Half mirror and image display apparatus

There is provided an image display apparatus which enables an image displayed on a display device to be viewed through a half mirror. The half mirror includes two translucent resin substrates formed from resin, a metallic film composed Ag and disposed between the resin substrates, and dielectric films disposed between the metallic film and the resin substrates, respectively. Each of the dielectric films includes an SiO2 layer provided on the corresponding one of the substrates, an Al2O3 layer provided on the SiO2 layer, and a ZrO2 layer provided on the Al2O3 layer.

BACKGROUND

1. Technical Field

The present invention relates to a half mirror and an image display apparatus including the half mirror.

2. Related Art

In typical, image display apparatuses, such as head mount displays, head up displays, and viewfinders of camcorders, include half mirrors to split a luminous flux into a plurality of beams. In the optical system of an observation system included in an image-recording apparatus, a half mirror reflects light of an image displayed on a display device, such as a liquid crystal display device, to eyes of a viewer while transmitting the external light, so that the viewer can view both image information displayed on the display device and external light in one visual field.

Examples of such a half mirror include a prism-type beam splitter including transparent substrates bonded to each other. In the prism-type beam splitter, a thin film composed of metal such as Ag is formed on a surface of one transparent substrate, and the other transparent substrate having a refractive index the same as that of one transparent substrate is bonded to the topmost layer of the thin layer such that the thin film is interposed between the two transparent substrates, which enables transmitted light to directly travel and enables external distortion of a transmission image to be reduced.

Example of the thin film include a multilayered dielectric film including a high-refractive-index film and low-refractive-index film composed of dielectric materials and a structure including dielectric films and a metallic film interposed therebetween (see, U.S. Pat. No. 3,559,090). The latter, in which a metallic film is interposed between dielectric films, can reduce polarization dependence and incident angle dependence within a broad wavelength range being approximately entire visible light region and is therefore suitable for use in see-through optical systems which enable image information to be displayed while allowing observation of transmitted external light.

Another metal, such as Al, may be used in place of Ag to form the thin film used in a half mirror; however, a problem of large optical loss by absorption is caused, and it is difficult to precisely and uniformly form a significantly thin layer having a thickness of 1 to 5 nm. The thin Ag film also has a problem in which Ag is readily allowed to react for degradation, such as diffusion by heat, granulation, and oxidation with a gas composition.

In typical, in order to suppress such reactivity of Ag, the film needs to be formed on a substrate kept at low temperature under high vacuum, and a protective layer needs to be formed to suppress degradation in air. In other known techniques, Ag is alloyed with a small amount of a stabilizer without impairing the optical properties of Ag, or an adjacent metallic layer composed of, for example, Cr is formed so as to have a slightly thin thickness for the purpose of stable film formation as is disclosed in Japanese Patent No. 3563955.

In display apparatuses, however, a resin substrate is used to form a prism in some cases because of demands for reductions in the weight of an optical component and its production cost. The resin substrate has particular water absorbability and contains unstable molecules, and moisture and organic components are therefore leaked from the resin substrate and then volatilized to be taken into the film during film formation under vacuum. Thus, density of the Ag layer and the dielectric films with the Ag layer interposed therebetween is reduced, which problematically causes diffusion or degradation in the Ag layer with ease as compared with use of the traditional glass substrates.

SUMMARY

An advantage of some aspects of the invention is that a substrate of a half mirror is formed from a resin material to reduce the weight of an optical component and its production cost, which can reduce loss in quantity of light and stabilize the optical characteristics of a metallic film included in the half mirror.

In order to provide the above advantage, a first aspect of the invention provides a half mirror including a pair of resin substrates formed from translucent resin, a metallic film at least containing Ag and disposed between the resin substrates, and a pair of dielectric films disposed between the metallic film and the resin substrates, respectively, wherein each of the dielectric films includes a high-refractive-index layer and a block layer, the high-refractive-index layer being composed of ZrO2and configured so as to contact the metallic film, and the block layer being disposed between each of the substrates and the corresponding one of the high-refractive-index layers; and each of the block layers includes an SiO2layer provided on the corresponding one of the resin substrates and an Al2O3layer provided on the SiO2layer.

In the first aspect of the invention, the metallic film composed of Ag is covered with the high-refractive-index layers composed of ZrO2, which enables a high refractive index to be securely exhibited. Furthermore, the block layers including the SiO2layers and the Al2O3layers can sufficiently secure the distance between the resin substrates and the metallic film, respectively, and the SiO2layers highly adhesive to resin directly contact the resin substrates, which can prevent a reduction in the density of the dielectric layers with the metallic layer interposed therebetween, such a reduction in the density of the dielectric layers being caused by leakage of moisture and organic components from the resin substrates and the subsequent volatilization thereof during film formation.

In the half mirror of the first aspect, it is preferable that each of the dielectric films further includes a protective film disposed between corresponding one of the resin substrates and the corresponding one of the block layers, the protective film being any one of a ZrO2layer, an Al2O3layer, an SiO2layer, and a laminate of ZrO2, Al2O3, and SiO2. In this case, the protective layers composed of any one of ZrO2, Al2O3, and SiO2or formed by laminating these materials are provided on the side of the resin substrates, which can secure a further appropriate refractive index and transmittance and further steadily prevent the resin substrates from typically degrading the metallic film.

In the half mirror of the first aspect of the invention, any one of an alloy of Zr and Ti and mixed oxide containing Zr and Ti may be used in place of ZrO2. In this case, use of the alloy of Zr and Ti or the mixed oxide containing Zr and Ti can secure a higher refractive index.

A second aspect of the invention provides a half mirror including a pair of resin substrates formed from translucent resin, a metallic film at least containing Ag and disposed between the resin substrates, and a pair of dielectric films disposed between the metallic film and the resin substrates, respectively, wherein each of the dielectric films includes a high-refractive-index layer and a block layer, the high-refractive-index layer being composed of Al2O3and configured so as to contact the metallic film, and the block layer being disposed between each of the substrates and the corresponding one of the high-refractive-index layers; and each of the block layers includes an SiO2layer provided on the corresponding one of the resin substrates and a ZrO2layer provided on the SiO2layer.

In the second aspect of the invention, the metallic film composed of Ag is covered with the high-refractive-index layers composed of Al2O3, which enables a high refractive index to be securely exhibited. Furthermore, the SiO2layers highly adhesive to resin directly contact the resin substrates, which can prevent a reduction in the density of the dielectric layers with the metallic layer interposed therebetween, such a reduction in the density of the dielectric layers being caused by leakage of moisture and organic components from the resin substrates and the subsequent volatilization thereof during film formation.

A third aspect of the present invention provides a half mirror including a pair of resin substrates formed from translucent resin, a metallic film at least containing Ag and disposed between the resin substrates, and first and second dielectric films disposed between the metallic film and the resin substrates, respectively, wherein the first dielectric film includes a high-refractive-index layer and a block layer, the high-refractive-index layer being composed of Al2O3and configured so as to contact the metallic film, and the block layer being disposed between one of the resin substrates and the high-refractive-index layer and including an SiO2layer provide on the resin substrate and an Al2O3layer provided on the SiO2layer; and the second dielectric film includes a high-refractive-index layer and a block layer, the high-refractive-index layer being composed of ZrO2and configured so as to contact the metallic film, and the block layer including an Al2O3layer provided on the other one of the resin substrates so as to be disposed between the resin substrate and the high-refractive-index layer.

In the third aspect of the invention, the metallic film composed of Ag is covered with the high-refractive-index layers composed of Al2O3and ZrO2in the first and second dielectric films, respectively, which enables a high refractive index to be securely exhibited. Furthermore, in the first and second dielectric films, the SiO2and Al2O3layers highly adhesive to resin directly contact the resin substrates, respectively, and the block layers sufficiently secures the distance from the resin, which can prevent a reduction in the density of the dielectric layers with the metallic layer interposed therebetween, such a reduction in the density of the dielectric layers being caused by leakage of moisture and organic components from the resin substrates and the subsequent volatilization thereof during film formation.

In the half mirror of any of the second and third aspects, it is preferable that each of the dielectric films further includes a protective film disposed between corresponding one of the resin substrates and the corresponding one of the block layers, the protective film being any one of a ZrO2layer, an Al2O3layer, an SiO2layer, and a laminate of ZrO2, Al2O3, and SiO2. In this case, the protective layers composed of any one of ZrO2, Al2O3, and SiO2or formed by laminating these materials are provided on the side of the resin substrates, which can secure a further appropriate refractive index and transmittance and further steadily prevent the resin substrates from typically degrading the metallic film.

In the half mirror of any of the second and third aspects, any one of an alloy of Zr and Ti and mixed oxide containing Zr and Ti may be used in place of ZrO2. In this case, use of the alloy of Zr and Ti or the mixed oxide containing Zr and Ti can secure a higher refractive index.

In the half mirror of any of the first to third aspects, it is preferable that the Al2O3layers have a thickness of not less than 5 nm, and the dielectric films have a thickness ranging from 200 nm to 1 μm. If the pair of dielectric films have a thickness higher than 1 μm, the films are cracked and separated because of a difference in a coefficient of linear expansion between the films and the resin substrates, whereas the dielectric films having an appropriate thickness of 200 nm to 1 μm can secure the distance between the resin substrates and the metallic film, respectively, and prevent the resin substrates from typically degrading the metallic film.

A fourth aspect of the invention provides an image display apparatus including a display device to display an image and the half mirror having any of the above-described advantages, wherein the half mirror transmits external light and reflects the image displayed on the display device to show the external light and the image combined with each other.

In the image display apparatus according to the fourth aspect of the invention which enables an image displayed on the display device, such as a liquid crystal display device, to be observed through the half mirror, the resin substrates of the half mirror are formed from a resin material, which can reduce the weight of an optical component and its production cost. Furthermore, the metallic film of the half mirror is composed of Ag, so that loss in quantity of light can be reduced and the block layers and protective layers of the dielectric films with the metallic film interposed therebetween can enhance durability and thermal stability over a long period of time to provide stable optical characteristics.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A head mount display (hereinafter referred to as HMD) of an embodiment of the invention will now be described in detail with reference to the accompanying drawings. In the drawings, sizes of components are appropriately changed as compared with the actual sizes thereof. Since the HMD to be described is merely an embodiment of the invention, the invention should not be limited thereto, and the HMD can be appropriately modified within the scope of the invention.

FIG. 1is a partial cross-sectional view illustrating an optical pass in an HMD100of an embodiment of the invention. As illustrated inFIG. 1, the HMD100is an image display apparatus which shows an image displayed on a display device1, such as a liquid crystal display device, in front of an eye of a viewer such that the viewer can see it. The HMD100is set on the head of a viewer, and various devices are incorporated in an HMD body to be disposed in front of an eye EY of the viewer. The HMD body is a housing to be disposed in front of an eye of a user and is provided with the display device1, an optical system6, and a light guide unit10which are each accommodated in the housing. The front side of the housing being the HMD body is opened or provided with a transparent component such as glass.

In this embodiment, the display device1is a transmissive liquid crystal panel, and image light emitted from the display device1passes the optical system6and then enters an image entrance plane10aof the light guide unit10. The optical system6is a group of lenses which magnify light emitted from the display device1to enable the light to enter the image entrance plane10aof the light guide unit10. In this embodiment, the optical system6includes an objective lens L1disposed in front of the display device1, an adjustment lens L2for adjustment, and a condenser lens L3to concentrate light toward the light guide unit10. In the case where a scale of an image does not need to be changed depending on the size of the display device1, the optical system6may not be provided. Although not illustrated, various optical components are provided between the display device1and the light guide unit10depending on optical requirements of the HMD100, such as a light-guiding medium, e.g., air, a transparent plastic member, or glass; or another lens.

The light guide unit10is a beam splitter and disposed in front of the eye EY of a viewer to combine a light beam reflected by an external object OBJ and a light beam emitted from the display device1, and the combined light beams enter the eye EY. Specifically, the light guide unit10includes a half mirror4incorporated therein and has an external light entrance plane10cthrough which external light passes, the image entrance plane10athrough which light emitted from the display device1passes, a reflection plane10bwhich reflects light which has passed through the image entrance plane10a, and an emission plane10dfrom which a light beam produced by combining external light and light reflected by the reflection plane10bis emitted.

The light guide unit10includes two resin substrates11and12formed from transparent resin, and the half mirror4is provided along the bonded surfaces of the two resin substrates11and12. Examples of the material of the resin substrates11and12include an acrylic material, urethane, polycarbonate, cycloolefin, and styrene, and each may be alone or in combination.

The half mirror4transmits external light which has passed through the external light entrance plane10cand reflects light which has been emitted from the display device1and then reflected by the reflection plane10b. In particular, a luminous flux (visible luminous flux) from an image displayed on the display device1enters the image entrance plane10athrough the optical system6and is then totally reflected by the reflection plane10b, and then this luminous flux is reflected by the half mirror4and guided to the eye EY of a viewer through the emission plane10d. In the half mirror4, an image of the object OBJ being scenery is spatially superimposed on a virtual image from the display device1, so that both the images can be observed in one visual field at the same visibility.

FIGS. 2A and 2Bare schematic cross-sectional views illustrating the configurations of the half mirror4inFIG. 1. With reference toFIG. 2A, the half mirror4schematically include the resin substrates11and12, a pair of dielectric films41formed on surfaces of the resin substrates11and12, respectively, and a metallic film40disposed between the dielectric films41. In this embodiment, the metallic film40is composed of Ag. The metallic film40may be composed of an ally of Ag and another metal, such as copper, gold, or palladium, to enhance bonding strength (adhesion) of the metallic film40to the dielectric films41provided on the resin substrates11and12, respectively, which leads to enhancements in thermal resistance, stability, and optical characteristics.

The dielectric films41include high-refractive-index layers41awhich form a pair and contact the upper and lower surfaces of the metallic film40and block layers41bwhich contact the resin substrates11and12, respectively. Materials of the high-refractive-index layers41aand the block layers41bcan be selected from SiO2, Al2O3, ZrO2, TiO2, mixed oxide containing Zr and Ti, mixed oxide containing La and Ti, mixed oxide containing La and Al, CeO2, Ta2O5, HfO2, and a mixture thereof, each being used alone or in combination.

Specifically, the high-refractive-index layers41aare primarily composed of ZrO2or TiO2having a high refractive index and a low optical absorption property and configured so as to have a thickness which enables reflection inside the metallic layer40composed of Ag to be selectively reduced, so that spectral characteristics are flattened. The high-refractive-index layers41amay be Zr-containing layers composed of an alloy of Zr and Ti or mixed oxide containing Zr and Ti, instead of ZrO2layers. Use of the alloy of Zr and Ti or mixed oxide containing Zr and Ti can contribute to securing a higher refractive index.

The block layers41bare composed of any one of ZrO2, Al2O3, and an oxide of silicon, such as SiO or SiO2, or formed by laminating these materials. The material to be used for the block layers41bexhibits high adhesion to the resin substrates11and12and enables deformation due to thermal expansion or hydroscopic expansion to be absorbed. Multiple materials may be laminated to form the block layers41b, or, for example, Al2O3layers may be formed so as to contact the resin layers11and12. In the case where Al2O3is used to form the block layers41b, the Al2O3layers have a thickness of not less than 5 nm. In addition, the pair of the dielectric films41each have a thickness ranging from 200 nm to 1 μm. The dielectric films41having such a thickness can sufficiently secure the distance between the resin substrates11and12and the metallic film40, respectively, and prevent the resin substrates11and12from typically degrading the metallic film40.

The dielectric films41may further include protective films41cwhich are disposed between resin substrates11and12and the block layers41bas illustrated inFIG. 2B, respectively, the protective films41cbeing composed of any one of ZrO2, Al2O3, and SiO2or formed by laminating these materials. The protective films41care provided on the side of the resin substrates11and12, respectively, which can secure further appropriate reflectance and transmittance and further steadily prevent the resin substrates11and12from typically degrading the metallic film40.

The dielectric films41illustrated inFIGS. 2A and 2Bmay be formed so as to have a difference in a film configuration between the upper side of the metallic film40and the lower side; in particular, the dielectric films41may be in the form of first and second dielectric films having different film configurations (composition and number of layers). Reflectance and transmittance of the entire film configuration may be adjusted depending on an image displayed by an image display apparatus and an incident direction or emission direction of external light.

Specific examples of the film configuration of the half mirror of the invention will now be described.

The resin substrates11and12were formed from acrylic resin (refractive index n=1.50, the value of refractive index n is a representative value at a wavelength of 550 nm being the central wavelength of visible light), and 10 layers were formed from materials shown in Table 1 so as to have refractive indexes and thicknesses shown in Table 1.

As shown in Table 1, in Example 1, the metallic film40(fifth layer) was an Ag film having a refractive index of 0.06 and a thickness of 18.0 nm, and the high-refractive-index layers41a(fourth and sixth layers) were formed from ZrO2such that the metallic film40was disposed therebetween. Furthermore, first Al2O3layers (third and seventh layers) were formed such that the above laminate was disposed therebetween, and SiO2layers (second and eighth layers) were formed such that the resulting laminate was disposed therebetween. Moreover, in Example 1, the protective layers41cwere formed such that the two SiO2layers were interposed therebetween. One of the protective films41cwas a single layer (first layer) composed of ZrO2and contacting one resin substrate, and the other one was a multilayer (ninth and tenth layers) including an SiO2layer contacting the other substrate and a second Al2O3layer covering the SiO2layer.

The Al2O3layers included in the block layers41bhad a thickness of not less than 5 nm, and the pair of dielectric films41had a thickness ranging from 200 nm to 1 μm. In other words, the total thickness of the first to fourth layers and the total thickness of the sixth to tenth layers were in the range of 200 nm to 1 μm.

FIG. 3is a graph illustrating a relationship of a refractive index R of a half mirror having the film configuration of Example 1 with a spectral transmittance T at an incident angle of 20 to 34°.FIG. 3demonstrates that Example 1 provided flat spectral characteristics at a wavelength from 400 to 660 nm within a visible light range and exhibited a reflectance of 62.0%, a transmittance of 35.0%, and optical absorption of 3.0% at a wavelength of 550 nm, which indicates that the half mirror exhibited low optical absorption and had excellent optical characteristics.

In Example 1, the metallic film40composed of Ag was covered with the high-refractive-index layers41acomposed of ZrO2, so that a high refractive index was able to be secured. Furthermore, the block layers41beach including the SiO2layer and the Al2O3layer were able to sufficiently secure the distances between the resin substrates11and the metallic film40and between the resin substrates12and the metallic film40, respectively. The protective films41cformed from ZrO2and formed by laminating Al2O3and SiO2were additionally provided on the side of the resin substrates11and12, respectively, which was able to secure a further appropriate refractive index and transmittance and further steadily prevent the resin substrates11and12from typically degrading the metallic film40. In particular, the ZrO2layer and the SiO2layer which were highly adhesive to resin directly contacted the resin substrates11and12, respectively, which was able to prevent a reduction in the density of the dielectric films41with the metallic layer40interposed therebetween, the reduction in the density of the dielectric films41being caused by leakage of moisture and organic components from the resin substrates11and12and the subsequent volatilization thereof during film formation.

The resin substrates11and12were formed from acrylic resin (refractive index n=1.50), and 7 layers were formed from materials shown in Table 2 so as to have refractive indexes and thicknesses shown in Table 2.

As shown in Table 2, in Example 2, the metallic film40(fourth layer) was an Ag film having a refractive index of 0.06 and a thickness of 17.9 nm, and the high-refractive-index layers41a(third and fifth layers) were formed from ZrO2such that the metallic film40was disposed therebetween. Furthermore, Al2O3layers (second and sixth layers) were formed such that the above laminate was disposed therebetween, and then SiO2layers (first and seventh layers) were formed such that the resulting laminate was disposed therebetween, thereby forming the block layers41b.

The Al2O3layers included in the block layers41bhad a thickness of not less than 5 nm, and the pair of dielectric films41had a thickness ranging from 200 nm to 1 μm. In other words, the total thickness of the first to third layers and the total thickness of the fifth to seventh layers were in the range of 200 nm to 1 μm.

FIG. 4is a graph illustrating a relationship of a refractive index R of a half mirror having the film configuration of Example 2 with a spectral transmittance T at an incident angle of 20 to 34°.FIG. 4demonstrates that Example 2 provided flat spectral characteristics at a wavelength from 400 to 660 nm within a visible light range and exhibited a reflectance of 64.0%, a transmittance of 34.5%, and optical absorption of 1.5% at a wavelength of 550 nm, which indicates that the half mirror exhibited low optical absorption and had excellent optical characteristics.

In Example 2, the metallic film40composed of Ag was covered with the high-refractive-index layers41acomposed of ZrO2, so that a high refractive index was able to be secured. Furthermore, the block layers41beach including the SiO2layer and the Al2O3layer were able to sufficiently secure the distances between the resin substrates11and the metallic film40and between the resin substrates12and the metallic film40, respectively, which was able to prevent a reduction in the density of the dielectric films41with the metallic layer40interposed therebetween, the reduction in the density of the dielectric films41being caused by leakage of moisture and organic components from the resin substrates11and12and the subsequent volatilization thereof during film formation.

The resin substrates11and12were formed from acrylic resin (refractive index n=1.50), and 5 layers were formed from materials shown in Table 3 so as to have refractive indexes and thicknesses shown in Table 3.

As shown in Table 3, in Example 3, the metallic film40(third layer) was an Ag film having a refractive index of 0.06 and a thickness of 17.4 nm, and the high-refractive-index layers41a(second and fourth layers) were formed from ZrO2such that the metallic film40was disposed therebetween. Furthermore, Al2O3layers (first and fifth layers) were formed as the block layers41bsuch that the above laminate was disposed therebetween.

The Al2O3layers as the block layers41bhad a thickness of not less than 5 nm, and the pair of dielectric films41had a thickness ranging from 200 nm to 1 μm. In other words, the total thickness of the first and second layers and the total thickness of the fourth and fifth layers were in the range of 200 nm to 1 μm.

FIG. 5is a graph illustrating a relationship of a refractive index R of a half mirror having the film configuration of Example 3 with a spectral transmittance T at an incident angle of 20 to 34°.FIG. 5demonstrates that Example 3 provided flat spectral characteristics at a wavelength from 400 to 660 nm within a visible light range and exhibited a reflectance of 64.0%, a transmittance of 35.5%, and optical absorption of 0.5% at a wavelength of 550 nm, which indicates that the half mirror exhibited low optical absorption and had excellent optical characteristics.

In Example 3, the metallic film40composed of Ag was covered with the high-refractive-index layers41acomposed of ZrO2, so that a high refractive index was able to be secured. Furthermore, the block layers41bcomposed of Al2O3were able to sufficiently secure the distances between the resin substrate11and the metallic film40and between the resin substrate12and the metallic film40, respectively, which was able to prevent a reduction in the density of the dielectric films41with the metallic layer40interposed therebetween, the reduction in the density of the dielectric films41being caused by leakage of moisture and organic components from the resin substrates11and12and the subsequent volatilization thereof during film formation.

The resin substrates11and12were formed from acrylic resin (refractive index n=1.50), and 10 layers were formed from materials shown in Table 4 so as to have refractive indexes and thicknesses shown in Table 4.

As shown in Table 4, in Example 4, the metallic film40(fifth layer) was an Ag film having a refractive index of 0.06 and a thickness of 17.3 nm, and the high-refractive-index layers41a(fourth and sixth layers) were formed from Al2O3such that the metallic film40was disposed therebetween. Furthermore, ZrO2layers were formed (third and seventh layers) such that the above laminate was disposed therebetween, and SiO2layers (second and eighth layers) were formed such that the resulting laminate was disposed therebetween, thereby forming the block layers41b. Moreover, in Example 4, the protective layers41cwere formed such that the block layers41bwere interposed therebetween. One of the protective films41cwas a single layer (first layer) composed of ZrO2and contacting one resin substrate, and the other one was a multilayer (ninth and tenth layers) including an SiO2layer contacting the other substrate and an Al2O3layer covering the SiO2layer.

The Al2O3layers as the high-refractive-index layers41ahad a thickness of not less than 5 nm, and the pair of dielectric films41had a thickness ranging from 200 nm to 1 μm. In other words, the total thickness of the first to fourth layers and the total thickness of the sixth to tenth layers were in the range of 200 nm to 1 μm.

FIG. 6is a graph illustrating a relationship of a refractive index R of a half mirror having the film configuration of Example 4 with a spectral transmittance T at an incident angle of 20 to 34°.FIG. 6demonstrates that Example 4 provided flat spectral characteristics at a wavelength from 400 to 660 nm within a visible light range and exhibited a reflectance of 64.0%, a transmittance of 34.0%, and optical absorption of 3.0% at a wavelength of 550 nm, which indicates that the half mirror exhibited low optical absorption and had excellent optical characteristics.

In Example 4, the metallic film40composed of Ag was covered with the high-refractive-index layers41acomposed of Al2O3, so that a high refractive index was able to be secured. Furthermore, the block layers41beach including the SiO2layer and the ZrO2layer were able to sufficiently secure the distances between the resin substrates11and the metallic film40and between the resin substrates12and the metallic film40, respectively. The protective films41cformed from ZrO2and formed by laminating Al2O3and SiO2were additionally provided on the side of the resin substrates11and12, respectively, which was able to secure a further appropriate refractive index and transmittance and further steadily prevent the resin substrates11and12from typically degrading the metallic film40. In particular, the ZrO2layer and the SiO2layer which were highly adhesive to resin directly contacted the resin substrates11and12, respectively, which was able to prevent a reduction in the density of the dielectric films41with the metallic layer40interposed therebetween, the reduction in the density of the dielectric films41being caused by leakage of moisture and organic components from the resin substrates11and12and the subsequent volatilization thereof during film formation.

The resin substrates11and12were formed from acrylic resin (refractive index n=1.50), and 7 layers were formed from materials shown in Table 5 so as to have refractive indexes and thicknesses shown in Table 5.

As shown in Table 5, in Example 5, the metallic film40(fourth layer) was an Ag film having a refractive index of 0.06 and a thickness of 17.0 nm, and the high-refractive-index layers41a(third and fifth layers) were formed from Al2O3such that the metallic film40was disposed therebetween. Furthermore, ZrO2layers (second and sixth layers) were formed such that the above laminate was disposed therebetween, and then SiO2layers (first and seventh layers) were formed such that the resulting laminate was disposed therebetween, thereby forming the block layers41b.

The Al2O3layers as the high-refractive-index layers41ahad a thickness of not less than 5 nm, and the pair of dielectric films41had a thickness ranging from 200 nm to 1 μm. In other words, the total thickness of the first to third layers and the total thickness of the fifth to seventh layers were in the range of 200 nm to 1 μm.

FIG. 7is a graph illustrating a relationship of a refractive index R of a half mirror having the film configuration of Example 5 with a spectral transmittance T at an incident angle of 20 to 34°.FIG. 7demonstrates that Example 5 provided flat spectral characteristics at a wavelength from 400 to 660 nm within a visible light range and exhibited a reflectance of 63.5%, a transmittance of 35.5%, and optical absorption of 1.0% at a wavelength of 550 nm, which indicates that the half mirror exhibited low optical absorption and had excellent optical characteristics.

In Example 5, the metallic film40composed of Ag was covered with the high-refractive-index layers41acomposed of Al2O3, so that a high refractive index was able to be secured. Furthermore, the block layers41beach including the SiO2layer and the ZrO2layer were able to sufficiently secure the distances between the resin substrates11and the metallic film40and between the resin substrates12and the metallic film40, respectively, which was able to prevent a reduction in the density of the dielectric films41with the metallic layer40interposed therebetween, the reduction in the density of the dielectric films41being caused by leakage of moisture and organic components from the resin substrates11and12and the subsequent volatilization thereof during film formation.

The different film configurations of Examples 1 to 5 may be separately provided at the upper side and the lower side of the metallic film40to adjust reflectance and transmittance of the entire film configuration depending on an image displayed by an image display apparatus and an incident direction or emission direction of external light.

Table 6 shows the film configuration of Example 6. The film configuration of Example 5 was provided as a first dielectric film41at the upper side of the metallic film40, and the film configuration of Example 3 was provided as a second dielectric film41at the lower side of the metallic film40. In particular, the resin substrates11and12were formed from acrylic resin (refractive index n=1.50), and 6 layers were formed from materials shown in Table 6 so as to have refractive indexes and thicknesses shown in Table 6.

As shown in Table 6, in Example 6, the metallic film40(fourth layer) was an Ag film having a refractive index of 0.06 and a thickness of 16.9 nm, and the first dielectric film41was formed at the upper side (top side) of the metallic film40. The first dielectric film41included the high-refractive-index layer41a(third layer) formed from Al2O3so as to cover the metallic layer40and the block layer41bincluding a ZrO2layer (second layer) formed so as to cover the high-refractive-index layer41aand an SiO2layer (first layer) formed so as to cover the ZrO2layer.

On the other hand, the second dielectric film41was formed at the lower side (bottom side) of the metallic film40, the second dielectric film41including the high-refractive-index layer41a(fifth layer) formed from ZrO2so as to cover the metallic layer40(third layer) and the block layer41b(sixth layer) formed from Al2O3so as to cover this high-refractive-index layer41a.

The Al2O3layers being the high-refractive-index layer41aof the first dielectric film41and the block layer41bof the second dielectric film41had a thickness of not less than 5 nm, and the first and second dielectric films41had a thickness ranging from 200 nm to 1 μm. In other words, the total thickness of the first to third layers and the total thickness of the fifth and sixth layers were in the range of 200 nm to 1 μm. The first and second dielectric films41may further include the protective films provided in other examples to produce a film configuration including 8 to 10 layers.

FIG. 8is a graph illustrating a relationship of a refractive index R of a half mirror having the film configuration of Example 6 with a spectral transmittance T at an incident angle of 20 to 34°.FIG. 8demonstrates that Example 6 provided flat spectral characteristics at a wavelength from 400 to 660 nm within a visible light range and exhibited a reflectance of 63.0%, a transmittance of 36.0%, and optical absorption of 1.0% at a wavelength of 550 nm, which indicates that the half mirror exhibited low optical absorption and had excellent optical characteristics.

In Example 6, the metallic film composed of Ag was covered with the high-refractive-index layers41acomposed of ZrO2and Al2O3, respectively, so that a high refractive index was able to be secured. Furthermore, the block layers formed from Al2O3and formed by laminating ZrO2and SiO2, respectively, were able to sufficiently secure the distances between the resin substrate12and the metallic film40and between the resin substrate11and the metallic film40, respectively, which was able to secure further appropriate reflectance and transmittance and further steadily prevent the resin substrates11and12from typically degrading the metallic film40. In particular, the Al2O3layer and the SiO2layer which were highly adhesive to resin directly contacted the resin substrates11and12, respectively, which was able to prevent a reduction in the density of the dielectric films41with the metallic layer40interposed therebetween, the reduction in the density of the dielectric films41being caused by leakage of moisture and organic components from the resin substrates11and12and the subsequent volatilization thereof during film formation.

Examples mentioned above have been described to exemplify the invention. The invention should not be therefore limited to Examples described above and can be variously modified within the scope of the invention, for example, in response to design requirement. In the embodiment described above, although the head mount display has been described to exemplify the image display apparatus, the invention may employ any other configuration which enables an image displayed on a display device, such as a liquid crystal display device, to be observed through the half mirror, such as head up displays and viewfinders of image-recording apparatuses, e.g., camcorders.

The entire disclosure of Japanese Patent Application No. 2011-256682, filed Nov. 24, 2011 is expressly incorporated by reference herein.