2-D software datum named as the 2-D mixed method has been constructed to measure mirror surface profiles. The probe of this method consists of two displacement and two angle sensors. Differential calculation of outputs of these four sensors eliminates pitch, roll and the z-directional translation errors of the scanning stages. An automomous procedure to estimate and compensate the zero-adjustment error among these four sensors is also proposed. By using this software datum, 2-D profiles of ultra-precision surfaces can be measured absolutely without using any external reference.
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