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A bar code is a self-contained, encrypted message with information
encoded within the bars and spaces of a specific pattern. Color is
not relevant and no characterizing information exists in the height
of the bars and spaces. Think of it as similar to the dots and dashes
in Morse Code.
Each bar code symbology contains a set of specifications that assigns
the specific widths of the bars and spaces and their allowable
tolerances. Depending on the symbology used, individual bar code
symbologies can use five, seven, eight, and nine elements to code a
single character.
Symbologies can encode either numbers (numeric) or alphanumeric
(alphabetic and numeric) information, while others support the Full
ASCII character set. There are many ways data can be encoded and
formatted within the pattern. Most bar codes have a unique start/stop
character combination; however, some allow multiple start/stop codes
that are used for a variety of purposes.
Bar codes are either discrete or continuous. As shown in, discrete
bar codes start with a bar, end with a bar, and have a space between
characters (intercharacter gap, which contains no information). Each
character can stand-alone and decoded independently from the adjacent characters.
Continuous bar codes always start with a bar, end with a space, and
have no intercharacter gap. Since there are no intercharacter gaps, a
continuous bar code requires less length to encode a given amount of
data. In this example, 1234 is encoded as an Interleaved 2 of 5 bar code.
Fixed or Variable Length
In fixed length configuration, certain symbologies limit the number
of data characters a specific bar code is allowed. Symbologies may
also use a fixed length environment due to data protection
considerations. Variable length refers to an unlimited number of data
characters that a bar code can have.
X Dimension
The width of a narrow element is called the X dimension. The amount
of data that can be encoded in a given unit of length differs from
symbology to symbology. In some codes, the width of each element is
specifically defined. However, some symbologies require only two
widths, one for the narrow elements and one for the wide. Other
symbologies have four widths or more. Some symbologies use five,
seven, eight, and nine elements to code a single character.
Density
The number of characters represented in a bar code, usually expressed
in characters per inch (CPI), is referred to as the X dimension. The
density of a bar code is determined by the minimum X dimension, the
wide-to-narrow element ratio, the number of elements required to
represent a character and the start/stop codes and check digits, if needed.
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