Modulation and decodability are easy to understand if you are familiar with the Scan Reflectance Profile or SRP. This is discussed in previous articles in this blog. The Scan Reflectance Profile is a powerful diagnostic tool for gauging what is going on with a barcode symbol.
The SRP makes it easier to talk about reflectivity attributes of the ANSI grading system.
Modulation grades well when the Scan Reflectance Profile is symmetrical
Modulation is beautifully illustrated graphically with the SRP. The north-to-south symmetry of the SRP is Modulation.
The modulation graph should be about six increments tall. The equatorial line is called the global threshold or GT and each bar and space must penetrate the GT for the scanner to count the elements and determine the symbology and decode.
Modulation is downgraded when the SRP is asymmetrical. When modulation problems become extreme, some elements (bars or spaces) will fail to penetrate the GT.
Modulation is downgraded most often due to excessive gain or spread of the barcode image. Several factors contribute to excessive gain or spread, including ink thickness or thinness, characteristics of the substrate and roller pressure. Modulation can also be impacted by a highly shiny or glossy ink or substrate.
Modulation probems are usually due to press gain
This as a good time to talk about Defects. This can also cause modulation problems. Now that you’re familiar with the SRP you can easily see how Defects throw a wrench into the works of a scanner.
Defects in a barcode are either voids in the bars or artifacts in the spaces. They could be caused by dirt or oils on the substrate that cause the ink not to adhere or particulate in the printing environment. When the scanner does the bar-space count at the GT, a void in a wide bar could be counted as two narrow bars; an artifact in a space could be counted as an extra bar. This can contribute to a modulation problem if a defect is detected but doesn’t intersect with the GT. Either way it throws the count off, making it impossible for the scanner to determine what the symbol is.
Modulation is tested and graded based on the same formula for every symbology. Decodability is calculated differently for each symbology. UPC uses a different formula than Code 128. In general, decodability is a measurement of the accuracy of the printed symbol as compared to its engineering specification—in other words, to its perfect form.
Decodability grades the symbol according to how much of the engineering tolerance (margin of error) is left for the scanner, after all the design, pre-press and on-press and post production activity (such as product packaging, shrink wrapping, etc.
Modulation measures reflectance; Decodability measures tolerance available for the scanner
Whereas modulation is a matter of reflectance, decodabiity can be a much more difficult fo figure out. Design mistakes can be a subtle and stealthy cause of decodability problems. A barcode that is designed at a resolution that is incompatible with the resolution of the output device (printer) it can cause bars and spaces locations and widths to be interpolated—moved around or re-sized. You can imagine how this messes up the engineering of the barcode.
ANSI-compliant verification and grading of barcodes—as comprehensive as it is—still does not cover all the bases when it comes to controlling barcode-related risk. There are other factors that can bite you. For example, what could possibly go wrong when a barcode receives a consistent ANSI A grade—but still fails at point-of-sale?
We will discuss some of these in future postings.
Please comment, ask questions and add stories of your own experiences.
