Why Do Barcodes Misread?
So much technology is involved with barcodes, from designing and printing them to scanning and decoding them, it is often overlooked that this is as much art as science. Sometimes there is not a readily evident technological explanation for why a barcode misreads—there is always a reason but it can be difficult to tease out into the light. In this sample, the right side human readables do not match the data encoded in the barcode.
Sometimes a barcode misread is the last stop on a path toward a barcode non-read. Usually this is caused by progressively deteriorating print quality during a print run–but not always. Ironically a barcode non-read can be the best possible outcome. Consider the ramifications of a barcode misread in the form of a barcode that scans and decodes perfectly—except that it is the wrong barcode. Yes this does happen and it is a train wreck. In a retail situation the store neither knows what it sold nor what to replenish. In other environments there are similar chasms of unknown. A non-read is a much better problem to have than a misread.
Of course there are instances where poor print quality is the obvious cause of a barcode problem. But poor print quality does not always cause a non-read. Differences in scanner aggressiveness can cause some scanners to substitute characters and misread; others may fail to read the symbol altogether.
Misreads in the form of a perfectly scanning but wrong barcode can be caused by scanner number substitution. Somehow the printed barcode image is wrongly decoded by the scanner. A poor quality printed barcode can contribute to this, but barcode structure also contributes. UPC is notoriously vulnerable to number substitution of 1 and 7, and 2 and 8 characters. This is because UPC is a modular symbology (learn more here) and the dimensional differences between the elements (bars or spaces) in these pairs is 1/13th of a module or .001” (at the nominal 100% magnification). The differences are even smaller in an 80% UPC. Usually (but not always) when a substitution error like this occurs, a check digit error also occurs, causing a decode failure.
This sample is a non-read which is the best possible problem to have. The cause: there is a bar missing on the right half of the symbol. There should be 30 bars in all, but here there are only 29. No scanner should decode this UPC.
Low ISO Decodability scores can contribute to barcode misreads, especially when they are caused by pixel rounding in the print process. This is most common in a thermal or thermal transfer printing but it can also occur in ink jet, DOD and other digital printing systems. Pixel rounding is caused by a resolution mismatch between the barcode design file and the printer: when the instructions being sent from the computer cannot be carried out by the printer, the printer relocates bars or spaces and/or modifies bar or space widths. This can confuse the scanner into misreading the modified pattern of bars and spaces.
Decodability due to pixel rounding might be the cause of this left-side mismatch. Each of encoded left side 3’s sit below their respective pattern of two bars and two spaces–but see how those two bar/space patterns are different. They should be identical.
Scanners can also misread otherwise acceptable barcodes. Most often the cause can be traced to damaged or dirty optics, but faulty decode algorithms area known cause, albeit a rare one. Older, high-mileage scanners such as lasers tend to be more prone to misreads than newer, more sophisticated digital area imager scanners.
Once a barcode misread has been detected, how can you isolate the cause? Here is another situation in which an ISO compliant verifier is an essential tool. Even in situations where a scanner misreads or fails to read a barcode, a verifier can often capture the data and analyze it. High quality verifiers can interpolate where decode takes place—they are not locked in to a single half-way point or global threshold reflectance level.
If a calibrated, ISO compliant verifier decodes and assigns an acceptable grade to a misreading barcode, one can be confident the cause lies with the scanner and not the barcode itself. This is yet another example of how a scanner cannot effectively serve as a verifier.
John helps companies resolve current barcode problems and avoid future barcode problems to stabilize and secure their supply chain and strengthen their trading partner relationships.