Rarely Asked Questions
Rarely Asked Questions
If you have a rarely asked question that is not listed below, email an engineer here at Webscan.
What is the effect of putting my film on a black, gray, or white background?
How often should I replace my calibration card?
Why do the calibration cards expire after two years?
What would happen if I continue to use my calibration card after it expires?
How can I tell if my expired calibration card yielded valid calibrations when I used it?
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What is the effect of putting my film on a black, gray, or white background?
The translucency of white ink on clear film results in the background having a large influence on the measurement of contrast (and to some extent modulation).
A dark background, such as black or clear glass with a large enough distance between the glass and anything reflective will eliminate any reflection from the background. This would result in a measurement that was only dependent on the white ink.
Some people use a background that is intended to simulate, or match, the color of the item directly behind the film, when the final product is wrapped in the film. For example, using white to simulate white paper towels would be a reasonable thing to do.
Using a neutral gray color may be intended to fill in for an unknown color. On the other hand, I believe that using a black background (or equivalently glass with a gap behind it) is the best and safest testing technique for clear film using white ink as a background for the bar code.
We have customer that is asking for a reverse code. It appears that the bars are the light color and the spaces are the dark color. We have not done this. Is it possible? Do the scanners then recognize the dark area as quite space? If this is possible; would we then just need to adjust the reduction for codes like this?
Most bar code symbologies specify which elements should be light and which should be dark. For example, a UPC code begins with a light quiet zone followed by a guard bar pattern which begins with a bar. Since most bar codes are printed on light colored backgrounds they usually begin and end with a dark bar.
Scanners will generally not read a bar code if it is printed in inverse colors. Of course, the inverse colors are sometimes encountered in pre-press on negative films and our verifiers can be set to recognize a bar code that is printed in reverse colors. But, such a bar code, if having reverse colors in its final form, would be incorrect and not readable by most scanners.
When printing on a dark background, it is necessary and correct to print the light elements (the spaces) with a light colored ink. Then, the dark background serves as the bars. This would not be a reverse color bar code; only the process of producing it is opposite of the typical process.
If your customer has a special reason to print the bar code in reverse, they should understand that such a bar code is not in conformance with industry standards and will not be read by most scanners. Perhaps that is their goal
– maybe they have a special scanner that is programmed to recognize these bar codes for a special application.
In such a case, you can set the Webscan TruCheck into a mode to check a bar code in an “Inverse Image” mode in which it expects the bars to be light and the and the spaces to be dark.
Why are my bar code grades varying even though the measured parameter values seem to be essentially the same every time?
You are noticing some variability in the overall resulting grade average and are noticing the range of values obtained for the individual parameters. The range for the individual parameters is very tight, and these results are considered very consistent. Note that the tolerances allowed with the ISO specifications are +-8 on DECODABILITY, SYMBOL CONTRAST and DEFECT and as you can see our measurements are tighter than this range of 16 levels.
The fact that relatively small variations in the measured values of the parameters can have a profound impact on the overall grade is a direct consequence of the way the overall grade is computed according to the spec. On each scan the parameters are assigned a grade level based on measured value. For example, if Symbol Contrast is between 50 and 62 it will get a B grade (equivalent to the number 3). The important thing to notice is that the values of several of your parameters can cross over the boundaries between grade levels. For example, the boundary between C and D for DEC (decodability) is at 37. So any time decodability is measured at 38 it will be a C but anytime it is measured at 36 it will be a D. Even a very small change in the actual value can result in a severe change in the assigned grade.
The actual grade result is thereby dependent upon the number of scans that fall on either side of the boundary which is somewhat random. I recommend that you concentrate instead on the consistency of the parameter values, and note when they are close to a grade boundary.
In order to consistently produce symbols that obtain passing (typically C) grades it is important to keep the print quality sufficient for all of the parameters to be above the minimum value for a C grade, taking into account the variability in both the printing process and the measurement process.
Since the measurement process variation therefore affects your required print quality, it is important to have the most accurate and repeatable measurement device possible. That is why our TruCheck units offer the most accurate and repeatable measurements available in the industry.
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How often should I replace my calibration card?
According to the manufacturer of the calibration cards, they should be replaced when they expire which is two years from the time the seal is broken on the package. If the card is not used and kept in its sealed package it should be good for a maximum of 4 years (2 years in the sealed package followed by 2 years of use).
Why do the calibration cards expire after two years?
While the calibration cards are produced on very stable materials, the color of the material does change over time, due to heat and exposure to light. After a very long time the reflectivity of the card may change from the calibrated values shown on the card that are used for calibration.
What would happen if I continue to use my calibration card after it expires?
If the card is still clean, it should continue to be accurate and useful for some time beyond the expiration date. However, an expired card should be replaced.
How can I tell if my expired calibration card yielded valid calibrations when I used it?
If you notice that your card is expired, and you obtain a new one, you may be curious to know whether all of the calibrations made with your expired calibration card were valid. A good way to check whether this is true is to calibrate your system using the new calibration card, and then verify the symbols on the old calibration card and check the Rmax and Rmin values that are measured, and confirm that the values match the calibration values written on the card within a small range of tolerance (+-5 for Rmax and +-3 for Rmin). This would prove that the old calibration card was still accurate and confirms that your calibrations using that card were valid.