This paper describes accurate surface profile measurement of an aspherical lens on an ultra-precision grinding machine using
the
error separation technique. At first, it is verified that the two-probe method, which has been previously developed for separation of the surface profile and the scanning error in
on-machine measurement of flat surface profiles, cannot be directly employed for aspherical surfaces because of the large measurement error associated with the alignment of the probes. A measurement model is then developed for solving this problem. In the model, the scanning error is accurately measured by the two-probe method, which employs two probes (Probes A and Probes B) to scan a flat surface. Another probe (Probe C) is employed to scan the aspherical surface. The accurate profile of the aspherical surface can then be obtained through extracting the scanning error from the output of Probe C. A technique in data processing of the two-probe method is also applied to the measurement of the scanning error. Experiments of scanning error measurement and aspherical profile measurement are conducted on a commercially available ultra-precision grinding machine.