Minimum Reflectance and Symbol Contrast

 In 301

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.

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Showing 6 comments
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    Which is better for print contrast in barcode scanner 30% reflectance or 20%

    • Avatar
      John Nachtrieb

      I do not understand your question. The print contrast is measured and reported in the barcode, not the scanner. It is measured and graded as Symbol Contrast, which is the high reflectance (background) value minus the low reflectance (bar) value. A Symbol Contrast of 20% or less is an ANSI F grade. The Symbol Contrast threshold to a C grade is 40%, so 30% is a D grade, still quite poor. I guess you could say that it is better.

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    What is the best scanner, a scanner that can read the contrast of 10 % or 30%?

    I’m trying to buy a scanner that can be code sensitive and cheap

    • Avatar
      John Nachtrieb

      Artur: It would depend on what kind of barcode (1D or 2D), why the contrast is so low and how the barcode is printed. In general you might find success with a DPM scanner such as the Datalogic Powerscan PD9500. Forewarning: don’t expect to find low contrast DPM scanning and a cheap price. You can get one or the other but not both.

  • Avatar
    Mithun Anirudhan

    I have two scanner specs with me one is Symbol LS2208 which shows : (Print Contrast: 20% minimum reflective difference) and second one is Symbol LI2208 which shows :(Print Contrast: 15% minimum reflective difference) so according to you which will be the better performing scanner LS2208 or LI2208? My understanding is that LI2208 will be better as its MRD is 15%. Am I right?

    • Avatar
      John Nachtrieb

      Mithun: I am more familiar with verifiers than with scanners, but here are my thoughts. Firstly whatever scanner you use should be compliant to ISO15426 (-1 for linear barcodes and -2 for 2D symbol such as Data Matrix). MRD is one of several opto-electronic performance attributes. Others include edge detection and scanning aperture. There are important ISO physical performance requirements such as power supply, ambient operating temperature and humidity range and sensitivity to ambient light. On the data-processing side, scanner performance is also a function of decode algorithm. Overly aggressive decode algorithms may incorrectly decode barcodes; hyper-conservative algorithms may fail to decode ISO compliant barcodes. Scanners are tested against these attributes using primary reference test symbols. Claiming “superior scanning performance” to a single operating parameter such as MRD is an over-simplification in my opinion.

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