Of all the ANSI/ISO parameters, decodability is the one that confounds and confuses most people. Attempts to define it in simple terms (“Decodability is the amount of tolerance left for the scanner after prepress and on-press operations.”) are unsatisfactory and unhelpful. Decodability is a complex parameter and will not yield to oversimplified explanations.
Decodability is the most difficult ANSI/ISO parameter to understand
Unlike many of the other parameters, decodability is not derived from reflectivity—it is calculated from measurements of the bars and spaces that form the encoded characters. Decodability grading is derived from how far these elements have deviated from their ideal measurements, in comparison to the decode formula or engineering specification for the symbology in question. Thus, decodability will be evaluated differently for different symbologies. UPC is graded differently than, for example, Code 128 or ITF14.
Decodability is determined differently for different symbologies
Decode formulas for symbologies like UPC work primarily on bar-to-bar measurements: the left edge of a bar to the left edge of the next bar, and so forth. This affectively cancels any influence of press gain since fattened bars steal their extra real estate from the spaces. The decode formula for UPC works on measurements of both bars in a character, making UPC is sensitive to press gain.
The decode formula for UPC varies in sensitivity to specific characters. For the characters 0, 3, 4, 5, 6 and 9, the decode formula requires measurement from left edge-to left edge of bars as described above, and press gain is neutralized.
But with UPC characters 1, 2, 7 and 8 the decode formula requires measurement of the width of both bars, from left edge of the first bar to the right edge of the second bar, because this is the only way these encoded characters can be distinguished from each other. Measured in this way, these encoded characters are sensitive to press gain, making UPC particularly susceptible to lower decodability grades.
When the decodability parameter in a UPC is downgraded, it usually occurs in a 1, 2, 7 or 8 character.
ITF14 and other symbologies with only two possible element widths use decode formulas based on left edge-to-left edge bar measurements. These symbologies are less vulnerable to press gain. This is one reason that ITF14 is a very forgiving symbology for printing of barcodes on corrugated.
UPC is uniquely vulnerable to decodability problems
Press gain is not the only reason for decodability problems, but it is undoubtedly the most frequent cause. Another cause is a mismatch of resolution settings the barcode design software and the printer. If the printer cannot print the barcode at its design resolution setting, it ”interpolates” those settings by relocating the bars to the nearest whole pixel, which will change the width of adjacent spaces. This is not unlike press gain but much more complicated and difficult to diagnose.
If you have questions other experiences with decodability, or want to suggest other topics you’d like to see addressed here, feel free to comment.