Understanding Modulation in the Barcode Verification Report
Recently we wrote about the ISO parameter Decodability. Now we will discuss Modulation, which is also somewhat difficult to understand. But first we must understand the various ways in which reflectivity is measured and graded in verification.
Modulation grades the variability in light and dark reflectance values
Minimum Edge Contrast is an expression of the contrast difference in every transition, from light to dark and back to light, across the barcode. Obviously the more contrast difference, the better, so lower (lesser) contrast differences will result in a lower verification grade for the parameter Minimum Edge Contrast—less than 15% will grade (ISO) 0.00 or (ANSI) F.
Symbol Contrast is the mathematical difference between the lowest reflectance value (RMin) and the highest reflectance value (RMax). Symbol Contrast does not take into consideration reflective differences in either Rmin or RMax values—it just measures the difference between the lowest and the highest reflectance values present.
Understanding Minimum Edge Contrast will help to understand Modulation, which is an expression of Minimum Edge Contrast as a ratio of Symbol Contrast. The word “modulation” describes differences in reflectivity, whether lower values such as bars or higher values such as spaces. In a perfect world, the dark reflectance values would all be the same; likewise light reflectance values. But in the real world they are not—and modulation is a measurement and a grade based on the magnitude of the reflective difference. If, for example, Minimum Edge Contrast is only 40% of Symbol Contrast, the resulting grade for Modulation would be (ISO) 0.00 or (ANSI) F.
What sorts of conditions aggravate Modulation? What sorts of conditions cause light or dark reflectance values to be variable? Printing a barcode against a screened, patterned or gradient background is a common mistake that causes problems with modulation. Printing the barcode on a transparent or translucent material, like a bag allows anything behind the barcode such as the product in the bag, to influence the reflectance modulation characteristics of the barcode—and cause it to fail.
The most common cause of light reflectance modulation problems is excessive bar or press gain. Excessively spread or “gained” bars will have a more profoundly negative effect on the reflectivity of narrow spaces than wide spaces, causing variation in their light reflectance values.
Excessive average bar gain is the most common cause of modulation problems
Dark reflectance values are not immune to modulation problems. If the barcode is printed in a marginal dark color such as one with high red content, narrow and wide bars can reflect differently, causing modulation downgrading. Likewise, ink which is not opaque or which is not uniformly opaque can cause modulation issues.
Ink that doesn’t “play nice” with the substrate can also cause modulation problems, if it doesn’t bond well to the surface or, worse still, if it bonds unevenly due to absorption variability. Sometimes this can be caused by a substrate that has been improperly stored or prepared, and is not clean or which has been contaminated with oils or solvents.
Look for causes such as these when verification results are downgraded due to the ISO parameter Modulation.
Your comments or experiences are always welcome.