Fixed Pattern Damage grading parameters, except for AG, are not renamed, but are functionally different since the global
threshold and modulation grading scale are different. As symbols obtain a significantly higher grade according to AIM-
DPM than ISO 15415, grading according to AIM-DPM is appropriate only when called for in an application specification.
Another difference is the allowance for a variety of illumination options. These include the four-sided 45˚ light that is the
default for ISO 15415. Additional lighting angles allowed are 30˚ lighting from four sides, 30˚ from two sides, which can
be either North/South or East/West, 30˚ from one side, and 90˚ diffuse on-axis lighting. The used light source is reported
using a notation that includes the angle, and a letter (Q for 4, T for two, S for single).
AIM-DPM also varies the size of the aperture until the symbol is decoded, then the grading is repeated with 50% and
80% aperture sizes. The better of the two is reported as the final grade.
Note
: When the reference decode algorithm fails to decode a symbol with both 50% and 80% aperture, the
DECODE grade is “F” and a note is printed on the grade section of the report, even if the symbol is recognized and
decoded with a different aperture size in an earlier phase of the grading procedure.
The following parameters are new or modified for AIM-DPM:
l
CC (Cell Contrast)
: The relative contrast value between bars and spaces, taken from the means of the light and
dark element CC = (Lmean - Dmean) / Lmean.
CC%
Grade
≥ 30%
4
≥ 25%
3
≥ 20%
2
≥ 15%
1
< 15%
0
l
CMOD (Cell Modulation)
: Similar to MOD in ISO 15415, CMOD measures the deviation in the reflectivity of dark
and light elements. A range for each light and dark group is created from the global threshold to the mean
reflectance of the elements. Each module is graded along the created range, then error correction capability is
considered to “discount” the effect of one or a few elements with low values, and a final grade is computed for this
parameter.
l
DDG (Distributed Damage Grade)
: Similar to AG in ISO 15415, DDG considers the effect of multiple segments of
the fixed pattern having imperfections. Where multiple segments have a low grade, the effect of this “distributed
damage” is reflected in a lower grade for DDG than the lowest of the individual segments.
l
MR (Minimum Reflectance)
: This is a requirement for at least 5% reflectance difference between light and dark
elements as a restraint on the purely relative CC parameter.
l
Decode
: Decode grade A or F depending upon whether the reference decode algorithm succeeds in decoding
the symbol with the required final aperture size.
Traditional (Non-Graded) Parameters
PCS
Ways of Quantifying Contrast: Print Contrast Signal (PCS) is an older and rarely used measure of contrast. Contrast is
intended to quantify the difference between the bars and the spaces in reflectance. PCS is defined mathematically as:
PCS = (Rmax-Rmin) / Rmax
44
Grading Standards and their Parameters
