Antireflection film and optical device
An object of the present invention is to provide an antireflection film and an optical device having excellent antireflection characteristics against a light beam having a wide wavelength range at a wide range of an incident angle, and also having practically sufficient durability. To achieve the above object, the present invention adopts an antireflection film having an optical two-layer structure composed of an intermediate layer provided on a base material and a low refractive index layer provided on a surface of the intermediate layer, wherein the low refractive index layer is a layer obtained by binding hollow silica by a binder, a refractive index n( 1 ) thereof is between 1.15 and 1.24 inclusive, and a refractive index n( 2 ) of the intermediate layer satisfies a relation in the expression (1) below assuming a refractive index of the base material n(sub). n (1)×√{square root over ( n (sub))}×0.930 ≦n (2)≦ n (1)×√{square root over ( n (sub))}×0.985 (1).
1. An antireflection film for a light beam having a wavelength of 400 nm to 800 nm inclusive, provided on a base material,
comprising an optical two-layer structure composed of an intermediate layer to be provided on the base material and a low refractive index layer provided on a surface of the intermediate layer, wherein a reflectance against a light beam having a wavelength of 400 nm to 800 nm inclusive at an incident angle of 0° is 0.5% or less, and a reflectance against a light beam having a wavelength of 400 nm to 680 nm inclusive at an incident angle of 45° or less is 0.7% or less;
wherein the low refractive index layer is a layer obtained by binding a hollow silica with a binder and the low refractive index layer has a refractive index n( 1 ) of between 1.15 and 1.24 inclusive;
wherein an optical film thickness of the low refractive index layer is between 100 nm to 180 nm inclusive;
wherein an optical film thickness of the intermediate layer is between 100 nm to 180 nm inclusive; and
a refractive index n( 2 ) of the intermediate layer satisfies a relation in an expression (1) below assuming a refractive index of the base material n(sub):
n (1)×√{square root over ( n (sub))}×0.930 ≦n (2)≦ n (1)×√{square root over ( n (sub))}×0.985 (1).
2. The antireflection film according to claim 1 , wherein occupied volume of the hollow silica in the low refractive index layer is between 30% by volume to 99% by volume inclusive.
3. The antireflection film according to claim 1 , wherein a void portion is provided in the low refractive index layer in addition to a hollow portion in the hollow silica.
4. The antireflection film according to claim 1 , wherein an average particle size of the hollow silica particles is between 5 nm to 100 nm inclusive, and outer sides of the hollow silica particles are coated with the binder.
5. The antireflection film according to any one of claims 1 to 4 , wherein the refractive index n( 1 ) of the low refractive index layer is between 1.17 and 1.23 inclusive.
6. The antireflection film according to claim 1 , wherein the refractive index n( 2 ) of the intermediate layer satisfies an expression (2) below:
n (1)×√{square root over ( n (sub))}×0.940 ≦n (2)≦ n (1)×√{square root over ( n (sub))}×0.970 (2).
7. The antireflection film according to claim 1 , wherein the intermediate layer is an equivalent stack having a multi-layer structure that satisfies the relation in the expression (1) at a reference wavelength.
8. The antireflection film according to claim 1 , wherein the low refractive index layer is a layer formed by using the hollow silica, and a resin material or metal alkoxide as a binder component.
9. The antireflection film according to claim 1 , wherein a functional layer having a refractive index n( 3 ) of 1.30 to 2.35 inclusive and a film thickness between 1 nm to 30 nm inclusive is provided on a surface of the low refractive index layer.
10. The antireflection film according to claim 1 , wherein the base material is a base material for an optical device.
11. An optical device provided with the antireflection film according to claim 1 .