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A barcode (also bar code) is a machine-readable representation of information in the widths and spacings of printed parallel lines (or concentric circles, in at least one symbology). They can be read by optical scanners called barcode readers or scanned from an image by special software

The idea for the barcode was developed by Norman Joseph Woodland and Bernard Silver. In 1948 they were graduate students at Drexel University. They developed the idea after hearing the president of a food sales company wishing to be able to automate the checkout process. One of their first ideas was to use Morse code printed out and extended verticaly, producing narrow and wide bars. Later, they switched to using a "bulls-eye" type barcode. The two filed U.S patent # 2,612,994 (click to see link) on October 20, 1949 for "Classifying Apparatus and Method". The patent was issued on October 7, 1952.

The first bar code reader was built by Woodland (who was an IBM employee at the time) and Silver in 1952 and included a 500 watt light bulb and a photomultiplier vacuum tube made by RCA for movie sound tracks (which were printed optically on film). This device was not very practical (the output was simply an oscilloscope, and the 500 watt bulb nearly caught the paper containing their first sample barcode on fire), and was not commercially produced. In 1962 they sold the patent to Philco, who later sold it to RCA. The later development of the laser allowed barcode readers to be made much more cheaply, and the development of the integrated circuit allowed the practical decoding of the scanned barcode. Sadly, Silver died in 1963 at age 38 before anything could come of the patent.

In 1972, a Kroger store in Cincinnati experimented with using a bull's-eye barcode reader, with help from RCA. Unfortunately, the bulls-eye barcodes were easy to smudge during printing, and were't very successful. In the meantime, Woodland at IBM was developping the linear barcode that was adopted on April 3, 1973 as Universal Product Code. On June 26, 1974, the first retail product (a pack of chewing gum) was sold using a barcode reader, at Marsh's supermarket in Troy, Ohio. (This pack of gum is now in the Smithsonian's Museum of American History.)
In 1992, Woodland was awarded the National Medal of Technology by president George H. W. Bush.

Barcodes (and other machine readable tags like RFID) are used wherever physical objects need to be tagged with information that is to be processed by computers. Instead of painstakingly typing long strings of data into a terminal, the operator only has to display the code to a barcode reader. It also allows for processing without the help of human operators in fully automated environments.

The amount of data contained in a barcode varies with the application. In the simplest case only an identification number is provided which is used to index into a central database where the complete information is kept. The EAN-13 and UPC codes commonly found on retail articles work this way.

In many cases it is more desirable to include the complete information in the barcode itself without the requirement for an external database. This led to the development of barcode symbologies that can express more than decimal digits, ranging from additionally encoding just the upper case alphabet to the complete ASCII character set and beyond. The drive to encode ever more information in combination with the space requirements of simple barcodes led to the development of matrix codes which are often also named 2D barcodes, although most do not consist of bars but rather a grid of square cells. Stacked barcodes are a compromise between true 2D barcodes and linear codes, and are formed by taking a traditional linear symbology and placing it in an envelope that allows multiple rows.

The mapping between messages and barcodes is called a symbology. The specification of a symbology includes the encoding of the single digits/characters of the message as well as the start and stop markers into bars and space, the size of the quiet zone required to be before and after the barcode as well as the computation of a checksum.

Symbologies can be classified mainly by two properties:

  • Continuous vs. discrete: Characters in continuous symbologies abut, with one character ending with a space and the next beginning with a bar, or vice versa. Characters in discrete symbologies begin and end with bars; the intercharacter space is ignored, as long as it is not wide enough to look like the code ends.
  • Two-width vs. many-width: Bars and spaces in two-width symbologies are wide or narrow; how wide a wide bar is exactly has no significance as long as the symbology requirements for wide bars are adhered to (usually two to three times as wide than a narrow bar). Bars and spaces in many-width symbologies are all multiples of a basic width called the module; most such codes use four widths of 1, 2, 3 and 4 modules.

Types of barcodes

Linear barcodes

Catalogs, store shelves, inventory
USA retail
Worldwide retail
Libraries, blood banks, airbills
Interleaved 2 of 5
Code 39
Code 93
Code 128
Code 11
Post office

Stacked barcodes

Stacked 1D barcodes.
Code 16K
Based on 1D Code 128.
Code 49
Stacked 1D barcodes from Intermec Corp.
The most common 2D barcode. Public domain.
Micro PDF417

2-D barcode

Developed by Lynn Ltd.
From ArrayTech Systems.
Aztec Code
Public domain.
Small Aztec Code
This was the barcode tested in a Kroger store in Cincinnati. It used concentric bars.
Code 1 Public domain.
CP Code From CP Tron, Inc.
Data Glyphs From Xerox PARC.
Data Matrix From RVSI Acuity CiMatrix. Now Public Domain
Datastrip Code From Datastrip, Inc.
Dot Code A  
HueCode From Robot Design Associates. Uses greyscale or colour.
INTACTA.CODE From INTACTA Technologies, Inc.
MaxiCode Used by United Parcel Service.
MiniCode From Omniplanar, Inc.
QR Code From Nippondenso ID Systems. Public domain.
SmartCode From InfoImaging Technologies.
Snowflake Code From Marconi Data Systems, Inc.
SpotCode Circular code from High Energy Magic Ltd.
SuperCode Public domain.
UltraCode Black-and-white & colour versions. Public domain.