USING BAR CODE - Why It's Taking Over, Second Edition, is in its fifth printing, and is perhaps the most widely read book on bar code in the world. It was written by David J. Collins and Nancy N. Whipple in response to the data collection problems and questions of our thousands of seminar participants. This 325-page book, with numerous illustrations, is easy to read and understand. We guarantee it will unlock the powerful rewards of bar code technology.

An excerpt from USING BAR CODE - Why It's Taking Over is provided below. For a complete list of topics covered in the book, please see the Table of Contents.



A symbology is a language in bar code technology. In certain countries people speak, write and read the French language in order to communicate. In other countries the language of choice and custom might be English, or German, or Chinese. In the technology field we call bar code, a symbology is a language which we use to print and read messages accurately and conveniently. When a symbology is used to print a message, we call that message a bar code label. Information in a label is read through the eyes of a scanner, but the scanner and the label have to communicate through the same symbology rules or the message will not be understood. There are a number of bar code symbologies, some primitive, and some quite sophisticated. This chapter will describe five symbologies in some detail, and explain why you might decide to use a certain symbology to control one portion of your business, and a different symbology to control another portion.

While there are, by some count, over forty bar code symbologies that have played a role in the history of bar code technology, we will only examine a few in this book. We think these symbologies give sufficient flexibility in providing efficient control for 99% of the automatic data collection applications adaptable to bar code use. Before presenting the interesting differences among these bar code symbologies, it is appropriate to present the similarities each share.

"X" Dimension

The smallest element of interest in a bar code is called the "X" dimension. This is the narrowest bar or space in the "picket fence like" array which carries your message. In designing your bar code driven data collection system, you must first decide what this dimension should be. In the range of practical applications of bar code in use today, you may choose from an "X" of 5 mils to an "X" of 50 mils. A mil is one thousandth of an inch. Each symbology you choose for your bar code language will have a number of its own rules for the representation of each character in your message. A character is made up of bars and spaces, some of which will be a single "X" in width, and others will be two, three, or four "X"s wide. Exhibit 2.1 illustrates a character in Code (or symbology) 128.

Generally speaking, the larger the "X" dimension, the more forgiving the bar code is when read by a scanner. On the other hand, a larger "X" dimension requires a larger label area. Most applications use "X" dimensions in the range of 10 mils to 20 mils, representing a trade off among a number of considerations in bar code technology, including printer resolution.

Quiet Zone and Contrast

In order to read a bar code label dependably, a scanner must make a series of sensitive measurements of the contrast between a black (or dark) bar and the white (or light) background space between each bar. To complicate this problem, the scanning instrument projects a spot of energy that can be moving very fast, challenging the capabilities of today's most responsive electronic circuits. Symbology designers have, therefore, specifies a zone just to the left and right of the bar code that shall be free of all printing. This "quiet zone" gives the scanning device time to adjust its opto-electronic measurement circuits in order to properly determine the critical width of each bar in the message.

Bar code is most frequently expressed as black bars on white paper. This combination gives the greatest contrast, and the most reliably scanned message. However, bar code continues to work effectively when certain other combinations of color and background are used. You see this in supermarket packaging. What it important is the contrast between bars and spaces as viewed by the scanner. Bar code technology is designed to function best with a red light as the scanning spot. The first red light scanner was a Helium-Neon laser with a beam projection of 633 nanometers in the light spectrum. Bar code standards have grown up with laser energy as the contrast point of reference. As a result, you must avoid using paper or background label stock that contains any significant blue component which reduces contrast with the bars. Black on white labels are best.

Start, Stop, and Parity Characters

From the earliest applications of bar code, system designers have insisted that bar code symbologies be bi-directional. This means that the bar code message can be read from left to right, or right to left, with no difference in performance. Actually, many bar codes have bars arranged vertically from top to bottom, or vice versa, when applied to cartons or conveyorized objects. The principle remains the same, these bar codes should be as easily read from top to bottom as from bottom to top.

To accomplish this bi-directionality, every symbology provides for a separate start and stop character, in addition to the message characters. By convention we refer to the start character as the unique character to the left of the bar code, and the stop character as the unique character to the right of the bar code. In a vertical label, the start would be the bottom character and the stop would be the top character. In some symbologies the start character may convey some additional information, as you will see later in this chapter.

The symbology designer may also specify a mandatory or optional parity, or check character, in the bar code message. This parity character is included in the bar code to minimize the risk of misreading the message. It does not add any information to the message itself. Usually this parity character is stripped by the decoding portion of the scanner, and not passed along to the information network supporting the bar code application. The start, stop, and parity character (if present) are collectively referred to as bar code message overhead. The quiet zone should be thought of as overhead also, and provided for in your label space even though no transaction information is carried there.


Bar Code Message Composition

Now that we've covered the prerequisites for a bar code message, lets look at how a bar code would typically appear. Exhibit 2.2 below represents a horizontal, or picket fence bar code message. Exhibit 2.3 represents a vertical, or ladder bar code.

As illustrated in these two examples, the zones to the left and right (or top and bottom) of the symbology are free from all printing so that the scanner can have time to calibrate. The distance set aside for this "quiet zone" is somewhat variable. Most symbologies require that the quiet zone be ten times the "X" dimension of the symbol, or one quarter of an inch, whichever is greater. Since most industrial bar codes are printed with an "X" dimension of 20 mils or less, the requirement for one quarter of an inch of quiet zone, each side, is the most common rule. Of course for large "X" dimensions of 40 or 50 mils, such as warehouse bin labels or shipping container labels, the quiet zone will grow proportionately. While the quiet zone is literally needed on only one side of the bar code, the side from which a scan is initiated, the specification requires a quiet zone on both sides since the scan can begin from either side.

After the quiet zone, the start character will appear. Following the start character, the message characters are represented with the most significant characters to the left (or bottom, in a vertical bar code). This means that the characters are arranged in a horizontal bar code message the same way they are presented in the roman alphabet. The bar code represented in Exhibit 2.2 is the message AB123. Finally, a stop character is placed at the end of the message, which means to the right side of the horizontal bar code, or at the top of the vertical bar code.

Note: In the authors' experience, a principle cause of poor performance in bar code systems is the failure of many bar code labels to allow for a sufficient quiet zone. Either the data content grew after the application was first initiated without an allowance for additional labeling space, or the label material shifted in the printer and the subsequent bar codes had their quiet zone clipped from one side.

In some symbologies the space between characters is permitted to vary slightly from the normally tight print tolerances of the bars which form a character. Other symbologies demand precise bar to bar tolerance throughout the label. Where the space between characters is allowed to vary somewhat, the symbology is referred to as a "discrete" symbology and the inter character variance is called an inter-character gap. Where the space between characters is held to tight character tolerances, the symbology is said to be "continuous". As a bar code user, you needn't worry about such details. When certain symbologies were developed, the selection of available bar code printers may have dictated the need for a variable inter-character gap. Today, for most print technologies, that need has disappeared, but the symbology feature remains.


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