Optical Methods in Engineering Metrology

ENGINEERING ASPECTS OF LASERS SERIES

Series Editor Dr T. A. Hall Reader in Physics University of Essex

SERIES EDITOR'S PREFACE

In the late 1960s and early 1970s the laser was still something of a scientific curiosity with only a limited practical use. The extent of the four volumes in this series shows the enormous change that has happened since that time. The laser is now an indispensable addition to the toolbox of the engineer and scientist. The progress from the time when the laser was often dubbed 'a solution in search of a problem' to today, when engineers of all disciplines frequently use lasers as a matter of course, is a remarkable transformation. Even so, the use of lasers in engineering and other walks of life is still in its infancy and has been held back partly by their relatively high cost and in some cases by their inconvenience in use. As these problems are overcome lasers will find wider and wider application and there is an ever increasing need for engineers and scientists, who perhaps have little interest in lasers them­selves, to have access to an authoritative source which not only acts as an intro­duction but also takes the reader up to the latest developments in laser applications.

The four books of the series 'Engineering Aspects of Lasers' arose from a series of laser workshop courses which have been held annually at the University of Essex since 1979. These courses have evolved very considerably since their inception but aspects of their organization have not changed - the contents of the courses have always been coordinated by the recognised international authority in each subject area and the lectures given by experts in the particular field from industry, government laboratories or universities. When the idea of publishing a series of books based upon the contents of these courses was first suggested the course coordinators at that time became the editors of each volume and the lecturers were asked to contribute.

The workshop courses are self-supporting courses which also form part of the MSc degree in Lasers and their Applications. There are many people who have con­tributed much to these courses over the years and have made them the success that they have been. I would like to express my gratitude to them all. The courses and the

MSc were the brainchild of T. P. Hughes who was then Reader in Physics at Essex University. Without his foresight, hard work and determination in setting up the courses, this series of books would not have been written.

Other titles in series

Laser Processing in Manufacturing Edited by R. C. Crafer and P. J. Oakley

Nonlinear Optics in Signal Processing Edited by R. W. Eason and A. Miller

Advances in Optical Communications Edited by N . Doran and I. Garrett

Optical Methods in Engineering Metrology

Edited by D. C. Williams Division of Mechanical and Optical Metrology UK National Physical Laboratory

m SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

First edition 1993

© 1993 Springer Science+Business Media Dordrecht Originally published by Chapman & Hall in 1993

Typset in 10/12 Times by Expo Holdings, Malaysia

ISBN 978-94-010-4683-1 ISBN 978-94-011-1564-3 (eBook) DOI 10.1007/978-94-011-1564-3 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page.

The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made.

A catalogue record for this book is available from the British Library

Library of Congress Cataloging-in-Publication data available

Contents

List of contributors

Preface

1 Introduction 1. M. Burch 1.1 The optical metrologist 1.2 Optical measurement 1.3 A wider view

References

2 Laser beam geometry and its applications D. C. Williams 2.1 Analysis of laser beam size

2.1.1 Characteristics of a laser beam 2.1.2 Analysis using skew rays 2.1.3 Some applications of the theory

2.2 Increasing the precision 2.2.1 Three-point alignment system 2.2.2 Fresnel zone plates 2.2.3 Variable-focus zone plate 2.2.4 Variable-focus lens systems 2.2.5 Afocal lens systems 2.2.6 Bessel beam

2.3 The effect of the atmosphere 2.3.1 Bending of a light beam 2.3.2 Effect of temperature and pressure gradients 2.3.3 Two-colour methods

2.4 Detection of beam position 2.4.1 Photoelectric detection methods 2.4.2 Phase-sensitive systems 2.4.3 Bore straightness measurements 2.4.4 Quadrant detectors

xv

xvii

1

3 7 9

11

II II 16 21 25 25 27 31 36 38 40 43 44 45 47 49 49 50 52 57

Contents VB

2.4.5 Axial position detection 58 2.4.6 Photopotentiometers 60 2.4.7 Intensity-modulation methods 61 2.4.8 Measurement of large movements 63

2.5 Problem solving with optical devices 65 2.5.1 Scanning and triangulation techniques 65 2.5.2 Two-mirror and related devices 67 2.5.3 Surface profilometer 74 2.5.4 Profilometry in two dimensions 76 2.5.5 Profilometry refinements 78 2.5.6 Roller parallelism measurements 79 2.5.7 Intensity-based measurements 82 References 85

3 Alignment metrology 87 B. S. Peam 3.1 Instrumentation for alignment 88

3.1.1 Micro-alignment telescope 88 3.1.2 Lasers 93

3.2 Telescope applications 94 3.2.1 Crankshaft bearings 95 3.2.2 Transport and process rollers 98 3.2.3 Coaxial spindles 99

3.3 Flatness or level 99 3.3.1 Flatness on-board ship 100 3.3.2 Alignment on an offshore platform 101

3.4 Autocollimation 1m 3.4.1 Straightness by autocollimation 103 3.4.2 Electronic autocollimators 105 3.4.3 Flatness of surface tables 106 3.4.4 Angle comparison 107

3.5 Practical problems 108 3.5.1 Environmental problems 108 3.5.2 Obstructed sight line 109 3.5.3 Parallax 110 3.5.4 Conclusion 112 Reference 112

4 Photogrammetry in industrial measurement 113 R. A. Hunt 4.1 The technique of photogrammetry 113 4.2 How to make a measurement 116

4.2.1 Choice of method 117

Vlll Contents

4.2.2 Mathematical modelling 117 4.2.3 Experimental design 118 4.2.4 Prediction of uncertainties 118 4.2.5 Assessment of the method 118 4.2.6 The experiment 118 4.2.7 Estimation of experimental uncertainties 119

4.3 Mathematical formulation 119 4.3.1 The bundle solution 123 4.3.2 Least squares revisited 123 4.3.3 The Newton-Raphson method 124 4.3.4 Treatment of distortions 125 4.3.5 Error ellipsoids: fact or fiction? 126 4.3.6 Singularities in the solution 127 4.3.7 Design of experiments 128 4.3.8 Caleulation of initial values 129

4.4 Imaging devices 129 4.4.1 Metric and semi-metric cameras 130 4.4.2 The NPL monocentric axicon camera 132 4.4.3 The role of the measuring machine 134 4.4.4 Finding the image centres 134

4.5 Towards real time 135 4.5.1 The coming of computer vision 136 4.5.2 Video theodolites 138 4.5.3 Location of images 139 4.5.4 True video photogrammetry 141

4.6 Applications for photogrammetry 144 4.6.1 Survey of an infrared telescope 145 4.6.2 The destruction of a bridge 148 4.6.3 Measurement of a fuel tank 149 References 152

5 Laser interferometry for precision engineering metrology 153 P. Gill 5.1 General principles 153

5.1.1 Optical interference by amplitude division 153 5.1.2 Limitations to fringe formation 154 5.1.3 Refractive index compensation 156 5.1.4 Single-wavelength fringe-counting interferometry 157

5.2 Stabilized laser wavelength sources 158 5.2.1 International definition of the metre 158 5.2.2 He-Ne stabilization techniques 159 5.2.3 Laser wavelength calibration and traceability 162

5.3 Heterodyne interferometry 163

Contents ix

5.4 Machine tool characterization 164 5.4.1 Angle measurement 164 5.4.2 Straightness measurement 165 5.4.3 Plane mirror interferometers 165

5.5 Laser interferometer measurement errors 166 5.5.1 System specification 166 5.5.2 Laser wavelength errors 166 5.5.3 Compensation errors 166 5.5.4 Interferometer verification 167 5.5.5 Operator errors 167

5.6 Multiple wavelength interferometry 168 5.6.1 Method of excess fractions 168 5.6.2 The NPL gauge-block interferometer 169

5.7 Multiple- and swept-wavelength interferometry using laser diodes 171 5.7.1 Laser diode properties and stabilization 171 5.7.2 EDM equivalence 174 5.7.3 Multiple-wavelength diode theory and application 175 5.7.4 Swept-wavelength diode theory and application 176 5.7.5 Future trends 176 References 177

6 Laser vibrometry 179 N. A. Halliwell 6.1 Laser Doppler velocimetry 179

6.1.1 Principles of operation 180 6.1.2 Frequency-shifting devices 183 6.1.3 Doppler signal processing 186

6.2 Solid surface vibration measurement 187 6.2.1 Laser speckle effects and theory of operation 188 6.2.2 Measurements on rotating targets 190 6.2.3 Choice of vibrometer: practical considerations 191

6.3 Torsional vibration measurement 194 6.3.1 Cross-beam torsional vibrometer 195 6.3.2 The laser torsional vibrometer 197 6.3.3 Practical considerations 201

6.4 Fibre optic vibration sensors 203 6.4.1 Intrinsic vibration sensors 204 6.4.2 Extrinsic vibration sensors 206 6.4.3 A practical all-fibre laser vibrometer 207 6.4.4 The fibre optic vibrometer: practical considerations 208 References 210

Laser Doppler ve10cimetry texts 210 Other references 210

x Contents

7 Industrial application of holographic interferometry 213 R. 1. Parker 7.1 Holography 213

7.1.1 Recording the whole picture 214 7.1.2 Two-beam interference and holography 214 7.1.3 Recording transmission holograms 216 7.1.4 Viewing in white light 217

7.2 Equipment for holography 218 7.2.1 Lasers 218 7.2.2 Laser safety 219 7.2.3 Recording materials 220 7.2.4 Other holographic equipment 226

7.3 Measurement with holograms 228 7.3.1 Holographic interferometry 228 7.3.2 Fringe pattern interpretation and analysis 231 7.3.3 Resolving components of displacement 232 7.3.4 Automatic analysis 233

7.4 Applications of holographic interferometry 235 7.4.1 Non-destructive testing 236

7.5 Vibration analysis by holography 242 7.5.1 Time-averaged holograms 242 7.5.2 Large-object or large-amplitude analysis 247 7.5.3 Holography of rotating objects 250

7.6 Flow visualization 253 7.6.1 Fringe interpretation 254 7.6.2 Two-dimensional flows 255 7.6.3 Three-dimensional flows 259 7.6.4 Real-time flow visualization 259 7.6.5 Rotating flows 261

7.7 Holographic contouring 262 7.7.1 Two-angled illumination 264 7.7.2 Two-refractive-index techniques 266 7.7.3 Two-wavelength techniques 268 7.7.4 Holographic contouring - new potential 270

7.8 Conclusion 270 References and bibliography 270

8 Television holography and its applications 275 1. C. Davies and C. H. Buckberry 8.1 Limitations of conventional holography 275

8.1.1 The need for holography 276 8.1.2 The benefits and limitations of holographic analysis 277

8.2 Principles of television holography 278

Contents Xl

8.2.1 Resolution requirements 278 8.2.2 Forming TV holograms 279 8.2.3 The effect of the aperture stop 281 8.2.4 Operating modes 284 8.2.5 Fringe formation 284

8.3 Systems and their operation 286 8.3.1 Optical arrangement 286 8.3.2 Operating parameters 289 8.3.3 Fibre optic systems 291 8.3.4 Setting up a system 292 8.3.5 Alternative operating geometries 293

8.4 Modulation techniques 295 8.4.1 Limitations of a basic system 296 8.4.2 Intensity modulation 296 8.4.3 Phase modulation 298 8.4.4 Combined modulation 300

8.5 Automatic fringe analysis 301 8.5.1 Digital processing 301 8.5.2 The engineering need 302 8.5.3 Phase-measurement algorithms 302 8.5.4 Application of phase stepping to TV holography 306 8.5.5 Practical considerations 308 8.5.6 Practical examples 311 8.5.7 Conclusions 316

8.6 Examples of applications 316 8.6.1 Automobile applications 316 8.6.2 In-situ measurement of stone degradation 325 8.6.3 Structural integrity testing 328 8.6.4 Pulsed TV holography 330 8.6.5 Surface contouring 332 8.6.6 Future developments 335 Acknowledgements 336 References 337

9 Moire methods in strain measurement 339 C.Forno 9.1 Conventional moire measurements 339

9.1.1 The moire phenomenon 339 9.1.2 Out-of-plane moire techniques 340 9.1.3 Imaged moire 342 9.1.4 In-plane measurement analysis 344 9.1.5 In-plane measurement techniques 347

9.2 High-resolution moire photography 348

xu Contents

9.2.1 Tuning the lens response 349 9.2.2 Photographic recording 352

9.3 Application of modified camera 354 9.3.1 Spatial filtering of moire fringe patterns 355 9.3.2 Fringe-pattern analysis 356 9.3.3 Examples of engineering structures 358 9.3.4 Deformation measurement in a timber structure 358

9.4 Types of surface pattern 362 9.4.1 Printed pattern 362 9.4.2 Stencilled patterns 366 9.4.3 Random patterns 369

9.5 Additional features of the technique 372 9.5.1 Optical differentiation 372 9.5.2 Three-dimensional movement 374 9.5.3 Pattern frequency multiplication 374 9.5.4 Optical regeneration of defective gratings 374 9.5.5 Displacement sensitivity direction 375

9.6 Moire interferometry 375 9.6.1 Two-beam interference 375 9.6.2 Surface grating 379 9.6.3 Moire fringe analysis 381 Acknow ledgement 382 References 383

10 Automatic analysis of interference fringes 385 G. T. Reid 10.1 Intensity-based methods 386

10.1.1 Dedicated systems 386 10.1.2 Fringe tracking 390

10.2 Phase-based methods 395 10.2.1 Electronic heterodyning 395 10.2.2 Phase stepping 397 10.2.3 Spatial phase measurements 405 References 410

11 Monomode fibre optic sensors 415 J. D. C. Jones 11.1 Measurement using optical fibres 415 11.2 Transduction mechanisms 417

11.2.1 Sensor transfer function 418 11.2.2 Phase modulated sensors 418 11.2.3 Polarization-modulated sensors 419

Contents X III

11.3 Optical processing 420 11.3.1 Two-beam interferometers 421 11.3.2 Multiple-beam interferometers 425 11.3.3 Polarimetric techniques 427 11.3.4 Spectral techniques 429

11.4 Modulators and components 430 11.4.1 Phase modulation 431 11.4.2 Polarization modulation 432 11.4.3 Frequency modulation 434 11.4.4 Directional couplers 436

11.5 Electronic processing 439 11.5.1 Active homodyne processing 440 11.5.2 Passive homodyne processing 442 11.5.3 Heterodyne processing 445 11.5.4 Noise considerations 445

11.6 Applications 450 11.6.1 Intrinsic sensors 450 11.6.2 Extrinsic sensors 457 References 459

Index 465

Contributors

C. H. Buckberry

Rover Group, Gaydon, Warwickshire, UK

J. M. Burch Cranfield Institute of Technology, UK

J. C. Davies Rover Group. Gaydon. Warwickshire. UK

C. Forno

National Physical Laboratory. Teddington. UK

P. Gill

National Physical Laboratory. Teddington. UK

N. A. Halliwell Loughborough University of Technology. UK

R. A. Hunt National Physical Laboratory. Teddington. UK

J. D. C. Jones Heriot-Watt University. Edinburgh. UK

R. J. Parker Rolls-Royce pic, Derby. UK

B. S. Pearn Formerly with Rank Taylor Hobson. Leicester, UK

G. T. Reid Formerly with National Engineering Laboratory, East Kilbride. UK

D. C. Williams

National Physical Laboratory. Teddington, UK

Preface

'Now I'm an engineer, and I come along and say "now look ... ".' This reaction came from an engineering graduate with many years of experience in tackling real-world industrial problems. His comment was prompted during the development of one of the optical devices described in this book, when he saw it being carefully aligned by a physicist who had surrounded it with a bewildering array of ancillary optical components.

The incident highlights a common dilemma. The physicist believes that his new creation has the potential to revolutionize an inspection process and when met with a cool reception he complains bitterly about the relu~tance of industry to adapt to new technology. The engineer already has an armoury of highly developed practical processes which have stood the test of time and which are well suited to the requirements of production. He does not wish to invest in alternatives unless

the benefits are clear. For the calibration and performance verification of three-axis measuring machines,

considerable effort was expended on an op.tical device which defined points in three­dimensional space by reflection from a flat plate placed on the table of the machine. The machine owners showed little interest, because the defined points were not tangible with well-defined locations; such calibrations are normally carried out using traditional mechanical artefacts such as ball plates and step gauges. However, optics has been able to make a genuinely useful contribution in this field by replacing the mechanical touch probe with an optical probe. Removing the need for physical contact between the probe and the artefact allows the machine to make settings without halting its motion, which gives considerable increases in both speed and reliability when measuring surface profiles.

Thus there are areas where optics is quite inappropriate, and others where it has much to offer. The most useful devices are often those which are ingenious and yet basically simple. It is my hope that this book may help to bridge the gap between optics and engineering by indicating how to approach problems which may be amenable to optical solution in a genuinely constructive way. The authors all have considerable experience in successfully applying optical methods to metrological problems of various kinds. As well as presenting a range of optical techniques and principles, ways to approach the solution of new problems are indicated. It is

xvi Preface

assumed that the reader already has a basic knowledge of optics; it is better for the engineer without this background to refer to one of the many textbooks that are available rather than being given an inferior and inadequate summary.

The programme of the Workshop Course on which the book is based was organized by the National Physical Laboratory and several of the authors are members of its staff. The content is therefore inevitably biased towards those areas of activity with which the NPL has been particularly associated, but the coverage is nevertheless quite broad. An attempt has been made at least to mention topics that are not treated in detail. The NPL has always been a centre for direction of enquiries about measurement problems which cannot be solved by established methods. In the parlance of today, such requests would be described as 'challenges', but for those of us who are fortunate enough to have memories of a less self-orientated society, they tend to be regarded as just pleasurable. Successful solutions depend not on management documents, objectives and targets, but on an enthusiasm for problem­solving coupled with a sense of dedication to meeting the needs of the customer.

A measure of cheating has taken place, in that while the majority of the text is concerned with laser methods, there is some discussion of optical techniques which use traditional light sources. However, this may serve to emphasize that the optimum technique is not always the one which uses the most up-to-date technology. In many areas, the fundamental principles have changed little in recent years; the latest developments have been in the application of new technology. The chapters differ markedly in their approaches to their subjects, but this serves to illustrate that individualism is the basis of creativity. I am grateful to all of the authors for their contributions; many 'f them were preoccupied with new responsibilities at the time of writing. Thanks are also due to various individuals and organizations who have provided information and illustrations.

It would be wrong to conclude this preface without special mention of Professor Jim Burch. He was the original organizer of the Workshop Course, and it is a measure of his knowledge of the subject that in the earlier years he delivered almost all of the lectures himself. Most organizations depend for their success on a few outstanding individuals, and Jim was one such at the NPL. His comments on the efforts of the rest of us were always stimulating and frequently astonishing. Many of the topics covered throughout the book owe their origin or success to his contriblltions. The breadth and depth of his knowledge of optics and his enthusiasm for it were complemented by warmth, humour and an unfailing courtesy shown to everyone with whom he came into contact. I am delighted that he has been able to contribute an introductory chapter, for it is he and not I who should be the editor. He will find many errors and omissions, but I hope that he will not be too displeased.

David C. Williams