The metrology of specular surfaces demands a continuing dialogue between the dual processes of inspection and measurement.
The above plea by the author is made to stress the essential difference between the two processes of inspection and measurement. The following definitions are taken from the Chambers Dictionary.
Inspect: to look into
Measure: the ascertainment of extent by comparison with a standard
The need for a clear understanding of the meaning of terms, often loosely applied in an industrial situation, arises unavoidably when drafting standards. A measurement standard aims to improve communication between a supplier and a customer by codifying measurement parameters typifying current good practice. It follows that purely subjective assessments, although essential for inspection, should be supported ultimately by an objective measurement traceable to national standards. To help further in understanding the subject, a glossary defining a selection of important technical terms is provided.
It should be noted here that an international standard is only published after receiving substantial agreement by an international community of experts. All standards are subject to review after five years but can be revised at any time, should the need arise through the advance of technology or due to the discovery of errors.
Optical components, with which this book is mostly concerned, usually require a degree of surface quality unsurpassed by most manufactured products. A traditional precision mechanical engineering workshop presented with a design requiring a surface-shape accuracy of 10 nm and residual RMS roughness of 1 nm, even accepting the vagueness of this specification, would probably be less than keen to quote. Optical workshops, however, have a long tradition of working to this level of drawing tolerance. The precise control of the passage of light through an optical system requires the use of tolerances related to its wavelength.
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