OPTICAL ELEMENT

摘要

An optical element (16) introduces a first wavefront deviation when a first radiation beam having a first wavelength passes through it and a second wavefront deviation when a second radiation beam having a second wavelength different from the first wavelength passes through it. One surface of the optical element comprises a phase structure in the form of annular areas (52, 53, 54, 55), the areas forming a non-periodic pattern of optical paths of different length. The optical paths for the first wavelength form the first wavefront deviation and the optical paths for the second wavelength form the second wavefront deviation, the difference between the two deviations being proportional to the difference between the first and second wavelength.