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 mirror-like 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 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 would like to thank all of my colleagues for their support and my wife, Dorothy, for her limitless understanding and patience.
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 techniques 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, noncontacting 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.