VERIFIERS

VERIFYING LINEAR BARCODES

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WHAT IS BARCODE VERIFICATION?

Linear barcode verification checks the quality of 1d barcodes using a standard method.

The first approach to this quality checking appeared in the early 1970's and is now known as the Traditional Method. This involved visual checks and an automated means of measuring the print contrast and the barwidths of the symbol. Barcodes were declared to be 'in spec' or 'out of spec'. Today we use a graded approach but some of the traditional parameters are still used to provide useful auxillary information.

The traditional method had a number of short comings and in 1990 a new standard was published in the US under ANSI X3.182. This approach looks at the barcode from the barcode scanner's point of view. This was followed in 1990 by the European standard EN 1635 and the International Standard ISO/IEC 15416 was published in 1995.

Apart from some minor presentational differences the ANSI and ISO/CEN methods are fully compatible.

WHY VERIFY?

Barcodes which meet the ISO/IEC 15416 quality standard can be read by any compliant barcode scanner anywhere in the world irrespective of reading technology or vintage.

The importance of verification is driven by the large scale players in the retail sector who require 'first time reads' at the checkout and the reliable reading of goods entering automated distribution warehouses.

In the retail sector failure to meet the agreed barcode quality standard can result in the rejection of goods and substantial financial costs. It is said that the average cost of a single incident of product rejection in the UK is currently around £10,000.

WHAT IS A BARCODE VERIFIER?

This section uses the term verifier to mean a device which tests a barcode fully against the ISO/IEC 15416 quality standard. To do this the verifier must comply with ISO/IEC 15426-1 (Part 1).

When selecting a verifier you should be aware that some do not comply with the standard. These typically are based on on barcode scanners which are held in the hand and pointed at the barcode at some distance. Whilst this mode of operation is extremely convenient, and some useful results can be obtained, the full set of tests which make up the ISO/IEC testing method cannot be performed.

Compliant verifiers use various forms of built-in barcode scanners which are held internally at a precise distance and angle from the barcode. A number of different reading technologies can be used. These include Wands, CCD, linear imagers and laser scanners.

Whichever technology is used the verifier effectively sweeps an accurately sized spot of red light (around 660 - 670nm) across the entire barcode, including the quiet zones on either side, and makes a series of reflectance measurements at precisely known distances.

The size of the spot is known as the aperture and in general should be about 80% of the width of the narrowest bar or space.

For retail purposes the required apertures have been standardised. The apertures are 6mil for EAN13 barcodes on retail packaging, and, in the case of ITF-14 or GS1 128 barcodes for traded units, 10mil on labels or 20mil on corrugated cartons.

With regards to the results of the tests some verifiers require connection to a computer (desktop,laptop or hand-held). Others are portable and print the verification details to a small portable printer.

Finally it should be stressed that verifiers are precision instruments which require calibration. Verifiers are supplied with a NIST traceable Calibration Card which should be used regularly in accord with the manufacturer's instructions.

HOW ARE VERIFIERS USED?

There are two main types of verifier, hand-scanned and static.

The hand-scanned verifier requires that a barcode pen, or similar device, is scanned evenly and at an appropriate speed, across the bars. This is quite difficult to do and inexperienced operators can obtain wildly inconsistent results.

In the case of static verifiers the device is simply placed over the barcode to be tested.

In either case a series of ten scans should be performed moving incrementally from 10% down from the top of the bars to 10% from the bottom of the bars.

WHAT ARE THE RESULTS OF USING A VERIFIER?

An ISO/IEC 15416 compliant verifier provides a graded overall result which is a numeric value between 0 and 4. The higher the number the better the quality. Zero is a fail.

The Final Grade is either a fail or the worst result from a set of graded tests.

If the Final Grade fails to meet your expectations you can use the details of the individual tests to diagnose the problem.

WHAT ISO TESTS ARE INVOLVED

ISO/IEC 15416 verifiers perform a set of seven tests.

Some are Pass (Grade = 4) or Fail (Grade = 0), others are graded from 0 to 4, where 0 is a fail and 4 is the best possible result.

The overall Grade is taken from the worst individual Grade.

1. Reflectance Minimum (Rmin/Rmax)

In this test the verifier compares the maximum reflectance found in the quiet zones before or after the bars against the refectance value of each bar.

If the miniumum reflectance Rmin is greater than 50% of the maximum reflectance Rmax the result is a Pass (Grade = 4) otherwise the barcode is Failed (Grade = 0).

2. Edge Determination and Decode

In this test the verifer sets a threshold midway between the highest and lowest reflectance value. The number of peaks and troughs which pass through this line are compared to that expected for the type of barcode.

If correct the result is a Pass (Grade = 4) otherwise the barcode is Failed (Grade = 0).

3. Symbol Contrast (SC)

In this test the verifier obtains the value for the Symbol Contrast (SC) which is difference between the maximum reflectance Rmax and the minimum reflectance Rmin

SC=Rmax - Rmin

and tests it against five four values to obtain the Grade.

SC => 70% Grade = 4

SC => 55% Grade = 3

SC => 40% Grade = 2

SC => 20% Grade = 1

SC   < 20% Grade = 0

4. Edge Contrast Minimum (ECmin)

In this test the verifier obtains the Edge Contrast which difference in reflectance between adjacent bars and spaces.

The lowest value obtained is the Edge Contrast Minimum (ECmin) and is the difference in contrast for the worst adjacent pair.

If the Edge Contrast Minimum is less than 15% the barcode is Failed (Grade = 0) otherwise the barcode is Passed (Grade = 4).

5. Modulation (MOD)

In this test the verifier divides the values found for the Edge Contrast Minimum (ECmin) by the Symbol Contrast (SC) to obtain the Modulation (MOD)

MOD=Ecmin/SC

and tests the result against against four preset values to obtain the Grade.

MOD => 0.70 Grade = 4

MOD => 0.60 Grade = 3

MOD => 0.50 Grade = 2

MOD => 0.40 Grade = 1

MOD   < 0.40 Grade = 0

6. Defects

Spots in the spaces and voids in the bars create additional peaks and valleys.

The verifier sees these effects as ripples in the peak and valley profile. The reflectance of these effects is measured and the largest deviation is known the maximum Element Reflectance Non-uniformity (ERNmax).

In this test the verifier divides the value found for largest Element Reflectance Non-uniformity (ERNmax) by the Symbol Contrast (SC) obtain the Defects

Defects=ERNmax/SC

and tests the result against four pre-set values to obtain the Grade.

Defects =< 0.15 Grade = 4

Defects =< 0.20 Grade = 3

Defects =< 0.25 Grade = 2

Defects =< 0.30 Grade = 1

Defects   > 0.30 Grade = 0

7. Decodability

In this test the verifier measures how close the barcode width dimensions are to their ideal values. As such it is a measure of dimensional accuracy. Decodability is calculated taking into account the decode algorithm for the particular barcode symbology and the result is tested against four pre-set values to obtain the Grade.

Decodability => 0.62 Grade = 4

Decodability => 0.50 Grade = 3

Decodability => 0.37 Grade = 2

Decodability => 0.25 Grade = 1

Decodability   < 0.25 Grade = 0

WHAT ABOUT TRADITIONAL TESTS?

Although the traditional tests are not part of the ISO/IEC standard they can be useful in that they can give a strong qualitative view of the 'health' of a barcode.

A high Print Contrast Signal (PCS) is generally considered to be good, as is a small Bar Width Deviation.

Some hold the view that the Bar Width Deviation is more useful in operational terms than ISO Decodability.

Other useful parameters can include the X-dimension measurement, which is the width of the smallest bar or space, and where appropriate, the magnification% which gives the overall scale of the symbol.

WHAT RESULTS ARE ACCEPTABLE?

It is obviously best to strive for Grade = 4 quality throughout.

In the real world it has to be accepted that some printing methods cannot achieve the highest quality and, in addition, printing processes must be expected to degrade with time and use.

The minimum acceptable grades are normally agreed between the parties issuing and reading the barcodes.

Unless otherwise agreed, the minimum acceptable barcode quality in the UK retail sector is Grade = 2 for all barcodes with the exception of ITF-14 barcodes printed directly on brown corrugated where Grade = 1 is sufficient.

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IMPROVING THERMAL TRANSFER BARCODES

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The following offers an approach to improving the grades of linear barcodes printed by thermal transfer label printers using the results available from ISO/IEC 15416 compliant barcode verifiers.

GENERAL GUIDELINES TO BARCODE QUALITY

From an ideal point of view barcodes should have the following characteristics:

• the bars should be perfectly black and the background should be perfectly white.
• the edges of the bars should be perfectly sharp.
• the widths of the bars and the spaces should be exactly in accord with that demanded by the type of barcode.
• the barcode should be positioned to provide unhindered white space on both sides and the label should be opaque.

In the real world we have accept compromise. Labels may not be white; there is usually some fuzziness to the barcode edges; bar widths will be affected by, less than perfect printer setup, bleed due to barcode orientation and the choice of labelling materials etc.

We must do what we can.

THE VERIFIER CANNOT SEE THE BARCODE

If the verifier cannot read the barcode at all, the possible causes and remedies are as follows:

• check for telltale 'white' lines along the roll. Heating wires in the print head have failed. Replace the print head.
• check to make sure that there is a clear area of white space on both sides of the barcode. Reposition the barcode.

USING THE ISO TESTS AND BAR WIDTH

This section presumes that the verifier is able to read the barcode and produce results for the individual ISO tests and some measurement of the bar width.

The Final Grade is the worst grade obtained for the individual ISO tests.

If you want to try to improve the result for a particular ISO test click on the test shown below and examine the possible causes and remedies.

• Reflectance Minimum (Rmin/Rmax)
• Symbol Contrast (SC)
• Edge Contrast Minimum (ECmin)
• Modulation (MOD)
• Defects
• Decodability

If you make any changes you should re-verify and look at the results of all of the tests.

If you would like to learn more about the ISO tests it may be worthwhile reviewing the technology guide on Linear Barcode Verifiers.

Reflectance Minimum (Rmin/Rmax)

Printing black barcodes on white labels will nearly always present a pass ie. Grade 4 for the Reflectance Minimum test. To fail this test the reflectance of the bars must be less than half of that of the background.

Typically the reflectance is around 5% for black ink and about 80% for white paper.

If the Reflectance Minimum test fails the possible causes and remedies are as follows:

• the barcode bars are too light. Clean print head.
• the barcode bars are too light. Increase print head temperature but be careful to avoid excessive bar width.
• the barcode bars are too light. Check the compatibility of the thermal transfer ribbon and the label surface.
• the background is too dark. Use a lighter background.
• detail beneath the label is showing through. Use an opaque label.

Note: If the Reflectance Minimum test fails Symbol Contrast will almost certainly be low.

Symbol Contrast

Printing black barcodes on white labels will nearly always present a Grade 4 for Symbol Contrast.

The reflectance of black ink is around 5% and for white paper is about 80%. This gives a Symbol Contrast of 75% which translates to Grade 4.

If Symbol Contrast is low the possible causes and remedies are as follows:

• the barcode bars are too light. Clean print head.
• the barcode bars are too light. Increase print head temperature but be careful to avoid excessive bar width.
• the barcode bars are too light. Check the compatibility of the thermal transfer ribbon and the label surface.
• the background is too dark. Use a lighter background.
• detail beneath the label is showing through. Use an opaque label.

Edge Contrast Minimum

If the Edge Contrast Minimum test Fails (Grade 0) the possible causes and remedies are as follows:

• the bars are fuzzy. Check print head.
• the bars and spaces are indistinct. Increase the barcode magnification or X-dimension.

If Edge Contrast is low, the possible causes and remedies are:

• detail beneath the label is showing through. Use an opaque label.
• excessive bleed. Change the thermal transfer ribbon and the label surface.
• the barcode is too small for the printing method. Increase the barcode magnification or X-dimension.

Note: If Edge Contrast is low, Modulation is likely to be adversely affected.

Modulation

If Modulation is low the possible causes and remedies are as follows:

• detail beneath the label is showing through. Use an opaque label.
• bar width is too wide due to bleeding. Change the thermal transfer ribbon or the label material.
• the barcode is too small for the printing method. Increase the barcode magnification or X-dimension.

Defects

If the value for Defects is too high the possible causes and remedies are as follows:

• dirty print head. Clean print head using a cleaning card.
• paper dust on labels. Replace labels or change supplier.

Decodability

If Decodability is low the possible causes and remedies are as follows:

• the bars are too wide. Reduce the print head temperature.
• the bars are too wide. Change the thermal transfer ribbon or the label material to reduce bleed.
• the bars are too narrow. Increase the print head temperature.

Note: The traditional print width deviation measurment is a very useful tool for this purpose.

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VERIFYING DATA MATRIX CODES

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Data Matrix is a two dimensional (2d) 'barcode'.

This code cannot be verified using a linear barcode verifier.

A special type of verifier is required.

DATA MATRIX

Data Matrix codes are made up of a small array of dots or square blocks with a distinctive 'L' shaped pattern on two sides and a regular pattern of 'clocking' dots on the other two sides.

Using the Data Matrix method it is possible to compact 50 characters of data into a symbol just 3mm square.

Data Matrix codes are increasingly found printed on labels and documents. Large amounts of data can be held in small discreet symbols. Printed Data Matrix symbols are widely used in Pharmaceutical packaging, letters and bills from Utility companies, forms from the Government, and labels involved in Military procurement.

Perhaps more interestingly this method lends itself to directly marking parts rather than using labels. These Direct Part Marked (DPM) codes are widely used in Automotive, Aerospace and Military applications

DPM Data Matrix codes can be marked on items by range of different methods. These include using a laser to engrave the symbol on to the surface, punching an array of small holes into the surface (dot peening), and etching the pattern on to the surface by electro-chemical means.

Laser engraving can be used with a wide range of materials including metals, plastic and glass. The other methods are primarily used with metal objects.

VERIFYING 2D CODES

The verification of 2d codes is more difficult than for linear barcodes.

Data Matrix symbols are usually verified using Machine Vision camera technology.

The verifier consists of an area imager which is housed in equipment at a fixed distance and angle from the symbol. A separate flood light source is also housed within the equipment and illuminates the symbol at a fixed angle.

The verifier does not scan a dot of light across the Data Matrix symbol but looks at the entire symbol and uses software to average groups of pixels together to simulate a pin hole camera.

The verifier software will then process the data obtained to provide the results required by the verification method.

The verifier is calibrated by using a reference symbol on a Calibration Card.

Verification can performed against a number of standards.

These include ISO/IEC 15415 and AIM DPM-1-2006 for printed and DPM Data Matrix codes, and in addition there are some industry standard methods such as AS9132.

ISO/IEC 15415

The ISO/IEC 15415 standard was published in 2000 and deals with the verification of 2d codes.

This is a good approach for Data Matrix codes which are printed on paper but is not generally suitable for DPM codes which are now covered by a different method.

The measured parameters are:

Reference Decode

The reference decode is applied to the Data Matrix symbol. If successful the result is a Pass (Grade = 4) otherwise the symbol is Failed (Grade = 0 ).

Symbol Contrast

The Symbol Contrast is the maximum reflectance difference between the light and dark regions of the symbol.

The result is graded between 0 and 4, where Grade = 4 is the best result and Grade = 0 is a Fail.

Fixed Pattern Damage

The Data Matrix symbol is examined for the presence of the quiet zone around the symvbol, the 'L' shaped finder pattern and the clocking pattern which is the regular series of dots on the other two sides.

If these are badly damaged the code will Fail (Grade = 0) otherwise the code will be Passed (Grade = 4).

Axial Non-uniformity

Axial Non-uniformity is a measure of symmetry in the cells and is usually associated with movement of the object while being marked.

The result is graded between 0 and 4, where Grade = 4 is the best result and Grade = 0 is a Fail.

Grid Non-Uniformity

Grid Non-uniformity is a measure of the actual grid against the ideal grid and the result is based on the largest deviation.

The result is graded between 0 and 4, where Grade = 4 is the best result and Grade = 0 is a Fail.

Modulation

Modulation relates to the reflectance uniformity of the symbols light and dark elements.

Unused Error Correction Capacity

Unused Error Correction Capacity relates to the amount of remaining error correction capacity following damage to the symbol.

The result is graded between 0 and 4, where Grade = 4 is the best result and Grade = 0 is a Fail.

Other

Verifiers usually determine some other useful parameters which can include:

• print growth eg. 10%
• symbol type eg. Data Matrix ECC200
• symbol size eg. 10x10
• pixels/element eg 10.1

Final Grade

The Final Grade is the worst result of any of the graded tests.

Grade = 4 is the best possible result and Grade = 0 is a fail.

AIM DPM-1-2006

DPM Data Matrix codes can be produced by a wide range of techniques and the appearance of the symbols can vary dramatically. Added to this is the background on which the code is marked. This can vary in colour and texture.

Analogous with linear barcode verification the ISO/IEC 15415 2d standard is ideally suited to situations where the Data Matrix code consists of regular homogeneous black blocks on a white background.

DPM Data Matrix codes rarely fit this description and may be unreadable by ordinary 2d barcode readers.

Special decoding algorithms are used for DPM Data Matrix codes. These cope with the poor difference in contrast and the effects of the surface.

In the AIM DPM verification method Symbol Contrast is replaced by Cell Contrast, Modulation is replaced by Cell Modulation and Minimum Reflectance is added.

Cell Contrast

Cell Contrast is the difference between the means of the reflectance of the light elements and the dark elements rather than the difference in reflectance between the lightest and darkest values.

Cell Modulation

Cell Modulation is a measure of the readability of the Data Matrix code. Low Cell Modulation indicates increasing difficulty in determining if individual cells are light or dark. The grading takes into account the error correction capacity.

Minimum Reflectance

Minimum Reflectance is a measure of the reflectance of the light cells of the Data Matrix symbol against the light cells of the reference symbol on the Calibration card.

Other

Cell Growth is a non-graded parameter which is useful for process control purposes.

Final Grade

As with ISO/IEC 15415 the Final Grade is the worst result of any of the graded tests. Grade = 4 is the best possible result and Grade = 0 is a fail.

Since careful lighting is often important in the reading and verifying of DPM Data Matrix codes details of both the angle of lighting and the wavelength used should be included with the Final Grade.

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