Comparison and Uncertainty Evaluation of Methods for Asphere Metrology

The goal of this project is a rigorous uncertainty assessment of generic methods for measuring aspheres. The evaluated methods include zonal and annular stitching, local curvature measurement, and computer generated holograms (CGHs). Anticipated results include standards-compliant uncertainty statements for typical measurement set-ups that act as templates which industry can easily adopt. Such statements reduce controversy in commerce over measurement results and specification compliance.

Aspheric surfaces are indispensable in modern optical systems through their combination of high optical performance, low system weight, and low cost. However, measuring aspheric surfaces poses formidable metrology problems because of the difficulty of obtaining a reference wavefront that closely matches the desired form of the asphere. No single, widely-recognized, general, validated way exists for measuring aspheric surfaces with nanometer-level uncertainties. This is a measurement barrier to the development, manufacture, and subsequent application of aspheric elements.

Several methods have been proposed and applied to measure aspheres. The application range and achievable accuracy of these methods are poorly understood. The goal of this project is a rigorous standards-compliant uncertainty assessment of generic methods for measuring aspheres, with a focus on zonal and annular stitching, local curvature measurement, and computer generated holograms (CGHs). The uncertainty assessment will be validated through comparison of the methods using measurement equipment at NIST and through collaboration with partners outside of NIST. Standards-compliant uncertainty assessments are rare in the optics industry, but increasingly required for ISO-certified quality systems and export. In this project we develop and verify 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 and that can form the basis for documentary standards.