Advertisement

In an effort to move the comparison to a more technically reliable validation basis, researchers at the Alabama Department of Forensic Sciences (ADFS) have developed a method for comparing land impressions. The measurement system used in the investigation was an optical profilometry system.Bullets fired from firearms carry a set of engraved areas (land impressions) that are characteristic of the rifling in the barrel of the firearm (Figure 1). Typically, the process of matching a bullet to a specific firearm involves firing a second bullet from the firearm and then making a direct visual comparison using a side by side comparative microscope. To guarantee a true match, all of the land impressions have to match in both form and sequence around both bullets. The task is thus to measure the land impressions and match them up from one bullet to the other. If a full match—with a high degree of reliability—is not possible, the bullets were perhaps not fired from the same firearm.

Figure 1: Land and groove impressions left on a bullet (right, source: ADFS).

Establishing a Match Using Optical Profilometry
In an effort to move the comparison to a more technically reliable validation basis, researchers at the Alabama Department of Forensic Sciences (ADFS), based in Hoover, Alabama, have developed a method1,2 for comparing land impressions. The measurement system used in the investigation was an optical profilometry system named PLu neox and an associated SensoMAP software. The system employs a dual technology sensor head combining confocal and interferometric techniques, allowing it to obtain fast, non-invasive measurements of the micro- and nanogeometry of technical surfaces.

Figure 2: Land impressions and scanned regions on two bullets fired with the same firearm.

The steps ADFS developed are as follows:

  • Measure an extended topography about 1mm wide and over the full length of the land impression (Figure 2).
  • After leveling using the least square plane, a region of interest (ROI) is selected and followed by removing the bullet’s cylinder (Figure 3).
  • A profile (section) is extracted for both bullets and these are then compared directly using functions specially developed for profile comparison.
Figure 3: SensoMAP data processing on a single scanned land impression: (left to right) true color rendering of the raw scan, false color height profile, region of interest, filtered profile, representation of the mean profile.        

Figure 4 shows mean profiles extracted from measurements obtained on matching land impressions of two bullets that are known to be fired from the same firearm. It is obvious that the two profiles are not identical: two bullets can be close but are never truly identical. Thus, a method is needed for validating the match between essentially identical land impressions.

Figure 4: Mean profiles for what should be two matching land impressions. CCFmax is 0.910 and the % value of Ds is 60.9.

Parameters for Assessing Similarity
NIST (National Institute for Standards and Technology3) has developed two reliable parameters to describe the degree of similarity.4,5

  • The cross-correlation function (CCF). The key parameter is CCFmax, which is 1 (unity) if the two profiles are identical. For the two mean profiles shown in Figure 5 the value of CCFmax is 0.910.
  • The signature difference Ds:
    Ds = Rq’2 / Rq2
    A known mean profile roughness Rq is used as a reference, and Rq’ is the roughness of the difference between two mean profiles. The term Ds yields the relative departure of the difference profile from the known profile. If the two profiles are exactly the same, Ds is obviously zero, but it increases with mounting discrepancy between the two. For the mean profiles in Figure 4 the % value of Ds is 60.9.
Figure 5: Visual comparison of the mean profile for a single land impression with the six land impressions of an effectively identical bullet.

Table 1 lists the values CCFmax and Ds for all six land impressions on two known bullets—36 comparisons in total. Looking at the two parameters CCFmax and Ds, proper assignment of the matching land impressions is now relatively straightforward. Note that, while it is common for Ds to be high even for very similar profiles, it is higher still for the other land impression comparisons. A visual comparison of the mean profiles (see rightmost image in Figure 3) enables the same result to be reached (Figure 5).

Table 1: Similarity parameters for two bullets fired from the same firearm.

Summary
This new scheme for comparing land impressions on bullets will initially serve as a secondary, independent technology method for confirming casework analyses, assisting with the confirmation of casework determinations made using traditional analyses by providing independent and objective data. Extensive accuracy and reproducibility testing according to ISO norms has already been carried out.

References

  1. D. S. McClarin, D. Stella: ‘The 10 Consecutive Ruger Barrel Study’
  2. A 3D look using the PLu neox from Sensofar’, 2nd Annual AFTE Training Seminar, Chicago, May 2011
  3. www.nist.gov/index.html
  4. L. Ma, J. Song, E. Whitenton, A. Zheng, T. Vorburger and J. Zhou: ‘NIST bullet signature measurement system for SMR (Standard Reference Material) 2460 standard bullets’, J. Forensic Sci., 49, No. 4, July 2004, pp. 649-659
  5.  J. Song, T. Vorburger, L. Ma, J. Libert and S. Ballou: ‘A metric for the comparison of surface topographies of standard reference material (SMR) bullets and casings’, 2005

Dr. Cristina Cadevall is Software Manager at Sensofar-Tech, a leading-edge technology company within the field of non-contact surface metrology. Sensofar-Tech is based in Terrassa, Spain. Niels Schwarz is Managing Partner of Sensofar, LLC, based in Carefree, Arizona.

 

Advertisement
Advertisement