A method is presented for ultra-precision two-degree-of-reedom measurement(TDFM)based on image processing of interferential fringes in real time

which are generated by three concentric-circle gratings and captured by CCD(Charge coupled Device)camera. The basic measuring principles are described in this paper including the generation of Moiré fringes of concentric circle gratings

three-grating-system

imaging optics

and the method of image processing. The actual measuring system is analyzed and calibrated through experiments and the measurement results are compared with laser interferometer system. Test shows that this system can be adopted to on-line measurement and compensation of ultra-precision process. It has a long measuring range of 200mm and can measure two dimensional error components at the same time with an accuracy of better than 0.1μm.