Interleaved 2 of 5 is a continuous two-width barcode symbology encoding numerical digits only. It is used commercially on 135 film and on cartons of some products, while the products inside are labeled with UPC or EAN.
Where "n" is a narrow line (bar or space) and "W" a wide line (2.0 to 3.0 times the width of a narrow line). The wide lines form a two-out-of-five code with consecutive values of 1, 2, 4, 7, and 0, where the code 0 has a value of 11. This is similar to the POSTNET bar code.
Before the actual pairs there is a start code consisting of nnnn, and after all symbols there is the stop code consisting of Wnn.
Only an even number of digits can be encoded. Typically an odd number of digits is encoded by adding a "0" as first digit, but sometimes an odd number of digits is encoded by using five narrow spaces in the last digit.
A checksum can be added as last digit, which is calculated in the same way as UPC checksums.
On 135 film (35 mm) canisters it is used to identify the manufacturer, the film type, number of exposures and proprietary information in 6 digits. The barcode is located between the electrically read silver and black DX Camera Auto-Sensing Code and the film can exit lip. It is optically scanned by many film-processing machines when the cartridge is inserted for developing
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