If you have a rarely asked question that is not listed below, email an engineer here at Cognex.
What is the effect of putting my film on a black, gray, or white background?
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 quiet space? If this is possible; would we then just need to adjust the reduction for codes like this?
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 a 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 quiet space? If this is possible; would we then just need to adjust the reduction for codes like this?
Most barcode 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 barcodes are printed on light colored backgrounds they usually begin and end with a dark bar.
Scanners will generally not read a barcode 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 barcode that is printed in reverse colors. But, such a barcode, 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 barcode; 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 barcode 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 barcodes for a special application.
In such a case, you can set the Webscan TruCheck into a mode to check a barcode in an “Inverse Image” mode in which it expects the bars to be light and the and the spaces to be dark.
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).
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.
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.
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.
How can I read unprintable ASCII codes?
ASCII Code Representation
Some barcode symbologies (especially 2D symbologies) can encode ASCII characters which are unprintable and therefore, difficult to represent in the printed reports. A special pattern is used to represent these characters using a consistent notation of two letters which represents the ASCII code, surrounded by brackets.
The codes are as follows:
|ASCII Code Value (decimal)||ASCII Code name||Special Pattern Used|
Note that the special patterns always consist of two letters, even though the name of the ASCII code sometimes contains more than two characters (example EOT, BEL, etc.). This is to preserve regular column spacing in the printed reports and to make it easier to parse the data strings.
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How does Minimum Reflectance (MR) effect DPM (direct part marking) grading?
MR is a measure of symbol contrast, similar to iso 15415 SC.
The new proprietary “Stability” measurement that is reported in “General Characteristics” when a Data Matrix bar code is verified, is a complicated, but potentially valuable tool. Many people are concerned about the stability of the grade that is reported by a verifier when the same symbol is verified again and again. In other words, it is greatly troubling when the same bar code results in different grades when you verify it again and again. We have studied this vexing problem and determined some things that cause it. The Stability Value can be used to guard against barcodes which have characteristics that may lead to fluctuations in grading in repeated verifications.
When we evaluate an image of a data matrix bar code, we arrive at its grade in accordance with the standardized process defined by specifications, namely ISOIEC 15415 or ISO/IEC TR 29158, etc. However, if we obtain another image of the same bar code, it is possible for slight variations in the image to result in a different result from the previous evaluation of the image. Even very small changes, almost imperceptible to a person looking at the image, can lead the verification process to obtain a different result. Usually, when this happens it is because the “grid” changes the location where the modules are evaluated. Even if the grid changes a very little bit, the grid can change enough to make a difference. For Data Matrix, the grid is determined by the clock tracks (both horizontal and vertical), in particular the locations of the edges between light and dark where the clock teeth begin and end.
In order to report “Stability”, we are evaluating the consistency of the clock tracks to give a stable grid even if there would be a little variation in the image. In this context, the term “variation in image” means a very small change in the value of the pixels. For example, if a pixel has a value of 100 in a certain image, normal tolerances dictate that the same pixel might be measured as 101 or 99 on a subsequent image. The “stability” is a measure of how close to a boundary condition the grid is, such that if a small variation in the image could cause the grid to change. Note that it is a characteristic of our verifiers that the internal noise is extremely low in order to minimize this effect as much as possible, but it still exists in any imaging system.
We have found that well printed symbols have very good “stability” but poor codes, typically low contrast ones (which may be laser etched or ink jet or really any printing process), will give low stability measurements. We have determined a level of stability that we feel is risky, meaning that it is possible for a subsequent verification to give a different result.
We have some customers who are afraid that the verifier can sometimes give a “C” or “B” grade on a symbol that later can be graded as “D” or “F” and they do not want to “pass” these codes. They prefer to improve the print quality to make sure the grade is stable. So we have provided a way to get a Warning or Error message when stability is low. You can choose whether to “Ignore”, “Warn”, or “Fail” when Stability is low, and this is in addition to any other grading and/or application standard requirements for Pass/Fail that are in effect according to your settings.
Please note, stability is a measure of the poor printing of the bar code and the way specifications such as ISO/IEC 15415 grading works on data matrix symbols in accordance with the reference decode algorithm for Data Matrix specified in ISOIEC 16022. Stability is not a measure of how good or bad the verifier is.
The value of stability, such as 70%, is pretty good. The level at which we “Warn” or “Fail” (according to your choice in the settings) is 30%. The symbols on the calibration card have Stability higher than 90% stability.