In 2 D and 3 D Topography Comparisons of Toolmarks Produced from Consecutively Manufactured Chisels and Punches

A 2009 report by the National Academies [1] recommended strengthening the scientific basis of procedures and criteria employed by the forensic science specialty of toolmark identification. The current method of comparison and determination of identity is conducted by a trained examiner using a comparison microscope. However, the ultimate conclusion of the comparison is subjective in nature and is affected by the examiner’s skill and experience. This study seeks to evaluate whether a mathematically objective metric, the maximum value of the normalized Cross Correlation Function (CCFMAX ), can be employed to identify the tool that generated a striated or impressed toolmark from a pool of consecutively manufactured tools. The metric will be applied to the measured surface topography of toolmarks generated under laboratory conditions on a near pristine surface. A device was designed for the controlled generation of toolmarks. Two types of representative tools were selected: chisels for making striated toolmarks and drift punches for making impressed toolmarks. For striated toolmarks, a 2D stylus instrument was used to capture the toolmark topography. Impressed toolmark topographies were captured using a 3D disc scanning confocal microscope. The comparisons were blind, with fully automated data analysis and identification. Based on the CCFMAX metric and a statistical analysis of the known match and known non-match scores, all the unknown toolmarks were correctly identified to the tool that created them. This study provides additional objective scientific support for the validity of toolmark identifications. general, toolmarks have class characteristics that are common to a certain tool brand and model, sub-class characteristics common to a certain batch of manufactured tools, and individual characteristics arising from random variations in tool manufacturing, use, and wear. The latter type of characteristics, usually microscopic in nature, forms the basis for toolmark identification. Currently, optical comparison microscopes are used to assist examiners in identifying toolmarks to potential tool sources. Through their many years of training and experience, examiners are able to judge whether or not toolmarks came from the same tool. This current practice of optical reflectance microscopy produces images representing optical contrast variations that provide, through slope variations and shadowing, only an indirect measure of surface topography. The images obtained are affected by lighting conditions, multiple reflections, exposure settings, and variations in surface reflectivity (including color) [3]. In 2009, the National Academies published the report “Strengthening Forensic Science in the United States: A Path Forward [1].” This report called into question, amongst other issues, the objectivity of conclusions based on visual toolmark identification by examiners. A major concern is the lack of precisely defined, and scientifically justified, protocols that yield objective determinations of a match or non-match with well-characterized confidence limits and/or error rates. Date Received: December 3, 2013 Peer Review Completed: December 30, 2013