Bar Code Labels
The physical properties of an element in a linear bar code are commonly described in terms of height and width. The unit for the width dimension is expressed in mil (0.001 inch) or mm (millimeter). Typically, the widths of both bars and spaces are significant. The width of the narrowest element is often the only one quantified, as the width of the other elements can be expressed based on the narrow element width. The element width is nominally the same for the narrow bar and the narrow space. The width is also referred to as the narrow bar width, element width, mil size, or the X-dimension. The X-dimension of a bar code is the theoretical width of the narrowest element.
The length of the symbol is the measurement perpendicular to the length of the bars, regardless of orientation. The height unit is often expressed in inches or millimeters. The height is measured along the length of the individual elements. If different heights are used within one symbol (height-modulated bar code), the smallest dimension is used. The element height is conceptually the same for bars and spaces. Unlike the width, this property is almost exclusively referred to as the bar height of the bar code. The taller the bar code, the more area the bar code reader has to scan the symbol. If part of the bar code is damaged, the scanner has a greater chance of still reading the symbol. Microscan recommends that the scanner receive a minimum of five looks at each symbol to achieve the optimal read performance possible from the bar code scanner.
Terms and Attributes
To create a good bar code, one must first understand the attributes of a bar code and how they can affect the readability in an application.
Quiet Zone/Margin: The quiet zone is the space preceding the first bar and trailing the last bar. This is also sometimes referred to as the "no print" zone. Generally, the quiet zone must be a minimum of 10 times the width of the narrow bar. If text or a graphic encroaches into this space, the bar code scanner may not be able to read the bar code.
Quiet zone intrusions are the most frequent violations in bar code applications. Even in space-constrained applications such as labels on a microplate, the bar code scanner will not reliably read the bar code if the human readable or other graphical elements encroach into this area. Do not violate the quiet zone (Figure 2).
Start/Stop Characters: Most common symbologies employ Start/Stop characters or patterns. The primary purpose of these characters is to indicate the direction of scan to the bar code reader. Unlike human-readable text that must be read in a specific direction to be understood, a bar code can be scanned left to right or right to left. The Start/Stop characters tell the bar code reader whether the data are to be decoded forward or backwards. This ensures that the bar code reader will output the data in the proper order. A few symbologies do not utilize Start/Stop characters and are very insecure as a result. The legacy symbology Pharma Code, for example, produces different decodes depending on the direction the bar code is presented to the scanner.
Data Characters: The data characters of a linear bar code contain the encoded information (Figure 2). For example, suppose that the user would like to encode a unique identifier into a bar code. The user defined this identifier as, e.g., ab123456. The data characters are the information sent to the bar code printer and the information decoded by the bar code scanner.
Check Character/Digit: Many symbologies employ check digits (Figure 2). They are optional in some and automatically built into others. Their function is to provide a higher level of data integrity and should always be enabled whenever possible, especially in applications where absolute data integrity is critical. The check character in a bar code works as follows:
- The bar code printer calculates a check character value based on the data to be encoded.
- This check character is included in the symbol.
- The bar code scanner decodes the symbol and extracts the check character.
- The bar code scanner performs the same calculation performed by the bar code printer and compares the result to the decoded value for the check character.
- If the calculated value and the actual value for the check character are identical, the data is outputted. If not, the bar code scanner will continue to decode in pursuit of a match.
To further increase the data security of the bar code, the modulo value or check character, is used to calculate the check sums of the bar code.
Narrow Bar Width: The narrow bar width is the nominal dimension of the smallest bars and spaces in a bar code symbol and is used for determining the size scale of the bar code (Figure 2). The most common unit of measure is either the mil (0.001 inch, primarily in the U.S.) or mm (millimeter). When determining the size of the bar code for a specific application, choose the largest narrow bar size the application will allow. A larger bar code is easier to print and read accurately and provides a greater depth of field for the bar code reader.
Contrast Ratio: This is the percentage of contrast between the bars and spaces of the bar code. The better the contrast, the easier it is for the bar code scanner to differentiate between the bars and spaces of the bar code. The bar code scanner will read the code more easily, and the chance of noise interference will be reduced in the process.
Bar Height: This is the measurement along the length of the bars regardless of orientation. Microscan recommends that the bar height be as high as possible. The taller the bar code, the more area the bar code reader has to scan the symbol. If part of the bar code is damaged, the scanner has a greater chance of still decoding the symbol on a tall bar code (Figure 3).
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