute or more, and have an incremental cost-per-page which is a fraction of that of printers.
The printing press remains the machine of choice for high-volume, professional publishing. However, as printers have improved in quality and performance, many jobs which used to be done by professional print shops are now done by users on local printers; see desktop publishing.
The world's first computer printer was a 19th-century mechanically driven apparatus invented by Charles Babbage for his Difference Engine.
In 2007, a study revealed that printers produced pollution as harmful as that from cigarettes

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