Patent ID: 7116473

Claim:
An optical element comprising: a fluorite substrate; and an antireflection film comprising: five low-refractive-index layers each of which has a refractive index ns satisfying the expression 1.50≦ns≦1.60 and an extinction coefficient ks satisfying the expression 0<ks≦1×10 −5 , for ultraviolet light having a wavelength of 193 nm; and four high-refractive-index layers each of which has a refractive index na satisfying the expression 1.68≦na≦1.74 and an extinction coefficient ka satisfying the expression 1×10 −5 ≦ka≦1×10 −3 , for ultraviolet light having a wavelength of 193 nm, a total geometrical thickness of the four high-refractive index layers being in a range of about 65 nm to about 75 nm, wherein the five low-refractive-index layers and the four high-refractive-index layers are alternately formed on the fluorite substrate such that one of the low-refractive-index layers is a most proximal layer of the antireflection film to the fluorite substrate, wherein the antireflection film has a design center wavelength λ 0 in a range of 180 nm to 220 nm, and wherein respective optical thicknesses d 1 , d 2 , d 3 , d 4 , d 5 , d 6 , d 7 , d 8 , and d 9 of the low-refractive-index and high-refractive-index layers at a first to ninth position from a substrate side satisfy the following expressions: 0.00λ 0 <d1≦0.15λ 0 , 0.05λ 0 ≦d2≦0.20λ 0 , 0.04λ 0 ≦d3≦1.50λ 0 , 0.05λ 0 ≦d4≦0.15λ 0 , 0.35λ 0 ≦d5≦0.45λ 0 , 0.15λ 0 ≦d6≦0.25λ 0 , 0.27λ 0 ≦d7≦0.37λ 0 , 0.16λ 0 ≦d8≦0.26λ 0 , and 0.23λ 0 ≦d9≦0.33λ 0 .