OSAC Forensic Metrology Standards Update

To support admissibility, periodic quality assurance processes should be applied to measuring instruments used in evidence collection. Terrestrial laser scanners (TLSs) are used extensivity in forensic practice to preserve and document evidence, but until recently the forensic community has lacked a standardized procedure by which to assess TLS performance interim to routine manufacturer calibrations. The Crime Scene Investigation and Reconstruction (CSIR) subcommittee within the Organization of Scientific Area Committees (OSAC) for Forensic Science has addressed this gap by developing a TLS Interim Performance Assessment (IPA) specifically designed to be performed in-house by forensic practitioners.

This talk outlines the motivation and challenges behind the IPA development efforts and describes its distinct testing procedures. There are two parts to the IPA, which separately evaluate instrument accuracy (“Part I”) and instrument precision (“Part II”). The IPA involves the measurement of 20 checkerboard targets from four TLS scan positions. Additionally, six specified lengths must be calibrated using an appropriate tape measure or other calibrated measurement instrument. In the Part I test procedure, the errors between TLS-measured lengths and the calibrated lengths are compared to a user-specified MPE. In the Part II test procedure, a statistical methodology is used to assess whether the instrument’s precision has significantly changed since a baseline state.

Importantly, the IPA does not replace periodic manufacturer-performed calibrations. Designed to be inexpensive, concise, and flexible, the IPA instead offers forensic TLS users an intermediate solution for assessing instrument performance between regularly scheduled maintenance services. Widespread adoption of the IPA will further strengthen forensic data integrity.