Patent ID: 7583443

Claim:
Reflective optical element comprising: a reflective surface, and a dielectric multilayer system formed on said reflective surface, said dielectric multilayer system comprising at least four successive pairs of layers, each of said pairs of layers consisting of a high refractive index layer alternating with a low refractive index layer, said dielectric multilayer system being reflective for a wavelength λ in the ultraviolet wavelength range, wherein said optical thickness of said low refractive index layers increases at least on average with increasing distance from said reflective surface, and wherein the total optical thickness of said high refractive index layers of the three pairs of layers having the closest distance to said reflective surface and the total optical thickness of said low refractive index layers of the three pairs of layers having the largest distance from said reflective surface each is higher than 3 times λ/4, and wherein the total optical thickness of said low refractive index layers of said three pairs having the closest distance to the reflective surface and the total optical thickness of said high refractive index layers of said three pairs of layers having the largest distance from said reflective surface each is lower than 3 times λ/4.