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.