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01-04

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What is Bar Code Print Quality

Understanding the difference between scannable and 

verifiable when dealing with bar codes is critical to achieving print quality.

Bar code print quality has become a critical issue over the past few years. Retailers require accountability of the printed bar code, with the potential penalty of fines for noncompliance levied against the manufacturers and printers.

The quality of a printed bar code should be such that the scanning equipment can read the symbol for the first time and every time thereafter. Sounds fairly easy, doesn't it? Well, it's not. Bar code quality can vary based on printing method, substrate, and printing equipment, just to mention a few variables. Bar code symbology specifications have been published, as well as print quality guidelines, to help direct printers and manufacturers towards higher bar code print quality. Industry specific requirements include a recommended American National Standards Institute (ANSI) print quality grade, which includes aperture size, and specific illumination wavelength.

What does it mean to verify a bar code, and why is it so important? Let's examine what could happen when a bar code that has not been verified enters the world of automatic data capture.

Automatic Data Collection

Bar code quality is not obvious to the human eye. Yes, we can see if a bar code has been severely over or underprinted (bars are too wide or too thin) or damaged, and we can also see if the color red (invisible to the scanning laser beam) has been used for the bars. We cannot see, however, how this printed code will scan at the point of sale, if it will scan reliably the first time and every time, or if this code will scan with errors resulting in a corrupted database. We all have watched our merchandise being checked out, and we have seen the operator's frustration resulting from trying to scan the code several times, only to see the product number manually keyed in to the register. This situation, which occurs daily, is a waste of time and money.

In today's world of automatic data capture, scanning systems are installed to increase productivity, accuracy, and data handling efficiency. A bad scan can result in data substitution errors, which can lead to incorrect inventory data or the wrong price lookup. The consequences of poorly printed bar codes put a tremendous strain on the customer/supplier relationship.

Need For Verification

I have been asked many times, "Why can't I use a scanner to check if my printed bar code will scan at the register?" This is not advisable due to the difference in design between scanners and verifiers. A scanner alone cannot determine the absolute quality of a printed bar code. The ability of a specific type of scanner to read the information encoded in the printed bar code is based on its decoding capability. Thus, a printed bar code of marginal quality may be decoded correctly by one scanner, but not by another. Verifiers check the quality of the printed bar code against key parameters as defined by bar code specifications or industry specific applications. Complete verification includes checking the bar code visually, as well as quantitatively, against traditional and ANSI parameters. Let's examine this in greater detail.

Visual Inspection

Visual inspection encompasses the human readable information, quiet zones, bar code location, and bar code height.

• Human readable generally refer to the characters below or above the bar code. The size and the placement of the characters should follow the published specifications.
• Quiet zones are the clear margins to the left, right, and sometimes top and bottom of the bar code. Insufficient quiet zone areas will decrease the scanner's ability to decode the printed bar code successfully.
• The location of the printed bar code on the package should follow published guidelines. The bar code should be the proper distance from the bottom and the side of the package for a scanning system, such as those on conveyors, to see the bar code.
• Bar code height is usually specified as a minimum, or as a certain fraction of the width of the bar code. Reducing the height of the bars (truncation) decreases the scanners chance for first pass decoding. Truncated symbols require rescanning more often than full height symbols.

Traditional Parameters

Traditional quality parameters are based on bar and space widths as measured by the human eye.

This also includes measuring the reflectance values of the bars and spaces as seen through a red filter or filtered light source, which simulates how a helium neon-based scanner would see the bar code.

Traditional parameters include encodation, print contrast signal (PCS), average bar deviation, quiet zone dimensions, wide-to-narrow ratio, and a check character calculation. These parameters can be measured by using a bar code verifier.

• Encodation checks that the bar code has the proper type (and number) of characters encoded in the symbol. The human readable characters normally match the encoded information.
• Print contrast signal looks at the difference in reflectivity between the bars and the spaces. This value must be above a certain minimum to guarantee that a scanner will be able to read the bar code successfully. Verifiers are designed to measure the PCS of a printed bar code.
• Average bar deviation represents the average amount of bar gain or loss throughout the bar code. Bar width variations result from the growth or shrinkage which occurs in printing processes.
• Quiet zones are a traditional as well as a visual parameter. Clear areas to the left, right, and sometimes top and bottom of the printed bar code are required so that the scanning equipment will be able to determine where the bar code starts and stops. Quiet zones should contain no dark marks, graphics, or type.
• Wide-to-narrow ratios apply to two-element width symbologies, such as code 39, Interleaved 2 of 5, and Codabar. Acceptable wide-to-narrow ratios are usually defined as ranges in either general symbology specifications or user-specific applications.
• Check character calculations are performed to ensure the accuracy of the decode. A check character is included within a bar code, usually at the end of a symbol, and it is the result of a calculation performed on all the characters preceding it. Its presence is normally mandatory, as defined in a bar code specification or user application.

ANSI Parameters

Now we will consider the ANSI parameters. What is ANSI Bar Code Print Quality? In 1990, the American National Standards Institute approved and published the Guideline for Bar Code Print Quality (X3. 182-1990). This guideline documents the results of eight years of extensive testing to evaluate how a bar code scanner sees a printed bar code. The ANSI X3A1 Technical Subcommittee determined which factors were pertinent to high first-read rate for scanner/decoders. All ANSI print quality parameters are based on bar/space reflectance values. Today, several industry-specific bar code applications require conformance to the ANSI X3. 182-1990 Bar Code Print Quality Guideline.

The aperture size and the illumination wavelength used when verifying printed bar codes, according to ANSI, can have a significant impact on the ANSI grade. The ANSI guideline specifies the aperture diameter based on the X-dimension (the width of the narrowest bar). Aperture size and illumination wavelength are included in industry-specific application standards.

Recommended Aperture Sizes

The following two graphics show what can happen when an incorrect aperture size is used to verify the bar code. In Figure 1, the aperture is too large, and it sees multiple bars and spaces at any given time. Thus, the verifier may think it is looking at a wide bar with voids instead of distinguishing the actual bars and spaces.

In Figure 2, the aperture is too small, and it sees too little of a bar or space at any given time. Thus, the verifier may see a void in a wide bar as two narrow bars and a narrow space, instead of integrating everything together into one actual wide bar.

The ANSI Bar Code Print Quality Guideline incorporates several parameters which are calculated from a Scan Reflectance Profile (Figure 3). This is a graphical representation of the reflectance values (optimally 00/o for the bars and 100% for the spaces) of a single scan across the entire width of a bar code. Each reflectance profile is evaluated and will either pass or fall (A or F), or be graded A, B, C, D, or F, depending on the parameter measured. The various tests, which lead to pass/fail or an ANSI grade, are described below.

ANSI GRADE

The first test is global threshold (GT)/edge determination. This test is evaluated as pass (A) or fail (F). To distinguish bars from spaces, a global threshold is drawn halfway between the highest (R~) and lowest (R,,,~) reflectance values of the scan reflectance profile (Figure 4). The quiet zones and spaces have high reflectance values. Only those reflectance transitions, which actually cross the global threshold line, are considered bar/space transitions. Edge determination counts the number of element edges, which cross the global threshold line, and checks if the count conforms to a legitimate bar code symbology.

The next parameter is minimum reflectance (Rmin). This test results in a pass (A) or a fail (F). In this test, a measurement of the highest reflectance values, which is typically found in the quiet zones, is made. Our example measures 78% (Figure 5). The reflectance value of at least one bar must be no more than half of the highest reflectance value (in our example: 39%) to pass the reflectance minimum test.

Edge contrast minimum (ECmin) is a test which also produces a pass (A) or fail (F). This test measures the difference in reflectivity from a bar to an adjacent space, or from a space to an adjacent bar (Figure 6). The reflectance of each element in a pair is measured, and the reflectance difference, which has the lowest value, is the minimum edge contrast (ECmin). The reflectance difference has to be at least 15% to pass the minimum edge contrast test.

Decode is the last of the ACSI specified pass (A) or fail (F) tests. This test is passed when a scanned bar code can be converted into a series of valid characters. Each symbology uses a different ANSI Reference Decode algorithm and applications can also specify special reference decode algorithms.

Our next four ANSI parameters use the grading system of _A, B, C, D, or F, or 4, 3, 2,1, or 0. The ANSI letter grades are converted into numeric values as shown.

• A = 4.0 for value range 3.5 - 4.0
• B = 3.0 for value range 2.5 - 3.4
• C = 2.0 for value range 1.5 - 2.4
• D = 1.0 for value range 0.5 - 1.4
• F = 0.0 less than 0.5

The graded parameters are symbol contrast (SC), modulation, defects, and decodability.

Symbol Contrast

This test measures the difference of the maximum and minimum reflectance values of all the bars and the spaces in the printed bar code (Figure 7). Symbol contrast is the difference between the highest (Rmax) and the lowest (Rmin) reflectance value. To achieve a grade of A, the symbol contrast has to be at least 70%; a grade of F is for symbol contrast less than 20%.

Modulation

This parameter looks at the difference in the widths of bars versus spaces and how it affects their reflectance's (Figure 8). Even if the actual element widths are identical to the human eye, bar code scanners typically perceive bars to be wider than spaces. Modulation also looks at the difference in reflectance values between wide and narrow elements. Even if the reflectivity of all spaces is the same, the scanner can perceive the narrow spaces to be less reflective than the wide spaces. To achieve an ANSI grade of A, modulation has to be at least 0.70; a grade of F is for modulation which is less than 0.40.

Detects

Defects are reflectance irregularities in a bar code, also known as voids in the bars or spots in the spaces or quiet zones. A defect profile (Figure 9) looks for these reflectance variations in each element of the bar code. Element reflectance nonuniformity (ERN) is the difference between the highest and the lowest of the reflection values found in each element. To achieve an ANSI grade of A, the maximum ERN can be no more than 0.15; a grade of F is for an ERN greater than 0.30.

Decodability

Decodability is calculated differently for each type of symbology, and is a measure of the accuracy of the printed bar code compared against the appropriate reference decode algorithm. It measures the amount of tolerance remaining for decoding, re., that which was not used up by the printing process.

Now that we have taken a look at all the different ANSI parameters, we need to examine the difference between a scan grade, often referred to as the scan profile grade, and the overall symbol grade. A scan grade is achieved with one scan by a verifier and the collection of one scan reflectance profile. The lowest ANSI grade achieved after calculation of all the test parameters is the scan grade. If there is one C grade and all the others are A, the scan grade will be a C.

The overall symbol grade is based on multiple scans of a printed bar code (ANSI X3. 182 recommends 10 scans). Since the quality of a printed bar code can vary throughout the symbol, ANSI suggests the 10 scans be evenly distributed from the top to the bottom of the bar code. The average of these scans is the overall symbol grade.

A printed bar code with an ANSI grade of A should enable high scanner performance, achieving the highest possible first read rate. A scan grade of B will scan in a single pass most of the time. ANSI grade C can require multiple passes of the scanner, whereas a D grade may not be read by some scanners. A grade of F represents failure, meaning that the bar code did not meet ANSI print quality standards, and reading equipment may not be able to decode these symbols.