Patent ID: 9302450
Filing Date: 2016-04-05
CPC Classification: B32B,G02B,Y10T

Claim Text:
1. An optical element comprising a substrate that is transparent to a d-line, and an antireflection film formed on the substrate, wherein the antireflection film includes a plurality of thin film layers that include an uppermost layer that is most distant from the substrate and has a refractive index nd of 1.20 to 1.30 to the d-line, a high refractive index layer having a refractive index of 2.00 to 2.40 to the d-line, and at least one of an intermediate refractive index layer having a refractive index of 1.55 to 1.70 to the d-line and a low refractive index layer having a refractive index of 1.40 to 1.52 to the d-line, and wherein an undercoat layer includes the substrate to the thin film layer adjacent to the uppermost layer, and the following expression is satisfied, where Y(θ, λ) is an optical admittance calculated from a refractive index to the d-line and a film thickness of the undercoat layer, θ is an incident angle in a range of 0° to 60°, and λ is a wavelength in a range of 420 nm and 680 nm: