Verifying 2D symbols: A Brief History
For those of us familiar with verifying 1D barcodes like UPC and Code 128, migrating into 2D symbols like QR Code or Data Matrix seems very complicated. Why is that? With 1D barcodes it was a fairly straightforward process, starting with the Uniform Code Council’s Universal Product Code Symbol Specification and UPC Verification Manuals (1970’s). Soon the American National Standards Institute got interested and in 1982 began studying bar code quality. Nine years later in 1990 ANSI X3-182 was published. Recognizing the importance of barcoding technology, the emergence of a global marketplace and the need for standards, the European organization CEN and the ISO soon created their own documents. In 1995 ISO/IEC published EN 1635 “Bar Code Print Quality Test Specification” and in 2000, ISO/IEC published ISO 15416. Each of these adapts and develops the specification in response to a growing and evolving set of needs and applications. This is a very organic process.
The history of the development of 2D symbol printed quality standards has been equally evolutionary, but there have been more steps in a shorter period of time. Many of the evolutionary steps have included imaging (of Data Matrix Codes) with dots—such as dot peen. Other alternative marking technologies include laser marking, electro-chemical etch and ink jet.
SAE AS9132 (2002) was developed by the Society of Automotive Engineers and is an aerospace standard (AS). It specifies six print quality and technical parameters for Data Matrix symbols that are direct part marked—specifically dot peen marking. Those parameters are:
- Quiet Zone: an unmarked perimeter of at least one element wide
- Symbol Contrast: the reflective difference of light and dark areas
- Cell Fill: akin to press or dot gain
- Dot Center Offset: element placement error or inaccuracy
- Dot Ovality: degree of non-roundness
- Angle of Distortion: an early version of Grid Non Uniformity
AS9132 failed to specify a methodology—much like Traditional Verification did in the early days of 1D barcodes. Also it imposed tolerances too stringent for laser etch imaging and does not test for other attributes that could cause the symbol of fail. Because of its focus on dot peen marking, AS9132 has become a Unique Item Identifier coding standard for dot peen marking.
ISO 16022 (2006) covers Data Matrix Code only, and defines print or marking method, code structure and decode algorithms. ISO 16022 was never intended to be a verification specification.
AIM DPM (2006) addresses the special and specific needs of direct part mark (DPM) of Data Matrix Codes onto a surface in such a way as to produce a sufficient reflectance difference as to produce a legible symbol. This was the first attempt at producing a complete quality system for DPM imaging. It differs from all other 2D specifications by addressing the very unique challenges of obtaining the image by optimizing the camera and lighting positioning as well as varying the type of lighting to include diffuse 90 degree lighting.
ISO 15415 (2011) expands upon ISO 16022. It uses the same grading parameters and adds Modulation, Grid Non Uniformity and Fixed Pattern Damage. But it does not provide for the special imaging requirements of DPM, causing good symbols to fail. Furthermore ISO 15415 does not accommodate codes created from dots and requires a 5 scan average to produce a grade.
ISO 15415 assumes the symbol is always marked in black over a white background, and requires white reflectance calibration of the verifier. It allows for only one lighting arrangement and does not accommodate the numerous marking methods, materials and non-white reflectance conditions in which Data Matrix Codes are often used. In short, ISO 15415 is too simplistic to encompass the wide (and widening) ways in which Data Matrix Code is used.
To summarize, verifying 2D symbols is much more complicated than verifying 1D barcodes because the methods of marking and reflective differences of the mark and its substrate are so widely varied and in some cases so minimal. As with 1D barcodes, the specifications for 2D symbols have evolved based on the needs of the industries and applications where they are used.