Certification, self-calibration and uncertainty in testing optical flats

Many different approaches may be taken in the certification of reference flats used for acceptance testing of optical quality surfaces. Measurement services offered by national measurement institutes cover a limited size 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. In-situ self-calibration using a full area variant of the 3-flat test enables the lowest possible uncertainty. The first part of this paper shows the options for external calibration and certification as a function of flat size, and orientation. Next the conditions that must be met to achieve traceability, according to the requirements of ISO 17025, will be discussed. Finally hardware and procedures will be described, and data presented, showing traceable measurement of a 450 mm aperture flat with nm level uncertainties.