2D Symbol Verification
Those of you who are familiar with 1D barcode verification will be familiar with some of the ISO parameters in 2D verification, but there are also some aspects that will be very unfamiliar. These differences are even greater than one might think considering that 2D symbols can be printed in two very different ways. 2D symbols conventionally printed on labels and cartons share the same reflectivity and contrast characteristics as 1D barcodes. But 2D symbols can also be imaged directly onto metal and other relatively low reflectivity substrates using a technology called DPM or Direct Part Mark.
DPM requires the verifier to have special lighting in order to create as much reflective difference as possible between the symbol and the background. The idea is to optimize the opportunity for the verifier to decode the symbol, which captures the data essential for compiling the ISO evaluation and grade. DPM verification is not an ISO specification—it is an AIM specification that can be used in conjunction with some ISO parameters in order to test and grade the symbol.
AIM DPM verification does not test or grade Symbol Contract, Modulation or Reflectance Margin, for obvious reasons. However, AIM DPM verification uniquely tests and grades a parameter called Minimum Reflectance. This parameter examines whether the symbol’s Rmax or light values (typically the background) meet a minimum reflectance threshold in order to provide sufficient reflective difference between the background and the symbol itself.
The ISO specification for print quality of 2D symbols is ISO 15415. The parameters that both ISO and AIM DPM test and grade in evaluating 2D symbols are:
- ANU or Axial Nonuniformity which measures the difference in X and Y distortion of the 2D symbol and grades it to a tolerance;
- Decodability which measures the amount of tolerance remaining for the scanner once the symbol is in its final form and ready for scanning;
- Fixed Pattern Damage which measures the integrity of the non-data carrying standard features of the 2D symbol which help the scanner orient itself to the symbol position;
- GNU or Grid Nonuniformity which measures how accurately the elements or dots that comprise the symbol fall on a uniform grid pattern; and
- UEC or Unused Error Correction; unlike 1D barcodes that have error detection capability, 2D symbols have data correction If none of it is used to decode the symbol, the symbol is not downgraded; if some of it is used, the symbol is downgraded based on how much as a percentage is used.
The common characteristic of 2D verification to 1D verification is its rationale: to predict the likelihood that the symbol will successfully scan in its intended usage, whether that be identifying a shipping container in a retail supply chain, a consumer item at point of sale or a drug at bedside dosing in a hospital. Barcode performance is important because bad barcodes are no longer just a blemish on the customer experience; they can be a matter of life and death.