Topography measurements and applications in ballistics and tool mark identifications.

The application of surface topography measurement methods to the field of firearm and toolmark analysis is fairly new. The field has been boosted by the development of a number of competing optical methods, which has improved the speed and accuracy of surface topography acquisitions. We describe here some of these measurement methods as well as several analytical methods for assessing similarities and differences among pairs of surfaces. We also provide a few examples of research results to identify cartridge cases originating from the same firearm or tool marks produced by the same tool. Physical standards and issues of traceability are also discussed.

The Second National Ballistics Imaging Comparison (NBIC-2).

In response to the guidelines issued by the American Society of Crime Laboratory Directors/Laboratory Accreditation Board (ASCLD/LAB-International) to establish traceability and quality assurance in U.S. crime laboratories, NIST and the ATF initiated a joint project, entitled the National Ballistics Imaging Comparison (NBIC). The NBIC project aims to establish a national traceability and quality system for ballistics identifications in crime laboratories utilizing ATF's National Integrated Ballistics Information Network (NIBIN). The original NBIC was completed in 2010. In the second NBIC, NIST Standard Reference Material (SRM) 2461 Cartridge Cases were used as reference standards, and 14 experts from 11 U.S. crime laboratories each performed 17 image acquisitions and correlations of the SRM cartridge cases over the course of about half a year. Resulting correlation scores were collected by NIST for statistical analyses, from which control charts and control limits were developed for the proposed quality system and for promoting future assessments and accreditations for firearm evidence in U.S. forensic laboratories in accordance with the ISO 17025 Standard.

The National Ballistics Imaging Comparison (NBIC) project.

In response to the guidelines issued by the American Society of Crime Laboratory Directors/Laboratory Accreditation Board (ASCLD/LAB-International) to establish traceability and quality assurance in U.S. crime laboratories, a NIST/ATF joint project entitled National Ballistics Imaging Comparison (NBIC) was initialized in 2008. The NBIC project aims to establish a National Traceability and Quality System for ballistics identifications in crime laboratories within the National Integrated Ballistics Information Network (NIBIN) of the U.S. NIST Standard Reference Material (SRM) 2460 bullets and 2461 cartridge cases are used as reference standards. 19 ballistics examiners from 13 U.S. crime laboratories participated in this project. They each performed 24 periodic image acquisitions and correlations of the SRM bullets and cartridge cases over the course of a year, but one examiner only participated in Phase 1 tests of SRM cartridge case. The correlation scores were collected by NIST for statistical analyses, from which control charts and control limits were developed for the proposed Quality System and for promoting future assessments and accreditations for firearm evidence in U.S. forensic laboratories in accordance with the ISO 17025 Standard.

Evaluation of polydimethylsiloxane modification methods for cell response.

Many methods exist in the literature to modify surfaces with extracellular matrix (ECM) proteins prior to cell seeding. However, there are few studies that systematically characterize and compare surface properties and cell response results among modification methods that use different bonding mechanisms. In this work, we compare cell response and physical characterization results from fibronectin or laminin attached to polydimethylsiloxane (PDMS) elastomer surfaces by physical adsorption, chemisorption, and covalent attachment to determine the best method to modify a deformable surface. We evaluate modification methods based on completeness and uniformity of coverage, surface roughness, and hydrophilicity of attached ECM protein. Smooth muscle cell adhesion, proliferation, morphology, and phenotype were also evaluated. We found that chemisorption methods resulted in higher amounts of protein attachment than physical adsorption and covalent bonding of the ECM proteins. Cell response to protein-modified surfaces was similar with respect to cell adhesion, area, aspect ratio, and phenotype. When all the data are considered, the chemisorption methods, most notably silane_70, provide the best surface properties and highest cell proliferation.

Measurements and predictions of light scattering by clear coatings.

Comparisons are made between calculated and measured angle-resolved light-scattering distributions from clear dielectric isotropic epoxy coatings over a range of rms roughness conditions, resulting in strongly specular scattering to diffuse scattering characteristics. Calculated distributions are derived from topography measurements performed with interferometric microscopes. Two methods of calculation are used. One determines the intensity of scattered light waves with a phase integral in the Kirchhoff approximation. The other is based on the reflection of light rays by locally flat surfaces. The angle-resolved scattering distributions for the coatings are measured with the spectral trifunction automated reference reflectometer (STARR) developed by the National Institute of Standards and Technology. Comparisons between measured and calculated results are shown for three surfaces with rms roughness values of approximately 3, 150, and 800 nm for an angle of incidence of 20 degrees .

Angular distribution of light scattered from a sinusoidal grating.

The angular distributions of light scattered by gold-coated and aluminum-coated gratings with amplitudes of approximately 90 nm and periods of 6.67 microm were measured and calculated for light incident from a He-Ne laser at an angle of 6 degrees. Experimental results are compared with predictions of Beckmann's scalar theory and Rayleigh's vector theory. The measured scattering pattern has a background of scattered light due mainly to residual surface roughness. Also the power in the higher-order peaks is larger by several orders of magnitude than the computed one, which can be attributed mainly to the low-order contributions of the harmonics in the profile.

Light scattering by sinusoidal surfaces: illumination windows and harmonics in standards.

Sinusoidal surfaces can be used as material standards to help calibrate instruments that measure the angular distribution of the intensity of light scattered by arbitrary surfaces, because the power in the diffraction peaks varies over several orders of magnitude. The calculated power in the higher-order diffraction peaks from sinusoidal surfaces expressed in terms of Bessel functions is much smaller than the values determined from angular distributions that are measured or computed from measured profiles, both of which are determined mainly by the harmonic contents of the profile. The finite size of the illuminated area, represented by an illumination window, gives rise to a background that is much larger than the calculated power in the higher-order peaks. For a rectangular window of a size equal to an even number of periods of the sinusoid, a computation of the power distribution produces minima at or near the location of the diffraction angles for higher-order diffraction angles.

Microform Calibration Uncertainties of Rockwell Diamond Indenters.

National and international comparisons in Rockwell hardness tests show significant differences. Uncertainties in the geometry of the Rockwell diamond indenters are largely responsible for these differences. By using a stylus instrument, with a series of calibration and check standards, and calibration and uncertainty calculation procedures, we have calibrated the microform geometric parameters of Rockwell diamond indenters. These calibrations are traceable to fundamental standards. The expanded uncertainties (95 % level of confidence) are ±0.3 µm for the least-squares radius; ±0.01° for the cone angle; and ±0.025° for the holder axis alignment calibrations. Under ISO and NIST guidelines for expressing measurement uncertainties, the calibration and uncertainty calculation procedure, error sources, and uncertainty components are described, and the expanded uncertainties are calculated. The instrumentation and calibration procedure also allows the measurement of profile deviation from the least-squares radius and cone flank straightness. The surface roughness and the shape of the spherical tip of the diamond indenter can also be explored and quantified. Our calibration approach makes it possible to quantify the uncertainty, uniformity, and reproducibility of Rockwell diamond indenter microform geometry, as well as to unify the Rockwell hardness standards, through fundamental measurements rather than by performance comparisons.

The Measurement and Uncertainty of a Calibration Standard for the Scanning Electron Microscope.

Standard Reference Material 484 is an artifact for calibrating the magnification scale of a Scanning Electron Microscope (SEM) within the range of 1000 × to 20000 ×. Seven issues, SRM-484, and SRM-484a to SRM-484f, have been certified between 1977 and 1992. This publication documents the instrumentation, measurement procedures and determination of uncertainty for SRM-484 and illustrates with data from issues 484e and 484f.

Autocorrelation functions from optical scattering for one-dimensionally rough surfaces.

The relationship between the height autocorrelation function of a one-dimensionally rough surface and the Fourier transform of the intensity distribution of the light scattered by that surface is tested experimentally. The theory is derived by using the Fraunhofer approximation, without recourse to the inconsistent Kirchhoff boundary conditions. In spite of the limitations imposed by the approximations used, the results obtained from optical data agree well with those obtained from stylus data, even for an autocorrelation length as small as the optical wavelength. However, this method should be limited to surfaces with rms roughness smaller than approximately 0.14 times the wavelength of light.

Regimes of surface roughness measurable with light scattering.

In this paper we summarize a number of previous experiments on the measurement of the roughness of metallic surfaces by light scattering. We identify several regimes that permit measurement of different surface parameters and functions, and we establish approximate limits for each regime. Using a straightforward criterion, we calculate that the smooth-surface regime, in which the angular distribution of scattered light is closely related to the power spectral density of the roughness, ranges over 0 < σ/λ ≲ 0.05, where σ is the rms roughness and λ is the opitcal wavelength. Above that the surfaceautocorrelation function may be calculated from a Fourier transform of the angular distribution over 0 < σ/λ ≲ 0.14. Then comes the specular regime where the specular beam can still be identified andmeasured over 0 < σ/λ ≲ 0.3. For all these regimes and for rougher surfaces too, the rms width of thescatter distribution is proportional to the rms slope of the surface.

Stylus profiling at high resolution and low force.

This paper describes experimental work to improve the lateral resolution of stylus instruments. Our efforts involve (1) use of a fine stylus, (2) low stylus load, (3) high magnification in the lateral direction, and (4) specimens with fine surface structure by which the lateral resolution of stylus instruments could be detected. By using styli with tip widths between 0.05 and 0.15-microm, a stylus load of 0.6-1.2 x 10(-6)-N (0.06-0.12-mgf), and a piezostage for lateral displacement, we detected 0.05-0.15-microm lateral resolution on the surfaces of different kinds of specimens. To get a high lateral resolution, the most important consideration is a fine stylus with small tip size.

Light-scattering measurement of the rms slopes of rough surfaces.

Angle-resolved light scattering (ARLS) is used to estimate the root-mean-square (rms) slopes of rough surfaces having a well-defined lay, and the effect on slope measurements caused by changing the angles of incidence and scattering is investigated. The ARLS patterns are taken with the Detector Array for Laser Light Angular Scattering (Dallas) research instrument, and the rms slopes are obtained from the angular widths of these patterns. In general, it was found that the angular width, and thus the estimated rms slope, is surprisingly insensitive to relatively large changes in both the incident and scattering angles of light. These results are independent of surface material and are valid for both sinusoidal and random rough surfaces with lay. The principles, experiments, analyses, and conclusions involved in using ARLS to estimate rms surface slopes are described.

Light scattering from glossy coatings on paper.

The application of angle-resolved light scattering (ARLS) to the measurement of the surface roughness of glossy coatings on paper was investigated. To this end, ARLS patterns were measured for laser light scattered from several glossy paper samples, and these patterns were compared with those calculated using a theoretical model based on plane-wave scattering from an isotropic rough surface. Mechanical stylus profilometry data for the rms roughnesses and the autocorrelation functions of the coatings were used as input to calculate the patterns. For all the paper samples measured, as well as for all the incidence angles used, there was good agreement between the experimental and the calculated patterns when all the rms roughnesses measured by profilometry were reduced by 30%. The indication from these experiments is that ARLS may be used to determine the roughness parameters of the coatings. As a check on these results, measurements were also performed with a commercial optical surface probe; these data agreed well with both the ARLS and the stylus profilometry results.

Standard Reference Specimens in Quality Control of Engineering Surfaces.

In the quality control of engineering surfaces, we aim to understand and maintain a good relationship between the manufacturing process and surface function. This is achieved by controlling the surface texture. The control process involves: 1) learning the functional parameters and their control values through controlled experiments or through a long history of production and use; 2) maintaining high accuracy and reproducibility with measurements not only of roughness calibration specimens but also of real engineering parts. In this paper, the characteristics, utilizations, and limitations of different classes of precision roughness calibration specimens are described. A measuring procedure of engineering surfaces, based on the calibration procedure of roughness specimens at NIST, is proposed. This procedure involves utilization of check specimens with waveform, wavelength, and other roughness parameters similar to functioning engineering surfaces. These check specimens would be certified under standardized reference measuring conditions, or by a reference instrument, and could be used for overall checking of the measuring procedure and for maintaining accuracy and agreement in engineering surface measurement. The concept of "surface texture design" is also suggested, which involves designing the engineering surface texture, the manufacturing process, and the quality control procedure to meet the optimal functional needs.

Direct and inverse problems for light scattered by rough surfaces.

Calculations are performed to relate the stylus profile of a one-dimensionally rough surface to the angular distribution of the light scattered by such a surface. In the direct problem, the angular distribution of the scattered light calculated from the profile is shown to agree with the measured one. In the inverse problem, the rms roughness and the autocorrelation function are found by a least-squares fit to the measured angular distribution. For the smoother surfaces, the rms roughness is mostly determined by the ratio between the power of the specular beam and the total power of the scattered light; the computed values are proportional to those calculated directly from the stylus profiles. The values of the parameters obtained by the least-squares fit are affected by a variety of errors and agree only partially with those obtained from the stylus profile.

Surface Roughness Studies with DALLAS-Detector Array for Laser Light Angular Scattering.

An instrument has been developed to study surface roughness by measuring the angular distributions of scattered light. In our instrument, a beam from a He-Ne laser illuminates the surface at an angle of incidence which may be varied. The scattered light distribution is detected by an array of 87 fiber optic sensors positioned in a semicircular yoke which can be rotated about its axis so that the scattered radiation may be sampled over an entire hemisphere. The output from the detector array is digitized, stored, and analyzed in a laboratory computer. The initial experiments have concentrated on measurements of stainless steel surfaces which are highly two-dimensional and which yield scattering distributions that are localized in the plane of incidence. The results are analyzed by comparing the angular scattering data with theoretical angular scattering distributions computed from digitized roughness profiles measured by a stylus instrument. The theoretical distributions are calculated by substituting the roughness profiles into the operand of an integral equation for electromagnetic scattering developed by Beckmann and Spizzichino. This approach directly tests the accuracy of the basic optical theory.

Ellipsometry of rough surfaces.

Ellipsometry measurements on several different kinds of rough surfaces were compared with stylus measurements of the surface texture. For steeply sloped periodic surfaces, the ellipsometric angles Delta and psi varied rapidly as the angle of incidence was varied near a diffraction minimum. This effect is interpreted in terms of the Kirchhoff theory and is ascribed to interference between singly and doubly reflected light waves. For a set of Ni replicas of machined surfaces with random surface profiles, Delta and psi varied systematically with surface texture. These variations persisted even after the surface composition was changed by evaporating first Al then Au on the surfaces. The systematic effects due to surface roughness are in disagreement with those of a previous experiment and are not readily explainable in terms of the Kirchhoff theory. The possible reasons for this are discussed along with the prospects for using ellipsometry as a tool for measuring surface roughness.