Patent ID: 6947209

Claim:
An optical element comprising: a quartz substrate; and an antireflection film comprising: four 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≦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 each of the four high-refractive index layers being in a range of about 65 to about 75 nm, wherein the four low-refractive-index layers and the four high-refractive-index layers are alternately formed on the quartz substrate such that one of the high-refractive index layers is a most proximal layer of the antireflection film to the quartz substrate, wherein the antireflection film has a design center wavelength λ o 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 , and d 8 of the high-refractive-index and low-refractive-index layers at a first to eighth positions from a substrate side satisfy the following expressions: 0.05λ o ≦d 1 ≦0.20λ o , 1.00λ o ≦d 2 ≦1.50λ o , 0.05λ o ≦d 3 ≦0.15λ o , 0.35λ o ≦d 4 ≦0.45λ o , 0.15λ o ≦d 5 ≦0.25λ o , 0.27λ o ≦d 6 ≦0.37λ o , 0.16λ o ≦d 7 ≦0.26λ o , and 0.23λ o ≦d 8 ≦0.33λ o .