Forensic Topography and Surface Metrology

Summary

We are working to improve the scientific infrastructure for objective forensic firearm and toolmark identification. We focus on the development of measurement methods, objective comparison metrics, and rigorous procedures to obtain quantitative uncertainty estimates for forensic conclusions. This work is part of the project's general mission to advance traceable surface topography measurements in the U.S. by developing methods to characterize and reduce measurement uncertainty and by providing best-in-class calibrations, standard reference materials (SRMs), and reference software.

Description

Measured three-dimensional topography of a firearm’s breech face impression on the bottom of a cartridge case. The outer diameter of the donut-shaped area is approximately 4.3 mm.

Surface topography affects the functionality of manufactured parts. To control surface topography, industry requires increasingly advanced measurement methods.

• The project provides best-in-class calibrations, SRMs, and reference software for surface roughness, step height, and microform. The SI-traceability of most topography measurements in the U.S. relies on this service.
• We provide surface and microform calibrations with uncertainty levels that are among the world's best. We lead the development of documentary standards, and conduct research to provide a sound basis for optical measurement methods.
• We provide an interactive website, the Surface Metrology Algorithm Testing System (SMATS), for quantitative assessment of surface topography software. The user can download or upload surface profiles and topography images and compare our calculated parameter values with those obtained by the user's software.

Forensic firearm and toolmark identification is based on comparing the surface topography of forensic samples, such as fired bullets and ejected cartridge cases. The examiner typically must determine whether microscopic marks on compared surfaces were made by the same firearm or tool. This determination is challenging, not only because there are differences between marks generated by the same firearm, but also because there are possible similarities between marks generated by different firearms. There is a fundamental need in forensics for objective, quantitative methods to evaluate the similarity between evidence and reference surfaces so that objective decisions about common origin can be made with uncertainty statements. Therefore:

• We conduct research to provide a scientific foundation for forensic identification. Our focus is on metrics and algorithms for objective identification, and on rigorous methods to estimate the uncertainty (or error rate) of a forensic conclusion.
• We collaborate on the characterization and improvement of measurement techniques and on documentary standards for measurement quality assurance. NIST SRM 2460 Standard Bullets and SRM 2461 Standard Cartridge Cases are used in both U.S. and foreign crime labs for quality control. We provide a crowd-sourced, open-access, research database of firearm toolmarks on bullets and cartridge cases. The database contains both traditional reflected light microscopy images and three-dimensional surface topography data. The data is used internationally for the development and validation of methods for objective identification and error rate estimation.

Major Accomplishments

2016

• Developed a crowd-sourced, open-access, research database of firearm toolmarks on bullets and cartridge cases. The database has a dynamic web interface enabling users to upload, search, and download datasets. We obtained and measured bullet and cartridge case samples from crime lab reference collections and foundational ballistics studies, such as studies involving consecutively manufactured firearm components and persistence firings.
• Refined the Congruent Matching Cells (CMC) identification method, including a new convergence algorithm, which improved specificity, sensitivity, and error rate estimation. We developed an approach using a “similarity map” to highlight areas of two compared samples that are highly similar or dissimilar, providing a visual aid to forensic examiners.
• Completed development and characterization of a replication process for replenishing the sold-out SRM 2460 Standard Bullet. We developed and tested a durable, sputter coated, metal surface layer for the polymer replicas. Produced 115 replica SRM bullets that are currently being certified. We furthermore provided replicas for a major international examiner proficiency study.

2015

• Initial statistical approach to estimate and express the uncertainty ('error rate') of a firearm identification result obtained using the Congruent Matching Cells (CMC) method.
• Prototype SRM bullet replicas for quality control of forensic measurements.
• A series of (draft) ISO standards describing performance parameters and sources of measurement uncertainty for all major optical measurement methods for surface texture – ISO 25178: Parts 601 to 607.

2014

• NBIC II study established SRM-based procedures for statistical quality control of ballistics toolmark measurements in forensic laboratories.
• Improved ISO standard for Rockwell hardness indenters based on NIST research results.
• Prototype NIST Ballistics Toolmark Research Database and characterization of over 1000 forensic samples.

2013

• Congruent Matching Cells (CMC) method for accurate and objective firearm identification. The method was tested using both topography data and reflectance microscopy images from studies involving consecutively manufactured firearms.
• Awarded SBIR Phase 2 project to address the need for smooth surface roughness SRMs through development of SRM 2076-2078 "High-precision random profile roughness specimens," with a roughness average (Ra) down to 1/10th of PTB specimens.
• Characterized the effects of the probe tip size on the measured roughness average (Ra) of specimens with rectangular profiles.

Associated Product(s)

• Surface Metrology Algorithm Testing Service (SMATS)
• NIST Ballistics Toolmark Research Database
• NIST microform calibration system for standard Rockwell diamond indenters.  

Forensic Science, Ballistics, Nanotechnology, Nanofabrication / manufacturing and Nanometrology