Bar Code Fundamentals
Confused about bar codes? Don’t be.
In the simplest definition, bar codes are simply a combination of lines and spaces that represent specific characters. While bar codes have similar characteristics as fonts (sizes, styles and languages), they are created by various algorithms for each “symbology” (language). Therefore, users who need code-specific, verifiable fonts for compliance labeling can be in trouble if they simply download a “bar code font” from the internet. Bar codes work much like human readable fonts, but are read by a machine—either a bar code scanner, mobile computer, imager or verifier. Because they are machine readable and used by millions of companies across the world, there are global standards established for the structure of bar code data, along with standardized measurements for the quality, or readability, of the bar code. Learn more about Bar Code Technologies.
Bar Code Glossary
1D: See Linear.
2D: Barcode symbols designed to increase data capacity beyond linear bar code symbols while still being readable by optical means. Two categories of two-dimensional symbols exist: multi-row (or stacked) and matrix. A 2D bar code can only be read be an imager, since a picture needs to be taken and then decoded. DataMatrix and Maxicode are examples.
Bar height: The height of the bars. The taller the bars, the easier it is for a person with a hand scanner to scan the bar code. Bearer bars: Some bar code symbologies are unsecure, meaning that if the bar code scan beam is partially off the bar code it may still result in a good read, although the data may be partial or incorrect. Bearer bars are bars at the top and bottom of the bar code bars that prevent partial reads. Code I 2 of 5 symbology contains bearer bars.
Bi-Directional: Characteristic of most bar code symbologies which permits scanning of the bar code symbol in either the left-to-right or right-to-left direction.
Check character: Extra digit added at the end of a bar code to allow the scanner to confirm that it read the bar code correctly. It is typically stripped from the data and not transmitted to the host.
Clear Area: See Quiet Zone.
Error Correction Level: Redundant data correction and/or extra error correction in some bar code symbologies, allowing for some of the bar code to be missing and still result in a correct scan.
Linear: A single line bar code. The UPC bar code is linear, as are Code 39, I 2 of 5, and Code 128.
Mil: A unit of measure equal to 1/1000 of one inch. For example, a line 1/4 inch wide would be 250 mils.
Narrow to Wide Ratio: The ratio of the narrowest bar (see “X dimension”) to the widest bar in a linear bar code symbol. Common values are 2, 2.5, and 3. Just like X dimension, a larger ratio generally means better readability by the bar code scanner.
Print contrast: The difference in reflectance between the bars and spaces of a bar code. Generally, the greater the contrast the easier it is for the reader to read the bar code, although some really reflective surfaces cause problems.
Quiet Zone: The minimum required space for bar code scannability, preceding the Start Character and following the Stop Character of a bar code symbol. The quiet zone should be free from any printing and be the same color and reflectance as the background of bar code symbol. The Quiet Zone should be ten times the width of the narrowest element in the bar code, or 0.25 inch minimum. Also known as Clear Area.
Stacked: A linear bar code that has been continued on the next line. Special commands are added so that the multiple rows are decoded in the same way a linear bar code would be. If you can drag a straight line through a symbol and hit all of the lines, it is stacked. PDF417 is technically a stacked symbology.
Start and Stop Characters: Special bar code characters that tell the scanner to start and stop reading a bar code symbol. The Start Character is typically found on the left side of a bar code symbol. The Stop Character is typically found on the right side of a bar code symbol. The characters signify to the scanner/reader that everything between them is data. Start and stop characters are usually stripped-off and not transmitted to the host. Compare to human readable text, where a capital letter indicates the beginning of a sentence and ending punctuation shows the end of the sentence.
Symbology: A defined method of representing numeric or alphabetic characters in a bar code; a type of bar code. Common symbologies include Code 39, Interleaved 2 of 5 (I 2 of 5), PDF417, Code 128, UPC and GTIN.
X-Dimension: The measure of the narrowest bar in a bar code, measured in thousandths of an inch. A 20 Mil bar code will have a narrow bar width of 20/1000 of an inch, or 0.0020”. Generally, the larger the mil value the easier the scanner can read it and from greater distances. Bar codes and scanners have different X dimensions so they must be matched. In addition to all of the variation within the bar codes themselves, there are also standards within industries. The automotive, electronics, and retail/distribution industries are just three of the many that have developed standardized label formats with specific bar codes and data structures so that all companies within that industry can read each other’s codes and use the data seamlessly. When developing a bar code system for use either in-house, with a vendor, or with a customer, it’s important to make sure that standardization is applied to ensure maximum usability. There are many additional sources of information on the Internet and in print, but the de facto standard is “The Bar Code Book”, by Roger C. Palmer, from Helmers Publishing.