Nanometer profile measurement of large aspheric optical surface by scanning deflectometry with rotatable devices: Error analysis and experiments

Scanning deflectometry is used for measuring optical near flat surface with uncertainty of sub-nanometres [1]. However, for measuring aspheric surface with large departure from perfect spherical surface this method is difficult to use. The key problem for scanning deflectometry is that high accuracy autocollimators usually have limited measuring range which is less than 1000 arc-sec, so it cannot be used for measuring surface with large slope. We have proposed a new method for measuring large aspheric surface with large slope based on scanning deflectometry method. Rotatable devices are used to enlarge the measuring range of autocollimator. The uncertainty propagation analysis of our proposed method is done. The result shows that when measuring a large aspheric surface with a diameter over 300 mm and with a slope of 10 arc-deg the uncertainty is less than 30 nm. For the verification of our proposed method, experimental devices are set up. A spherical optical surface with diameter of 35 mm and curvature radius of 5000 mm is measured. Measuring range of autocollimator is successfully enlarged by our proposed method. Experimental result shows that the standard deviation of 10 repeated measurement is less than 30 nm which is larger than the analysis value. Random drift and systematic error is found in the experiment result. Temperature drift is considered to be the main reason for the systematic error.