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Why is my barcode failing for Modulation and Reflectance Margin?


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Issue:
When using an LVS-95XX Series barcode verifier, a user may come across labels that produce significantly lower grade scores that exhibit two specific symptoms:
1. Modulation and Reflectance Margin grades dramatically lower than most other graded criteria.
2. Modulation and Reflectance Margin grades that are exactly or nearly identical to each other.
While these symptoms may still be present, a different grade may even be generated if the label is graded on another LVS-95XX model, such as an LVS-9510 vs. an LVS-9580. Labels exhibiting these symptoms are most likely using a problematic label substrate material.
Explanation:
The reason for this is that ISO standards were written with the expectation of high contrast diffusely reflecting labels being under inspection. In other words, dark print on white (matte finish) background. To illustrate, the images below come from a 3.0 FOV 5MP 9580 (left) and a 3.0 FOV 5MP 9510 (right) to ensure consistent perspective and remove issues like resolution, focal length, window size, etc. All those parameters are essentially made uniform using the same camera and FOV. Look at the image below of a standard non-glossy, diffusely reflecting high contrast label as defined under typical ISO circumstances. Notice how the images look very much the same from both verifiers. The grading result from both verifiers is essentially the same also.
 

image.png.dce22a347b5fc81055284827c086e054.png

Now, notice in the images of labels using silver reflective paper below how the 9510 lighting
configuration gives the label an overall darker appearance (while the liner/backing remains
more consistent in color with the 9580), whereas the label image from the 9580 is brighter and
more uniform. Also notice how dark the white cells appear in the 9510 images compared to the
9580 image. The darker image means lower maximum reflectance, lower global threshold, and
lower cell modulation resulting in a lower overall grade, most often a result of the symptoms
outlined at the start of this technical note. The paper color appears considerably darker as it
reflects light back into the camera altering the appearance of the paper. Reflective materials
play “tricks” with the light and change appearances based on the angle of incidence of light.
This can be seen also in symbols printed directly on fibrous materials such as Tyvek, where the
large fibers can also interact with the light.

image.png.e3ed6fea14e2aec11c5b840f165ba358.png

ISO15415 states the following about glossy substrates:
“The reference illumination conditions specified for the measurement of reflectance should enable the maximum rejection of specular reflection while giving a representative assessment of the diffuse reflectance of the symbol and substrate. Highly glossy materials and those whose diffuse reflectance characteristics vary with the angle of incident and/or collected light - as may be the case with many materials on which symbols are directly marked - may yield grades differing from those obtained by the use of the reference optical arrangement with illumination at 45°, and for this reason sub-clause 7.3.4 provides alternative angles of illumination to enable apparent symbol contrast to be maximized.”
Using reflective labels borders on the realm of DPM as metallic parts produce highly spectral reflections also. This deviation is related to the physics of light and its interaction with the substrate. These images and accompanying explanation are presented to illustrate the impact of label material selections that deviate from what is outlined in the relevant ISO standards as required when grading a barcode using these ISO standards.
Possible Solutions:
1. Whenever possible, select a label substrate material and print combination that is high contrast and diffusely reflecting, i.e. dark print on white (matte finish) background.
2. If (1) is not possible, be aware that the labels no longer strictly adhere to the anticipated label specs outlined by ISO earlier in this technical note. As ISO-compliant barcode verifiers (such as the LVS-95XX Series) must adhere to the state design guidelines, there are no adjustments that can be made to the verifier to improve grades. Verifiers are, by definition, intended to be a fixed, “known quantity”.

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