Certification, self-calibration and uncertainty in optical surface testing

Abstract

Conventional approaches to achieving traceability in measurements of optical surfaces are based on a series of intercomparisons ultimately linking the measurement result to a realisation of the unit (metre) at a national measurement institution. Measurement services offered by national measurement institutes (NMIs) cover a limited size and shape range and the uncertainties associated with the transfer of a calibration must be considered when data from any testing service is used in quality assurance. Local realisation of the unit, typically using CIPM recognised wavelengths and their associated uncertainties, combined with in–situ self–calibration of geometry and the development of a rigorous uncertainty analysis meet the conditions to achieve traceability according to the requirements of ISO 17025. This paper reviews the underlying basis for achieving traceability without a direct chain of comparisons to NMIs and gives some examples using a range of optical measurements.