Surface metrology is commonly used to characterize functional engineering surfaces. The technologies developed offer opportunities to improve forensic toolmark identification. Toolmarks are created when a tool comes into contact with a surface and causes plastic deformation. Toolmarks are commonly found on fired bullets and cartridge cases. Trained firearms examiners use these toolmarks to link an evidence bullet or cartridge case to a specific tool (gun), which can lead to a criminal conviction. Currently, the identification is typically based on qualitative visual comparison by a trained examiner using a comparison microscope. In 2009, a report by the National Academies called into question, amongst others, the objectivity of visual toolmark identification by firearms examiners. The National Academies recommended development of objective toolmark identification criteria and characterization of error rates. The National Institute of Standards and Technology (NIST) uses its experience in surface metrology to develop objective identification criteria, measurement methods, and reference artefacts. NIST developed the Standard Reference Material SRM 2460 standard bullet and SRM 2461 standard cartridge case to facilitate quality control and traceability of identifications performed in crime laboratories. Objectivity is improved through measurement of areal surface topography and application of unambiguous surface similarity metrics, such as the maximum value (ACCFMAX) of the areal cross correlation function. Case studies were performed on consecutively manufactured tools, such as gun barrels and breechfaces, to demonstrate that even in this worst case scenario, all the tested tools imparted unique surface topographies that were identifiable. These studies provide scientific support for toolmark evidence admissibility in criminal court cases.
Citation: Surface Topography: Metrology and Properties
Pub Type: Journals
Toolmark, Metrology, Cross Correlation Function, Forensics