A short story about the ANSI specification for linear barcode quality.
Once upon a time, in the olden days before the ANSI spec, there was something called Traditional verification. This method of verifying a barcode was based on physical measurement of the bars and spaces, along with measurement of some reflective properties.
Traditional verification was replaced by ANSI verification because barcodes were passing the Traditional tests but still failing at the scanner
Traditional verification has been replaced by ANSI verification as the basis for determining barcode performance and the reason is, barcodes were found to be passing all the Traditional tests but still failing at the point-of-use scanner…and the reason or this is that linear measurement of bars and spaces is not how scanners work. They work based on reflectivity—they don’t measure the bars and spaces in order to decode the symbol.
Traditional verification is based primarily on linear measurements
The ANSI specification is based almost entirely on reflectivity, so this method of testing and grading a symbol is much more real-life. This makes the ANSI method of verification much more reliable as a way to predict whether or not the barcode is going to perform its function at the scanner.
Eight of the nine ANSI parameters are based of reflectivity. Only one is not directly related to reflectivity: the parameter called Decode. Do not confuse the parameter Decode with the parameter Decodability—they are completely different. Decodability will be discussed in a later installment in this tutorial series.
Decode is a pass-fail parameter and tests whether the barcode is decipherable as a known symbology or not. Of course this relates to reflectivity, but only indirectly.
Traditional verification doesn’t see the barcode the way the scanner does
The remaining parameters are all based on two reflectivity measurements—the light reflective value and the dark reflective value of the symbol. The light reflective value measures the background which represents the left and right quiet zones and the spaces between the bars. The dark reflectance value measures the bars themselves. Each of the eight ANSI parameters is evaluated and graded based on mathematical calculations made from the light and dark reflectance values.
Note that the UPC system, now known as the GS1 Global system which encompasses virtually all barcodes used in global commerce utilizes the Print Contrast Signal or PCS system which dictates that the light reflectance value is always the background for the barcode; the dark reflectance value is always the bars. Symbols that are printed in the opposite way violate the specification and will not work.
You may have observed certain symbols that seem to violate this rule—for example certain soft drink cans have symbols that are reverse printed: the spaces are printed and the bars are the unprinted, bare aluminum. In actuality, this does adhere to the PCS system because the printed quiet zones are highly reflective and the bare aluminum scatters the light, acting very much like low reflectance, printed black bars.
Traditional verification methods would have missed all of this.
Since I’ve already mentioned quiet zones, I’ll finish with them before digging into the more technical stuff in a later article. Every linear barcode requires a blank, unprinted space leading and trailing the barcoded message. There is never a quiet zone above or below the barcode—just left and right. This helps the scanner determine what kind of barcode it’s scanning, count the right number of bars and capture the whole encoded message. If there are graphics or text or screened pixels encroaching on the symbol, this can throw off the element count and confuse the scanner.
Traditional verification still has its place in barcode quality–all ISO compliant verifiers test and report traditional as well as ISO-ANSI parameters.
I hope this is interesting and helpful to you so far.
More to come.
