THE BARCODE EXPERTS
This section will give you an understanding of the factors involved in ISO/IEC verification, so that you can understand the verifier results.
How easily a barcode can be read depends upon how well the code has been printed, and on certain parameters of the scanner. Specifications lay down the ideal dimensions and suggest methods of production, but it is not always possible to adhere to the ideal conditions.
In practice, barcodes can be produced that are not perfect, and they may or may not scan successfully. In addition there are other problems that can occur in the printing or production process which may contribute to the difficulty of scanning.
So how can the chances of a code scanning be assessed?
Verification equipment has been around for some years to assess the dimensional accuracy of printed codes and to give an indication of how easily a barcode can be scanned.
All Axicon verifiers conform to the relevant ISO/IEC standards, and for linear barcodes this is ISO/IEC 15426-1. This ISO/IEC grading system takes into account seven parameters:
The print quality standard recommends that ten scans across 80% of the height of each symbol are used as the basis for verification. For each scan the lowest single grade of any one of these seven parameters (being the weakest link in the chain) is the result. After the recommended ten scans have been taken, the results are averaged to produce the overall grade. Each parameter has an integer value from 4 to 0, with 4 being the highest, and 0 the worst. The overall grade will be expressed to one decimal place. The grade bands for these parameters are shown later in this section. The historical ANSI grading system uses letters rather than numbers , with the grades being A,B,C,D, or F for fail. The linear print quality assessment method is ISO/IEC 15416 specification.
The required pass grade for most major retailers is a grade 1.5 (C) or above for all barcodes, with the exception of those printed directly onto corrugated fibreboard, for which a grade 0.5 (D) is normally acceptable.
Scan Reflectance Profile window
All verifiers use the reflectance data from the barcode to calculate these parameters. To get a better idea of what is happening, the reflectance data for a barcode is shown on a graph - the Scan Reflectance Profile. On this graph the peaks represent spaces (the background reflectance) while the troughs represent the bars. An ideal code would have all the peaks at the same level (as near as possible to the high reflectance end of the graph), and all the troughs at the same level (at the bottom, low reflectance end), but in practice narrower white spaces will not appear as white as the wider spaces, and the narrower black bars will not appear to be as dark as the wider bars.
The verifier uses measurements from the Scan Reflectance Profile to calculate several of the ISO/IEC parameter values. The measurements made by the verifier, and how they are used to calculate these parameter values are explained below.
The Axicon verifier shows the position along the graph in a scale of millimetres along the top of the graph and sample position along the bottom.
Minimum Reflectance (RMin)
This is the lowest amount of reflectance recorded, and is measured from the darkest bar. This will never be 0% but will often be a figure closer to 3 -5%.
Maximum reflectance (RMax)
This is the greatest amount of reflectance recorded and will be measured from the widest light space. The highest values normally recorded are about 90% with a white background for a barcode. On fibreboard cases, the figure will often be about 40%.
Minimum Edge Contrast (MEC)
This is the least difference between a bar and a space, so the smallest measurement between a peak and a trough on the Scan Reflectance Profile.
The Global Threshold
This is the halfway point between the highest and lowest reflectance values, and appears on the SRP as a horizontal blue line.
The verifier will determine whether the image it sees is a barcode, and will check that all the troughs (the bars) are below the Global Threshold and that all the peaks (the spaces) are above it. The verifier will also check to ensure that the barcode does not break any requirements for that barcode symbology including the presence of the following components:
Quiet zones (Left and right margins)
Check digits (where they are mandatory)
Start, Stop and any other mandatory characters.
A symbol passes or fails this test, with a pass scoring 4 or A, and a fail scoring 0 or F. If a code fails, the ‘Scan’ tab, at the top of the Details window will describe in more detail the reason for the failure.
Minimum Edge Contrast
The least difference between a bar and a space must be at least 15%. If the MEC is greater or equal to 15%, the symbol passes the test and scores 4 or A. If it fails it scores 0 or F.
Minimum Reflectance (RMin)
The value for RMin is compared with that for Maximum Reflectance (RMax). RMin must be less than or equal to half the value of RMax. If it passes this test, the score is 4 or A, and if it fails it scores 0 or F.
Symbol contrast is the difference between the highest and the lowest levels of reflectance i.e. the darkest bar and the lightest space. This is the difference between the horizontal orange and green lines shown on the Scan Reflectance Profile. Barcodes with a high value are more easily read by a scanner, so the ISO/IEC standard grades this parameter from 4 to 0, or A to F. The table that shows how this is done is shown below:
|Symbol contrast percentage value||ISO/IEC grade (ANSI in brackets)|
|≥ 70%||4 (A)
Modulation is a measure of the variability of the symbol contrast throughout the barcode. It is calculated by comparing the Minimum Edge Contrast with the Symbol Contrast, and expressing this as a percentage.
Modulation = Minimum Edge Contrast/Symbol Contrast x 100 %.
This is graded from 4 to 0 (A to F) as shown in this table:
|Modulation percentage value||ISO/IEC grade (ANSI in brackets)|
|≥ 70%||4 (A)
Defects are the presence of small marks in the uniform pale background of the barcode or small absences in the dark bars. Their presence is shown by uneven peaks or troughs on the Scan Reflectance Profile. The size of the largest variation at any peak or trough is measured and compared to the Symbol Contrast. This is graded as shown below:
|Defects percentage value||ISO/IEC grade (ANSI in brackets)|
|≤ 15%||4 (A)
|≤ 25%||2 (C)|
|≤ 30%||1 (D)|
|> 30%||0 (F)|
This is a measure of how easily a scanner can determine the different widths of the bars and spaces, taking into account the tolerances allowed for their variation from the ideal measurements. The verifier determines the edges between each bar and space at the mid-point between a peak and trough, and not where the Global Threshhold is crossed. Decodability is a measure of the dimensional accuracy of the barcode, and the verifier uses several calculations to determine this value. The percentages are again converted to grades as shown below:
|Decodability percentage value||ISO/IEC grade (ANSI in brackets)|
|≥ 62 %||4 (A)
|≥ 50 %
|≥ 37 %||2 (C)|
|≥ 25 %||1 (D)|
|< 25 %||0 (F)|
Calculating the final grade
The lowest grade (of the seven different parameters) from each scan is averaged to produce the overall grade. As the scans are taken at different heights through the barcode the final grade will give a good indication of whether a barcode will or will not scan.
Finally there is an important factor to consider concerning the reading head. In most scanning situations an appropriate scanner will have been selected for the size of bars and spaces in the code, and the ISO/IEC method takes this into account. For instances where very narrow bars and spaces exist in the barcode, a special high resolution scanner will be needed, and it makes sense to verify the code with this in mind. Conversely when a barcode is going to be used that has very wide bars and spaces, a much lower resolution scanner can be used, ensuring that fine details, such as very small holes or extra specks of ink, do not affect the results unnecessarily.
This situation is exactly what we see when codes are printed on fibreboard outer cases. Here ink coverage is often not perfect, and holes or ink spots may exist that do not affect the scanning because a low resolution scanner is used.
The ISO/IEC standards specify that the size of a verifier's measuring aperture should be chosen to match the size of the bars in the printed code.
Aperture sizes are often quoted in units of 1/1000 of an inch. This unit is sometimes referred to as a ‘mil’. Note that this is not the same as a millimetre.
The grade for a barcode should ideally state the aperture used for the verification and the wavelength of light being used by the verifier, like this 3.7/06/600, where 3.7 (A) is the measured grade, 06 the aperture reference number, and 660 the wavelength in nanometers. The aperture reference number is equal to the diameter of the aperture measured in mils.
The Axicon 6015 will choose the correct aperture for most applications automatically, but the user can choose whether the EAN/UPC barcode will be scanned only at the retail point of sale, or in general distribution. If the verifier cannot use the correct recommended aperture it will select the closest available, and the Summary window will show a warning, along with both the used and recommended apertures.