Understanding Barcodes

Background Information:

A bar code is a series of black bars and white spaces of varying widths that are printed on labels to identify items. A scanner is used to decode the bar code. The scanner measures the reflected light and interprets the code as numbers and letters that are passed to a computer.

To make decoding bar codes easier, think of every bar (black stripe) as being a 1, and every space, (white strip) as being a 0.

A UPC-A barcode starts on the left with “bar-space-bar” (101). This is the signal for the scanner to start reading, and it lets the scanner know the width of the binary digits within the symbol. The set of long bars that follows is the first digit of the barcode. This digit identifies the type of product.

The next five digits identify the manufacturer. This is called the left field. All of the products made by the same manufacturer have the same code in the left field.

The long bars in the middle separate the left field from the right field. They are called the center guard. These long lines represent the digits 01010.

The next set of bars is called the right field. This code also has five characters.

The long bars at the end contain the signal to stop reading (101), as well as to check the sum. This sum allows the scanner to check and see if all the other numbers are correct.

To determine what the check digit should be, find the sum of the odd-spaced digits (the 1st, 3rd, 5th, 7th, 9th, and 11th digits). Then multiply this sum by 3. Save that as answer #1. Add up the even-spaced digits (the 2nd, 4th, 6th, 8th, and 10th digits). Add this answer to answer #1. The check digit is the number you have to add to your answer to find the next multiple of 10.