Measurement Methods and Services for Optical Reference Surfaces

Interferometry is the leading method for measuring ultra-precision surfaces and optical elements. The accuracy of the measurement results is to a large extent determined by the errors in the shape of the reference surfaces used. In this project we develop measurement methods and services for flatness, sphericity, and radius of curvature of these reference surfaces. We contribute to the development of relevant international standards and are leading an international comparison of flatness measurement capabilities.

Phase-shifting interferometry with computer-aided data analysis is the leading method for measurement of ultra-precision surfaces and optical elements. Interferometric form measurements of flat, spheric, and aspheric surfaces with uncertainties at the nm-level have become the state of the art. The measurement uncertainty is to a large extent determined by the form errors of the reference surfaces used. Optical flats and spheres are used for this purpose, and are critical components in the traceability chain for measurements of optical elements.

In this project we develop measurement methods and services for flatness, sphericity, and radius of curvature of the reference surfaces. Where possible, we seek to eliminate the requirement of calibrated transfer artifacts through the development of absolute calibration methods. Absolute calibration methods enable the separation of instrument errors from errors of the part by exploiting the invariance of some systematic errors during reversals and shifts. This approach is well suited to phase-measuring interferometry due to the high repeatability of the measurement process. Absolute calibration techniques reduce cost while addressing the challenge of ever increasing stability requirements for reference artifacts.

Standards-compliant uncertainty assessments are increasingly required for ISO-certified quality systems and export. In this project we develop uncertainty statements, compliant with the Guide to the Expression of Uncertainty in Measurement (GUM), for typical measurement set-ups that act as templates which industry can easily adopt. The project contributes to the ANSI/OEOSC Optics and Electro-Optics Standards Council Technical Advisory Group to ensure that standards developed by the ISO/TC172 Technical Committee on Optics and Photonics reflect the state-of-the-art.

NIST has initiated an international comparison of flatness measurement capability of 300 mm diameter optical flats. Several National Measurement Institutes (NMIs) provide calibration services for flat reference surfaces. The approaches used to calibrate the master flats vary. To ensure the consistency of measurement results required for international commerce, NIST initiated an international comparison. NIST will act as the pilot laboratory, and the comparison will be executed in a star pattern. The NMIs planning to participate are: CSIRO (Australia), NPL (UK), NMIJ (Japan), KRISS (Republic of Korea), and PTB (Germany).

• Conducted a preliminary international comparison of flatness measurement capability of 300 mm diameter optical flats in a horizontal orientation. Designed the comparison protocol, reference artifact, fixturing, and instrumented shipping container. The comparison was conducted with two international organizations and served to evaluate the test artifact and comparison protocol.