MANUFACTURERS Guide to Bar Code, Common Forms and EC/EDI
7. Bar Code Print Quality
7.1 Overview to Printing and Applying Bar Codes
One reason that bar code technology has grown so rapidly is that bar codes can be printed with many different methods. Bar codes can be printed with impact printers, thermal printers, toner-based printers (lasers), ink jet printers, and virtually any photographic or wet ink process on almost any surface.
7.2 Deciding Which Printing Technology to Use
One of the first decisions a project team must make is how they are going to print each different type of bar code they need to create. For economic, logistical or technical reasons, different printing methods might be used to print each of the three different types of labels described in the U.P.C. standards:
|The 12 digit U.P.C.-A item label applied to individual consumer units.|
|The 14 digit SCC applied to intermediate packs and shipping containers.|
|The 20 digit serial number applied to cartons and shipping containers.|
In general terms, each symbol / label can be:
|Printed directly (no label) on the item, package or the carton.|
|Printed on a label applied to the item or carton.|
Labels can be produced using a variety of printing techniques including:
|Impact printers including dot matrix and industrial quality shuttle printers.|
|Thermal printers, primarily thermal transfer.|
|Toner-based printers including laser and LED printers.|
Furthermore, labels can be produced in-house or purchased through a service bureau.
This chapter discusses each of these techniques and the very important topic of Bar Code Print Quality.
7.3 Bar Code Quality
Before getting into the different methods of printing bar codes, it's important to discuss bar code print quality. Just because a bar code looks good to our eye, doesn't mean that it is actually printed properly and should be readable. This is such an important issue that some retailers have resorted to fines or "charge backs" for out-of-specification bar codes.
Bar code print quality is always the responsibility of the supplier. Regardless of whether bar code symbols are printed by the supplier or a service bureau provides bar codes to the supplier; in any event, the supplier needs to be sure that the bar codes are readable.
The cost of creating bad bar codes can be extremely high. Ask the candy company that printed hundreds of thousands of red bar codes on candy wrappers for a Christmas promotion and had to call them all back because red bars are invisible to most bar code scanners. Or ask the company that was charged $30,000 by its customer for marking its products with unreadable bar codes.
Bar code "quality" involves the actual print quality and several other important issues that combine to determine if a bar code is in or out of specification.
|Image quality and dimensional accuracy.|
|Encodation of the data.|
|Placement of the symbol / label on the item.|
|As mentioned in the appendix, bar codes encode data in much the same way that Morse Code works, using a series of wide and narrow bars and spaces instead of dots and dashes to encode each character.|
|The widths of the bars and spaces are critical. If the widths are wrong, the bar code could be unreadable or, even worse, could be decoded improperly.|
|Bar code readers need to be able to tell the difference between a bar and a space. If the contrast between them is not great enough, the bar code may not be readable.|
|Bar codes are supposed to have "quiet zones" on either side of the symbol. Quiet zones are blank areas, free of any printing, typically 10 times the width of the narrowest bar or space in the bar code. Failure to allow adequate space on either side of the symbol for quiet zones can make it impossible to read the bar code.|
|Bar code verifiers are instruments that analyze the bar code print quality to determine if it should be readable by a bar code scanner using a specified light source and spot size. They cost between $1,000 and $3,500. Considering the penalties for supplying out-of-spec. bar codes, this is a necessary piece of equipment that should be used often enough to ensure that your printing processes are working properly and that service bureaus are supplying in-spec. bar codes.|
Some retailers have selected a specific brand of verification equipment for their testing and donít accept results from any other brand of verifier. This situation is not common and is usually a legacy from before the ANSI X3.182 Quality Standard, but itís a good idea to check with the customer to see if they have a preferred brand of testing equipment.
ANSI Print Quality
The American National Standards Institute has published a bar code print quality specification. Named ANSI X3.182, this specification uses 8 categories of measurement criteria to grade bar code print quality using letter grades ranging from A to F. Any grade other than an "F" is considered passing. As a rule of thumb, the higher the letter score, the easier it will be to read. Some bar code specifications reference this specification and specify that labels must receive a specific letter grade or better, i.e. "C" or better. Some bar code verifiers can test to the ANSI print quality test. As standards are updated, inclusion of ANSI print quality references will grow. Verifier manufacturers are offering more options that incorporate the ANSI print quality measurement.
The ANSI print quality grading method is based on the relationship between the printed symbol and the way bar code scanners interpret the symbol. Scanning performance is key to the process. In order to determine how well a bar code will perform in typical situations, a number of measurements are performed by a verifier to determine the final grade of the symbol.
The first key measurement is decode. This is a test to decipher the bars and spaces into meaningful data. Assuming that the decode test passes, the other checks are graded from A through F. If the symbol cannot be decoded, it fails and automatically receives a grade of F. Other measurements include:
This is a measurement that tests the printing accuracy of the symbol. In general, the higher level of decodability, the better a scanner can be expected to perform.
A measurement between the largest and smallest reflectances in a scan profile. In general, the whiter the spaces and blacker the bars, the better the symbol contrast. Colored symbols are acceptable if they provide adequate contrast.
The ratio of the minimum edge contrast to the symbol contrast. This test looks at the edges of the bars and compares them to the "clarity" of the spaces and also looks at the edges to be sure that they are not wavy or fuzzy.
The irregularities found within elements and quiet zones. Most often these are spots or voids in the symbol.
This a measure of the amount of light reflected from an illuminated source. Light is reflected from the spaces and absorbed by the bars and this measurement checks the spaces and quiet zones to be sure that ample light is being reflected so that a scanner can pick up the bar and space elements in a bar code.
Other factors also take item in the determination of how a symbol is graded such as label material opacity, surface gloss measurement, and the effect of over-laminates.
For most compliance applications, hand held laser scanners will be used. This is an application where ANSI Grade "C" or better is acceptable and the mandate will typically specify this measurement.
|Even if the bar code is printed properly, if the human readable data below it doesn't match the decoded bar code, the label is out of specification because the human readable interpretation and the bar code must agree.|
|Likewise, even if the bar code and human readable match, if the symbol is affixed to the wrong item, itís also out of specification.|
|If the label adhesive is not correct, the label can fall off during storage or shipment.|
|If the label materials are too translucent, too much of a dark background can show through the material, making the spaces difficult to identify and causing the label to fail a print contrast test. Therefore, print quality testing should be done after the label has been applied to the package.|
|Label materials are selected for their required durability and abrasion resistance, surface smoothness to create a sharp image, the surface they will be applied to for adhesive selection, maximum and minimum exposure temperatures and application temperatures for substrate selection, opacity to ensure adequate print contrast and, finally, budget.|
Placement of the Label
|If the label is placed improperly, making it unreadable, it's out of specification.|
|Allowing the label inside a clear package containing many items to show through is also not permitted because the wrong symbol could inadvertently be scanned.|
|Bar code labels placed on the top of a shipping container that will be stacked will be inaccessible.|
|Most specifications define where the label / symbol should be affixed on an item or carton. Failure to comply results in an out-of-spec. label.|
Verification tests the symbol for its characteristics but to comply with the specification, quality procedures that check for the correct human readable interpretation and label placement as well as test for label adhesion are needed.
All quality checks need to be documented and kept on file in case future quality questions arise.
In summary, procedures to monitor the quality of the bar code printing and placement must be written and approved by MIS and Operations.