Non-contact optical metrology for forensic applications

A common task in forensic inspection is to associate a bullet found at a crime scene with a specific firearm. Due to rifling inside a gun’s barrel, striated marks are imparted onto bullets during the firing process. These marks provide detailed information that permits confirmation of whether or not a particular bullet was fired through a suspected firearm.

Traditionally, skilled forensic scientists use a comparison microscope to draw subjective conclusions regarding pattern agreement between an unknown evidence bullet and a known test fire bullet. To enhance the scientific basis for these conclusions, researchers at the Alabama Department of Forensic Sciences (ADFS), based in Hoover (AL, USA), employed an optimized PLu neox optical profiling system from Sensofar (Barcelona, Spain). The PLu neox utilizes a dual technology sensor head that combines both confocal and interferometric techniques, making it a very flexible and ideal tool for obtaining fast, reliable and non-invasive measurements of the micro- and nanogeometry of bullet surfaces.

To accomplish the task of objectively comparing bullet surface features, a customized, forensic-specific and user-friendly software extension was designed for the SensoMAP software of the PLu neox system. A conclusion about the quality of the comparison is drawn from a single objective correlation value that is modeled after the value successfully employed by NIST (National Institute for Standards and Technology, USA).

This new scheme for comparing marks on bullets is designed to compliment current workflows in a forensic lab setting serving as a secondary, independent and technological approach to confirming casework analyses. The power of the technology is that it enhances casework determinations through the association of an independent and objective value, adding a level of confidence to every analysis performed. Extensive accuracy and reproducibility testing of the PLu neox system according to ISO specifications has already been carried out, providing support for the practicalities of implementing this instrument into a forensic laboratory environment.

Visit us at AAFS
February 18-23, 2013, Marriott Wardman Park Hotel, Washington DC, USA