An image scanner (often abbreviated to just scanner) is a device that optically scans images, printed text, handwriting, or an object and converts it to a digital image. The most common type of scanner used in the home and the office is the flatbed scanner, where the document is placed on a glass bed. A sheetfed scanner, which moves the page across an image sensor using a series of rollers, may be used to scan one page of a document at a time or multiple pages, as in an automatic document feeder. A handheld scanner is a portable version of an image scanner that can be used on any flat surface. Scans are typically downloaded to the computer that the scanner is connected to, although some scanners are able to store scans on standalone flash media (e.g., memory cards and USB drives). Modern scanners typically use a charge-coupled device (CCD) or a contact image sensor (CIS) as the image sensor, whereas drum scanners, developed earlier and still used for the highest possible image quality, use a photomultiplier tube (PMT) as the image sensor. Document cameras, which use commodity or specialized high-resolution cameras, photograph documents all at once.
History
Precursors
Fax and wirephoto
Image scanners are considered the successors of early facsimile (fax) and wirephoto machines. Unlike scanners, these devices were used to transmit images over long distances rather than for processing and storing images locally. The earliest attempt at a fax machine was patented in 1843 by the Scottish clockmaker Alexander Bain but never put into production. In his design, a metal stylus linked to a pendulum scans across a copper plate with a raised image. When the stylus makes contact with a raised part of the plate, it sends a pulse across a pair of wires to a receiver containing an electrode linked to another pendulum. A piece of paper impregnated with an electrochemically sensitive solution resides underneath the electrode and changes color whenever a pulse reaches the electrode. A gear advances the copper plate and paper in tandem with each swing of the pendulum; over time, the result is a perfect reproduction of the copper plate. In Bain's system, it is critical that the pendulums of the transceiver and receiver are in perfect step, or else the reproduced image will be distorted. In 1847, the English physicist Frederick Bakewell developed the first working fax machine. Bakewell's machine was similar to Bain's but used a revolving drum coated in tinfoil, with non-conductive ink painted on the foil and a stylus that scans across the drum and sends a pulse down a pair of wires when it contacts a conductive point on the foil. The receiver contains an electrode that touches a sheet of chemically treated paper, which changes color when the electrode receives a pulse; the result is a reverse contrast (white-on-blue) reproduction of the original image. Bakewell's fax machine was marginally more successful than Bain's but suffered from the same synchronization issues. In 1862, Giovanni Caselli solved this with the pantelegraph, the first fax machine put into regular service. Largely based on Bain's design, it ensured complete synchronization by flanking the pendulums of both the transceiver and receiver between two magnetic regulators, which become magnetized with each swing of the pendulum and become demagnetized when the pendulum reaches the maxima and minima of each oscillation. In 1893, the American engineer Elisha Gray introduced the telautograph, the first widely commercially successful fax machine that used linkage bars translating x- and y-axis motion at the receiver to scan a pen across the paper and strike it only when actuated by the stylus moving across the transceiver drum. Because it could use commodity stationery paper, it became popular in business and hospitals. In 1902, the German engineer Arthur Korn introduced the phototelautograph, a fax machine that used a light-sensitive selenium cell to scan a paper to be copied, instead of relying on a metallic drum and stylus. It was even more commercially successful than Gray's machine and became the basis for wirephoto (also known as telephotography) machines used by newspapers around the world from the early 1900s onward.
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