Abstract: A reference space includes a plurality of reference graphics. An image space is obtained by imaging the reference space. Affine transformation series are determined from the reference and image spaces for transcribing graphic cores of imaged graphics on graphic cores of reference graphics. If an affine-transformed positional error falls outside a tolerance, the reference space is divided into sub-spaces. Affine transformations and space divisions are repeated for out-of-tolerance sub-spaces until their estimations of distortion finally fall inside the tolerance.

Abstract: An interference optical system (18) leads parallel beam to a pair of opposed test surface (M1, M2). It then leads, from the test surfaces via different optical paths (C1, C2), interference fringe images formed by radiation of the parallel beam to the test surfaces (M1, M2), respectively. The interference optical system (18) has a pair of opposite reference surfaces (S1, S2) formed thereon and defined with a highly accuracy parallelism and distance. A measurement head (27) is provided with these reference surfaces (S1, S2), which are interposed between and oppose to the test surfaces (M1, M2), respectively. Imaging devices (19a, 20a) take interference fringe images that are created through interference between a light reflected at each of the test surfaces (M1, M2) and a light reflected at the corresponding reference surface (S1, S2) opposing thereto.