UDI Barcode Challenges: Difficult Substrates

 In 101

Some substrates can be very challenging for barcodes, and the UDI requirement has introduced some heretofore unseen situations. Recently we were invited to a company that prints linear barcodes on those clear plastic drip bags that hang at hospital bedsides, infusing a patient with a solution through the IV. Sounds straightforward enough, but it is surprisingly challenging.

White on Clear Hydration Bag Barcode CROPPEDNot unlike the world of barcodes on aluminum soda cans, some barcodes on infusion bags are reverse printed, treating the bare bag as the low reflectance value and printing the quiet zones and spaces in white. While this sounds like a solid solution, it doesn’t consistently work well.

Recently we were invited to a manufacturer doing the exact opposite: printing black barcodes on infusion bags, treating the bare substrate as the high reflectance value. And here again, it doesn’t consistently work well. How could this be? Shouldn’t one method or the other work well?

First, what constitutes “working well’? At the company we recently visited, the scanners on their vision system read their black-on-clear barcodes perfectly but their verifiers were often unable to decode them and when they did, they always failed. Which would you believe to predict how the barcodes would perform out in the world?Black on Clear Barcode CROPPED

We don’t have manufacturer insight into the white-on-clear barcodes, but the nursing staff at a local hospital says they successfully scan some of the time. Our verifiers are unable to read them. In other words, either method of printing barcodes on the bare bag fails. What’s going on here?

Haemonetics SRP CROPPED

Notice the fairly consistent high reflectivity of the left quiet zone, and the much lower and inconsistent reflectivity of the narrow, medium space and wide spaces. This is because the adjacent bars restrict the return signal from the semi-refractive clear plastic substrate.

The problem is obviously the plastic bag, but why does it behave so inconsistently? The answer is subtle—and interesting. The plastic material, like an aluminum soda can, is neither entirely reflective nor refractive. It is a little of both and there lies the problem—the beginning of the problem. Here’s the really interesting part. When adjacent bars and spaces are present, the amount of direct-reflected light coming back into the scanner (or verifier) we be less—significantly less—than in areas where adjacent bars and spaces are not present—specifically the quiet zones.

No doubt a materials expert could explain why occurs: possibly having to do with the thickness of the material, or specular properties of the material. The verification report and in particular, the Scan Reflectance Profile, shows exactly what is happening and why these barcodes fail.  The reflectance values for the bare substrate will be extremely variable, and this reflectance difference will trigger the Modulation parameter and cause the barcode to fail.

What is the solution? Pretty simple really. Print a white background over which the barcode can be printed in black, or if you really insist on reverse printing, print a black background before printing the quiet zones and spaces in white. Or, affix a pre-printed thermal or thermal transfer label. But don’t rely on the bare plastic substrate—its spectral behavior is too variable.

Please share your comments or experiences.


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