Multilayer film, optical element comprising the multilayer film, and method for producing the multilayer film

Abstract

A multilayer film that sufficiently exhibits a self-cleaning function by a photocatalytic reaction on a surface and can maintain hydrophilicity in the dark for a long period of time even when the thickness of a low-refractive-index layer on the surface is adjusted to a certain thickness or more to reduce light reflection; an optical element including the multilayer film; and a method for producing the multilayer film. Specifically, a multilayer film to be formed on glass or a resin, the multilayer film including a layer containing specific cerium oxide and a layer containing specific silicon oxide, or a layer containing specific magnesium fluoride, formed on the layer containing specific cerium oxide directly or via another layer; an optical element including the multilayer film; and a method for producing the multilayer film.

Description

Technical area

[0001] The present disclosure relates to a multilayer film having excellent self-cleaning property and hydrophilicity, an optical element including the multilayer film, and a method for producing the multilayer film. State of the art

[0002] An optical element, such as an optical lens, a mirror, or an optical filter, has a film formed from an inorganic material to increase or decrease the transmittance and reflectance of light. The film formed from an inorganic material generally has a high surface free energy and therefore exhibits high hydrophilicity immediately after film formation. However, due to self-reaction or the adhesion of contaminants from humans or the environment, the surface free energy is reduced in a relatively short period of time, resulting in a decrease in hydrophilicity.

[0003] For example, if the adhesion of water droplets occurs in a state where the hydrophilicity of the surface of an optical product to be used in an automobile, an optical product to be used outdoors such as a surveillance camera or a spectacle lens, or a related protective cover decreases, visibility may deteriorate, so that there is a fear that the functions of the above-mentioned optical products and protective covers cannot be sufficiently exhibited.

[0004] As a means to solve the above problem, a hydrophilic film is used in which a thin silica film is formed on a thin crystalline titanium dioxide film (Patent Literature 1, Patent Literature 2, and Non-Patent Literature 1). When the surface of the crystalline titanium dioxide is irradiated with near-ultraviolet light, active oxygen is generated through a photocatalytic function, and the generated active oxygen decomposes an organic substance on the surface of the hydrophilic film. This restores the hydrophilicity of the hydrophilic film, and the dirt is washed off by rain. Thus, the hydrophilic film is self-cleaning.In addition, it is known that when silica is arranged on titania, unlike the thin film formed of titania alone, the resulting thin film is not hydrophobicized in a short time even if the light irradiation is stopped, and the hydrophilicity remains even in the dark for about 1 week to about 2 weeks.

[0005] However, when the thickness of the thin silica film formed on the thin crystalline titanium dioxide film is less than 50 nm, there is a problem that the reflectance is increased and the transmittance is decreased because titanium dioxide has a high refractive index. Therefore, there is a problem that although this hydrophilic film can be applied to a vehicle door mirror without any problems even if the reflectance is increased, the hydrophilic film is not suitable for an optical element such as a lens that includes an anti-reflection film. In addition, when the thickness of the thin silica film formed on the surface of the thin crystalline titanium dioxide film is 50 nm or more, there is also a problem that the self-cleaning function by a photocatalytic reaction is not sufficiently exhibited.In addition, there is a problem that although the performance of maintaining hydrophilicity in the dark is improved compared to the case of only crystalline titanium dioxide, the performance of maintaining hydrophilicity is not sufficient.

[0006] In addition, when using a crystalline titanium dioxide layer as a photocatalyst, there is also a problem that, in order to crystallize a thin film formed of titanium dioxide, the thin film and a base material on which the thin film is deposited must be heated to a high temperature, so a base material made of a resin with low heat resistance cannot be used. Film formation of titanium dioxide by some wet methods does not require high-temperature heating. However, when using a wet method, there are the following problems: it is difficult to perform fine thickness control in units of 1 nm, formation of a thin film with a uniform thickness, and film formation on any base material except for a base material with a simple shape such as a flat plate; and the storage time of a coating liquid is short. Citation listPatent literature PTL 1: Japanese Patent Application Publication No. 09-057912 PTL 2: Japanese Patent Application Publication No. 2000-053449 Non-patent literature

[0007] NPL 1: Journal of the Ceramic Society of Japan 110 [5] 450-454 (2002) Brief description of the inventionTechnical problem

[0008] With regard to application to an optical element, even if the thickness of the low-refractive index layer on the surface of the optical element is selected to be large so that the low-refractive index layer can be designed as an anti-reflection film, there is a need for a multilayer film that exhibits sufficient self-cleaning function through a photocatalytic reaction. Furthermore, a multilayer film that maintains its hydrophilicity in the dark for an extended period of time has been sought.

[0009] The present disclosure has been made in view of the above-mentioned problems and provides a multilayer film that has a sufficient self-cleaning function by a photocatalytic reaction on a surface and can maintain hydrophilicity in the dark for a long period of time even when the thickness of a low refractive index layer on the surface is set to a certain thickness or more to reduce light reflection, an optical element including the multilayer film, and a method for producing the multilayer film. Solution to the problem

[0010] According to the present disclosure, there is provided a multilayer film including: a layer containing cerium oxide; and a layer containing silicon oxide or a layer containing magnesium fluoride formed directly or via another layer on the layer containing cerium oxide, wherein the layer containing cerium oxide contains cerium oxide having a cubic polycrystalline structure, wherein the layer containing cerium oxide has a thickness of 70 nm or more and 300 nm or less, wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a thickness of 50 nm or more and 240 nm or less, and wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a refractive index of 1.65 or less at a wavelength of 500 nm.

[0011] Furthermore, in the multilayer film of the present disclosure, the multilayer film includes a silicon dioxide-containing layer formed directly or through another layer on the silicon oxide-containing layer or the magnesium fluoride-containing layer. The silicon dioxide-containing layer has a thickness of 30 nm or less.

[0012] Furthermore, in the multilayer film of the present disclosure, the multilayer film further includes a layer formed of a first metal oxide between the layer containing cerium oxide and the layer containing silicon oxide or the layer containing magnesium fluoride. The layer formed of a first metal oxide contains a metal oxide having a polycrystalline structure. The layer formed of a first metal oxide has a thickness of 0.5 nm or more and 15 nm or less.

[0013] Furthermore, in the multilayer film of the present disclosure, the multilayer film further includes a layer formed of a second metal oxide between the layer containing silicon oxide or the layer containing magnesium fluoride and the layer containing silicon dioxide. The layer formed of a second metal oxide contains a metal oxide having a polycrystalline structure. The layer formed of a second metal oxide has a thickness of 0.5 nm or more and 7 nm or less.

[0014] Furthermore, according to the present disclosure, there is provided an optical element including the above-mentioned multilayer film of the present disclosure.

[0015] Furthermore, according to the present disclosure, there is provided a method for producing a multilayer film, the method comprising: forming a layer containing cerium oxide on a base material directly or via another layer by a vacuum evaporation method;and forming a silicon oxide-containing layer or a magnesium fluoride-containing layer on the cerium oxide-containing layer directly or via another layer by the vacuum vapor deposition method, wherein the cerium oxide-containing layer contains cerium oxide having a cubic polycrystalline structure, the cerium oxide-containing layer having a thickness of 70 nm or more and 300 nm or less, the silicon oxide-containing layer and the magnesium fluoride-containing layer each having a thickness of 50 nm or more and 240 nm or less, and the silicon oxide-containing layer and the magnesium fluoride-containing layer each having a refractive index of 1.65 or less at a wavelength of 500 nm.; Advantageous effects of the invention

[0016] According to one aspect of the present disclosure, the multilayer film that has a sufficient self-cleaning function by a photocatalytic reaction on a surface and can maintain hydrophilicity even in the dark for a long period of time even when the thickness of the low refractive index layer on the surface is set to a certain thickness or more to reduce light reflection, the optical element including the multilayer film, and the method for producing the multilayer film can be obtained. Brief description of the drawings Fig. 1 is a schematic sectional view illustrating the configuration of a multilayer film according to a first embodiment of the present disclosure. Fig.2 is a schematic sectional view illustrating the configuration of a multilayer film according to a second embodiment of the present disclosure. Fig. 3 is a schematic sectional view illustrating the configuration of a multilayer film according to a third embodiment of the present disclosure. Fig. 4 is a schematic sectional view illustrating the configuration of a multilayer film according to a fourth embodiment of the present disclosure. Fig. 5 is a schematic sectional view illustrating the configuration of a multilayer film according to a fifth embodiment of the present disclosure. Fig. 6 is a schematic sectional view illustrating the configuration of a multilayer film according to a sixth embodiment of the present disclosure. Fig.7 is a schematic view illustrating an optical element according to an embodiment of the present disclosure. Fig. 8 is a schematic view illustrating an optical element according to an embodiment of the present disclosure. Description of the embodiments

[0017] Embodiments of a multilayer film, an optical element including the multilayer film, and a method for forming the multilayer film according to the present disclosure are described below with reference to exemplary embodiments.

[0018] Furthermore, the present disclosure is not limited to the following embodiments.

[0019] Furthermore, in the present disclosure, the description [XX or more and YY or less] or [XX to YY] representing a numerical range means a numerical range including a lower limit and an upper limit as endpoints, unless otherwise specified. When the numerical ranges are described in stages, the upper limit and lower limit of each numerical range can be freely combined.

[0020] In the present disclosure, the term [multilayer film] refers to a configuration including two or more layers formed on the surface of a base material. The multilayer film according to the present disclosure may be disposed directly on the base material or through another layer. The layers for forming the multilayer film may also be referred to as "films" hereinafter.

[0021] In the present disclosure, the term [base material] refers to a solid material as an object.

[0022] In the present disclosure, the term [optical element] refers to an optical element including the above-mentioned multilayer film. Examples of the optical element include an optical filter, an optical lens, a daylight lens, an optical film, an optical prism, an eyeglass lens, a photographic lens, a security camera cover, a vehicle camera cover, a vehicle sensor cover, a vehicle door mirror, a glass panel, a condenser lens, a display cover glass, a touch panel, and various types of films.

[0023] Before specifically describing the multilayer film according to the present disclosure, for a better understanding of the present disclosure, the following describes the estimation of a mechanism by which the effects thereof are achieved. However, the following description is merely a hypothesis, and the present disclosure is in no way limited by the following hypothesis.

[0024] The inventors found that when a layer containing cerium oxide having a cubic polycrystalline structure is disposed directly or through another layer on a base material, and a layer containing magnesium fluoride or silicon oxide with a certain thickness is further disposed directly or through another layer on the cerium oxide layer, a hydrophilicity restoration function is achieved through a self-cleaning property with a photocatalyst. The following content is considered the mechanism of the above.

[0025] When a near-ultraviolet light-responsive photocatalyst film is irradiated with near-ultraviolet light, holes and electrons are generated by photoexcitation. If the generated holes and electrons can reach the surface of the film, a chemical reaction occurs, thus achieving a hydrophilicity-restoring function through a self-cleaning property. However, when a low-refractive index layer with a specific thickness of more than 50 nm is formed on crystalline titanium dioxide, the holes and electrons generated by photoexcitation are blocked by the thickness, so the holes and electrons cannot reach the surface of the multilayer film. Therefore, the holes and electrons are recombined and deactivated, thus failing to achieve the hydrophilicity-restoring function through a self-cleaning property.Meanwhile, when a low-refractive-index layer with a thickness of more than 50 nm is formed on the cerium oxide-containing layer of the present disclosure, the ratio of holes and electrons reaching the surface of the multilayer film is increased to cause a chemical reaction, compared to the case where the low-refractive-index layer is formed on crystalline titanium dioxide, and thus the hydrophilicity-restoring function is provided by a self-cleaning property. At the interface between the cerium oxide-containing layer of the present disclosure and the upper layer in contact with the cerium oxide-containing layer, there is a region where the respective layers are diffused with each other, such as a compound or mixture.Trivalent cerium and tetravalent cerium are easily mixed in this region, so the electrons generated by photoexcitation can be easily held. Therefore, holes and electrons are less likely to recombine. Furthermore, the cerium oxide in the layer containing cerium oxide has a cubic polycrystalline structure, so the conductivity is high and there are a large number of interfaces. Therefore, holes and electrons are less likely to move and recombine.

[0026] Furthermore, the inventors found that when a layer containing cerium oxide with a cubic polycrystalline structure is disposed directly or through another layer on a base material, and further a layer containing magnesium fluoride or silicon oxide is disposed on top of the layer, the ability to maintain hydrophilicity in the dark is significantly improved. The following content is considered to be the mechanism of the above.

[0027] When a layer containing cerium oxide having a cubic polycrystalline structure is exposed to ultraviolet light or a light beam whose energy is higher than that of ultraviolet light, electrical energy, chemical energy, and the like are stored in the cerium oxide and at the interface. It is believed that when the layer containing cerium oxide of the present disclosure supplies the stored energy to the surface side of the multilayer film, the surface energy of the multilayer film is maintained in a high state and the hydrophilicity in the dark is improved. In particular, in the layer containing cerium oxide of the present disclosure formed by vacuum vapor deposition, trivalent cerium and tetravalent cerium are easily mixed, and therefore, there is a possibility that the layer is brought into a state where energy is easily stored. <<Erste Ausführungsform> >

[0028] Fig. 1 is a schematic sectional view illustrating a multilayer film according to a first embodiment of the present disclosure, which is disposed on a base material. In this embodiment, a configuration example is shown in which a layer 13 containing cerium oxide of the present disclosure is formed on a base material 11, and a layer 14 containing magnesium fluoride is formed on the layer 13 containing cerium oxide. In each of Fig. 1 to Fig. 6 schematically illustrates the configuration of the multilayer film in the present disclosure. Therefore, the area, thickness, and the like of each layer are not shown in precise proportions.

[0029] The base material 11 is described.

[0030] It is only necessary that another layer 12 or the layer 13 containing cerium oxide of the present disclosure may be coated on the base material 11, and glass, ceramics, resin, metal, or the like may be used. The shape of the base material is not limited, and the base material may have, for example, a flat surface shape, a curved surface shape, a concave surface shape, a convex surface shape, or a film shape. Furthermore, the base material 11 may include a hard layer or a barrier layer. Furthermore, the size and thickness of the base material 11 are not particularly limited and can be appropriately adjusted according to applications and the like.

[0031] The cerium oxide-containing layer 13 according to the present disclosure will be described. The cerium oxide-containing layer 13 of the present disclosure is a layer containing cerium oxide (CeO x) with a cubic polycrystalline structure. Cracks may occur in a layer formed from cerium oxide with a single-crystal structure. If the layer is formed from cerium oxide with an amorphous structure, the self-cleaning function with a photocatalyst and the ability to maintain hydrophilicity in the dark are significantly reduced.

[0032] The term [polycrystalline structure] as defined in the present disclosure means that a unique peak for cerium oxide appears in X-ray diffraction (XRD) measurement of a film after film formation.

[0033] The cerium oxide-containing layer 13 of the present disclosure has a thickness of 70 nm or more and 300 nm or less. When the thickness of the cerium oxide-containing layer 13 is less than 70 nm, the self-cleaning function with a photocatalyst and the ability to maintain hydrophilicity in the dark are significantly reduced. When the thickness of the cerium oxide-containing layer 13 is more than 300 nm, cracks may occur, and the heterogeneity and surface roughness may become excessive, which may adversely affect the optical properties.

[0034] The composition of cerium oxide in the cerium oxide-containing layer 13 of the present disclosure is CeO x , and the “x” preferably represents 1.5 or more and 2.0 or less. When the value of “x” in the composition CeO xof cerium oxide falls within the above range, a more transparent film can be obtained in a wavelength range from visible light to near infrared light.

[0035] The content of cerium oxide in the one cerium oxide-containing layer 13 of the present disclosure is preferably 85 mass% or more with respect to the entire cerium oxide-containing layer 13. When the content of cerium oxide is 85 mass% or more, the ability to maintain hydrophilicity in the dark is further improved.

[0036] The layer 13 containing cerium oxide of the present disclosure may be disposed directly on the base material 11 or by means of another layer 12, which will be described later.

[0037] The magnesium fluoride-containing layer 14 according to the present disclosure is described. The thickness of the magnesium fluoride-containing layer 14 is 50 nm or more and 240 nm or less. If the thickness of the magnesium fluoride-containing layer 14 is less than 50 nm, the reflectance of the multilayer film may become too high. Conversely, if the thickness of the magnesium fluoride-containing layer 14 is more than 240 nm, the self-cleaning function with a photocatalyst on the surface of the multilayer film may not be achieved. In addition, the refractive index of the magnesium fluoride-containing layer 14 at a wavelength of 500 nm is 1.65 or less. If the refractive index of the magnesium fluoride-containing layer 14 is more than 1.65, the reflectance of the multilayer film may become too high.

[0038] The content of magnesium fluoride in the total amount of the substances for forming the magnesium fluoride-containing layer 14 is preferably 65% by mass or more. When the content falls within the above range, the ability to maintain hydrophilicity in the dark is further improved. <<Zweite Ausführungsform> >

[0039] Fig.2 is a schematic sectional view illustrating a multilayer film according to a second embodiment of the present disclosure. In this embodiment, the multilayer film according to the present disclosure is disposed on the other layer 12 disposed on the base material 11. That is, in the second embodiment, a configuration example is shown in which the other layer 12 is formed on the base material 11, further, the layer 13 containing cerium oxide of the present disclosure is formed on the other layer 12, and additionally, a layer 15 containing silicon oxide is formed on the layer 13 containing cerium oxide.

[0040] The base material 11 and the layer 13 containing cerium oxide of the present disclosure are as described in the above-mentioned first embodiment.

[0041] In this embodiment, the magnesium fluoride-containing layer 14 described in the first embodiment may be disposed instead of all or part of the silicon oxide-containing layer 15. Furthermore, the cerium oxide-containing layer 13 of the present disclosure may be disposed directly on the base material 11 without another layer 12 therebetween.

[0042] A layer containing a metal, a fluoride, an oxide, a carbide, or a nitride can be arranged as the other layer 12. Specific examples of the layer that can be used as the other layer 12 include: a metal layer containing an element such as aluminum (Al), chromium (Cr), gold (Au), silver (Ag), copper (Cu), silicon (Si), germanium (Ge), titanium (Ti), or nickel (Ni); a layer containing a fluoride such as magnesium fluoride (MgF2) or calcium fluoride (CaF2); a layer containing an oxide such as silicon oxide (SiO x ), aluminum oxide (Al2O x ), yttrium oxide (Y2O x ), zirconium oxide (ZrO x ), hafnium oxide (HfO x ), zinc oxide (ZnO x ), tantalum oxide (Ta2O x ), niobium oxide (Nb2O x ), indium oxide (In2O x ), tin oxide (SnO x ), tungsten oxide (WO x ), cerium oxide (CeO x ), titanium oxide (TiO x ), lanthanum titanate (Lax Ti y O z ), aluminum titanate (La x Al y O z ) or alumina-doped silicon dioxide (SiO2+Al2O3), a layer containing a nitride such as silicon nitride (Si3N4); and a layer containing a carbide such as tungsten carbide (WC). The other layer 12 may be a single layer or a multilayer of two or more layers. When the other layer 12 is a multilayer of two or more layers, the other layer 12 may be formed from a combination of multiple types of layers from the above-mentioned layers. Furthermore, the other layer 12 may be a layer containing a mixture of two or more types of compounds in the above-mentioned layers.

[0043] A method for forming the other layer 12 is not particularly limited. As the method for forming the other layer 12, for example, a dry film formation method such as a sputtering method, a vacuum evaporation method, or an ion plating method, or a wet film formation method such as a dipping method, a coating method, a spraying method, a spin coating method, a bar coating method, a printing method, or a flow coating method can be used.

[0044] When the composition, refractive index, thickness, number of layers, and the like of the other layer 12 are adjusted according to purposes and functions, a multilayer film having a specific function such as an anti-reflection layer, a half-mirror layer, a light-absorbing layer, an alkali diffusion preventing layer, an adhesive layer, an antistatic layer, or a heating layer can be formed.

[0045] The refractive index of layer 15 containing silicon oxide is 1.65 or less at a wavelength of 500 nm. If the refractive index is more than 1.65, the reflectance of the multilayer film may become too high.

[0046] The layer 15 containing silicon oxide is a layer containing silicon oxide (SiO x). The silicon oxide content in the silicon oxide-containing layer 15 is preferably 65 mass% or more with respect to the entire silicon oxide-containing layer 15. When the silicon oxide content in the silicon oxide-containing layer 15 falls within the above-mentioned range, the ability to maintain hydrophilicity in the dark is further improved.

[0047] In the layer 15 containing silicon oxide, the composition of silicon oxide is SiO x , and the “x” preferably represents 1.5 or more and 2.0 or less. When the value of “x” in the composition SiO x of the silicon oxide falls within the above-mentioned range, the refractive index of the silicon oxide-containing layer 15 can be set to 1.65 or less. Furthermore, a more transparent film can be obtained in a wavelength range from visible light to near-infrared light.

[0048] The layer 15 containing silicon oxide may, in addition to silicon oxide (SiO x ) may also contain alumina. In this case, the content of alumina in the silicon oxide-containing layer 15 is preferably 0.1 mass% or more and 10 mass% or less with respect to the entire silicon oxide-containing layer 15. When the silicon oxide-containing layer 15 contains 0.1 mass% to 10 mass% of alumina, the durability of the multilayer film, such as scratch resistance and moisture resistance, can be improved while maintaining the self-cleaning function with a photocatalyst and the ability to maintain hydrophilicity in the dark of the multilayer film.

[0049] The layer 15 containing silicon oxide may, in addition to silicon oxide (SiO x) may also contain cerium oxide. In this case, the content of cerium oxide in the silicon oxide-containing layer 15 is preferably 0.1 mass% or more and 35 mass% or less with respect to the entire silicon oxide-containing layer 15. When the silicon oxide-containing layer 15 contains 0.1 mass% to 35 mass% of cerium oxide, the self-cleaning function can be improved with a photocatalyst while maintaining the ability to maintain the hydrophilicity of the multilayer film. <<Dritte Ausführungsform> >

[0050] Fig.3 is a schematic sectional view illustrating a multilayer film according to a third embodiment of the present disclosure, which is disposed on the base material. In the third embodiment, a configuration example is shown in which the cerium oxide-containing layer 13 of the present disclosure is formed on the base material 11, the magnesium fluoride-containing layer 14 is further formed on the cerium oxide-containing layer 13, and a silicon dioxide-containing layer 16 is additionally formed on the magnesium fluoride-containing layer 14.

[0051] The base material 11, the cerium oxide-containing layer 13 of the present disclosure, and the magnesium fluoride-containing layer 14 are as described in the above-mentioned first and second embodiments. The silicon oxide-containing layer 15 described in the second embodiment may be disposed instead of all or part of the magnesium fluoride-containing layer 14. Furthermore, the cerium oxide-containing layer 13 of the present disclosure may be disposed directly on the base material 11 or via another layer 12 described in the second embodiment.

[0052] The silicon dioxide-containing layer 16 according to the present disclosure will be described. The multilayer film according to the present disclosure preferably includes the silicon dioxide-containing layer 16 with a thickness of 30 nm or less on top of the magnesium fluoride-containing layer 14 or the silicon oxide-containing layer 15. With this configuration, the ability to maintain hydrophilicity in the dark can be further improved.

[0053] The refractive index of the silicon dioxide-containing layer 16 is preferably 1.65 or less at a wavelength of 500 nm. If the refractive index of the silicon dioxide-containing layer 16 is above 1.65, the reflectance of the multilayer film may become too high. <<Vierte Ausführungsform> >

[0054] Fig.4 is a schematic sectional view illustrating a multilayer film according to a fourth embodiment of the present disclosure, which is disposed on the base material. In the fourth embodiment, a configuration example is shown in which the layer 13 containing cerium oxide of the present disclosure is formed on the base material 11, further a layer 17 formed of a first metal oxide of the present disclosure is disposed on the layer 13 containing cerium oxide, and additionally the layer 15 containing silicon oxide is formed on the layer 17 formed of a first metal oxide.

[0055] The base material 11, the cerium oxide-containing layer 13 of the present disclosure, and the silicon oxide-containing layer 15 are as described in the above-mentioned first to third embodiments. The magnesium fluoride-containing layer 14 described in the first embodiment may be disposed instead of all or part of the silicon oxide-containing layer 15. Furthermore, the cerium oxide-containing layer 13 of the present disclosure may be disposed directly on the base material 11 or via another layer 12 described in the second embodiment.

[0056] The first metal oxide layer 17 according to the present disclosure will be described. The first metal oxide layer 17 contains a metal oxide having a polycrystalline structure, and the thickness of the first metal oxide layer 17 is preferably 0.5 nm or more and 15 nm or less. In the multilayer film according to the present disclosure, it is preferable that the first metal oxide layer 17 of the present disclosure is disposed between the cerium oxide-containing layer 13 of the present disclosure and the magnesium fluoride-containing layer 14 or the silicon oxide-containing layer 15. When the multilayer film according to the present disclosure has such a configuration, the ability to maintain hydrophilicity in the dark and the self-cleaning function with a photocatalyst are improved.The metal oxide in the first metal oxide layer 17 may be a simple oxide or a complex oxide. Furthermore, the first metal oxide layer 17 may contain a plurality of two or more types of metal oxides or only one type of metal oxide.

[0057] The metal oxide in the first metal oxide layer 17 of the present disclosure described above is preferably cerium oxide represented by the composition CeO x (x=1.5 or more and 2.0 or less) or copper oxide represented by the composition CuO x (x=0.5 or more and 1.0 or less). By using these oxides, the ability to maintain hydrophilicity in the dark and the self-cleaning function with a photocatalyst can be further improved.

[0058] The thickness of the first metal oxide layer 17 of the present disclosure described above is preferably 0.5 nm or more and 15 nm or less. When the thickness of the first metal oxide layer 17 falls within the above-mentioned range, the ability to maintain hydrophilicity in the dark and the self-cleaning function with a photocatalyst can be further improved without adversely affecting light transmittance. <<Fünfte Ausführungsform> >

[0059] Fig.5 is a schematic sectional view illustrating a multilayer film according to a fifth embodiment of the present disclosure, which is disposed on the base material. In the fifth embodiment, a configuration example is shown in which the cerium oxide-containing layer 13 of the present disclosure is formed on the base material 11, further the silicon oxide-containing layer 15 is formed on the cerium oxide-containing layer 13, additionally a second metal oxide-containing layer 18 of the present disclosure is disposed on the silicon oxide-containing layer 15, and finally the silicon dioxide-containing layer 16 is formed on the second metal oxide-containing layer.

[0060] The base material 11, the cerium oxide-containing layer 13 of the present disclosure, and the silicon oxide-containing layer 15 are as described in the above-mentioned first to fourth embodiments. The magnesium fluoride-containing layer 14 described in the first embodiment may be disposed instead of all or part of the silicon oxide-containing layer 15. Furthermore, the cerium oxide-containing layer 13 of the present disclosure may be disposed directly on the base material 11 or via another layer 12 described in the second embodiment.

[0061] The second metal oxide layer 18 having a polycrystalline structure according to the present disclosure will be described. The second metal oxide layer 18 contains a metal oxide having a polycrystalline structure, and the thickness of the second metal oxide layer 18 is preferably 0.5 nm or more and 7 nm or less. In the multilayer film according to the present disclosure, it is preferable that the second metal oxide layer 18 is disposed between the silicon oxide layer 15 and the silicon dioxide layer 16. When the multilayer film according to the present disclosure has such a configuration, the self-cleaning function with a photocatalyst is further improved. The metal oxide in the second metal oxide layer 18 may be a simple oxide or a complex oxide.Furthermore, the layer 18 formed from a second metal oxide may contain two or more types of metal oxides or only one type of metal oxide.

[0062] In the fifth embodiment, a layer other than the second metal oxide layer 18 of the present disclosure may be disposed between the silicon oxide-containing layer 15 and the silicon dioxide-containing layer 16. One or more layers containing various substances, such as a fluoride and a nitride, may be disposed so that the ability to maintain hydrophilicity in the dark and the self-cleaning function with a photocatalyst are not adversely affected.

[0063] The metal oxide in the second metal oxide layer 18 of the present disclosure is preferably cerium oxide of the present disclosure having a cubic polycrystalline structure represented by the composition CeO x (x=1.5 or more and 2.0 or less) or copper oxide represented by the composition CuO x (x=0.5 or more and 1.0 or less). By using these oxides, the self-cleaning function of a photocatalyst can be particularly improved. <<Sechste Ausführungsform> >

[0064] Fig.6 is a schematic sectional view illustrating a multilayer film according to a sixth embodiment of the present disclosure, which is disposed on the base material. In the sixth embodiment, a configuration example is shown in which the other layer 12 is formed on the base material 11, and then the layer 13 containing cerium oxide of the present disclosure, the layer 17 formed of a first metal oxide of the present disclosure, the layer 15 containing silicon oxide, the layer 18 formed of a second metal oxide of the present disclosure, and the layer 16 containing silicon dioxide are formed in the order shown.

[0065] The base material 11 and each of the layers are as described in the above-mentioned first to fifth embodiments. The layer 14 containing magnesium fluoride described in the first embodiment may be arranged instead of all or part of the layer 15 containing silicon oxide.

[0066] Each layer of the multilayer film of the present disclosure may contain a different compound or atom as long as the present disclosure is not affected. That is, in addition to the impurities inevitably present in the composition of the present disclosure, a different compound or atom may be added as needed to the extent that the present disclosure is not affected. <<Optisches Element> >

[0067] Fig. 7 and Fig.8 are each a schematic view illustrating the configuration of an optical element according to an embodiment of the present disclosure. Fig. 7 is an illustration of a lens cover for a surveillance camera in which the multilayer film of the present disclosure is formed on the surface of a dome-shaped resin substrate 21. In addition, Fig. 8, eyeglasses are illustrated, including: eyeglass lenses 31, each being the optical element according to an embodiment of the present disclosure, and an eyeglass frame 32. The multilayer film of the present disclosure is formed on each surface of the eyeglass lens 31.

[0068] The multilayer film of the present disclosure can be used as an optical thin film, such as an anti-reflection film, various types of multilayer films for optical filters, or a multilayer film for optical mirrors. Furthermore, the multilayer film of the present disclosure can be used, for example, in optical elements such as an optical filter, an optical lens, a daylight lens, an optical film, an optical prism, an eyeglass lens, a photographic lens, a surveillance camera cover, a vehicle camera cover, a vehicle sensor cover, a vehicle door mirror, a glass panel, a condenser lens, a display cover glass, a touch panel, and various types of films or covers for protecting optical elements.When the surface of the base material 11 except the surface on which the above-mentioned layers are arranged is coated with a layer having the composition, refractive index, thickness, number of layers, and the like according to the purposes and functions, the multilayer film can be formed as an optical element to which a specific function is assigned, such as a mirror layer, a half-mirror layer, a light-absorbing layer, a transparent heating layer, or an anti-reflection layer. <<Verfahren zum Herstellen von Mehrschichtfilm> >

[0069] Furthermore, a method for producing a multilayer film according to the present disclosure is characterized in that a multilayer film is formed by a method including at least the following steps (A) and (B): (A) a step of forming the layer 13 containing cerium oxide on the base material directly or through another layer by a vacuum deposition method; and (B) a step of forming the layer 14 containing magnesium fluoride or the layer 15 containing silicon oxide on the above-mentioned layer 13 containing cerium oxide of the present disclosure by the vacuum evaporation method.

[0070] The cerium oxide-containing layer 13 formed in step (A) contains cerium oxide having a cubic polycrystalline structure, and the thickness of the cerium oxide-containing layer 13 formed in step (A) is 70 nm or more and 300 nm or less. Moreover, the thickness of the silicon oxide-containing layer 14 formed in step (B) and the magnesium fluoride-containing layer 15 formed in step (B) are each 50 nm or more and 240 nm or less, and the refractive index of the silicon oxide-containing layer 14 formed in step (B) and the magnesium fluoride-containing layer 15 formed in step (B) is each 1.65 or less at a wavelength of 500 nm.

[0071] The temperature of the base material in the vapor deposition process is preferably a temperature at which cerium oxide crystallizes. Although it depends on the heat-resistant temperature of a base material to be used and other film-forming conditions, the temperature can generally be selected within a range of 0°C or more to 500°C or less.

[0072] The evaporation method used in the vacuum vapor deposition process is not limited, as long as it is a method that evaporates a film-forming material. Evaporation means such as an electron gun, resistance heating, or a laser can be used as the evaporation method. Furthermore, ion-assisted deposition, plasma-assisted deposition, or the like can be used in combination with the evaporation means as needed.

[0073] The multilayer film of the present disclosure can be suitably produced by the above-mentioned method. Examples

[0074] The present disclosure will be described in more detail below with reference to examples. The present disclosure is by no means limited to the following examples.

[0075] The materials used to prepare and evaluate multilayer films are described below with examples. (base material)

[0076] Flat plates made of the materials described below were used as base materials. When the base material temperature was 350°C at the time of deposition, two types of base materials were used: borosilicate glass and synthetic quartz. At any other base material temperature, all of the following base materials were used. -Borosilicate glass: thickness of 3 mm -Synthetic quartz: thickness of 3 mm -Polycarbonate resin: thickness of 2 mm -Polymethyl methacrylate resin: thickness of 2 mm (Film-forming material)

[0077] The materials described below were used. -Ce: granular, purity 99.9%. -CeO2: columnar, purity of 99.9 -La2O3: columnar, purity 99.9%. -Sm2O3: columnar, purity of 99.9%. -SiO: granular, purity 99.9%. -SiO2: granular, purity of 99.9%. -Al2O3: granular, purity 99.9%. -MgF2: granular, purity 99.9%. -CaF2: granular, purity of 99.9 -ZrO2: columnar, purity 99.9 -Ti3O5: granular, purity 99.9%. -CuO: granular, purity of 99.9% -Cu2O: granular, purity of 99.9% -O2: Gas, purity 99.999%. (reagents and the like) -Pure water -Stearic acid: JIS K 8585 Special Grade, purity 99.9%. -Heptane: JIS K 9701 Special Grade, purity 99.9%. (Production of multilayer film)

[0078] A film-forming method and film-forming conditions for producing a multilayer film common to Examples and Comparative Examples are described. A vacuum evaporation apparatus (dome diameter: ϕ900 mm, vapor deposition distance: 890 mm) was used as the film-forming apparatus. The above-mentioned film-forming materials and various types of clean base materials were placed in the apparatus, and the apparatus was vacuumed to a degree of 7.0 × 10 -4Pa), at which film formation was initiated. The substrate temperature at the time of film formation was -10°C or more and 350°C or less. Next, a multilayer film of the film-forming materials was formed on the adjusted base material by a vacuum evaporation method, as shown in Table 1 (Table 1-1 to Table 1-4), to prepare a test piece. In this case, the material for each layer was deposited at a deposition rate of 0.5 nm / sec. A film formed of two components, such as CeO2+Al2O3 or SiO2+Al2O3, was formed by a double vacuum evaporation method in which two types of film-forming materials were placed in heating sources at two locations, and the film-forming materials were evaporated simultaneously.In each of Examples 1 to 47 and Comparative Examples 1 to 8, there was no significant difference between the multilayer films obtained by changing the type of base material, and therefore only one example is listed in Table 1. Furthermore, the term "polycrystalline" in Table 1 means a cubic polycrystalline structure.

[0079] The individual conditions in the examples and comparative examples for the production of a multilayer film are described below. [Examples 1 to 3]

[0080] A CeO2 film was formed on each of the various types of base materials heated to 350°C using CeO2 as the film-forming material. Subsequently, a MgF2 film was formed on top of the film using MgF2 as the film-forming material to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 4 to 6]

[0081] A CeO2 film was formed on each of the various types of base materials heated to 350°C using CeO2 as the film-forming material. Subsequently, an SiO2 film was formed on it using SiO2 as the film-forming material to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 7]

[0082] A CEO 1,8A film was formed on each of the various types of base materials heated to 350°C using CeO2 and Ce as film-forming materials. Subsequently, a MgF2 film was formed thereon using MgF2 as the film-forming material to prepare a multilayer film. Other film-forming conditions are described in the section (Preparation of Multilayer Film). [Example 8]

[0083] A CEO 1,8 A film was formed on each of various types of base materials heated to 350°C by using CeO2 and Ce as film-forming materials. Subsequently, an SiO2 film was formed thereon by using SiO2 as the film-forming material to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 9]

[0084] A CEO 1,5(85%) + La2O3 (15%) film was formed on each of the various types of base materials heated to 350°C by using two materials of CeO2 and La2O3 as film-forming materials to obtain the composition shown in Table 1. Subsequently, a MgF2 film was formed thereon by using MgF2 as the film-forming material to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 10]

[0085] A CEO 1,8+Al2O3 films were formed on each of various types of base materials heated to 350°C by using two materials, CeO2 and Al2O3, as film-forming materials to obtain the composition shown in Table 1. Subsequently, a SiO2 film was formed thereon by using SiO2 as the film-forming material to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 11]

[0086] A CEO 1,5A +Sm2O3 film was formed on each of various types of base materials heated to 350°C by using two materials, CeO2 and Sm2O3, as film-forming materials to obtain the composition shown in Table 1. Subsequently, a SiO2 film was formed thereon by using SiO2 as the film-forming material to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 12 to 17]

[0087] A CeO2 film was formed on each of various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, a SiO2+Al2O3 film was formed thereon using SiO2 and Al2O3 as film-forming materials to obtain the composition shown in Table 1, thereby preparing a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 18 to 23]

[0088] A CeO2 film was formed on various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, a SiO2+CeO2 film was formed thereon by using two materials, SiO2 and CeO2, as film-forming materials to obtain the composition shown in Table 1, thereby preparing a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 24 and 25]

[0089] A CeO2 film was formed on various types of base materials heated to 350°C using CeO2 as the film-forming material. Subsequently, an MgF2 film was formed thereon using MgF2 as the film-forming material. Furthermore, an SiO2 film was formed thereon using SiO2 to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 26 and 27]

[0090] A CeO2 film was formed on various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, an SiO2+Al2O3 film was formed thereon by using two materials of SiO2 and Al2O3 as the film-forming materials to obtain the composition shown in Table 1. Furthermore, an SiO2 film was formed thereon by using SiO2 to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 28 to 30]

[0091] A CeO2 film was formed on each of various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, an SiO2+CeO2 film was formed thereon by using SiO2 and CeO2 as the film-forming materials to obtain the composition shown in Table 1. Furthermore, an SiO2 film was formed thereon by using SiO2 to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 31 to 33]

[0092] A CeO2 film was formed on various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, a CuO film was formed thereon by using CuO as the film-forming material. Furthermore, an SiO2+CeO2 film was formed thereon by using SiO2 and CeO2 as film-forming materials to obtain the composition shown in Table 1, thereby preparing a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 34]

[0093] A CEO 1,5A +La2O3 film was formed on each of various types of base materials heated to 350°C by using two materials of CeO2 and La2O3 as film-forming materials to obtain the composition shown in Table 1. Subsequently, a CeO2 film was formed thereon by using CeO2 as the film-forming material. Further, a SiO2+CeO2 film was formed thereon by using two materials of SiO2 and CeO2 as film-forming materials to obtain the composition shown in Table 1, thereby producing a multilayer film. Other film-forming conditions are as described in the section (Production of Multilayer Film). [Example 35]

[0094] A CEO 1,8+Al2O3 film was formed on each of various types of base materials heated to 350°C by using CeO2 and Al2O3 as film-forming materials to obtain the composition shown in Table 1. Subsequently, a CeO2 film was formed thereon by using CeO2 as the film-forming material. Further, an SiO2+CeO2 film was formed thereon by using SiO2 and CeO2 as film-forming materials to obtain the composition shown in Table 1, thereby preparing a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 36]

[0095] A CEO 1,5A +Sm2O3 film was formed on each of various types of base materials heated to 350°C by using two materials of CeO2 and Sm2O3 as film-forming materials to obtain the composition shown in Table 1. Subsequently, a CeO2 film was formed thereon by using CeO2 as the film-forming material. Further, a SiO2+CeO2 film was formed thereon by using SiO2 and CeO2 as film-forming materials to obtain the composition shown in Table 1, thereby preparing a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 37 to 39]

[0096] A CeO2 film was formed on each of various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, an SiO2+CeO2 film was formed thereon by using SiO2 and CeO2 as film-forming materials to obtain the composition shown in Table 1. Furthermore, a CuO film was formed thereon by using CuO. Furthermore, an SiO2 film was formed thereon by using SiO2 to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Examples 40 to 42]

[0097] A CeO2 film was formed on each of various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, an SiO2+CeO2 film was formed thereon by using SiO2 and CeO2 as the film-forming materials to obtain the composition shown in Table 1. Further, a CeO2 film was formed thereon by using CeO2. Additionally, an SiO2 film was formed thereon by using SiO2 to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 43]

[0098] A multilayer film was prepared in the same manner as in Example 5, except that the heating temperature of the substrate was changed from 350°C in Example 5 to no heating (room temperature). The temperature near the substrate during film formation averaged 28°C. [Example 44]

[0099] A multilayer film was prepared in the same manner as in Example 16, except that the heating temperature of the substrate was changed from 350°C in Example 16 to no heating (room temperature). The temperature near the substrate during film formation averaged 28°C. [Example 45]

[0100] A multilayer film was prepared in the same manner as in Example 29, except that the heating temperature of the substrate was changed from 350°C in Example 29 to no heating (room temperature). The temperature near the substrate during film formation averaged 29°C. [Example 46]

[0101] A multilayer film was prepared in the same manner as in Example 41, except that the heating temperature of the substrate was changed from 350°C in Example 41 to no heating (room temperature). The temperature near the substrate during film formation averaged 33°C. [Example 47]

[0102] A CeO2 film was formed on each of the various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, a CuO film was formed thereon using CuO. Further, a SiO1,5 +CeO2 film was formed by using SiO and CeO2 as film-forming materials at a film-forming rate of 2.5 nm / s to obtain the composition shown in Table 1. Furthermore, a CuO film was formed thereon by using CuO. Furthermore, an SiO2 film was formed thereon by using a SiO2 melt to prepare a multilayer film. Other film-forming conditions are as described in the section (Preparation of Multilayer Film). [Example 48]

[0103] A CeO2 film was formed on each of various types of base materials heated to 350°C by using CeO2 as the film-forming material. Subsequently, a CuO film was formed thereon by using CuO. Furthermore, a MgF2+CaF2 film was formed thereon by using a MgF2 melt and a CaF2 melt as film-forming materials to obtain the composition shown in Table 1. Other film-forming conditions are as described in the section (Preparation of Multilayer Films). [Comparison example 1]

[0104] An anatase-type TiO2 layer was formed instead of the cubic polycrystalline CeO2 layer in Example 2. In this case, a film was formed by using Ti3O5 as the film-forming material while introducing an O2 gas so that the degree of vacuum was 1.8 × 10 -2Pa using an automatic pressure control (APC) device. A multilayer film was prepared by the same procedure as in Example 2, except for the above points.

[0105] Here, the APC device was used to set a gas partial pressure in the vapor deposition device. [Comparison example 2]

[0106] A film was formed while introducing an O2 gas by changing the film-forming material to a Ti3O5 material in the same manner as in Comparative Example 1, so that an anatase-type TiO2 layer was obtained instead of the cubic polycrystalline CeO2 layer in Example 5. A multilayer film was prepared by the same method as in Example 5, except for the above points. [Comparison example 3]

[0107] A film was formed by changing the substrate temperature to -10°C, so that an amorphous CeO2 layer was obtained instead of the cubic polycrystalline CeO2 layer in Example 2. A multilayer film was prepared by the same method as in Example 2, except for the above points. [Comparison example 4]

[0108] A film was formed by changing the substrate temperature to -10°C, so that an amorphous CeO2 layer was obtained instead of the cubic polycrystalline CeO2 layer in Example 5. A multilayer film was prepared by the same method as in Example 5, except for the above points. [Comparison Example 5]

[0109] The CeO2 layer and the MgF2 layer in Example 1 were each prepared so that their thickness was smaller than that in Example 1. A multilayer film was prepared by the same method as in Example 1 except for the above points. [Comparison Example 6]

[0110] The CeO2 layer and the MgF2 layer in Example 3 were each prepared so that their thickness was larger than that in Example 3. A multilayer film was prepared by the same method as in Example 3 except for the above points. [Comparison Example 7]

[0111] The CeO2 layer and the SiO2 layer in Example 4 were each prepared so that their thickness was smaller than that in Example 4. A multilayer film was prepared by the same method as in Example 4 except for the above points. [Comparison Example 8]

[0112] The CeO2 layer and the SiO2 layer in Example 6 were each prepared so that their thickness was larger than that in Example 6. A multilayer film was prepared by the same method as in Example 6 except for the above points. (Measurement of thickness)

[0113] The thickness of each layer of the multilayer films of Examples and Comparative Examples was measured using spectroscopic ellipsometry (ESM300, manufactured by JA Woollam Co.). When the number of layers and thickness were complicated and the analysis was difficult, the analysis was performed using the refractive index of a single-layer film obtained separately during film formation of the same batch, and the like. (Measurement of composition)

[0114] The composition of the layer formed from two components, such as SiO2+CeO2 or SiO2+Al2O3, in the multilayer films of Examples and Comparative Examples was determined by measurement with a wavelength-dispersive fluorescence spectroscopic analyzer (ZSX Primus II, manufactured by Rigaku Corporation). (Measurement of crystallinity)

[0115] Each of the multilayer films of Examples and Comparative Examples was measured in a range of 2θ=20° to 100° with an XRD diffraction device (Smart Lab, manufactured by Rigaku Corporation), and the layers were identified and the crystallinity was determined based on the intensity of the diffraction line and the like. (Evaluation of hydrophilicity maintenance)

[0116] Each of the multilayer films of the examples and comparative examples was stored in the dark for 180 days, and then its contact angle was measured with water. A CA-X150 model from Kyowa Interface Science Co., Ltd. was used as the contact angle measuring device. 2.5 mL of pure water was dropped onto a test specimen using a microsyringe, and the contact angle was determined 5 seconds after dropping using a θ / 2 method.

[0117] The hydrophilicity of a surface can be quantified by its contact angle with water. Generally, a case where the contact angle is less than 20° is called "hydrophilic," and a case where the contact angle is less than 10° is called "superhydrophilic." Subsequently, a case where the contact angle is less than 10° is rated as [A], a case where the contact angle is 10° or more and less than 20° is rated as [B], and a case where the contact angle is 20° or more is rated as [C]. (Evaluation of self-cleaning performance)

[0118] Stearic acid was coated onto each of the multilayer films of Examples and Comparative Examples using a heptane solution of stearic acid (0.3 mass%) according to JIS R1753-1 and dried in a dryer at 70°C for 30 minutes. The contact angle of the stearic acid-coated test specimen was measured using the same method as described in the section (Evaluation of Hydrophilicity Maintenance), and it was found to be 20° or more. The test specimen was then irradiated with ultraviolet light for 6 hours, and the contact angle was measured again to determine the water contact angle after ultraviolet light irradiation. A blue fluorescent lamp (FL20SBL-B, manufactured by Hotalux, Ltd.) was used as the ultraviolet light source. The test specimen was irradiated with ultraviolet light at an illuminance of 2.0 mW / cm. 2 irradiated.

[0119] In the same manner as the above evaluation in the section (Evaluation of Hydrophilicity Maintenance), a case where the contact angle was less than 10° was evaluated as [A], a case where the contact angle was 10° or more and less than 20° was evaluated as [B], and a case where the contact angle was 20° or more was evaluated as [C].

[0120] The evaluation results in the sections (Evaluation of Hydrophilicity Maintenance) and (Evaluation of Self-Cleaning Performance) are shown in Table 1. In each of the examples and comparative examples, the evaluation results were the same regardless of the type of base material. Table 1-1 Temperature of base material at the time of evaporation [°C] Layer 13, which contains cerium oxide Layer 17, which is formed from the first metal oxide composition Crystallinity Thickness [nm] composition Thickness [nm] Example 1 350 CeO2 Polycrystalline 72 - - Example 2 350 CeO2 Polycrystalline 123 - - Example 3 350 CeO2 Polycrystalline 296 - - Example 4 350 CeO2 Polycrystalline 72 - - Example 5 350 CeO2 Polycrystalline 123 - - Example 6 350 CeO2 Polycrystalline 291 - - Example 7 350 CeO 1.8 Polycrystalline 125 - - Example 8 350 CeO 1.8 Polycrystalline 125 - - Example 9 350 CeO 1,5 (85%)+La2O3 (15%) Polycrystalline 179 - - Example 10 350 CeO 1,8 (85%)+Al2O3 (15%) Polycrystalline 190 - - Example 11 350 CeO 1,5 (85%)+Sm2O3 (15%) Polycrystalline 177 - - Example 12 350 CeO2 Polycrystalline 71 - - Example 13 350 CeO2 Polycrystalline 124 - - Example 14 350 CeO2 Polycrystalline 298 - - Example 15 350 CeO2 Polycrystalline 70 - - Example 16 350 CeO2 Polycrystalline 121 - - Example 17 350 CeO2 Polycrystalline 297 - - Example 18 350 CeO2 Polycrystalline 74 - - Example 19 350 CeO2 Polycrystalline 125 - - Example 20 350 CeO2 Polycrystalline 298 - - Example 21 350 CeO2 Polycrystalline 72 - - Example 22 350 CeO2 Polycrystalline 128 - - Example 23 350 CeO2 Polycrystalline 300 - - Example 24 350 CeO2 Polycrystalline 71 - - Example 25 350 CeO2 Polycrystalline 130 - - Example 26 350 CeO2 Polycrystalline 299 - - Example 27 350 CeO2 Polycrystalline 72 - - Table 1-2 Temperature of base material at the time of evaporation [°C] Layer 13, which contains cerium oxide Layer 17, which is formed from the first metal oxide composition Crystallinity Thickness [nm] composition Thickness [nm] Example 28 350 CeO2 Polycrystalline 122 - - Example 29 350 CeO2 Polycrystalline 298 - - Example 30 350 CeO2 Polycrystalline 72 - - Example 31 350 CeO2 Polycrystalline 121 CuO 0,5 Example 32 350 CeO2 Polycrystalline 127 CuO 5 Example 33 350 CeO2 Polycrystalline 123 CuO 15 Example 34 350 CeO 1,5 (85%)+La2O3 (15%) Polycrystalline 150 CeO2 0,6 Example 35 350 CeO 1,8 (85%)+Al2O3 (15%) Polycrystalline 156 CeO2 5 Example 36 350 CeO 1,5 (85%)+Sm2O3 (15%) Polycrystalline 151 CeO2 15 Example 37 350 CeO2 Polycrystalline 120 - - Example 38 350 CeO2 Polycrystalline 124 - - Example 39 350 CeO2 Polycrystalline 129 - - Example 40 350 CeO2 Polycrystalline 123 - - Example 41 350 CeO2 Polycrystalline 126 - - Example 42 350 CeO2 Polycrystalline 122 - - Example 43 28 CeO2 Polycrystalline 131 - - Example 44 28 CeO2 Polycrystalline 133 - - Example 45 29 CeO2 Polycrystalline 300 - - Example 46 33 CeO2 Polycrystalline 129 - - Example 47 350 CeO2 Polycrystalline 115 CuO 10 Example 48 350 CeO2 Polycrystalline 120 CuO 7 Comparison example 1 350 TiO2 Polycrystalline 114 - - Comparison example 2 350 TiO2 Polycrystalline 111 - - Comparison example 3 -10 CeO2 Amorphous 139 - - Comparison example 4 -10 CeO2 Amorphous 141 - - Comparison example 5 350 CeO2 Polycrystalline 65 - - Comparison example 6 350 CeO2 Polycrystalline 333 - - Comparison example 7 350 CeO2 Polycrystalline 65 - - Comparison example 8 350 CeO2 Polycrystalline 323 - - Table 1-3 Layer 14 containing magnesium fluoride or layer 15 containing silicon oxide Layer 18, which is formed from a second metal oxide composition Refractive index [-] Thickness [nm] composition Thickness [nm] Example 1 MgF2 1,38 50 - - Example 2 MgF2 1,38 97 - - Example 3 MgF2 1,38 240 - - Example 4 SiO2 1,47 51 - - Example 5 SiO2 1,47 96 - - Example 6 SiO2 1,47 236 - - Example 7 MgF2 1,38 98 - - Example 8 SiO2 1,47 93 - - Example 9 MgF2 1,38 133 - - Example 10 SiO2 1,47 130 - - Example 11 SiO2 1,47 131 - - Example 12 SiO2 (99.9%) + Al2O3 (0.1%) 1,47 52 - - Example 13 SiO2 (99.9%) + Al2O3 (0.1%) 1,47 92 - - Example 14 SiO2 (99.9%) + Al2O3 (0.1%) 1,47 240 - - Example 15 SiO2 (90%) + Al2O3 (10%) 1,53 52 - - Example 16 SiO2 (90%) + Al2O3 (10%) 1,53 89 - - Example 17 SiO2 (90%) + Al2O3 (10%) 1,53 237 - - Example 18 SiO2 (99.9%) + CeO2 (0.1%) 1,48 53 - - Example 19 SiO2 (99.9%) + CeO2 (0.1%) 1,48 92 - - Example 20 SiO2 (99.9%) + CeO2 (0.1%) 1,48 237 - - Example 21 SiO2 (65%) + CeO2 (35%) 0,64 51 - - Example 22 SiO2 (65%) + CeO2 (35%) 1,65 87 - - Example 23 SiO2 (65%) + CeO2 (35%) 1,65 236 - - Example 24 MgF2 1,38 50 - - Example 25 MgF2 1,38 97 - - Example 26 SiO2 (97.5%) + Al2O3 (2.5%) 1,48 238 - - Example 27 SiO2 (95%)+Al2O3 (5%) 1,49 50 - - Table 1-4 Layer 14 containing magnesium fluoride or layer 15 containing silicon oxide Layer 18, which is formed from a second metal oxide composition Refractive index [-] Thickness [nm] composition Thickness [nm] Example 28 SiO2 (95%) + CeO2 (5%) 1,48 90 - - Example 29 SiO2 (95%) + CeO2 (5%) 1,65 240 - - Example 30 SiO2 (95%) + CeO2 (5%) 1,51 51 - - Example 31 SiO2 (95%) + CeO2 (5%) 1,51 90 - - Example 32 SiO2 (95%) + CeO2 (5%) 1,51 91 - - Example 33 SiO2 (95%) + CeO2 (5%) 1,51 92 - - Example 34 SiO2 (95%) + CeO2 (5%) 1,51 109 - - Example 35 SiO2 (95%) + CeO2 (5%) 1,51 121 - - Example 36 SiO2 (95%) + CeO2 (5%) 1,51 111 - - Example 37 SiO2 (95%) + CeO2 (5%) 1,51 91 CuO 0,5 Example 38 SiO2 (95%) + CeO2 (5%) 1,51 89 CuO 4 Example 39 SiO2 (95%) + CeO2 (5%) 1,51 88 CuO 7 Example 40 SiO2 (95%) + CeO2 (5%) 1,51 91 CeO2 0,6 Example 41 SiO2 (95%) + CeO2 (5%) 1,51 91 CeO2 4 Example 42 SiO2 (95%) + CeO2 (5%) 1,51 90 CeO2 7 Example 43 SiO2 1,47 93 - - Example 44 SiO2 (90%) + Al2O3 (10%) 1,53 88 - - Example 45 SiO2 (95%) + CeO2 (5%) 1,65 239 - - Example 46 SiO2 (95%) + CeO2 (5%) 1,51 82 CeO2 5 Example 47 SiO2 (95%) + CeO2 (5%) 1,51 75 CeO2 3 Example 48 MgF2 (85%) + CaF2 (15%) 1,36 93 - - Comparison example 1 MgF2 1,38 98 - - Comparison example 2 SiO2 1,47 93 - - Comparison example 3 MgF2 1,36 99 - - Comparison example 4 SiO2 1,44 95 - - Comparison example 5 MgF2 1,38 47 - - Comparison example 6 MgF2 1,38 256 - - Comparison example 7 SiO2 1,47 46 - - Comparison example 8 SiO2 1,47 261 - - Table 1-5 Layer 16, which contains silicon dioxide Assessment of hydrophilicity maintenance Evaluation of self-cleaning performance composition Refractive index [-] Thickness [nm] Contact angle after storage in the dark [°] Evaluation Contact angle after UV irradiation [°] Evaluation Example 1 - - - 11 B 8 A Example 2 - - - 7 A 7 A Example 3 - - - 6 A 7 A Example 4 - - - 11 B 8 A Example 5 - - - 6 A 7 A Example 6 - - - 5 A 8 A Example 7 - - - 7 A 5 A Example 8 - - - 6 A 6 A Example 9 - - - 7 A 6 A Example 10 - - - 6 A 7 A Example 11 - - - 5 A 7 A Example 12 - - - 13 B 7 A Example 13 - - - 6 A 7 A Example 14 - - - 5 A 7 A Example 15 - - - 12 B 7 A Example 16 - - - 7 A 7 A Example 17 - - - 6 A 7 A Example 18 - - - 12 B 6 A Example 19 - - - 6 A 6 A Example 20 - - - 6 A 6 A Example 21 - - - 16 B 7 A Example 22 - - - 5 A 5 A Example 23 - - - 5 A 5 A Example 24 SiO2 1,45 1 15 B 7 A Example 25 SiO2 1,47 29 5 A 7 A Example 26 SiO2 1,47 29 5 A 5 A Example 27 SiO2 1,45 2 14 B 5 A Table 1-6 Layer 16, which contains silicon dioxide Assessment of hydrophilicity maintenance Evaluation of self-cleaning performance composition Refractive index [-] Thickness [nm] composition Refractive index [-] Thickness [nm] composition Example 28 SiO2 1,45 1 5 A 5 A Example 29 SiO2 1,47 30 5 A 5 A Example 30 SiO2 1,47 30 10 B 5 A Example 31 - - - 7 A 5 A Example 32 - - - 8 A 4 A Example 33 - - - 9 A 3 A Example 34 - - - 5 A 5 A Example 35 - - - 6 A 5 A Example 36 - - - 6 A 5 A Example 37 SiO2 1,46 15 7 A 4 A Example 38 SiO2 1,46 15 9 A 4 A Example 39 SiO2 1,46 15 9 A 3 A Example 40 SiO2 1,46 15 5 A 5 A Example 41 SiO2 1,46 15 6 A 4 A Example 42 SiO2 1,46 15 6 A 3 A Example 43 - - - 5 A 3 A Example 44 - - - 5 A 4 A Example 45 SiO2 1,47 30 5 A 4 A Example 46 SiO2 1,46 15 5 A 5 A Example 47 SiO2 1,46 15 9 A 2 A Example 48 - - - 9 A 5 A Comparison example 1 - - - 49 C 55 C Comparison example 2 - - - 33 C 41 C Comparison example 3 - - - 14 B 61 C Comparison example 4 - - - 12 B 41 C Comparison example 5 - - - 33 C 50 C Comparison example 6 - - - 5 A 51 C Comparison example 7 - - - 32 C 52 C Comparison example 8 - - - 4 A 53 C [Example 49]

[0121] A flat glass sheet containing the multilayer film obtained in Example 3 was processed and attached to the outside of a near-infrared sensor of a commercial vehicle to serve as a protective cover for the sensor. [Example 50]

[0122] A dome-shaped transparent substrate made of a polymethyl methacrylate resin was used as the base material, and a SiO2 film (200 nm) was formed as the first layer by using SiO as the film-forming material. A ZrO2 film (15 nm) was formed as the second layer by using ZrO2. A SiO2 film (35 nm) was formed as the third layer by using SiO2. A CeO2 film (117 nm) was formed as the fourth layer by using CeO2. A CuO film (7 nm) was formed as the fifth layer by using CuO. A SiO2 (95%) + CeO2 (5%) film (80 nm) was formed as the sixth layer by using SiO2 and CeO2. A SiO2 film (13 nm) was formed as the final seventh layer by using SiO2. In this way, a multilayer film was fabricated.During film formation, the multilayer film was formed at a deposition rate of 0.5 nm / s, while the base material was subjected to planetary rotation without heating. Additionally, during the formation of the first, fourth, fifth, and sixth layers, ion-assisted deposition was performed using an RF ion source under the condition of an O2 gas flow rate of 40 sccm. In this case, ion-assisted deposition was performed in the first layer under the conditions of an accelerating voltage of 250 V and an accelerating current of 250 mA. In the fourth, fifth, and sixth layers, ion-assisted deposition was performed under the conditions of an accelerating voltage of 500 V and an accelerating current of 500 mA. The resulting dome-shaped resin substrate with the multilayer film was attached to a surveillance camera to serve as a cover for the surveillance camera.

[0123] The surveillance camera with the manufactured cover was stored in a dark packaging container for 3 months. After that, the surveillance camera was installed outdoors when it rained at night. Even when water adhered to the surveillance camera due to the rain, the water droplets spread wetly on the cover, ensuring satisfactory visibility. Even after the moisture dried, no water stains remained, so visibility remained satisfactory. Even when it rained 6 days after the surveillance camera was installed outdoors, the water droplets spread wetly on the cover, ensuring satisfactory visibility. [Example 51]

[0124] A resin substrate (MR-8, manufactured by Mitsui Chemicals, Inc.) with a silicon-based hard film formed thereon was used as the base material, and an Al2O3 film (82 nm) was formed as the first layer using Al2O3. A CeO2 film (119 nm) was formed as the second layer using CeO2. A CuO film (7 nm) was formed as the third layer using CuO. A SiO2 (95%) + CeO2 (5%) film (79 nm) was formed as the fourth layer using SiO2 and CeO2. A SiO2 film (15 nm) was formed as the final fifth layer using SiO2. In this way, a multilayer film was fabricated. During film formation, the multilayer film was formed at an evaporation rate of 0.5 nm / s by heating the base material to a temperature of 80°C. The resulting resin substrate with the multilayer film was processed and mounted on an eyeglass frame to produce a pair of glasses.The resulting glasses were stored in a dark place in a case for three months. Afterward, when water droplets from the air were allowed to adhere to a lens, the water droplets spread wetly on the lens, ensuring satisfactory vision. The contact angle of the water in this case was 5°. Furthermore, even after the moisture dried, no water stains remained, thus maintaining satisfactory vision. Industrial applicability

[0125] The multilayer film of the present disclosure can be used, for example, in optical elements such as an optical filter, an optical lens, a daylight lens, an optical film, an optical prism, an ophthalmic lens, a photographic lens, a vehicle door mirror, a glass panel, a condenser lens, a display cover glass, a touch panel, and various types of films or covers for protecting optical elements such as a surveillance camera cover, a vehicle camera cover, and a vehicle sensor cover.

[0126] Furthermore, the optical element of the present disclosure can be used, for example, in optical devices such as a digital camera, a digital video camera, an action camera, an endoscope, a lens barrel, eyeglasses, a sensor, binoculars, a telescope, a surveillance camera, a vehicle camera, a smartphone, a tablet PC, a weather camera, a live camera, safety goggles, swimming goggles, a head-mounted display, sunglasses, smart glasses, a face shield, a helmet shield, a vehicle mirror, and a bathroom mirror, or covers for protecting the optical devices.

[0127] The present disclosure includes the following embodiments. (1) A multilayer film comprising: a layer containing cerium oxide; and a layer containing silicon oxide or a layer containing magnesium fluoride formed directly or by means of another layer on the layer containing cerium oxide, wherein the layer containing cerium oxide contains cerium oxide having a cubic polycrystalline structure, wherein the layer containing cerium oxide has a thickness of 70 nm or more and 300 nm or less, wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a thickness of 50 nm or more and 240 nm or less, and wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a refractive index of 1.65 or less at a wavelength of 500 nm. (2) The multilayer film according to (1), wherein the cerium oxide in the layer containing cerium oxide has a composition CeO x where x=1.5 or more and 2.0 or less. (3) The multilayer film according to (1) or (2), wherein a content of the cerium oxide in the layer containing cerium oxide is 85 mass% or more with respect to the entire layer containing cerium oxide. (4) The multilayer film according to any one of (1) to (3), wherein a content of the silicon oxide in the silicon oxide-containing layer is 65 mass% or more with respect to the entire silicon oxide-containing layer. (5) Multilayer film according to (4), wherein the layer containing silicon oxide contains aluminum oxide, and wherein a content of the aluminum oxide in the silicon oxide-containing layer is 0.1 mass% or more and 10 mass% or less with respect to the entire silicon oxide-containing layer. (6) Multilayer film according to (4), wherein the layer containing silicon oxide contains cerium oxide, and wherein a content of the cerium oxide in the layer containing silicon oxide is 0.1 mass% or more and 35 mass% or less. (7) The multilayer film according to any one of (1) to (6), further comprising a layer containing silicon dioxide formed directly or via another layer on the layer containing silicon oxide or the layer containing magnesium fluoride, wherein the layer containing silicon dioxide has a thickness of 30 nm or less. (8) The multilayer film according to any one of (1) to (7), further comprising a layer formed of a first metal oxide between the layer containing cerium oxide and the layer containing silicon oxide or the layer containing magnesium fluoride, wherein the layer formed of a first metal oxide contains a metal oxide having a polycrystalline structure, and wherein the layer formed of a first metal oxide has a thickness of 0.5 nm or more and 15 nm or less. (9) The multilayer film according to (8), wherein the metal oxide is copper oxide represented by a composition CuO x , where x=0.5 or more and 1.0 or less. (10) The multilayer film according to (8), wherein the metal oxide is cerium oxide having a cubic polycrystalline structure represented by a composition CeO x , where x=1.5 or more and 2.0 or less. (11) The multilayer film according to (7), further comprising a layer formed of a second metal oxide between the layer containing silicon oxide or the layer containing magnesium fluoride and the layer containing silicon dioxide, wherein the layer formed of a second metal oxide contains a metal oxide having a polycrystalline structure, and wherein the layer formed of a second metal oxide has a thickness of 0.5 nm or more and 7 nm or less. (12) The multilayer film according to (11), wherein the metal oxide is copper oxide represented by a composition CuO x , where x=0.5 or more and 1.0 or less. (13) The multilayer film according to (11), wherein the metal oxide is cerium oxide having a cubic polycrystalline structure represented by a composition CeO x , where x=1.5 or more and 2.0 or less. (14) An optical element comprising the multilayer film according to any one of (1) to (13). (15) A method for producing a multilayer film, the method comprising: Forming a layer containing cerium oxide on a base material directly or through another layer by a vacuum deposition method; and Forming a layer containing silicon oxide or a layer containing magnesium fluoride on the layer containing cerium oxide directly or by means of another layer by the vacuum deposition method, wherein the layer containing cerium oxide contains cerium oxide having a cubic polycrystalline structure, wherein the layer containing cerium oxide has a thickness of 70 nm or more and 300 nm or less, wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a thickness of 50 nm or more and 240 nm or less, and wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a refractive index of 1.65 or less at a wavelength of 500 nm.

[0128] This application claims priority based on Japanese Patent Application No. 2022-144038 filed on September 9, 2022, the entire contents of which are incorporated herein by reference. List of reference symbols 11 Base material 12 other shift 13 layer containing cerium oxide 14 layer containing magnesium fluoride 15 layer containing silicon oxide 16 layer containing silicon dioxide 17 Layer formed from the first metal oxide 18 Layer formed from a second metal oxide 21 dome-shaped resin substrate 31 lenses 32 glasses frame QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] JP 2022-144038

[0128] Cited non-patent literature

[0000] Journal of the Ceramic Society of Japan 110 [5] 450-454 (2002

[0007]

Claims

[1] Multilayer film comprising: a layer containing cerium oxide; and a layer containing silicon oxide or a layer containing magnesium fluoride formed directly or by means of another layer on the layer containing cerium oxide, wherein the layer containing cerium oxide contains cerium oxide having a cubic polycrystalline structure, wherein the layer containing cerium oxide has a thickness of 70 nm or more and 300 nm or less, wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a thickness of 50 nm or more and 240 nm or less, and wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a refractive index of 1.65 or less at a wavelength of 500 nm. [2] The multilayer film according to claim 1, wherein the cerium oxide in the layer containing cerium oxide has a composition CeO x where x=1.5 or more and 2.0 or less. [3] The multilayer film according to claim 1, wherein a content of the cerium oxide in the cerium oxide-containing layer is 85 mass% or more with respect to the entire cerium oxide-containing layer. [4] The multilayer film according to claim 1, wherein a content of the silicon oxide in the silicon oxide-containing layer is 65 mass% or more with respect to the entire silicon oxide-containing layer. [5] Multilayer film according to claim 4, wherein the layer containing silicon oxide contains aluminum oxide, and wherein a content of the aluminum oxide in the silicon oxide-containing layer is 0.1 mass% or more and 10 mass% or less with respect to the entire silicon oxide-containing layer. [6] Multilayer film according to claim 4, wherein the layer containing silicon oxide contains cerium oxide, and wherein a content of the cerium oxide in the layer containing silicon oxide is 0.1 mass% or more and 35 mass% or less. [7] The multilayer film according to claim 1, further comprising a silicon dioxide-containing layer formed directly or through another layer on the silicon oxide-containing layer or the magnesium fluoride-containing layer, wherein the silicon dioxide-containing layer has a thickness of 30 nm or less. [8] The multilayer film according to claim 1, further comprising a layer formed of a first metal oxide between the layer containing cerium oxide and the layer containing silicon oxide or the layer containing magnesium fluoride, wherein the layer formed of a first metal oxide contains a metal oxide having a polycrystalline structure, and wherein the layer formed of a first metal oxide has a thickness of 0.5 nm or more and 15 nm or less. [9] The multilayer film according to claim 8, wherein the metal oxide is copper oxide represented by a composition CuO x , where x=0.5 or more and 1.0 or less. [10] The multilayer film according to claim 8, wherein the metal oxide is cerium oxide having a cubic polycrystalline structure represented by a composition CeO x , where x=1.5 or more and 2.0 or less. [11] The multilayer film according to claim 7, further comprising a layer formed of a second metal oxide between the layer containing silicon oxide or the layer containing magnesium fluoride and the layer containing silicon dioxide, wherein the layer formed of a second metal oxide contains a metal oxide having a polycrystalline structure, and wherein the layer formed of a second metal oxide has a thickness of 0.5 nm or more and 7 nm or less. [12] The multilayer film according to claim 11, wherein the metal oxide is copper oxide represented by a composition CuO x , where x=0.5 or more and 1.0 or less. [13] The multilayer film according to claim 11, wherein the metal oxide is cerium oxide having a cubic polycrystalline structure represented by a composition CeO x , where x=1.5 or more and 2.0 or less. [14] An optical element comprising the multilayer film according to any one of claims 1 to 13. [15] A method of producing a multilayer film, the method comprising: Forming a layer containing cerium oxide on a base material directly or through another layer by a vacuum deposition method; and Forming a layer containing silicon oxide or a layer containing magnesium fluoride on the layer containing cerium oxide directly or by means of another layer by the vacuum deposition method, wherein the layer containing cerium oxide contains cerium oxide having a cubic polycrystalline structure, wherein the layer containing cerium oxide has a thickness of 70 nm or more and 300 nm or less, wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a thickness of 50 nm or more and 240 nm or less, and wherein the layer containing silicon oxide and the layer containing magnesium fluoride each have a refractive index of 1.65 or less at a wavelength of 500 nm.

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