Patent ID: 7724376

Claim:
A measurement method for measuring a wavefront aberration of a target optical system using an object plane mask that is arranged on an object plane side of the target optical system and includes a first object plane mask slit and a second object plane mask slit, and an image plane mask that is arranged on an image plane side of the target optical system and includes a first image plane mask slit and a second image plane mask slit, the first object plane mask slit having a width in a shorter direction, which is equal to or smaller than a diffraction limit of the target optical system, the second object plane mask slit having a width in a shorter direction, which is close to or greater than the width of the first object plane mask slit, the first image plane mask slit having a width in a shorter direction, which is equal to or smaller than the diffraction limit of the target optical system, the second image plane mask slit having a width in a shorter direction, which is greater than the diffraction limit of the target optical system, said measurement method comprising the steps of: measuring a first reference wavefront generated from the first object plane mask slit by interference between light from a slit having the same shape as that of the first object plane mask slit and light from a pinhole; measuring a second reference wavefront generated from the first image plane mask slit by interference between light from a slit having the same shape as that of the first image plane mask slit and light from a pinhole; measuring a wavefront of the target optical system including a reference wavefront error from an interference pattern formed by interference between light from the first image plane mask slit and light from the second image plane mask slit; and subtracting the measured first and second reference wavefronts from the target optical system including the measured reference wavefront error.