Display optical system and image display device

Abstract

To provide a polarization-based display optical system with reduced external light ghosting.SOLUTION: A display optical system provided herein guides light from a display element 108 to a viewing side via a partially transmissive / reflective surface 112 and a polarization separating surface 114. The partially transmissive / reflective surface has a reflectance of 35% or less for visible light. The partially transmissive / reflective surface is made of a dielectric multilayer film with 5 to 10 layers, inclusive.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a display optical system suitable for an image display device such as a head-mounted display (HMD), which displays an enlarged version of an original image displayed on a display element. [Background technology]

[0002] As an example of such a display optical system, an optical system that folds the optical path by using polarized light and that uses a polarization selection element (polarization separation element) and a half mirror is disclosed in Patent Documents 1 and 2. These optical systems use reflection by the half mirror, so external light is easily reflected inside the optical system and directed toward the viewer's eye, causing external light ghosts.

[0003] Patent Documents 1 and 2 disclose the reflectance of half mirrors. Patent Document 1 also discloses that a curved polarization selection element is used in the optical system to achieve both a wider angle of view and a thinner optical system. Patent Document 2 discloses that the reflectance of the half mirror in the optical system is made greater than the transmittance, thereby reducing ghosts caused by birefringence of the lens. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2020-515903 [Patent Document 2] Japanese Patent Publication No. 2021-124539 Summary of the Invention [Problem to be solved by the invention]

[0005] However, although Patent Document 1 states that the reflectance of the half mirror is 20% or more, it does not mention anything about external light ghosts or details about the film that makes up the half mirror. Also, in the optical system of Patent Document 2, the reflectance of the half mirror is high, which may increase external light ghosts.

[0006] The present invention provides a display optical system and an image display device that are optical systems that use polarized light and are capable of reducing external light ghosts. [Means for solving the problem]

[0007] A display optical system according to one aspect of the present invention guides light from a display element to a viewing side via a partial transmission reflecting surface and a polarization separation surface. The partial transmission reflecting surface has a reflectance for visible light of 35% or less. The partial transmission reflecting surface is formed of a dielectric multilayer film, and the number of layers in the dielectric multilayer film is 5 to 10. Note that an image display device including the above-described display optical system and a display element also constitutes another aspect of the present invention. [Effects of the Invention]

[0008] According to the present invention, it is possible to reduce external light ghosts in a display optical system that uses polarized light. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a plan view showing the configuration of an image display device according to a first embodiment. [Figure 2] FIG. 1 is a cross-sectional view showing the configuration of a display optical system according to a first embodiment. [Figure 3] 1 is an external view of an image display device according to a first embodiment. [Figure 4] FIG. 2 is a diagram showing the relationship between the display optical system and the eyeball in the first embodiment. [Figure 5] FIG. 3 is a diagram showing the optical path of an external light ghost in the display optical system according to the first embodiment. [Figure 6] FIG. 3 is a diagram showing the optical path of an internal ghost in the display optical system according to the first embodiment. [Figure 7] FIG. 3 is a diagram showing the optical path of an external light ghost in the display optical system according to the first embodiment. [Figure 8] FIG. 3 is a diagram showing the reflectance characteristics of a half mirror in the first embodiment. [Figure 9] FIG. 10 is another diagram showing the reflectance characteristics of the half mirror in the first embodiment. [Figure 10]10 is yet another diagram showing the reflectance characteristics of the half mirror in the first embodiment. FIG. [Figure 11] FIG. 10 is a plan view showing the configuration of an image display device according to a second embodiment. [Figure 12] FIG. 10 is a diagram showing the configuration of a display optical system according to a second embodiment. [Figure 13] FIG. 10 is a diagram showing the optical path of an internal ghost in the display optical system according to the second embodiment. [Figure 14] FIG. 10 is a diagram showing the reflectance characteristics of a half mirror in the second embodiment. [Figure 15] FIG. 10 is another diagram showing the reflectance characteristics of the half mirror in the second embodiment. [Figure 16] 10 is yet another diagram showing the reflectance characteristics of the half mirror in the second embodiment. FIG. [Figure 17] FIG. 10 is a diagram showing a cemented lens in a display optical system according to a second embodiment. [Figure 18] FIG. 10 is another diagram showing the reflectance characteristics of the half mirror in the second embodiment. [Figure 19] 10A and 10B are diagrams illustrating gaze detection in a display optical system according to a second embodiment. [Figure 20] FIG. 10 is still another diagram showing the reflectance characteristics of the half mirror in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [Example]

[0011] FIG. 1 shows the configuration of an HMD 101 as an image display device using the display optical system of Example 1, viewed from above. Reference numeral 102 denotes the observer's right eye, and 103 denotes the observer's left eye. Lenses 104 and 105 constitute part of the display optical system for the right eye, and lenses 106 and 107 constitute part of the display optical system for the left eye. Reference numeral 108 denotes a display element for the right eye, and 109 denotes a display element for the left eye, which in this example use an organic EL element. Each display element emits three colors of light: red light, blue light, and green light.

[0012] The right-eye display optical system magnifies light (virtual image) from an original image displayed on a right-eye display element 108 and guides it to the right eye 102, which is located at the exit pupil on the observation side of the display optical system. The left-eye display optical system magnifies light from an original image displayed on a left-eye display element 109 and guides it to the left eye 103, which is located at the exit pupil on the observation side of the display optical system.

[0013] The focal length f1 of the right-eye display optical system and the left-eye display optical system is 12 mm, the horizontal display angle of view is 45°, the vertical display angle of view is 34°, and the diagonal display angle of view is 54°. The distance E1 between the HMD 101 and the observer's eyes (eye relief) is 18 mm.

[0014] The display optical system of this embodiment is an optical system that folds the optical path by using polarized light, and its specific configuration will be explained using the display optical system for the right eye shown in Fig. 2. The display optical system for the right eye has a first polarizing plate 110 and a first phase plate 111, which are arranged between a right-eye display element 108 and a lens 105 in this order from the display element side. The first polarizing plate 110 and the first phase plate 111 are each formed in a planar shape and are stacked on top of each other.

[0015] Furthermore, a half mirror 112 constituting a partial transmission / reflection surface is formed by vapor deposition on the surface of the lens 104 facing the display element (lens 105). Furthermore, between the lens 104 and the right eye 102, a second phase plate 113 and a polarization separation element (PBS) 114 constituting a polarization separation surface are arranged in this order from the display element side (lens 104 side). The polarization separation surface is an optically functional surface whose transmittance and reflectance change depending on the polarization direction of incident light. The second phase plate 113 and the PBS 114 are each formed in a planar shape, stacked on top of each other, and bonded to the plane of the lens 104 facing the exit pupil (eyeball side). The first phase plate 111 and the second phase plate 113 are each wave plates with a phase difference of λ / 4.

[0016] The polarization direction of the polarized light transmitted by first polarizer 110 and the slow axis of first phase plate 111 are inclined at 45° to each other. The polarization direction of the polarized light transmitted by first polarizer 110 and the slow axis of second phase plate 113 are inclined at −45° to each other. The polarization direction of the polarized light transmitted by first polarizer 110 and the polarization direction of the polarized light transmitted by PBS 114 are perpendicular to each other.

[0017] In the above configuration, unpolarized light emitted from the right-eye display element 108 passes through the first polarizing plate 110 and becomes linearly polarized light, and this linearly polarized light passes through the first phase plate 111 and becomes circularly polarized light. The circularly polarized light that passes through the half mirror 112 passes through the second phase plate 113 and becomes linearly polarized light, and because the polarization direction of this linearly polarized light is perpendicular to the polarization direction of light passing through the PBS 114, it is reflected by the PBS 114. The reflected linearly polarized light then passes through the second phase plate 113 and becomes circularly polarized light.

[0018] The circularly polarized light reflected by the half mirror 112 passes through the second phase plate 113 and becomes linearly polarized light. Since the polarization direction of this linearly polarized light matches the polarization direction of light passing through the PBS 114, it passes through the PBS 114 and is guided to the right eye 102. The above configuration is the same for the display optical system for the left eye.

[0019] By folding the optical path using polarized light as in this embodiment, the display optical system can be made thinner and the focal length can be shortened, thereby realizing image observation at a wide angle of view.

[0020] FIG. 3 shows the appearance of the HMD 101. Because the HMD 101 is worn on the viewer's head, it is desirable that it be lightweight. For this reason, the lenses that make up the display optical system are desirably made of resin, which has a lower specific gravity than glass, and in this embodiment, the lenses 104 and 106 are made of resin lenses. Furthermore, by making the lenses 104 and 106 aspherical lenses with a plano-convex shape, the aberration correction effect can be improved. The lenses 105 and 107 are double-sided aspherical lenses made of resin.

[0021] As shown in FIG. 4, the exit pupil of the display optical system in this embodiment is located at a distance of 28 mm, which is the sum of an eye relief of 18 mm and the radius of rotation of the eyeball (102) of 10 mm, and the exit pupil diameter is 6 mm. This ensures that even when the eyeball rotates to observe up, down, left, or right, light in that direction is incident on the eyeball. It is desirable that the eye relief be 15 mm or more so that observers wearing eyeglasses can also wear the HMD 101. Furthermore, since a longer eye relief increases the outer diameter of the lens and therefore the size of the HMD 101, it is desirable that the eye relief be 25 mm or less.

[0022] In the display optical system of this embodiment, the surface on which the half mirror 112 is vapor-deposited (the surface of the lens 104 on the display element side) is a surface that is convex toward the display element side. By vapor-depositing the half mirror 112 on this convex surface, it is possible to achieve a wide angle of view while reducing the thickness of the display optical system. Furthermore, by making the convex surface on which the half mirror 112 is vapor-deposited aspherical, it is possible to improve the aberration correction effect.

[0023] On the other hand, the display optical system of this embodiment is internally provided with a half mirror 112. Therefore, as shown in Fig. 5, external light 115, which is unwanted light entering the display optical system from outside the eyeball (102), is multiple-reflected by the half mirror 112 and the PBS 114 and is guided to the eyeball, becoming an external light ghost. The external light ghost is reflected on the displayed image, which is distracting to the viewer and makes it difficult to observe a good image.

[0024] Therefore, in this embodiment, the reflectance of the half mirror 112 is set to 35% to reduce the amount of external light ghost light. Because the reflectance of a conventional half mirror is 50%, the efficiency (amount of external light incident on the eyeball) when external light is reflected twice is 25%. In contrast, if the reflectance of the half mirror 112 is set to 35%, the efficiency when reflected twice is 12%, which is approximately half of the conventional efficiency. For this reason, in order to reduce external light ghost light, it is desirable to set the reflectance of the half mirror 112 to 35% or less.

[0025] The reflectance of the half mirror in this embodiment (and in the second embodiment described later) is a numerical value in the wavelength range of visible light (for example, wavelengths of 400 nm to 700 nm). However, if the reflectance differs for each wavelength, it may be the average value of the reflectance in the visible light range. It may also be the reflectance for a representative wavelength with high luminosity, such as green light, among red, blue, and green light. If the reflectance characteristics of the half mirror differ depending on the angle of incidence, it may be the reflectance at a specific angle of incidence (for example, an angle of incidence of 0°) or the reflectance at the angle of incidence of ghost light, which will be described later.

[0026] On the other hand, it is desirable that the reflectance of the half mirror 112 is 20% or more. This is because if the reflectance is less than 20%, the display efficiency for the light emitted from the display element 108 will be too low, making it impossible to observe a bright display image. When a conventional half mirror with a reflectance of 50% is used, the display efficiency is 25% (reflectance x transmittance). In contrast, when the reflectance of the half mirror 112 is 20%, the reflectance x transmittance becomes 16%, which is approximately 60% of the conventional efficiency.

[0027] Table 1 shows the film configuration of the half mirror 112 in this embodiment, and FIG. 8 shows the reflectance characteristics (spectral characteristics) of the half mirror 112. As shown in Table 1, by forming the half mirror 112 from a dielectric multilayer film, a half mirror with lower reflectance than conventional half mirrors can be realized. Table 1 also shows the refractive index and film thickness of the substrate and each layer (film) of the dielectric multilayer film formed thereon (this also applies to other tables described later). The dielectric multilayer film that constitutes the half mirror 112 contains alternating layers of silicon oxide (SiO2) and niobium oxide (Nb2O5). If a low-reflectance half mirror is to be formed by vapor deposition using a metal film such as silver, the metal film will be too thin, making vapor deposition difficult. For this reason, it is desirable to form a low-reflectance half mirror using a dielectric multilayer film.

[0028] [Table 1]

[0029] When using a half mirror 112 with low reflectivity as in this embodiment, variations (changes) in the reflectivity within the mirror surface have a significant impact on the display efficiency of light from the display element 108. For this reason, it is desirable that the variations in the reflectivity (and transmittance) within the mirror surface of the half mirror 112 be ±5% or less. When a conventional half mirror with a reflectivity of 50% is used, the display efficiency is 25% (reflectance x transmittance), as described above. However, if the variations in reflectance and transmittance are ±10%, the display efficiency becomes 24%, a decrease of 1%. In contrast, when a half mirror 112 with a reflectivity of 35% is used, the display efficiency (reflectance x transmittance) is 22.75%, but if the variations in reflectance and transmittance are ±10%, the display efficiency becomes 18.75%, a decrease of 4%. For this reason, in this embodiment, the variations in the reflectivity and transmittance are kept to ±5% by reducing the variations in the thickness of the dielectric multilayer film. As a result, the display efficiency (reflectance x transmittance) is 21%, a decrease of only 1.75%.

[0030] In the dielectric multilayer half mirror 112, the polarization characteristics change depending on the angle of incidence of light onto the half mirror 112. FIG. 9 shows the reflectance characteristics of the half mirror 112 when the angle of incidence of light onto the half mirror 112 is 45°. As the angle of incidence increases, the difference in reflectance between S-polarized light and P-polarized light (first linearly polarized light and second linearly polarized light) incident on the half mirror 112 increases. In this case, unpolarized external light incident on the display optical system becomes linearly polarized when it passes through the PBS 114, and this linearly polarized light becomes circularly polarized when it passes through the second phase plate 113. When the circularly polarized external light is reflected by the half mirror 112, if the half mirror 112 does not have polarization characteristics, it is reflected as circularly polarized light. However, if the half mirror 112 has polarization characteristics, it is reflected as elliptically polarized light. The elliptically polarized and reflected external light passes through the second phase plate 113 and enters the PBS 114, where it is separated into light that passes through the PBS 114 and exits the display optical system and light that is reflected by the PBS 114.

[0031] 7, light that is reflected once by the half mirror 112 and emitted outside the display optical system follows an optical path that is less likely to be guided to the eyeball, and is therefore less likely to cause external light ghosts. In addition, the amount of light that is reflected twice by the half mirror 112 and emitted outside the display optical system is reduced by the amount of light that is reflected once by the half mirror 112 and emitted outside the display optical system, thereby reducing external light ghosts.

[0032] It is desirable that the difference in reflectance between S-polarized light and P-polarized light as visible light on the half mirror 112 be 20% or more. This increases the effect of reducing external light ghosts caused by external light that has been reflected twice by the half mirror 112.

[0033] In a display optical system using polarized light as in this embodiment, ghost light, which is unwanted light that does not follow the optical path of normal light (effective visible light that contributes to image display) shown in FIG. 1, is generated due to birefringence of lenses 104 to 107 and polarization characteristics of polarizing plate 110, first and second phase plates 111 and 113, and PBS 114. That is, as shown in FIG. 6, internal ghost light is generated by ghost light that is guided to the eyeball without being reflected by PBS 114. Specifically, unpolarized light emitted from display element 108 passes through first phase plate 111 and becomes circularly polarized light, but this circularly polarized light is converted to elliptically polarized light by birefringence in lenses 105 and 104. When the elliptically polarized light enters second phase plate 113, the polarization direction of the linearly polarized light that passes through it is tilted relative to the polarization direction of the polarized light reflected by PBS 114. As a result, ghost light is generated that passes through PBS 114 and is guided to the eyeball (102), generating internal ghost light. Furthermore, even if there is no birefringence in the lens, if the polarization characteristics of the polarizer 110, the first and second phase plates 111 and 113, and the PBS 114 are not good, internal ghosts will occur.

[0034] 1 and the optical path of the ghost light in Fig. 6, in the optical path of the normal light, the light emitted from the display element 108 is both transmitted and reflected by the half mirror 112, but in the optical path of the ghost light, the light is only transmitted by the half mirror 112. For this reason, if the reflectance of the half mirror 112 is low and the transmittance is high, as in this embodiment, the intensity of the internal ghost increases.

[0035] For this reason, it is desirable that the birefringence in the region near the center of each of the lenses 104 to 107 is small. In this embodiment, the retardation amount per mm of the lens thickness of the lenses 104 and 106 is 2 nm / mm, and the retardation amount per mm of the lens thickness of the lenses 105 and 107 is 5 nm / mm. The retardation amount per mm of the lens thickness is desirably 10 nm / mm or less. Furthermore, because light passes through the lenses 104 and 106 three times, it is desirable that the birefringence be even smaller, for example, that the retardation amount per mm of the lens thickness be 5 nm / mm or less.

[0036] Furthermore, if the reflectance of the half mirror 112 differs for each wavelength, a difference will occur between the spectral characteristics of the half mirror 112 in the optical path of normal light and the spectral characteristics of the half mirror 112 in the optical path of ghost light. As described above, comparing the optical path of normal light in Fig. 1 with the optical path of ghost light in Fig. 6, in the optical path of normal light, light emitted from the display element 108 is both transmitted and reflected by the half mirror 112, but in the optical path of ghost light, light is only transmitted by the half mirror 112. For this reason, if the reflectance of the half mirror 112 differs for each wavelength, a difference will occur between the spectral characteristics of the half mirror 112 for normal light and the spectral characteristics of the half mirror 112 for ghost light, resulting in a larger color shift between the normal light and the ghost light and making the ghost light more noticeable.

[0037] Therefore, in this embodiment, the reflectance and transmittance characteristics of the half mirror 112 at an incident angle of 0° are set as shown in FIG. 10. In other words, by reducing the difference in reflectance of the half mirror 112 for each wavelength within the visible light range, the difference between the spectral characteristics of the half mirror 112 for normal light and the spectral characteristics of the half mirror for ghost light is reduced. In FIG. 10, the spectral characteristics of the half mirror 112 for normal light are shown in a graph of "transmittance x reflectance," and the spectral characteristics of the half mirror 112 for ghost light are shown in a graph of "transmittance." The small difference in the spectral characteristics shown in these graphs reduces the color shift between normal light and ghost light, making the ghost light less noticeable.

[0038] For example, the dominant wavelength of blue light emitted from the display element 108 is 450 nm, the dominant wavelength of green light is 525 nm, and the dominant wavelength of red light is 610 nm. The reflectance of the half mirror 112 for the first wavelength of 450 nm, the second wavelength of 525 nm, and the third wavelength of 610 nm is 22%, 32%, and 35%, respectively. In this case, the difference between the maximum and minimum reflectance values ​​for the dominant wavelengths of the blue, green, and red light emitted from the display element 108 is 13%. This difference is preferably 15% or less. A difference of 15% or less reduces the color shift between normal light and ghost light due to differences in the spectral characteristics of the half mirror 112, making the ghost light less noticeable. Furthermore, to minimize the color shift between normal light and ghost light, this difference is preferably 5% or less.

[0039] The first to third wavelengths that define the difference in reflectance between the half mirror 112 for normal light and ghost light are desirably set to match the dominant wavelengths of the blue, green, and red light emitted from the display element 108. This makes it possible to reduce the color shift between the normal light and ghost light in accordance with the spectral characteristics of the light emitted from the display element 108. However, when the difference in reflectance between the half mirror 112 for each wavelength is small as in this embodiment, the first to third wavelengths do not necessarily have to match the dominant wavelengths of the blue, green, and red light from the display element 108.

[0040] Generally, the dominant wavelength of blue light emitted from a display element is often in the range of 430 to 480 nm, and the dominant wavelength of green light is often in the range of 520 to 570 nm. The dominant wavelength of red light is often in the range of 600 to 650 nm. Therefore, it is desirable that the first wavelength be in the range of 430 to 480 nm, the second wavelength be in the range of 520 to 570 nm, and the third wavelength be in the range of 600 to 650 nm. This can reduce color shifts among blue, green, and red.

[0041] In this embodiment, the dielectric multilayer film serving as the half mirror 112 has a five-layer structure. The number of layers in the dielectric multilayer film is preferably 5 or more and 10 or less. If the number of layers is less than 5, the reflectance varies greatly for each wavelength, and the color difference between the ghost light and the normal light increases, making the ghost light more noticeable. Furthermore, if the number of layers is more than 10, the durability of the dielectric multilayer film decreases, which is not preferable.

[0042] In this embodiment, the lenses 105 and 107 are made of resin as described above, but the lenses 105 and 107 may be made of glass lenses because their small outer diameters have little effect on the weight of the display optical system. The birefringence of glass lenses is very small, making it possible to observe high-quality images.

[0043] In addition, in order to reduce external light ghosts and increase the contrast of the displayed image, a second polarizing plate (not shown) may be disposed between the PBS 114 and the eyeball (102).

[0044] In this embodiment, the eyeball-side surface of the lens 104 on which the second phase plate 113 and the PBS 114 are provided is flat. This is to achieve both a long eye relief and a slim display optical system. If this surface has a concave shape toward the eyeball side, the lens 104 would have to be thick to ensure the eye relief at the periphery of the surface, which is not preferable. Also, if this surface has a convex shape toward the eyeball side, the lens 104 would have to be thick to ensure the thickness of the edge of the lens 104, which is not preferable. For this reason, in this embodiment, the lens 104 is a plano-convex lens.

[0045] As described above, the phase difference between the first and second phase plates 111 and 113 in this embodiment is λ / 4, but the phase difference may be shifted from λ / 4 to cancel the birefringence of the lenses 104 and 105. In this case, the sum of the phase differences between the lens 104 and the second phase plate 113 is preferably 3λ / 20 or more and 7λ / 20 or less. The sum of the phase differences between the lens 105 and the first phase plate 111 is also preferably 3λ / 20 or more and 7λ / 20 or less. If the sum of the phase differences is outside these ranges, the intensity of ghost light increases, making it impossible to observe a good image.

[0046] In this embodiment, an organic EL element that emits unpolarized light is used as the display element, but by using a liquid crystal element that emits linearly polarized light, the first polarizer 110 is not required, and the display optical system can be made even thinner. [Example]

[0047] 11 shows the configuration of an HMD 201 as an image display device using the display optical system of Example 2, viewed from above. Reference numeral 202 denotes the right eye of the observer, and 203 denotes the left eye of the observer. Lenses 204 and 205 are cemented together to form part of the display optical system for the right eye, and lenses 206 and 207 are cemented together to form part of the display optical system for the left eye. Reference numeral 208 denotes a display element for the right eye, and 209 denotes a display element for the left eye, and organic EL elements are used in this example as well.

[0048] The right eye display optical system magnifies light (virtual image) from the original image displayed on the right eye display element 208 and directs it to the right eye 202 located on the observation side, and the left eye display optical system magnifies light from the original image displayed on the left eye display element 209 and directs it to the left eye 203 located on the observation side.

[0049] The focal length f2 of the right-eye display optical system and the left-eye display optical system is 13 mm, the horizontal display angle of view is 60°, the vertical display angle of view is 60°, and the diagonal display angle of view is 78°. The eye relief E2 is 20 mm.

[0050] The display optical system of this embodiment also folds the optical path using polarized light. Its specific configuration will be described using the right-eye display optical system shown in FIG. 12. The right-eye display optical system includes a first polarizing plate 210 and a first phase plate 211 disposed between a right-eye display element 208 and a lens 205, and a half mirror 212 configured as a partially transmissive and reflective surface disposed at the junction between the lens 204 and the lens 205, which are cemented together. The half mirror 212 is deposited on the surface of the lens 204 facing the lens 205. The right-eye display optical system also includes a second phase plate 213 and a PBS 214 disposed between the lens 204 and the right eye 202, in that order from the display element side. The second phase plate 213 and the PBS 214 are stacked on top of each other and bonded to the flat surface of the lens 204 facing the eyeball side. Both the first and second phase plates 211 and 213 are wave plates with a phase difference of λ / 4.

[0051] The polarization direction of the polarized light transmitted by first polarizer 210 is tilted at 45° from the slow axis of first phase plate 211. The polarization direction of the polarized light transmitted by first polarizer 210 is tilted at -45° from the slow axis of second phase plate 213. The polarization direction of the polarized light transmitted by first polarizer 210 and the polarization direction of the polarized light transmitted by PBS 214 are orthogonal to each other.

[0052] In the above configuration, unpolarized light emitted from the right-eye display element 208 passes through the first polarizing plate 210 and becomes linearly polarized light. The linearly polarized light passes through the first phase plate 211 and becomes circularly polarized light. The circularly polarized light passes through the half mirror 214 and then passes through the second phase plate 213 and becomes linearly polarized light. Because the polarization direction of this linearly polarized light is perpendicular to the polarization direction of the polarized light passed through the PBS 214, it is reflected by the PBS 214 and then passes through the second phase plate 213 and becomes circularly polarized light. The circularly polarized light reflected by the half mirror 214 passes through the second phase plate 213 and becomes linearly polarized light. Because the polarization direction of this linearly polarized light matches the polarization direction of the polarized light passed through the PBS 214, it passes through the PBS 214 and is directed to the right eye 202.

[0053] By folding the optical path using polarized light as in this embodiment, the display optical system can be made thinner and the focal length can be shortened, thereby realizing image observation at a wide angle of view.

[0054] In this embodiment, the lenses 204, 205, 206, and 207 are all made of resin to reduce weight, and aspherical lenses are used to enhance the aberration correction effect. The exit pupil position of the display optical system in this embodiment is 30 mm, which is the eye relief of 20 mm plus the radius of rotation of the eyeball of 10 mm, and the exit pupil diameter is 6 mm.

[0055] In this embodiment, the surface of the lens 204 on which the half mirror 212 is vapor-deposited is convex toward the display element. By vapor-depositing the half mirror 212 on this convex surface, a wide angle of view is achieved while reducing the thickness of the display optical system. Furthermore, by making the convex surface on which the half mirror 212 is vapor-deposited aspherical, the aberration correction effect can be improved.

[0056] On the other hand, since the display optical system of this embodiment has the half mirror 212 arranged inside, external light incident on the display optical system, as in the first embodiment, is reflected multiple times by the half mirror 212 and the PBS 214 and is directed to the eyeball (202), generating external light ghosts and making it impossible to observe a good image.

[0057] Therefore, in this embodiment, the reflectance of the half mirror 212 is set to 25% to reduce the amount of external light ghost light. Because the reflectance of a conventional half mirror is 50%, the efficiency when external light is reflected twice is 25%. However, when the reflectance of the half mirror 212 is set to 25%, the efficiency when reflected twice becomes 6%, which is reduced to approximately 25%. For this reason, as described in the first embodiment, in order to reduce external light ghost light, it is desirable to set the reflectance of the half mirror 212 to 35% or less, or even 30% or less.

[0058] Table 2 shows the film configuration of the half mirror 212 in this example, and Fig. 14 shows the reflectance characteristics of the half mirror 212. As shown in Table 2, by forming the half mirror 212 from a dielectric multilayer film, it is possible to realize a half mirror with lower reflectance than conventional half mirrors. In this example, too, the dielectric multilayer film that constitutes the half mirror 212 contains alternating layers of silicon oxide and niobium oxide.

[0059] [Table 2]

[0060] As in the first embodiment, when a half mirror 212 with low reflectance is used, variations in the reflectance within the mirror surface have a large effect on the display efficiency of light from the display element 208. For this reason, it is desirable that the variations in the reflectance (and transmittance) within the mirror surface of the half mirror 112 be ±5% or less.

[0061] When a half mirror 212 with a reflectance of 25% is used as in this embodiment, the display efficiency (reflectance x transmittance) is 18.75%, but if the reflectance and transmittance vary by ±10%, the display efficiency becomes 12.75%, a decrease of 6%. For this reason, in this embodiment, the variation in the thickness of the dielectric multilayer film is reduced to ±3%, thereby reducing the variation in reflectance and transmittance. As a result, the display efficiency (reflectance x transmittance) becomes 17.16%, a decrease of approximately 1.6%.

[0062] As in Example 1, the polarization characteristics of the dielectric multilayer film half mirror 212 change depending on the angle of incidence of light onto the half mirror 212. FIG. 15 shows the reflectance characteristics of the half mirror 212 when the angle of incidence of light onto the half mirror 212 is 45°. As the angle of incidence increases, the difference in reflectance between S-polarized light and P-polarized light incident on the half mirror 212 increases. In this case, unpolarized external light incident on the display optical system becomes linearly polarized when it passes through the PBS 214, and this linearly polarized light becomes circularly polarized when it passes through the second phase plate 213. When the circularly polarized external light is reflected by the half mirror 212, if the half mirror 212 does not have polarization characteristics, it remains circularly polarized. However, if the half mirror 212 has polarization characteristics, it is reflected as elliptically polarized light. The elliptically polarized and reflected external light passes through the second phase plate 213 and enters the PBS 214, where it is separated into light that passes through the PBS 214 and exits the display optical system and light that is reflected by the PBS 214.

[0063] 7 in the first embodiment, the light reflected once by the half mirror 212 and emitted outside the display optical system follows an optical path that is less likely to be guided to the eyeball, and therefore is less likely to cause external light ghosts. In addition, the amount of light reflected twice by the half mirror 212 and emitted outside the display optical system is reduced by the amount of light that is reflected once by the half mirror 212 and emitted outside the display optical system, thereby reducing external light ghosts.

[0064] It is desirable that the difference in reflectance between S-polarized light and P-polarized light on the half mirror 212 be 30% or more, which increases the effect of reducing external light ghosts caused by external light that has been reflected twice by the half mirror 112.

[0065] Furthermore, as in Example 1, due to the birefringence of the lenses 204 to 207 of the display optical system of this Example and the polarization characteristics of the first polarizing plate 210, the first and second phase plates 211 and 213, and the PBS 214, ghost light, which is unnecessary light that does not follow the optical path of the normal light shown in Figure 11, is generated, as shown in Figure 13.

[0066] 11 and the optical path of the ghost light in Fig. 13, in the optical path of the normal light, the light emitted from the display element 108 is both transmitted and reflected by the half mirror 212, but in the optical path of the ghost light, the light is only transmitted by the half mirror 212. For this reason, if the reflectance of the half mirror 212 is low and the transmittance is high, as in this embodiment, the intensity of the internal ghost increases.

[0067] For this reason, it is desirable that the birefringence in the region near the center of each of the lenses 204 to 207 be small. In this embodiment, the retardation amount per mm of the lens thickness of the lenses (one lens) 205 and 207 on the display element side is 8 nm / mm, and the retardation amount per mm of the lens thickness of the lenses (the other lens) 204 and 206 on the observation side is 1 nm / mm. It is desirable that the retardation amount per mm of the lens thickness of each lens be 10 nm / mm or less. Furthermore, because light passes through the lenses 204 and 206 three times, it is desirable that the birefringence be even smaller, for example, that the retardation amount per mm of the lens thickness be 5 nm / mm or less.

[0068] Furthermore, similar to the first embodiment, if there is a difference in the reflectance of the half mirror 212 for each wavelength, a difference occurs between the spectral characteristics of the half mirror 212 in the optical path of the normal light and the spectral characteristics of the half mirror 212 in the optical path of the ghost light. As a result, the color difference between the normal light and the ghost light becomes large, making the ghost light more noticeable.

[0069] Therefore, in this embodiment, the reflectance and transmittance characteristics of the half mirror 212 at an incident angle of 0° are set as shown in FIG. 16. That is, by reducing the difference in reflectance of the half mirror 212 for each wavelength within the visible light range, the difference between the spectral characteristics of the half mirror 212 for normal light and the spectral characteristics of the half mirror 212 for ghost light is reduced. In FIG. 16, the spectral characteristics of the half mirror 212 for normal light are shown in a graph of "transmittance x reflectance," and the spectral characteristics of the half mirror 212 for ghost light are shown in a graph of "transmittance." The small difference in the spectral characteristics shown in these graphs reduces the color shift between normal light and ghost light, making the ghost light less noticeable.

[0070] For example, the dominant wavelength of blue light emitted from display element 208 is 470 nm, the dominant wavelength of green light is 545 nm, and the dominant wavelength of red light is 605 nm. The reflectance of half mirror 212 for the first wavelength of 470 nm, the second wavelength of 545 nm, and the third wavelength of 605 nm is 26%, 25%, and 24%, respectively. In this case, the difference between the maximum and minimum values ​​of the reflectance for the dominant wavelengths of blue light, green light, and red light emitted from display element 108 is 2%. This difference is 5% or less.

[0071] In this embodiment, the dielectric multilayer film serving as the half mirror 212 has a seven-layer structure. As described in the first embodiment, the number of layers in the dielectric multilayer film is preferably 5 to 10.

[0072] In this embodiment, the lenses 204 and 205 are cemented together. Therefore, the half mirror 212 may be deposited on the surface of the lens 205 facing the eyeball. Even in this case, the surface on which the half mirror 212 is deposited is convex toward the display element. Furthermore, by forming the lenses 204 and 205 as cemented lenses, these lenses can be easily held.

[0073] When bonding lenses 204 and 205 together, arranging the bonding surfaces of lenses 204 and 205 so that they are convex downward (concave upward) as shown in FIG. 17 makes it easier to apply adhesive to the bonding surfaces. Generally, ultraviolet curing resin is used as the adhesive. In this case, when ultraviolet light (e.g., wavelength less than 400 nm) 215 is irradiated from above as shown in FIG. 17, the ultraviolet light 215 is reflected by the half mirror 212, internally reflected by the upper surface (flat surface) of lens 204, and concentrated on the adhesive between lenses 204 and 205. Because the intensity of the ultraviolet light 215 is high in the portion of the adhesive where the ultraviolet light 215 is concentrated, the ultraviolet curing resin cures faster than in the portion where the ultraviolet light 215 is not concentrated. As a result, unnecessary stress is generated in the adhesive due to the difference in curing speed between the portion where the ultraviolet light 215 is concentrated and the portion where the ultraviolet light 215 is not concentrated. Therefore, to reduce the generation of unnecessary stress, it is desirable for the half mirror 212 to have a low reflectance in the ultraviolet range.

[0074] FIG. 18 shows the reflectance characteristics of the half mirror 212 when the film configuration of the dielectric multilayer film serving as the half mirror 212 is as shown in Table 3.

[0075] [Table 3]

[0076] 18, the number of layers in the dielectric multilayer film is nine, and the reflectance in the ultraviolet range of 350 nm is set to a low 10%. By setting the reflectance of the half mirror 212 lower in the ultraviolet range than in the visible light range in this way, it is possible to reduce the occurrence of unnecessary stress at the bonding surfaces of the lenses 204 and 205.

[0077] 19 , infrared light sources 216 and 218 and infrared cameras 217 and 219 may be arranged in the HMD 201 as detection means for detecting the line of sight of the observer's right eye 202 and left eye 203, respectively. Infrared light (for example, wavelength longer than 700 nm) emitted from the infrared light source 216 passes through lenses 205 and 204 and is irradiated onto the right eye 202, and the infrared light reflected by the right eye 202 passes through lenses 204 and 205 and is imaged by the infrared camera 217. Infrared light emitted from the infrared light source 218 passes through lenses 207 and 206 and is irradiated onto the left eye 203, and the infrared light reflected by the left eye 203 passes through lenses 206 and 207 and is imaged by the infrared camera 219.

[0078] Because the infrared light from the infrared light sources 216 and 218 each passes through the half mirror 212 twice, it is desirable that the reflectance of the half mirror 212 in the infrared range be lower than its reflectance for visible light. If the reflectance of the half mirror 212 in the infrared range is low, the infrared light from the infrared light sources 216 and 218 can be irradiated onto the right eye 202 and the left eye 203 with high efficiency, and bright reflected light can be captured.

[0079] For example, it is preferable that the film configuration of the dielectric multilayer film serving as the half mirror 212 be as shown in Table 4. Fig. 20 shows the reflectance characteristics of the half mirror 212 in this case.

[0080] [Table 4]

[0081] As shown in FIG. 20, by making the number of layers of the dielectric multilayer film nine, the reflectance in the infrared region of 900 nm is reduced to 10%.

[0082] In this embodiment, a second polarizing plate may also be placed between the PBS 214 and the eyeball in order to reduce external light ghosts and increase the contrast of the displayed image.

[0083] In this embodiment, the eyeball-side surface of the lens 204, on which the second phase plate 213 and the PBS 214 are provided, is flat. That is, the lens 204 is a plano-convex lens. This allows the eye relief to be increased and the display optical system to be made thinner.

[0084] Also in this embodiment, the phase difference of each of the first and second phase plates 211 and 213 may be shifted from λ / 4 so as to cancel the birefringence of the lenses 204 and 205. The sum of the phase differences of the lens 204 and the phase plate 213 is preferably 3λ / 20 or more and 7λ / 20 or less. The sum of the phase differences of the lens 205 and the phase plate 211 is also preferably 3λ / 20 or more and 7λ / 20 or less.

[0085] In this embodiment, an organic EL element that emits unpolarized light is used as the display element, but by using a liquid crystal element that emits linearly polarized light, the first polarizing plate 210 is no longer necessary, and the display optical system can be made even thinner.

[0086] The above embodiment includes the following configurations.

[0087] (Configuration 1) A display optical system that guides light from a display element to an observation side via a partial transmission / reflection surface and a polarization separation surface, the reflectance of the partially transmissive reflective surface for visible light is 35% or less; the partially transmitting and reflecting surface is formed of a dielectric multilayer film, The display optical system is characterized in that the number of layers of the dielectric multilayer film is 5 to 10. (Configuration 2) 2. The display optical system according to configuration 1, wherein the reflectance of the partially transmitting and reflecting surface is 20% or more. (Configuration 3) 3. The display optical system according to configuration 1 or 2, wherein the light from the display element is guided to the observation side after being transmitted through the partial transmission / reflection surface, reflected at the polarization separation surface, reflected at the partial transmission / reflection surface, and transmitted through the polarization separation surface. (Configuration 4) 4. The display optical system according to any one of configurations 1 to 3, wherein the partially transmitting and reflecting surface has a convex shape facing the display element side and an aspherical shape. (Configuration 5) 5. The display optical system according to any one of configurations 1 to 4, wherein the variation in the reflectance within the plane of the partially transmitting and reflecting surface is ±5% or less. (Configuration 6) Includes a resin lens, 6. The display optical system according to any one of configurations 1 to 5, wherein the amount of phase difference per mm of thickness of the resin lens is 10 nm / mm or less. (Configuration 7) Includes two resin lenses, The display optical system according to configuration 6, wherein the amount of phase difference per mm of thickness of one of the two resin lenses, which is disposed closer to the observation side than the other resin lens, is 5 nm / mm or less. (Configuration 8) 8. The display optical system according to any one of configurations 1 to 7, characterized in that, at an incident angle when external light incident from the observation side and transmitted through the polarization separation surface is incident on the partial transmission reflection surface, the difference in reflectance of the partial transmission reflection surface for first linearly polarized light and second linearly polarized light, whose polarization directions are orthogonal to each other, is 20% or more. (Configuration 9) The display optical system according to any one of configurations 1 to 8, wherein the difference between the maximum and minimum values ​​of the reflectance of the partially transmissive reflective surface for each of light of a first wavelength, light of a second wavelength, and light of a third wavelength, which are different from one another as the visible light, is 15% or less. (Configuration 10) 10. The display optical system according to configuration 9, wherein the light from the display element includes three colors of light having the first wavelength, the second wavelength, and the third wavelength as dominant wavelengths, respectively. (Configuration 11) It includes two lenses bonded together using an ultraviolet curable resin; the partially transmissive and reflective surface is disposed at the joint of the two lenses; 11. The display optical system according to any one of configurations 1 to 10, wherein the reflectance of the partially transmitting and reflecting surface for ultraviolet light is lower than the reflectance for visible light. (Configuration 12) a detection means for detecting the line of sight of an eyeball by irradiating the eyeball arranged on the observation side with infrared light and capturing an image of the infrared light reflected by the eyeball; the infrared light is transmitted through the partially transmissive and reflective surface, 12. The display optical system according to any one of configurations 1 to 11, wherein the reflectance of the partially transmitting and reflecting surface for the infrared light is lower than the reflectance for the visible light. (Configuration 13) the light from the display element is unpolarized light, 13. The display optical system according to any one of configurations 1 to 12, wherein a polarizing plate and a phase plate are disposed in this order between the display element and the partial transmission / reflection surface. (Configuration 14) a display optical system according to any one of configurations 1 to 13; and An image display device comprising the display element.

[0088] The embodiments described above are merely representative examples, and various modifications and alterations are possible to each embodiment when implementing the present invention. [Explanation of symbols]

[0089] 101,201 HMD 102,202 Observer's right eye 103,203 Observer's left eye 104~107, 204~207 lenses 108,208 Display element for right eye 109,209 Display element for left eye 112,212 Half mirror (partially transmissive and reflective surface) 114,214 Polarization separation element (polarization separation surface)

Claims

1. A display optical system that guides light from a display element to an observation side via a partial transmission / reflection surface and a polarization separation surface, the reflectance of the partially transmissive reflective surface for visible light is 35% or less; the partially transmitting and reflecting surface is formed of a dielectric multilayer film, The display optical system is characterized in that the number of layers of the dielectric multilayer film is 5 to 10.

2. 2. The display optical system according to claim 1, wherein the reflectance of the partially transmitting and reflecting surface is 20% or more.

3. 2. The display optical system according to claim 1, wherein the light from the display element is guided to the observation side after being transmitted through the partial transmission reflecting surface, reflected through the polarization separating surface, reflected through the partial transmission reflecting surface, and transmitted through the polarization separating surface.

4. 2. The display optical system according to claim 1, wherein the partially transmitting and reflecting surface has a convex shape facing the display element side and an aspherical shape.

5. 2. The display optical system according to claim 1, wherein the variation in the reflectance within the plane of the partially transmitting and reflecting surface is within ±5%.

6. Includes a resin lens, 2. The display optical system according to claim 1, wherein the amount of phase difference per mm of thickness of the resin lens is 10 nm / mm or less.

7. It includes two resin lenses, 7. The display optical system according to claim 6, wherein the phase difference per mm of thickness of one of the two resin lenses, which is arranged closer to the observation side than the other resin lens, is 5 nm / mm or less.

8. 2. The display optical system according to claim 1, wherein the difference in reflectivity of the partially transmissive reflective surface for first linearly polarized light and second linearly polarized light, the polarization directions of which are orthogonal to each other, is 20% or more at an incident angle when external light incident from the observation side and transmitted through the polarization separation surface is incident on the partially transmissive reflective surface.

9. 2. The display optical system according to claim 1, wherein the difference between the maximum and minimum values ​​of the reflectance of the partially transmissive reflecting surface for each of light having a first wavelength, light having a second wavelength, and light having a third wavelength, which are different from each other as the visible light, is 15% or less.

10. 10. The display optical system according to claim 9, wherein the light from the display element includes three colors of light having the first wavelength, the second wavelength, and the third wavelength as dominant wavelengths, respectively.

11. two lenses cemented together using an ultraviolet curable resin; the partially transmissive and reflective surface is disposed at the joint of the two lenses, 2. The display optical system according to claim 1, wherein the reflectance of the partially transmitting and reflecting surface for ultraviolet light is lower than the reflectance for the visible light.

12. a detection means for detecting the line of sight of an eyeball by irradiating the eyeball arranged on the observation side with infrared light and capturing an image of the infrared light reflected by the eyeball; the infrared light is transmitted through the partially transmissive and reflective surface, 2. The display optical system according to claim 1, wherein the reflectance of the partially transmitting and reflecting surface for the infrared light is lower than the reflectance for the visible light.

13. the light from the display element is unpolarized light, 2. The display optical system according to claim 1, wherein a polarizing plate and a phase plate are disposed in this order between the display element and the partial transmission / reflection surface.

14. A display optical system according to any one of claims 1 to 13; An image display device comprising the display element.

Images