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The first published document concerning the issue of printed bar code quality were during the development of the Uniform Code Council (UCC) Universal Product Code (U.P.C.) Symbol Specifications and U.P.C. Verification manuals. Quality parameters for checking the quality of bar codes in the original U.P.C. print quality requirement had to do with:

Did the background and bar contrast (color) or reflectance meet the correct criteria for a bar code scanner to "see" the bar code? (At that time, scanners where primarily based on helium neon lasers which "sees" everything as if it had red glasses on.)

 In 1982 the American National Standards Institute, (ANSI) X3A1 Technical Sub-committee with the assistance of other ANSI and industry committees and bar code authorities, began studying the issue of bar code print quality in other symbologies for all types of printing methods. Through the years, bar codes had been printed that met the existing standards, but would not scan. More often bar codes printed out of specified standards did scan.

This combined group knew that the existing specifications for quality control of bar codes were evaluating criteria based on the way the human eye "viewed" the bar codes. This was not the way any bar code scanner would "see" the bar code. A bar code scanner is an optical device and does not incorporate human eye optical properties when "looking" at a bar code. The ANSI X3A1 group evaluated what factors were important to the many different types of bar code scanners/decoders for high first read rates and readability. After eight years of extensive testing, American National Standard X3.182-1990 Bar Code Print Quality Guideline was published. That document outlines quality parameters based on the optics of bar code scanning systems.

 Today many groups including the UCC, ANSI/Material Handling Institute, Automotive Industry Action Group and the ABCD Microcomputer Industry have specified conformance to ANSI X3.182 1990 Bar Code Print Quality Guideline. The present literature will outline the parameters of bar code quality from the ANSI document. It will discuss the importance of these parameters, and what corrective action is necessary to greatly improve bar code print quality.

APERTURE AND WAVELENGTH 
The aperture size and wavelength has a significant impact as to the grade results obtained. For instance, a symbol checked with a 5 mil aperture with a 633 nm (red) light source might achieve a grade of D. The same symbol could be verified with a 10 mil aperture at the same wavelength and receive a grade of B, and then receive a grade F (fail) if verified with a 10 mil aperture with a 900 nm (non-visible) light source. The ANSI guideline also recommends the aperture diameter based on the "X" dimension of the bar code being verified. Industry specifications will also indicate what wavelength of light the bar code should be verified with. 

03 .004" to .007" 
05 .0071" to .013 
10 .0131" to .025" 
20 .0251" and larger 

DEFINITION:  
ANSI X3.182-1990 Bar Code Print Quality Guideline outlines several parameters that greatly effect the quality of the printed bar code. These parameters are tested by creating a SCAN REFLECTANCE PROFILE. A SCAN REFLECTANCE PROFILE is a record of the reflectance values (00% to 100%) measured along a single line across the entire width of the bar code. These values are charted to create an analog representation of the bar code. Each SCAN REFLECTANCE PROFILE will either PASS, FAIL or be graded as A, B, C, D, or F (referred to as a SCAN GRADE) for one or possibly more of specified criteria as described in the ANSI document and further explained in this document. Ten Scan Reflectance Profiles are required to determine SYMBOL GRADE (See figure 1 for a PASSING profile and figure 1A for a FAILING profile.)

After creating the SCAN REFLECTANCE PROFILE, a count of the elements (bars and spaces) is done to determine if the bar code conforms to some type of symbology. But before this can be accomplished, EDGE DETERMINATION must be done.

 GT = Global Threshold Rmin = Reflectance Minimum

SC = Symbol Contrast 

GT = Rmin + SC/2

Definition:   In order to discern bars and spaces, a GLOBAL THRESHOLD is established on the scan reflectance profile by drawing a horizontal line half way between the highest reflectance value and the lowest relectance value seen in the profile. EDGE DETERMINATION can then be done by counting the number of crossings at the global threshold confirming whether the count conforms to or is considered non-conforming to a legitimate bar code symbology. If the bar code conforms it PASSES; if it is considered non-conforming it FAILS.

(see figure 2 for a PASS on EDGE DETERMINATION and figure 2A for a FAIL on EDGE DETERMINATION.) 

Rmin = REFLECTANCE MINIMUM Rmax = REFLECTANCE MAXIMUM

 Formula: Rmin <= .5 Rmax = PASS

Rmin > .5 Rmax = FAIL

Definition:   The reflectance value for at least one bar must be half or less than the highest reflectance value for a space. If the highest space reflectance value is equal to 80% the relectance value of at least one bar in the profile must be 40% or less. (See figure 3 for a PASS on Minimum Reflectance and figure 3A for a FAIL on Minimum Reflectance.)

 Suggestions for improving MINIMUM REFLECTANCE:

Making bars darker, ie. darker ink or for thermal printing increasing heat.

 Rs = Space Reflectance Rb = Bar Reflectance 

ECmin = Rs min - Rb max (worst pair)

> = 15% = PASS

< 15% = FAIL 

Definition:   Each transition from a bar to a space, or back again, 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 from the transition from space reflectance to bar reflectance, or from bar to space, is the MINIMUM EDGE CONTRAST or ECmin. (See figure 4 for a pass on ECmin and figure 4A for a FAIL on ECmin.)

Using a "lighter" substrateand darker ink, or increasing the X dimension (minimum element width), 
assuming the appropriate aperture size is used.

SYMBOL CONTRAST (SC) 
Symbol contrast is graded A, B, C, D or F. Symbol contrast is the difference between the highest reflectance value (Rmax) and the lowest reflectance value (Rmin) in a scan profile. The higher the value, the better the grade. To achieve grade A the SC must be equal to or greater than 70%, grade B is equal to or greater than 60%, grade C equal to or greater than 50%, grade D is equal to or greater than 40% and a value less 40% grade F (FAIL). This parameter is calculated by subtracting the highest reflectance value minus the lowest reflectance value measured in the scan profile. (see figure 5)

Improving the symbol contrast grade usually requires making the bars darker and/or making the spaces lighter or less shiny. Shiny materials are a special case because they usually fail to reflect much light back in the direction received. This causes the reflectance to be a lower value in shiny areas. 

MODULATION  
Modulation is also graded A through F. Modulation has to do with how a scanner "sees" wide elements (bars or spaces) in relationship to narrow elements, as represented by reflectance values in the scan profile. Scanners usually "see" spaces narrower than bars and scanners typically "see" narrow spaces being even less intense or not as reflective as wide spaces. Modulation is calculated as MINIMUM EDGE CONTRAST divided by SYMBOL CONTRAST. If results are equal to 70% or greater then the bar code's modulation grade is grade A, equal to or greater than 60% is grade B, equal to or greater than 50% is grade C, equal to or greater than 40% is grade D. Below 40% is grade F and FAILS the modulation test. (See figure 6)

Making narrow spaces wider than the narrow bars usually will increase the Modulation grade. Measuring with a smaller aperture will often increase the modulation grade, but the measurement aperture should always be the correct one for the application.

DEFECTS 
Defects are graded A through F. Defects are voids found in the bars or voids found in the spaces and quiet zones of the code. According to the ANSI print quality guideline, each element is individually evaluated for its' reflectance non-uniformity. Element reflectance non-uniformity is the difference between the highest reflectance value and the lowest reflectance value found within a given element. Many elements will have zero non-uniformity. DEFECTS are measured as the maximum Element Reflectance Non-uniformity (ERNmax) divided by SYMBOL CONTRAST. To achieve a grade A, defects can equal or be less than 15%, B equal or less than 20%, C equal or less than 25%, D equal or less than 30% and greater than 30% FAILS the defects test. (see figure 7)

DECODABILITY
Decodability, which can also be referred to as ANSI PEAK DECODABILITY, is a graded parameter. Different decodability calculation methods are needed for each type of symbology being tested, but the concept is basically the same for all symbologies. This tests for consistency in element widths throughout the 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. The higher the percent, the higher the grade and thus the larger the margin for the scanning system.

For instance, in Code 39 there are two element widths, either wide or narrow. Decodability looks for wide elements to be the same widths and likewise all narrow elements should be the same widths. It also looks for sufficient difference in the measured or perceived wide and narrow elements. This is controlled by the wide to narrow ratio, and ANSI X3.182-1990 terms this as Reference Threshold. Decodability measures this printing accuracy and compares it to how a scanner would be able to read the bar code against a reference decode algorithm. The result is the available margin for the reading process.

DECODE
Decode is either a PASS or FAIL parameter. Decode is the ability to convert the bars and spaces into the proper series of digits when checked against the ANSI reference decode algorithm for a given symbology and/or application.

SCAN GRADE 
The scan grade is the lowest grade received out of the eight parameters tested from a given scan profile. Even though grade A is received on all parameters except modulation where grade C is received, the overall symbol grade is C.

 OVERALL SYMBOL GRADE 
ANSI states that the overall symbol grade is based on ten scan profiles, and the average of their resultant scan grades as defined above. The reason for averaging ten scans is purely for vertical redundancy. Quality levels could change within the height of the bar code being verified.

Übernommen von ADES, 6. Juni 2001, Heinrich G. Merki

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