Image scanner
An image 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.
Modern scanners typically use a charge-coupled device or a contact image sensor as the image sensor, whereas drum scanners, developed earlier and still used for the highest possible image quality, use a photomultiplier tube 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 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 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 machines used by newspapers around the world from the early 1900s onward.
Analog scanners
Before the advent of digital image processing in the middle of the 20th century, the term scanner originally referred to analog equipment used within offset printing presses. These analog scanners varied in design depending on their purpose: some scanned images stored as color transparency film onto color separation plates that could be used to print the original image en masse; while others were used to convert simple cyan, magenta, and yellow plates into cyan, magenta, yellow, and black in order to produce prints with darker, richer colors—a process known then in the trade as color correction. Converting from CMY to CMYK used to be a highly manual affair involving techniques such as masking. Analog scanners automated this process to a large extent.Alexander Murray and Richard Morse invented and patented the first analog color scanner at Eastman Kodak in 1937. Their machine was of a drum scanner design that imaged a color transparency mounted in the drum, with a light source placed underneath the film, and three photocells with red, green, and blue color filters reading each spot on the transparency to translate the image into three electronic signals. In Murray and Morse's initial design, the drum was connected to three lathes that etched CMY halftone dots onto three offset cylinders directly. The rights to the patent were sold to Printing Developments Incorporated in 1946, who improved on the design by using a photomultiplier tube to image the points on the negative, which produced an amplified signal that was then fed to a single-purpose computer that processed the RGB signals into color-corrected CMYK values. The processed signals are then sent to four lathes that etch CMYK halftone dots onto the offset cylinders.
In 1948, Arthur Hardy of the Interchemical Corporation and F. L. Wurzburg of the Massachusetts Institute of Technology invented the first analog, color flatbed image scanner, intended for producing color-corrected lithographic plates from a color negative. In this system, three color-separated plates are prepared from a color negative via dot etching and placed in the scanner bed. Above each plate are rigidly fixed, equidistant light beam projectors that focus a beam of light onto one corner of the plate. The entire bed with all three plates moves horizontally, back and forth, to reach the opposite corners of the plate; with each horiztonal oscillation of the bed, the bed moves down one step to cover the entire vertical area of the plate. While this is happening, the beam of light focused on a given spot on the plate gets reflected and bounced off to a photocell adjacent to the projector. Each photocell connects to an analog image processor, which evaluates the reflectance of the combined CMY values using Neugebauer equations and outputs a signal to a light projector hovering over a fourth, unexposed lithographic plate. This plate receives a color-corrected, continuous-tone dot-etch of either the cyan, magenta, or yellow values. The fourth plate is replaced with another unexposed plate, and the process repeats until three color-corrected plates, of cyan, magenta and yellow, are produced. In the 1950s, the Radio Corporation of America took Hardy and Wurzburg's patent and replaced the projector-and-photocell arrangement with a video camera tube focusing on one spot of the plate.
Use in digital imaging
The first digital imaging system was the Bartlane system in 1920. Named after the pair who invented it, Harry G. Bartholomew and Maynard D. McFarlane, the Bartlane system used zinc plates etched with an image from a film negative projected at five different exposure levels to correspond to five quantization levels. All five plates are affixed to a long, motor-driven rotating cylinder, with five equidistant contacts scanning over each plate at the same starting position. The Bartlane system was initially used exclusively by telegraph, with the five-bit Baudot code used to transmit the grayscale digital image. In 1921, the system was modified for offline use, with a five-bit paper tape punch punching holes depending on whether its connections to the contacts are bridged or not. The result was a stored digital image with five gray levels. Reproduction of the image was achieved with a lamp passing over the punched holes, exposing five different intensities of light onto a film negative.The first scanner to store its images digitally onto a computer was a drum scanner built in 1957 at the National Bureau of Standards by a team led by Russell A. Kirsch. It used a photomultiplier tube to detect light at a given point and produced an amplified signal that a computer could read and store into memory. The computer of choice at the time was the SEAC mainframe; the maximum horizontal resolution that the SEAC was capable of processing was 176 pixels. The first image ever scanned on this machine was a photograph of Kirsch's three-month-old son, Walden.
In 1969, Dacom introduced the 111 fax machine, which was the first digital fax machine to employ data compression using an on-board computer. It employed a flatbed design with a continuous feed capable of scanning up to letter paper in 1-bit monochrome.
The first flatbed scanner used for digital image processing was the Autokon line introduced by ECRM Inc. in 1975. The inaugural Autokon 8400 used a laser beam to scan pages up to 11 by 14 inches at a maximum resolution of 1000 lines per inch. Although it was only capable of scanning in 1-bit monochrome, the on-board processor was capable of halftoning, unsharp masking, contrast adjustment, and anamorphic distortions, among other features. The Autokon 8400 could either be connected to a film recorder to create a negative for producing plates or connected to a mainframe or minicomputer for further image processing and digital storage. The Autokon series was expanded over the following two decades and enjoyed widespread use in newspapers and prepress.
In 1977, Raymond Kurzweil, of his start-up company Kurzweil Computer Products, released the Kurzweil Reading Machine, which was the first flatbed scanner with a charge-coupled device imaging element. The Kurzweil Reading Machine was invented to assist blind people in reading books that had not been translated to braille. It comprised the image scanner and a Data General Nova minicomputer—the latter performing the image processing, optical character recognition, and speech synthesis.
The first scanners for personal computers appeared in the mid-1980s, starting with ThunderScan for the Macintosh in December 1984. Designed by Andy Hertzfeld and released by Thunderware Inc., the ThunderScan contains a specialized image sensor built into a plastic housing the same shape as the ink ribbon cartridge of Apple's ImageWriter printer. The ThunderScan slots into the ImageWriter's ribbon carrier and connects to both the ImageWriter and the Macintosh simultaneously. The ImageWriter's carriage, controlled by the ThunderScan, moves left-to-right to scan one 200-dpi line at a time, with the carriage return serving to advance the scanner down the print to be scanned. The ThunderScan was the Macintosh's first scanner and sold well but operated very slowly and was only capable of scanning prints at 1-bit monochrome. In 1999, Canon iterated on this idea with the IS-22, a cartridge that fit into their inkjet printers to convert them into sheetfed scanners.
In early 1985, Datacopy released the first flatbed scanner for the IBM PC, the Datacopy Model 700. Based on a CCD imaging element, the Model 700 was capable of scanning letter-sized documents at a maximum resolution of 200 dpi at 1-bit monochrome. The Model 700 came with a special interface card for connecting to the PC, and an optional, aftermarket OCR software card and software package were sold for the Model 700. In April 1985, LaserFAX Inc. introduced a CCD-based color flatbed scanner, the SpectraSCAN 200, for the IBM PC. The SpectraSCAN 200 worked by placing color filters over the CCD and taking four passes per scan to build up a color reproduction. The SpectraSCAN 200 took between two and three minutes to produce a scan of a letter-sized print at 200-dpi; its grayscale counterpart, the DS-200, took only 30 seconds to make a scan at the same size and resolution. The SpectraSCAN was the first flatbed scanner capable of scanning in color.
The first relatively affordable flatbed scanner for personal computers appeared in February 1987 with Hewlett-Packard's ScanJet, which was capable of scanning 4-bit grayscale images at a maximum resolution of 300 dpi. By the beginning of 1988, the ScanJet had accounted for 27 percent of all scanner sales in terms of dollar volume, per Gartner Dataquest. In February 1989, the company introduced the ScanJet Plus, which increased the bit depth to 8 bits while costing only US$200 more than the original ScanJet's $1990. This led to a massive price drop in grayscale scanners with equivalent or lesser features in the market. The number of third-party developers producing software and hardware supporting these scanners jumped dramatically in turn, effectively popularizing the scanner for the personal computer user. By 1999, the cost of the average color-capable scanner had dropped to $300. That year, Computer Shopper declared 1999 "the year that scanners finally became a mainstream commodity".