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Spie Press Book

Optical Glass
Author(s): Peter Hartmann
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Book Description

For more than 400 years, optical glass has provided mankind with a window into both the hidden microcosm and vast outer cosmos of the known universe, transforming philosophy, science, and engineering through its visage and, thus, shaping modern civilization. Its high transmittance, homogeneity, and precisely defined light refraction properties are the preconditions for highly resolved true-color imaging, making it an intrinsic component of technology in general. From consumer products, such as cameras and binoculars, to microscopes and telescopes—the most essential tools of research in many fields—the role of optical glass is integral to the very foundations of modern science and industry.

In contrast to its fundamental importance, there is often a lack of knowledge regarding the properties of optical glass by engineers and designers, causing misunderstandings in purchasing and fabrication, and ultimately limiting the potential and application of this dynamic material. This book will serve as an invaluable resource of technical information, including the index of refraction and its dependence on wavelength (dispersion), optical homogeneity and transmittance (presented together with restrictions imposed by the manufacturing processes and chemical resistance), as well as mechanical, thermal, and environmental properties. Measurement methods with their achievable accuracy are given, along with a wide scope of overview diagrams illustrating properties and main uses, as well as diagrams ranking optical glass types with respect to their properties. The wide scope and lucid organization of this volume will prove to be highly valuable across a wide range of design, engineering, and purchasing applications within the many fields dependent on this incredible material.

Book Details

Date Published: 11 August 2014
Pages: 180
ISBN: 9781628412925
Volume: PM249

Table of Contents
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Table of Contents

List of Symbols

1 Optical Glass: Significance and Definitions
1.1 Key Enabling Material
1.2 The History of Optical Glass
1.3 General Glass Properties
1.4 Optical Glass: Definition
1.5 Optical Glass Types
1.6 Optical Glass Types: Denominations
1.7 Optical Glass Types: Glass Codes
1.8 Optical Glass Types: Portfolio
1.9 Optical Glass Types: Availability
1.10 Selected Types of Optical Glass

2 Production of Optical Glass
2.1 Melting
2.2 Annealing
2.3 Reheat Pressing
2.4 Precision Molding

3 Refractive Index and Dispersion
3.1 Snell's Law of Refraction
3.2 Dispersion: Abbe Number
3.3 Characteristic Light Wavelengths
3.4 Partial Dispersion
3.5 Dispersion Formulas
3.6 Refractive Index Tolerance
3.7 Refractive Index Variation Tolerances
3.8 Abbe Number Tolerances
3.9 Annealing Influence on Refractive Index and Dispersion
3.10 Annealing Influence on Different Glass Types
3.11 Annealing Schedule
3.12 Refractive Index Measurement: V-Block
3.13 Standard Test Certificate: V-Block
3.14 Test Certificate for a Delivery Lot
3.15 Refractive Index Measurement: Prism Goniometer
3.16 Sellmeier Data Fit Quality
3.17 Test Certificate: Prism Goniometer
3.18 Refractive Index: Temperature Influence
3.19 Temperature Coefficient Measurement Reproducibility and Melt Variations
3.20 Temperature Coefficients of the Refractive Index
3.21 Thermo-optical Coefficient

4 Homogeneity
4.1 Optical Homogeneity versus Striae
4.2 Optical Homogeneity: Tolerances
4.3 Wavefront Measurement
4.4 Optical Homogeneity: Measurement of Glass Items
4.5 Optical Homogeneity in Glass Items
4.6 Optical Homogeneity of Glass Types
4.7 Striae Appearance
4.8 Striae Measurement: The Shadowgraph Method
4.9 Striae Specification
4.10 Stress Birefringence: Refractive Index Homogeneity in Polarized Light
4.11 Stress-Optical Coefficient
4.12 Stress Birefringence: Limit Values for Typical Applications
4.13 Stress Birefringence: Size Effect
4.14 Transient Stress Birefringence
4.15 Birefringence Measurement
4.16 Bubbles and Other Inclusions
4.17 Bubbles and Inclusions: Inspection
4.18 Bubbles and Inclusions: Specification

5 Transmittance
5.1 Internal Transmittance
5.2 Measurement of Internal Transmittance
5.3 Overall Internal Transmittance of Optical Glass
5.4 Internal Transmittance: Color Code
5.5 Internal Transmittance Tolerances and High Transmittance Quality Grade
5.6 Fluorescence
5.7 Solarization
5.8 Ionizing-Irradiation-Induced Transmittance Loss: Radiation Damage
5.9 Use of Optical Glass in the UV and IR Wavelength Ranges

6 Chemical Resistance
6.1 General Remarks on Chemical Resistance of Optical Glasses
6.2 Chemical Resistance: Measurement and Classification
6.3 Chemical Resistance of the Optical Glass Types: Overview Diagrams

7 Mechanical Properties
7.1 Density
7.2 Elasticity: Young's Modulus
7.3 Knoop Hardness
7.4 Grindability
7.5 Bending Strength
7.6 Mechanical Properties of Selected Optical Glass Types

8 Thermal Properties
8.1 Viscosity
8.2 Thermal Expansion
8.3 Transformation Temperature
8.4 Thermal Conductivity and Heat Capacity
8.5 Thermally Induced Stress
8.6 Thermal Properties of Selected Glass Types

9 Environmental Properties

10 Specification of Optical Elements: Recommendations for Optical Glass Properties and Optical Element Manufacturing
10.1 Small, Thin Lenses
10.2 Medium-Sized Lenses and Prisms
10.3 Large Lenses and Prisms

11 Other Optical Materials
11.1 General Requirements on Materials for Optical Elements
11.2 Other Materials Used for Optical Elements



Very few books on optical glass are available in the literature. Especially lacking is a compilation of the basic properties of optical glass for readers who are not specialists in glass but need some information for their work with optical glass elements. For example, ISO has plans to write a new part (Part 18) of the international standard ISO 10110 (Optics and Photonics— Preparation of Drawings for Optical Elements and Systems) that will replace Parts 2, 3, and 4 on material imperfections. Providing only the formal specification tolerances for material properties of optical elements leaves the user with the problem of how to translate these requirements into the optical raw glass delivery forms needed for the production of optical elements. Such a translation needs to include some information about optical glass, its properties, and its production. A number of specific issues are encountered when dealing with optical glass that, if ignored, can easily lead to problems in daily practice. These problems result in, for example, unexpected availability difficulties, long delivery times, and high prices for special tolerances, sizes, and shapes.

In the past, such problems have originated from the fact that instructions in the way of optical element drawings, alone, have been simply copied to glass purchase orders. The example of the zero-bubble requirement for a small lens transferred to an entire large strip of 800-mm length may seem odd, but this actually happened. The question, "Will you throw away an entire 800-mm strip just because there is a single bubble inside?" solved the problem instantly. But such an incident illustrates the need for information on optical glass and its properties that also relates to glass item sizes. This book is meant to reduce communication problems between the glass supplier and the customers.

Many years of responding to customer's questions on optical glass has led to the SCHOTT Technical Information Exchange (TIE) series, which treats special aspects in a set of articles. However, it is not possible to simply assemble these exchanges and publish them as a book. A considerable amount of additional information is needed to provide a general overview, which this book provides.

Another observation is that optical glass is deemed to be a commodity product in the view of many people, even leaders in the optics industry. Therefore, a short chapter points out the key enabling character of this material for science and technology as a whole. Optical glass manufacturing needs outstanding capabilities; it is not a technology for which one can simply purchase a standard production plant and begin delivery.

Optical glass is a material with a high leverage effect. As a rule, the value of optical systems is 100 times higher than that of the glass used in the systems. Thus, there is little room for cost-cutting strategies. On the other hand, each dollar of missing optical glass will lead to the loss of $100 in turnover. Delivery of a glassless microscope is a 99% deal for the system supplier and a 100% loss for his customer. Optical glass purchasing strategies should take this fact into account.

Optical glass is a high-tech product. Its main property, refractive index, is specified and monitored to the fifth decimal place, whereas technical glasses need only one to three digits in their refractive indices. Everyone who has ever improved production tolerances or has related measurement methods knows that each digit or each factor of ten of higher accuracy means not only another digit to be read from the display but also a factor of ten of more precise equipment and stabilization, a more highly mastered environment, and more highly skilled personnel. Optical glass is two orders of magnitude away from the highest-quality technical glass; thus, its production and quality assurance is in another category altogether.

Chemistry plays the decisive role in melting. This is especially the case for the most sought-after extreme glass types, which present particular challenges during the melting and casting process. If these challenges are not overcome, the material can end up as polycrystalline pieces that are useless for optics. A variety of melting facilities must be maintained in high standards in order to produce the many different optical glass types. Even though optical glasses end up looking very much the same as technical glasses, their production requires different setups with different resistances of the facilities against high temperature and highly chemically aggressive melts. This involves developing high-precision chemistry under adverse conditions.

But even these mastered processes can define refractive index only to the third decimal place. Therefore, it is not only the melting processes that must be mastered, but also the subsequent heat treatment, the so-called fine annealing. During this annealing process, the refractive index is adjusted to the required fifth decimal place.

Optical glass without the data given in a test report is useless. For reliable provision of actual test results from glass items, sophisticated sample logistics are necessary to obtain melt-specific glass data. These data must be determined with precise measurement methods. For fine control of annealing processes, carefully determined typical glass data are needed.

Optical homogeneity is another critical requirement on optical glass items. High homogeneity becomes an increasing challenge with larger sizes of glass pieces. This holds not only for production but also for measurement. Generally, size increase exacerbates issues pertaining to many optical glass properties such as stress birefringence, striae, and transmittance. Detrimental effects not only increase linearly with size but also grow stronger for most properties. This will be pointed out accordingly.

For a set of other properties, overview diagrams similar to the one at the end of this Preface are provided that rank glass types with respect to these properties. The last chapter of the book gives practical advice for transferring material requirements on optical elements to the raw glass forms to be purchased from the glass supplier.

This book concentrates on optical glass and its principal purpose of imaging with visible light. Other materials with core applications outside of the visible light range, such as fused silica and calcium fluoride, or infrared light materials such as chalcogenides, zinc sulfide, zinc selenide, sapphire, and germanium, are outside the scope of this book. These materials are very different from optical glasses in many aspects. Of these materials, only fused silica and chalcogenides are also classified as glasses. However, these two materials consist of only a few components (3 or fewer) in contrast to optical glasses, which consist of 6–12 components. The other materials are crystals or microcrystalline materials. In any case, the production processes and properties of the mentioned materials are very different from those of optical glasses.

Peter Hartmann
August 2014

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