Recent developments in the measurement of aspheric surfaces by contact stylus instrumentation

Contact stylus instruments with sufficient range to resolution that could measure and analyze aspheric surfaces first appeared in the 1980s. These where limited to the measurement of a single profile over a rotationally symmetric aspheric surface. Although limited in use they proved to be very useful for the characterization of both optical and non-optical aspheric surfaces, correction of tool paths for aspheric generators etc. The work described here reviews recent developments in the measurement and characterization of aspheric surfaces by contact stylus instrumentation and includes measurement over an area rather than a single profile and measurement and the characterization of non-rotationally symmetric aspheric surfaces. Some of the challenges involved in the areal measurement of aspheric surfaces by contact stylus instrumentation will be described together with the techniques and considerations used to overcome these challenges. Specifically we will describe the mathematical models used to describe the aspheric surfaces and how these can be used to eliminate measurement set-up errors. How the finite size of the stylus can be corrected using techniques developed for image analysis. The automatic detection and removal of asperities using wavelet technology will be described. Finally the benefits and limitations of data fusion techniques to improve the range of the instrument will be reviewed.