Patent ID: 8610876

Claim:
An optical element configured to be used at an operating wavelength, the optical element comprising: a substrate; and a reflective surface supported by the substrate, the reflective surface comprising a multilayer system comprising layers of at least first and second alternating materials having different real parts of the refractive index at the operating wavelength, wherein: the reflective surface comprises a first portion which comprises a plurality of stacks; each stack in the first portion comprises a layer of the first material and a nearest layer of the second material; each stack in the first portion has the same thickness; the reflective surface comprises a second portion which comprises a plurality of stacks; the second portion comprises first and second regions; each stack in the first region comprises a layer of the first material and a nearest layer of the second material; each stack in the first region has the same thickness; each stack in the second region comprises a layer of the first material and a nearest layer of the second material; each stack in the second region has the same thickness; the thickness of each stack in the first region is different from the thickness of each stack in the second region; the thickness of each stack in the first region is different from the thickness of each stack in the first portion; and an arrangement of the first and second portions across the reflective surface is adapted to an incident angle bandwidth distribution of radiation at the operating wavelength that can impinge on the reflective surface.