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Table of contents :
Part I: Theory and Materials
1.2 Defining Quality
1.3 Relating Quality with Optics Inspections and Testing
1.4 The Purpose of This Book
2.1 History and Development
2.2 The Nature of Light
2.3 Geometrical Optics
2.4 Physical Optics
2.5 Optical Aberrations
2.7 Optical System Design
2.8 Types of Optical Components
3 Raw Materials for Producing Optical Elements
3.1 What Is an Optical Material?
3.2 Materials for Optical Elements
3.3 Classification of Optical Materials
3.4 Main Characteristics of Optical Materials
4 Manufacturing Processes of Optical Materials
4.2 Glass Manufacturing Process
4.4 Chemical Vapor Deposition
5 Methods for Producing Optical Components
5.2 Conventional Method: Spindle Grinding and Polishing
5.3 Diamond Turning
5.4 Precision Glass Molding and Precision Molded Optics
5.5 Additional Methods for Improving Optical Elements
5.6 Additional Shaping Methods and Those that Produce Specular Surfaces
6 Optical Coatings
6.1 Classification of Optical Coatings
6.3 AR Coating
6.4 Reflective Coatings
6.5 Optical-Coating-Deposition Technologies
6.7 Typical Spectral Curves
7 Optical Adhesives
7.2 Production Bond Failures
7.3 Incoming Failure Identification
8 Optics Standards and General Technical Specifications
8.2 The Importance and Utility of Standards and Specifications
8.3 Defining a Standard
8.4 Defining a Specification
8.5 MIL-HDBK, MIL-STD, and Milspecs
8.6 International Organization for Standardization
8.7 ANSI, ASTM, and ASME
8.8 Deutsches Institut für Normung
8.9 General Standards for Technical Drawings
9 Metrology: Measurement Theory
9.2 Scientific or Fundamental Metrology
9.3 Applied, Technical, and Industrial Metrology
9.4 Legal Metrology
9.5 Geometric Dimensioning and Tolerancing
9.6 Rules of Thumb for Measurement Tools
Part II: Methods and Tools
10 Testing and Examining Optical Components
10.2 Overview of Production Requirement Documents
10.3 A Review of Quality Production and Inspection Records
11 Inspection and Testing of Raw Materials
11.1 Index of Refraction
11.3 Bubbles and Inclusions
11.5 Strain (Stress)
11.7 Resistivity of Silicon or Germanium
References Refractive index
12 Inspection and Testing of Components
12.1 Radius of a Spherical Surface
12.2 Sag (Sagitta)
12.4 Dial Gauges and Indicators
12.5 Roundness (Circularity)
12.6 Central Thickness of a Lens
12.7 Thickness and Parallelism of Windows
12.8 Length between Ground Surfaces
12.12 Inside Edges
12.13 Surface Texture
13 Inspection and Testing of Surface Shape and Figure
13.1 Test Plate
13.2 Analyzing the Interference Pattern Revealed by the Test Plate
13.3 Example Surface Patterns of Flat and Spherical Surfaces
13.4 Principles of Manual Analysis of Interferograms
13.5 Analyzing the Interference of Simple Patterns
13.6 Analyzing the Interference of Various Patterns
13.7 Important Considerations when Analyzing Interference
13.8 Interferometric Measurements for Flat and Spherical Surfaces
13.9 Testing Cylindrical Surfaces
14.1 Optical Properties
14.2 Environmental Durability
14.3 Visual Inspections
14.4 Witness Samples
14.5 Durability of Coatings with Primers and Silicon Removers
15 Special Properties of Aspheric Surfaces, Diffractive Surfaces, and Sapphire
15.1 Aspheric Surfaces
15.2 Diffractive Surfaces
Part III: Inspection and Quality Assurance
16 Acceptance Sampling (Standards and Methods)
16.2 Terms and Definitions
16.3 How to Determine if 100% Inspection is Necessary
16.4 Sampling Plan Procedure
16.5 Types of Decisions
16.6 Main Statistical Inspection Sample Table References
17 Location and Process of the Inspection/Test
17.2 Basic Needs
18 Visual Inspection
18.3 Requirements for Good Visual Inspections
18.4 Kinds of Defects
18.5 Visual Inspection Methods
18.6 Main Defects in Optical Elements
18.7 Illustrations of Visible Defects in Optical Elements
18.8 Measuring and Calculating Visual Defects
19 Handling Optical Components
19.2 Cleaning and Handling
19.4 Packaging, Storage, and Shipping
19.5 Health and Safety Aspects
19.6 Environmental and Additional Health and Safety Aspects
19.7 First Contact
20 Testing of Optical Systems
20.2 Main Optical System Parameters
20.3 Additional Required Tests of Optical Systems
21 Handling Nonconforming Optical Elements
21.3 MRB Decisions
21.4 Other Considerations
22 Quality Assurance
22.2 Terms and Definitions
22.3 Quality Management Theories
22.4 Product Manufacturing Steps
22.5 Quality Management Standards
22.6 Quality Audits
22.7 Lean Manufacturing
22.8 Quality Costs
22.9 Controlling Manufacturing Processes
22.10 Communication with Suppliers and Customers
22.11 Manufacturer Qualifying Process
22.12 First Article Inspection
22.13 Professionalism and Organizational Conduct
Library of Congress Cataloging-in-Publication Data Names: Hausner, Michael, author. Title: Optics inspections and tests : a guide for optics inspectors and designers / Michael Hausner. Description: Bellingham, Washington : SPIE Press,  | ©2017 | Includes bibliographical references and index. Identifiers: LCCN 2016006977 (print) | LCCN 2016009108 (ebook) | ISBN 9781510601796 (softcover) | ISBN 1510601791 (softcover) | ISBN 9781510601802 (pdf) | ISBN 9781510601819 (epub) | ISBN 9781510601826 (mobi) Subjects: LCSH: Optical materials Quality control. | Optical materials Testing. Classification: LCC QC374 .H38 2016 (print) | LCC QC374 (ebook) | DDC 621.36028/7 dc23 LC record available at http://lccn.loc.gov/2016006977
Published by SPIE P.O. Box 10 Bellingham, Washington 98227-0010 USA Phone: + 1 360.676.3290 Fax: + 1 360.647.1445 Email: [email protected] Web: http://spie.org
Copyright © 2017 Society of Photo-Optical Instrumentation Engineers (SPIE) All rights reserved. No part of this publication may be reproduced or distributed in any form or by any means without written permission of the publisher. The content of this book reflects the work and thought of the author. Every effort has been made to publish reliable and accurate information herein, but the publisher is not responsible for the validity of the information or for any outcomes resulting from reliance thereon. Printed in the United States of America. First Printing. For updates to this book, visit http://spie.org and type “PM269” in the search field.
Contents Preface Acknowledgments
I Theory and Materials
1 Introduction 1.1 1.2 1.3 1.4
Prologue Defining Quality Relating Quality with Optics Inspections and Testing The Purpose of This Book
03 04 05 06
2.1 2.2 2.3
07 09 11 12 13 15 16 16 17 20 22 22 22 25 27 32
History and Development The Nature of Light Geometrical Optics 2.3.1 Scattering 2.3.2 Critical angle and total internal reflection 2.4 Physical Optics 2.5 Optical Aberrations 2.5.1 Chromatic aberrations 2.5.2 Monochromatic aberrations 2.5.3 Correcting (reducing) optical aberrations 2.5.4 Surface and material aberrations 2.5.5 Optical system aberrations 2.6 Interference 2.7 Optical System Design 2.8 Types of Optical Components References
3 Raw Materials for Producing Optical Elements 3.1 3.2
What Is an Optical Material? Materials for Optical Elements 3.2.1 Glass 126.96.36.199 Optical glass 188.8.131.52 Color optical filter
33 33 33 34 34 36
184.108.40.206 Special glasses for molding 3.2.2 Crystal 3.2.3 Plastic 3.2.4 Metals (for mirrors only) 3.2.5 Special materials 3.3 Classification of Optical Materials 3.3.1 According to molecular structure 3.3.2 According to atomic orientation 3.3.3 According to the working spectral range 3.3.4 According to colors 3.3.5 According to the refraction index (for glasses) 3.4 Main Characteristics of Optical Materials 3.4.1 Optical properties 3.4.2 Internal (bulk) quality 3.4.3 Chemical properties 3.4.4 Mechanical properties 3.4.5 Electrical properties References 4 Manufacturing Processes of Optical Materials 4.1 4.2 4.3
Introduction Glass Manufacturing Process Crystal 4.3.1 Sapphire manufacturing methods 4.3.2 Gradient solidification method 4.3.3 Czochralski method 4.4 Chemical Vapor Deposition 4.4.1 Types of CVD processes 4.4.2 Basic steps of the CVD process 4.4.3 CVD system 4.4.4 Hot isostatic press 4.5 Plastic 4.5.1 CR-39 4.5.2 Related concepts 4.6 Aluminum 4.6.1 Related concepts References 5 Methods for Producing Optical Components 5.1 5.2 5.3 5.4 5.5
Introduction Conventional Method: Spindle Grinding and Polishing Diamond Turning Precision Glass Molding and Precision Molded Optics Additional Methods for Improving Optical Elements
36 38 39 40 40 41 41 41 42 43 44 44 44 45 46 48 49 50 51 51 51 54 55 56 58 59 60 60 60 61 62 63 64 64 65 67 69 69 69 71 73 76
5.5.1 Magneto rheological finishing 5.5.2 Hybrid molding 5.5.3 Computer-numerical-control grinding and polishing method 5.5.4 Freeform polishing method 5.5.5 Ion beam figuring 5.6 Additional Shaping Methods and Those that Produce Specular Surfaces References 6 Optical Coatings 6.1 6.2 6.3 6.4 6.5
Classification of Optical Coatings Materials AR Coating Reflective Coatings Optical-Coating-Deposition Technologies 6.5.1 Evaporation (deposition) methods 6.5.2 Sputter deposition methods 6.5.3 Advanced plasma reactive sputtering (APRS) 6.6 Requirements 6.7 Typical Spectral Curves References 7 Optical Adhesives 7.1 Introduction 7.2 Production Bond Failures 7.3 Incoming Failure Identification References 8 Optics Standards and General Technical Specifications 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9
Introduction The Importance and Utility of Standards and Specifications Defining a Standard Defining a Specification MIL-HDBK, MIL-STD, and Milspecs International Organization for Standardization ANSI, ASTM, and ASME Deutsches Institut für Normung General Standards for Technical Drawings
9 Metrology: Measurement Theory 9.1 9.2 9.3 9.4 9.5
Definition Scientific or Fundamental Metrology Applied, Technical, and Industrial Metrology Legal Metrology Geometric Dimensioning and Tolerancing
76 78 79 80 82 83 84 87 87 88 88 90 92 92 94 95 95 97 97 99 99 101 103 104 105 105 105 106 107 108 110 114 115 115 117 117 118 118 118 119
9.6 Rules of Thumb for Measurement Tools References
II Methods and Tools
10 Testing and Examining Optical Components
10.1 Introduction 10.2 Overview of Production Requirement Documents 10.3 A Review of Quality Production and Inspection Records 10.3.1 Process report 10.3.2 Inspection report 10.3.3 Raw material certificates (or certificate and meld data) 10.3.4 Routing card 10.3.5 COC, COT, and COA 11 Inspection and Testing of Raw Materials 11.1 Index of Refraction 11.2 Homogeneity 11.2.1 Designation of required homogeneity in production files 11.2.2 Homogeneity designation according to ISO 10110-4 11.3 Bubbles and Inclusions 11.4 Striae 11.4.1 Designation of required striae in production files 11.5 Strain (Stress) 11.5.1 Designation of required stress birefringence in production files 11.6 Transmission/transmittance 11.7 Resistivity of Silicon or Germanium References 12 Inspection and Testing of Components 12.1 Radius of a Spherical Surface 12.2 Sag (Sagitta) 12.3 Centration 12.4 Dial Gauges and Indicators 12.5 Roundness (Circularity) 12.6 Central Thickness of a Lens 12.7 Thickness and Parallelism of Windows 12.8 Length between Ground Surfaces 12.9 Concentricity 12.10 Perpendicularity 12.11 Chamfer 12.12 Inside Edges 12.13 Surface Texture
125 126 126 127 127 128 132 135 139 139 145 150 152 152 152 156 158 167 167 170 172 177 179 204 209 224 224 233 242 247 249 255 259 263 265
12.14 Angularity 13 Inspection and Testing of Surface Shape and Figure 13.1 Test Plate 13.2 Analyzing the Interference Pattern Revealed by the Test Plate 13.3 Example Surface Patterns of Flat and Spherical Surfaces 13.4 Principles of Manual Analysis of Interferograms 13.5 Analyzing the Interference of Simple Patterns 13.6 Analyzing the Interference of Various Patterns 13.7 Important Considerations when Analyzing Interference 13.8 Interferometric Measurements for Flat and Spherical Surfaces 13.9 Testing Cylindrical Surfaces References 14 Coatings 14.1 Optical Properties 14.2 Environmental Durability 14.3 Visual Inspections 14.4 Witness Samples 14.5 Durability of Coatings with Primers and Silicon Removers References 15 Special Properties of Aspheric Surfaces, Diffractive Surfaces, and Sapphire 15.1 Aspheric Surfaces 15.1.1 Profile plots 15.1.2 Contact profilometer method by OptiPro 15.1.3 Noncontact 3D method 15.1.4 Roughness 15.1.5 Slope error 15.2 Diffractive Surfaces 15.2.1 Measuring and testing diffractive surfaces 15.3 Sapphire References III Inspection and Quality Assurance 16 Acceptance Sampling (Standards and Methods) 16.1 16.2 16.3 16.4 16.5 16.6 16.7
Introduction Terms and Definitions How to Determine if 100% Inspection is Necessary Sampling Plan Procedure Types of Decisions Main Statistical Inspection Sample Table References Summary
272 283 285 288 290 295 298 302 306 308 314 321 325 325 334 337 338 345 346 347 347 348 359 361 362 367 369 371 375 381 383 385 385 385 386 387 387 388 391
References 17 Location and Process of the Inspection/Test 17.1 Location 17.2 Basic Needs 17.3 Process 18 Visual Inspection 18.1 Introduction 18.2 Definitions 18.2.1 Visual inspection 18.2.2 Beauty defects 18.2.3 Cosmetic defects 18.3 Requirements for Good Visual Inspections 18.4 Kinds of Defects 18.5 Visual Inspection Methods 18.6 Main Defects in Optical Elements 18.7 Illustrations of Visible Defects in Optical Elements 18.8 Measuring and Calculating Visual Defects 18.8.1 Scratches and digs 18.8.2 Defects according to military specifications 18.8.3 Defects according to ISO standards 18.8.4 Edge chips according to military specifications 18.8.5 Glass defects, bubbles, and inclusions according to military specifications and standards 18.8.6 Stains 18.8.7 Cement defects 18.8.8 Drawing C7641866: surface quality standards for optical elements 18.8.9 Common sense and consideration References 19 Handling Optical Components 19.1 Introduction 19.2 Cleaning and Handling 19.2.1 Cleaning solvents (solutions) 19.2.2 Supplemental materials and accessories 19.2.3 Cleaning and handling procedure 19.2.4 Cleaning and handling assembled optical elements 19.2.5 Cleaning and handling procedure for outer optical elements during maintenance 19.3 Guidelines for Cleaning or Handling Optical Elements 19.4 Packaging, Storage, and Shipping 19.4.1 Packaging
391 393 393 393 394 397 397 398 398 399 399 400 402 402 404 409 409 410 420 423 425 425 428 429 431 435 438 441 441 441 441 443 443 447 448 448 449 450
19.4.2 Storage 19.4.3 Shipping 19.5 Health and Safety Aspects 19.6 Environmental and Additional Health and Safety Aspects 19.7 First Contact™ Cleaning Technology References 20 Testing of Optical Systems 20.1 Introduction 20.2 Main Optical System Parameters 20.2.1 Resolving power (or resolution) 20.2.2 Modulation transfer function 20.2.3 Boresight 20.2.4 Noise equivalent temperature difference 20.2.5 Minimum resolvable temperature difference 20.2.6 Minimum resolvable contrast 20.2.7 Blur circle (blur spot) 20.3 Additional Required Tests of Optical Systems References 21 Handling Nonconforming Optical Elements 21.1 21.2 21.3 21.4
Introduction Procedure MRB Decisions Other Considerations
22 Quality Assurance 22.1 Introduction 22.2 Terms and Definitions 22.3 Quality Management Theories 22.3.1 Deming’s theory 22.3.2 Crosby’s theory 22.3.3 Juran’s theory 22.3.4 Ishikawa’s theory 22.3.5 Feigenbaum’s theory 22.3.6 Shewhart’s theory 22.3.7 Garvin’s theory 22.4 Product Manufacturing Steps 22.5 Quality Management Standards 22.6 Quality Audits 22.7 Lean Manufacturing 22.7.1 The seven wastes 22.8 Quality Costs 22.9 Controlling Manufacturing Processes
451 451 452 452 453 454 459 459 459 459 460 462 462 464 464 465 466 466 467 467 467 468 469 471 471 471 472 472 474 475 476 478 479 481 483 483 485 485 486 486 487
22.10 Communication with Suppliers and Customers 22.11 Manufacturer Qualifying Process 22.12 First Article Inspection 22.13 Professionalism and Organizational Conduct References
487 488 489 489 491
Preface This book is the culmination of the knowledge I have acquired during my work in the optics industry over 30 years. During that time, I have collected a vast amount of information dealing with the inspection of optical elements, as well as other related topics, such as optical materials, production methods, and standards and specifications used to determine requirements in drawings and tests. Because the demand for more-complicated optical elements is growing, the need for adequate quality is growing, as well, which means that both the manufacturer’s and the customer’s optics inspectors must be well aware of the requirements stated in relevant drawings and specifications. Furthermore, they need to be well trained with the methods and tools used for tests and inspections. Knowledge of basic optics, raw materials used in the production of optical components, methods of manufacturing optical elements and coatings, sampling methods, and quality assurance theories is an essential part of inspector professionalism. An optics inspector must also complete inspection reports, conduct failure investigations by request, and fulfill the requirements of other organizational procedures. Good professional relations with the inspectors of customers or suppliers help promote workflow and serve the organization’s needs. This book serves as a tool for optics quality inspectors at the beginning and at a more advanced stage of their work. It also serves optical designers and anyone else who is interested in optics inspection and related issues. Michael Hausner September 2016
Acknowledgments Many of the photographs, pictures, and figures included in this book were used with permission of their owners, and I am very thankful for their help. Without their approval, the book would have been very different and much poorer. In the caption of each such image, the owner, source, and their rights are mentioned. I would like to express my gratitude in alphabetic order to the following companies/producers/manufacturers and their representatives, and to article writers I have contacted and received their permission: • • • • • • • • • • • • • • • • • • • • • • • •
ABTech, Inc. (www.abtechmfg.com). Aero Compliance (http://www.aerocompliance.com.au). Alcatel-Lucent (http://www.alcatel-lucent.com). Ametek Precitech, Inc. (www.precitech.com). Armstrong Optical Metrology (http://www.armstrongoptical.co.uk). American Society for Quality (ASQ) (http://asq.org/index.aspx). Astro-foren.de http://astro-foren.de). Atago Co. Ltd. (http://www.atago.net). Carl Zeiss AG (http://www.zeiss.com). Crystaltechno Ltd. (http://www.crystaltechno.com). Custom Scientific, Inc. (www.CustomScientific.com). CVI Melles Griot (http://mellesgriot.com). Edmund Optics Inc. (http://www.edmundoptics.com). Electro Optical Components, Inc. (http://www.eoc-inc.com). Enggwave.Com (http://www.enggwave.com). Fisba Optic AG (http://www.fisba.ch/english/company/default.cfm). Fraunhofer Institute for Production Technology IPT (http://www.ipt. fraunhofer.de). Gaertner Scientific Corporation (http://www.gaertnerscientific.com). Gage-Line Technology, INC (http://www.gage-line.com). Guild Optical Associates, INC (http://guildoptics.com). GT Advanced Technologies. (http://www.gtat.com). Harvard Business School (and David A. Garvin). (http://www.hbs.edu). Holmarc Opto-Mechatronics Pvt Ltd. (www.holmarc.com). Hoya Corporation (http://www.hoya.co.jp/english).
• ilis GmbH (http://www.ilis.de/en/products.html). • Instrument Technology Research Center (ITRC) (http://www.itrc.narl. org.tw/index-e.php). • ISP Optics Corp. (http://www.ispoptics.com). • James C. Wyant, Professor of Optical Sciences, Professor of Electrical and Computer Engineering, University of Arizona. • J.M. Derochette. (http://jm-derochette.be). • Juran Institute, Inc. (http://www.juran.com). • Kugler GmbH (http://www.kugler-precision.com/index.php?Home-EN). • Kurt J. Lesker Company (http://www.lesker.com) • Lambda Research Corporation (http://www.lambdares.com) • Larsen Equipment Design, Inc. (http://larsenequipment.com). • Le Compendium (www.leCompendium.com). • Luceo Co., Ltd. (http://www.luceo.co.jp/en). • Mahr GmbH (http://www.mahr.com). • Marcel-Aubert SA (http://www.marcel-aubert-sa.ch). • Massachusetts Institute of Technology (http://www.jollystar.com). • MaxMax (http://www.MaxMax.com). • Miller Design and Equipment (http://www.miller-design.com/index.htm). • Mitutoyo (http://www.mitutoyo.com). • Moeller-Wedel Optical GmbH (www.moeller-wedel-optical.com). • Moore Nanotechnology Systems, LLC (http://www.nanotechsys.com). • National Academy of Engineering (http://www.nae.edu/Home.aspx). • Neoark Corporation (www.neoark.co.jp/english). • NewOpto Technology Corporation (www.NewOpto.com). • Nikon Corporation (www.nikonmetrology.com). • NIST Research Library (http://www.nist.gov). • Norland Products Inc. (www.norlandprod.com). • NTG Neue Technologien GmbH & Co KG (http://www.ntg.de). • Ocean Optics Inc (http://oceanoptics.com). • OEG GmbH (www.oeggmbh.com). • Ohara Corporation (www.oharacorp.com). • Olympus Corporation (http://www.olympus-global.com). • OptiPro Systems. (www.optipro.com). • Opto-Alignment Technology, Inc. (http://optoalignment.com). • Panasonic (http://www.panasonic.com.sg/industry). • Prometra GmbH (www.prometra.li). • QED Technologies International Inc (//www.qedmrf.com). • Qioptiq (http://www.qioptiq.com). • Rotem Industries (http://www.rotemi.co.il). • Roger Weller/Cochise College (http://skywalker.cochise.edu/wellerr/ mineral/calcite/calcite1.htm). • Rubicon Technology, Inc. (http://www.rubicontechnology.com).
• • • • • • • • • • • • • • • • • • • • • • • • • •
Savvy Optics Corp. (http://www.savvyoptics.com). Schott AG (http://www.schott.com/english/index.html). Scientific Materials Corp. (http://www.scientificmaterials.com). Shimadzu Corporation (http://www.shimadzu.com). Success Infrared, the IR division of Success Optics Inc. (www.opticsinfrared.com). Summers Optical, a division of EMS Acquisition Corp. (https://www. optical-cement.com/default.htm) Tangram Technology Ltd. (www.tangram.co.uk) Taylor-Hobson Ltd., part of the Ultra Precision Technologies Division of AMETEK Inc. (http://www.taylor-hobson.com). Temmek Optics Ltd. (http://temmek.com). TESA Group (http://www.tesagroup.com). Thermotron Industries (http://www.thermotron.com). Thorlabs, Ltd. (http://www.thorlabs.de/index.cfm). Trioptics, Optical Test Instruments (http://www.trioptics.com). ToolsHero (http://www.toolshero.com/kaoru-ishikawa). Tydex (http://www.tydexoptics.com). United Lens Company Inc. (http://www.unitedlens.com). University of Rochester (http://www.rochester.edu). Vecor (http://www.vecorus.com). Vermont Photonics Technologies Corp. (http://www.vermontphotonics. com). Vitron Spezialwerkstoffe GmbH (http://www.vitron.de). Winter Park History & Archive Collection (http://www.wppl.org/ wphistory/philipcrosby). W. Edwards Deming Institute. (https://deming.org). WEO Corporation. (http://weocorp.com). Xonox Technology GmbH (www.xonox-tec.com). Zygo Corporation (http://www.zygo.com). II-VI Infrared (http://www.iiviinfrared.com).
Additional thanks are offered to the following people, each of whom has helped me by reading, correcting, and advising this project: • Orna M., Chemist with a Ph.D. in Physical Chemistry from the Technion, Institute of Technology, Haifa, Israel. Expertise in design, processes development, production, characterization and quality control of optical coatings for all spectral regions and for various optical materials and components. • Sylvie W., Senior Optical Designer with a M.Sc. in Physics from the Technion, Institute of Technology, Haifa, Israel. Involved in Design of Optical Components and Systems and Characterization of Optical Materials for Design.
• Tali M., Optical Engineer with a M.Sc. from the Technion, Institute of Technology, Haifa, Israel, Faculty of Physics. Specializes in designing optical systems for imaging and non-imaging applications in the IR and VIS range. • Gadi K., Electro-Optics R&D Quality Manager who holds a B.A of Natural Science from the Open University of Israel and a Ph.D. of Science Philosophy from the University of Haifa. • Karnit M., Optical Designer with a M.Sc. in Applied Physics from the Hebrew University, Jerusalem, Israel. Involved in design of optical systems in the IR and VIS range. • David M., Fine Mechanics and Automation Technician (retired), over 40 years in the industry. Involved in the design, practical uses, and repair of many different systems, including optical systems. • Scott M., Sr. Editor at SPIE Press. Assisted with revisions and composition.
Part I: Theory and Materials
Introduction 1.1 Prologue Quality is a very important issue in production or service; it makes the product attractive. Satisfied customers bring profit to the manufacturer and enable company growth by providing repeat business and by encouraging other potential customers. Of course, quality is not the only issue in business, but it is a major one. The quality of products and services has been discussed widely by “quality gurus” and others who have published their theories about “quality” and “quality management” in books and articles. Those theories, which supplied the tools for behavior and improvement, helped further improve the quality of products and services and the growth of companies. Notable quality gurus include Walter Shewhart and his “Statistical Control Charts,” Edwards Deming and his 14 principles for “Total Quality Management” (TQM), Joseph Juran and his “Quality Trilogy,” Armand Feigenbaum and his “Total Quality Control” (TQC), Kaoru Ishikawa and his “Company-Wide Quality Control” (CWQC), Philip Crosby and his “Zero Defects,” and David Garvin with his “Eight Quality Dimensions” (see Chapter 17). At the same time and afterwards, national and international quality standards, based widely on quality theories and standards related to designations in optical drawings and/or specifications, were established and improved. They became an integral part of the manufacturing and managing of quality issues of optical elements and assemblies in all existing organizations involved in the optics industry. Writing this guide used all of the experience I gathered while inspecting and testing optical elements over the years, in house and at manufacturer plants (source inspection), handling various failures and nonconformance due to bad production, failures or mistakes in design or in production files, and during necessary corrective actions (handled in house, by suppliers or producers, and even according to customer requests). My collaboration with
optics designers as a technical consultant involved training and solving problems in the domain of optics QA, which had a special influence on my knowledge in the field of inspection of optical elements. Important supporting sources for this book include relevant international standards of optics, professional literature (books and articles), and photographs and figures. The information this book presents is very broad and appears in standards, professional literature, books and articles based on the knowledge and experience of people working in this area. Some of those resources are available for purchase, and some are free to download from the Internet. The online information may be changed or removed, and other web sources will appear. Thus, people working in these areas (designing and inspection) should regularly avail themselves of the aforementioned pertinent information to support their current work and develop their knowledge for the future.
1.2 Defining Quality The following are definitions of quality based on the aforementioned gurus and standards: • “Quality consists of the capacity to satisfy wants” (C. D. Edwards, “The Meaning of Quality,” Quality Progress, Oct. 1968). • “Quality is fitness for use” (J. M. Juran, Quality Control Handbook, 1988). • “Quality [means] conformance to requirements” (P. B. Crosby, “Quality Is Free,” 1979). • “Uniformity around a target value,” i.e., lower the standard deviation in outcomes and keep the range of outcomes to a certain number of standard deviations (with rare exceptions). • “The loss a product imposes on society after it is shipped.” This definition of quality is based on a more-comprehensive view of the production system (G. Taguchi, “Introduction to Quality Engineering,” 1986). • “A way of managing a business organization” or “The total composite product and service characteristics of marketing, engineering, manufacturing, and maintenance through which the product and service in use will meet the expectation of the customer” (A. Feigenbaum, “Total Quality Control,” 1961). • David Garvin identified “eight dimensions of quality,” which he maintained describes the meaning of quality to managers, operators, and customers. By accepting that customers have a different perception of quality than a manager, the quality effort can be focused. The eight dimensions are performance, features, reliability, conformance, durability, serviceability, aesthetics, and perceived quality.
• “Totality of characteristics of an entity that bear on its ability to satisfy stated and implied need” (ISO 8402:1994, ANSI/ISO/ASQC A8402:1994). • “The totality of features and characteristics of a product or service that bears on its ability to satisfy given needs” (BS 4778: PART 1:1987). • “A subjective term for which each person has his or her own definition. In technical usage, quality can have two meanings: (1) The characteristics of a product or service that bear on its ability to satisfy stated or implied needs, and (2) A product or service free of deficiencies” (ASQ Quality Glossary, http://asq.org/glossary/q.html).
1.3 Relating Quality with Optics Inspections and Testing Prior to the process of integrating an optical assembly, there are two kinds of customers: one who buys the product (optical elements, such as lenses or mirrors) and the manufacturer of the optical elements who buys the raw materials of the product. Both customers want to get a product (or a material) that satisfies their needs and wants. The products must fit their use and conform to the requirements stated in the written specifications, drawings, and contracts. In order to fulfill all of these expectations, some necessary steps that involve a number of bodies in the organization must be taken: • Engineering department, which is responsible for establishing the technical requirements to be stated in drawings and specifications. • Purchasing department, which is responsible for the outlay and lead time. • Quality assurance (QA), which is responsible for verifying the matching of the products’ parameters to the stated requirements. The engineering and QA departments play a major role in this chain, emphasizing the purpose of this text: ensure that the product that is planned, produced, and sent to the customer fits the requirements. Thus, the elements that are produced by our suppliers at our plant must be inspected and tested, and the product delivered to the customers must be verified to fit all requirements, which is a major issue in the business. The inspection activities in most organizations belong to the QA department. In some organizations, inspections and tests belong to the purchasing department. Regardless, the ownership is less important than the familiarity of the inspector or the tester with all of the stated designations and requirements and their ability to perform their duties objectively, e.g., producing trustworthy results without managerial oversight. The optical designer who is responsible for designating the requirements stated in drawings or specifications must be familiar with the accepted standard
designations, as well as with optical standards (which play a major role in determining the requirements in optical elements and assemblies). There is a technical language that connects (in almost all cases) the designer and the quality inspector without needing frequent communication.
1.4 The Purpose of This Book This book is intended to instruct inspectors and designers of optical elements (mainly) and assemblies about the major systems of relevant inspections and tests, and introduce the corresponding tools and instruments.
Because the inspection and testing of optical elements consist of quality and optics, this text also refers to the major subjects of optics that are important for inspectors and discusses QA and inspection issues for designers. A professional optics inspector must not only hold the inspection tool, measure the required parameter, and document the result on the inspection report but also understand the meanings of all of the optical, mechanical, and quality requirements of the optical element and be aware of all existing standards. When a deviation from the requirement has been found, a welltrained optics inspector should be able to identify the source of the observed deviation and recommend a corrective action. Conversely, an optics designer responsible for determining the requirements stated in drawings and/or specifications must be familiar with the producer’s or the customer’s inspection capabilities and with the limitations and existence of the needed tools or equipment for measuring the stated requirements. This awareness helps the designer clearly determine the requirements and either guides the manufacturer and inspector toward verifying their fulfillment or recommends an alternative inspection method. • This book presents the important equipment and tools used in the optics industry for the manufacturing (grinding and polishing), processing (coatings and their durability), inspection, and testing of mechanical and optical parameters of optical elements. • It is not the purpose of this text to teach how to use these tools or equipment. This responsibility belongs to the manufacturers, either their representatives or the proper manuals. It is also not the intention of this book to recommend specific equipment or tools. • Each equipment or tool has its limitations and advantages, and it is the obligation of the users or buyers to choose the proper option according to their needs.
Optics 2.1 History and Development Optics is the branch of physical science that deals with the nature and properties of electromagnetic radiation called light and its interaction with matter. Many optical elements, such as lenses, beamsplitters, and mirrors, are widely used in the military, medical, and industrial fields for many commercial applications. The word optics is derived from Greek and refers to matters of vision. Optics usually describes the behavior of visible, infrared, and ultraviolet light. Two main branches describe light: • Geometrical optics, which describes light in terms of straight rays, and • Physical optics, which describes light in terms of waves. The history of optical science can be divided into the following ages and topics: 1. The beginning in the Islamic world, ~300 BCE to ~1000 CE. In ~300 BCE, Euclid (Alexandria) noted that light travels in straight lines and described the law of reflection. 2. The continuation in the Middle Ages (medieval period) in Europe from the ~5th to the ~15th century. 3. The Renaissance and modern optics in Europe from the ~14th to the ~17th century introduced diffraction to light theory. 4. From the ~17th century until the Modern Age, further research, theories, and discoveries occurred: the discovery of infrared and ultraviolet radiation, development of interferometry, measurement of the velocity of light in media, theory of aberrations of optical instruments, theory of resolution of optical instruments, x rays, holography, lasers, etc. 5. The Modern Age (the 20th century) refers to “modern optics” developments, such as wave optics and quantum optics, as well as many optical instruments for military, medical, communications, and industrial uses based on lasers and optical fibers. The development of optical science went hand in hand with the development of lenses, beginning in the ancient period with the Egyptians,
Greeks, Romans, and Chinese. These lenses (spheres filled with water) were used only as magnifying glasses or burning glasses. A breakthrough in lens development was made in the Middle Ages when the principles of lens functionality were properly described by an Arabic physicist, Ibn al-Haytham (965–1020), also known as Alhacen or Alhazen. He was the first to account correctly not only how light is refracted by a lens but also how the eye functions in principle. His investigations used spherical and parabolic mirrors, and he was aware of spherical aberration. Ibn al-Haytham also investigated the magnification produced by lenses and atmospheric refraction. His work was translated into Latin and became accessible to later European scholars. The widespread use of lenses in Europe started with the invention of spectacles around 1284 in Italy, credited to Salvino D’Armate (1258–1312) for inventing the first wearable eyeglasses. Additional milestones in lenses and their usage: • Reading stones were invented between the 11th and 13th century. They were plano-convex lenses that were placed on top of text to magnify the letters for people with presbyopia, a condition where an aging eye exhibits a progressively diminished ability to focus on near objects. • The microscope was invented around 1595 in the Dutch Republic. The credit was given to three different eyeglass makers: Hans Lippershey (1570–1619), and Hans Janssen (15??–1638) and his son, Zacharias (1585–1632). • The telescope was invented in the Netherlands in 1608, credited to three individuals: Hans Lippershey, Zacharias Janssen, and Jacob Metius (after 1571–1624 or 1631). The creation of the microscope and the telescope led to improvements in the correction of natural errors/deviations (chromatic and spherical) caused by lenses and, later, to improvements of the results received from the apparatus being used. More study and research in the field of optics led to additional performance improvements and the construction of devices needed in all industries (military, medical, and civilian). Such improvements include different types of coatings of optical lenses and other optical items (prisms, beamsplitters, filters, and windows), different kind of surfaces (aspheric, parabolic, and diffractive), and new materials for different wavelengths; in order to achieve all of this, the optical industry had to build the necessary tools. Furthermore, new instruments were constructed to verify the results to stated requirements: • • • •
Spectrophotometers for verifying a coating’s spectral conformance, Refractometers for verifying the index of refraction of optical materials, Autocollimators for verifying the angles of prisms, and Interferometers for verifying the optical surface quality.
2.2 The Nature of Light The term “light” is usually used to mean wavelengths visible to human eyes, even though it sometimes refers to the electromagnetic radiation of any wavelength, visible or not. Light is electromagnetic radiation and has a wavelength range (in the visible, VIS) of ~400 nm (0.4 mm) to ~700 nm (0.7 mm), as shown in Fig. 2.1. It is a part of the electromagnetic spectrum that also includes the infrared range and the ultraviolet range. The main properties of light include the following: • Speed: The speed of light in vacuum is 299,792,458 meters per second (m/s), one of the fundamental constants of nature. • Reflection: When light is reflected off any surface, the angle of incidence Qi is always equal to the angle of reflection Qr (see Fig. 2.2). • Refraction: Refraction is the bending of light (wave) as it passes between materials with different optical densities (different speeds), as illustrated
The electromagnetic spectrum. (See color plate section.)
The reflection property of light.
Figure 2.3 indices.
Refraction of light at the interface between two media of different refractive
in Fig. 2.3. The refraction of light passing from a fast medium (e.g., air) to a slow medium (e.g., BK7, Schott optical glass) bends the light direction toward the normal of the boundary between the two media. The amount of bending depends on the refractive index of the two media and is described quantitatively by Snell’s law, named for Willebrord Snell, who discovered the law in 1621. Snell’s law states that n1 sin ur v2 ¼ ¼ , n2 sin ui v1
where n1 is the refractive index of medium 1, n2 is the refractive index of medium 2, ui is the angle of incidence, ur is the angle of refraction, v1 is the velocity of light in medium 1, and v2 is the velocity of light in medium 2. Note that n ¼ c/v, where c is the speed of light in vacuum, and v is the speed of light in the medium. • Dispersion: Dispersion in optics describes the splitting of light into its composed colors. Light comprises waves of different wavelengths that represent different colors. When light passes through a transparent medium, each wavelength refracts in a different direction, separating all of the colors. This phenomenon can be seen when light passes through a triangular prism, as in Fig. 2.4, creating a rainbow (see Fig. 2.5 for natural occurances).
Figure 2.4 Dispersion behavior of white light. (See color plate section.)
Figure 2.5 Dispersion behavior as seen by (a) a rainbow and (b) on stairs at a train station. (See color plate section.)
• Diffraction: Diffraction phenomenon of light refers to the bending of light as it passes around an edge, opening, or slit and then spreads out. See Fig. 2.6 for an illustration. • Interference: Interference is discussed in detail in Section 2.6.
2.3 Geometrical Optics Geometrical optics, or ray optics, describes light behavior in terms of “rays,” which is an abstracted description for the behavior of light when it travels in a homogeneous medium, bends or splits in two at an interface between two dissimilar media, and then is reflected, refracted or absorbed. The light ray is perpendicular to the light’s wavefronts (and is therefore collinear with the wave vector).
Diffraction behavior of a plane wave.
The main characteristics that refer to the geometrical optics are reflection, refraction, and dispersion (covered in Section 2.2), as well as two other factors: • scattering, and • critical angle and total internal reflection. 2.3.1 Scattering The scattering of light (an electromagnetic radiation) means that when light impinges on or passes through a medium that is not uniform (fully or localized), it scatters in all directions. Theoretically, a perfect mirror’s surface or 100% homogenous optical material (without bubbles or inclusions and with a perfectly polished surface) will reflect or transmit light, respectively, perfectly without scattering; however, in the real world, there is always some kind of inhomogeneity that creates scattering. Scattered light from or in an optical element or assembly mostly affects the following optical characteristics: • Transmission of the optical element (more scattering means less transmission), and • Reflection from exposed (not blackened) surfaces in optical assemblies might confuse the image or the wavefront. Reducing the scattering is made by the optical engineering by determining the polishing surface quality (by scratch–dig and roughness requirements) and raw material quality, or purity (by bubbles and inclusions class). Thus, the scattering may occur mainly out of the following reasons: a poor polish surface (Fig. 2.7) or bubbles and inclusions (Fig. 2.8). Furthermore, the finished optical surfaces of the optical elements must be cleaned during each step of production and assembly in order to eliminate the scattering from different kinds of particles attached to the optical surfaces.
Figure 2.7 Scattering due to a poor polishing surface, which can occur on the front and back surfaces.
Figure 2.8 Scattering due to inclusions or bubbles in the raw material.
2.3.2 Critical angle and total internal reflection When a light ray passes from one transparent medium to another, there are four possibilities for the light ray behavior: 1. The light ray is traveling normal to the surface, and it is not bent when passing from one medium to another, but part of it is reflected back. In glasses, each surface reflects about 4% of the incoming energy (light). See possibility (1) in Fig. 2.9. 2. In ordinary refraction, the incident light ray passing from media n2 in angle uin is reflected and comes out of medium n1 at angle uout. See possibility (2) in Fig. 2.9. 3. Critical angle: The minimum angle of incidence light ray beyond which total internal reflection occurs for light traveling from a medium with a higher refractive index n2 to a medium with a lower refractive index n1. In this case, the refracted light travels along the
Figure 2.9 Light ray behaviors when passing from one medium to another.
Figure 2.10 Light ray behaviors for the critical angle and for internal reflection.
boundary between the two media and will not cross that boundary. See possibility (3) in Fig. 2.10. 4. Total internal reflection (TIR): A phenomenon that occurs when a light ray moving in a medium with a higher index of refraction n2 strikes the boundary of a medium with a lower index of refraction n1 at an angle that is greater than a particular critical angel. In such case, the entire light ray is reflected back to the original media. The incident ray u1 is equal to the reflected ray u2. See possibility (4) in Fig. 2.10. The critical angle can be calculated from Snell’s law; for Fig 2.10, it will be n2 sin uc ¼ n1 sin90 deg and
sin uc ¼ ðn1 sin90 degÞ∕n2 :
If, for example, the critical angle of a light ray traveling from glass (nglass ≈ 1.5) to air (nair ¼ 1.0) is calculated, the critical angle will be uc ¼ arcsinð1.0∕1.5Þ ¼ 41.8 deg :
2.4 Physical Optics Physical optics, or wave optics, describes light behavior in terms of “waves.” This branch of optics addresses phenomena such as interference, diffraction, and polarization because waves are electromagnetic with an equal amount of energy stored in the electric and magnetic fields perpendicular to each other (see Figs. 2.11 and 2.12).
Schematic demonstration of the electromagnetic wave.
Figure 2.12 The connection between a wavelength and its energy.
2.5 Optical Aberrations An optical aberration is a natural distortion in the front wave or image formed by an optical element (medium), primarily a lens. These distortions include two groups: • Chromatic aberrations [longitudinal and lateral (transverse)] and • Monochromatic aberrations (Seidel and wave aberrations). Two other groups of aberration should be noted: • Surface and material aberrations, and • Optical assembly aberrations, i.e., those formed by design and/or assembly mistakes, or a combination of all existing aberrations. One of the major tasks of an optics designer is to eliminate or minimize any kind of existing or possible aberration to produce the required optical assembly with the best possible performance. 2.5.1 Chromatic aberrations The longitudinal chromatic aberration (Fig. 2.13) occurs due to diffraction phenomenon. When white light travels along the optical axis and goes through two media (air and glass in the simplest case), different wavelengths of light are refracted by different amounts and to different directions. The blue light is refracted more strongly than the green light, and the green refracted more strongly than the red. Thus, the focus of each light lies at a different point along the optical axis in the longitudinal direction. The lateral (transverse) chromatic aberration (Fig. 2.14) occurs due to the same diffraction phenomenon as the longitudinal chromatic aberration but from a white ray light coming not along the axis but at an angle to it, refracted and focused at different positions along the same focal plane.
Figure 2.13 Behavior of the longitudinal chromatic aberration. (See color plate section.)
Behavior of the lateral (transverse) chromatic aberration. (See color plate section.)
2.5.2 Monochromatic aberrations Five monochromatic aberrations are created because of the nature of a lens and the behavior of monochromatic light passing through it: • • • • •
Spherical aberration, Coma, Astigmatism, Curvature of field, and Distortion.
Spherical aberration is an axial aberration that occurs due to the different directions of refraction of light rays when they strike and pass through a lens at a different height relative to the optical axis or when monochromatic light rays are reflected from a spherical mirror. Figures 2.15 through 2.18 illustrate the difference between the ideal and realistic behavior of light. Coma is an optical aberration that occurs due to a tilted, incoming incident wavefront (with respect to the optical surface or the optical axis) that creates an image with a tail (coma) like a comet (Fig. 2.19). A lens with considerable coma may produce a sharp image in the center of the field that becomes increasingly blurred toward the edges. Astigmatism is an aberration resulting of different curvatures in perpendicular planes. Rays propagate in two planes and are focused at different places on the principal axis of the ray. At intermediate points between the two focused points, rays combine to form a compromise image that sometimes looks like a small plus sign (Fig. 2.20). Curvature of field is an aberration where the focused image is accepted on a curve surface. If the sharpness is set along the edges, the center of the image
Figure 2.15 Behavior of the ideal monochromatic rays passing through a lens and focused on one point of the optical axis of the lens.
Figure 2.16 Behavior of the real monochromatic rays passing through a lens and spread on different points on the optical axis of the lens.
Figure 2.17 Behavior of the ideal monochromatic rays impinging on a mirror and reflected on one point of the optical axis of the mirror.
Figure 2.18 Behavior of the real monochromatic rays impinging the mirror, reflected and spread on different points on the optical axis of the mirror
Figure 2.19 The coma aberration effect of incoming monochromatic tilted light passes through a lens and creates an image with a comet-like tail.
accepted on a flat detector surface will be blurry (not as sharp as the edges), and if the sharpness is set on the center, then the edges of the image accepted on a flat detector surface will be blurry. Figure 2.21 depicts this behavior. This aberration phenomenon is produced for an off-axis beam only. Distortion is an aberration that results from the difference in the magnification over the field of the lens. Because the focal lens varies over the image surface (transverse magnification), an image with different magnification areas is created (i.e., parts of the image are more magnified than others).
Figure 2.20 Principle of the astigmatism aberration affect.
Figure 2.21 The principle of curvature field aberration affect.
There are two types of distortion (see Figs. 2.22 and 2.23): • Barrel distortion, where the magnification decreases with radial distance from the optical axis; and • Pincushion distortion, where the magnification increases with radial distance from the optical axis. 2.5.3 Correcting (reducing) optical aberrations Optical aberrations cannot be eliminated completely, but they can be reduced to a size that allows the optical performance to meet requirements. The optical designer accounts for all of the existing aberrations and takes appropriate steps to reduce them.
Figure 2.22 Barrel and pincushion image distortion compared to an object free of distortion.
Figure 2.23 The real-life objects from which the names for barrel and pincushion distortion were derived.
Chromatic aberrations can be corrected by the following: • An achromatic lens (achromat), comprising two cemented lenses (crown glass and flint glass) that focus two wavelengths sharply. • An apochromatic lens (apochromat), comprising three cemented lenses that focus three wavelengths sharply. • A combination of extra-low-dispersion (ED) glass with other glasses. • An element with a diffractive surface. Monochromatic aberrations require specialized treatment: • Spherical aberration can be minimized by using a small aperture stop, by using a parabolic lens or lenses made of gradient-index material, or by using lenses with aspheric surfaces. • Coma can be reduced or canceled by an appropriate choice of radii of curvature of the lens, by using a small aperture stop (for wide angle aberration), or by using a combination of optical elements.
• Astigmatism can be reduced by choosing appropriate radii of the lenses, by stopping down the lens (smaller lens clear aperture, larger F-number), or by appropriate lens combinations in optical assembly. • Curvature of field can be reduced by choosing appropriate radii of the lenses, by relocating the stop position, or by using a different combination of lenses. • Distortion can be reduced by a so-called orthoscopic doublet. 2.5.4 Surface and material aberrations Surface irregularity and power, not a homogenous index of refraction and raw material imperfections (e.g., stones and bubbles), lead to wavefront aberration. All of the mentioned parameters are defined in the drawing or specification of the lens (or optical element) and should meet the stated requirements. 2.5.5 Optical system aberrations Optical systems (assemblies) include (besides the optical elements) a metal housing, electronic parts, and lens adapters that connect the lenses to the housing. The optical assembly should withstand environmental tests such as vibration, humidity, and temperature. In addition to the optical designer, other people from different disciplines (such as mechanical design and electrical engineering) help fulfill the stated requirements of the optical system. Assuming that the specifications were correctly defined, however, some aberration or failure may occur during the final test. An investigation of this failure may lead to the following findings: • incorrect assembly, • a deviation in some parameter that was not found during inspection (optics, housing, or electronic devices), or • design error. The discovery of an aberration or any other nonconformance during the final test of the optical system may require spending precious extra time and money, and delay delivery to the customer. A nonconformance in optical performance may mean the optical system must be disassembled to find a mounting error. Corrective actions to find and eliminate the source of the aberration or any nonconformance should be taken to prevent similar situations.
2.6 Interference Interference is a phenomenon in which two light waves coincide; it can create either a constructive or destructive interference pattern (Fig. 2.24), depending on how the crest and trough of each wave coincide. The term usually refers to the interaction of waves that are correlated or coherent with each other, either
Figure 2.24 Destructive and constructive interference phenomena.
because they come from the same source or because they have the same or nearly the same frequency. Interference effects can be observed with all types of waves. Matters such as the inspection and testing of optical elements deal with light interference for all kinds of light waves when testing optical surfaces or assemblies in the visible (VIS), infrared (IR), or ultraviolet (UV) region. These tests are conducted with a monochromatic or laser light. The interference phenomenon is used widely in optics in three matters: 1. Coatings: Interference enables optical coatings to reduce the reflection from optical surfaces and increase the light transmission passing through the media (lens, window, etc.). 2. Testing the quality of optical surfaces: • An interference pattern is created by matching the test plate to the optical surface and illuminating them with monochromatic light. This interference pattern (fringes or Newton rings) allow one to visually evaluate or calculate their amount by accepted techniques and compare to stated requirements (see Fig. 2.25, interference pattern #1). • A laser interferometer can create interference that enables one to see an interference pattern (fringes or Newton rings) and visually evaluate them or automatically measure their precise amount and compare to stated requirements (see Figs. 2.26 and 2.27). 3. Testing optical assemblies by analyzing their wavefront interferogram created by an interferometer. When measurements are conducted with a test plate, the following VIS monochromatic lights are primarily used: • Yellow/orange sodium light source, l ¼ 575 nm (0.575 mm); • Yellow/orange sodium light source, l ¼ 589.3 nm (0.5893 mm);
Figure 2.25 Interference patterns: #1 is made by a test plate with a monochromatic green mercury light source, and #2 is made by a Zygo interferometer with a HeNe laser light source. (See color plate section.)
Figure 2.26 An interferogram made by a Zygo GPI XP/D interferometer (632.8-nm HeNe laser light source) shows the interference pattern and computerized measured results.
• Yellow helium gas source, l ¼ 586.7 nm (0.5867 mm); and • Green mercury light source, l ¼ 546.1 nm (0.5461 mm). When measurements are performed with an interferometer in the VIS, the red HeNe laser light source [l ¼ 632.8 nm (0.6328 mm)] is mostly used. Other light sources are used for IR or UV measurements with an interferometer, depending on the specific needs. Table 2.1 lists the available
Figure 2.27 Simple example of calculating the interferogram of an interference pattern with an accepted technique.
Table 2.1 Various wavelengths available for Zygo interferometers. Data reprinted courtesy of Zygo Metrology Solutions Division. Wavelength 266 355 405 532
nm nm nm nm
1053 nm 1064 nm 1319 nm 1550 nm 3.39 mm 10.60 mm
Application General UV lens system testing and higher precision testing General UV lens system testing and higher precision testing Testing lenses used in DVD optical storage and audio visual devices General testing applications, including optical surfaces, lenses, prisms, corner cubes, homogeneity, and more Testing in laser fusion research Laser rod testing, military imaging systems testing, general near IR optical testing General near IR optical testing Telecommunications optics testing General IR optical system testing IR optical system testing, rough surface testing
wavelengths available for Zygo interferometers, in addition to the common 632.8-nm wavelength.
2.7 Optical System Design An optical system starts with an idea—the recognition of market needs—or a customer request for a specific optical system. In either case, the requirements of the optical system are determined and should include the following parameters: • • • • •
Purpose (civilian or military use), Size and structure, Weight (minimal), Spectrum operation (VIS, IR, UV), Environment operation conditions (temperature, humidity, vibration),
• Performance (resolution, MTF, etc., see Chapter 15), and • Price limitations or price target. These parameters include numeric values that should be fulfilled and taken into account in the next step. An optical system is never created in isolation. It includes a mechanical housing and electronic devices, which means that different disciplines are involved during development: • Optical physicists for the basic optical system, • Optical chemical engineers for the optical coatings, • Mechanical engineers for the mechanical housing and jigs (for testing purposes), • Electrical (hardware and software) engineers, • Testing engineers for constructing test equipment, and • Support staff, i.e., purchasing, quality assurance, and assembly. When designing an optical system, the designer may use different optical design software, such as OSLO, CODE V, LightTools, RSoft, Optica 3.0, Zemax, and WinLens 3D (see Figs. 2.28 and 2.29 for examples). The software enables optimization, tolerancing, and environmental analysis during the design process. The optical system assembly is inspected and tested at the prototype stage to verify its conformance to stated requirements and whether any changes are needed. A request for a change, including changes in tolerances, can come from inspectors, design engineers, and even from the assemby workers. Any
Figure 2.28 Pure optical design (optical elements with ray tracing) using Zemax software. Image reprinted courtesy of Temmek Optics Ltd., all rights reserved.
Figure 2.29 Cross-section of an optomechanical assembly, including ray tracing, produced by TracePro illumination software. Image reprinted courtesy of the Lambda Research Corporation, all rights reserved.
necessary adjustments should be made during the development stage (prototype); therefore, prior to serial production, all required documentation of the optical element should be updated, approved, and made final.
2.8 Types of Optical Components The main optical components that are part of an optical system include • Lens: An optical device that transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element. • Singlet: A simple lens consisting of a single simple element. See Fig. 2.30. • Doublet: A type of lens combination comprising two simple (singlet) lenses, cemented or attached to each other (air spaced). Doublets are used to correct chromatic aberrations. See Fig. 2.31.
Figure 2.30 Different kinds of singlet lenses.
Figure 2.31 A doublet lens.
Figure 2.32 A triplet lens.
• Triplet: A type of lens combination comprising three simple (singlet) lenses, cemented or attached to each other (air spaced). Triplets are used to correct chromatic aberrations. See Fig. 2.32. • Prism: A transparent optical element with at least two polished flat surfaces that refract light and have an angle between them. See Fig. 2.33.
• Window: A transparent optical element with two flat and polished parallel surfaces for transmitting light of a particular wavelength range of interest (visible and infrared). The window is used as a cover to protect the inner optical elements or as a substrate to produce filters. See Figs. 2.34 and 2.35. • Dome: A transparent optical element with bent parallel (in most cases) polished surfaces that is used as a cover to protect the inner optical elements. A dome with no surfaces that are not parallel behaves like a lens. See Fig. 2.36. • Filter: An optical element that selectively transmits light in a specific wavelength range. The filter is usually implemented as a plane material. The material can be specially colored for a needed wavelength or made of
Figure 2.34 Windows of different materials for different regions (coated or uncoated). Images reprinted courtesy of Edmund Optics, Inc., all rights reserved. (See color plate section.)
Figure 2.35 Windows of different materials for different regions and/or applications (coated or uncoated). Images reprinted courtesy of II-VI Infrared, all rights reserved.
Figure 2.36 TECHSPEC® glass domes made of N-BK7 glass. Image courtesy of Edmund Optics, Inc., all rights reserved.
Figure 2.37 Optical filters. Image reprinted courtesy of Ocean Optics, Inc., all rights reserved.
transparent clear material that is coated to get the needed filter characteristics (selective transmittance or blocking ranges). See Fig. 2.37. • Beamsplitter: An optical element or device that splits the incoming beam of light in two. See Figs. 2.38–2.40. The split is typically performed with one of three implements:
Figure 2.38 A cube acting as a beamsplitter.
Figure 2.39 A coated window acting as a beamsplitter.
Figure 2.40 Examples of beamsplitters for different regions and/or applications. Image reprinted courtesy of Edmund Optics, all rights reserved.
Figure 2.41 Different kinds of mirrors. Image reprinted courtesy of Electro Optical Components, Inc., all rights reserved.
• A cube comprising two triangular cemented prisms, • A half-aluminized mirror on a two-surface, polished optical material, or • A dichroic optical coating. In all cases, the optical reflected or transmitted surfaces are coated to increase the transmittance or improve the other optical characteristics. • Mirror: An optical element that reflects light. The reflection is typically made by a metal coating (e.g., aluminum, gold, or silver) on flat or
Figure 2.42 reserved.
Different kind of mirrors. Image reprinted courtesy of Edmund Optics, all rights
curved surfaces. For special applications, the reflection can be made by a metal (e.g., aluminum) polished surface. See Figs. 2.41 and 2.42.
References OPTICS 1, Inc., “History of Optics,” http://www.optics1.com/optics history. php (2013). Olympus Microscopy Resource Center, “Common Optical Defects in Lens Systems (Aberrations),” http://www.olympusmicro.com/primer/ lightandcolor/opticalaberrations.html.
Raw Materials for Producing Optical Elements
Metal is not usually considered an optical material, but metals are used for optical elements (e.g., in mirrors), which is why this chapter’s title is more specific than “Optical Materials.”
3.1 What Is an Optical Material? An optical material is the raw material used to fabricate optical elements that are assembled into optical systems. Those elements are lenses, windows, mirrors, filters, and domes. Optical elements transmit or reflect the electromagnetic radiation in one or more spectral regions: IR, VIS, or UV, as designed and specified in the relevant drawing and/or specification by the optics designer. Optical materials include different kind of glasses, crystals, and plastics. Because some metals (aluminum, for example) are also used to fabricate mirrors, they can be classified for this matter as an optical material.
3.2 Materials for Optical Elements The materials used for optical elements can be divided into five major groups: 1. Glass (VIS, IR, UV) • Optical glass • Color optical filter • Special glass for molding 2. Crystal (isotropic and anisotropic) 3. Plastic 4. Metal (for mirrors only) 33
5. Special materials • Chalcogenide material • Glass ceramics 3.2.1 Glass Glass is an amorphous (non crystalline) solid, typically brittle and optically transparent. The most familiar type of glass, used for centuries in windows and drinking vessels, is made by fusing soda with sand, lime, and other ingredients. However, the quality of common glass is insufficient for precision optics applications. The wide range of requirements for various and special needs has led to the development of many types of optical glasses. 220.127.116.11 Optical glass
Optical glass is a special glass that is developed for use in optical devices such as telescopes, microscopes, binoculars, laser equipment, optical guided missiles, eyeglasses, and more. This glass is made very precisely for known and needed characteristics, and it must be produced with care and tight control to ensure its purity and the right planned parameters. Adding impurities, materials, or compounds to the basic glass changes the colors and characteristics of the glass and enables designers to design and develop different kinds of optical systems according to customer requirements and needs. Nearly all commercial glasses fall into one of six basic categories or types based on chemical composition. Within each type, except for fused silica, there are numerous distinct compositions. • Fused silica glass (SiO2) has very low thermal expansion, is very hard, and resists high temperatures (1000–1500 °C). It is also the most resistant against weathering. It is used for high-temperature applications, such as furnace tubes, melting crucibles, etc. • Soda-lime-silica glass is transparent, easily formed, and most suitable for window glass. It has a high thermal expansion and poor resistance to heat (500–600 °C). This material is used for windows, containers, lightbulbs, and tableware. • Sodium borosilicate glass withstands heat expansion much better than window glass. It is used for chemical glassware, cooking glass, car headlamps, reagent bottles, optical components, and household cookware. • Lead-oxide glass has a high refractive index, which makes the glassware look more brilliant (crystal glass). It also has a high elasticity, which makes the glassware “ring.” It is more workable in the factory but cannot stand heating very well. This type of glass is favored for electrical applications because of its excellent electrical insulating properties.
Raw Materials for Producing Optical Elements
Thermometer tubing and art glass are also made from lead-alkali glass, commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature. • Aluminosilicate glass is similar to borosilicate glass, but it has greater chemical durability and can withstand higher operating temperatures. Extensively used for fiberglass, it is included in glass-reinforced plastics (boats, fishing rods, etc.). Another application is halogen bulb glass. • Oxide glass is extremely clear and used for fiber optic wave guides in communication networks. Light loses only 5% of its intensity through 1 km of glass fiber. Optical glasses are classified by their main chemical components and are identified by refractive index nd and Abbe number Vd. They are divided into groups, and each type of glass is designated by abbreviated group symbol and a number. This designation of optical glass varies from manufacturer to manufacturer; a sample is provided in Table 3.1. The identification of the glass by refractive index and Abbe number is known as the International Glass Code, a six-digit number based on the MIL-G-174 (U.S. Military Specification). For example, BK7 (Schott) has nd ¼ 1.5168 and Vd ¼ 64.17, which has a glass code of 517642 [d is the spectral line of the yellow helium (He) line, i.e., a 587.5618-nm wavelength]. Table 3.2 provides some example glasses and their codes. Note that the glass properties can vary slightly between glass types of different manufacturers.
Table 3.1 Group/designation collation of Hoya and Schott optical materials. Table reprinted courtesy of Hoya Corp., all rights reserved. Group Fluor Crown Dense Fluor Crown Phosphate Crown Special Phosphate Crown Dense Phosphate Crown Boro Silicate Crown Light Barium Crown Crown Zinc Crown Barium Crown Dense Barium Crown Extra Dense Barium Crown Light Lanthanum Crown Lanthanum Crown Tantalum Crown Crown Flint Antimony Flint Light Barium Flint
FC FCD PC PCS PCD BSC BaCL C ZnC BaC BaCD BACED LaCL LaC TaC CF SbF BaFL
FK FK PK PK PSK BK BaLK K ZK BaK SK SSK LaK LaK LaK KF KzF BaLF
Extra Light Flint Barium Flint Light Flint Flint Dense Barium Flint Dense Flint Special Dense Flint Fluor Flint Light Lanthanum Flint Lanthanum Flint Niobium Flint Tantalum Flint Dense Niobium Flint Dense Tantalum Flint Abnormal Dispersion Crown Abnormal Dispersion Flint Athermal Crown Athermal Flint
FEL BaF FL F BaFD FD FDS FF LaFL LaF NbF TaF NbFD TaFD ADC ADF ATC ATF
LLF BaF LF F BaSF SF SFS TiF LaF LaF LaF LaF, LaSF LaF, LaSF LaSF KzFS
Chapter 3 Table 3.2
Glass types and the corresponding manufacturer codes. Manufacturer Code
Glass Type Borosilicate crown Barium crown Dense crown Lanthanum flint Dense flint
1.5168 1.5688 1.6204 1.7439 1.7847
64.17 56.05 60.32 44.85 25.76
517642 569561 620603 744448 785258
Schott N N N N N
BK7 BaK4 SK16 LaF22 SF11
BSC517642 MBC569561 DBC620603 LAF744447 DEDF785258
BSC7 BaC4 BaCD16 LaF2 FD11
Ohara S S S S S
BSL7 BAL14 BSM16 LAM12 TIH11
18.104.22.168 Color optical filter
Color optical filters (also called absorption filters) are devices that selectively transmit light of different wavelengths (i.e., colors) while blocking the remainder via internal absorption of the raw material; see Fig. 3.1. For example, Schott’s RG are red and black glasses that transmit IR, and Hoya’s B-370 transmits light in the blue spectrum. A combination of a glass and coating (single or multilayer interference filter) or a lens and filter device (filter lens) can also match filtering requirements; see Chapter 6. 22.214.171.124 Special glasses for molding
Glasses specifically developed for precision molding are called low-Tg glasses (see Figs. 3.2 and 3.3). The Tg is expressed as the temperature above which a glass changes from a solid state to a plastic state. For the molding process, a polished form is shaped into final or almost-final geometry, including preservation of its surface quality. The typical temperature range for the molding process is between 500 °C and 700 °C.
Figure 3.1 Optical plane glass filters. Image reprinted courtesy of Ocean Optics, Inc., all rights reserved.
Raw Materials for Producing Optical Elements
Figure 3.2 Molded lenses for VIS applications. Image reprinted courtesy of Fisba Optik AG, all rights reserved.
Figure 3.3 Molded IR lenses made of chalcogenide glasses. Image reprinted courtesy of Fisba Optik AG, all rights reserved.
• Some of the Schott materials suitable for molding process are N-FK5, PSK57, N-LAF33, SF57, P-SF67, and others. A full list of these materials can be found in Schott’s catalog “Optical Materials for Precision Molding.” • Some of the Ohara materials suitable for molding process are L-BAL42, L-LAH84, L-LAM60, L-TIM28, and others. A full list of these materials can be found in Ohara’s catalog “Low-Softening-Temperature Optical Glass.” • Some of the Hoya materials suitable for molding process are MC-BACD12, MC-FD32, MC-PCD51-70, MP-FDS1, and others. A full list of these materials can be found in Hoya’s catalog “Preforms for Precision Molding.”
3.2.2 Crystal Crystal is a piece of a homogeneous solid substance that has a naturally geometrically regular form with symmetrically arranged plane faces due to the regular internal structure of its atoms, ions, or molecules. A monocrystalline material is a single-crystal material in which the crystal structure has a unique arrangement of atoms, continuous and unbroken. Single crystals have no grain boundaries. A polycrystalline material is composed of many crystallites (small microscopic crystals) of different sizes and orientation with grain boundaries between individual crystals. The variation in direction can be random (called random texture) or directed, possibly due to growth and processing conditions. Two such optical polycrystalline materials are zinc sulfide and zinc selenide. • Zinc sulfide (ZnS), or zinc sulphide, is an inorganic compound used mostly as an infrared optical material, transmitting from the visible wavelength to 12 mm. It can be used planar as an optical window or shaped into a lens. It is produced as microcrystalline sheets by the synthesis of hydrogen sulfide gas and zinc vapor. This material is sold as forward-looking infrared (FLIR) grade, where the zinc sulfide takes an opaque, milky yellow form. When it is hot isostatically pressed (HIPed), it can be converted to a water-clear form known as CleartranTM. • Zinc selenide (ZnSe) is a light-yellow, binary solid compound. It is used as an IR optical material with a remarkably wide transmission wavelength range (0.45–21.5 mm). The refractive index is ~2.67 at 550 nm (green) and ~2.40 at 10.6 mm (long-wavelength infrared, or LWIR). An optical crystal is any natural or synthetic crystal that is used in visible, infrared, and ultraviolet optics to produce optical elements. These crystals can appear in either a monocrystalline or polycrystalline state. Such crystals are sapphire (Al2O3, for VIS and IR) and calcium fluoride (CaF2, for VIS and IR); zinc compounds for IR include ZnSe and ZnS; and semiconductors for IR include germanium (Ge) and silicon (Si). (For large elements, it is difficult to produce large monocrystalline materials.) If the spacing and arrangement of the atoms in a crystal are the same in each of the three planes (x, y, and z), i.e., the properties of the crystal are identical in all directions, then the crystal is isotropic. Plain table salt (sodium chloride), which has a lattice structure called “cubic,” is an example of an isotropic crystal. When the properties of a material vary with different crystallographic orientations, meaning that the material has two or more distinct directions with different optical properties, then the material is said to be anisotropic [a single crystal of magnesium fluoride (MgF2) appears in an anisotropic state], as shown in Fig. 3.4.
Raw Materials for Producing Optical Elements
Figure 3.4 Calcite double refraction: a line viewed through a cubic calcite crystal (optically anisotropic) is split into two separate rays and creates two separated images. Photo reprinted courtesy of Roger Weller/Cochise College, all rights reserved.
3.2.3 Plastic A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are moldable. Plastics are typically organic polymers with a high molecular mass, but they often contain other substances. Plastics may be used for optical applications such as windows and lenses (Fig. 3.5). Plastic optics has cost and weight benefits when compared to ground- and polished-glass optics, but it also has limitations due to functional requirements, especially for a broad thermal working range (lack of stability with temperature).
Plastic lenses. Photo reprinted courtesy of Edmund Optics, all rights reserved.
Plastic lens materials include the following: • CR-39 is a plastic polymer that is transparent in the visible spectrum and almost completely opaque in the ultraviolet range. It has high abrasion resistance—in fact, the highest abrasion/scratch resistance of any uncoated optical plastic. CR-39 has about half the weight of glass with an index of refraction only slightly lower than that of crown glass, and its high Abbe number yields low chromatic aberration (CR-39 is a product of PPG Industries, introduced in the early 1940s). • Polycarbonate is a thermoplastic polymer (plastic) known by trademarks names such as Lexan (a registered trademark of the General Electric Company), Makrolon or Makroclear (developed and produced by Bayer MaterialScience), and others. Polycarbonate is easily worked, molded, and thermoformed, and may therefore be found in some applications of optical elements. • Zeonex® E48R (by Zeon Chemicals LP) is an organic transparent polymer with excellent light transmission from ~300 nm to ~1200 nm and excellent resistance to common solvents. 3.2.4 Metals (for mirrors only) Metal substrates are used widely for mirrors in optics. Some of the substrates used for this application include beryllium, aluminum 6061-T6, and aluminum-beryllium. The mirror surface of the metal substrate can be fabricated directly, replicated, or coated. Metals have important advantages as mirror substrates. Existing metal mirrors exploit the bulk properties of the material for mechanical and thermal purposes, and some mounting details and useful optical features that are very difficult to achieve with glass can be more readily produced in metal. Additional metals that are used for these applications are stainless steel, molybdenum, and aluminum-silicon carbide. 3.2.5 Special materials Special optical materials are those materials for optical elements with special characteristics and for special uses. Two such examples include chalcogenide glass and glass ceramics. A chalcogenide glass (pronounced with a hard “k,” as in “chemistry”) is a glass that contains one or more chalcogenide elements, which make up Group 16 in the periodic table, i.e., sulfur (S), selenium (Se), and tellurium (Te). Such glasses are covalently bonded materials and may be classified as network solids; the entire glass matrix behaves like an infinitely bonded molecule. Lenses made of chalcogenide glass are used in the IR region. Their main advantage is a low thermal expansion value, which allows them to be incorporated in a thermal optical system (see Sections 3.4.4 and 3.4.5).
Raw Materials for Producing Optical Elements
Some examples of chalcogenide glasses include IG 2–6 (produced by Vitron), Gasir (produced by Umicore), and As2S3 (arsenic trisulfide). Glass ceramics are polycrystalline materials tailored by controlling the composition and the heat treatment/crystallization of base glass. Glass ceramics have the fabrication advantage of glass as well as special properties of ceramics. Schott’s Zerodur® product, for example, is a glass ceramic material used for mirror substrates for segmented and large monolithic astronomical telescopes and for ultra-lightweight mirror blanks.
3.3 Classification of Optical Materials 3.3.1 According to molecular structure Amorphous materials • Glass is an optical (not crystalline) material, i.e., without any molecular order. The molecules move coincidently in the glass medium. Examples include BK7, SF11, and K5 by Schott; S-BSL7, S-BAL14, S-BSM16 by Ohara; FD11, LaF2, BaCD16 by Hoya; and BSC517642, MBC569561, and DBC620603 by Pilkington. (Liquids, gases, and common glasses are also amorphous materials.) • Plastic is a typically organic polymer of high molecular mass. The material can be either flexible or hard. Examples include CR-39 and Zeonex® 350R (hard), and polycarbonate (flexible). Crystalline materials Crystalline optical materials are materials whose molecules are stable and in order, e.g., sapphire (Al2O3), calcium fluoride (CaF2), germanium (Ge), and silicon (Si). 3.3.2 According to atomic orientation Optical isotropy means the optical properties, especially the refraction index, are the same in all directions. The velocity of light in isotropic materials is the same in all directions. The chemical bonds holding the material together are the same in all directions, such that light passing through the material sees the same electronic environment in all directions regardless of the direction the light takes through the material. Glass (e.g., Schott’s BK7) is an isotropic material. Optical anisotropy means having different optical properties especially regarding the refraction index such as birefringence. Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. Birefringence is also a synonym for double refraction, i.e., the decomposition of a ray of light into two rays (called ordinary and extraordinary rays) when it passes through a
Figure 3.6 A calcite crystal displays double refraction (or birefringence) while sitting on a paper with crossed lines. Photo reprinted courtesy of J. M. Derochette, all rights reserved.
birefringent material. Crystals, such as sapphire (Al2S3), calcite (CaCO3), and quartz (SiO2), are birefringent. Figure 3.6 demonstrates a piece of calcite lying on a paper with crossed lines. When viewing through the calcite, the one crossed line appears to be two crossed lines. If an isotropic mineral is deformed or strained, then the chemical bonds holding the mineral together will be affected: some will be stretched, and others will be compressed. The result is that the mineral may appear to be anisotropic. 3.3.3 According to the working spectral range • Materials for the VIS range (0.39–0.75 mm). Examples include fused quartz (silica, or SiO2), BK7, K5, and SF11. • Materials for the IR range (0.75–1000 mm). Examples include calcium fluoride (CaF2), germanium (Ge), and silicon (Si). The infrared range is usually arranged according to one of the following three divisions: Five subdivisions • Near infrared (NIR): 0.75–1.4 mm/750–1400 nm, • Short-wavelength infrared (SWIR): 1.4–3.0 mm/1400–3000 nm, • Mid-wavelength infrared (MWIR): 3–7 mm/3000–8000 nm, • Long-wavelength infrared (LWIR): 7–15 mm/8000–15000 nm, and • Far infrared (FIR): 15–1000 mm/15000 nm to 1 m. Three CIE (International Commission on Illumination) subdivisions • IR-A: 0.7–1.4 mm/700–1400 nm, • IR-B: 1.4–3.0 mm/1400–3000 nm, and • IR-C: 3–1000 mm/3000 nm to 1 mm.
Raw Materials for Producing Optical Elements
Electromagnetic spectrum ranges of optical interest.
Three ISO 20473 subdivisions • Near infrared (NIR): 0.78–3.00 mm/780–3000 nm, • Mid infrared (MIR): 3–50 mm/3000–50000 nm, and • Far infrared (FIR): 50–1000 mm/50000 nm to 1 m. • Materials for the ultraviolet (UV) range (0.1–0.4 mm/100–400 nm). Examples include calcium fluoride (CaF2), lithium fluoride (LiF), and magnesium fluoride (MgF2). The divisions defined here (and charted in Fig. 3.7) are not precise for every use. What is more important is that different optical materials are suitable for different wavelengths; for example, calcium fluoride (CaF2) has a high broadband transmission from the deep UV to IR (130 nm up to 8 mm). 3.3.4 According to colors • Transparent materials (to human eyes), such as BK7, SF11, and K5 by Schott, and calcium fluoride (CaF2). • Semitransparent materials (to human eyes), such as zinc sulfide (ZnS), zinc selenide (ZnSe), and arsenic trisulphide (As2S3). • Opaque materials (to human eyes), such as germanium (Ge), silicon (Si), IG-6 by Vitron, and Gasir by Umicore.
3.3.5 According to the refraction index (for glasses) • Crown glass, which usually has a refractive index greater than 1.52 and lower than 1.6. • Flint glass, which usually has a refractive index greater than 1.6 and lower than 1.75. The classifications presented throughout Section 3.3 are mostly important to the optical designers, who choose the proper material for their design needs, but it is always beneficial to know the differences of the materials’ characteristics while inspecting, testing, or handling them or their finished products (lenses, prisms, or windows).
3.4 Main Characteristics of Optical Materials 3.4.1 Optical properties The refractive index n is when light enters a non-absorbing homogeneous materials reflection and refraction occurs at the boundary surface. It is given by the ratio of the velocity of light in vacuum c to that of the medium v: c n¼ : v
The refractive index data given in glass catalogues are measured relative to the refractive index of air, which is very close to 1. The refractive index is a function of the wavelength. The most common characteristic of an optical glass is the refractive index n in the middle range of the visible spectrum. This principal refractive index is usually denoted as nd, the refractive index at the wavelength 587.56 nm (0.58756 mm). Regarding materials in the IR spectral range (amorphous or crystalline), the refractive index may vary from 1.37 (for MgF2) to 4.02 (for Ge). Dispersion is a measure of the change of the refractive index with wavelength. The difference (nF – nC) is called the principal dispersion; nF and nC are the refractive indices at the 486.13-nm and 656.27-nm wavelengths, respectively. The most common characterization of the dispersion of optical glasses is the Abbe number, which is defined as V d ¼ ðnd
Sometimes the Abbe number is defined according to the e line as V e ¼ ðne
nC 0 Þ:
Optical glasses in the range of Vd > 50 are traditionally called crown glasses, otherwise they are considered as flint glasses. Many glasses with a low refractive index also have a low dispersion behavior, e.g., a high Abbe
Raw Materials for Producing Optical Elements
number. Glasses with a high refractive index have mainly a high dispersion behavior and a low Abbe number. The refractive index homogeneity is used to designate deviations of the refractive index within individual pieces of glass. Special efforts in melting and fine annealing make it possible to produce pieces of glass with high homogeneity. Homogeneity is achievable for a given glass type depending on the volume and the form of the individual glass pieces. The internal (or bulk) transmittance (Ti) refers to the transmittance obtained by excluding scattering and reflection losses from the entrance and exit surfaces of the glass. 3.4.2 Internal (bulk) quality Internal properties, when referring to different kind of imperfections (defects) that are mentioned in this section, might influence the quality of the optical element or system at the highest level and should be taken into account when determining the requirements of the item. Glasses are well defined by quality standards, as noted here. However, these standards also apply to optical crystals and plastics. Striae are internal imperfections of glass that appear as wavy distortions, according to MIL-STD-1241A. The striae quality grades are defined by the U.S. military standard MIL-G-174B, the ANSI/OEOSC OP3.001-2001 standard, and the ISO 10110-4. Standard MIL-G-174B notes four grades of striae, as shown in Fig. 3.8. Striae are spatially short-range variations of the homogeneity—thread-like veins or cords that are visual indications of abruptly varying densities in a glass. The ANSI/OEOSC OP3.001-2001 standard defines striae grades the same way as MIL-G-174B, whereas ISO 10110-4 defines striae in five classes with numerical values of optical path differences in nanometers (see each standard for more details).
Striae grades according to MIL-G-174B.
According to MIL-STD-1241A, a bubble is a gaseous inclusion in glass. An inclusion is the presence, within the body of the glass, of an extraneous or foreign material. Stress birefringence means the grade of birefringence (double refraction) caused by the residual stress within the glass. Excessively large stress birefringence may affect optical performance or cause breakage. There are two main reasons for mechanical stress inside a glass part. Mechanical stress can be generated due to the annealing process and/or variations of the chemical composition within a melt. 3.4.3 Chemical properties The chemical properties of glass determine its resistance to attack by water, moisture, acids, and alkalis. Different optical glass manufacturers use different approaches. Glass containing larger amounts of substances such as silicon dioxide (SiO2), aluminum oxide (Al2O3), titanium oxide (TiO2), or oxides of the rare earths is more resistant to being leached by aqueous and acidic solutions. They are also usually more resistant to local corrosion. If the glass contains large quantities of more readily soluble substances such as alkalis, then reactions of varying degrees can be expected to depend on the operating conditions. These reactions are sufficient for layer formation or removal of the glass surface. Schott uses five test methods to assess the chemical behavior of polished glass surfaces. Table 3.3 shows an example of the specific chemical properties classification for N-BK7 and N-FK51A. The meanings of the properties and numbers presented in Table 3.3 are as follows: • Climatic resistance (CR) is the resistance to moisture in the air, expressed in classes 1 (high) to 4 (low). • Stain resistance (FR) is the resistance to stain formation, expressed in classes 0 (high) to 5 (low). • Acid resistance (SR) is the resistance to acid solutions, expressed in classes 1 (high) to 4 (low) and 51 to 53 (very low). • Alkali resistance (AR) is the resistance to alkaline solutions, expressed in classes 1 (high) to 4 (low).
Designations for the chemical properties of two Schott glasses. Chemical Properties
Material N BK7 N FK51A
Raw Materials for Producing Optical Elements
Table 3.4 Designations for chemical properties of two Ohara glasses, where (p) indicates a powder group, and (s) indicates a surface group. Chemical Properties Material
1 2 2
S BSL7 S FFPM2
• Phosphate resistance (PR) is the resistance to alkaline-phosphatecontaining solutions, expressed in classes 1 (high) to 4 (low). Ohara uses five test methods in order to assess the chemical behavior of crushed glass or polished glass surfaces. Table 3.4 shows an example of the specific chemical properties classification for S-BSL7 and S-FFPM2. The meanings of the properties and numbers presented in Table 3.4 are as follows: • Water resistance (RW) is the resistance of crushed glass (powder, p) to distilled water, expressed in classes 1 (low loss of weight) to 6 (high loss of weight). • Acid resistance (RA) is the resistance of crushed glass (powder, p) to nitric acid expressed, in classes 1 (low loss of weight) to 6 (high loss of weight). • Weathering resistance (W) is the resistance of freshly polished glass plates (surface, s) to humidity at þ 50 °C, expressed in classes 1 (good resistance, no fading) to 4 (bad resistance, notable fading). • Acid resistance (SR) is the resistance of freshly polished glass plates (surface, s) to a nitric acid solution (pH 0.3) or to an acetic acid buffer solution (pH 4.6) that results in a loss of mass, expressed in classes 1 (low loss) to 5 (high loss) for pH 0.3 and 5 (low loss) through 51, 52 and 53 (high loss) for pH 4.6. • Phosphate resistance (PR) is the resistance of freshly polished glass plates (surface, s) to an aqueous solution that results in a loss of mass, expressed in classes 1 (low loss) to 4 (high loss). Hoya uses six different test methods in order to assess the surface deterioration caused by chemical reactions of the glass constituents Table 3.5
Designations for chemical properties of two Hoya glasses. Chemical Properties
LAC7 S FD60