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Spie Press Book

Metrics for High-Quality Specular Surfaces
Author(s): Lionel R. Baker
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Book Description

This book supplies the optical component and systems designer, and quality assurance engineers and managers with the definitions, measurement principles, and standard metrics used to characterize high-quality specular surfaces. The author covers both the traditional visual methods as well as newer (but not necessarily better) computer-aided techniques and describes the metrics adopted by the new ISO standards, including the setting of form and finish tolerances. Key issues of industry are raised, to help stimulate research and development of new methods and standards that blend the best of the old and new approaches to surface assessment.

Book Details

Date Published: 3 November 2004
Pages: 166
ISBN: 9780819455765
Volume: TT65

Table of Contents
SHOW Table of Contents | HIDE Table of Contents
List of abbreviations
List of symbols
1.1 Introduction
1.2 Why measure surfaces?
1.2.1 System function
1.2.2 Appearance
1.2.3 Manufacturing efficiency
1.2.4 Benefits
1.3 Definition of surface metrics
1.3.1 Surface metrics influencing quality
1.3.2 Causes of defects in surface topography
1.4 Chapter 1 conclusions
2.1 Introduction
2.2 Optical height probes
2.3 Optical slope probe
2.4 Interferometers
2.4.1 Twyman-Green interferometer
2.4.2 Laser Fizeau interferometer
2.4.3 Four-step measurement of phase
2.4.4 Inter-laboratory comparison of workshop interferometers
2.4.5 Interferometer error sources and calibration
2.5 Surface form tolerances
2.5.1 Sagitta error
2.5.2 Irregularity
2.5.3 RMS Irregularity, RMSi
2.5.4 Rotationally symmetric irregularity
2.5.5 RMS asymmetry, RMSa
2.5.6 Total RMS deviation, RMSt
2.5.7 Wavefront form indication on drawings
2.6 Chapter 2 conclusions
3.1 Introduction
3.2 Typical optical component roughness values
3.3 Deterministic methods
3.3.1 Stylus method
3.3.2 Profilometry parameters
3.3.3 Microinterferometer
3.4 Parametric methods
3.4.1 Surface spread functions
3.4.2 Total integrated scatter measurement
3.5 Surface roughness indications in drawings
3.6 Chapter 3 conclusions
4.1 Introduction
4.2 Fourier analysis of height profile
4.3 Spatial frequency zones
4.4 Computation of texture
4.5 Chapter 4 conclusions
5.1 Introduction
5.2 Imperfections and subconscious thoughts
5.3 Effect of surface imperfections
5.4 Impact of imperfections on market access
5.5 Description of imperfections
5.5.1 Terminology
5.5.2 Size of imperfections
5.5.3 Substrates/materials
5.5.4 Location
5.5.5 Characteristics
5.6 Influence of imperfections on quality
5.6.1 Cosmetic influence
5.6.2 Functional influence
5.7 Causes of imperfections
5.8 Reduction of damage
5.9 Imperfection measurement
5.9.1 Why measure imperfections?
5.9.2 Characterization/measurement of imperfections
5.10 Comparison of measurement methods
5.11 Imperfection size versus visibility
5.11.1 Surface step as an imperfection
5.11.2 Step measurement by interferometry
5.12 The eye as a sensor
5.12.1 Benefits
5.12.2 Disbenefits
5.13 Disbenefits of inspection
5.14 National standards for scratch assessment
5.14.1 USA
5.14.2 Germany
5.14.3 France
5.14.4 UK
5.15 Level of agreement achieved using national standards
5.16 Scratch reference standards
5.17 Target specification for imperfection measurement
5.18 Need for standards
5.19 ISO TC 172 optics and optical instruments
5.20 Comparison of two methods proposed by ISO in 1997
5.20.1 Area method l
5.20.2 Visibility method ll
5.20.3 Comparison of methods I and ll
5.21 Chapter 5 conclusions
6.1 Introduction
6.2 Optical component inspection
6.3 Radiometric obscuration by imperfections
6.4 Calibration graticules
6.5 LEW and SED measurement requirements
6.6 LEW and SED simple viewing system
6.7 Analogue microscope image comparator (AMIC)
6.7.1 Description
6.7.2 Theory
6.7.3 Method of operation
6.8 Digital microscope image comparator (DMIC)
6.8.1 Description
6.8.2 Results and discussion
6.9 Chapter 6 conclusions.
7.1 Introduction
7.2 Survey of tolerances
7.2.1 British Standard BS4301 (1991)
7.2.2 American Standard MIL-0-13830:1963
7.2.3 German Standard DIN 3140:Part 7, 1978
7.2.4 French Standard
7.3 Acceptable thresholds for scratches and roughness
7.4 Inspection and measurement flow diagram
7.5 Chapter 7 conclusions
8.1 Introduction
8.2 Comparison between subjective and objective measurements of imperfections
8.3 Relative contrast of standard scratches
8.4 Measurement of imperfections and contamination in assemblies
8.5 Measurement of imperfections in coatings
8.6 Use of MIC to measure surface texture
8.7 Use of MIC to examine phase objects
8.8 Use of MIC in AC mode
8.10 Use of MIC on-machine
8.11 Chapter 8 conclusions
9.1 Introduction
9.2 Laser beam scanners
9.3 Camera inspection systems
9.4 Chapter 9 conclusions
Appendix 1: Quality metrics for digital cameras
Appendix 2: Surface cleaning
Contacts and Further Reading


This book provides a basic working knowledge of the definition, measurement, and standardization of a number of different metrics used to characterize high-quality specular surfaces. It should be of interest to optical component and systems designers, quality assurance engineers, and designers of quality assurance instrumentation, as well as those with a need to set acceptance thresholds for surface form and finish in accordance with ISO standards. The quality of finish of specular or mirrorlike surfaces is also of more general interest to engineers from the electronics and precision mechanical industries. These include the automotive, defense, pharmaceutical, and biotechnology sectors, as well as those concerned with the new technologies of integrated optics and microfabrication. Products with a specular finish also requiring surface quality control include, for example, glossy computer printing paper, plastics laminates, and rolled strip. The quality of nonspecular surfaces that may be ground or painted and diffuse light are mostly excluded from this study.

The design and working of traditional, as well as some new, techniques and instrumentation for the inspection of specular surfaces and for the measurement of quality metrics applied to high- quality surfaces are described. It is hoped that an understanding of the provisions and methods of operation of recently available international standards, including the setting of form and finish tolerances, will provide an insight into the changing needs of those concerned with the design and manufacture of specular surfaces with characteristics defined either by function or appearance.

A parametric approach to the characterization of the total topography of a surface leads on to a comparison of methods for the measurement of form and finish and to a better understanding, based on recent research, of the calibration and practical use of instruments for measuring the basic metrics of form, texture, and imperfections. New metrics for the objective measurement of imperfections, adopted in a recently published ISO standard, are defined, and designs of analogue and digital comparison microscopy systems for their measurement are described. Their embodiment in optical component quality control procedures is also covered. It is anticipated that some of the new tools described here may have application in the characterization of the surface appearance of a wide range of products with a specular finish.

The potential for use of relatively low cost comparison microscopes for measuring other parameters such as contamination, microtopography, and surface texture is explored and supported by practical sections involving the measurement of image luminosity by digital camera. Due to the nanometric sensitivity to surface height variations of these methods, the term "far- field nanoscopy (FFN)," as opposed to near-field nanoscopy (NFN), has been used as a generic title involving the use of an instrument called a nanoscope. A study of methods for measuring the spatial image quality of digital cameras includes, in an appendix, a proposal for a new spatial image quality metric based on the measurement of optimum print width. A simple method for assessing the contrast resolution of digital cameras is also described. A review of surface cleaning techniques needed for the realization of these procedures is included in a second appendix. The book ends with a chapter on the latest automated laser beam scanning techniques used for inspecting very wide specular surfaces found typically in strip product manufacture.

The content of this book is based on collaborative research and discussions, extending over several years, with workers from industries around the world and with colleagues serving on committees of the British Standards Institution (BSI) and the International Organization for Standardization (ISO). Much of the original research on the measurement of imperfections, by analogue comparison microscopy, reported here, was undertaken while I was working at Sira Ltd., although new digital methods, also described, have been developed since. I should like to thank all my colleagues for their support.

The principal objective in writing this book is to stimulate and motivate others to carry forward research on metrics for characterizing the function and appearance of specular surfaces that has occupied much of my own time and thoughts in recent years. Much previous work, excellent publications, and a variety of instruments already exist in the field of surface metrology. The niche I have started to address here, on behalf of the industrial user, occurs between traditional visual methods, still applied in most surface quality assessments, and that employing modern, slow, computer-aided, off-machine, and costly but precise instrumentation requiring laboratory accreditation. The driver has been the repeated request from industry for fast, low-cost, non- contacting methods with traceability in support of subjective assessments.

It is hoped that describing the methods here will encourage their further validation by industry and lead on to the drafting of future surface measurement standards. I have tried to apply a physical and practical approach related to the needs of industry by minimizing the mathematical complexity of the subject. To enable an extended study, references are provided together with a glossary defining new and frequently used technical terms.

Orpington, Kent, UK

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