Micrometer Level Surface Finish Metrology Project

Provide optical three-dimensional surface topography measurement at the micrometer level. Conventional surface topography involves contact stylus instruments and are considered the "gold standard" in surface metrology. Advances in optical instrumentation for surface topography allows rapid three dimensional characterization of surfaces. Characterization of these instruments and establishing their traceability at the micrometer scale is imperative to many industries and government agencies. Applications include the traceable measurement of hardness, where the shape of the indenter strongly affects results and the measurement of topography of bullets and casings in crime labs, where the fine individual characteristics that produce positive identification of individual weapons need to be separated from longer scale characteristics that characterize overall shape.

This project will establish the limits of validity for surface texture measurement obtainable by optical methods and developing standards for measurement of surface texture and topography derived from optical measurements. The research directions include: 1) the measurement of surfaces using optical techniques and the comparison of the results with stylus methods, 2) leadership in the development of documentary standards for optical techniques, and 3) the development of physical standards and measurement parameters for optical microscopes used in crime labs to examine bullets and casings.

Specifically, the project will investigate the large errors arising with the use of coherence scanning interferometry for measurement of roughness average in the 50 nm to 300 nm range. This technique will be compared with that of confocal microscopy and the more fundamental stylus techniques. Additionally, documentary standards for phase shifting interferometric microscopy, coherence scanning microscopy, and confocal chromatic probing, will be developed. As a specific application of optical techniques, the project will continue to evolve the physical standard, SRM 2460, for optical examination of bullets and an analogous standard, SRM 2461, for examination of the casings. This work has application to the infrastructural work on optical techniques, discussed above, because the similarity between surface topography as measured by stylus and optical techniques is of crucial importance to establishing the validity of optical techniques.

Challenge/Problem Addressed: U.S. manufacturing needs improved speed, resolution, and accuracy of surface finish measurements to enable improved productivity. Criminologists need a traceability system for optical inspection devices of bullets and casings in crime laboratories according to recently developed guidelines of the American Society of Crime Laboratory Directors (ASCLD).

• The NIST Report, Surface Topography Analysis for a Feasibility Assessment of a National Ballistics Imaging Database, was heavily cited by the National Academies in their publication, entitled Ballistic Imaging, one of the reports leading to a substantial ongoing re-evaluation of the use of firearms evidence from fired bullets and casings; 2008.
• The On-line Surface Metrology Algorithm Testing System has received thousands of hits by users and has been used in an international comparison of Surface Metrology Software, recently published by NPL (UK).