DPM Barcode Verification
Direct Part Mark or DPM barcodes pose a special set of challenges for verification. As the name implies, DPM barcodes are imaged directly onto a (usually) metal substrate using a variety of marking techniques. This alone presents challenges since both the method of marking can vary and the substrate can also vary. Added to that is the very low contrast difference between the barcode symbol and the substrate or background. Because the reflectance values are very similar, the DPM verification standard differs from the ISO15415 specification used to verify 2D symbols printed on paper labels and product packaging.
The standard used to verify DPM barcodes is the AIM-DPM standard (ISO29158), which tests the same structural attributes of the symbol as does ISO15415 except for Symbol Contrast. Instead, the AIM-DPM standard tests and grades Cell Contrast, Cell Modulation and Reflectance Margin, but of course any reflectance reading is highly dependent upon capturing the image of the barcode. The AIM-DPM specification as created by a committee of barcode industry leaders, many of them scanner manufacturers who are very familiar with the challenges in this technology.
Very special lighting is required to enable the verifier to capture and decode the barcode so that it can generate a verification report. A verifier could have 10 or more different lighting configurations—all of this just to capture the image in order to process the data and generate a verification report and a grade. And scanner manufacturers are routinely working on yet other lighting configurations not as yet covered by the AIM-DPM standard in order to successfully decode DPM symbols.
Occasionally we are told by a manufacturer or end user that they are able to successfully scan a DPM symbol with a smart phone when their regular scanner or even their verifier cannot, and they are left to wonder what they should believe. This points to one of the greatest challenges in DPM verification: relating the verification report to the performance of the end-user’s scanner. This, after all, is the whole idea behind an industry standard: to establish an internationally agreed-upon testing protocol that enables trading partners to predict, with some degree of accuracy and confident, that a barcode symbol will decode successfully no matter where or with what scanner it is being read.
To do this requires a verifier. It cannot be done with anything else. But how strong is the implied connection between a verification grade and the performance of that barcode no matter where it goes and what is used to scan it? The same question pertains to any barcode including conventionally printed barcodes entering the retail supply chain. But the verifier-to-scanner implied connection is very much challenged with DPM barcodes because there are some any variables.
Does this suggest that verification s less meaningful in direct part marking? Not at all. While DPM scanner technology continues to advance with more aggressive decode algorithms and lighting options, the need to accurately predict DPM barcode performance remains an important capability in preserving supply chain integrity.
The AIM-DPM standard will also continue to evolve, but more slowly and with arguably more rigor than the pace of scanner technology advancements. This is as it should be.