How Magnification Affects the 3D Imaging and Magnification of Fired Cartridge Cases with TopMatch-GS 3D

When a firearm is discharged, the bullet and cartridge case acquire unique microscopic markings called toolmarks. Firearm forensics involves comparing two objects with similar toolmarks. Fired cartridge cases are examined with a comparison light microscope and entered into a database. 2D digital images may be entered into a database of previously imaged casings to compare and identify connections with other incidents. The comparison assigns a match score for the casings and the examiner directly examines the evidence and test to determine if there is a true match. Lighting conditions may obscure or illuminate the casing’s toolmarks. Comparing the 3D geometry rather than its 2D representation avoids potential ambiguities from lighting effects. Once captured, these new 3D systems let the examiner view the surface topography under various lighting conditions, reducing the amount of time an examiner spends on a single case. The prototype TopMatch-GS 3D system (Cadre Forensics; Chicago, IL) lets the examiner measure 3D surface topographies, compute a match score, and note geometrically similar areas of the casing. Several studies investigate various methods of producing a 3D image, yet few studies explore the impact of magnification for comparing 3D images. The prototype TopMatch-GS 3D system analyzed 12 pairs of test fires, where each pair was fired in the same firearm. These test fires cause the casings to range in matching difficulty. This small set of casings may reveal how well the prototype system matches cartridge casings at a variety of magnifications (4x, 3x, 2x, 1.5x, and 1.x). It is hypothesized that higher magnification will capture additional detail than lower magnification; however, it is unclear if additional resolution is unnecessary. All 12 pairs of test fires are scanned at each magnification, compared against each other, and the match score recorded for the known match and the highest known nonmatch. This study is one step closer towards understanding scanning resolution requirements for cartridge case comparison in firearm forensics.