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Forensic Topography and Surface Metrology

Summary:

We seek to build the scientific infrastructure for objective forensic firearm and toolmark identification by developing rigorous procedures to obtain quantitative uncertainty estimates for forensic conclusions, which will provide fundamental support to ballistics identification in forensic science. This principal objective is closely aligned with the project's general mission to advance traceable surface topography measurements in the U.S. by improving methods and standards to characterize and reduce measurement uncertainty, and by providing best-in-class calibrations, SRMs, and reference software.

Description:

Forensic firearm and toolmark identification is based on comparing the surface topography of forensic samples. There is a fundamental need for objective metrics and uncertainty estimates to quantify the similarity between two surfaces so that objective decisions about common origin can be made with clear statements of uncertainty. 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 assurance. NIST SRM bullets, SRM cartridge cases, and control procedures have been used in US and foreign crime labs for quality control, that has made NIST world leader in this field.

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

  • The project provides best-in-class calibrations, SRMs, and reference software for surface roughness, step height, and parameters quantifying microform. The SI-traceability of most topography measurements in the U.S. relies on this service.
  • We provide world-class surface and microform calibrations among world's best uncertainty levels. We are the leader in developing documentary standards, and conducting research to provide a sound basis for optical measurement methods.

Microform calibration of a standard Rockwell hardness diamond indenter.

Figure 1. Microform calibration of a standard Rockwell hardness diamond indenter.

Major Accomplishments:

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.
Measured three-dimensional topography of a firearm’s breech face impression on the bottom of a cartridge case.
Figure 2. 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. The vertical range of the data

Lead Organizational Unit:

pml

Customers/Contributors/Collaborators:

Our collaborators are interested in our measurement expertise, unique measurement capabilities, and unbiased standards contributions. Our forensics work is considered to be game changing. Collaborators include:

  • Forensics: ATF, FBI, NIJ, State crime labs, Leeds Forensic Systems, Cadre Forensics, John Jay College of Criminal Justice.
  • Surface metrology: PTB, NPL, Timken, Federal-Mogul, Professional Instruments, Michigan Metrology, Zygo, Zeiss, Bruker, Apple, UNCC, Worcester Polytechnic.

Facilities/Tools Used:

Specialized Equipment:

  • Talystep stylus instrument for step height measurement (0.04 nm repeatability).
  • Form Talysurf stylus instrument for surface topography measurement (12 mm z-range,1 nm z-resolution, 200 mm x-range, and 0.125 μm x-resolution).
  • Disk scanning confocal microscope for three-dimensional surface topography measurement, in particular of forensic samples (3 nm z-resolution).
  • Interferometric microscope for three-dimensional surface topography measurement (phase shifting and coherence scanning).

Specialized, Best-In-Class, Measurement Capabilities:

  • Surface roughness calibrations with nanometer level uncertainty (Ra).
  • Step height calibrations down to 0.15 nm uncertainty.
  • Microform calibration with the world's lowest uncertainty for the microform calibration of standard Rockwell hardness diamond indenters.

Specialized NIST Facilities:

  • Forensic comparison microscope (Special Programs Office).
  • Firing range (Special Programs Office).

Staff:

Jun-Feng (John) Song, Project Leader
Rick Silver
Daniel Ott
Thomas Brian Renegar
Johannes A. Soons
Robert Thompson
J. Yen
Nien-Fan Zhang
Xiaoyu Alan Zheng

Associates:

Dr. Theodore V. Vorburger
Wei Chu
Mingsi Tong
Hao Zhang
Hui Zhou

Contact

Physical Measurement Laboratory (PML)
Semiconductor & Dimensional Metrology Division (683)

General Information:
301-975-5609 Telephone
301-869-0822 Facsimile

100 Bureau Drive, M/S 8212
Gaithersburg, Maryland 20899-8212