Profile measurement of large aspheric optical surface by scanning deflectometry with rotatable mirror - Method for enlarging measuring range of autocollimator

Interferometers are widely used for measuring large reflecting flat surfaces and spherical surfaces because of its high accuracy and high efficiency. Because interferometry needs standard reference surface for measurement, when measuring a large aspheric surface it needs null lens or hologram to make an aspheric wave front. However, if the perfect profile of the aspheric surface is not known before test, interferometry cannot be used to measure. When measuring a large flat, deflectometry is also proposed and proved to have high precision of sub-nanometers. However, it cannot be used because of the limit of the measuring range of the autocollimator. We proposed a new scanning deflectometry method using an autocollimator and a rotatable mirror fixed on a scanning stage. The rotatable mirror and a fixed mirror are supposed to have the same function as a pentagon prism which can eliminate the pitching angle error of the scanning stage as it moves. We scan slope of the aspheric mirror by autocollimator, and when the angle becomes close to exceeding the measureable range of the autocollimator, the rotatable mirror is turned a certain angle to fit the slope of the aspheric mirror and continue scanning. The error analysis of the method is done, and the result show that if the condition is well controlled the uncertainty of this method is 10nm. Pre-experiments is done, which proved that the rotatable scanning mechanism is able to eliminate pitching error of the scanning stage.