Decoding the ISO 15415 Verification Report
Scanning a barcode, whether a 1D like UPC or a 2D like Data Matrix, is based on reflectivity, specifically the reflective differences between the barcode image and its background. At the top of the verification report are the readings for these two essential values.
Minimum Reflectance is the value for the low reflectance image of the barcode.
Maximum Reflectance is the value for the high reflectance value of the background, including quiet zones and spaces.
The Global Threshold* is the halfway point between the Minimum Reflectance and Maximum Reflectance values. This is where a scanner decodes the barcode.
X and Y Dimensions and Print Gain are not ISO parameters. They are reported because they are helpful in understanding ISO parameter grading results. On 2D barcodes such as Data Matrix, where data is encoded on 2 axes, these measurements indicate axial differences in imaging accuracy, which can lead to scanning problems.
ISO Parameter Grades
Decode is a pass/fail parameter. The barcode either scans or does not. It is possible for the verifier to read and grade a barcode that fails this parameter. That is because the barcode fails other essential quality attributes such as Quiet Zone (Fixed Pattern Damage on 2D symbols).
Symbol Contrast evaluates and grades the reflectance difference between the barcode and background. It is calculated by subtracting the minimum contrast value from the maximum contrast value. The threshold is 50%. Symbol Contrast is graded on a scale (it is not a pass/fail parameter).
Axial Non-Uniformity evaluates the amount of variation between the height and width of elements (dots or squares) in a 2D symbol. It is a graded parameter. Accurate decoding depends upon elements being dimensionally accurate, within a range of acceptability or tolerance.
Grid Non-Uniformity evaluates the amount of variation in element placement of 2D symbols to a grid. Accurate decoding depends upon accurate element location, within a tolerance. This is a graded parameter.
Modulation evaluates the uniformity of Minimum and Maximum reflectance values. Variations caused by show-through from a non-opaque background or other patterns, or uneven dark elements (dots or squares) will lower this graded parameter.
Reflectance Margin evaluates the extremes of the Minimum and Maximum Reflectance values. When the lightest Minimum Reflectance value, or the darkest Maximum Reflectance value is 5% or less from the Global Threshold*, the symbol is near to failing to decode.
Unused Error Correction evaluates how much error correction remains. 2D barcodes have user-determinable amount of error correction, which provides the barcode the ability to recover from varying degrees of damage or print inaccuracy.
Contrast Uniformity is the worst-case contrast measurement within the symbol. It is not a graded parameter.
Fixed Pattern Damage is an evaluation of the common features that every 2D symbol includes. On a 1D symbol these are quiet zones and start/stop patterns. On a 2D symbol this parameter includes finder patterns (square targets in a QR Code, L and Clock Tracks in a Data Matrix Code) and quiet zones. The verification report further itemizes each sub-attribute within the broader Fixed Pattern parameter.
How do you correct for one or more of these parameters when they are downgraded? We will discuss that in depth in future articles. Your comments are always welcome.