Bar Code Symbologies
Authored by: Niels Wartenberg and Susan Snyder; Microscan Systems Inc.
To create the best possible bar code for the application, one must first understand the advantages and disadvantages of the available symbologies. Since several of the symbologies share similar characteristics, they can be grouped into families. The most common families are Ratio and Modular bar codes and Discrete and Continuous bar codes.
Character Construction: Ratio and Modular Bar Codes
Two methods are commonly used for encoding data into a bar code: ratio and modular. The ratio-based method utilizes only elements of two widths to create characters (Figure 4). The narrow element width is nominally equal to X. The wider element width is defined as a ratio of X, such as 2X or 3X. Ratio-based symbologies were the first symbologies to be widely adopted, and several remain very popular.
Modular symbologies divide the area for each character into a number of equally sized modules. The width of the modules is nominally equal to X. Each module is either light or dark. Identical adjacent modules make up bars and spaces wider than 1X. The width of each bar or space is a whole multiple of the module width (Figure 5).
The bar code symbologies developed over the last few years, such as Code 128, have been almost exclusively modular because the modular structure creates a much more compact bar code. More data can be encoded into a much smaller space. The advantage to using ratio-based symbologies is that they are more impervious to printing inaccuracies. The bar code reader must only discriminate between two different widths, as opposed to reading a modular symbol that is constructed with four different element widths. This is usually not an issue unless the narrow bar width approaches the maximum resolution of the bar code printer.
Symbol Construction: Continuous and Discrete Bar Codes
The characters of both modular and ratio-based bar codes are formatted in one of two ways: discrete or continuous. In a discrete symbology, each character begins and ends with a bar. The individual characters are separated by spaces that do not contain information. The space between characters is referred to as the Intercharacter Gap, similar to the spaces in between the following letters: A B C D. Generally, the Intercharacter Gap should not be greater than four times the Narrow Bar Width (4x).
In a continuous symbology, each character begins with a bar and ends with a space. There is no space between characters. For example, the data would resemble the format of the following letters: abcde.
Linear Bar Codes
Literally hundreds of linear symbologies are in use today. However, only a few are widely adopted and appropriate for use in the Life Sciences. Listed below are some of the more common symbologies employed.
UPC/EAN was developed by the Uniform Code Council (UCC) for use in the retail grocery industry. It has several good features, including a mandatory check digit and good density. Advantage: The UPC code has good data security and immunity to ink spread when "similar-edge" algorithm is used. Disadvantage: It has a limited number of characters (numeric characters only) and is used for point-of-sale applications.
Code 39 was the first alphanumeric symbology and is still widely used today, Code 39 is also recommended by the NCCLS for specific applications. Advantage: Code 39 offers good security and an alphanumeric character set. Disadvantage: Since it is not as dense as Code 128, Code 39 is not recommended for applications requiring long data identifiers.
Code 128 is a very reliable symbology offering three different character sets. Advantage: Its large character set combined with high density and flexibility has earned it the recommendation of NCCLS and ISBT, among others. Code 128’s mandatory check character maximizes data integrity. Disadvantage: Since Code 128 is a modular symbology, users must be aware that problems can arise when the narrow bar width approaches the resolution of the bar code generation system.
Codabar was originally developed for the blood bank industry, but the industry has now started to use Code 128. While Codabar is still used in a number of applications, it offers no advantages and several disadvantages. The code’s most significant drawbacks are very low security and a limited character set.
Interleaved 2 of 5 is widely used in shipping applications. Advantage: It is the highest density linear bar code for numeric only strings of fewer than 10 characters. Disadvantage: Interleaved 2 of 5 has very low security. The symbology uses two bar patterns for start and stop instead of full characters, which can lead to truncation errors.
Code 93 uses the same character set as Code 39. Because Code 93 is a modular symbology, it produces a much more compact bar code than ratio-based Code 39. Advantage: Code 93 always uses two check digits to ensure data security. It is also the densest linear symbology for random alphanumeric strings. Disadvantage: The symbology is not a standard symbology read by most scanners, and its varying dimensions can be difficult to print accurately.
RSS is a new family of linear symbologies recently introduced by the UCC for small, space-constrained applications. Advantage: The RSS family is the most compact linear bar code symbology offered today and has limited orientation requirements. Disadvantage: RSS uses multiple bar code widths. As with other modular symbologies, users must be aware that difficulties can arise when the narrow bar width approaches the resolution of the bar code generation system.
Stacked bar codes were developed in the 1980s to encode more data in a smaller footprint than allowed by the then current linear bar codes. Today, the primary advantage to using a stacked symbology is to encode more data into a symbol that can still be read by a laser bar code scanner. While matrix codes are much more space efficient and offer increased data capacity than stacked 2D codes, matrix codes must be read with an image-based reader.
One significant drawback to stacked symbologies is their sensitivity to scanner tilt. To decode the symbol, the laser beam must be aligned so that the beam passes through the rows one at a time. The stacked symbology PDF417 provides the user with a little more tolerance. Since PDF417 has three different encoding schemes that read every three rows, the laser beam can pass through up to three rows at a time and still decode the symbol (Figure 6).
Code 49 was introduced in 1987 and was designed for labeling small objects. Advantage: It has a smaller footprint than most standard linear bar codes. Disadvantage: It has a maximum capacity of 49 alphanumeric characters and does not offer built-in error correction.
Code 16K was introduced a year after Code 49 and has an inverted Code 128 character set and has much more data capacity than Code 49. Advantage: It is much more compact than standard linear bar codes. Disadvantage: It is not widely used and does not offer built-in error correction.
PDF417 symbology stands for Portable Data File and unlike the other stacked symbologies offers the robust Reed-Solomon form of error correction. This enables the symbol to sustain considerable damage and still be readable.PDF417 has three different encoding schemes that repeat every three rows. So, for example, the bar pattern for the character ‘‘A’’ in row one is not the same pattern for ‘‘A’’ in row 2. However, it is the same pattern in row 4. Since the encoding scheme repeats every three rows, the symbology can sustain a certain degree of scanner tilt and still be readable. Advantage: PDF417 has high data capacity and offers Reed-Solomon error correction, which makes it highly secure. It can also be read with a laser scanner in many applications. Disadvantage: Like other stacked symbologies, it is sensitive to scanner tilt and is not as space efficient as matrix codes.
RSS/Composite Symbology The linear symbology RSS has a two-dimensional option called a Composite component. If additional information is needed, the Composite component can be attached to the top of the RSS symbol. The composite options are based on PDF417 and Micro-PDF417. Advantage: The composite component increases the data capacity of the RSS symbol. Disadvantage: Because the composite symbol is printed so small, the symbol’s sensitivity to scanner tilt becomes even more of a concern and can affect ease of use. An image-based reader is recommended for reading RSS/Composite symbols.
Matrix codes are becoming increasingly popular in the life sciences for their data capacity and exceptionally small footprints. The most popular code in use today is Data Matrix (Figure 6). Originally developed by a NASA engineer for marking and tracking space shuttle parts, the symbol was introduced into the public domain in 1995, and the AIM standard released in 1996.
Data Matrix is primarily used for its small size and exceptional data capacity, capable of encoding up to 2335 alphanumeric characters. If there is 1 square inch of space on the bottom of a well or a slide, the item can be marked with a five- or six-digit Data Matrix symbol. Data Matrix has additional benefits as well. Data Matrix offers the robust Reed-Solomon method of error correction and has no orientation requirements.
The Data Matrix symbol consists of elements that resemble a checkerboard pattern. Since both the light and dark elements are the same size throughout the symbol, it can accommodate a variety of marking methods (Figure 7). The border of the symbol—or finder pattern—is composed of two sides forming an ‘‘L’’ shape and two sides alternating light and dark elements. The Quiet Zone requirement for Data Matrix is a one-element width on each side of the symbol. Data Matrix can have multiple data regions, separated by the ‘‘alignment pattern’’ of each symbol. Depending on the needs of the application, Data Matrix symbols can be either square or rectangular, making them exceptionally flexible for space-constrained applications.
Advantages and Disadvantages: The primary drawback to using Data Matrix, like other matrix codes, is that it must be read with an image-based reader. Imagers are typically more expensive than a laser bar code scanner. However, for certain applications, Data Matrix may prove to be the only solution available. For applications such as marking each individual well on a microplate, Data Matrix provides the only viable option. A lab might also consider using Data Matrix for the high data security the symbol provides. Data Matrix can sustain a higher percentage of damage than a bar code can and still be readable.
Figure 7. 1. Element (module): Square-shaped cell that encodes one bit of binary data, ‘‘0’’ or ‘‘1.’’ 2. Quiet zone: The AIM specification calls for a minimum of one element width (1X) on each side of the symbol. 3. Structure Finder Pattern: The outermost rows and columns, composed of two solid lines and two alternating dark / light lines. The finder pattern is also used to define physical size, orientation, distortion, and the number of rows and columns in the symbol. 4. Data Region: The area inside the finder pattern, which contains the data and error correction codewords.
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