Using the Verification Report to Improve 2D Barcode Quality
In a recent article we talked about using the verification report to improve 1D barcode quality. Doing the same for 2D symbols is not altogether different—mostly the nomenclature is different. Many of the ISO15415 parameters for 2D symbols will be familiar to someone who has worked with linear barcodes.
Matrix symbols such as Data Matrix will nearly always have concerns about data structure, whereas it will only be an issue—or will be a much simpler issue—with some 1D barcodes. While data structure is not parameter in the ISO specification for either type of barcode, it is an important consideration in overall symbol quality. A perfectly printed symbol but with data format errors will fail just as quickly as a poorly printed symbol—in fact it could be worse than a symbol that fails to scan altogether.
The ISO specification for 2D symbols is also concerned with the parameter Symbol Contrast, but this is a very different issue than with 1D barcodes, where the operant assumption has been that the scanner uses 660nm laser light. While most commercial and industrial camera-based scanners also use red spectrum light for 2D symbols, in many cases the “scanner” is a smart phone camera using ambient lighting or sometimes a white spectrum flash. Consequently the concern in 2D symbols is reflectance difference between the symbol and substrate.
Because of its two-dimensional structure, matrix symbols are also subject to axial distortion that can render them unreadable. Symbols that are differentially distorted in the X and T axes are downgraded, as are symbols whose elements do not uniformly fall on grid lines. Such distortions are often the result of attempting to print the 2D symbol as the substrate (carton or package) moves at too high a speed past the inkjet printer, not unlike those distorted, forward leaning images of fast moving cars or trains in early photographs.
Fixed pattern damage is an ISO parameter that covers several 2D symbol attributes. Whether a QR Code or a Data Matrix Code, 2D symbols contain certain features that are always present, attributes such as finder patterns or targets and clock track patterns that help the scanner orient itself to the symbol. Quiet Zone is also a fixed pattern and yes, 2D symbols do have quiet zones. QR Code is particularly confusing in this regard since there is an informal acceptance of a smaller, non-spec quiet zone on symbols meant for smart phone scanning. Some verifier manufacturers allow for this in their device configuration area, others simply accept a technically violated quiet zone as acceptable.
Unlike linear barcodes which can have error detection capability through use of check digits, 2D symbols are capable of error correction using algorithms such as the Reed Solomon method. The symbol is downgraded when a significant percentage of the error correction capability is used. Numerous factors can contribute to this. One of the more common with QR Code is the unwise practice of customizing the symbol by imposing logos or other graphics within the symbol. This self-inflicted wound ends up having the opposite effect of what is intended: rather than making it more likely that the QR Code will be scanned by a prospect, it cripples or even kills it.