ISO 29158 (AIM-DPM) 2006 Grading Parameters
The ISO 29158 (AIM-DPM) method of grading Data Matrix symbols modifies the process of ISO-15415 and is appropriate for direct part marking applications. This standard is more representative of the scanning performance of modern readers that are designed in some cases for these demanding applications. In this method, the image brightness is adjusted to produce an image of the symbol that fills most or all of the dynamic range of the imager, resulting in an image that is easy to see. The threshold between dark and light is calculated from the statistics of the image brightness histogram. Therefore, the measurements calculated by AIM-DPM differ from those of ISO 15415 significantly. Some of the parameters reported in ISO 15415 are changed so drastically and are renamed to avoid confusion between the two methods:
| AIM-DPM Parameter Name | ISO 15415 Parameter Name | Summary of Change(s) |
|---|---|---|
| CC (Cell Contrast) | SC (Symbol Contrast) | Made relative to light background. |
| CM (Cell Modulation) | MOD (Modulation) | Threshold calculated from statistics rather than the maximum and minimum reflectance. Grading scale range set to Mean of distribution, rather than maximum and minimum reflectance. |
| DD (Distributed Damage) | AG (Average Grade) | Modulation overlay uses only A, B, D, and F levels instead of A, B, C, D, and F. |
| MR (Minimum Reflectance) | Not necessary, since SC is measured on an absolute scale. | An absolute limit on SC of 5% added to temper the relative nature of CC and “catch” extremely low contrast symbols. |
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.
The following parameters are new or modified for AIM-DPM:
- 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 - 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.
- 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.
- 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.
- Decode: Decode grade A or F depending upon whether the reference decode algorithm succeeds in decoding the symbol with the required final aperture size.