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Behind bars

There is a sequentially updated bar code appearing on the front cover of copies of New Scientist. I have tried without success to decipher
these. How does the coding system (or any bar-coding system) work and how do bar code readers work?

Andrew Pettit

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    John Pearce Article Number Association(UK) London

    The bar coding of New Scientist and other publications forms part
    of the EAN (international article numbering) system that is used to identify
    and bar code items that appear in open trade. The bar code simply represents
    the number shown below in a machine readable form. The then European article
    numbering system was founded in 1976, and incorporated the numbering system
    for periodicals already established worldwide, the ISSN (international standard
    serial numbering) system. The EAN system prefixes all ISSN numbers (less
    their eighth digit – the ISSN check digit) with the digits 977, adds two
    digits normally used to indicate changes in price and a final EAN check
    digit. This 13 digit number identifies the publication uniquely, but not
    the week in which it appears. The two-digit supplementary code identifies
    each particular issue.

    John Pearce Article Number Association(UK) London

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    D. R. Wilkins Camberley, Surrey

    The reason why you can’t decipher bar codes is that the encoding
    is not as you might expect. All of the digits are encoded as four stripes,
    two white and two black in alternating sequence, so there is no correlation
    between digit value and stripe width. All digits are encoded within seven
    units of width, although the widths of the black and white bits vary. Furthermore,
    bar-code scanners have to know which order the digits are in, so they have
    to know which way up the bar code is. To do this, the code is split. Bars
    on the left half of the code have an odd number of black elements (odd parity)
    and bars on the right half have an even number (even parity). The longer
    bars which project down into the human-readable part of the bar code are
    guard bars. These allow scanning equipment to work out where the code starts
    and finishes and where the middle is.

    D. R. Wilkins Camberley, Surrey

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    Neil Attard Wokingham, Berkshire

    Bar codes consist of patterns of light and dark areas. When a beam
    of light is shone at the bar code, the light areas reflect the light back
    to the reader while the dark areas absorb the light. Most bar code readers
    consist of a low-powered laser as the light source, and a sensor to detect
    when the light is reflected back. When the laser beam is passed across a
    code, the sensor detects the presence of light spaces because they reflect
    well, and black bars because they reflect badly. By recording the pattern
    of reflections, the reader can construct an image of the bar code and tries
    to match the pattern with one it recognises. Although there seems to be
    a variety of different bar code readers the only difference is the way they
    move the beam about.

    Neil Attard Wokingham, Berkshire

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    Rachel Coleman Wiltshire

    I work part-time on a checkout and so spend several hours a week
    contemplating bar codes. The bars of a bar code are a digital version of
    the numbers underneath. If the sensor fails to ‘read’ the bar code, I type
    in the number. The number matches with one in the store computer database.
    This provides the price and a description of the product, which my till
    computer prints onto the receipt. The main computer logs the sale and re-orders
    the product.

    Rachel Coleman Wiltshire





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