Optical Methods of Surface 

Measurement

Ted Vorburger, Guest Researcher

National Institute of Standards and Technology 

(NIST)

Measurement Science and Standards

in Forensic Firearms Analysis 2012

NIST, Gaithersburg, MD

July 10, 2012
Contents

• Three optical methods for measuring surface topography:

– Confocal microscopy

– Focus variation microscopy

– Coherence scanning interferometry

• For each method, will discuss:

– Brief description

– Documentary standard

– Some strengths and limitations

– Applications in firearms examinations and research

• Some comparisons

• Observations on possible research with these methods for firearms 

• Will not discuss applications of optical reflection microscopy –

already widely known and used
Sources 

• John Song, Alan Zheng, T.B. Renegar (NIST)

• ISO Standards and drafts

• R. Artigas, Sensofar

• B. Bachrach, Intelligent Automation, Inc.

• F. Helmli, Alicona

• R. Krueger-Sehm, (PTB)

• P. de Groot, Zygo Corp.

• M. Johnson et al. (MIT)

• K. Garrard et l. (NCSU)

• Nanofocus

• Forensic Technology Inc.

• Pyramidal Technologies

Note: Certain commercial equipment may be identified in this presentation in order to 

specify certain experimental procedures. This does not imply recommendation or 

endorsement by NIST, nor does it imply that the equipment are the best available for 

the purpose.
Observation:

Conventional Optical Microscopy vs. Topographic Microscopy

Surface topography (roughness height variation Z (x, y)) is likely the primary 

effect produced by firearms on bullets and casings. 

Conventional Microscopy:

The optical image contrast

I (x, y) Is primarily a function of:

-Slope,

-Shadowing,

-Multiple reflections

-Optical properties

-Illumination direction

-local height variations, 

indirectly 

Topographic Microscopy, such as 

interferometric and confocal 

microscopy, can measure local 

height variations Z (x, y) directly, 

independent of illumination and 

shadowing effects

But there are signal-to-noise issues 

and data dropouts

Optical image I (x, y) ≠ Topography Z (x, y)
From Classification of Methods for Measuring Surface 

Texture, ISO International Standard (IS) 25178-6

Line Profiling,

Senses Z(X)

Areal Topography,

Senses Z(X,Y) or

Z(X) as a function of Y*

Contacting Stylus,

Phase Shifting Interferometry,

Coherence Scanning Interferometry,

Confocal Microscopy,

Chromatic Probe, 

Structured Light and Triangulation,

SEM Stereoscopy,

Scanning Tunneling Microscopy,

Atomic Force Microscopy,

Optical Differential Profiling,

Angle Resolved SEM,

Focus Variation Microscopy

Contacting Stylus,

Phase Shifting Interferometry,

Circular Interferometric Profiling,**

Optical Differential Profiling

Total Integrated Scatter,

Angle Resolved Scatter,

Parallel Plate Capacitance,

Pneumatic

Area-Integrating

*The accuracy of Z(Y) profiling depends on the method and should 

be assessed for each method.

**Relies on circular scanning to produce a Z() profile.
Important Properties of Microscopes for 

Measuring Surface Topography 

• Vertical Resolution 

• Lateral Resolution

What can the instruments achieve and what vertical and lateral 

resolutions are required to resolve important individual 

characteristics of bullet and cartridge case surfaces? 

• Maximum Measurable Slope (Slope Range)

Important for measuring firing pin impressions

• Cost

• Speed 
http://www.nanofocusag.com/de/html/3dmicro.html

Confocal 

Microscopy:

Schematic diagram of a disk 

scanning confocal microscope

Surface
ISO Working Draft 25178-607

ISO/Technical Committee 213/ Working Group 16 

Geometrical product specification (GPS) – Surface texture: Areal – Part 

607: Nominal characteristics of non-contact (imaging confocal microscopy) 

instruments 

Figure B.1 – A series of confocal images through the depth of focus of a 

confocal microscope’s objective. 

Figure B.2 – Left: axial response of a single pixel along the z-direction. Right: the three 

dimensional surface of the series of images of B.1.
Strength

• Vertical resolution ≈ 3 nm

Lateral resolution ≈ 1 μm (depends on the magnification and the objective, 

typical for an optical microscope)

Limitation

• Signal decreases and becomes unreliable for high surface slopes

≈ 15 ° leading to dropouts and outliers

Topography image of a 

firing pin impression 

obtained with a confocal 

microscope
From P. Murphy et al., Three-Dimensional Virtual Comparison Microscope for Bullets, 

http://www.forensictechnology.com/Portals/71705/docs /technote_3dvcmbullets_20100429.pdf

(May, 2010)

Application of 

Confocal Microscopy 

to Firearms 

Research

Topography image of a pair of fired 

bullets

BulletTrax-3D

Forensic Technology, Inc.
Focus Variation Microscopy

ISO Committee Draft 

25178-606

Geometrical product 

specification (GPS) —

Surface texture: Areal 

— Part 606: Nominal

characteristics of noncontact (focus 

variation) instruments,

Figure A.1
Strength of Focus Variation Microscopy

Ability to measure steeply sloped surfaces

Firing pin 

impression on 9 

mm cartridge case 

measured with 

focus variation. 

Overlay of 

reflectance and 

topography images. 
A Limitation of Focus Variation Microscopy

Vertical 

resolution 

≈ 100 nm

Lateral 

resolution

≈ several pixels 

Table A.1

From

ISO DIS

25178―606
Applications in Forensic Science

• BrassTrax-3D “depth from focus (DFF)”

http://www.forensictechnology.com/publications/

• Alicona Infinite Focus

• See also review article by R.S. Bolton-King et al. in 

AFTE Journal 42 (1) , 23 (2010) 
Principle of Coherence 

Scanning Interference 

Microscopy

aka:

Vertical scanning interferometry, 

White light interferometry,

Scanning white light 

interferometry, 

Optical coherence tomography... 

Scan 

Direction

MIRAU CONFIG.
ISO Draft International Standard (DIS) 25178-604

Geometrical product specification (GPS) – Surface texture: Areal –

Part 604: Nominal characteristics of non-contact (coherence 

scanning interferometric microscopy instruments

z

B

A

(c)

(b)

(a)

Fig. B.1
Coherence Scanning Interferometry

• Strength: Vertical resolution ≈ 3 nm

• Lateral resolution ≈ 1 μm, comparable to confocal 

microscopy

• Limitation: Has difficulty with steeply sloped surfaces

• Issue: Complex sensor signal 

D

E

A B

Signal

Height
Application in Firearms Research

ALIAS, from Pyramidal Technology in cooperation with 

Heliotis (Switzerland)

-CSI with high-speed camera 

Topographic image of breech face and firing pin impressions. 
Contents

• Four optical methods for measuring surface topography:

– Confocal microscopy

– Focus variation microscopy

– Interferometry

– Chromatic confocal microscopy

• For each method, will discuss:

– Brief description

– Documentary standard

– Some strengths and limitations

– Applications in firearms examinations and research

• Some comparisons

• Observations on possible research with these methods for firearms 

• Will not discuss applications of optical reflection microscopy –

already widely known and used
Comparison of 2D Profiles of a Standard Bullet Measured 

by Four Techniques

Stylus,

Master Profile

CSI

Disc Scanning 

Confocal

Microscope

Laser Scanning

Confocal

Microscope
Areal Cross Correlation of Firing Pin Impressions of 

Two Casing Replicas with Confocal Microscopy
Profiles of a 100 nm Ra 

sinusoidal grating 

obtained with four 

techniques

-400

-200

0

200

400

0 20 40 60 80 100 120

-400

-200

0

200

400

0 20 40 60 80 100 120

-400

-200

0

200

400

0 20 40 60 80 100 120

-400

-200

0

200

400

0 20 40 60 80 100 120(μm)

(μm)

(μm)

(μm)

Stylus

PSI

WLI

Confocal

(nm)

Stylus

PSI

CSI

Confocal

m

nm
Test of Confocal Microscopy:

Measurement of chirped roughness standard from PTB, 

Germany

≈ 1 μm

≈ 2 mm

Stylus Profile

Confocal, 50X

Confocal, 10X
Other Methods

Confocal chromatic probe

Polaris – 3D, 

K. Garrard et al.,NCSU

Contacting + Optical Method

Gelsight, M.K. Johnson et al.

MIT
Concluding Observations

• Calibration standards are available for testing different 

methods

– Smooth surfaces for assessing vertical resolution

– Set of periodic surfaces for testing lateral resolution

– Rough surface standards for testing the algorithms

– Standard bullet and cartridge case for testing capabilities to 

measure steep slopes and specific geometries 

• Would be beneficial to test vertical and lateral resolution 

of each method and optimize vs. needs in ballistics 

identification, speed, and cost → could use a standard 

set of cartridge cases and bullets (Tulleners-De Kinder 

Cartridge Cases, NIST Cartridge Cases, IAI Bullets,...)
Concluding Observations

• Data Fusion:

How best to combine 

reflection microscopy and 

topography data 

quantitatively 

• Can apply scanning electron microscopy to obtain 

images with sub-micrometer resolution (Note: BKA 

research)