Patent Description:
Japanese Patent Application Laid-Open No. ("<CIT> discloses an antireflection film having an outermost layer, which is a layer (silicon oxide layer) including SiO<NUM> or SiO as a principal component, and a next layer, which is a layer (magnesium fluoride layer) including MgF<NUM> as a principal component. <CIT> discloses an antireflection film consisting of a multi-laminated layer in which silicon oxide layers and tantalum oxide layers are alternately laminated, a magnesium fluoride layer formed on the multi-laminated layer, and a silicon oxide layer as an outermost layer formed on the magnesium fluoride layer.

However, with the configuration of the antireflection film described in <CIT> or <CIT>, it is difficult to increase mechanical strength and environmental durability of an optical element because tensile stress of magnesium fluoride is strong. As a result, film cracking or film peeling may occur in the antireflection film on a lens made from resin material. <CIT> relates to an antireflection film that is provided on a substrate and includes at least nine layers, wherein an outermost layer of the nine layers is formed by SiO<NUM> or MgF<NUM>.

The present disclosure provides an optical element, an optical system, and an optical apparatus each of which has high mechanical strength and high environmental durability.

The present disclosure in a first aspect provides an optical element as specified in claims <NUM> to <NUM>.

The present disclosure in a second aspect provides an optical system as specified in claim <NUM>.

The present disclosure in a further aspect provides an optical apparatus as specified in claim <NUM>.

Referring now to the accompanying drawings, a detailed description will be given of embodiments and examples according to the present disclosure. Examples <NUM> and <NUM> are reference examples outside of the scope of the present invention as claimed.

First, a description will be given of a schematic configuration of an optical element <NUM> in this embodiment with reference to <FIG> is a schematic sectional view illustrating the optical element <NUM>. The optical element <NUM> includes a transparent resin substrate <NUM> which is a base material consisting of resin material, and an antireflection film <NUM> formed on the transparent resin substrate <NUM>. The antireflection film <NUM> includes a multilayer film <NUM> as a first film and a multilayer film <NUM> as a second film, in order from the side closer to the transparent resin substrate <NUM>. That is, the antireflection film <NUM> consists of a multilayer film <NUM> formed on the transparent resin substrate <NUM> and a multilayer film <NUM> formed on the multilayer film <NUM>, i.e., formed at a position farthest from the transparent resin substrate <NUM>.

The multilayer film <NUM> consists of layers <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, in order from the side closer to the transparent resin substrate <NUM>. In this embodiment, the multilayer film <NUM> consists of six layers, but the present disclosure is not limited to this. The number of layers of the multilayer film <NUM> may be any number as long as the multilayer film <NUM> consists of one or more layer. The multilayer film <NUM> consists of three layers of a layer <NUM> as a first layer, a layer <NUM> as a second layer, and a layer <NUM> as a third layer, in order from the side closer to the multilayer film <NUM>. The layers <NUM> and <NUM> forming the multilayer film <NUM> each include silicon oxide (SiO<NUM>), and the layer <NUM> includes magnesium fluoride (MgF<NUM>). More specifically, the layers <NUM> and <NUM> each consist of silicon oxide, or each include silicon oxide as a principal component, that is, are layers each including silicon oxide at a weight ratio of <NUM>% or more. The layer <NUM> consists of magnesium fluoride or includes magnesium fluoride as a principal component.

When the antireflection film <NUM> is vapor-deposited on the transparent resin substrate <NUM>, it is necessary to form films with the transparent resin substrate <NUM> in a state of non-heated, or of heated at low temperature of <NUM> degrees or less. Magnesium fluoride vapor-deposited in the state of non-heated, or of heated at low temperature of <NUM> degrees or less, has low film strength and strong tensile stress. On the other hand, even when vapor-deposited in the state of non-heated, or of heated at a low temperature of <NUM> degrees or less, silicon oxide has high film strength and strong tensile stress. In this embodiment, a magnesium fluoride layer is sandwiched between silicon oxide layers as in the configuration of the multilayer film <NUM>, so that the film strength can be improved and adhesion can be improved by canceling the stress.

In this embodiment, the following conditional expression (<NUM>) may be satisfied where n1, n2, and n3 respectively represent refractive indexes at a d-line of the layers <NUM>, <NUM>, and <NUM>, d1, d2, and d3 (nm) respectively represent physical film thicknesses of the layers <NUM>, <NUM>, and <NUM>, and λ represents a wavelength of the d-line.

In this embodiment, the numerical range of the conditional expression (<NUM>) may be set to that in the following conditional expression (1a).

In the antireflection film <NUM>, antireflection performance is improved when the outermost layer, i.e., a top layer, is made from material having a low refractive index and an optical film thickness of the outermost layer is set to about λ/<NUM>. In this embodiment, the antireflection performance can be improved by regarding the multilayer film <NUM> as a layer substantially equivalent to the low refractive index material of the outermost layer and satisfying the conditional expression (<NUM>).

In this embodiment, the following conditional expressions (<NUM>) and (<NUM>) may be satisfied. <MAT> <MAT>.

In this embodiment, the numerical ranges of the conditional expressions (<NUM>) and (<NUM>) may be set to those in the following conditional expressions (2a) and (3a). <MAT> <MAT>.

If the film thickness of the layer <NUM> of the multilayer film <NUM> increases, an average refractive index of the multilayer film <NUM> decreases, but the film strength decreases, and tensile stress increases and makes it difficult to ensure adhesion. On the other hand, if the film thicknesses of the layers <NUM> and <NUM> increase, the film strength improves and the tensile stress decreases and makes it easy to ensure the adhesion, but the average refractive index of the multilayer film <NUM> increases. When the films are formed so that the respective film thicknesses satisfy the conditional expressions (<NUM>) and (<NUM>), it is possible to ensure both antireflection performance and film strength.

The transparent resin substrate <NUM> expands as the temperature rises. Magnesium fluoride has a large tensile stress. Generally, vapor-deposited films made from silicon oxide have compressive stress, but stronger compressive stress is required to offset the stress of magnesium fluoride. In this embodiment, the compressive stress is enhanced by using silicon oxide material including a small amount of aluminum. Therefore, material forming the layers <NUM> and <NUM> is a material including silicon oxide as a principal component and a small amount of aluminum. The refractive indexes n1 and n3 may satisfy the following conditional expressions (<NUM>) and (<NUM>), respectively. <MAT> <MAT>.

Each of the layers <NUM> and <NUM> includes aluminum at a weight ratio of <NUM>% or less. The addition of aluminum is effective even when the added amount is very small. Even a silicon oxide film including aluminum at a weight ratio of <NUM>% can hinder film cracking and film peeling from occurring when combined with a magnesium fluoride film.

The multilayer film <NUM> may be a combination of high refractive index material and medium refractive index material, which has a refractive index at the d-line of about <NUM> to <NUM>, but may be a layer (alternate layer) formed by alternately laminating high refractive index material and low refractive index material. The high refractive index material used in the multilayer film <NUM> may be one of tantalum oxide, titanium oxide, lanthanum oxide, and zirconium oxide, or may be material whose principal component is a mixture of more than one of them. The high refractive index material has tensile stress. The low refractive index material used in the multilayer film <NUM> may be the same material as the layers <NUM> and <NUM> so as to be easily manufactured and to offset the stress of the antireflection film <NUM>. Since the high refractive index material has the tensile stress and the low refractive index material has the compressive stress, the stress is offset by forming the alternate layer. The alternate layer is not limited to the six layers as illustrated in <FIG>, as long as the alternate layer includes at least one layer for each of two types of layers made from material different from each other.

A layer <NUM> which is a bottom layer of the multilayer film <NUM> may be made from the same material as the layers <NUM> and <NUM>. The transparent resin substrate <NUM> generally has a thermal expansion coefficient larger than that of glass. When material having strong compressive stress is used on the transparent resin substrate <NUM>, the layer <NUM> can follow a shape variation of the transparent resin substrate <NUM> expanding due to high temperature, and hindering film cracking from occurring.

A film forming method for the antireflection film <NUM> consisting of the multilayer film <NUM> and the multilayer film <NUM> is not particularly limited as long as it is physical vapor deposition such as vapor deposition, a sputtering method, and an ion plating method. In particular, vapor deposition may be used because fluoride is less likely to decompose. In vapor deposition, heating methods for vapor deposition material include electrical resistance heating, electron-beam physical vapor deposition, pulsed laser deposition, and the like. Electron-beam physical vapor deposition may be used because it can form a film with a substrate in a non-heated state by directly heating film material, and can provide a film of relatively high quality with small amount of contamination. An ion beam assist method may also be used. By an independent ion source playing a role of assisting vapor deposition, it is possible to form a dense film with low absorption and scattering and high strength.

In this embodiment, the following conditional expression (<NUM>) may be satisfied where nd represents a refractive index (average refractive index at the d-line) of the transparent resin substrate <NUM>.

Further, in this embodiment, the following conditional expression (<NUM>) may be satisfied where α (<NUM>-<NUM>/°C) represents a coefficient of linear expansion of the transparent resin substrate <NUM>.

A detailed description will be given below of each example.

<FIG> is a schematic sectional view of an optical element <NUM> in an Example <NUM> of the present disclosure. The optical element <NUM> in this example is an optical element in which an antireflection film <NUM> is formed on a transparent resin substrate <NUM>. The transparent resin substrate <NUM> is made from COP resin (Zeon Corporation, "ZEONEX") having a refractive index of <NUM> (at the d-line). As layer material, layers <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM>, layers <NUM>, <NUM>, and <NUM> use mixture of ZrO<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>).

A film forming method for the antireflection film <NUM> in this example is as follows. The antireflection film <NUM> is formed by vapor deposition. An electron beam was used to heat evaporation material. Ion beam-assisted vapor deposition was performed to form a denser film. The inside of a vacuum chamber of a vapor deposition apparatus was exhausted in a non-heated state up to a high-vacuum range of about <NUM>×<NUM>-<NUM> (Pa). After it was ensured that the inside of the vacuum chamber was in the high vacuum state, Ar as inert gas was introduced into an ion gun and the ion gun was discharged. After the ion gun became a stable state, oxygen was introduced into the vacuum chamber, and ion assisted vapor deposition using oxygen ion was performed at a vacuum pressure of about <NUM> × <NUM>-<NUM> (Pa).

<FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

The layer <NUM> made from magnesium fluoride is formed by non-heating vapor deposition, and has low strength and strong tensile stress. On the other hand, a layer of silicon oxide formed by non-heating vapor deposition has high strength and has compressive stress. The multilayer film <NUM> with magnesium fluoride film sandwiched between silicon oxide films as a whole has a structure in which strength is high and stress is offset, and is a film with good environmental reliability that does not cause cracking or peeling.

The antireflection film <NUM> was subjected to the following durability tests for confirming its durability under various conditions.

A prepared sample was left for <NUM> hours in a constant temperature bath set to a temperature of <NUM> degrees and a humidity of <NUM>%, and thereafter the appearance of the antireflection film <NUM> was visually observed.

A prepared sample was left for <NUM> hours in a constant temperature bath set to a temperature of -<NUM> degrees, and thereafter the appearance of the antireflection film <NUM> was visually observed.

A prepared sample was left for <NUM> hours in a constant temperature bath set to <NUM> degrees, and thereafter the appearance of the antireflection film <NUM> was visually observed.

Adhesive tape was put on a surface of the antireflection film <NUM> of a prepared sample, and the tape was peeled off in a direction perpendicular to the film surface. It was repeated five times and whether or not the film had peeled off was confirmed by visual observation.

After the antireflection film <NUM> was rubbed for ten times back and forth with a lens-cleaning paper soaked with solvent with a load of about <NUM> applied, the appearance of the antireflection film <NUM> was visually observed.

Table <NUM> indicates the results of the durability tests. It could be confirmed that no film cracking or film peeling occurred in every test, and that a good antireflection film was formed.

An optical element in an Example <NUM> is made by using the same transparent resin substrate, the same vapor deposition material, and the same vapor deposition condition as those in the Example <NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

A transparent resin substrate <NUM> in an Example <NUM> is made from special PC resin (Mitsubishi Gas Chemical Company, Inc. , "EP-<NUM>"). As layer material, layers <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM>, and <NUM> use mixture of Ta<NUM>O<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). A film forming method for an antireflection film <NUM> in this example is electron-beam physical vapor deposition and ion-assisted vapor deposition as in the Example <NUM>. <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

In this example, the layer <NUM> and the layer <NUM> are SiO<NUM> layers each including Al at the weight ratio of <NUM>%. These layers have very strong compressive stress. MgF<NUM> has very strong tensile stress, and resin material is very likely to expand. According to this example, these stresses are offset, and thus it is possible to provide an antireflection film having a very high environmental durability.

A transparent resin substrate <NUM> in an Example <NUM> is made from special PC resin (Mitsubishi Gas Chemical Company, Inc. , "EP-<NUM>"). As layer material, layers <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM>, and <NUM> use mixture of ZrO<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

A transparent resin substrate <NUM> in an Example <NUM> is made from COP resin (Zeon Corporation, "ZEONEX"). As layer material, layers <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM>, and <NUM> use mixture of Ta<NUM>O<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

A transparent resin substrate <NUM> in an Example <NUM> is made from COP resin (Zeon Corporation, "ZEONEX"). As layer material, layers <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM>, and <NUM> use mixture of Ta<NUM>O<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

A transparent resin substrate <NUM> in an Example <NUM> is made from special PC resin (Mitsubishi Gas Chemical Company, Inc. , "EP-<NUM>"). As layer material, layers <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM> and, <NUM> use mixture of Ta<NUM>O<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

A transparent resin substrate <NUM> in an Example <NUM> is made from special PC resin (Mitsubishi Gas Chemical Company, Inc. , "EP-<NUM>"). As layer material, layers <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM>, and <NUM> use mixture of Ta<NUM>O<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

A transparent resin substrate <NUM> in an Example <NUM> is made from special PC resin (Mitsubishi Gas Chemical Company, Inc. , "EP-<NUM>"). As layer material, layers <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM>, and <NUM> use mixture of Ta<NUM>O<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

A transparent resin substrate <NUM> in an Example <NUM> is made from polyester film (PET resin) (Toray Industries, Inc. , "Lumirror T60"). As layer material, layers <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> use SiO<NUM> including Al at a weight ratio of <NUM>%, layers <NUM>, <NUM>, and <NUM> use mixture of Ta<NUM>O<NUM> and TiO<NUM>, and a layer <NUM> uses MgF<NUM>. Table <NUM> indicates details of a film configuration of the optical element in this example. A refractive index and a film thickness of each material satisfy the expressions (<NUM>), (<NUM>), and (<NUM>). <FIG> indicates a reflectance characteristic of the optical element in this example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM>% or less in a wavelength range of <NUM> to <NUM>, which is a very good characteristic.

Next, a description will be given of an optical system in an Example <NUM> with reference to <FIG> is a sectional view of an optical system <NUM>. The optical system <NUM> includes a plurality of optical elements G401 to G411. A reference numeral <NUM> denotes a diaphragm and a reference numeral <NUM> denotes an image plane. The optical elements G401 to G411 are lenses, respectively. Of these lenses, at least one of entrance surfaces and emission surfaces is provided with the antireflection film according to any one of the Examples <NUM> to <NUM>. That is, the optical system <NUM> includes the plurality of optical elements G401 to G411, and the plurality of optical elements G401 to G411 includes the optical element <NUM> provided with the antireflection film according to any one of the Examples <NUM> to <NUM>.

The optical system <NUM> in this Example is not limited to an image pickup optical system used in an image pickup apparatus described later, and may be applied to optical systems for various purposes such as binoculars, projectors, and telescopes.

Next, a description will be given of an image pickup apparatus in an Example <NUM> with reference to <FIG> is an external perspective view illustrating the image pickup apparatus (digital camera <NUM>).

The digital camera <NUM> includes a camera body <NUM> and a lens apparatus <NUM> which is integrally configured with the camera body <NUM>. However, this example is not limited to this, and the lens apparatus <NUM> may be an interchangeable lens, which is detachably attachable to the camera body <NUM>, such as a lens for a single-lens reflex camera and a lens for a mirrorless camera. The lens apparatus <NUM> includes an optical system <NUM> according to any one of the Examples <NUM> to <NUM>. The camera body <NUM> includes an image sensor <NUM> such as a CMOS sensor and a CCD sensor. The image sensor <NUM> is disposed on an image plane <NUM> in the optical system <NUM>.

A Comparative Example <NUM> uses the same vapor deposition material, the same transparent resin substrate, and the same vapor deposition condition as those in the Example <NUM>. Table <NUM> indicates a film configuration of an optical element in this comparative example. A multilayer film <NUM> is made only of a magnesium fluoride layer. <FIG> indicates a reflectance characteristic of the optical element in this comparative example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is <NUM> or less in a wavelength range of <NUM> to <NUM>, achieving lower reflectance than those in the Examples <NUM> to <NUM>.

Table <NUM> indicates the results of the durability tests. An outermost layer in this comparative example is made of a magnesium fluoride film having low strength. Therefore, in the configuration of this comparative example, film cracking and film peeling occur in each durability test, which is not suitable for use as an antireflection film.

A Comparative Example <NUM> uses the same vapor deposition material, the same transparent resin substrate, and the same vapor deposition condition as those in the Example <NUM>. Table <NUM> indicates a film configuration of an optical element in this comparative example. A multilayer film <NUM> is made only of a silicon oxide layer.

Table <NUM> indicates the results of the durability tests. An outermost layer in this comparative example is made of a silicon oxide film having high strength. Therefore, no film cracking and no film peeling occurred in every durability test.

<FIG> indicates a reflectance characteristic in this comparative example. In <FIG>, a horizontal axis represents wavelength (nm) and a vertical axis represents reflectance (%). The reflectance is about <NUM>% in a wavelength range of <NUM> to <NUM>, which is higher than those in the Examples <NUM> to <NUM>. Thus, in the configuration of this comparative example, ghost and flare may be caused.

According to each example, it is possible to provide an optical element, an optical system, and an optical apparatus each of which has high mechanical strength and high environmental durability.

Claim 1:
An optical element (<NUM>) comprising a base material (<NUM>) and an antireflection film (<NUM>),
wherein the antireflection film (<NUM>) consists of a first film (<NUM>) formed on the base material (<NUM>) and a second film (<NUM>) formed on the first film (<NUM>),
wherein the second film (<NUM>) consists of a first layer (<NUM>), a second layer (<NUM>), and a third layer (<NUM>), in order from a side closest to the first film (<NUM>),
wherein the first layer (<NUM>) and the third layer (<NUM>) each include silicon oxide,
wherein the second layer (<NUM>) includes magnesium fluoride; and
wherein the first layer and the third layer each include silicon oxide at a weight ratio of <NUM>% or more;
characterized in that
the base material consists of resin material, and
the first layer and the third layer each include aluminum at a weight ratio of <NUM>% or less.