Wednesday, October 19, 2011

Verifying datamatrix codes

An engineer called in a panic. His customer had rejected a delivery because they couldn’t read the datamatrix code, yet when he ran the offending parts through his system he got good reads every time.

This is an all too common problem and it boils down to the difference between “reading” and “verifying”. “Reading” happens when you point the scanner at the code and it outputs a character string. The problem is that the customer almost certainly does this at a different working distance and with different lighting, so what works under your ideal conditions may fail in his suboptimal environment.

It was to overcome this that “verifying” was introduced. Verification is done under standardized conditions and with the objective of assessing the quality of the marking. I sent the panicked engineer a link to a useful article explaining the difference, and if you’re interested I suggest you look at it too. (I would cut ‘n’ paste the relevant paragraphs but was too intimidated by the threatening copyright notice.)

If you click the link above you’ll find that the article references various standards, most notably ISO 15415. This provides a standard for code verification but unfortunately there is a problem. As explained in “New standard improves verification of Data Matrix codes” (Test & Measurement World, February 2007,) the ISO standard is geared to black codes printed on white paper. It doesn’t work quite so well with codes laser-marked on metal surfaces. And that’s where AS9132 comes in to play.

But putting aside details of which standard to follow, the point remains: the engineer and his customer needed to agree a common way to assess code quality. Without this, arguments over readability, or lack thereof, devolve into shouting contests that no one will ever win.

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