Scanners page

Scanner characteristics

Resolution and interpolation: Resolution is the degree of detail that a scanner reproduces. Resolution is measured in dots per inch, the higher the resolution the more detailed a scanned image will be. Computer images and text are made up of little bits of light called pixels. The scanner's resolution tells you how many of those pixels it can scan at one time, per square inch. For example, a 300x600-resolution scanner will break your image up into 300 pixel squares by 600 pixel squares. The higher the number of squares, the more detail can be read, and the better the image will look. The resolution of a scanner is determined by the Optical Resolution of the CCD and the Stepping Speed of the scanner's motor. A 300 x 600 dpi scanner has a 300 dot per inch CCD and a motor that goes slow enough to scan 600 lines per inch as it travels the length of the bed. In addition most of scanners have hardware and software interpolation for resolution. Software Interpolation is performed by the Twain driver and calculated by the computer.. An integrated circuit chip inside the scanner can generate new data through an algorithm by averaging the color of adjacent dots. Interpolated resolution can be higher than the maximum resolution. given in the passport or manual of the scanner.

Bit-depth: Pixels are made up of layers of information that relate to its color. The higher the bit-depth number, the more information your scanner can gather, and thus the more colors it can pick up. A 1-bit scanner, for example, records only black and white, while a 30- or 36-bit scanner can capture millions of colors. So again, the higher the number the better - but keep in mind that many older monitors and printers won't be able to display as many colors as you can scan. Actual resolution and bit depth are usually lower than scanner manufacturers claim. In particular, many so-called 36-bit scanners really realize only 24-bits of useful information. That's enough for most users. Bit depth gives the number of different gradations of color that a scanner can see. The advantage of true 36-bit depth shows up only if you scan transparencies, slides, or negatives.

Speed of scanning: Scanning speed really depends on a combination of factors:

Scanned document's file size: File Size = (Resolution x Horizontal Size) x (Resolution x Vertical Size) x Scan Mode. Where Scan Mode = 1/8 for both Line Art and Halftone, 1 for Grayscale and 3 for Color. Here are the file sizes for a 4" x 4" photo scanned under four different scan modes and resolutions.

Scan Mode

Resolution (dpi)





Line Art

19.5 Kb

44 Kb

175 Kb



19.5 Kb

44 Kb

175 Kb



156 Kb

352 Kb

1.37 Mb

5.5 Mb


469 Kb

1 Mb

4.12 Mb

16.5 Mb

Software packages: Many scanners with identical features are selling at   different prices. This is generally because of the software that is included with the scanner.  Most scanners include OCR (Optical Character Recognition) software. OCR converts the scanned image (usually it is bitmap file) into something that you can edit with a word processor for further editing. OCR software does this in three primary ways: Pattern Matching, Feature Extraction and Spell Checking. Software packages can range from simple computer drivers that allow you to use your scanner, to full-fledged image manipulating software and print programs. Very often versions of OCR are nearly useless on pages with multiple columns or inserted graphics.  Computers control scanners through drivers, so you must make sure the scanner comes with a driver that's compatible with your operating system.  The scanner manufacturer's Web site is a good place to check it.

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