Patent ID: 8098432

Claim:
An optical thin film formed of only two layers, namely: an outermost layer which is stacked above an optical surface of an optical substrate, and which has a lower refractive index than that of the optical substrate; and a certain refractive index layer which is disposed between the optical surface of the optical substrate and the outermost layer, and which has a refractive index adjustable in a predetermined range higher than that of the outermost layer, wherein the refractive index of the certain refractive index layer at 550 nm is from 1.45 to 1.9, the refractive index of the outermost layer at 550 nm is from 1.20 to 1.30, the outermost layer is formed by a wet film-formation method using: low-refractive-index fine particles made of a low-refractive-index material having a lower refractive index than that of the optical substrate; and a first binder which binds the low-refractive-index fine particles together, and the certain refractive index layer is formed by a wet film-formation method using: high-refractive-index fine particles made of a high-refractive-index material having a higher refractive index than that of the low-refractive-index material; and a second binder which has a refractive index different from that of the high-refractive-index material, and which binds the high-refractive-index fine particles together, and wherein the reflectance of substantially an entire surface of the optical surface is 2% or lower over an entire wavelength range of 400 nm to 1100 nm.