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 more appropriate for direct part marking applications. This standard was developed to be more representative of the scanning performance of modern readers that in some cases are specifically designed 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 easier to see. Additionally, the threshold between dark and light is calculated from the statistics of the image brightness histogram. Thus, 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 that to remove the possibility of confusion between these two methods, the parameters have been renamed. These parameters are:

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.

All of the Fixed Pattern Damage grading (other than AG shown above) are not renamed, but are functionally different since the global threshold and modulation grading scale are different. In general, symbols will obtain a significantly higher grade according to AIM-DPM than ISO 15415. Therefore, grading according to AIM-DPM is appropriate only when called for in an application specification.

Another significant 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 light source that is used 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, and then the grading is repeated with two different aperture sizes (50% and 80%) and the better of the two grades is reported as the final grade. Note that when the reference decode algorithm fails to decode a symbol with both 50% and 80% aperture, the DECODE grade will be “F” and a note will be 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 parameters, which are new or significantly modified for AIM-DPM, are explained below:

  • CC (Cell Contrast): This is the relative contrast value between bars and spaces, taken from the means of the light and dark element CC = (Lmean - Dmean) / Lmean.

    ≥ 30%4
    ≥ 25%3
    ≥ 20%2
    ≥ 15%1
    < 15%0
  • CMOD (Cell Modulation): Similar to MOD in ISO 15415, this parameter measures the deviation in the reflectivity of dark and light elements. A range for each group (light and dark) is created from the global threshold to the mean reflectance of the elements. Each module is graded along this range, then error correction capability is considered to “discount” the effect of one or a few elements with low values, and a final grade for this parameter is computed.
  • DDG (Distributed Damage Grade): Similar to AG in ISO 15415, this parameter takes into account 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.