Forensic Topography and Surface Metrology

Summary

This project provides methods, standards, and services for characterizing surface texture and microform in manufacturing, forensics, and science. We seek to advance SI-traceable measurements by developing methods to characterize and reduce measurement uncertainty, and by providing best-in-class calibrations, standard reference materials (SRMs), and reference software. A major application area addressed by the project is forensic firearm and toolmark identification, which is based on comparing microscopic marks in the topography of forensic samples. Here we contribute to the scientific infrastructure needed for objective forensic evaluations by developing traceable measurement methods, objective comparison metrics, and statistically rigorous procedures to obtain quantitative estimates for uncertainty or weight of evidence.

Description

Random profile roughness artifact to characterize the accuracy of a surface roughness measurement instrument. We are developing an updated SRM with low roughness average (Ra of 0.05 µm).

Surface texture and microform

Surface texture affects the functionality of many products, ranging from bearings to semiconductors and optics. Affected properties include aesthetics, friction, wear, lubrication, sealing, light scattering, and conductivity. Improvements in parameters and metrology to characterize surface texture enable innovations and facilitate commerce. Major challenges are the large variety of applications, the large number of parameters available to specify and characterize surface texture, and the large array of mechanical and optical measurement techniques. The project seeks to advance SI-traceable metrology of surface texture and microform through:

• Best-in-class, highly leveraged, calibrations of transfer standards for surface roughness, step height, lateral spacing, and microform. We furthermore provide SRMs for instrument calibration and quality control. Millions of surface topography measurements in the U.S. are SI-traceable through these calibrations and SRMs.

• 

  Mechanical stylus instrument at NIST for surface texture and microform measurements. The insert shows examples of specialized artifacts used for its calibration.

  Leadership in the development of national and international documentary standards. These standards provide definitions and procedures for surface texture specification, topography measurement, instrument calibration, and measurement uncertainty evaluation.
• Reference software and datasets for validating the values of surface texture parameters calculated by surface analysis software. We provide an interactive website, the Surface Metrology Algorithm Testing System (SMATS), that enables the user to compare our calculated parameter values with those obtained by the user's software.
• Parameters and calibrations for the microform of standard hardness Rockwell-C (HRC) diamond indenters. Our world-leading low measurement uncertainty benefits the calibration of hundreds of NIST SRM 2810-2812 HRC hardness standard blocks.

False color 3D image of the measured topography of the primer surface at the bottom of a fired cartridge case. The color indicates surface height. The diameter of the imaged area is approximately 3.8 mm. The dark blue center is the impression from the firearm’s firing pin.

Firearm and toolmark examination

Forensic firearm and toolmark examination is based on comparing the surface topography of forensic samples, such as fired bullets and ejected cartridge cases. When a firearm is fired, it leaves microscopic toolmarks on the bullet and cartridge case. Forensic examiners compare these marks to determine whether two cartridge cases or bullets were fired from the same firearm. Currently, this evaluation is subjective, relying on an examiner’s expertise and judgement. The evaluation 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 are two major shifts occurring in this forensic area: 1) a transition from two-dimensional (2D) reflectance microcopy images of toolmarks to three-dimensional (3D) topography images, and 2) a desired transition from subjective match/no-match assessments to objective comparisons with a rigorous evaluation of uncertainty to quantify the weight of evidence. We address these challenges through:

• Characterization and improvement of measurement techniques, and the development of standards and best practice guides for measurement quality assurance in forensic labs. Many forensic labs use the NIST SRM 2460 Standard Bullet and SRM 2461 Standard Cartridge Case for for quality control.
• Research to improve the scientific foundation for firearm and toolmark examination. Our focus is on metrics and algorithms for objective comparisons, and on statistical methods to quantify the weight of evidence.
• Reference and research databases of firearm toolmark images of bullets and cartridge cases. The databases are needed for the development and validation of methods for objective identification and the development of statistical models for weight of evidence estimation.

Major Accomplishments

2020

• Leadership in completing the 2019 update of the ASME B46.1 standard “Surface Texture (Surface Roughness, Waviness, and Lay).” The standard provides a comprehensive overview of surface texture parameters and measurement methods. 
• Pilot study on the objective comparison of fragmented or severely deformed bullets. The study showed improved comparison results through the application of an image reconstruction procedure of the deformed striation marks in combination with the Congruent Matching Profile Segments (CMPS) comparison method developed at NIST. 
• Feasibility study on the application of objective comparison methods to scenarios where the evidence firearm is inoperable. In the study, the Congruent Matching Cells (CMC) method was successfully applied to compare the firearm breech face impression on cartridge cases with casts of the breech face of “inoperable” firearms. No significant change in comparison performance was observed.
• Application of CMC comparison score distribution models for firearm breech face impressions on cartridge cases to obtain a quantitative expression for the weight of evidence of a comparison result as a likelihood ratio.  

2019

• Leadership in completing the new ISO standard 25178-600:2019 “Geometrical product specifications (GPS) – Surface texture: Areal – Part 600: Metrological characteristics for areal topography measuring methods.” The standard defines generic metrological characteristics of measurement instruments that are important for instrument performance evaluation and measurement uncertainty estimation.
• High-resolution 2D images and 3D topography models of ballistic samples from the President John F. Kennedy assassination. The National Archives and Records Administration (NARA) requested the measurement data for digital preservation and to facilitate public access. The project extended the limits of toolmark measurement and representation for complex bullet fragments.
• Congruent Matching Profile Segments (CMPS) method for objective comparison of striated tool marks and its application to bullet comparisons. The method is derived from the Congruent Matching Cells (CMC) method developed at NIST for the comparison of impressed tool marks. The CMPS method is designed to increase comparison accuracy by addressing the challenges of differences in same-source striae profiles caused by varying lateral/vertical scales and sections of striae that are poorly marked.
• Re-calibration of two NIST primary standard Rockwell diamond indenters using the NIST microform calibration system established in 1995. The results demonstrated a long-term reproducibility for both the NIST standard indenters and the calibration system, which support NIST’s leading position in international hardness standardization.

2018

• Development and certification of the NIST SRM 2460a Standard Bullet Replica for quality control of bullet measurements in forensic labs. The polymer SRM with a sputter-coated metal surface is a high-precision, low-cost, replica of our sold-out, diamond-turned, copper SRM 2460 standard bullet.
• Overview paper on the quality assurance challenges faced by forensic laboratories when integrating 3D topography metrology into casework.
• Statistical distribution models to estimate the error rate of firearm identification results obtained using Congruent Matching Cells (CMC). The CMC method was developed at NIST for the objective comparison of impressed toolmarks, such as firearm breech face impressions on cartridge cases. The method holds promise for very low false positive error rates.  

Associated Product(s)

• Dimensional Metrology
• Forensic science
• Ballistics
• Calibration Services
• Documentary Standards
• Standard Reference Materials