Reflection reducing film, optical member and optical system   

Abstract: A reflection reducing film is provided on a surface of an optical substrate, and is sequentially provided with a buffer layer and a reflection reducing layer. The reflection reducing layer includes first to eighth layers sequentially laminated from the side of the buffer layer. The first and sixth layers are made of a low-refractive index material having a refractive index in the range of 1.35 to 1.50 at the d-line, the third, fifth and seventh layers are made of an intermediate-refractive index material having a refractive index in the range of 1.55 to 1.85 at the d-line, and the second, fourth and eighth layers are made of a high-refractive index material having a refractive index in the range of 1.70 to 2.50 at the d-line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending U.S. application Ser. No. 12/470,216 filed on May 21, 2009, which claims priority under 35 USC 119 from Japanese Patent Application No. 2008-134224 filed May 22, 2008 and Japanese Patent Application No. 2008-134225 filed May 22, 2008; the entire contents of which are incorporated herein by reference.

1. Technical Field

The present invention relates to a reflection reducing film which is formed on one face of an optical member such as a lens, a filter and the like to exhibit a reflection reducing effect with respect to light having a predetermined wavelength band, and an optical member and an optical system which have the reflection reducing film.

2. Related Art

Generally, an image pickup apparatus such as a photographic camera or a TV broadcasting camera has a number of optical members such as lenses, prisms, filters and the like disposed on an optical path thereof. When light is incident on surfaces of the respective optical members, a portion of the light becomes reflection light. Herein, when the number of the optical members increases, a total amount of the reflection light increases and thus defects, for example, flare and ghost occur in images of a broadcasting camera. In addition, since reflectances on the surfaces of the optical members have distribution with respect to a wavelength of the incident light and wavelength dependences of various reflectances are shown in accordance with constituent materials for the respective optical member, a chromaticity balance deteriorates, and thus it is necessary to adjust a white balance in an entire image pickup apparatus.

Accordingly, a surface of each optical member may be provided with a reflection reducing film (also referred to as the antireflection film). The reflection reducing film is a multilayer film in which dielectric films having different refractive indices are combined, and a configuration thereof is disclosed in, for example, the following Non-Patent Document 1. In the Non-Patent Document 1 (“Light•Thin Film Manual”, The Optronics Co., Ltd, Oct. 9, 1989, P 246-247), an antireflection film having a 5-layer structure is disclosed and an attempt has been made to achieve a low reflectance over a wider band.

In addition to the Non-Patent Document 1, Patent Document 1 (JP-A-2002-267801) discloses an antireflection film having a 9-layer structure.

For example, the antireflection film disclosed in the above Non-Patent Document 1 exhibits relatively excellent low reflectance characteristics when being provided on an optical substrate having a refractive index lower than 1.70 at the d-line. However, it is found that there is a tendency in which flatness in reflectance distribution is lost as described later and a reflectance is thus increased at a specific wavelength when the antireflection film is provided on an optical substrate having a high refractive index higher than 1.75.

Tables 1 and 2 show basic data (constituent materials, refractive indices and optical film thicknesses) of antireflection films (Related examples 1 and 2) having a laminate structure corresponding to the antireflection film disclosed in Non-Patent Document 1. And FIG. 45 shows reflectance distributions of the antireflection films having the basic data of Tables 1 and 2.

TABLE 1 OPTICAL FILM RELATED CONSTITUENT REFRACTIVE THICKNESS EXAMPLE 1 MATERIAL INDEX N INDEX Nxd OPTICAL BK-7 1.51 — SUBSTRATE 1st LAYER SUB-M1 1.67 0.25 λ 0 2nd LAYER ZrO2 2.05 0.50 λ 0 3rd LAYER Al2O3 1.62 0.50 λ 0 4th LAYER SUB-H4 2.05 0.50 λ 0 5th LAYER MgF2 1.39 0.25 λ 0 AIR — 1.00 — CENTER WAVELENGTH λ0 = 500 nm

TABLE 2 OPTICAL FILM RELATED CONSTITUENT REFRACTIVE THICKNESS EXAMPLE 2 MATERIAL INDEX N INDEX Nxd OPTICAL S-LAH58 1.88 — SUBSTRATE 1st LAYER SUB-M3 1.78 0.25 λ 0 2nd LAYER ZrO2 2.05 0.50 λ 0 3rd LAYER Al2O3 1.62 0.50 λ 0 4th LAYER SUB-H4 2.05 0.50 λ 0 5th LAYER MgF2 1.39 0.25 λ 0 AIR — 1.00 — CENTER WAVELENGTH λ0 = 500 nm

Related example 1 of Table 1 is a constitutional example when a refractive index of an optical substrate on which the antireflection film is formed is relatively low, and Related example 2 of Table 2 is a constitutional example when a refractive index of an optical substrate is relatively high. In Tables 1 and 2, “SUB-M1” represents a substance M1 (Merck & Co., Inc.) including PrAlO3 as a main component, “SUB-M3” represents a substance M3 (Merck & Co., Inc.) including lanthanum aluminate (La2xAl2YO3(X+Y)) as a main component, and “SUB-H4” represents a substance H4 (Merck & Co., Inc.) including LaTiO3 as a main component. In FIG. 45, a curved line 23A indicates the reflectance distribution of Related example 1 and a curved line 23B indicates the reflectance distribution of Related example 2. In Related example 2 in which the refractive index of the optical substrate is 1.8830 as above, the reflectance distribution is not flat and peaks are generated at wavelengths of about 420 nm, 510 nm and 650 nm.

Moreover, recently, demand for a day and night vision camera (day-night-camera) as a broadcasting camera increases. However, the antireflection film described in the above Patent Document 1 has insufficient antireflection performance in a near-infrared region. Thus, an optical system capable of excellently performing photographing in a near-infrared region at a wavelength of about 700 to 900 nm in addition to in a visible light region is strongly required.

Accordingly, a reflection reducing film, which has a sufficiently low reflectance for light at a wavelength of about 400 to 900 nm even when being provided on an optical substrate having a relatively high refractive index, is desired.

SUMMARY

The invention is contrived in view of the problem and a first object of the invention is to provide a reflection reducing film which has a sufficiently reduced reflectance in a wider wavelength band. A second object of the invention is to provide an optical member and an optical system which have the reflection reducing film.

A reflection reducing film of the invention includes: a reflection reducing layer which is provided on a substrate and includes first to eighth layers sequentially laminated from the opposite side to the substrate. The first and sixth layers are made of a low-refractive index material having a refractive index equal to or higher than 1.35 and equal to or lower than 1.50 at the d-line, the third, fifth and seventh layers are made of an intermediate-refractive index material having a refractive index equal to or higher than 1.55 and equal to or lower than 1.85 at the d-line, and the second, fourth and eighth layers are made of a high-refractive index material having a refractive index higher than that of the intermediate-refractive index material in the range of 1.70 to 2.50 at the d-line. An optical member of the invention has the reflection reducing film provided on a surface thereof, and an optical system of the invention is provided with the optical member.

In the reflection reducing film of the invention, the first to eighth layers of the reflection reducing layer provided on the substrate are made of materials having predetermined refractive indices and thus reflectance distribution is sufficiently reduced in a wider wavelength band.

In the reflection reducing film of the invention, all the following conditional expressions (1) to (8) are preferably satisfied. λ0 is a center wavelength, N1 to N8 are refractive indices of the first to eighth layers at the center wavelength λ0, and d1 to d8 are physical film thicknesses of the first to eighth layers.

0.23×λ0≦N1×d1≦0.25×λ0  (1)

0.11×λ0≦N2×d2≦0.13×λ0  (2)

0.03×λ0≦N3×d3≦0.05×λ0  (3)

0.25×λ0≦N4×d4≦0.29×λ0  (4)

0.22×λ0≦N5×d5≦0.24×λ0  (5)

0.22×λ0≦N6×d6≦0.26×λ0  (6)

0.21×λ0≦N7×d7≦0.24×λ0  (7)

0.45×λ0≦N8×d8≦0.53×λ0  (8)

In the reflection reducing film of the invention, a buffer layer, which is provided between the substrate and the reflection reducing layer to reduce reflection light generated between the substrate and the reflection reducing layer, may be further included. The buffer layer preferably has a single-layer structure made of the intermediate-refractive index material or a multilayer structure in which a layer coming into contact with the reflection reducing layer is made of the intermediate-refractive index material. The buffer layer moderates a sudden change in refractive index between the substrate and the reflection reducing layer to reduce the reflection on the surface of the substrate. When the buffer layer has the multilayer structure, it preferably has, for example, 3 to 5 layers. Specifically, a refractive index of the substrate at the d-line is preferably equal to or higher than 1.66 and equal to or lower than 2.2, the buffer layer preferably includes ninth to thirteenth layers sequentially laminated from the side of the reflection reducing layer, the ninth, eleventh and thirteenth layers are preferably made of the intermediate-refractive index material, and the tenth and twelfth layers are preferably made of the high-refractive index material. Or, a refractive index of the substrate at the d-line is preferably equal to or higher than 1.51 and equal to or lower than 1.72, the buffer layer preferably includes ninth to eleventh layers sequentially laminated from the side of the reflection reducing layer, the ninth and eleventh layers are preferably made of the intermediate-refractive, index material, and the tenth layer is preferably made of the high-refractive index material. Or, a refractive index of the substrate at the d-line is preferably equal to or higher than 1.40 and equal to or lower than 1.58, the buffer layer preferably includes ninth to twelfth layers sequentially laminated from the side of the reflection reducing layer, and the ninth and twelfth layers are preferably made of the intermediate-refractive index material, the tenth layer is preferably made of the high-refractive index material, and the eleventh layer is preferably made of the low-refractive index material, or the ninth layer is preferably made of the intermediate-refractive index material, the tenth and twelfth layers are preferably made of the high-refractive index material, and the eleventh layer is preferably made of the low-refractive index material.