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How does a barcode work?

Each character is represented by a pattern of wide and narrow bars.

A barcode reader uses a photosensor to convert the barcode into an electrical signal as it moves across a barcode.  The scanner then measures the relative widths of the bars and spaces, translates the different patterns back into regular characters, and sends them on to a computer or portable terminal.

how_it_works.jpg (14454 bytes)Every barcode begins with a special start character and ends with a special stop character. These codes help the reader detect the barcode and figure out whether it is being scanned forward or backward.

Some barcodes may include a checksum character just before the stop character. A checksum is calculated when the barcode is printed using the characters in the barcode. The reader performs the same calculation and compares its answer to the checksum it read at the end of the barcode. If the two don't match, the reader assumes that something is wrong, throws out the data, and tries again.

There are different barcode symbologies, each with its own particular pattern of bars. The UPC code used on retail products is an all-numeric code; so is the Interleaved 2 of 5 Code. Code 39 includes upper case letters, digits, and a few symbols. Code 128 includes every printable and unprintable ASCII character code.