Minimum Reflectance and Symbol Contrast
Minimum Reflectance and Symbol Contrast are tested by taking reflectance readings from the symbol—specifically the maximum reflectance value and the minimum reflectance value, and applying a calculation to those values.
Minimum Reflectance is a Pass/Fail parameter
A barcode image must have a minimum amount of reflective difference between the barcode and the background for the scanner to be able to see it at all. If the background is too dark or the barcode is too light, the scanner will struggle or fail to see it.
Minimum Reflectance (MR) is a Pass/Fail parameter. The formula is as follows:
PASS = MR that is less than or equal to half
the maximum reflectance
FAIL = MR is greater than half the maximum reflectance
Another way to say this is, the reflectance of the bar must be equal to or less than half the reflectance of the background.
As you can see this is not an absolute scale. The minimum and maximum reflectance values are interrelated. If the minimum reflectance parameter is failing, it can be improved by making the dark bars darker and less reflective or making the substrate lighter and more reflective.
Symbol Contrast (SC) is a graded parameter, not a pass/fail parameter. It is the simplest parameter to calculate. The formula is as follows:
SC= maximum reflectance minus minimum reflectance.
Symbol Contrast is an A-F graded parameter
Grading is as follows:
A = SC of greater than or equal to 70%
B = SC of greater than or equal to 55%
C = SC of greater than or equal to 40%
D = SC of greater than or equal to 20%
F = SC of less than 20%
SC is a straight and simple subtraction. Whatever is the remainder is the grade according to the scale above.
SC can be improved the same way Minimum Reflectance is improved—make the bars darker or the spaces lighter.
Note that a high gloss or shiny surface does not necessarily result in higher reflectivity. Glossy surfaces often scatter the light and this does not reflect more light back into the scanner.
These parameters are often at play in corrugated containers, where ITF-14 or other barcodes are printed onto the bare kraft. The unbleached, brown substrate which should provide the maximum reflectance value actually has a relatively low reflectance value. This makes it imperative that the minimum reflectance value–the bars in the barcode–be as dark and dense as possible, but not excessively spread: not an easy task on the highly absorbent kraft.
Are you still with me? It will be helpful to you when we turn our attention to improving print quality.
Hang in there—it will be worth it.