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 4.0 or A.  If it fails it scores 0.0 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 4.0 or A, and if it fails it scores 0.0 or F.

Reflectance margin
This is the value of the least reflectance difference of a bar of space from the global threshold. If this value is less than 5%, when measured as a percentage of the symbol contrast, there is a possibility that the symbol is close to failing because the edges of the bars and spaces cannot be determined. If it fails, the symbol will score 0.0 or F for decode. This parameter is not graded, but some verifiers may provide a warning if reflectance margin fails to less than 5%.

Symbol Contrast
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 4.0 to 0.0, or A to F.  The table that shows how this is done is shown opposite:

Modulation
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 as shown in this table:

Defects
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 opposite:

Decodability
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 opposite:

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.

Aperture
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.