Display optical system and display unit

Abstract

To provide a display optical system having a triple-path configuration, and satisfactorily correct chromatic aberration, curvature of field, and astigmatism, while providing a reflection surface with a strong condensing effect to increase magnification and reduce thickness.SOLUTION: A display unit has a display element, and a display optical system that guides light from a display surface ID of the display element to an observation side SP. The display optical system has a first semi-transmissive reflection surface PBS and a second semi-transmissive reflection surface HM arranged on a joint surface of two optical elements, and guides the light from the display surface to the observation side through transmission through the second semi-transmissive reflection surface, reflection on the first semi-transmissive reflection surface, reflection on the second semi-transmissive reflection surface, and transmission through the first semi-transmissive reflection surface. The curvature of the second semi-transmissive reflection surface is larger than the curvature of the first semi-transmissive reflection surface. The display surface has a convex shape toward the observation side on at least one of two cross sections including an optical axis of the display optical system and orthogonal to each other.SELECTED DRAWING: Figure 1

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 original image displayed on a display element. [Background technology]

[0002] Such display devices are required to be small and have a wide viewing angle. Patent Document 1 discloses a display optical system that uses a reflecting surface and has a small size and a wide viewing angle. This display optical system achieves both a wide viewing angle (high magnification) and a thin design by adopting a triple-pass configuration using two reflecting surfaces that reflect light from a display element (display surface). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-148627 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the display optical system of Patent Document 1, the optical material of the lenses is selected so that the Petzval curvature of field occurring on the reflecting surface (concave mirror) is offset by the Petzval curvature of field occurring on the refracting surface, thereby correcting the curvature of field in the entire optical system to accommodate a flat display surface.

[0005] However, the range of refractive index and Abbe number of optical materials is finite, and if the optical material is selected with priority given to the correction of Petzval sum, it may be difficult to correct chromatic aberration. Magnification chromatic aberration can be reduced by performing electronic correction of the image on the display element.

[0006] However, when displaying high-definition images with high spatial frequencies, the optical performance is degraded due to lateral chromatic aberration in each of the RGB color channels, and it is difficult to electronically correct longitudinal chromatic aberration.

[0007] Furthermore, in the display optical system of Patent Document 1, in order to balance the curvature of field and chromatic aberration, it is difficult to give a strong light-collecting effect to the reflecting surface, and there is a limit to increasing the magnification and reducing the size of the optical system. In addition, since the display optical system is composed of a cemented lens consisting of a plano-convex lens and a concave-plano lens with a reflecting surface sandwiched between them, there is also a limit to the correction of astigmatism.

[0008] The present invention provides a display optical system having a triple-path configuration, which has a strong light-gathering effect on the reflecting surface for high magnification and thinness, while being capable of satisfactorily correcting chromatic aberration, field curvature, and astigmatism. [Means for solving the problem]

[0009] A display device according to one aspect of the present invention includes a display element and a display optical system that guides light from a display surface of the display element to an observation side. The display optical system includes a first semi-transmissive reflective surface and a second semi-transmissive reflective surface disposed at a bonding surface between two optical elements, and guides light from the display surface to the observation side via transmission at the second semi-transmissive reflective surface, reflection at the first semi-transmissive reflective surface, reflection at the second semi-transmissive reflective surface, and transmission at the first semi-transmissive reflective surface. The second semi-transmissive reflective surface has a larger curvature than the first semi-transmissive reflective surface, and the display surface has a convex shape toward the observation side in at least one of two cross sections that are orthogonal to each other and include an optical axis of the display optical system. Effect of the Invention

[0010] According to the present invention, in a display optical system having a triple-path configuration, it is possible to provide a reflecting surface with a strong light-collecting effect in order to achieve high magnification and a slim design, while at the same time making good corrections for chromatic aberration, field curvature, and astigmatism. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a ZX sectional view of the display optical system according to the first embodiment. [Diagram 2] FIG. 3 is a YX sectional view of the display optical system according to the first embodiment. [Diagram 3] 4A to 4C are diagrams showing longitudinal aberration in the horizontal viewing angle direction of the display optical system according to the first embodiment. [Figure 4] 4A to 4C are diagrams showing longitudinal aberration in the vertical viewing angle direction of the display optical system according to the first embodiment. [Diagram 5] FIG. 11 is a YX sectional view of the display optical system according to the second embodiment. [Figure 6] 6A to 6C are diagrams showing longitudinal aberration in the horizontal / vertical viewing angle directions of the display optical system according to the second embodiment. [Figure 7] FIG. 11 is a YX sectional view of the display optical system according to the third embodiment. [Figure 8] 11A to 11C are longitudinal aberration diagrams in the horizontal / vertical viewing angle directions of the display optical system according to the third embodiment. [Figure 9] FIG. 13 is a YX sectional view of the display optical system according to the fourth embodiment. [Figure 10] 13A to 13C are longitudinal aberration diagrams in the horizontal / vertical viewing angle directions of the display optical system according to the fourth embodiment. [Figure 11] FIG. 13 is a YX sectional view of the display optical system according to the fifth embodiment. [Figure 12] 13A to 13C are longitudinal aberration diagrams in the horizontal / vertical viewing angle directions of the display optical system according to the fifth embodiment. [Figure 13] FIG. 1 is a diagram for explaining an optical path using polarized light. [Figure 14] FIG. 1 is a diagram showing an HMD using the display optical systems of Examples 1 to 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, before describing the display devices of Examples 1 to 5, matters common to the respective embodiments will be described. Figures 1 and 2 show the ZX cross section and the XY cross section of the display device of Example 1. Figures 5, 7, 9, and 11 show the YX cross sections of the display devices of Examples 2 to 5, respectively.

[0013] The display device of each embodiment has a display element and a display optical system, and displays an image displayed on a display surface ID of the display element in an enlarged manner toward a pupil plane SP on the observation side. The display optical system has a configuration that forms a folded optical path (triple path) by two reflections inside. Specifically, the display optical system has, arranged in order from the pupil plane side (observation side) to the display element side, a first semi-transmissive reflective surface PBS, a first quarter-wave (λ / 4) plate QWP1, a first lens G1, a second semi-transmissive reflective surface HM, a second lens G2, a second λ / 4 plate QWP2, and a polarizing plate POL. The pupil plane SP is an observation surface, and the observer's pupil is located here.

[0014] The first semi-transmissive reflective surface PBS is formed of, for example, a wire grid polarizer, which reflects linearly polarized light having the same polarization direction as the linearly polarized light passing through the polarizing plate POL and transmits linearly polarized light having a polarization direction perpendicular to the linearly polarized light. In the wire grid polarizer, the surface on which the wire grid is formed functions as a semi-transmissive reflective surface.

[0015] The first λ / 4 plate QWP1 and the second λ / 4 plate QWP2 are arranged with their slow axes tilted at 90° to each other. The slow axis of the first λ / 4 plate QWP1 is tilted at 45° with respect to the polarized light transmission axis of the polarizer POL. The second semi-transmissive reflective surface HM is a half mirror formed by, for example, a dielectric multilayer film or metal deposition. A liquid crystal display element (LCD), an organic EL element, or the like is used as the display element. CG is a cover glass that covers the display surface ID of the display element.

[0016] Fig. 13 shows the use of polarized light in the display optical system of each embodiment. Among the light emitted from the display surface ID, linearly polarized light parallel to the polarization transmission axis of the polarizing plate POL (E) passes through the polarizing plate POL, is converted to circularly polarized light by the second λ / 4 plate QWP2 (D), and enters the second semi-transmissive reflective surface HM (C). A part of the light that reaches the second semi-transmissive reflective surface HM is reflected and becomes circularly polarized light with a direction opposite to that before reflection, and returns to the second λ / 4 plate QWP2. The reverse circularly polarized light that has returned to the second λ / 4 plate QWP2 is converted by the second λ / 4 plate QWP2 into linearly polarized light with a polarization direction perpendicular to the polarization direction when it first passed through the polarizing plate E, returns to the polarizing plate POL, and is absorbed by the polarizing plate POL.

[0017] On the other hand, the light that reaches and transmits through the second semi-transmissive reflective surface HM is converted by the first λ / 4 plate QWP1(B) into linearly polarized light having the same polarization direction as the light that transmitted through the polarizing plate POL, and then enters the first semi-transmissive reflective surface PBS(A). The linearly polarized light that entered the first semi-transmissive reflective surface PBS is reflected by the polarization selectivity of the first semi-transmissive reflective surface PBS described above, and is converted by the first λ / 4 plate QWP1 into circularly polarized light having the opposite rotation to the circularly polarized light that initially exited from the second λ / 4 plate QWP2, and then enters the second semi-transmissive reflective surface HM.

[0018] The circularly polarized light incident on the second semi-transmissive reflecting surface HM is reflected by the second semi-transmissive reflecting surface HM and becomes circularly polarized light in the opposite direction to that before reflection. The circularly polarized light then enters the first λ / 4 plate QWP1, where it is converted into linearly polarized light with a polarization direction perpendicular to the polarization direction of the linearly polarized light that first passed through the polarizing plate POL, and then enters the first semi-transmissive reflecting surface PBS. The linearly polarized light that entered the first semi-transmissive reflecting surface PBS passes through the first semi-transmissive reflecting surface PBS due to the polarization selectivity of the first semi-transmissive reflecting surface PBS described above, and reaches the pupil plane SP.

[0019] By using the above polarization, light emitted from the display surface ID travels a triple path, passing through the second semi-transmissive reflective surface HM, reflecting off the first semi-transmissive reflective surface PBS, reflecting off the second semi-transmissive reflective surface HM, and again passing through the first semi-transmissive reflective surface PBS, before being directed to the pupil surface SP.

[0020] Next, the characteristics of the display optical system of each embodiment will be described. The display optical system employs a triple-path configuration using two semi-transmissive reflective surfaces (a first semi-transmissive reflective surface PBS and a second semi-transmissive reflective surface HM), and has one cemented optical element in which two optical elements (first and second lenses G1, G2) are cemented together. The semi-transmissive reflective surface (the second semi-transmissive reflective surface) having the larger curvature of the two semi-transmissive reflective surfaces is disposed on the cemented surface of the cemented lens. The display surface of the display element has a convex shape toward the observation side in at least one of two cross sections (ZX cross section and XY cross section) that are orthogonal to each other and include the optical axis.

[0021] In each embodiment, the display optical system is made thinner by adopting a triple-pass configuration. Of the two semi-transmissive reflective surfaces, the semi-transmissive reflective surface that shares the main optical power (the reciprocal of the focal length) of the entire display optical system is disposed at the bonding surface. This configuration avoids total reflection at the semi-transmissive reflective surface, which is an issue when the display optical system is made high-magnification. In addition, the bonding surface has a low risk of exposure to the external environment, which is advantageous in terms of environmental resistance when a half mirror made of a metal film or a dielectric multilayer film is used as the semi-transmissive reflective surface.

[0022] Here, in order to increase the magnification of a display optical system, it is effective to strengthen the optical power of the semi-transmissive reflective surface, that is, to give the semi-transmissive reflective surface a larger curvature. In this case, the problem of total reflection can be avoided by placing a semi-transmissive reflective surface at the cemented surface. However, it becomes difficult to correct the Petzval sum in the entire optical system, and the curvature of field remains. By compensating for the remaining curvature of field using a display element having a display surface formed into a convex curved surface on the observation side, a display optical system with good optical performance over the entire display screen can be realized.

[0023] One known method for achieving a curved display surface is to curve the end face of a fiber optic plate (FOP). There are also known examples of making the display surface cylindrical or foldable using technology for processing LCDs and OLEDs to make them flexible. Here, the use of display elements such as LCDs and OLEDs with curved display surfaces is advantageous in terms of resolution and cost compared to the use of FOPs.

[0024] Also, there is known an example in which a curved display surface is combined with a display optical system constituted by a refractive optical system. In this example, the main optical power of the entire optical system is borne by the refractive surface. In contrast, the display optical systems of the embodiments are different in that the main optical power of the entire optical system is borne by the reflective surface. Since the Petzval terms generated at the refractive surface and the reflective surface have opposite signs, the sign of the generated curvature of field is also opposite. For this reason, in the combination of a curved display surface and a display optical system constituted by a refractive optical system, it is preferable that the curvature of the display surface is convex toward the display optical system.

[0025] As described above, in each embodiment, a display optical system having high optical performance, combining high magnification with a thin design, and a display element having a curved display surface are used to realize a small-sized display device with a wide viewing angle.

[0026] In each embodiment, the two optical elements bonded at the bonding surface where the semi-transmissive reflective surface is provided are made of a resin material, which is advantageous in terms of weight reduction compared to when the optical elements are made of a glass material.

[0027] In the above configuration, the focal length of the first optical element (G1) in air, calculated from the radius of curvature of the reference spherical surface of the first optical element arranged on the observation side (hereinafter, the radius of curvature of the reference spherical surface is referred to as the reference radius of curvature), of the two optical elements, is fG1. The focal length of the second optical element (G2) arranged on the display element side, calculated from the reference radius of curvature, of the second optical element, is fG2. The refractive index of the optical element having the positive focal length described above at the d-line is Ndp, and the Abbe number based on the d-line is νdp. The refractive index of the optical element having the negative focal length described above at the d-line is Ndn, and the Abbe number based on the d-line is νdn. The reference radius of curvature of the semi-transmissive reflecting surface (second semi-transmissive reflecting surface) having the larger curvature of the two semi-transmissive reflecting surfaces is Rhm. The focal length of the entire display optical system is f. The reference radius of curvature of the display surface is Rp. The reference radius of curvature of an optical element and a semi-transmissive reflective surface is the radius of a reference sphere determined by the surface vertex on the optical axis and the maximum optically effective diameter (effective diameter). The reference radius of curvature of a display surface is the minimum value of the radii of curvature (absolute values) of a reference sphere determined by the surface vertex on the optical axis and the edge of the effective area of ​​the display surface.

[0028] In this case, it is preferable that the display optical system of each embodiment satisfies at least one of the following expressions (1) to (5).

[0029] fG1 / fG2<0 (1) 1.4≦Ndp <Ndn≦1.9 (2) 1.0<νdp / νdn≦4.0 (3) 1.0<|Rhm| / f≦5.0 (4) 0.1≦|Rhm| / |Rp|≦2.0 (5) Equation (1) shows an appropriate range of the ratio of the focal lengths of the positive lens and the negative lens when the two optical elements having the semi-transmissive reflective surface on the cemented surface are composed of a cemented lens of a positive lens and a negative lens. By forming the cemented lens with a positive lens and a negative lens, it is possible to correct the chromatic aberration occurring on the refractive surface of the cemented lens. When fG1 / fG2 exceeds the upper limit of Equation (1), both of the two optical elements become positive lenses or negative lenses, and achromatism is not achieved. Although no lower limit is set in Equation (1), when fG1 / fG2 becomes too small, the optical power is too biased toward one lens, which is disadvantageous for correcting chromatic aberration. It is preferable that the cemented surface has a convex shape toward the display element side. By arranging the semi-transmissive reflective surface in a concentric shape with respect to the pupil plane, it is advantageous for correcting astigmatism. In this case, it is preferable that the lens on the observation side is arranged as a positive lens, and the lens on the display element side is arranged as a negative lens.

[0030] The condition of formula (2) shows the appropriate relationship and range of the refractive index of the two optical elements. By making the refractive index of the positive lens smaller than that of the negative lens, it is easy to select a low dispersion material for the positive lens and a high dispersion material for the negative lens. This refractive index configuration is disadvantageous from the viewpoint of compensating for the Petzval term occurring at the reflecting surface (concave mirror) with the Petzval term occurring at the refracting surface. However, in the display optical system of each embodiment combined with a display element having a curved display surface, it is preferable to adopt a configuration in which correction of chromatic aberration is prioritized over correction of curvature of field. If Ndp or Ndn is below the lower limit of formula (2), the refractive index of the optical element becomes too low, making it difficult to correct monochromatic aberrations such as spherical aberration, which is not preferable. If Ndp or Ndn is above the upper limit of formula (2), the refractive index of the optical element becomes too high. In this case, a material with a large specific gravity is selected as the optical material, making it difficult to reduce the weight of the display optical system. This is not preferable.

[0031] The condition of formula (3) indicates the appropriate range of the ratio of the Abbe numbers of the two optical elements. Axial chromatic aberration and lateral chromatic aberration are corrected by selecting a low dispersion material for the positive lens and a high dispersion material for the negative lens to configure the cemented lens as an achromat. In a display optical system with a triple-pass configuration, achromatism in the lens is important because chromatic aberration does not occur on the reflecting surface. If νdp / νdn is below the lower limit of formula (3), the Abbe number of the positive lens becomes too small compared to the Abbe number of the negative lens, which is not preferable because chromatic aberration is not sufficiently corrected. If νdp / νdn is above the upper limit of formula (3), the Abbe number of the positive lens becomes too large compared to the Abbe number of the negative lens. In this case, a material with an extremely low refractive index is selected for the positive lens, or a material with an extremely high dispersion is selected for the negative lens, which is not preferable because it causes problems in correcting monochromatic aberration and reducing the transmittance of the optical system.

[0032] The condition of formula (4) indicates an appropriate range of the ratio between the radius of curvature of the semi-transmissive reflective surface arranged at the cemented surface and the focal length of the entire optical system. In each embodiment, the semi-transmissive reflective surface is arranged at the cemented surface, and the semi-transmissive reflective surface is made to share strong optical power while avoiding the total reflection condition, thereby increasing the magnification of the display optical system. If |Rhm| / f is below the lower limit of formula (4), the radius of curvature of the semi-transmissive reflective surface becomes too small compared to the focal length of the entire optical system, which is not preferable because it is difficult to compensate for the curvature of field generated in the optical system with the curved display surface. If |Rhm| / f is above the upper limit of formula (4), the radius of curvature of the semi-transmissive reflective surface becomes too large compared to the focal length of the entire optical system. In this case, the optical power shared by the reflecting surface (concave mirror) at the semi-transmissive reflective surface decreases, which is not preferable because it is difficult to increase the magnification of the optical system.

[0033] The condition of formula (5) indicates an appropriate range of the ratio between the radius of curvature of the semi-transmissive reflective surface arranged on the bonding surface and the radius of curvature of the display surface. By optimizing both the radius of curvature of the semi-transmissive reflective surface and the radius of curvature of the display surface, it is possible to obtain an amount of curvature that allows the display element to be manufactured as a device while realizing correction of field curvature in the entire display device. If |Rhm| / |Rp| is below the lower limit of formula (5), the radius of curvature of the semi-transmissive reflective surface becomes too small compared to the radius of curvature of the display surface, which is not preferable because the correction of field curvature in the entire display device is insufficient. If |Rhm| / |Rp| is above the upper limit of formula (5), the radius of curvature of the semi-transmissive reflective surface becomes too large compared to the radius of curvature of the display element surface. In this case, it becomes difficult to increase the magnification of the optical system, and the correction of field curvature in the entire display device becomes excessive, and it becomes difficult to manufacture the display element, which is not preferable.

[0034] It is more preferable that the numerical ranges of the formulas (1) to (5) are as follows:

[0035] -3.0≦fG1 / fG2≦-0.2 (1a) 1.45≦Ndp <Ndn≦1.8 (2a) 1.4≦νdp / νdn≦3.5 (3a) 1.0<|Rhm| / f≦4.0 (4a) 0.2≦|Rhm| / |Rp|≦1.7 (5a) Moreover, it is more preferable that the numerical ranges of the formulas (1) to (5) are within the following ranges.

[0036] -1.5≦fG1 / fG2≦-0.4 (1b) 1.5≦Ndp <Ndn≦1.7 (2b) 1.8≦νdp / νdn≦3.0 (3b) 1.0<|Rhm| / f≦3.6 (4b) 0.3≦|Rhm| / |Rp|≦1.4 (5b) Moreover, the display optical systems of Examples 1, 2, and 4 have an aspheric refractive surface (lens surface) that includes an inflection point within the optical effective diameter. The inflection point is a point (line) where the refractive power of the refractive surface switches between positive and negative. In Examples 1, 2, and 4, a display element having a curved display surface is used to compensate for the field curvature of the display optical system, but astigmatism needs to be corrected within the display optical system. An aspheric refractive surface that includes an inflection point can greatly change the divergence and convergence action of the refractive surface near the optical axis and off the axis, and is effective in correcting astigmatism while allowing the field curvature in the display optical system.

[0037] In addition, in Examples 1 to 4, the semi-transmissive reflective surface having the smaller curvature of the two semi-transmissive reflective surfaces is configured as a flat surface. When adopting the configuration using polarized light shown in FIG. 13, a thin display optical system can be realized by bonding a polarizing element to the flat surface. Furthermore, by making the bonding surface with the polarizing element flat, bonding processing of the polarizing element can be facilitated. Furthermore, when both of the two semi-transmissive reflective surfaces are configured as curved surfaces as in Example 5, two reflecting surfaces having optical power can be arranged in the display optical system, which increases the design freedom and is advantageous for further increasing the magnification of the display optical system.

[0038] In addition, in Examples 1 to 3 and 5, the optical element having optical power included in the display optical system is composed of only a cemented lens, which makes it possible to achieve the required optical performance with a minimum configuration and reduce the weight of the display optical system.

[0039] In each embodiment, the absolute value of the angle between the pupil center ray (the ray passing through the center of the pupil surface) and the normal to the display surface at an eye relief of 15 mm, diopter of 0, and viewing angle of 30 degrees is set to 30 degrees or less. This angle is evaluated in the direction where the radius of curvature (absolute value) of the sphere determined by the apex of the display surface on the optical axis and the effective area of ​​the display surface takes the minimum value when the display surface has a rotationally asymmetric shape with respect to the optical axis.

[0040] In a conventional display optical system having a triple-pass configuration with two semi-transmissive reflective surfaces, increasing the magnification poses the problem that the angle of incidence of light rays on the display surface increases as well as the field curvature.

[0041] By using a display element with a curved display surface, it is possible to reduce the angle of incidence of light with respect to the normal to the display surface, improving brightness shading and color shift in the peripheral areas of the screen. More preferably, the angle (absolute value) between the pupil center ray and the normal to the display element surface is set to 20 degrees or less, as in each embodiment. Even more preferably, the angle (absolute value) between the pupil center ray and the normal to the element surface is set to 10 degrees or less, as in embodiments 1 and 3 to 5.

[0042] In each embodiment, at least one of the two semi-transmissive reflecting surfaces is configured by a polarization-selective semi-transmissive reflecting element. This makes it possible to easily realize the configuration using polarized light shown in FIG. 13. That is, the unnecessary light from the display surface that passes through the two semi-transmissive reflecting surfaces without being reflected and reaches the pupil surface can be blocked by using polarized light. Examples of the polarization-selective semi-transmissive reflecting element include a wire grid element such as WGF (registered trademark) manufactured by Asahi Kasei Corporation, a reflective linear polarizing element such as IQP-E manufactured by 3M, and a circular polarizing reflecting element using cholesteric liquid crystal. When a reflective linear polarizing element is used, a λ / 4 plate is placed between the two semi-transmissive reflecting surfaces.

[0043] Below, Examples 1 to 5 and their corresponding Numerical Examples 1 to 5 will be explained. Numerical Examples 1 to 5 are collectively described after Example 5. The various numerical values ​​in the Numerical Examples will be described later. EXAMPLES

[0044] The display device of Example 1 (Numerical Example 1) shown in FIG. 1 and FIG. 2 has a horizontal viewing angle of about 90 degrees. The display optical system of this example includes a cemented lens consisting of a plano-convex lens G1 arranged on the observation side and an aspherical lens G2 arranged on the display element side, and the cemented surface of the cemented lens has a second semi-transmissive reflective surface HM of an aspherical shape with a concave surface facing the observation side. In addition, a first semi-transmissive reflective surface PBS is arranged on the observation side surface (flat surface) of the lens G1. With this configuration, the lens G1 can be used in triple pass, and the display optical system can be made high-magnification and thin. Furthermore, by using resin materials for the lenses G1 and G2, the display optical system can be made lighter. Furthermore, by cementing the lens G1 as a positive lens formed of a low dispersion material and the lens G2 as a negative lens formed of a high dispersion material, the chromatic aberration of the display optical system can be well corrected. Furthermore, by making the display element side surface of the lens G2 an aspherical surface having an inflection point within the optical effective diameter, the astigmatism can be well corrected.

[0045] The display element has a rectangular display surface ID with an aspect ratio of approximately 16:9 in the effective area, and the display surface ID has a cylindrical shape with a convex surface facing the observation side in the ZX cross section (horizontal viewing angle direction parallel to the longitudinal direction of the display surface) shown in Figure 1. This makes it possible to compensate for the field curvature remaining in the display optical system with the curved display surface ID, and to realize a display device with good image surface characteristics in the horizontal viewing angle direction.

[0046] The display optical system of Numerical Example 1 satisfies all of the conditions of the above-mentioned expressions (1) to (5).

[0047] 3 and 4 respectively show longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) in the horizontal and vertical viewing angle directions of the display optical system of Numerical Example 1. In the spherical aberration diagrams, EPD / 2 indicates the radius of the pupil surface SP, the solid line indicates the spherical aberration for the d-line (wavelength 587.6 nm), the two-dot chain line indicates the spherical aberration for the C-line (wavelength 656.3 nm), and the one-dot chain line indicates the spherical aberration for the F-line (wavelength 486.1 nm). In the astigmatism diagrams, the solid line ΔS indicates the sagittal image plane, and the dashed line ΔM indicates the meridional image plane. The distortion aberration is shown for the d-line. The chromatic aberration diagrams show the lateral chromatic aberration for the C-line and the F-line. ω is the half angle of view (°). EXAMPLES

[0048] 5 has horizontal and vertical viewing angles of about 85 degrees. The basic configuration of the display device of this embodiment is the same as that of the first embodiment, but the specifications of the display element and the display optical system are different from those of the first embodiment.

[0049] The display element has a square display surface ID with an aspect ratio of approximately 1:1 in the effective area, and the display surface ID has a spherical shape with a convex surface facing the observation side in the YX cross section shown in Fig. 5 and in the ZX cross section (not shown). This makes it possible to compensate for the field curvature remaining in the display optical system with the curved display surface ID, and to realize a display device with good image surface characteristics in the horizontal viewing angle direction.

[0050] The display optical system of Numerical Example 2 satisfies all of the conditions of the above-mentioned expressions (1) to (5). Fig. 6 shows the longitudinal aberration in the horizontal and vertical viewing angle directions of the display optical system of Numerical Example 2. EXAMPLES

[0051] 7 has horizontal and vertical viewing angles of about 120 degrees. The basic configuration of the display device of this embodiment is the same as that of the second embodiment, but the specifications of the display element and the display optical system are different from those of the second embodiment.

[0052] The display element has a square display surface ID with an aspect ratio of approximately 1:1 in the effective area, and the display surface ID has a spherical shape with a convex surface facing the observation side in the YX cross section shown in Fig. 7 and in the ZX cross section (not shown). This makes it possible to compensate for the field curvature remaining in the display optical system with the curved display surface ID, and to realize a display device with good image surface characteristics in the horizontal viewing angle direction.

[0053] The display optical system of Numerical Example 3 satisfies all of the conditions of the above-mentioned expressions (1) to (5). Fig. 8 shows the longitudinal aberration in the horizontal and vertical viewing angle directions of the display optical system of Numerical Example 3. EXAMPLES

[0054] 9 has horizontal and vertical viewing angles of about 90 degrees. The basic configuration of the display device of this embodiment is the same as that of the first embodiment, but the specifications of the display element and the display optical system are different from those of the first embodiment.

[0055] The display optical system of this embodiment has a cemented lens with a second semi-transmissive reflective surface HM located on the cemented surface, and the entrance and exit surfaces are configured as flat surfaces. Aberration correction is shared by an aspheric lens G3 made of a resin material that is located on the display element side of the cemented lens.

[0056] In this embodiment, a first semi-transmissive reflective surface PBS is disposed on the observation side surface (flat surface) of lens G1, and lens G1 is used in a triple pass configuration, thereby realizing a high magnification and thin display optical system. In addition, the display element side surface of lens G3 is made aspheric with an inflection point within the optical effective diameter, thereby effectively correcting astigmatism.

[0057] The display element has a square display surface ID with an aspect ratio of approximately 1:1 in the effective area, and the display surface ID has a spherical shape with a convex surface facing the observation side in the YX cross section shown in Figure 9 and in the ZX cross section (not shown). This makes it possible to compensate for the field curvature remaining in the display optical system with the curved display surface ID, and to realize a display device with good image surface characteristics in the horizontal viewing angle direction.

[0058] The display optical system of Numerical Example 4 satisfies all of the conditions of the above-mentioned expressions (1) to (5). Fig. 10 shows the longitudinal aberration in the horizontal and vertical viewing angle directions of the display optical system of Numerical Example 4. EXAMPLES

[0059] The display device of Example 5 (Numerical Example 5) shown in Fig. 11 has horizontal and vertical viewing angles of about 100 degrees. The basic configuration of the display device of this example is the same as that of Example 1, but differs from Example 1 in that the two semi-transmissive reflective surfaces PBS, HM are curved. By making the first semi-transmissive reflective surface PBS arranged on the observation side surface of lens G1 a curved surface, an even higher magnification is achieved in the display optical system.

[0060] The display element has a square display surface ID with an aspect ratio of approximately 1:1 in the effective area, and the display surface ID has a spherical shape with a convex surface facing the observation side in the YX cross section shown in Fig. 11 and in the ZX cross section (not shown). This makes it possible to compensate for the field curvature remaining in the display optical system with the curved display surface ID, and to realize a display device with good image surface characteristics in the horizontal viewing angle direction.

[0061] The display optical system of Numerical Example 5 satisfies all of the conditions of the above-mentioned expressions (1) to (5). Fig. 12 shows the longitudinal aberration of the display optical system of Numerical Example 5 in the horizontal and vertical viewing angle directions.

[0062] The aspect ratio and shape of the effective area of ​​the display element (display surface) of each embodiment can be set arbitrarily. For example, the shape of the effective area of ​​the display element can be a circle, a polygon, or the like, in addition to the above-mentioned quadrangle. Also, the curved shape of the display element can be an anamorphic surface shape, an aspheric surface shape, a free-form surface shape, or the like, in addition to the above-mentioned cylindrical shape or spherical shape.

[0063] Numerical examples 1 to 5 are shown below. In each numerical example, surface number i indicates the order of the surface when counted from the object side. r is the radius of curvature (mm) of the i-th surface from the object side, d is the lens thickness or air space (mm) between the i-th and (i+1)-th surfaces, and nd is the refractive index at the d-line of the optical material between the i-th and (i+1)-th surfaces. νd is the Abbe number based on the d-line of the optical material between the i-th and (i+1)-th surfaces.

[0064] The Abbe number νd based on the d-line is expressed as νd=(Nd-1) / (NF-NC), where Nd, NF, and NC are the refractive indices at the Fraunhofer d-line (wavelength 587.6 nm), F-line (wavelength 486.1 nm), and C-line (wavelength 656.3 nm), respectively. The effective diameter is the diameter (mm) of the area where light rays that contribute to imaging (image display) from the display surface on the i-th surface enter. Each numerical example also shows the focal length, pupil diameter, and total viewing angle of the display optical system.

[0065] An "*" next to a surface number means that the surface has an aspheric shape. An aspheric shape is expressed by the following formula, where x is the displacement from the apex of the surface in the optical axis direction, h is the height from the optical axis in a direction perpendicular to the optical axis, the direction of light travel is positive, R is the paraxial radius of curvature, K is the conic constant, and A4, A6, A8, A10, and A12 are aspheric coefficients. The "ex" of the conic constant and aspheric coefficient is x10 -x means.

[0066] x=(h 2 / R) / [1+{1-(1+k)(h / R) 2} 1 / 2 ] +A4·h 4 +A6·h 6 +A8·h 8 +A10·h 10 +A12·h 12 Moreover, values ​​corresponding to the conditions of the above-mentioned formulas (1) to (5) in Numerical Examples 1 to 5 are summarized in Table 1. Furthermore, Tables 2 and 4 show various values ​​in Numerical Examples 1 to 5 (the exit angles and incident angles in Table 4 are when the light rays are reverse traced), and Table 3 shows the lens surfaces having inflection points in Numerical Examples 1, 2, and 4. (Numerical example 1) Unit: mm Surface Data Surface number rd nd νd Effective diameter (Z,Y) 1 (pupil plane) ∞ (variable) 6.00 2∞0.201.4900050.0 (39.00,27.00) 3∞0.101.5100050.0 (39.00,27.00) 4 ∞ 0.20 1.58000 50.0 (39.00,27.00) 5 ∞ 7.00 1.54500 56.2 (39.00,27.00) 6* -44.529 -7.00 (39.00,27.00) 7 ∞ -0.20 1.58000 50.0 (39.00,27.00) 8∞0.20 (39.00,27.00) 9 ∞ 7.00 1.54500 56.2 (39.00,27.00) 10* -44.529 3.50 1.64000 22.5 (39.00,27.00) 11* -332.092 1.08 (32.00,27.00) 12 ∞ 0.20 1.58000 50.0 30.00 13 ∞ 0.20 1.49000 50.0 30.00 14 ∞ 1.20 30.00 15 ∞ 0.70 1.51633 64.1 30.00 16∞0.0030.00 Display surface (Z,Y)=(37.000,∞) Aspheric Data Page 6 K = 0.00000e+00 A 4= 1.07930e-07 A 6=-1.20647e-09 A 8= 9.57432e-12 A10=-2.62069e-14 A12= 3.48383e-17 Side 10 K = 0.00000e+00 A 4= 1.07930e-07 A 6=-1.20647e-09 A 8= 9.57432e-12 A10=-2.62069e-14 A12= 3.48383e-17 Page 11 K = 0.00000e+00 A 4= 5.45637e-05 A 6=-6.07318e-07 A 8= 2.74780e-09 A10=-6.65150e-12 A12= 6.85403e-15 Various data Focal length 14.48 Pupil diameter 6.00 Total viewing angle (°) (Z,Y)=(90.00,58.72) d 1 15.00 Entrance pupil position 0.00 Exit pupil position 38.81 Front principal point position 19.89 Back principal point position -14.48 Lens Group Data Group starting plane Focal length Lens length Front principal point position Rear principal point position 1 2 14.48 14.38 4.89 -14.48 Single lens data Lens starting surface focal length Pupil plane 1 0.00 POL2 0.00 PBS 3 0.00 QWP1 4 0.00 G1 5 81.71 G2(HM) 10 -80.73 QWP2 12 0.00 POL 13 0.00 CGI 15 0.00 (Numerical example 2) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1 (pupil plane) ∞ (variable) 6.00 2 ∞ 0.20 1.49000 50.0 37.00 3 ∞ 0.10 1.51000 50.0 37.00 4 ∞ 0.20 1.58000 50.0 37.00 5 ∞ 7.00 1.57500 55.4 37.00 6* -43.000 -7.00 37.00 7 ∞ -0.20 1.58000 50.0 37.00 8 ∞ 0.20 37.00 9 ∞ 7.00 1.57500 55.4 37.00 10* -43.000 3.00 1.64000 23.5 37.00 11* -42.050 0.68 29.00 12 ∞ 0.20 1.58000 50.0 30.00 13 ∞ 0.20 1.49000 50.0 30.00 14 ∞ 1.20 30.00 15 ∞ 0.70 1.51633 64.1 30.00 16∞0.0030.00 Display surface 100.000 Aspheric Data Side 6 K = 0.00000e+00 A 4=-1.32805e-06 A 6= 3.76169e-09 Side 10 K = 0.00000e+00 A 4=-1.32805e-06 A 6= 3.76169e-09 Page 11 K = 0.00000e+00 A 4= 4.12993e-04 A 6=-4.91892e-06 A 8= 2.28817e-08 A10=-3.78901e-11 Various data Focal length 13.20 Pupil diameter 6.00 Total viewing angle (°) 85.00 d 1 15.00 Entrance pupil position 0.00 Exit pupil position 20.26 Front principal point position 21.80 Back principal point position -13.20 Lens Group Data Group starting plane Focal length Lens length Front principal point position Rear principal point position 1 2 13.20 13.48 6.80 -13.20 Single lens data Lens starting surface focal length Pupil plane 1 0.00 POL2 0.00 PBS 3 0.00 QWP1 4 0.00 G1 5 74.78 G2(HM) 10 1332.20 QWP2 12 0.00 POL 13 0.00 CGI 15 0.00 (Numerical example 3) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1 (pupil plane) ∞ (variable) 6.00 2 ∞ 0.20 1.49000 50.0 60.00 3 ∞ 0.10 1.51000 50.0 60.00 4 ∞ 0.20 1.58000 50.0 60.00 5 ∞ 9.60 1.54400 56.2 60.00 6* -61.371 -9.60 60.00 7 ∞ -0.20 1.58000 50.0 60.00 8 ∞ 0.20 60.00 9 ∞ 9.60 1.54400 56.2 60.00 10* -61.371 3.00 1.66000 21.0 60.00 11* -63.802 4.46 54.00 12 54.400 0.20 1.58000 50.0 40.00 13 54.200 0.20 1.49000 50.0 40.00 14 54.000 0.70 1.51633 64.1 40.00 15 53.300 0.00 40.00 Display surface 53.300 Aspheric Data Page 6 K = 0.00000e+00 A 4= 1.69732e-07 A 6=-4.58512e-10 A 8= 1.04438e-12 A10=-1.18179e-15 A12= 5.77673e-19 Side 10 K = 0.00000e+00 A 4= 1.69732e-07 A 6=-4.58512e-10 A 8= 1.04438e-12 A10=-1.18179e-15 A12= 5.77673e-19 Page 11 K = 0.00000e+00 A 4= 2.90606e-05 A 6=-6.94977e-08 A 8= 9.77055e-11 A10=-7.27403e-14 A12= 1.95997e-17 Various data Focal length 18.94 Pupil diameter 6.00 Total viewing angle (°) 120.00 d 1 15.00 Entrance pupil position 0.00 Exit pupil position 74.07 Front principal point position 23.78 Back principal point position -18.94 Lens Group Data Group starting plane Focal length Lens length Front principal point position Rear principal point position 1 2 18.94 18.66 8.78 -18.94 Single lens data Lens starting surface focal length Pupil plane 1 0.00 POL2 0.00 PBS 3 0.00 QWP1 4 0.00 G1 5 112.81 G2(HM) 10 -4793.67 QWP2 12 -40160.33 POL 13 -44499.31 CG 14 -12075.06 (Numerical example 4) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1(Pupil plane) ∞ (variable) 30.00 2 ∞ 0.20 1.49000 50.0 40.00 3 ∞ 0.10 1.51000 50.0 40.00 4 ∞ 0.20 1.58000 50.0 40.00 5 ∞ 6.50 1.54000 56.0 40.00 6* -47.217 -6.50 40.00 7 ∞ -0.20 1.58000 50.0 40.00 8 ∞ 0.20 40.00 9∞6.50 1.54000 56.0 40.00 10* -47.217 3.00 1.60700 27.0 40.00 11 ∞ 0.20 1.58000 50.0 40.00 12 ∞ 0.20 1.49000 50.0 40.00 13 ∞ 0.50 40.00 14* 33.118 3.14 1.53000 55.0 32.50 15* 63.161 1.33 32.50 16 43.200 0.70 1.51633 64.1 25.00 17 42.500 0.00 25.00 Display surface 42.500 Aspheric Data Side 6 K = 0.00000e+00 A 4=-1.38181e-07 A 6= 7.61577e-10 A 8= 1.97719e-12 A10=-1.54347e-14 A12= 2.91621e-17 Side 10 K = 0.00000e+00 A 4=-1.38181e-07 A 6= 7.61577e-10 A 8= 1.97719e-12 A10=-1.54347e-14 A12= 2.91621e-17 Side 14 K = 0.00000e+00 A 4=-1.07263e-04 A 6= 2.05131e-07 Page 15 K = 0.00000e+00 A 4=-1.04715e-04 A 6= 2.21415e-07 Various data Focal length 14.67 Pupil diameter 6.00 Total viewing angle (°)90.00 d 1 15.00 Entrance pupil position 0.00 Exit pupil position 38.05 Front principal point position 20.32 Back principal point position -14.67 Lens Group Data Group starting plane Focal length Lens length Front principal point position Rear principal point position 1 2 14.67 16.07 5.32 -14.67 Single lens data Lens starting surface focal length Pupil plane 1 0.00 POL2 0.00 PBS 3 0.00 QWP1 4 0.00 G1 5 87.44 G2(HM) 10 -77.79 QWP2 11 0.00 POL 12 0.00 G3 14 126.78 CG 16 -7702.66 (Numerical example 5) Unit: mm Surface Data Surface number rd nd νd Effective diameter 1 (pupil plane) ∞ (variable) 6.00 2 -37.898 0.20 1.49000 50.0 33.00 3 -37.698 0.10 1.51000 50.0 33.00 4 -37.598 0.20 1.58000 50.0 33.00 5 -37.398 6.50 1.54000 56.0 33.00 6* -24.733 -6.50 36.00 7 -37.398 -0.20 1.58000 50.0 33.00 8 -37.598 0.20 33.00 9 -37.398 6.50 1.54000 56.0 33.00 10* -24.733 2.00 1.64000 22.0 36.00 11* -1305.271 0.50 31.00 12 ∞ 0.20 1.58000 50.0 30.00 13 ∞ 0.20 1.49000 50.0 30.00 14 ∞ 0.76 30.00 15 44.660 0.70 1.51633 64.1 25.00 16 43.960 0.00 25.00 Display surface 43.960 Aspheric Data Side 6 K = 0.00000e+00 A 4= 7.86898e-08 A 6=-9.99204e-09 A 8= 6.36425e-11 A10=-2.20795e-13 A12= 2.88649e-16 Side 10 K = 0.00000e+00 A 4= 7.86898e-08 A 6=-9.99204e-09 A 8= 6.36425e-11 A10=-2.20795e-13 A12= 2.88649e-16 Page 11 K = 0.00000e+00 A 4=-1.14287e-04 A 6= 5.91339e-07 A 8=-2.28815e-09 A10=5.16921e-12 A12=-5.11210e-15 Various data Focal length 13.74 Pupil diameter 6.00 Total viewing angle (°) 100.00 d 1 15.00 Entrance pupil position 0.00 Exit pupil position 157.16 Front principal point position 14.94 Back principal point position -13.74 Lens Group Data Group starting plane Focal length Lens length Front principal point position Rear principal point position 1 2 13.74 11.36 -0.06 -13.74 Single lens data Lens starting surface focal length Pupil plane 1 0.00 POL2 10970.68 PBS 3 20775.06 QWP1 4 8866.70 G1 5 114.62 G2(HM) 10 -39.42 QWP2 12 0.00 POL 13 0.00 CG 15 -8236.54

[0067] [Table 1]

[0068] [Table 2]

[0069] [Table 3]

[0070] [Table 4]

[0071] 14 shows a head mounted display (HMD) as a display device in Examples 1 to 5. The HMD is worn on the head (in front of the eyes) of a viewer by means of a wearing gear (not shown).

[0072] The HMD has display elements RID and LID for the right and left eyes, a right-eye display optical system ROS that guides display light from the right-eye display element RID to the observer's right eye, and a left-eye display optical system LOS that guides display light from the left-eye image display element LID to the observer's left eye.

[0073] By using the configurations described in the first to fifth embodiments, it is possible to realize a small, lightweight HMD that enables viewing of images with good image quality.

[0074] The above embodiment includes the following configurations.

[0075] (Configuration 1) A display device having a display element and a display optical system that guides light from a display surface of the display element to a viewing side, The display optical system includes: a first semi-transmissive reflective surface; a second semi-transmissive reflective surface disposed on the bonding surface of the two optical elements; guide light from the display surface to the observation side via transmission through the second semi-transmissive reflective surface, reflection through the first semi-transmissive reflective surface, reflection through the second semi-transmissive reflective surface, and transmission through the first semi-transmissive reflective surface; the curvature of the second semi-transmissive reflective surface is greater than the curvature of the first semi-transmissive reflective surface; A display device, wherein the display surface has a convex shape facing the observation side in at least one of two cross sections that are perpendicular to each other and contain the optical axis of the display optical system. (Configuration 2) 2. The display device according to configuration 1, wherein the two optical elements are made of a resin material. (Configuration 3) Of the two optical elements, a first optical element on the observation side has a focal length in air calculated from a radius of curvature of a reference spherical surface of the first optical element, and a second optical element on the display element side has a focal length in air calculated from a radius of curvature of a reference spherical surface of the second optical element, and fG1 / fG2<0 2. The display device according to claim 1, wherein the display device satisfies the following conditions: (Configuration 4) Of the two optical elements, the refractive index at the d-line of the optical element having a positive focal length in air calculated from the radius of curvature of the reference sphere is Ndp, and the refractive index at the d-line of the optical element having a negative focal length in air calculated from the radius of curvature of the reference sphere is Ndn. 1.4≦Ndp <Ndn≦1.9 3. The display device according to configuration 1 or 2, which satisfies the following conditions: (Configuration 5) 5. The display device according to configuration 4, wherein of the two optical elements, the focal length of the optical element on the observation side is positive, and the focal length of the optical element on the display element side is negative. (Configuration 6) Of the two optical elements, the Abbe number based on the d-line of the optical element having a positive focal length in air calculated from the radius of curvature of the reference sphere is νdp, and the Abbe number based on the d-line of the optical element having a negative focal length in air calculated from the radius of curvature of the reference sphere is νdn. 1.0<νdp / νdn≦4.0 6. The display device according to any one of configurations 1 to 5, which satisfies the following conditions: (Configuration 7) Let Rhm be the radius of curvature of the reference spherical surface of the second semi-transmissive reflective surface, and f be the focal length of the display optical system. 1.0<|Rhm| / f≦5.0 7. The display device according to any one of configurations 1 to 6, which satisfies the following conditions: (Configuration 8) When the radius of curvature of the reference spherical surface of the second semi-transmissive reflective surface is Rhm and the radius of curvature of the reference spherical surface of the display surface is Rp, 0.1≦|Rhm| / |Rp|≦2.0 8. The display device according to any one of configurations 1 to 7, which satisfies the following conditions: (Configuration 9) 9. The display device according to any one of configurations 1 to 8, wherein the display optical system has an aspheric refractive surface that includes an inflection point within an optical effective diameter. (Configuration 10) 10. The display device according to any one of configurations 1 to 9, wherein the first semi-transmissive reflective surface is a flat surface. (Configuration 11) 11. The display device according to any one of configurations 1 to 10, wherein the display optical system includes only a cemented lens formed of the two optical elements as a refractive element. (Configuration 12) 12. A display device according to any one of configurations 1 to 11, characterized in that the absolute value of the angle between the pupil center ray and the normal to the display surface when the eye relief is 15 mm, the visual acuity is 0 diopters, and the viewing angle is 30 degrees is 30 degrees or less. (Configuration 13) 13. The display device according to any one of configurations 1 to 12, wherein at least one of the first and second semi-transmissive reflective surfaces is configured by a polarization-selective semi-transmissive reflective element.

[0076] The embodiments described above are merely representative examples, and various modifications and alterations are possible for each embodiment when implementing the present invention. [Explanation of symbols]

[0077] SP pupil plane PBS First semi-transparent reflective surface QWP1 First quarter wave plate G1 First Lens ID display surface HM Second semi-transparent reflective surface G2 Second Lens QWP2 Second quarter wave plate POL polarizing plate G3 Third Lens CG cover glass

Claims

1. A display device having a display element and a display optical system that guides light from a display surface of the display element to a viewing side, The display optical system includes: a first semi-transmissive reflective surface; a second semi-transmissive reflective surface disposed on the bonding surface of the two optical elements; guide light from the display surface to the observation side via transmission through the second semi-transmissive reflective surface, reflection through the first semi-transmissive reflective surface, reflection through the second semi-transmissive reflective surface, and transmission through the first semi-transmissive reflective surface; the curvature of the second semi-transmissive reflective surface is greater than the curvature of the first semi-transmissive reflective surface; A display device, wherein the display surface has a convex shape facing the observation side in at least one of two cross sections that are perpendicular to each other and contain an optical axis of the display optical system.

2. 2. The display device according to claim 1, wherein the two optical elements are made of a resin material.

3. Of the two optical elements, a first optical element on the observation side has a focal length in air calculated from a radius of curvature of a reference spherical surface of the first optical element, and a second optical element on the display element side has a focal length in air calculated from a radius of curvature of a reference spherical surface of the second optical element, and fG1 / fG2<0.0 2. The display device according to claim 1, wherein the following conditions are satisfied:

4. Of the two optical elements, the refractive index at the d-line of the optical element having a positive focal length in air calculated from the radius of curvature of the reference sphere is Ndp, and the refractive index at the d-line of the optical element having a negative focal length in air calculated from the radius of curvature of the reference sphere is Ndn, 1.4≦Ndp<Ndn≦1.9 2. The display device according to claim 1, wherein the following conditions are satisfied:

5. 5. The display device according to claim 4, wherein the focal length of the optical element on the observation side of the two optical elements is positive, and the focal length of the optical element on the display element side is negative.

6. Of the two optical elements, the Abbe number based on the d-line of the optical element having a positive focal length in air calculated from the radius of curvature of the reference spherical surface is denoted by νdp, and the Abbe number based on the d-line of the optical element having a negative focal length in air calculated from the radius of curvature of the reference spherical surface is denoted by νdn. 1.0<νdp / νdn≦4.0 2. The display device according to claim 1, wherein the following conditions are satisfied:

7. When the radius of curvature of the reference spherical surface of the second semi-transmissive reflective surface is Rhm and the focal length of the display optical system is f, 1.0<|Rhm| / f≦5.0 2. The display device according to claim 1, wherein the following conditions are satisfied:

8. When the radius of curvature of the reference spherical surface of the second semi-transmissive reflective surface is Rhm and the radius of curvature of the reference spherical surface of the display surface is Rp, 0.1≦|Rhm| / |Rp|≦2.0 2. The display device according to claim 1, wherein the following conditions are satisfied:

9. 2. The display device according to claim 1, wherein the display optical system has an aspheric refractive surface that includes an inflection point within an effective optical diameter.

10. 2. The display device according to claim 1, wherein the first semi-transmissive reflective surface is a flat surface.

11. 2. The display device according to claim 1, wherein the display optical system includes only a cemented lens made up of the two optical elements as a refractive element.

12. 2. The display device according to claim 1, wherein the absolute value of the angle between the pupil center ray and the normal to the display surface when the eye relief is 15 mm, the diopter is 0 diopter, and the viewing angle is 30 degrees is 30 degrees or less.

13. 2. The display device according to claim 1, wherein at least one of the first and second semi-transmissive reflective surfaces is formed of a polarization-selective semi-transmissive reflective element.

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