Measuring surface slope error on precision aspheres

Optical designers are becoming increasingly aware of the importance of specifying and tolerancing slope errors on optical surfaces, especially aspheric surfaces. Slope errors can degrade system optical performance - in some cases even if the peak to valley surface figure errors meets the optical design tolerance analysis. With this awareness, more optical engineers are putting requirements for peak surface slope on optical element drawings. This puts pressure on optical fabricators to understand slope specifications and react to these requirements, and use the appropriate metrology instrumentation to ensure final system performance. This paper will discuss appropriate ways to specify slope errors, and the challenges and limitations of measuring slope errors with commercial interferometers. The optical designer should be aware of how slope errors are measured on Fizeau interferometers and should specify the spatial intervals of interest when tolerancing aspheric elements. Peak slope error measurement is prone to erroneous measurement errors due to surface contamination, environmental errors, and pupil focus. Finally, filtering has a strong influence on surface slope calculations. Practical examples of slope specifications and experimental results will be presented.