Patent Description:
In recent years, as a next-generation video display device, a head mounting type "wearable display", such as a head-mounted display, has been actively developed.

The head-mounted display has an optical system comprising a light guide that guides a video output from a video light generation unit to eyes of a wearer. The light guide is classified roughly into a reflection type using a partially reflecting surface, a volumetric hologram type, and a diffractive element type. For example, <CIT> and <CIT> disclose a reflection type light guide. The light guide disclosed in <CIT>, <CIT>, and <CIT> is a light guide that propagates video light while totally reflecting the video light, and has a configuration in which a plurality of partially reflecting surfaces that reflect a part of the video light and output the part to an outside, and that transmit a part of the video light are disposed substantially parallel to each other along a propagation direction of the video light inside the light guide. <CIT> shows glasses comprising a mirrored optical waveguide. The optical waveguide has an array of reflective surfaces, which can be produced by coating glass layers with a partially reflective coating and bonding the glass layers to each other, so that the reflective surfaces are created at the layer bonding surfaces. For example, borosilicate glass can be coated with a reflective coating, such as silicon dioxide. The laminated stack of glass layers is diced obliquely to the reflective surfaces to produce the optical waveguide. Projected imagery enters the optical waveguide and is reflected to a viewer. <CIT> relates to light-guide optical elements. Illumination light entering a substrate is reflected on a first reflecting surface and a second reflecting surface. An array of selective reflecting surfaces couples the light out of the substrate toward a viewer. Light can partly pass through the selective reflecting surfaces. The selective reflecting surfaces are arranged in an inclined manner in the substrate with distance from each other. Dielectric coatings can be provided on the selective reflecting surfaces. The reflectance of one of the selective reflecting surfaces used for eyeball tracking can be approximately zero for less than <NUM>° for light having a wavelength of <NUM>. <NPL> discloses that video light incident on both the first surface and the second surface of at least one of the plurality of half mirrors causes unwanted stray light.

An object of the technology of the present invention is to provide a light guide and a video display device capable of displaying a video having high contrast.

A light guide according to the present invention is defined by the features of claim <NUM>.

In the light guide according to the present invention, it is preferable that the refractive indices of the outermost layers of the dielectric multi-layer film are <NUM>. 95n to <NUM>. 10n, and it is more preferable that the refractive indices of the outermost layers of the dielectric multi-layer film are <NUM>. 00n to <NUM>.

In the light guide according to the present invention, it is preferable that in a case in which film thicknesses of the outermost layers of the dielectric multi-layer film are d [nm], the refractive indices of the outermost layers are n1, and Δn = {(n - n1)/n}×<NUM> [%], Δn·d [%·nm] is in a range of -<NUM> to +<NUM>.

In the light guide according to the present invention, it is preferable that a tilt angle of the half mirror with respect to the first reflecting surface and the second reflecting surface is such that in a case in which the video light made incident into the base is incident on the first surface at an incidence angle of <NUM>° to <NUM>°, transmitted light transmitted through the half mirror of the incident video light is able to be re-incident from the second surface after being reflected by the first reflecting surface or the second reflecting surface.

In the light guide according to the present invention, it is preferable that the tilt angle of the half mirror is an angle in which an incidence angle in a case in which the transmitted light is re-incident from the second surface is in a range of <NUM>° to <NUM>°.

In the light guide according to the present invention, it is preferable that in the dielectric multi-layer film, a layer of low refractive index having a relatively low refractive index and a layer of high refractive index having a relatively high refractive index are alternately laminated.

In the light guide according to the present invention, it is preferable that at least one of the two outermost layers of the dielectric multi-layer film and the base are bonded by optical contact.

In the light guide according to the present invention, it is preferable that the two outermost layers of the dielectric multi-layer film and the base are in direct contact with each other.

In the light guide according to the present invention, it is preferable that no adhesive is present between the half mirror and the base.

In the light guide according to the present invention, each layer of the dielectric multi-layer film may contain silicon, oxygen, and nitrogen.

In the light guide according to the present invention, each layer of the dielectric multi-layer film may be a metal oxide layer containing at least one of silicon, niobium, tantalum, aluminum, titanium, tungsten, or chromium.

In the light guide according to the present invention, it is preferable that the refractive index of the base is <NUM> or more.

In the light guide according to the present invention, it is preferable that the base is a parallel flat plate in which the first reflecting surface and the second reflecting surface are parallel to each other.

In the light guide according to the present invention, it is preferable that the plurality of half mirrors are arranged in parallel to each other in a direction in which the video light propagates.

A video display device according to the present invention comprises: a video light generation unit that generates video light; the light guide according to the present invention, which propagates the incident video light; and an optical coupling member that makes the video light generated by the video light generation unit incident into the light guide.

According to the light guide and the video display device according to the present invention, it is possible to obtain a video having high contrast.

In the present specification, a numerical range represented by "to" means a range including numerical values before and after "to" as a lower limit value and an upper limit value.

<FIG> shows an appearance of a head-mounted display (HMD) <NUM> which is an embodiment of a video display device of the present disclosure. The HMD <NUM> comprises an embodiment of a light guide of the present disclosure. The HMD <NUM> is used, for example, by being worn on a head of a user <NUM>. <FIG> is a view of the user <NUM> wearing the HMD <NUM> as viewed from above the head. <FIG> is an enlarged view showing a light guide <NUM>.

The HMD <NUM> comprises a video light generation unit <NUM>, a light guide <NUM>, and an optical coupling member <NUM>.

The video light generation unit <NUM> generates video light and emits the video light toward the optical coupling member <NUM>. The video light generation unit <NUM> comprises, for example, a light source unit, a video light generation element that generates video light, and a projection optical unit for projecting the video light.

The light source unit comprises, for example, a light source including each of red, green, and blue light emitting diodes (LEDs) or laser diodes (LDs), and a lens for irradiating the video light generation element with light from the light source.

The video light generation element comprises a display element that displays a video based on a video signal, and generates video light by modulating the light incident from the light source unit with the display element. As the display element, for example, a liquid crystal panel or a digital mirror device (DMD) is used.

The projection optical unit comprises a projection lens consisting of one or a plurality of lenses, and projects the video light generated by the video light generation element onto the optical coupling member <NUM>.

The light guide <NUM> is positioned in front of an eye <NUM> of the user <NUM> in a case of wearing the light guide <NUM>, and the video light generated by the video light generation unit <NUM> is incident into the light guide <NUM>. The light guide <NUM> propagates the incident video light, and emits the video light toward the user <NUM>, thereby making the user <NUM> visually recognize a video. The video visually recognized by the user <NUM> may be a still image or a video image. As shown in <FIG>, the light guide <NUM> comprises a base <NUM> and a plurality of half mirrors 30a, 30b, 30c, and 30d disposed in the base <NUM>. The base <NUM> has a first reflecting surface <NUM> and a second reflecting surface <NUM>, and video light L0 propagates in the base <NUM> by repeated total reflection on the first reflecting surface <NUM> and the second reflecting surface <NUM>. The half mirrors 30a, 30b, 30c, and 30d reflect a part of the incident light and transmit the others. As an example, a reflectivity of each of the half mirrors 30a, 30b, 30c, and 30d is about <NUM>% to <NUM>%. The video light L0 is partially reflected by each of the plurality of half mirrors <NUM> disposed in the base <NUM> and emitted from the base <NUM> as emitted light L1 to make the user <NUM> visually recognize the video.

In addition, in the present specification, a reflectivity is shown as an average value of a reflectivity for p-polarized light and a reflectivity for s-polarized light.

The optical coupling member <NUM> makes the video light L0 generated by the video light generation unit <NUM> incident into the light guide <NUM>. The optical coupling member <NUM> is an optical coupling prism in the present example. In the present embodiment, one surface of the optical coupling member <NUM> is disposed in contact with the first reflecting surface <NUM> of the light guide <NUM>. The optical coupling member <NUM> introduces the video light L0 into the light guide <NUM> such that the video light L0 is incident on the first reflecting surface <NUM> and the second reflecting surface <NUM> at an angle at which the video light L0 is totally reflected and propagates in the light guide <NUM>. In addition, the optical coupling member <NUM> introduces the video light L0 into the light guide <NUM> such that the video light L0 is incident on a first surface 31a of the half mirror 30a at a desired incidence angle θ1. The optical coupling member <NUM> introduces the video light L0 into the light guide <NUM> such that the incidence angle θ1 of the video light L0 on the first surface 31a of the half mirror 30a in the light guide <NUM> is, for example, <NUM>° to <NUM>°. Here, the incidence angle means an angle formed by a normal line of a surface on which light is incident and a ray.

Hereinafter, details of the light guide <NUM> will be described.

The light guide <NUM> is an embodiment of the light guide according to the present disclosure. As described above, the light guide <NUM> comprises the base <NUM> and the plurality of half mirrors 30a, 30b, 30c, and 30d. The base <NUM> has the first reflecting surface <NUM> and the second reflecting surface <NUM>, and propagates the incident video light L0 while totally reflecting the video light L0 by the first reflecting surface <NUM> and the second reflecting surface <NUM>. In the present embodiment, the base <NUM> is a parallel flat plate in which the first reflecting surface <NUM> and the second reflecting surface <NUM> are parallel to each other. Here, the parallel flat plate means that the first reflecting surface <NUM> and the second reflecting surface <NUM> by which the video light L0 is reflected and propagates are plate-shaped members disposed in parallel to each other. Of course, the term "parallel flat plate" according to the technology of the present disclosure also includes a shape having unevenness in a region that does not affect the propagation of light in a part of an outer peripheral surface of the parallel flat plate or a shape having a portion where the first reflecting surface <NUM> and the second reflecting surface <NUM> are non-parallel. The use of the parallel flat plate facilitates the optical path design. In the base <NUM>, the first reflecting surface <NUM> and the second reflecting surface need not necessarily be parallel to each other in a case in which the video light L0 propagates between the first reflecting surface <NUM> and the second reflecting surface <NUM> by repeatedly being totally reflected and a video is visible by the emitted light L1 reflected by the half mirror <NUM>.

The base <NUM> is not particularly limited as long as it is a transparent member. A refractive index n of the base <NUM> is preferably <NUM> or more, more preferably <NUM> or more, and still more preferably <NUM> or more. As the refractive index is higher, light leak from the light guide to an outside can be reduced, and a good video can be obtained.

In the present embodiment, the plurality of half mirrors 30a, 30b, 30c, and 30d are arranged in parallel to each other in a direction in which the video light propagates. The half mirrors 30a, 30b, 30c, and 30d have first surfaces 31a, 31b, 31c, and 31d and a second surface 32a on a back side of the first surfaces 31a, 31b, 31c, and 31d, respectively. In a case in which the plurality of half mirrors are not distinguished individually below, the subscripts attached to the reference numerals, such as a, b, c, and d, are omitted and are simply referred to as a half mirror <NUM>, a first surface <NUM>, and a second surface <NUM>.

The plurality of half mirrors <NUM> are disposed in the base <NUM> such that the half mirrors are spaced from each other by being tilted with respect to the first reflecting surface <NUM> and the second reflecting surface <NUM>. A tilt angle α of the half mirror <NUM> with respect to the first reflecting surface <NUM> and the second reflecting surface <NUM> is referred to as a tilt angle α of the half mirror <NUM>.

As shown in <FIG>, the video light L0 made incident into the base <NUM> is repeatedly totally reflected by the first reflecting surface <NUM> and the second reflecting surface <NUM> of the base <NUM> and propagates in a direction A parallel to the first reflecting surface <NUM> and the second reflecting surface <NUM>. In this case, the video light L0 propagates through the plurality of half mirrors <NUM> provided in the base <NUM> once or a plurality of times. In a case in which the video light L0 is incident on the first surface <NUM> of the half mirror <NUM>, a part of the video light L0 is reflected by the half mirror <NUM> and emitted as emitted light L1.

The base <NUM> and the plurality of half mirrors <NUM> are configured such that the video light L0 made incident into the base <NUM> is incident on each of the first surface <NUM> and the second surface <NUM> of at least one of the plurality of half mirrors <NUM> one or more times. For example, as shown in <FIG>, the video light L0 incident on the first reflecting surface <NUM> of the base <NUM> via the optical coupling member <NUM> at an optical coupling angle θ0 is incident on the first surface 31a of the half mirror 30a. In this case, a part of the video light L0 is reflected by the half mirror 30a and emitted from the base <NUM> as emitted light L1. The video light L0 that has passed through the half mirror 30a without being reflected is incident on a point 22a of the second reflecting surface <NUM>. The video light L0 incident on the second reflecting surface <NUM> is totally reflected and is incident on the second surface 32a of the half mirror 30a at an incidence angle θ2. The video light L0 incident on the half mirror 30a from the second surface 32a and transmitted through the half mirror 30a is incident on a point 21a on the first reflecting surface <NUM> and is totally reflected. The video light L0 reflected by the first reflecting surface <NUM> is incident again on the first surface <NUM> of the half mirror 30a, and a part of the video light L0 is emitted from the base <NUM> as emitted light L1. As described above, in the example shown in <FIG>, the video light L0 incident on the base <NUM> is incident on the first surface <NUM> of the half mirror 30a twice and is incident on the second surface <NUM> once.

<FIG> is a diagram schematically showing a configuration of one half mirror <NUM> provided in the base <NUM>. As shown in <FIG>, the half mirror <NUM> includes a dielectric multi-layer film <NUM> formed by laminating a plurality of dielectric layers <NUM> to <NUM>. In the present embodiment, the half mirror <NUM> is made of a dielectric multi-layer film <NUM>. In <FIG>, the dielectric multi-layer film <NUM> comprises seven dielectric layers <NUM> to <NUM>, but the number of the dielectric layers is not limited as long as the dielectric multi-layer film <NUM> functions as the half mirror <NUM>.

The dielectric multi-layer film <NUM> is formed by laminating a plurality of dielectric layers having different refractive indices. Refractive indices of two outermost layers <NUM> and <NUM> out of the plurality of dielectric layers constituting the dielectric multi-layer film <NUM> are <NUM>. 90n to <NUM>. 15n in a case in which the refractive index of the base <NUM> is n. The refractive indices of the outermost layers <NUM> and <NUM> are preferably <NUM>. 95n to <NUM>. 10n, and more preferably <NUM>. 00n to <NUM>. The outermost layer in the dielectric multi-layer film <NUM> means the outermost layer among the layers sensed by the incident video light. Here, the layer sensed by the video light conceptually means the layer which affects the video light such as refraction, and specifically refers to a layer where an optical path length n·d, which is indicated by a product of a refractive index n of the dielectric film and a physical film thickness d of the dielectric film, is greater than <NUM>. Therefore, even though a layer having n·d of <NUM> or less is disposed between the dielectric multi-layer film <NUM> and the base <NUM>, such a layer does not correspond to the outermost layer of the dielectric multi-layer film <NUM>.

As described above, in the light guide <NUM> of the present embodiment, the base <NUM> and the plurality of half mirrors <NUM> are configured such that the video light L0 made incident into the base <NUM> is incident on each of the first surface <NUM> and the second surface <NUM> of at least one of the plurality of half mirrors <NUM> one or more times. Therefore, as shown in <FIG>, the video light L0 incident and transmitted from the first surface 31a of the half mirror 30a is reflected by the second reflecting surface <NUM> and is incident on the half mirror 30a again from the second surface 32a. In this case, the light reflected by the half mirror 30a, which is incident from the second surface 32a, is stray light LM, and a part of the stray light LM is emitted to the outside as uncontrollable unnecessary light LM1.

In a case in which the amount of the stray light LM reflected by being incident from the second surface 32a of the half mirror 30a is large, the amount of the video light L0 propagating to the half mirror 30b disposed in a rear stage and the amount of light reflected by the first surface 31b of the half mirror 30b and emitted as the emitted light L1 are greatly reduced. This is repeated, and the emitted light L1 in the half mirror disposed in a further rear stage shows a marked decrease in light amount. In a case in which the amount of the emitted light L1 is reduced, the visually recognized video becomes dark. In addition, a part of the stray light LM is emitted from the base <NUM> without angle control, so that the image may be blurred or appear to be duplicated. As described above, in a case in which the reflected light amount of the video light L0 reflected by the second surface <NUM> of the half mirror <NUM> is large, a problem arises in that the contrast of the video decreases. This is a problem peculiar to the light guide <NUM> configured such that the video light L0 is incident on one half mirror <NUM> a plurality of times, that is, the video light L0 is incident on each of the first surface <NUM> and the second surface <NUM> at least one or more times.

The half mirror <NUM> provided in the light guide <NUM> is designed to have a desired reflectivity in a case in which the video light L0 is incident on the first surface <NUM>, from which the reflected light is emitted to the outside as the emitted light L1, at the incidence angle θ1. In this case, in the related art, the video light L0 is generally incident only once on the half mirror <NUM>, so that a reflectivity of the incidence angle θ2 of the video light L0 on the second surface <NUM> was not taken into consideration.

On the other hand, the present inventors have found that it is possible to effectively suppress the reflectivity in a case in which the video light L0 is incident on the second surface <NUM> of the half mirror <NUM> by setting the refractive indices of the two outermost layers <NUM> and <NUM> out of the plurality of dielectric layers constituting the dielectric multi-layer film <NUM> of the half mirror <NUM> to <NUM>. 90n to <NUM>. 15n in a case in which the refractive index of the base <NUM> is n (see Design Example described below).

In the light guide <NUM> of the present embodiment, the refractive indices of the outermost layer <NUM> on the first surface <NUM> side and the outermost layer <NUM> on the second surface <NUM> side of the dielectric multi-layer film <NUM> constituting the half mirror <NUM> are <NUM>. 90n to <NUM>. 15n, so that the reflectivity on the second surface <NUM> of the half mirror <NUM> can be suppressed. Since the reflectivity of the half mirror <NUM> with respect to the incidence on the second surface <NUM> can be suppressed, a decrease in the amount of the video light L0 and the generation of stray light can be suppressed, and a high-contrast video can be obtained.

The incidence angle θ1 on the first surface <NUM> of the half mirror <NUM> is preferably <NUM>° to <NUM>°. In addition, the incidence angle θ2 on the second surface <NUM> of the half mirror <NUM> is preferably <NUM>° to <NUM>° (see Verification Example described below).

The incidence angle θ1 of the video light L0 incident on the first surface <NUM> of the half mirror <NUM> changes depending on the optical coupling angle θ0 which is the incidence angle of the video light L0 into the light guide. For example, in a case in which the tilt angle α of the half mirror <NUM> is <NUM>°, a relationship between the optical coupling angle θ0 and the incidence angle θ1 is as shown in <FIG>. A relationship between the incidence angle θ1 and the incidence angle θ2 of the video light L0, which is incident on the first surface <NUM> at the incidence angle θ1 and then totally reflected by the second reflecting surface <NUM> and incident on the second surface <NUM>, is as shown in <FIG>.

That is, in the example shown in <FIG>, in a case in which the optical coupling angle θ0 is <NUM>°, the incidence angle θ1 is <NUM>° and the incidence angle θ2 is <NUM>°. In addition, in a case in which the optical coupling angle θ0 is <NUM>°, the incidence angle θ1 is <NUM>° and the incidence angle θ2 is <NUM>°. The relationship between the optical coupling angle θ0 and the incidence angle θ1 and the incidence angle θ2 changes depending on the tilt angle α of the half mirror <NUM>.

In an actual system, the tilt angle α and the optical coupling angle θ0 of the half mirror are selected such that the incidence angle θ1 and the incidence angle θ2 are desired values. The tilt angle α is, for example, <NUM>° to <NUM>°. As shown in the example shown in <FIG>, in general, the incidence angle θ1 of the video light L0 on the first surface <NUM> of the half mirror <NUM> and the incidence angle θ2 of the video light L0 on the second surface <NUM> are significantly different from each other. In addition, in general, the reflectivity of the half mirror <NUM> made of the dielectric multi-layer film <NUM> with respect to light has an incidence angle dependence. As described above, in the related art, only the reflectivity at the incidence angle θ1 was taken into consideration, so that the reflectivity at the incidence angle θ2 is increased, resulting in a decrease in contrast of the visually recognized image. A more specific configuration of the half mirror <NUM> for setting the reflectivity at the incidence angle θ1 to a desired value and sufficiently suppressing the reflectivity at the incidence angle θ2 will be described below.

It is preferable that in the dielectric multi-layer film <NUM> forming the half mirror <NUM>, a layer of low refractive index having a relatively low refractive index and a layer of high refractive index having a relatively high refractive index are alternately laminated. Each layer may have a different refractive index, but layers of low refractive index having the same refractive index and layers of high refractive index having the same refractive index may be alternately laminated. In addition, for example, the dielectric multi-layer film <NUM> may have a configuration comprising an intermediate region <NUM> in which layers of low refractive index <NUM>, <NUM>, and <NUM> having a refractive index lower than that of the base <NUM> and layers of high refractive index <NUM> and <NUM> having a refractive index higher than that of the base <NUM> are alternately laminated, and the outermost layers <NUM> and <NUM> having a refractive index of <NUM>. 90n to <NUM>. 15n with respect to the refractive index n of the base <NUM>. By alternately providing the layer of low refractive index and the layer of high refractive index, it is easy to design and produce a half mirror having a desired incidence angle-dependent reflectivity.

Each of the layers <NUM> to <NUM> of the dielectric multi-layer film <NUM> can contain silicon (Si), oxygen (O), and nitrogen (N). In a case in which each of the layers <NUM> to <NUM> is a silicon oxynitride film, a desired refractive index can be obtained by changing a content ratio of Si:O:N.

In addition, each of the layers <NUM> to <NUM> of the dielectric multi-layer film <NUM> may be a metal oxide layer containing at least one of silicon, niobium (Nb), tantalum (Ta), aluminum (Al), titanium (Ti), tungsten (W), or chromium (Cr). A metal oxide containing one or more metals can be appropriately used depending on a desired refractive index.

It is preferable that in a case in which film thicknesses of the outermost layers <NUM> and <NUM> of the dielectric multi-layer film <NUM> are d, the refractive indices of the outermost layers are n1, and a percentage of a difference between the refractive index n of the base <NUM> and the refractive index n1 is Δn = {(n - n1)/n}×<NUM> [%], Δn·d [%·nm] is in a range of -<NUM> to +<NUM>. Δn·d is more preferably in a range of -<NUM> to +<NUM>. Δn·d is still more preferably in a range of -<NUM> to <NUM>, and still more preferably in a range of -<NUM> to +<NUM>. In a case in which Δn·d is in the range of -<NUM> to +<NUM>, the reflectivity of the video light L0 to the second surface <NUM> of the half mirror <NUM> can be effectively suppressed.

As shown in <FIG>, it is preferable that the two outermost layers <NUM> and <NUM> of the dielectric multi-layer film <NUM> are disposed to be in direct contact with the base <NUM>. That is, it is preferable that no adhesive is present between the dielectric multi-layer film <NUM> and the base <NUM>. It is preferable that at least one of the two outermost layers <NUM> and <NUM> of the dielectric multi-layer film <NUM> and the base <NUM> are bonded by optical contact. Here, the term "bonded by optical contact" means a state of being bonded without using an adhesive. By bonding at least one of the two outermost layers <NUM>, <NUM> of the dielectric multi-layer film <NUM> and the base <NUM> together by optical contact, there is no adhesive between the two outermost layers <NUM> and <NUM> of the dielectric multi-layer film <NUM> and the base <NUM>, and the two outermost layers <NUM> and <NUM> can be brought into contact with the base <NUM>.

Details of a manufacturing method of the light guide will be described below, but an optical adhesive is generally used for bonding the optical members. However, the refractive index of a general-purpose optical adhesive is <NUM>, and, in a case in which the refractive index n of the base <NUM> is <NUM> or more, a difference from the refractive index of the base becomes too large to make a design of the dielectric multi-layer film unviable. In addition, in a case of performing the bonding using an adhesive, a probability that a parallelism of the surfaces exceeds a target value increases, resulting in a decrease in productivity. By performing the bonding by optical contact, it is possible to solve a problem that arises in the case of performing the bonding using such an adhesive.

In the half mirror <NUM>, the reflectivity with respect to the video light L0 incident on the first surface <NUM> at the incidence angle θ1 is <NUM>% to <NUM>% In addition, in the half mirror <NUM>, the reflectivity with respect to the video light L0 incident on the second surface <NUM> at the incidence angle θ2 is <NUM>% or less, preferably <NUM>% or less, still more preferably <NUM>% or less, and still more preferably <NUM>% or less. For the incidence of the video light L0 on the first surface <NUM>, at least a part of the video light L0 needs to be reflected and emitted to the outside, so that it is necessary to reflect the video light L0 to a certain extent, but, for the incidence on the second surface <NUM>, the smaller reflectivity is preferable from the viewpoint of suppressing the decrease of the video light L0 and suppressing the stray light LM.

By setting the reflectivity with respect to the video light L0 incident on the second surface <NUM> at the incidence angle θ2 to <NUM>% or less, it is possible to effectively suppress the generation of the stray light LM, and to stably obtain a high-contrast video.

In order to realize the above configuration, the half mirror <NUM> in the light guide <NUM> has an average reflectivity of <NUM>% to <NUM>% with respect to light with a wavelength of <NUM> to <NUM> that is incident at an incidence angle of <NUM>° to <NUM>°, and has an average reflectivity of <NUM>% or less with respect to the light with the wavelength of <NUM> to <NUM> that is incident at an incidence angle of <NUM>° to <NUM>°. The video light is visible light and includes light with a wavelength of <NUM> to <NUM>, and in the present specification, the reflectivity with respect to the video light means an average reflectivity with respect to light with a wavelength of <NUM> to <NUM>.

As described above, the half mirror <NUM> is disposed at the tilt angle α at which the video light propagating in the light guide <NUM> by repeated total reflection by the first reflecting surface <NUM> and the second reflecting surface <NUM> is incident on the first surface <NUM> of the half mirror <NUM>, and then incident again from the second surface <NUM>. Here, the video light is made incident into the light guide <NUM> is set so as to be incident on the first surface <NUM> at the incidence angle θ1 = <NUM>° to <NUM>°, and then incident on the second surface <NUM> at the incidence angle θ2 = <NUM>° to <NUM>°. Since the half mirror <NUM> in the light guide <NUM> has an average reflectivity of <NUM>% to <NUM>% with respect to light with a wavelength of <NUM> to <NUM> that is incident at an incidence angle of <NUM>° to <NUM>°, and has an average reflectivity of <NUM>% or less with respect to the light with the wavelength of <NUM> to <NUM> that is incident at an incidence angle of <NUM>° to <NUM>°, it is possible to effectively suppress the reflected light on the second surface <NUM>, so that it is possible to suppress the stray light and to obtain a video having a higher contrast. In a half mirror configured of a multi-layer film, the reflectivity changes depending on the wavelength even at the same incidence angle. In addition, even at the same wavelength, the reflectivity changes as the incidence angle changes. As used herein, the term "average reflectivity" means an average value of the reflectivity with respect to light with a wavelength of <NUM> to <NUM> at a specific incidence angle. In addition, in a case in which the average reflectivity of the reflected light on the second surface is <NUM>% or less, it is possible to obtain a video having a higher contrast.

An example of a manufacturing method of the light guide <NUM> will be described below.

The light guide <NUM> is produced through a process of forming a dielectric multi-layer film on a plurality of substrates (see <FIG>), a process of bonding the substrates on which the dielectric multi-layer film is formed (see <FIG> and <FIG>), and a process of cutting out a light guide from a bonded body in which the plurality of substrates are bonded (see <FIG>).

A plurality of plate-shaped transparent substrates <NUM> are prepared, and as shown in <FIG>, the dielectric layers <NUM> to <NUM> are sequentially formed on one surface <NUM> of each substrate <NUM> to form the dielectric multi-layer film <NUM>. A method for forming the dielectric layers <NUM> to <NUM> is not particularly limited, but a method of forming a film in plasma, such as sputtering or a plasma chemical vapor deposition (CVD) method, is suitable.

In a case in which each of the layers <NUM> to <NUM> of the dielectric multi-layer film <NUM> is made of silicon oxynitride consisting of silicon, oxygen, and nitrogen, for example, each layer can be formed by a sputtering method in which an argon (Ar) gas, an oxygen gas, or a nitrogen gas is introduced into a chamber using a target. By changing a flow rate ratio of oxygen and nitrogen, an Si:O:N ratio in the film changes. Then, a refractive index of the film can be changed by changing the Si:O:N ratio. Therefore, each layer of the dielectric multi-layer film need only be formed by changing the flow rate ratio of oxygen and nitrogen so as to obtain a desired design refractive index.

<FIG> shows a nitrogen/oxygen flow rate ratio dependence of a refractive index of an oxynitride film with respect to light with a wavelength of <NUM>. In the present specification, the nitrogen/oxygen flow rate ratio is shown as a ratio of oxygen in the nitrogen (N<NUM>) gas + the oxygen (O<NUM>) gas. From <FIG>, it can be seen that in a case in which the nitrogen/oxygen flow rate ratio is changed, the refractive index of the film can be changed in a range from n = <NUM> in a case of nitrogen: oxygen = <NUM>:<NUM> to n = <NUM> in a case of nitrogen: oxygen = <NUM>:<NUM>. Sputtering conditions for <FIG> are as follows: Ar gas flow rate = <NUM> sccm, O<NUM> + N<NUM> gas flow rate = <NUM> sccm, sputtering power = <NUM> W, target diameter = <NUM> inches, substrate temperature (setting) = <NUM>, and sputtering gas pressure = <NUM> Pa. In addition, the refractive index of the film was measured using Ellipsometer VASE (registered trademark) manufactured by J.

In this way, a silicon oxynitride film having a desired refractive index can be obtained by changing the flow rate ratio of nitrogen: oxygen in a case of the sputtering film formation.

In a case of forming a dielectric multi-layer film, it is common to use a metal oxide with a chemical quantitative ratio. Therefore, it is necessary to design a dielectric multi-layer film using a refractive index of a metal oxide with a chemical quantitative ratio. However, as described above, since the refractive index can be changed by changing the Si:O:N ratio, a film with an optional refractive index can be formed of silicon oxynitride, and a design of a dielectric multi-layer film has a high degree of freedom.

In addition, in a case in which each of the layers <NUM> to <NUM> of the dielectric multi-layer film <NUM> is a metal oxide layer containing at least one of Si, Nb, Ta, Al, Ti, W, or Cr, the sputtering method can be similarly used. In co-sputtering using two or more metal targets, the refractive index can be controlled by adjusting a target voltage. In addition, the refractive index of each of the layers <NUM> to <NUM> may be controlled using a method of alternately depositing and forming films containing any of the metals with a thickness of <NUM>/<NUM> or less of a wavelength λ of the video light (see, for example, <CIT>).

An optical contact method is suitable for bonding the plurality of substrates <NUM> on which the dielectric multi-layer film <NUM> is formed.

As shown in <FIG>, an outermost surface 40A of the dielectric multi-layer film <NUM>, which is provided on one surface of the plurality of substrates <NUM> each comprising the dielectric multi-layer film <NUM>, and the other surface <NUM> of the substrate <NUM> on which the dielectric multi-layer film <NUM> is not formed (hereinafter, collectively referred to as bonding surfaces 40A and <NUM>) are irradiated with an ion beam <NUM> in vacuum. An ion beam irradiation device <NUM> is used for irradiation of the ion beam <NUM>. Specifically, the bonding surfaces are irradiated with argon ions as an ion beam <NUM> in a vacuum chamber. The irradiation of the ion beam <NUM> removes stains such as organic substances adhering to the bonding surfaces 40A and <NUM> and activates the bonding surfaces 40A and <NUM>.

After that, as shown in S1 of <FIG>, the substrates <NUM> are sequentially stacked such that the dielectric multi-layer film <NUM> provided on one substrate <NUM> and the surface <NUM> of the other substrate <NUM> on which the dielectric multi-layer film <NUM> is not formed face each other. In this way, as shown in S2 of <FIG>, the activated bonding surfaces are brought into contact with each other. By stacking the substrate <NUM> on which the dielectric multi-layer film <NUM> is not provided on the top, a laminate <NUM> in which each dielectric multi-layer film <NUM> is interposed between the substrates <NUM> is formed.

After that, as shown in S3 of <FIG>, the laminate <NUM> is held under a constant load P, for example, <NUM>/cm<NUM> for a certain period of time, for example, <NUM> hour, to obtain a bonded body <NUM> (see <FIG>).

After that, as shown in <FIG>, the bonded body <NUM> is cut at a cut surface that is tilted by a predetermined angle α with respect to a substrate surface, and a light guide <NUM> comprising the plurality of half mirrors <NUM> in the base <NUM> is cut out. In <FIG>, a broken line constitutes one side of the cut surface. <FIG> is a view of the light guide <NUM> cut out from the bonded body <NUM> as viewed from a direction of an arrow 12B of the bonded body <NUM> of <FIG>. The light guide <NUM> corresponds to the light guide <NUM> comprising the plurality of half mirrors <NUM> that are tilted at a tilt angle α with respect to the first reflecting surface <NUM> and the second reflecting surface <NUM> in parallel to each other. The cut surface is set according to a desired tilt angle α of the half mirror <NUM>. The angle α is preferably, for example, about <NUM>° to <NUM>°.

Hereinafter, specific design examples and verification results of the dielectric multi-layer film constituting the half mirror used in the light guide according to the present disclosure will be shown. In the design examples and the verification examples, a film thickness and a wavelength dependence were obtained by simulation using commercially available thin film calculation software. In the following, the refractive index is a refractive index at a wavelength of <NUM>.

Table <NUM> shows Design Example <NUM> of a dielectric multi-layer film in a case in which SF11 (manufactured by Shott Corporation) having a refractive index n = <NUM> was used as the base. In the simulation, the thickness of each layer was optimized by designing a reflectivity of <NUM>±<NUM>% at an incidence angle of <NUM>° and the lowest reflectivity at an incidence angle of <NUM>°.

In Design Example <NUM>, a refractive index n1 of a layer <NUM> and a layer <NUM> as the outermost layer is <NUM>, and n1 = <NUM>.

<FIG> shows an incidence angle dependence of the reflectivity for light with a wavelength of <NUM> with respect to the dielectric multi-layer film of Design Example <NUM>. As shown in <FIG>, a reflectivity of <NUM>% or less is achieved in a range of an incidence angle of <NUM>° or less, a reflectivity of <NUM>% to <NUM>% is achieved in a range of an incidence angle of <NUM>° to <NUM>°, and a reflectivity of <NUM>% or less is achieved in a range of an incidence angle of <NUM>° to <NUM>°.

With respect to the dielectric multi-layer film of Design Example <NUM>, <FIG> shows a wavelength dependence of a reflectivity for an incidence angle of <NUM>°, and <FIG> shows a wavelength dependence of a reflectivity for an incidence angle of <NUM>°.

As shown in <FIG>, an average reflectivity at an incidence angle of <NUM>° is <NUM>% and an average reflectivity at an incidence angle of <NUM>° is <NUM>% with respect to light with a wavelength of <NUM> to <NUM>. Assuming that an incidence angle θ1 with respect to the first surface <NUM> is <NUM>° and an incidence angle θ2 with respect to the second surface <NUM> is <NUM>° in the half mirror <NUM> provided in the base <NUM>, a video having a high contrast can be obtained with a very small reflectivity of <NUM>% or less in a case in which video light is incident on the second surface <NUM>.

Table <NUM> shows Design Example <NUM> of a dielectric multi-layer film in a case in which S-BSM25 (manufactured by OHARA INC. ) having a refractive index n = <NUM> was used as the base. In the simulation, the thickness of each layer was optimized by designing a reflectivity of <NUM>±<NUM>% at an incidence angle of <NUM>° and the lowest reflectivity at an incidence angle of <NUM>°.

In Design Example <NUM>, a refractive index n1 of a layer <NUM> and a layer <NUM> as the outermost layer is <NUM>, and n1 = <NUM>.

Table <NUM> shows Design Example <NUM> of a dielectric multi-layer film in a case in which BK7 (manufactured by Shott Corporation) having a refractive index n = <NUM> was used as the base. In the simulation, the thickness of each layer was optimized by designing a reflectivity of <NUM>±<NUM>% at an incidence angle of <NUM>° and the lowest reflectivity at an incidence angle of <NUM>°.

In Design Example <NUM>, a refractive index n1 of a layer <NUM> and a layer <NUM> as the outermost layer is <NUM>, and n1 = <NUM>.

As shown in <FIG>, an average reflectivity at an incidence angle of <NUM>° is <NUM>% and an average reflectivity at an incidence angle of <NUM>° is <NUM>% with respect to light with a wavelength of <NUM> to <NUM>.

Results of examining an allowable range of the refractive index n1 of each outermost layer with respect to Design Examples <NUM> to <NUM> described above are shown. For Design Examples <NUM> to <NUM> described above, results obtained for an average reflectivity at an incidence angle of <NUM>° and an average reflectivity at an incidence angle of <NUM>° for light with a wavelength of <NUM> to <NUM> in a case in which the refractive index of the outermost layer is changed from <NUM>. 85n to <NUM>. 20n with respect to the refractive index n of the base are shown in Tables <NUM> to <NUM>, respectively. In the simulation, for each of Design Examples <NUM> to <NUM>, only the refractive index of the outermost layer was changed, the refractive index of the layers <NUM> to <NUM> was not changed, and the reflectivity at an incidence angle of <NUM>° was set to <NUM>±<NUM>% as a target value, and, in this case, the reflectivity at <NUM>° was optimized to be as small as possible.

Table <NUM> shows the results in a case in which the base (SF11) having a refractive index n = <NUM> of Design Example <NUM> was used.

In this example, in a range of n1 = <NUM>. 90n to <NUM>. 20n, the average reflectivity at the incidence angle of <NUM>° was in a range of <NUM>±<NUM>%, and the average reflectivity at the incidence angle of <NUM>° was <NUM>% or less. In addition, the average reflectivity at the incidence angle of <NUM>° with n1 = <NUM>. 00n could be set to <NUM>% or less.

Table <NUM> shows the results in a case in which the base (S-BSM25) having a refractive index n = <NUM> of Design Example <NUM> was used.

In this example, in a range of n1 = <NUM>. 90n to <NUM>. 15n, the average reflectivity at the incidence angle of <NUM>° was in a range of <NUM>±<NUM>%, and the average reflectivity at the incidence angle of <NUM>° was <NUM>% or less. In addition, in a range of n1 = <NUM>. 95n to <NUM>. 05n, the average reflectivity at the incidence angle of <NUM>° could be set to <NUM>% or less.

Table <NUM> shows the results in a case in which the base (BK7) having a refractive index n = <NUM> of Design Example <NUM> was used.

In this example, in a range of n1 = <NUM>. 90n to <NUM>. 15n, the average reflectivity at the incidence angle of <NUM>° was in a range of <NUM>±<NUM>%, and the average reflectivity at the incidence angle of <NUM>° was <NUM>% or less. In addition, in a range of n1 = <NUM>. 00n to <NUM>. 10n, the average reflectivity at the incidence angle of <NUM>° could be set to <NUM>% or less.

In Design Examples <NUM> and <NUM>, in a case in which the refractive index of the outermost layer was set to <NUM>. 85n, no solution was obtained, so the calculation was not performed.

From the above results, it is clear that the average reflectivity at the incidence angle of <NUM>° can be suppressed to <NUM>% or less in a case in which the refractive index of the outermost layer is in a range of <NUM>. 9n to <NUM>. 15n, regardless of which base is used, from low to high refractive index. The refractive index of the outermost layer is preferably <NUM>. 95n to <NUM>. 10n, and more preferably <NUM>. 00n to <NUM>. A still more preferable range of the refractive index of the outermost layer varies slightly depending on the refractive index of the base.

Next, with respect to a case in which the thickness d of the outermost layer of the dielectric multi-layer film was set to <NUM>, <NUM>, or <NUM> in Design Examples <NUM> to <NUM>, a Δn dependence, which is represented by the refractive index n1 of the outermost layer and the refractive index n of the base, of an average reflectivity in a case in which light with a wavelength of <NUM> to <NUM> was incident at the incidence angle of <NUM>° (hereafter, simply referred to as an average reflectivity (<NUM>°) was examined. Here, Δn [%] = {(n - n1)/n}·<NUM>. In the simulation, in film configurations of Design Examples <NUM> to <NUM>, the thickness of the outermost layer (layer <NUM> and layer <NUM>) was fixed and the thicknesses of the other layers <NUM> to <NUM> were optimized such that the reflectivity at the incidence angle of <NUM>° was <NUM>±<NUM>% and the reflectivity at the incidence angle of <NUM>° was the lowest. The results are shown in <FIG>.

<FIG> show the results in a case in which the base (SF11) having a refractive index n = <NUM> of Design Example <NUM> was used. <FIG> shows the Δn dependence of the average reflectivity (<NUM>°), and <FIG> shows a Δn·d dependence of the average reflectivity with respect to light incident at the incidence angle of <NUM>°. As shown in <FIG>, there is a minimum value in a range of -<NUM> to <NUM> for Δn% in all of <NUM>, <NUM>, and <NUM>. As shown in <FIG>, it can be seen that in a case in which a horizontal axis is Δn·d, regardless of the thickness of the outermost layer, the reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>, the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>, and the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>.

<FIG> show the results in a case in which the base (S-BSM25) having a refractive index n = <NUM> of Design Example <NUM> was used. <FIG> shows the Δn dependence of the average reflectivity (<NUM>°), and <FIG> shows a Δn·d dependence of the average reflectivity with respect to light incident at the incidence angle of <NUM>°. As shown in <FIG>, there is a minimum value in a range of -<NUM> to <NUM> for Δn% in all of <NUM>, <NUM>, and <NUM>. As shown in <FIG>, it can be seen that in a case in which a horizontal axis is Δn·d, regardless of the thickness of the outermost layer, the reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>, the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>, and the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>. Further, in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>, the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less.

<FIG> show the results in a case in which the base (BK7) having a refractive index n = <NUM> of Design Example <NUM> was used. <FIG> shows the Δn dependence of the average reflectivity (<NUM>°), and <FIG> shows a Δn·d dependence of the average reflectivity with respect to light incident at the incidence angle of <NUM>°. As shown in <FIG>, there is a minimum value in a range of -<NUM> to +<NUM> for Δn% in all of <NUM>, <NUM>, and <NUM>. As shown in <FIG>, it can be seen that in a case in which a horizontal axis is Δn·d, regardless of the thickness of the outermost layer, the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>, the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>, and the average reflectivity (<NUM>°) can be set to approximately <NUM>% or less in a case in which Δn·d [%·nm] is -<NUM> to +<NUM>.

From the above results, it is possible to set the average reflectivity (<NUM>°) to <NUM>% or less by setting Δn·d [%·nm] to be approximately -<NUM> to +<NUM>, to set the average reflectivity (<NUM>°) to <NUM>% or less by setting Δn·d [%·nm] to be approximately -<NUM> to +<NUM>, and to set the average reflectivity (<NUM>°) to <NUM>% or less by setting Δn·d [%·nm] to be approximately -<NUM> to +<NUM> without selecting the refractive index of the base.

Table <NUM> shows Design Example <NUM> of a dielectric multi-layer film in a case in which S-LAH79 (manufactured by OHARA INC. ) having a refractive index n = <NUM> was used as the base.

In Design Example <NUM>, a refractive index n1 of a layer <NUM> and a layer <NUM> as the outermost layer is <NUM>, and n1 = <NUM>. In a case in which the half mirror comprising the dielectric multi-layer film of Design Example <NUM> is disposed in the base at a tilt angle of <NUM>°, the refractive index of the base is high, so that the video light propagates in the base by repeated total reflection even though the video light is incident on the dielectric multi-layer film at an incidence angle θ1 = <NUM>°.

In addition, for the dielectric multi-layer films of Design Examples <NUM> to <NUM>, a case in which refractive index of a first layer and an eleventh layer, which are the outermost layers, is the same as the refractive index n of the base is referred to as Design Examples 1A to 4A, and, from the incidence angle dependence of the average reflectivity for the light with a wavelength of <NUM> to <NUM> in each example, a preferred incidence angle range of the video light with respect to the first surface and the second surface of the half mirror was verified.

For the half mirror of each of Design Examples 1A to 4A, the average reflectivity with respect to the light with a wavelength of <NUM> to <NUM> at each of incidence angles θ1 and θ2 in a range of the incidence angle θ1 = <NUM>° to <NUM>° and the incidence angle θ2 = <NUM>° to <NUM>° was evaluated according to the following criteria. The results are shown in Table <NUM>.

As the incidence angle θ1, a good average reflectivity of more than <NUM>% and <NUM>% or less was obtained in a range of <NUM>° to <NUM>°. On the other hand, as the incidence angle θ2, an average reflectivity of <NUM>% or less could be obtained in a range of <NUM>° to <NUM>°, and in Design Examples 1A to 3A, an average reflectivity of <NUM>% or less could be obtained in a range of <NUM>° to <NUM>°. It is preferable to set the tilt angle α and the optical coupling angle θ0 of the half mirror such that the incidence angle θ1 is <NUM>° to <NUM>° and the incidence angle θ2 is <NUM>° to <NUM>°. Since the average reflectivity can be <NUM>% or less, the incidence angle θ2 is more preferably <NUM>° to <NUM>°. In a real system, the incidence angle θ1 is preferably <NUM>° or more because of the constraints on the configuration.

Here, a sensory evaluation was made on a state of a light-dark pattern of a video in a case in which the reflectivity with respect to the light incident on the second surface of the half mirror in the light guide was changed. In the present specification, the light-dark pattern of the video means a light-dark pattern based on a light amount intensity distribution that appears in a video visually recognized through the light guide. It is considered that this light-dark pattern is generated by an interference between the video light and the stray light. An ideal high-contrast image is obtained in a case in which no light-dark pattern appears, and the higher the degree of visibility of the light-dark pattern is, the lower the contrast of the image is.

A light guide comprising six half mirrors made of the dielectric multi-layer film shown in Design Example <NUM> was produced (a production method thereof will be described below). In the half mirror of Design Example <NUM>, the average reflectivity can be changed from <NUM>% to <NUM>% or less by changing the incidence angle of the second surface in a range of <NUM>° to <NUM>°. In addition, as an example in which the average reflectivity of the second surface exceeds <NUM>%, a light guide comprising six half mirrors made of the dielectric multi-layer film of Comparative Example <NUM> below was produced. For the light guide comprising the half mirror of Design Example <NUM>, a sensory evaluation was made on the degree of the light-dark pattern due to the interference between the video light and the stray light in a case in which the incidence angle on the first surface was changed such that the incidence angle on the second surface of the half mirror is <NUM>° to <NUM>° and the average reflectivity of the second surface was set to <NUM>% or less to <NUM>%. In addition, for the light guide comprising the half mirror of Comparative Example <NUM>, the same sensory evaluation was made for a case in which the incidence angle on the first surface was set such that the incidence angle on the second surface of the half mirror is <NUM>°, and the average reflectivity of the second surface exceeded <NUM>%.

Table <NUM> shows a layer configuration of Comparative Example <NUM> of a dielectric multi-layer film in a case in which SF11 (manufactured by Shott Corporation) having a refractive index n = <NUM> was used as the base. A refractive index n1 of a first layer and an eleventh layer, which are two outermost layers of the dielectric multi-layer film, was set to <NUM>. 83n, which is a value outside a range of <NUM>. 9n to <NUM>. In the simulation, the thickness of each layer was optimized by designing a reflectivity of <NUM>±<NUM>% at an incidence angle of <NUM>° and the lowest reflectivity at an incidence angle of <NUM>°.

For the half mirror of Comparative Example <NUM>, the average reflectivity with respect to the light with a wavelength of <NUM> to <NUM> at each of incidence angles θ1 and θ2 in a range of the incidence angle θ1 = <NUM>° to <NUM>° and the incidence angle θ2 = <NUM>° to <NUM>° is shown in Table <NUM>.

In Comparative Example <NUM>, the average reflectivity with respect to the light with a wavelength of <NUM> to <NUM> that is incident in a range of the incidence angle θ2 = <NUM>° to <NUM>° exceeds <NUM>%.

As shown in Table <NUM>, in Comparative Example <NUM>, as the incidence angle θ1, a good reflectivity of more than <NUM>% and <NUM>% or less can be obtained in a range of <NUM>° to <NUM>°. On the other hand, in a case in which the light is incident at the incidence angle θ2 of <NUM>° or more, the average reflectivity with respect to the light with a wavelength of <NUM> to <NUM> exceeds <NUM>%.

Table <NUM> shows results of the sensory evaluation in a case in which the average reflectivity of the half mirror to the second surface (second surface reflectivity in Table <NUM>) was changed from <NUM>% or less to more than <NUM>%.

As shown in Table <NUM>, results were obtained that in a case in which the average reflectivity of the second surface of the half mirror exceeds <NUM>%, the light-dark pattern is clearly visible, but in a case in which the average reflectivity is <NUM>% or less, the appearance of the light-dark pattern is somewhat suppressed. From the results of the sensory evaluation, it can be said that the average reflectivity of the half mirror on the second surface is preferably <NUM>% or less, more preferably <NUM>% or less, still more preferably <NUM>% or less, and still more preferably <NUM>% or less.

Hereinafter, an example of a multi-layer film constituting the half mirror in which the average reflectivity with respect to the light with a wavelength of <NUM> to <NUM> that is incident at an incidence angle of <NUM>° to <NUM>° is <NUM>% to <NUM>%, and the average reflectivity with respect to the light incident at an incidence angle of <NUM>° to <NUM>° is <NUM>% or less is shown as Reference Examples <NUM> to <NUM>, which do not fall under the scope of the claims.

Table <NUM> shows a layer configuration of Reference Example <NUM> of a dielectric multi-layer film in a case in which SF11 (manufactured by Shott Corporation) having a refractive index n = <NUM> was used as the base. A refractive index n1 of a first layer of two outermost layers of the dielectric multi-layer film was set in a range of <NUM>. 9n to <NUM>. 15n of the refractive index n of the base, and a refractive index n1 of an eleventh layer was set outside the range of <NUM>. 9n to <NUM>. In the simulation, the thickness of each layer was optimized by setting a target value of the reflectivity at the incidence angle of <NUM>° to <NUM>±<NUM>% and designing the reflectivity at the incidence angle of <NUM>° to have a lowest value.

With respect to the dielectric multi-layer film of Reference Example <NUM>, <FIG> shows a wavelength dependence of a reflectivity for an incidence angle of <NUM>°, and <FIG> shows a wavelength dependence of a reflectivity for an incidence angle of <NUM>°.

As shown in <FIG>, an average reflectivity at an incidence angle of <NUM>° is <NUM>% and an average reflectivity at an incidence angle of <NUM>° is <NUM>% with respect to light with a wavelength of <NUM> to <NUM>. Assuming that an incidence angle θ1 with respect to the first surface <NUM> is <NUM>° and an incidence angle θ2 with respect to the second surface <NUM> is <NUM>° in the half mirror <NUM> provided in the base <NUM>, the average reflectivity in a case in which video light is incident on the second surface <NUM> exceeds <NUM>%, and the average reflectivity increases as compared with Design Examples <NUM> to <NUM>. However, since the average reflectivity in a case in which the video light is incident on the second surface is <NUM>% or less, it is possible to obtain an effect of suppressing the light-dark pattern appearing due to interference between the stray light and the video light, that is, an effect of improving the contrast of the video can be obtained.

Table <NUM> shows a layer configuration of Reference Example <NUM> of a dielectric multi-layer film in a case in which S-BSM25 (manufactured by OHARA INC. ) having a refractive index n = <NUM> was used as the base. A refractive index n1 of a first layer of two outermost layers of the dielectric multi-layer film was set in a range of <NUM>. 9n to <NUM>. 15n of the refractive index n of the base, and a refractive index n1 of an eleventh layer was set outside the range of <NUM>. 9n to <NUM>. In the simulation, the thickness of each layer was optimized by setting a target value of the reflectivity at the incidence angle of <NUM>° to <NUM>±<NUM>% and designing the reflectivity at the incidence angle of <NUM>° to have a lowest value.

As shown in <FIG>, an average reflectivity at an incidence angle of <NUM>° is <NUM>% and an average reflectivity at an incidence angle of <NUM>° is <NUM>% with respect to light with a wavelength of <NUM> to <NUM>. Assuming that an incidence angle θ1 with respect to the first surface <NUM> is <NUM>° and an incidence angle θ2 with respect to the second surface <NUM> is <NUM>° in the half mirror <NUM> provided in the base <NUM>, the average reflectivity in a case in which video light is incident on the second surface <NUM> exceeds <NUM>%, and the average reflectivity increases as compared with Design Examples. However, since the average reflectivity in a case in which the video light is incident on the second surface is <NUM>% or less, it is possible to obtain an effect of suppressing the light-dark pattern appearing due to interference between the stray light and the video light, that is, an effect of improving the contrast of the video can be obtained.

Table <NUM> shows Reference Example <NUM> of a dielectric multi-layer film in a case in which BK7 (manufactured by Shott Corporation) having a refractive index n = <NUM> was used as the base. A refractive index n1 of a first layer of two outermost layers of the dielectric multi-layer film was set in a range of <NUM>. 9n to <NUM>. 15n of the refractive index n of the base, and a refractive index n1 of an eleventh layer was set outside the range of <NUM>. 95n to <NUM>. In the simulation, the thickness of each layer was optimized by setting a target value of the reflectivity at the incidence angle of <NUM>° to <NUM>±<NUM>% and designing the reflectivity at the incidence angle of <NUM>° to have a lowest value.

As described above, in Reference Examples <NUM> to <NUM>, the reflectivity of the second surface is larger than that in Design Examples <NUM> to <NUM>. That is, by setting the refractive index of the two outermost layers of the dielectric multi-layer film to <NUM>. 90n to <NUM>. 15n in a case in which the refractive index of the base is n, as in Design Examples <NUM> to <NUM>, the reflectivity of the second surface can be more effectively suppressed, and by setting the refractive index of the two outermost layers of the dielectric multi-layer film to <NUM>. 95n to <NUM>. 15n, the reflectivity of the second surface can be still more effectively suppressed. On the other hand, even though a condition that the refractive index of the two outermost layers of the dielectric multi-layer film is <NUM>. 90n to <NUM>. 15n or <NUM>. 95n to <NUM>. 15n in a case in which the refractive index of the base is n, as in Design Examples <NUM> to <NUM>, is not satisfied, the average reflectivity of the video light to the second surface satisfies <NUM>% or less, so that the contrast improvement effect of the video can be obtained in comparison with a case in which the average reflectivity of the video light to the second surface exceeds <NUM>%.

A production method of the light guide used in a sensory evaluation test will be described.

Seven substrates (SF11) having a thickness of <NUM> × <NUM> × <NUM> were prepared, and a half mirror made of a dielectric multi-layer film was formed on one surface of six substrates out of the seven substrates. Specifically, the dielectric multi-layer film shown in Design Example <NUM> was formed. Each layer shown in Design Example <NUM> was a silicon oxynitride film.

In this case, the nitrogen/oxygen flow rate ratio was set as shown in Table <NUM> below according to the nitrogen/oxygen flow rate ratio dependence of the refractive index shown in <FIG>. The film thickness was as shown in Table <NUM>. Sputtering conditions were as follows: Ar gas flow rate = <NUM> sccm, O<NUM> + N<NUM> gas flow rate = <NUM> sccm, sputtering power = <NUM> W, target diameter = <NUM> inches, substrate temperature (setting) = <NUM>, and sputtering gas pressure = <NUM> Pa.

After the film formation, the film was cut to have a size of <NUM> × <NUM> with a slicer.

Next, the bonding surface of the substrate on which the dielectric multi-layer film was formed and the bonding surface of the substrate on which the dielectric multi-layer film was not formed were irradiated with an ion beam to perform cleaning and activation. As a device for irradiation with the ion beam, a device shown in Table <NUM> was used.

Conditions for ion beam irradiation were as shown in Table <NUM>.

After the ion beam irradiation, six substrates on which the dielectric multi-layer film was formed and one substrate on which the dielectric multi-layer film was not formed were stacked in the atmosphere, and then a load of <NUM>/cm<NUM> was applied thereto and held for <NUM> hour, thereby obtaining a bonded body.

After that, the bonded body was cut as shown in <FIG>, and then a light guide for a sensory test was obtained in which six half mirrors were disposed in the base at equal intervals with an inclination of <NUM>° with respect to the first reflecting surface and the second reflecting surface.

Next, results of verification of the mechanical strength and environmental durability of the light guide formed by performing bonding by optical contact as in the above-described manufacturing method will be described. In addition, for a durability test, a light guide <NUM> having a side parallelogram shape was cut out from a bonded body produced in the same manner as in the above-described manufacturing method, and as shown in <FIG>, both ends of the light guide <NUM> were cut to produce <NUM> rectangular sample pieces S. An area of the cut end surface was <NUM><NUM>.

A mechanical strength test was executed according to a JIS K <NUM> test. A strength tester (model number DS2-500N) manufactured by IMADA Corporation was used. As shown in <FIG>, after the sample piece S was placed between a probe <NUM> of the tester and a stainless steel table <NUM>, a load P was gradually applied until fracture occurred. A strength test was performed on <NUM> sample pieces S for a strength test <NUM>. Table <NUM> shows results of the fracture load for each sample piece.

As a result of confirming the breakage of each sample piece, the number of samples in which the fracture occurred from the bonding surface was <NUM>. In general, in a bonded member in which optical members are bonded to each other, the fracture occurs at a bonding surface, but in all the sample pieces, the fracture occurred at a portion other than the bonding surface. From this result, the average adhesion strength (= average fracture load/sample area) for a plurality of samples is calculated as <NUM> N/cm<NUM> from the fracture strength of <NUM> kgf, the sample area of <NUM><NUM>, and <NUM> kgf = <NUM> N. Therefore, it can be estimated that the adhesion strength of the bonding surface by the optical contact is <NUM> N/cm<NUM> or more.

Further, as a reliability test, assuming that the product is to be placed in a poor environment, a high-temperature and high-humidity test and a thermal shock test were performed, followed by a strength test using the same method as described above.

As the high-temperature and high-humidity test, six sample pieces S for a strength test <NUM> were stored in an environment of <NUM> and <NUM> RH% for <NUM> hours. After that, a strength test was performed in the same manner as described above. Table <NUM> shows results of the fracture load for each sample piece.

As the thermal shock test, with respect to six sample pieces S for a strength test <NUM>, storage in a temperature tank at <NUM> for <NUM> minutes and storage in a temperature tank at -<NUM> for <NUM> minutes as one cycle were repeated for <NUM> cycles with a temperature tank transfer time of <NUM> minutes or less. After that, a strength test was performed in the same manner as described above. Table <NUM> shows results of the fracture load for each sample piece.

In both the strength tests <NUM> and <NUM>, as a result of confirming the breakage of the sample, the number of samples in which the fracture occurred from the bonding surface was <NUM> as in the case of the strength test <NUM>. From this result, it was found that the adhesion strength of the bonded surfaces bonded by the optical contact hardly deteriorated even in an environment of high temperature and high humidity and thermal shock. In addition, also in these strength tests, the bonding strength was estimated to be <NUM> N/cm<NUM>.

Claim 1:
A light guide (<NUM>) comprising:
a base (<NUM>) that has a first reflecting surface (<NUM>) and a second reflecting surface (<NUM>) and that propagates incident video light (L0) while totally reflecting the video light (L0) by the first reflecting surface (<NUM>) and the second reflecting surface (<NUM>); and
a plurality of half mirrors (<NUM>), each of which has a first surface (<NUM>) and a second surface (<NUM>) on a back side of the first surface (<NUM>) and is configured to include a dielectric multi-layer film (<NUM>),
wherein the plurality of half mirrors (<NUM>) are disposed in the base such that the half mirrors (<NUM>) are spaced from each other by being tilted with respect to the first reflecting surface (<NUM>) and the second reflecting surface (<NUM>),
the base (<NUM>) and the plurality of half mirrors (<NUM>) are configured such that the video light (L0) made incident into the base is incident on each of the first surface (<NUM>) and the second surface (<NUM>) of at least one of the plurality of half mirrors (<NUM>) one or more times, characterised in that:
(i) refractive indices of two outermost layers (<NUM>, <NUM>) of the dielectric multi-layer film (<NUM>) on a first surface side and a second surface side are <NUM>.90n to <NUM>.15n in a case in which a refractive index of the base (<NUM>) is n; and
(ii) the half mirror (<NUM>) has an average reflectivity of <NUM>% to <NUM>% with respect to light with a wavelength of <NUM> to <NUM> that is incident at an incidence angle of <NUM>° to <NUM>°, and has an average reflectivity of <NUM>% or less with respect to the light incident at an incidence angle of <NUM>° to <NUM>°, wherein the video light (L0) is partially reflected by each of the plurality of half mirrors (<NUM>) disposed in the base (<NUM>) and emitted from the base (<NUM>) as emitted light (L1) to make a user (<NUM>) visually recognize the video.