Patent ID: 7564763

Claim:
An optical arrangement for interacting with a radiation beam, the optical arrangement comprising an optical system and a compensator, the compensator including a first optical element, the first optical element having a phase structure comprising stepped annular areas forming a non-periodic pattern of optical paths of different lengths, the compensator being arranged to generate: a first wavefront deviation introduced by the variation of a first parameter during interaction of the radiation beam with the compensator, the first wavefront deviation being arranged to counteract a wavefront deviation introduced by the variation of the first parameter during interaction of the radiation beam with the optical system; and a second wavefront deviation introduced by the variation of a second, different, parameter during interaction of the radiation beam with the compensator, wherein the compensator further includes a second optical element formed from a different material than the first optical element and having a phase structure comprising stepped annular areas forming a non-periodic pattern of optical paths of different lengths, the second optical element being arranged to reduce said second wavefront deviation, wherein the annular areas of the first optical element are stepped by a step height of h j and the annular areas of the second optical element are stepped by a step height of b j and wherein the first optical element is arranged such that, for each said annular area, the step height h j is substantially equal to: h j = m j ⁢ λ n 1 - 1 where m j is an integer, λ the wavelength and n 1 is a refractive index of a first material from which the first optical element is made, and wherein the second optical element is arranged such that, for each said annular area, the step height b j is substantially equal to: b j = q j ⁢ λ n 2 - 1 where q j is an integer, λ the wavelength and n 2 a refractive index of a second material of which the second optical element is made.