1. Field of the Invention
The present invention relates to an optical element having an antireflective film, an optical system, and an optical apparatus.
2. Description of the Related Art
In general, in order to form a high-performance antireflective film on a substrate having a high refractive index, the outermost layer of the antireflective film needs to have a low refractive index. It is known that an inorganic material such as silicone resin or a magnesium fluoride or an organic material such as a silicone resin or an amorphous fluorine resin is used as a material of the layer having a low refractive index. It is also known that vacant spaces configured to further suppress the reflectance are formed in a silicone resin layer or in a magnesium fluoride layer. For example, a thin-film magnesium fluoride layer having a refractive index of 1.38 has a porocity of 30% (volume), so that the refractive index can be reduced down to 1.27. It is known that a sol-gel method is used as a method of forming vacant spaces to deposit magnesium fluoride nanoparticles and an antireflective film is formed by using a low refractive index material where the vacant spaces are formed between the nanoparticles (Japanese Patent Laid-Open No. (“JP”) 2010-15186). Another known method of forming vacant spaces includes aging a mixture of a solvent, an acidic catalyst, and a surfactant, hydrolyzing and poly-condensing alkoxy silane, coating the resultant material with a sol solution added with a basic catalyst, followed by drying, removing the solvent, and calcining (JP 2010-55060).
However, the antireflective films disclosed in JP 2010-15186 and JP 2010-55060 are applied to a substrate having a refractive index from 1.52 to 1.60, and the documents do not disclose or suggest an antireflective film which is appropriate for a high refractive index substrate having a refractive index of 1.80 or more.
The present invention provides an optical element, an optical system, and an optical apparatus having an excellent low reflectance characteristic and having a high antireflection performance for a high reflective index glass.
An optical element according to the present invention includes a substrate that is transparent to light in a wavelength range to be used, and an antireflective film laminated on the substrate. The antireflective film includes, in order from the substrate, a first layer, a second layer, and a third layer. The substrate has a refractive index of 1.80 to 2.05 for a d-line. The first layer is an inorganic oxide film having a refractive index of 1.43 to 1.47 for the d-line and a physical film thickness of 29.0 to 40.0 nm and containing silica as a main component. The second layer is an inorganic oxide film having a refractive index of 2.00 to 2.20 for the d-line and a physical film thickness of 12.0 to 41.0 nm. The third layer is a film having a refractive index of 1.23 to 1.26 for the d-line and a physical film thickness of 110.0 to 130.0 nm and containing silica nanoparticles.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
The substrate 11 is a glass substrate having a refractive index of 1.80 to 2.05 for the d-line (wavelength 587.6 nm). The shape of the substrate 11 is not particularly limited, and the shape may be planar, curved, concave, convex, or film-shaped. The substrate 11 having a high refractive index is appropriate for a high-accuracy digital camera. Note that the substrate 11 is not particularly limited to a glass, and the substrate 11 may be configured by using other materials such as a resin.
The first layer 12 is an inorganic oxide film having a refractive index of 1.43 to 1.47 for the d-line and a physical film thickness of 29.0 to 40.0 nm and containing silica as a main component and is made of, for example, SiO2. Thereby, an adhesive force between the first layer 12 and the silica as a main component of the glass substrate 11 increases and an adhesion strength of the film increases.
The second layer 13 is an inorganic oxide film having a refractive index of 2.00 to 2.20 for the d-line and a physical film thickness of 12.0 to 41.0 nm. The physical film thickness may range from 12.0 to 30.0 nm. The second layer 13 is an adjustment film for the first layer 12 and the third layer 102 in order to obtain a low reflectance characteristic of the antireflective film, and the above ranges of the refractive index and the physical film thickness may be provided. The second layer 13 may be made of a zirconium oxide, a titanium oxide, a tantalum oxide, a niobic oxide, a hafnium oxide, a lanthanum oxide, alumina, silica, or a mixture of at least two of these materials.
The first layer 12 and the second layer 13 are inorganic films made of an oxide. A dielectric antireflective film constructed with an inorganic film may be formed through a vacuum evaporation method. The formation method of the dielectric antireflective film is not particularly limited thereto, but a sputtering method may be used.
The third layer 102 is closest to air 115. The third layer 102 is a film having a refractive index of 1.23 to 1.26 for the d-line and a physical film thickness of 110.0 to 130.0 nm and containing silica nanoparticles. In
The cavity 16 may be a single cavity or multiple cavities and this structure may be properly selected. The shell 17 may be made of a material having a low refractive index. An inorganic material such as SiO2, MgF2 or an organic material such as fluorine, silicone may be used as the material of the shell 17. Since the particles of SiO2 can be easily manufactured, SiO2 may be used. The average particle diameter of the hollow nanoparticle 14 may be 20 nm or more and 70 nm or less, or 30 nm or more and 60 nm or less. If the average particle diameter of the hollow nanoparticle 14 is less than 20 nm, the size of the cavity 16 decreases and it is difficult to decrease the refractive index. If the average particle diameter is nm or more, the vacant space size among the particles increases and scattering occurs due to the size of the particles.
The binder of this embodiment may be made of an organic material or an inorganic material depending upon optical characteristics of films, the abrasion resistance, the adhesion force, and the environmental reliability. However, in terms of the refractive index and the abrasion resistance, it may be made of a silane alkoxy hydrolysis condensate or a partial-hydrolysis condensate. Assume that a coating material of a mixture of the hollow nanoparticles 14 and the silane alkoxy hydrolysis condensate as the binder 15 is used for the third layer 102. Then, as the weight ratio of solid contents of the hollow silica nanoparticles and the silane alkoxy hydrolysis condensate contained in the coating material, (weight of solid content of hollow silica nanoparticle): (weight of solid content of binder) may be in a range of 7:3 to 8:2. This range provides the layer with a low refractive index and a strong film strength and maintains the abrasion resistance.
The formation method of the third layer is not particularly limited, and it is possible to use a general coating method for a liquid coating solution such as a dip coating, a spin coating, a spray coating, or a roll coating. The drying after the coating may be performed by using such as a drier, a hot plate, or an electric furnace. The temperature and time in the dry condition are set to a degree that the drying can be performed to evaporate an organic solution in the hollow particles without influencing on the substrate 11. In general, the temperature of 300° C. or less may be used. The number of coating processes is not particularly limited.
The optical element according to the present embodiment is appropriate for an imaging optical system of an imaging apparatus (optical apparatus) such as a digital still camera, a digital video camera, or a television camera. The antireflective film 101 is installed on two surfaces or one surface of the optical element to effectively increase an amount of transmitting light and to effectively avoid ghost and flare caused by unnecessary light. Of course, the optical apparatus is not particularly limited to imaging apparatuses such as binoculars, telescopes, or microscopes.
An optical element according to a first embodiment has the configuration illustrated in
An optical element of a second embodiment also has the configuration illustrated in
An optical element according to a third embodiment also has the configuration illustrated in
An optical element according to a fourth embodiment also has the configuration illustrated in
An optical element according to a fifth embodiment 5 also has the configuration illustrated in
An optical element according to a sixth embodiment 6 also has the configuration illustrated in
An optical element according to a seventh embodiment also has the configuration illustrated in
An optical element according to an eighth embodiment also has the configuration illustrated in
An optical element according to a comparative example 1 also has the configuration illustrated in
An optical element according to a comparative example 2 also has the configuration illustrated in
The present invention can provide an optical element, an optical system, and an optical apparatus having an excellent low reflectance characteristic and having a high antireflection performance for a high reflective index glass.
The optical element is applicable to lenses or the like.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2012-248743, filed Nov. 12, 2012, which is hereby incorporated by reference herein in its entirety.
Number | Date | Country | Kind |
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2012-248743 | Nov 2012 | JP | national |