Patent ID: 8437083

Claim:
An optical element comprising: a substrate; and an antireflection film provided on a surface of the substrate, wherein the antireflection film is a graded layer having a refractive index that is progressively decreased from the substrate side towards an outer surface of the antireflection film, wherein the graded layer has a reflectivity characteristic occupying ⅔ of a usable wavelength range around the center of the usable wavelength range, wherein reflectivity of the graded layer at a maximum value of the reflectivity characteristic is a peak value equal to or less than 0.4%; wherein the reflectivity characteristic of the optical element has only one peak value within the usable wavelength range, wherein at least one of reflectivities at both ends of the usable wavelength range is equal to or less than half the reflectivity at the peak value, wherein an optical thickness d of the graded layer satisfies a condition in the following expression, ( 2 ⁢ m + 1 ) - 0.25 4 ⁢ λ < d < ( 2 ⁢ m + 1 ) + 0.30 4 ⁢ λ , where λ is a usable wavelength located within the ⅔ of the usable wavelength range, and m is a natural number, and wherein a refractive index gradient of the graded layer satisfies a condition in the following expression,  n β - n β + 1 n β + n β + 1  <  n α - n α + 1 n α + n α + 1  , where n α is a refractive index at a position at which the optical thickness d of the graded layer is d/4 when viewed from the substrate side, n α+1 is a refractive index at a position next to the position of n α , n β is a refractive index at a position at which the optical thickness of the graded layer is 3d/4 when viewed from the substrate side, and n β+1 is a refractive index at a position next to the position of n β .