Patent ID: 9684252
Date: 2017-06-20
CPC Classifications: G02B,G03F

Claim:
1. Optical element transparent for radiation with a wavelength λ in the ultraviolet wavelength range below 250 nm, comprising: a substrate with a refractive index n an antireflection coating formed on at least part of the surface of the substrate between the substrate and an ambient medium with a refractive index n wherein the antireflection coating comprises low refractive index layers alternating with medium refractive index layers, such that a first low refractive index layer is provided adjacent to the substrate and at least one further low refractive index layer alternates with at least one medium refractive index layer, and such that the first low refractive index layer lies closer to the substrate than do any of the medium refractive index layers, wherein all the medium refractive index layers have optical thicknesses that differ from each other, and all the low refractive index layers have optical thicknesses that differ from each other, wherein the optical thickness of the first low refractive index layer is less than 0.45λ, where λ is 193 nm, wherein the optical thickness of a second low refractive index layer is between 0.26λ and 0.34λ, where λ is 193 nm, and where the second low refractive index layer is arranged on the first low refractive index layer with a medium refractive index layer arranged between the first low refractive index layer and the second low refractive index layer, wherein the optical thicknesses of all the medium and low refractive index layers are such that reflection of the radiation is minimized, and wherein the substrate comprises a material selected from the group consisting of: barium fluoride (BaF