An Explanation of the ISO 15416 1D Barcode Grading Process
A barcode verifier is the only device that can assign a quality letter grade to a barcode and generate an official quality report. A series of quality parameters are used to determine a barcode’s overall quality grade. The individual grades from each of those parameters determines the overall grade of the code. The International Organization for Standardization (ISO) 15416 governs one-dimensional (1D) barcodes. By first understanding the decode and grading process, users can get the most out of their verifiers and verification software.
1D Barcode Decode Process
A 1D barcode is made of a sequence of bar and space elements that create a set of data based on the pattern of their widths. Each individual bar or space is referred to as an element. Groups of bars and spaces that form a specific pattern are called characters. Each symbology has a different set of patterns that translate into characters.
To be decoded, a scan line crosses the code horizontally measuring reflectivity levels to determine the size and pattern of the bar and spaces. As the scan line moves across the code, light is reflected as it crosses over space modules and is absorbed by the bar modules. The light path going from dark to light is what creates a scan reflectance profile, or SRP. As the scan line crosses the dark bars it will cause the SRP to drop below the global threshold and then it will rise again above the global threshold as it crosses over the light spaces. The dotted line in the image represents the global threshold, which is what defines the gray scale values dividing between light and dark.
Every graded parameter is based off the SRP. Ideally the SRP represents the bars and spaces just as they were intended when printed. The code in this image shows an example of a code with defect and modulation issues. When the scan line goes across the defect spot the light path begins to drop, interrupting the smooth curve with a dip. There is also a space that is not as bright as the other spaces, so the reflectance profile does not reach the same height as other space elements, revealing an issue with modulation.
ISO 15416 Grading Process
Using the ISO 15416 grading process, 1D barcodes are graded by averaging the results of 10 scan lines. Each scan line is graded against nine different parameters to assess readability. The lowest graded parameter becomes the scan grade for the scan line. All the scan grades for each scan line are then averaged to compute the overall grade.
1D Aperture Size
The scan reflectance profile consists of samples across the length of the code. The diameter of the sampled area is the aperture size. Apertures are referred to in mils which is equivalent to a 1000th of an inch. Aperture size is specified by the application standard in use. If aperture is set at ‘auto’ and an application standard is selected, the software will automatically set the aperture recommended in that application standard.
If you do not have an application standard to reference, the following chart provides a general recommendation of aperture sizes to use.
|Code X-Dimension||Aperture Size|
|4-7 mil||3 mil|
|7-13 mil||5 mil|
|13-25 mil||10 mil|
|>25 mil||20 mil|
To learn about each quality parameter in the ISO 15416 standard and common solutions to undesirable results, download the Understanding Verification Results Whitepaper.