Bar Code Quality

By Dr. Brad Paulson, Thor Engineering, ICMA Standards Representative

The first published document concerning the quality of printed bar codes was the Universal Product Code (U.P.C.) Symbol Specification and U.P.C. Verification manuals of the Uniform Code Council (UCC). These “traditional” metrics were based on linear measurements of bar/space widths as seen by the human eye, reflectance values of background vs. bar color when measured through a red filter, and the correct data encodation. However, visual measurement results do not necessarily correlate with bar code scanning systems. In fact, bar codes had been printed that met the existing standards, but would not scan, while bar codes printed outside the standard specifications would scan successfully.

In 1982, a committee of the American National Standard Institute (ANSI) began development of a bar code specification, published in 1990 as American National Standard X3.182:1990 Bar Code Print Quality Guideline. The ANSI document, as well as the Committee for European Normalization standard, CEN 1635, has since been superceded by adoption of ISO/IEC 15416:2000 Information technology -- Automatic identification and data capture techniques – Bar code print quality test specification -- Linear symbols. The ISO Print Quality Guideline recommends a method of measuring the quality parameters of a printed bar code symbol. The evaluation is based on how the scanning or reading equipment “sees” the bar code, rather than how the bar code appears to the human eye.

Light source wavelength and measuring aperture size have a significant impact on measuring the reflectivity of the bar code components. Therefore, unless a nominal aperture diameter is specified by the user application, ISO/IEC 15416 suggests the aperture diameter be based on the specified width of the narrow elements, or “X” dimension, of the bar code to be verified. However, apertures and wavelengths specified in Industry Applications Standards take precedence over the ANSI guideline, even if some X dimensional ranges do not agree with the ANSI recommendations. A Scan Reflectance Profile (SRP) is generated by measuring the reflectance characteristics of an entire bar code symbol, which is a record of the reflectance values (from 0 to 100 percent) measured along a single line across the entire length of the bar code. Eight quality parameters are calculated using the SRP reflectance values, shown in the table.

The lowest grade received by any individual ISO parameter becomes the overall ISO grade for that Reflectance Profile. Ten scan grades are required to calculate the overall grade of a bar code symbol. These ten grades are averaged to determine the final ISO grade for the symbol.

Decode

Decode is simply whether the bar and space pattern matches a valid symbology based on the decode algorithm. Passing indicates the pattern conforms to some symbology.

Minimum Reflectance (Rmin)

The lowest reflectance value for at least one bar (Rmin) must be half or less than the highest reflectance value for a space (Rmax). For example, if the highest space reflectance value is equal to 80 percent, the reflectance value of at least one bar in the profile must be 40 percent or less.

Edge Contrast Minimum (ECmin)

Each transition from a bar to a space, or a space to bar, is an “edge,” whose contrast is determined as the difference between peak values in that space and that bar. Each edge in the scan profile is measured, and the edge that has the minimum contrast between the transition from space reflectance to bar reflectance, or from bar to space, is the Minimum Edge Contrast. In other words, the minimum space reflectance adjacent to the maximum bar reflectance is used to determine ECmin. An ECmin less than 15 percent fails.

Edge Determination

In order to discern bars and spaces, a Global Threshold is established on the scan reflectance profile by drawing a horizontal line halfway between the highest reflectance value and the lowest reflectance value. The barcode conforms if every bar-to-space and space-to-bar transition crosses this horizontal line. If one or more transitions do not cross this line, the bar code fails the Global Threshold.

Symbol Contrast (SC)

Symbol Contrast is the difference between the highest reflectance value (Rmax) and the lowest reflectance value (Rmin) in the SRP. Higher values provide better grades; a value greater than 70 percent is an A grade, and less than 20 percent is an F grade.

Modulation (MOD)

Modulation is a calculated value, and has to do with how a scanner “sees” wide elements in relationship to narrow elements, as represented by reflectance values in the scan reflectance profile. Typically, scanners “see” spaces narrower than bars and narrow spaces less reflective, or less intense, than wide spaces. Modulation is ECmin divided by SC, and is a graded parameter; a value greater than 70 percent is an A grade, and a value less than 40 percent is an F.

Defects

Defects are voids (light areas) found in bars, or spots (dark areas) found in spaces and quiet zones of the printed bar code. Each element is individually evaluated for reflectance non-uniformity, and is the difference between the highest reflectance value and the lowest reflectance value found within a given element. Less than 15 percent non-uniformity is graded an A, and greater than 30 percent non-uniformity is graded an F.

Decodability

Decodability tests for consistency in element widths throughout the printed bar code and compares the readability against a reference decode algorithm. Decodability measures the amount of “safe” margin left for the reading process after any errors in the printing of the bar code. A larger margin for the scanning system is the result of higher percent consistency, and thus, receives a higher grade. Different decodability calculations are used for each symbology; the appropriate standard should be referenced to obtain the correct formulas used to calculate decodability.

In conclusion, it is important to recognize the difference between validation and verification: validation is checking to see if the bar code symbol will decode and if the data in it exists in the database, while verification is checking the overall print quality against a set of standards or specifications. To continue the point, scanners are not verifiers; scanners are designed to read, while verifiers are analytical instruments used to evaluate bar code quality.

Brad Paulson, Ph.D., is the ICMA Official Standards Representative and concurrently serves as principal and founder of Thor Engineering, a company he started in 2001 to test and evaluate media and materials to determine failure modes and recommend material and process improvements. Views expressed here are his own. Questions? Contact Brad at Tpaulson@rconnect.com.