Patent ID: 7792366

Claim:
An decentration amount measuring method which sets a sample of an optical element on a mounting member which is rotatable around an axis, irradiates two-sided surfaces of the sample with a light from a light source through a measuring optical system including an indicator of a shape, leads a reflection light from the two-sided surfaces of the sample on an image forming surface, obtains a plurality of indicator images formed on an image forming surface by the reflection light, relating to the two-sided surfaces for each of at least three rotary positions on the mounting member, obtains a decentration amount of each of the two-sided surfaces on the basis of the at least three indicator image positions, and measures the decentration amount of the optical element on the basis of difference in decentration amount between the two-sided surfaces, comprising the steps: creating a matching reference image m(x, y) which forms a reference shape of one of the plurality of indicator images when the indicator image is detected for each of the rotary positions on the mounting member; performing matching processing between an image n(x, y) on the image forming surface and the matching reference image m (x, y); acquiring a first indicator image having a maximum matching strength as one of the plurality of indicator images of one of the two-sided surfaces of the sample; in case that a position of a second indicator image having a second largest matching strength is specified, acquiring the second indicator image as one of the plurality of indicator images of the other of the two-sided surfaces of the sample; and in case that it is difficult to specify the position of the second indicator image, after deleting the first indicator image from the acquired picture image, performing matching processing between an image n′(x, y) on the image forming surface after the deletion and the matching reference image m(x, y), and acquiring another first indicator image having a maximum matching strength at this time as one of the plurality of indicator images of the other of the two-sided surfaces of the sample.