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Practical Optical Dimensional Metrology
Author(s): Kevin G. Harding
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Book Description

Practical Optical Dimensional Metrology provides basic explanations of the operation and application of the most common methods in the field and in commercial use. The first half of the book presents a working knowledge of the mechanism and limitations of optical dimensional measurement methods that use: light level changes, two-dimensional imaging, triangulation, structured-light patterns, interference patterns, optical focus, light characteristics such as polarization, and hybrid methods with mechanical or other measurement tools. The book concludes with a series of manufacturing application examples that look at measurements from the centimeter range down to the nanometer range.

Book Details

Date Published: 2 January 2019
Pages: 228
ISBN: 9781510622937
Volume: TT119

Table of Contents
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Table of Contents


1 Introduction to Metrology
1.1 Basic Terms
1.2 Methods of Optical Metrology

2 Light-Intensity-based Metrology
2.1 Light, Optics, and Machine Vision Technology
      2.1.1 Lighting methods for machine vision
      2.1.2 Optical components for machine vision
2.2 Where To Use Intensity-based Methods
2.3 Sources of Errors

3 Triangulation- and Shift-based Metrology
3.1 Stereo Imaging
      3.1.1 Photogrammetry
      3.1.2 Optical flow
3.2 Active Triangulation
      3.2.1 Point gages
      3.2.2 Structured-line-of-light gages
      3.2.3 Line triangulation gage limitations
3.3 3D Phase-based Measurements
      3.3.1 Phase shift analysis
      3.3.2 Structured-light triangulation
      3.3.3 Triangulation gage pros and cons
      3.3.4 Moiré contouring
      3.3.5 Interferometry (laser or white-light based)
      3.3.6 Holography (including shearography and speckle methods)
3.4 Summary of Triangulation and Phase Shift Methods

4 Focus-based Optical Metrology
4.1 Introduction to Focus-based Methods
4.2 Point-based Distance Measurement
      4.2.1 Conoscopic imaging
      4.2.2 Confocal imaging
      4.2.3 Chromatic confocal imaging
4.3 Area-based Focus Metrology Methods
      4.3.1 Depth from focus
      4.3.2 Structured-pattern, focus-based methods
      4.3.3 Depth from defocus
      4.3.4 Image focus-based method summary
4.4 Focused-based Metrology Summary

5 Light-Characteristic-based Dimensional Measurements
5.1 Introduction to Light Characteristics
5.2 Polarization-based Dimensional Metrology
      5.2.1 Ellipsometry
      5.2.2 Photo-elastic methods
      5.2.3 Pixelated polarization masks
5.3 Light-Scatter-based Measurements
      5.3.1 Light scattering pros and cons
5.4 Color-based Measurements
      5.4.1 Color-based measurement pros and cons

6 Portable and Hybrid Gages
6.1 Introduction to Portable and Hybrid Gages
6.2 Measurement of Large Structures
      6.2.1 Point trackers
      6.2.2 Handheld area scanners
      6.2.3 Laser radar
6.3 Measurement of Mid- to Large-Size Durable Assets
      6.3.1 Laser-based tracker systems
      6.3.2 Robot/gantry-mounted scanners
6.4 High-Precision Hybrid Systems
6.5 Summary of Hybrid Gages

7 Finding the Right Technology for the Application
7.1 Introduction
7.2 Low-Precision Applications < 10 mm
      7.2.1 Limitations of low-precision optical methods
7.3 Large Objects and Assemblies < 1 mm
      7.3.1 Limitations of large-part optical methods
7.4 General Manufacturing Applications < 0.1 mm
      7.4.1 Limitations of general manufacturing optical methods
7.5 Precision-Manufactured Parts < 0.01 mm
      7.5.1 Limitations of method for precision parts
7.6 Micro-feature Metrology < 0.001 mm
      7.6.1 Limitations of micro-feature metrology methods
7.7 Nano-features < 0.0001 mm
      7.7.1 Limitations of nano-feature metrology methods
7.8 Summary of Application Comparisons

8 Part Location
8.1 Part Location Applications
8.2 Large Parts Measured to < 10-mm resolution
      8.2.1 Machine vision
      8.2.2 Stereo imaging
      8.2.3 Triangulation laser line
      8.2.4 Phase shift 3D
      8.2.5 Laser radar
      8.2.6 Large-part pickup summary
8.3 Composite Layup Monitoring
      8.3.1 Lighting enhancement methods
      8.3.2 Structured light
      8.3.3 Tape layup summary
8.4 Part Location Summary of Options

9 Optimized Measurement of Gaps
9.1 The Application < 0.1 mm
9.2 Elimination of Methods that Are Not Suitable
9.3 Laser Line Triangulation
9.4 3D Triangulation
9.5 Chromatic Confocal Method
9.6 Comparison Tests
9.7 Comparison of Methods
9.8 Summary of Options

10 Measurement of Small Holes
10.1 The Application < 0.01 mm
10.2 Laser Line Structured Light (Static)
10.3 Scanning-Laser-Line or Multiple-Laser-Line Probe
10.4 Phase-Shifted Structured Light
10.5 Conoscopic Point Probe
10.6 Confocal Point Probe
10.7 Digital Optical Comparator (2D)
10.8 Depth from Focus Microscopy
10.9 Depth from Defocus Microscopy
10.10 Summary of Options

11 Three-Dimensional Metrology for Printed Electronics
11.1 The Application < 0.001 mm
11.2 Laser Line Structured Light (Static)
11.3 Phase-Shifted Structured Light
11.4 Confocal Point Probes
11.5 Depth from Focus or Defocus Microscopy
11.6 Artifact-based Verification
11.7 Conclusions

12 Industrial Surface Finish Method Comparison for Fine Finish Measurements
12.1 The Application < 0.0001 mm
12.2 Interferometry
12.3 Focus-based Systems
12.4 Confocal Systems
12.5 Scatter-based Systems
12.6 Comparison of Methods
12.7 Summary of Options



This book is based on 40 years of working with, evaluating, testing, using, and learning about a wide range of optical dimensional metrology techniques and products. The applications have ranged from consumer products such as electronics to measuring gears and sheet metal in the automotive industry to measuring airfoils from turbine engines. Over this time, I needed to understand both what a technique can and cannot measure, as well as which applications would simply be easier or less expensive to measure by some other means. I have found that for many applications, the established theory and calculations indicate that one optical metrology method or another is suitable. However, for practical reasons of environment, measurement restrictions, or commercial availability, this method may not be a viable solution without more work or development. I have tried to capture the practical knowledge gained from hands-on experience that is useful to others who later attempt to address a similar measurement need. In many cases, the insights and diagrams were the result of a colleague coming to my office to ask how to do some measurement and the resulting discussion on a white board.

There is a lot of theory, math, and science behind the way that many of these optical measurement methods work, all of which has been well covered in the publications referenced in this book. The objective of this book is not to make the reader an expert on metrology, optics, or any of these methods, but rather to impart the practical knowledge that enables the successful use of optical metrology tools to address a measurement need in production manufacturing. I encourage those readers interested in more in-depth analysis of how these methods work to read the many excellent references cited at the end of each chapter.

This book is organized into two primary sections. The first six chapters provide basic, working explanations of how each of the optical measurement methods works. The chapters are organized according to the basic mechanisms of measurement, including light intensity changes, two-dimensional imaging, triangulation, structured-light patterns, interference patterns, optical focus, light characteristics such as polarization, and hybrid methods with mechanical or other measurement tools. The basic explanations presented do not necessarily include all of the details needed to build your own product or all of the variations in the way the method has been employed in the past, but rather represent the core operating principles of each method. With these explanations, I include some insights into the limitations as well as application mistakes to avoid.

Chapter 7 begins the second half of the book, which looks at optical metrology methods from the perspective of real applications, working from relatively course measurements on the centimeter scale down to very fine measurements on the nanometer scale. I summarize the key application assumptions for each measurement range in a table, ranking the relative capability of each optical dimensional measurement method to address these application assumptions on a practical basis based on my experience. Thus, Chapter 7 is a summary chapter to set the stage for the discussion of real-world applications.

The remaining five chapters pull information from real application examples that I have published over the years to illustrate the considerations that my colleagues and I have evaluated in finding a solution to a production measurement need. The applications were chosen to be representative of the measurement range (from coarse to fine) discussed in Chapter 7, showing what worked and what did not work based on experiments and extended qualification tests. These example applications are by no means exclusive of the possible uses of the optical measurement technologies discussed. Even small application changes might change the outcome of the evaluations. The evaluation process shown in these chapters is intended to help guide users in their own evaluations.

New improvements are continuously emerging for any high-technology tool, and optical metrology is no exception. My experience suggests that the time period between the development of a new method (or a variation to an existing method) in the metrology field and its commercial use can be 10 to 20 years. Metrology tools such as gage blocks, calipers, electronic gages, and coordinate measurement machines are all proven and known measurement tools that have been around for a long time and will continue to be used for some time to come. My hope is that this book will help measurement system users find those applications where optical metrology can help achieve the speed or another performance parameter that will meet their needs and perhaps advance the state of manufacturing.

Kevin G. Harding
January 2019

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