Printing Barcodes on Difficult Surfaces
Printing barcodes on difficult surfaces can be confusing because we don’t expect some substrates to be problematic. We received a call recently from a client who was printing 1D barcodes with a thermal transfer printer on what they described as a metallic silver label. We suspect it was a silver-colored mylar, but no matter—the barcodes were failing. Some of them weren’t even decoding. The client was mystified—the barcode was being imaged in a dense black on the silver label which they were convinced was highly reflective, which he described as “…equivalent to the reflectivity of white—probably better.”
Metallic or metal-emulating substrates such as this can behave differently that we assume. Sometimes we get caught in a disconnect between what we believe and what is actually happening with how light responds. The same can also be true of glass substrates or highly glossy inks (including black inks) on a glossy base material. Think about how a scanner works: light is bounced off the barcode surface; presumably dark areas (bars) reflect less light than light areas (quiet zones and spaces), allowing the scanner to decode the symbol.
But metallic substrates often do not reflect the light back to the scanner, as a white or light-colored material usually does. Instead of reflecting the light, metalized surfaces such as bare aluminum refract the light and scatter it in all directions. Very little light is reflected back to the scanner. If the metallic substrate is the background, it can behave as the dark bars rather than the expected light background. Symbol contrast will be downgraded, perhaps even fail. The barcode can be virtually invisible to the scanner.
Glass can behave similarly—even colored class. The human eye sees it as highly reflective, but in fact it behaves as refractive or absorptive of light, acting more like minimum reflective bars than as maximum reflective background (quiet zones and spaces).
What to do it you are printing barcodes on difficult surfaces such as metal or glass? There are three choices. Depending upon the refractive qualities of the base material, one solution would be to reverse print the barcode, leaving the bare substrate to represent the dark reflectance bars and print the light reflectance quiet zones and spaces—as is often done with soda pop cans. But be careful with this solution because the refractive behavior of the substrate must be uniform and not varying across the surface.
Before considering this possibility, it would be wise to make a static reflectance test of the substrate using a reflectance densitometer. What you are looking for is not only the reflectance value of the substrate, but also its uniformity over a wide area. If the material is uniformly reflective at a relatively low level (Rmin), calculate what the Symbol Contrast value would be if the barcode was reverse printed, printing the quiet zones and the spaces instead of the bars. Use the reflectance densitometer to take a high reflectance (Rmax) reading on the proposed white ink.
Symbol contrast is a simple subtraction of Rmin from Rmax and is graded as follows:
- 70% = A
- 55% = B
- 40% = C
- 20% = D
…and Less than 20% = F
Here is a static reflectance test report. This verifier even calculates the Symbol Contrast (see Details window, right side at the top).
Another solution is to print the barcode conventionally but first print a white or other acceptable light color background on the substrate. This can be a great solution but not without some considerations, including the added cost of a second color and the challenge of printing the barcode positioned accurately within the white patch.
Abandoning the challenge of printing the barcode on the challenging substrate altogether, a third choice is to apply a pre-printed label. This is also a viable solution but it brings additional cost both in the label and its application.