High resolution flatness measurements on diffuse and scattering surfaces

The use of interferometry for very high accuracy area mapping of the flatness of optical surfaces is a well-known, high-performance tool. However, it is difficult to make a high-performance measurement over a full area of a diffuse or scattering surface such as a machined metal or metal powder used in manufacturing. This challenge is generally addressed with scanning methods using laser lines, confocal or other point scanners of mechanical means such as coordinate measuring machines. Data collection with these method can be very slow, due in part from the flexibility to measure a large change in contour that is not critical to most flatness applications. Previously, we reported on a moire method for diffuse or powder surfaces that is not unlike the Ronchi tests using in process of ground glass. This paper discussed some of the challenges of making this type of measurement and suggests a new hybrid method that trades of the capability for providing profiles over relatively large measurement ranges for obtaining very high sensitivity to errors from the ideal flat surface