Scanners, Digital

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Scanners, Digital

The scanning process

Color scanning

Digital scanners for publishing

Scanners that can read


A digital scanner is a computer accessory (peripheral) used to digitize pictures. It analyzes and then converts a visual image (such as text or a photograph) to a digital image. Most digital scanners are flatbed scanners. Other types of digital scanners include 3D scanners (which reproduce three-dimensional [3D] images) and planetary scanners (which are used to reproduce books and other similar documents). The scanning signal is interpretable by computer software, which allows the image to be recorded, manipulated, and even sent electronically to another computer.

Even in the early 1990s, computers were used more for in-house functions, such as preparing documents. But, with the expanding power of the Internet and the development of powerful and sophisticated software, computers became important for the preparation and publication of professional quality documents that included graphics.

One means by which photographs can be efficiently transferred from one computer to another is by scanning the image and digitizing the information as a computer file.

In the recording of music, compact discs (CDs) convert the spectrum of instrumental or vocal sound into the 1s and 0s of digital code. Scanners perform an analogous function for images. A digital scanner converts the continuous tones in a photograph (light) into the digital code that is the language understood by computers. All words, numbers, images, and instructions to the computer ultimately consist of series of ones and zeroes.

Digital scanners can use laser light to scan an image. These scanners, while offering superb quality, are much more expensive that other scanners. The scanners that use conventional visible light are more affordable and, hence, far more popular.

The scanning process

A scanner initially operates much like a photocopier. An image such as a photograph or a drawing is placed on a transparent plate. The lid of the scanner is closed to keep stray outside light from entering. When the scan is begun, an incandescent or fluorescent light illuminates the image on the transparent plate. The light that reflects off the image enters a lens. At this point, the scanner operates differently from a photocopier. In a photocopier the lens focuses the light onto a plate that creates an electric charge, which attracts the toner particles that produce the mirror image of whatever document is being copied. In a scanner, however, the lens focuses the reflected light onto a row containing many electronic light sensors.

The sensors convert the light to electric current. The strength of the current produced by each sensor (in volts) is proportional to the intensity of light striking the particular sensor. The electrical output is in digital form. The signal, which now represents the tonal values of the original image in digital form, is ready to be read by a computer.

After being scanned by a digital scanner, a photograph consists of a grid of points called pixels. A pixel is the smallest unit of information in a digital image. Each pixel has data attached that tells the computer what color to assign the pixel. For black-and-white-images, that data is usually a level of gray from 0, which is black, to 255, which is white. It sometimes may have more or fewer levels, depending on the sensitivity of the scanner.

More expensive scanners often have more pixels per square inch of the scanned image than do less expensive scanners. More pixels translate to more information, which produces a digital image that more closely mirrors the actual image. In other words, the resolution of the denser pixel system is greater.

Color scanning

Depending on the configuration of the machine, a scanner can produce images that are black and white or color. In a color scanner, the digital color image consists of three gray-scale images. These images are often called layers. One layer defines which areas will be green in the final color image. The other two layers do the same for red and blue.

To create the differently colored layers, the scanner must be able to distinguish those parts of the color image that are blue, red or green.

The three colors can be created optically. Here, the scanners light is shown through a red, green, or blue color filter before it strikes the item that is being scanned. An image is scanned three times, once with each filter, to create the three layers. The actual process of shining the light through a filter is simple. However, overall, the method is relatively time-consuming because each image must be scanned three times. As well, problems can arise if the three scans are not aligned precisely. For example, if one of the scans is misaligned by even a pixel of less, discoloration and other problems can occur in the final image.

Instead of using filters, some scanners use three different light sources to create red, green, and blue light. The lights turn on and off in a regular and coordinated way for each line of pixels scanned. The advantage with this approach is that the three images are obtained in a single scan of the document or photograph. Thus, the three images are aligned correctly.

A third approach uses a single white light. However, filters separate the red, green, and blue wavelengths of the visible light spectrum after the light reflects from the image being scanned. Each color of light is then focused onto its own row of light sensors. The three colors are read at the same time during one scan.

Digital scanners for publishing

High-end (technically complex and expensive) digital scanners are typically used where a professional quality image is necessary, such as in magazine publishing. These scanners often use lasers to read the original images. The image is placed in a transparent drum, which rotates past the laser. The document is scanned in a precise pattern, and is scanned in great detail (i.e., one pixel at a time). After computer processing to ensure the image will print correctly, the images are printed onto film in a process that again uses a laser.

A primary reason that digital scanners became popular beginning in the 1980s was that they created better-published results more cheaply. Photographs reproduced in magazines and newspapers are converted into patterns of dots, called halftones, for publication. Before digital scanners, halftones were produced using cameras. With scanners, halftones in most cases could be made more cheaply and easily. Using digital scanners also allowed for adjusting the size, sharpness, and type of halftone screen to a degree not possible with cameras.

As well, because the digital images are maintained as computer files, they are much easier to store than photographic prints and negatives, and are hardier than negatives (which can be scratched or decay over time). The files can also be easily shared between computers via electronic mail. Thus, in a publishing company, each branch does not need to have its own repository of photographs. A central data bank of filed images can be useful to employees all over the globe.

Scanners that can read

Scanning artwork and photographs for reproduction is only one reason to use a digital scanner. Another important use is to enable computers to


Digital code Binary code, the series of ones and zeroes that make up all information used by a computer.

Halftone screen Pattern of minute dots that defines a color or a black-and-white tone on a printed page.

Light sensor Device that translates light into an electric current, the stronger the light the stronger the current.

Optical character recognition Process in which a typewritten document is scanned as an image, and translated into words using computer software.

Pixel The smallest unit of color or tonality in a digital image; a single point.

Resolution The number of pixels a scanner can read per square inch.

read printed documents. This process is called optical character recognition (OCR).

In optical character recognition, a black-and-white digital scan is first made of a document. Using various software programs, a computer is able to recognize this image as various letters and words. The text can then be edited in a word-processing program, just like text typed in by a person at a keyboard.

Because converting paper documents to digital format is very important to many businesses; some scanners have been made for this specific purpose. The primary technology is the same as for desktop scanners, but these special scanners use a mechanism that feeds pieces of paper one-by-one onto the scanning plate.

See also Computer, digital; Digital recording.



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Lew, Michael S. Principles of Visual Information Retrieval (Advances in Pattern Recognition). Berlin, Germany: Springer Verlag, 2001.

Marshall, Gerald F. Handbook of Optical and Laser Scanning. New York: Marcel Dekker, 2004.

Reid, Constance. From Zero to Infinity: What Makes Numbers Interesting. Wellesley, MA: A.K. Peters, 2006.

Schroeder, Manfred Robert. Number Theory in Science and Communications: With Applications in Cryptography, Physics, Digital Information, Computing, and Self-similarity. Berlin, Germany, and New York: Springer, 2006.


PC Tech Guide. Input-Output Scanners. December 1, 2001. <> (accessed November 19, 2006).

Scott M. Lewis