Explainer: The basics of barcodes

Barcodes work based on light reflected back to scanner optics: black does not reflect light, while the white (blank) spaces do reflect light. Because characters (alphabetic, numeric, special) are represented by symbols (most familiarly, the narrow and wide bars and the short and long spaces in-between), the different “languages” of barcodes are known as “symbologies.” The same character on a keyboard can be represented differently based on the symbology used, that is to say, the same keyboard character can be represented by a different combination of narrow and wide bars and short and long spaces depending upon the symbology. Some symbologies are dedicated to certain uses, e.g., the UPC (Universal Product Code) which is now known as the GTIN-12 (Global Trade Identification Number 12) which is used to identify a unique sellable item in US retail.

Related: The Perfect Order: Introduction

Some symbologies may have a restricted character set. The GTIN-12/UPC is limited to only numbers, and is further constrained to being only 12 characters (more specifically, digits) in length, no more and no less. The SSCC-18 (Serial Shipper Container Code) is another barcode that is only numeric, has a specific use, and is constrained to a specific length.

Traditional barcodes composed of bar symbols are also known as 1D (one-dimensional) barcodes. These barcodes are read horizontally across the bars. Other barcodes like the PDF-417 and QR code are known as 2D (two-dimensional) barcodes and are read in whole as an image. Letters, numbers, and special characters are still represented by symbols, and are represented by different sets of symbols depending on the 2D symbology; it’s just that the symbols used for 2D barcodes are not the bars used in 1D barcodes.

The check digit—common in most 1D barcodes—is used to validate the integrity of the barcode by verifying the other digits of the barcode. The check digit on a 1D barcode is typically the right-most digit if optionally displayed. (2D barcodes use a type of error correction at the symbol level.) The most common check digit is the Modulo-10 (MOD-10) calculation, which works like this for a GTIN-12/UPC, where the 12th digit (the right-most digit) is the check digit:

1. Sum the digits of the odd-numbered positions (e.g., characters 1, 3, 5, 7, 9, and 11).
2. Multiply this number by 3.
3. Sum the digits in the even number positions (e.g., characters 2, 4, 6, 8, and 10).
4. Add the two numbers together. (Add the result of Step # 2 and Step # 3.)
5. The MOD-10 is the number that is required to be added to make this resultant number (Step # 4) evenly divisible by 10.

The GTIN-12/UPC (the GTIN-13 in Europe) is used to identify an individual sellable item, effectively an “each” unit of measure. The GTIN‑14 is used to identify a case of more than one of the same item, (e.g., 6, 12, 15, 18, 24) as the GTIN-14 is based on the GTIN-12 identifier. The SSCC-18 is used to identify a shipping container such as a carton or a pallet. All three of these symbologies use the MOD-10 check digit for validation.

The global organization that sets and maintains barcode standards is GS1 (www.gs1.org). GS1 operates in over 100 countries worldwide. The US affiliate’s website of GS1 is: www.gs1us.org.

GS1US has a convenient online check digit calculator for different barcode symbologies that can be found at: https://www.gs1us.org/tools/check-digit-calculator

Barcode labeling projects require that the label size, label media (material), adhesive, label surface (e.g., matte versus glossy), and print methodology (e.g., direct thermal versus thermal transfer) be thoughtfully considered before any money is spent. Should you integrate a verifier with your barcode printer? For some industries or circumstances, this might be a very good idea.

If RFID (radio frequency identification) tagging is required, which type of tag (active—those with an embedded power source, or passive—those without an embedded power source) needs to be determined, along with the frequency range for the tag. Will the tag be embedded in the product package or a (barcode) label? If the latter, ensuring that barcode printers that can encode an RFID tag are purchased will be a requirement.

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