Method for measuring plane mirror absolute surface shape based on conjugate difference method

The invention discloses a method for measuring plane mirror absolute surface shape based on a conjugate difference method. The method comprises the following steps: a plane mirror to be measured and a standard plane mirror forming interference images on a CCD of an interferometer respectively, judging the sizes of light spots of the plane mirror to be measured and the standard plane mirror formed on the CCD of the interferometer and enabling the light spot of the standard plane mirror to be larger than the light spot of the plane mirror to be measured; determining translation amount delta of the plane mirror to be measured corresponding to one pixel in the CCD of the interferometer, delta>0; defining the direction of optical axis of the interferometer as the z axis and carrying four-step test in the positive and negative directions of the x axis and y axis respectively and recording wavefront data phi (x+delta, y), phi (x-delta, y), phi (x, y+delta) and phi (x, y-delta) respectively; obtaining surface shape gradient of the plane mirror to be measured according to the measured wavefront data; and recovering the surface shape w of the plane mirror to be measured according to the surface shape gradient of the plane mirror to be measured by utilizing a wave surface recovering method. The method improves precision of differential approximation and prevents coupling between step-by-step tests, thereby improving stability of the test; and the method can also be used for measuring the absolute surface shape of a cylindrical mirror, a conical mirror and a spherical mirror.