When a Verified Barcode Fails to Scan–What To Do
What about when the verifier reads the barcode but the scanner does not? Now you are really in the dark! How do you figure out what’s wrong? Here are some points that can help you sort it out.
If the verifier can read the barcode and grades it an ANSI F, all bets are off about the likelihood it will scan. An F grade does not “guarantee” that the barcode will fail to scan in all circumstances and all possible scanners. But it indicates a likelihood of scanning problems. Likewise a D grade. Some scanners will read the barcode, others will not. The ISO standard for barcode verification are a method for predicting likely scanner performance based on key print quality parameters. Some scanners are more forgiving than others, some are less forgiving for some of those parameters. Scanner performance degrades over time. Rough handling or challenging environments influence scanner performance. There are a lot of variables.
When a verifier fails a barcode, obviously it successfully scanned the barcode to grade it. If the verifier can scan the barcode, why would a scanner be unable to scan it? Verifiers have the unique ability to decode barcodes that fail certain ISO parameters. Verifier software manipulates how the verifier decodes the barcode. This specialized ability makes a verifier so useful and important.
The ISO verification standards are not a perfect judge of barcode performance with so many unanticipated and uncontrollable variables. But there is no better way of predicting whether or not a barcode will scan.
A verifier could grade a barcode a C or better, but a scanner could not read it. What can cause that?
Consider this: barcode scanning relies on the reflective differences between the barcode and its background. Think about how different scanner technologies detect the reflective differences:
- A laser scanner draws a very thin, bright line across the barcode, and the reflective differences between the bars and spaces of the barcode are reflected back into the receptor in a very directional way.
- CCD and digital camera-based scanners are different than laser scanners. They flood the barcode with a blast of relatively diffuse light. Camera verfiers detect the reflective differences between the bars and spaces using an algorithm that captures and processes the digital image file as a whole. Lasers detect them as a linear timing sequence. A CCD array captures reflective differences with a row of light-sensitive pixels that detect the reflective differences sequentially, somewhat like a laser but without moving parts.
Usage factors and environmental variables can also influence scanning:
- The angle and distance of the scanner from the barcode will influence the high and low reflectance values of light reflecting back into the scanner sensor
- Ambient light can influence the levels of light reflected back to the sensor
- Seasonal, process-related or other lighting factors, such as intense side-lighting from an adjacent window can affect scanning
- barcodes printed over glossy substrates can cause scanning problems, or barcodes printed in a glossy black
- lamination, shrink wrap or other membranes installed over the barcode after verification can cause unexpected problems
Some scanning problems are not caused by the scanner optics or the reflectivity of the barcode. Scanner programming could be the culprit. When setting up a scanning system, limiting the number of digits that can be accepted in the barcode data field can cause an A grade barcode to not scan.
Symbologies with optional check digits, such as ITF (Interleaved Two-of-Five) and Code 39 can be an unexpected trap. Likewise, EAN barcodes that should be compatible with a 12 digit UPC-friendly scanning system—but encode a 13th digit that a scanning system may not be programmed to accept. If the verifier says the barcodes are good but the scanner cannot read them, check the scanner configuration for barcode data field size and check digit.
Finally, barcodes can be legitimately printed with an X dimension that is incompatible with the scanner. The X dimension is the width of the narrow element (bar or dot) in a barcode. It is considered the “building block” of a symbology, and the scanner must have an aperture small enough to accommodate it. Most commonly this can occur with a high density barcode. A mid-range or long distance scanner will have an aperture capable of reading barcodes with X dimensions of 40 mils (.040″) or even larger, but not barcodes with smaller X dimensions, which can be as small as 10 mils and even smaller. They could be high quality barcodes from an ISO standards perspective and still fail to scan.
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