by Scott Sabreen
Question: We presently print and read data matrix codes on several parts that we manufacture. If the codes are readable, can we assume they will pass verification? Which verification standard do you suggest?
Answer: A common misconception is that if a 2-dimensional data matrix code is “readable” then it also is presumed “verified” as a passable code. This is not true! A readable data matrix code by one person does not guarantee that another will be able to read the same code using an identical or dissimilar reader device. Reading is not the same as verification. Proper lighting is important to achieve robust read-verification. Using special lighting and software on verification systems provides statistical measurements which help ensure that the code can be properly read by other data matrix readers.
|These markings illustrate how proper lighting illuminates the entire symbol and delivers enough contrast for the reader to distinguish between light and dark data cells for readability. In this case, dome lighting (one of the 10 possible illumination techniques) makes this symbol readable.|
Statistical measurement provides important information. As an example, one photographer can take a picture using lighting and proper angles to produce a flattering image. Another photographer, however, can change the lighting and angles to yield unflattering results. While the figure in the image has not changed, the lighting and angles have altered the quality of the appearance. The same principles apply with data matrix codes, wherein different lighting brings out certain features while hiding others.
The current 2D data matrix verification standard published by AIM Global, the AIM Direct Part Mark Quality Guide (AIM DPM-1-2006), states that the proper verification of Direct Part Marks (DPM) requires multiple illumination techniques. These are needed because parts have different surface characteristics, texture and gloss which reflect light back to the camera differently. The verification standard requires illumination using 10 different lighting angles to cover all surface possibilities. Curved items can be particularly difficult to light uniformly for readability.
AIM Verification standards, which are part of the Data Matrix specifications, measure print contrast, modulation, axial non-uniformity and unused error correction to grade marks on an A to F scale. A grade of A is excellent and a grade of F is failure. AIM DPM-1-2006 now is Verification Standard in MIL STD 130N.
The quality metrics when using the AIM standard as a basis for verification are defined in Section 8 and Annex N of the International Symbology Specification — Data Matrix (ISO 16022). When using the AIM standard, each of the quality metrics that are defined in the table below yield a “grade” of “A” (4.0) through “F” (0.0). The overall quality of a symbol is the lowest grade achieved by any of the tests.
Data Matrix Verification is highly recommended to extend tracking through the full life of a part so that it can be identified from the beginning of its life to the end, and to identify high-value parts that are subject to theft or counterfeiting. Verification reduces costs associated with rejected parts due to unreadable codes. If a part loses its identity due to the quality of the mark, then the part cannot be used. Verification equipment systems can also detect problems with marking machines which could be due to poor part fixturing, damage to the marking machine (such as a broken tip on a dot peen system) or incorrect settings during part changeover.
Scott Sabreen is the founder and president of The Sabreen Group, Inc., a plastics engineering consulting firm. He is a board member for the Society of Plastics Engineers Decorating/Assembly Division, technical editor for Plastics Decorating and expert engineer for Omnexus/SpecialChem, Intota-Guideline and Nerac. Sabreen may be reached via email at firstname.lastname@example.org.