Novel method of measuring optical freeform surface based on laser focusing probe without calibrating focus error signal

Abstract

A new method was developed for measuring an optical freeform surface with a noncontact laser focusing probe. The measurement process does not require calibration of the focus error signal (FES). Without the calibration process when measuring the workpiece topography greatly increases the measurement efficiency compared to the traditional astigmatism method, which extends the measurement range while providing high accuracy and stability. The new method was used to design a low-cost, noncontact, high-accuracy, and autofocusing laser probe. A dynamic focusing scheme was designed for measurement control software that can eliminate environmental interference and other noise to achieve fast and accurate focusing. Experimental results for a 10 µm standard step height verified the accuracy of the developed measurement system with an average error of 0.0141 µm, root mean squared error of 0.0432 µm, and standard deviation of 0.0201 µm. The developed system can be applied to measuring the optical freeform surface of low-reflectivity materials.