Optical system

Abstract

To provide an optical system having good optical characteristics, which satisfies size reduction and improvement request for light quantity efficiency.SOLUTION: An optical system comprises in order from a pupil surface EP side toward a display surface IMG side: a first lens L1 with positive refractive power, a first reflection polarization plate 11; a second lens L2 with positive refractive power; a half mirror HM; a third lens L3 with positive refractive power; a second reflection polarization plate 12; a fourth lens L4; a first 1 / 4 wavelength plate 21 arranged between the pupil surface EP and the half mirror HM; and a second 1 / 4 wavelength plate 22 disposed between the half mirror HM and the display surface IMG. The display surface IMG side of the first lens L1 is convex in a paraxial state and satisfies a predetermined conditional expression.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical system for magnifying an image (for example, an image displayed on an image display device). [Background technology]

[0002] 2. Description of the Related Art Known display devices using image display elements include electronic viewfinders, electronic binoculars, and head-mounted displays.

[0003] In such a display device, the distance between the image display element and the eye must be as short as possible, and the optical system for magnifying the image displayed on the image display element must be housed in a limited space. This often makes it difficult to eliminate various aberrations in the optical system, and the range of correction is also limited.

[0004] As is well known, visual acuity depends on the density of the cones in the photoreceptor cells, and the eye has the characteristic of forming a clear image near the fovea of ​​the macula, i.e., the center of the pupil. Therefore, by utilizing this physiological optical property of the eye to complement the aberration correction of the optical system, it is possible to achieve good optical performance. Specifically, narrowing the pupil increases the depth of focus and reduces the effects of spherical aberration and coma, thereby reducing sensitivity to blur even if the aberration and refractive correction are insufficient or excessive. Furthermore, by utilizing the so-called Stiles-Crawford effect, which is a phenomenon in which the sensitivity of light rays incident from the periphery is lower than that of light rays passing through the center of the pupil, the effects of spherical aberration, coma, and chromatic aberration can be reduced. Furthermore, by maintaining this state, the user gradually becomes accustomed to the condition, which can also reduce the effects of distortion and other aberrations.

[0005] The optical systems installed in such display devices are required to be compact and have high light efficiency, where light efficiency refers to the ratio of the amount of light reaching the eye (pupil plane) when the amount of light on the display surface of the image display element is taken as 100%.

[0006] Known examples of conventional optical systems include the optical system described in Patent Document 1. Patent Document 1 discloses an eyepiece lens that includes at least one stationary lens group and at least two movable lens groups that are movable relative to the stationary lens group along an optical axis between an image plane and an exit pupil, the lens groups being arranged in a positive-negative-positive configuration. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2020-522022 Summary of the Invention [Problem to be solved by the invention]

[0008] If an attempt is made to reduce the size of the optical system described in Patent Document 1, it becomes extremely difficult to correct aberrations in the peripheral area, and good optical performance cannot be obtained.

[0009] The present invention has been made in view of the above-mentioned problems, and aims to provide an optical system that has high resolving power with various aberrations well corrected, while satisfying the demands for compactness and improved light efficiency in a balanced manner. [Means for solving the problem]

[0010] The optical system according to the present invention comprises, in order from the pupil plane side to the display surface side, a first lens having positive refractive power, a first reflective polarizer, a second lens having positive refractive power, a half mirror, a third lens having positive refractive power, a second reflective polarizer, and a fourth lens, as well as a first quarter-wave plate disposed between the pupil plane and the half mirror, and a second quarter-wave plate disposed between the half mirror and the display surface, the first lens having a convex surface facing the display surface in a paraxial direction. In this specification, the terms "convex," "concave," and "flat" refer to the shape of a lens in a paraxial direction, and the term "refractive power" refers to the refractive power in a paraxial direction unless otherwise specified.

[0011] The first lens has positive refractive power and has a convex surface on the display surface side in the paraxial direction, thereby suppressing spherical aberration, coma, astigmatism, curvature of field, and distortion.

[0012] A reflective polarizer reflects linearly polarized light having one polarization direction and transmits linearly polarized light having an orthogonal polarization direction.

[0013] A quarter-wave plate converts linearly polarized light into circularly polarized light and circularly polarized light into linearly polarized light by delaying the phase of polarized light by 1 / 4λ.

[0014] The second lens has a positive refractive power, and thereby effectively corrects spherical aberration, astigmatism, curvature of field, and distortion.

[0015] A half mirror transmits 50% of the light and reflects the remaining 50%.

[0016] The third lens has a positive refractive power, and thereby effectively corrects spherical aberration, astigmatism, curvature of field, and distortion.

[0017] The fourth lens element effectively corrects astigmatism, field curvature, and distortion.

[0018] In the optical system of the present invention, a first reflective polarizer, a first quarter-wave plate, and a second lens having positive refractive power are arranged in a group on the pupil plane side, and a third lens having positive refractive power, a second quarter-wave plate, and a second reflective polarizer are arranged in a group on the display surface side. These groups on the pupil plane side and the display surface side are arranged substantially symmetrically with respect to the semi-transmissive surface of the half mirror. Therefore, by finally superimposing two beams of light, that is, light reflected by and light transmitted through the half mirror, it is possible to achieve an improvement in light efficiency while miniaturizing the optical system.

[0019] In the optical system having the above configuration, it is desirable that the surface of the second lens on the pupil plane side in the paraxial direction be a convex surface.

[0020] By making the surface of the second lens on the pupil plane side a convex surface in the paraxial direction, it becomes possible to make good corrections for astigmatism, field curvature, and distortion.

[0021] In the optical system having the above configuration, it is desirable that the third lens has a convex surface on the display surface side in the paraxial direction.

[0022] By making the surface of the third lens on the display surface side a convex surface in the paraxial direction, it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0023] It is desirable that the optical system having the above configuration satisfy the following conditional expression (1): (1)-1.25 <r3 / r6<-0.75 Here, r3 is the paraxial radius of curvature of the surface of the second lens on the pupil plane side, and r6 is the paraxial radius of curvature of the surface of the third lens on the display plane side.

[0024] By satisfying the range of conditional expression (1), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0025] It is desirable that the optical system having the above configuration satisfy the following conditional expression (2): (2) 0.75 <f2 / f3<1.25 Here, f2 is the focal length of the second lens, and f3 is the focal length of the third lens.

[0026] By satisfying the range of conditional expression (2), it becomes possible to make good corrections for spherical aberration, astigmatism, curvature of field, and distortion.

[0027] It is desirable that the optical system having the above configuration satisfy the following conditional expression (3). (3)2 <r3 / f<8 Here, r3 is the paraxial radius of curvature of the surface of the second lens on the pupil plane side, and f is the focal length of the entire optical system.

[0028] By satisfying the range of conditional expression (3), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0029] It is desirable that the optical system having the above configuration satisfy the following conditional expression (4): (4)-1.00 <r2 / r3<-0.25 Here, r2 is the paraxial radius of curvature of the surface of the first lens on the display surface side, and r3 is the paraxial radius of curvature of the surface of the second lens on the pupil plane side.

[0030] By satisfying the range of conditional expression (4), it becomes possible to make good corrections for coma, astigmatism, curvature of field, and distortion.

[0031] It is desirable that the optical system having the above configuration satisfy the following conditional expression (5): (5)-3.00 <r2 / |r8|<-0.02 Here, r2 is the paraxial radius of curvature of the surface of the first lens on the display surface side, and r8 is the paraxial radius of curvature of the surface of the fourth lens on the display surface side.

[0032] By satisfying the range of conditional expression (5), it becomes possible to make good corrections for coma, astigmatism, curvature of field, and distortion.

[0033] It is desirable that the optical system having the above configuration satisfy the following conditional expression (6): (6)-0.85 <r2 / f1<-0.25 Here, r2 is the paraxial radius of curvature of the surface of the first lens on the display surface side, and f1 is the focal length of the first lens.

[0034] By satisfying the range of conditional expression (6), it becomes possible to make good corrections for coma, astigmatism, curvature of field, and distortion.

[0035] It is desirable that the optical system having the above configuration satisfy the following conditional expression (7): (7)-8 <r6 / f<-2 Here, r6 is the paraxial radius of curvature of the surface of the third lens on the display surface side, and f is the focal length of the entire optical system.

[0036] By satisfying the range of conditional expression (7), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0037] It is desirable that the optical system having the above configuration satisfy the following conditional expression (8). (8)4 <f2 / f<15 Here, f2 is the focal length of the second lens, and f is the focal length of the entire optical system.

[0038] By satisfying the range of conditional expression (8), it becomes possible to make good corrections for spherical aberration, astigmatism, curvature of field, and distortion.

[0039] It is desirable that the optical system having the above configuration satisfy the following conditional expression (9): (9)39<νd1<73 Here, νd1 is the Abbe number for the d-line of the first lens.

[0040] By satisfying the range of conditional expression (9), chromatic aberration can be corrected satisfactorily.

[0041] It is desirable that the optical system having the above configuration satisfy the following conditional expression (10). (10)39<νd3<73 Here, νd3 is the Abbe number for the d-line of the third lens.

[0042] By satisfying the range of conditional expression (10), chromatic aberration can be corrected satisfactorily.

[0043] It is desirable that the optical system having the above configuration satisfy the following conditional expression (11). (11)-5 <r2 / f<-1 Here, r2 is the paraxial radius of curvature of the surface of the first lens on the display surface side, and f is the focal length of the entire optical system.

[0044] By satisfying the range of conditional expression (11), it becomes possible to make good corrections for coma, astigmatism, curvature of field, and distortion.

[0045] It is desirable that the optical system having the above configuration satisfy the following conditional expression (12). (12)0.4 <r3 / |r7|<6.0 Here, r3 is the paraxial radius of curvature of the surface of the second lens on the pupil plane side, and r7 is the paraxial radius of curvature of the surface of the fourth lens on the pupil plane side.

[0046] By satisfying the range of conditional expression (12), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0047] It is desirable that the optical system having the above configuration satisfy the following conditional expression (13). (13) 0.5<|r7| / f<7.0 Here, r7 is the paraxial radius of curvature of the pupil plane side surface of the fourth lens, and f is the focal length of the entire optical system.

[0048] By satisfying the range of conditional expression (13), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0049] It is desirable that the optical system having the above configuration satisfy the following conditional expression (14). (14) 13<|r7| / T3<50 Here, r7 is the paraxial radius of curvature of the pupil plane side surface of the fourth lens, and T3 is the distance on the optical axis from the display plane side surface of the third lens to the pupil plane side surface of the fourth lens.

[0050] By satisfying the range of conditional expression (14), it becomes possible to achieve a low profile and to make good corrections for astigmatism, curvature of field, and distortion.

[0051] It is desirable that the optical system having the above configuration satisfy the following conditional expression (15). (15)3<|r7| / D4<37 Here, r7 is the paraxial radius of curvature of the pupil plane side surface of the fourth lens, and D4 is the thickness of the fourth lens on the optical axis.

[0052] By satisfying the range of conditional expression (15), it becomes possible to achieve a low profile and to make good corrections for astigmatism, curvature of field, and distortion.

[0053] It is desirable that the optical system having the above configuration satisfy the following conditional expression (16): (16) 0.5<|r8| / f<105.0 Here, r8 is the paraxial radius of curvature of the surface of the fourth lens element on the display surface side, and f is the focal length of the entire optical system.

[0054] By satisfying the range of conditional expression (16), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0055] It is desirable that the optical system having the above configuration satisfy the following conditional expression (17). (17) 0.1<|r8 / f4|<11.5 where r8 is the paraxial radius of curvature of the surface of the fourth lens on the display surface side, and f4 is the focal length of the fourth lens.

[0056] By satisfying the range of conditional expression (17), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion.

[0057] It is desirable that the optical system having the above configuration satisfy the following conditional expression (18). (18)4<|r8| / D4<540 Here, r8 is the paraxial radius of curvature of the surface of the fourth lens on the display surface side, and D4 is the thickness of the fourth lens on the optical axis.

[0058] By satisfying the range of conditional expression (18), it becomes possible to achieve a low profile and to make good corrections for astigmatism, curvature of field, and distortion.

[0059] It is desirable that the optical system having the above configuration satisfy the following conditional expression (19): (19) 1.0<(D² / f²)×100<4.5 Here, D2 is the thickness of the second lens on the optical axis, and f2 is the focal length of the second lens.

[0060] By satisfying the range of conditional expression (19), it becomes possible to achieve a low profile and to make good corrections for spherical aberration, astigmatism, curvature of field, and distortion.

[0061] It is desirable that the optical system having the above configuration satisfy the following conditional expression (20). (20) 1.0<(D3 / f3)×100<4.5 Here, D3 is the thickness of the third lens on the optical axis, and f3 is the focal length of the third lens.

[0062] By satisfying the range of conditional expression (20), it becomes possible to achieve a low profile and to make good corrections for astigmatism, curvature of field, and distortion.

[0063] It is desirable that the optical system having the above configuration satisfy the following conditional expression (21). (21)0.25 <f1 / f2<1.00 Here, f1 is the focal length of the first lens, and f2 is the focal length of the second lens.

[0064] By satisfying the range of conditional expression (21), it becomes possible to make good corrections for spherical aberration, coma, astigmatism, curvature of field, and distortion.

[0065] It is desirable that the optical system having the above configuration satisfy the following conditional expression (22). (22)0.20 <f1 / |f4|<1.05 Here, f1 is the focal length of the first lens, and f4 is the focal length of the fourth lens.

[0066] By satisfying the range of conditional expression (22), it becomes possible to make good corrections for spherical aberration, coma, astigmatism, curvature of field, and distortion.

[0067] It is desirable that the optical system having the above configuration satisfy the following conditional expression (23). (23)0.45 <f2 / |f4|<1.65 Here, f2 is the focal length of the second lens, and f4 is the focal length of the fourth lens.

[0068] By satisfying the range of conditional expression (23), it becomes possible to make good corrections for spherical aberration, astigmatism, curvature of field, and distortion.

[0069] It is desirable that the optical system having the above configuration satisfy the following conditional expression (24). (24)4 <f3 / f<15 Here, f3 is the focal length of the third lens, and f is the focal length of the entire optical system.

[0070] By satisfying the range of conditional expression (24), it becomes possible to make good corrections for spherical aberration, astigmatism, curvature of field, and distortion.

[0071] It is desirable that the optical system having the above configuration satisfy the following conditional expression (25). (25)4<|f4| / f<14 Here, f4 is the focal length of the fourth lens, and f is the focal length of the entire optical system.

[0072] By satisfying the range of conditional expression (25), it becomes possible to make good corrections for astigmatism, curvature of field, and distortion. [Effects of the Invention]

[0073] The present invention makes it possible to obtain an optical system with high resolution and excellent correction of aberrations while satisfying the requirements for miniaturization and improved light efficiency in a well-balanced manner. Furthermore, the optical system according to the present invention improves light efficiency, thereby reducing power consumption in the image display element and providing an environmentally friendly optical system. [Brief explanation of the drawings]

[0074] [Figure 1] FIG. 1 is a cross-sectional view showing a schematic configuration of an optical system according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a partially enlarged view of the optical system shown in FIG. [Figure 3] 2A to 2C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the optical system shown in FIG. [Figure 4] FIG. 10 is a cross-sectional view showing a schematic configuration of an optical system according to Example 2 of the present invention. [Figure 5] FIG. 5 is a partially enlarged view of the optical system shown in FIG. [Figure 6] 5A to 5C are aberration diagrams showing spherical aberration, astigmatism, and distortion of the optical system shown in FIG. [Figure 7] 7A is a cross-sectional view showing a path 1 in the optical system shown in FIG. 4. FIG. 7B is a cross-sectional view showing a path 2 in the optical system shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0075] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described in detail with reference to the accompanying drawings.

[0076] 1 and 4 are cross-sectional views showing the schematic configuration of the optical systems of Examples 1 and 2 according to the present embodiment, respectively. Also, Fig. 2 and Fig. 5 show enlarged partial views of the optical systems of Examples 1 and 2, respectively. The optical system according to the present embodiment will be described in detail below with reference to the optical system of Example 1.

[0077] As shown in Figure 1, the optical system of this embodiment includes, in order from the pupil plane EP toward the display surface IMG of the image display element, a first lens L1 having positive refractive power, a first reflective polarizer 11, a second lens L2 having positive refractive power, a half mirror HM, a third lens L3 having positive refractive power, a second reflective polarizer 12, and a fourth lens L4, as well as a first quarter-wave plate 21 arranged between the pupil plane EP and the half mirror HM, and a second quarter-wave plate 22 arranged between the half mirror HM and the display surface IMG, and the first lens L1 has a convex surface on the display surface IMG side in the paraxial direction.

[0078] In this optical system, a first reflective polarizer 11, a first quarter-wave plate 21, and a second lens L2 having positive refractive power are arranged in a group on the pupil plane EP side, and a third lens L3 having positive refractive power, a second quarter-wave plate 22, and a second reflective polarizer 12 are arranged in a group on the display surface IMG side. The group on the pupil plane EP side and the group on the display surface IMG side are arranged approximately symmetrically with respect to the semi-transmitting surface of the half mirror HM.

[0079] The optical system according to this embodiment may be mounted on any object, but may be mounted on a head-mounted display, for example, as an optical system for enlarging an image displayed on a display surface IMG. In this case, the pupil of the observer is located on the pupil plane EP. An aperture stop may be disposed on the pupil plane EP.

[0080] In the optical system according to this embodiment, anti-reflection films may be attached to both sides of the half mirror HM. Since anti-reflection films have the function of preventing light reflection, this configuration can prevent a decrease in image contrast due to reflection of external light.

[0081] As shown in Fig. 1, the first lens L1 has positive refractive power, and the surface on the pupil plane EP side is flat in the paraxial direction, and the surface on the display surface IMG side is convex in the paraxial direction. This shape of the first lens L1 can suppress spherical aberration, coma, astigmatism, field curvature, and distortion. The shape of the surface of the first lens L1 on the pupil plane EP side is not limited to the shape according to the first embodiment, and may be a shape that is convex or concave in the paraxial direction.

[0082] As shown in FIG. 2, the surface of the first reflective polarizer 11 facing the display surface IMG and the surface of the second lens L1 facing the pupil surface EP have the same shape, and they are attached with an adhesive or the like. The second lens L2 has a convex surface on the pupil surface EP side paraxially. The second lens L2 of this embodiment has positive refractive power, a convex surface on the pupil surface EP side paraxially, and a flat surface on the display surface IMG side paraxially. This shape of the second lens L2 allows for excellent correction of spherical aberration, astigmatism, field curvature, and distortion.

[0083] 2 and 5, the optical system according to this embodiment is an example in which the first quarter-wave plate 21 is disposed between the first reflective polarizer 11 and the second lens L2, and the second quarter-wave plate 22 is disposed between the third lens L3 and the second reflective polarizer 12. When the first quarter-wave plate 21 and the second quarter-wave plate 22 are disposed in such positions, a composite film formed by bonding the first reflective polarizer 11 (second reflective polarizer 12) and the first quarter-wave plate 21 (second quarter-wave plate 22) together can be attached to the second lens L2 (third lens L3), which facilitates industrial production. This reduces costs and improves the axial accuracy of the first reflective polarizer 11 (second reflective polarizer 12) and the first quarter-wave plate 21 (second quarter-wave plate 22). However, the positions of the first quarter-wave plate 21 and the second quarter-wave plate 22 are not limited to this. The first quarter-wave plate 21 may be located between the pupil plane EP and the half mirror HM, and the second quarter-wave plate 22 may be located between the half mirror HM and the display surface IMG.

[0084] 1, the third lens L3 has positive refractive power, a paraxial plane surface facing the pupil plane EP, and a paraxial convex surface facing the display surface IMG. This shape of the third lens L3 enables excellent correction of spherical aberration, astigmatism, field curvature, and distortion.

[0085] The surface of the third lens L3 on the display surface IMG side and the surface of the second reflective polarizer 12 on the pupil plane EP side have the same shape, and the two are attached with an adhesive or the like.

[0086] As shown in FIG. 1, the fourth lens L4 has positive refractive power, a convex surface on the pupil plane EP side paraxially, and a concave surface on the display surface IMG side paraxially. This shape of the fourth lens L4 allows for excellent correction of spherical aberration, astigmatism, field curvature, and distortion. The shape of the fourth lens L4 may also be negative refractive power, a concave surface on the pupil plane EP side paraxially, and a convex surface on the display surface IMG side paraxially, as in the optical system described in Example 2. In this case, chromatic aberration, astigmatism, field curvature, and distortion can be excellently corrected.

[0087] As described above, in the optical system according to this embodiment, the second lens L2 and the third lens L3 are arranged to sandwich the half mirror HM from both sides. In addition to this basic configuration, the first reflective polarizer 11, the first quarter-wave plate 21, the second quarter-wave plate 22, and the second reflective polarizer 12 are each arranged in appropriate positions to improve the light efficiency of the optical system. This will be described in detail below.

[0088] As a device having a display surface IMG, for example, a liquid crystal display or a micro OLED (Organic Light Emitting Diode) display can be adopted.

[0089] As shown in Fig. 7A of Figure 7, light emitted from the display surface IMG is converted into linearly polarized light by the second reflective polarizer 12, then converted into circularly polarized light by the second quarter-wave plate 22, and enters the half mirror HM. A portion of the light that enters the half mirror HM passes through it and is converted by the first quarter-wave plate 21 into linearly polarized light with the same polarization direction as when it passed through the second reflective polarizer 12, and enters the first reflective polarizer 11. This linearly polarized light is reflected by the polarization selectivity of the first reflective polarizer 11. The light reflected by the first reflective polarizer 11 is converted into circularly polarized light by the first quarter-wave plate 21, enters the half mirror HM, and is reflected there. The light reflected by the half mirror HM becomes circularly polarized in the opposite direction to the light before reflection. For convenience, the light traveling this path will be referred to as "light on path 1" below. In the cross-sectional views of the optical system according to this embodiment shown in FIGS. 1 and 4, only the light of this path 1 is shown in order to clarify the schematic configuration of the optical system.

[0090] On the other hand, as shown in Fig. 7B of Fig. 7, a portion of the light that has been converted into circularly polarized light by the second quarter-wave plate 22 and entered the half mirror HM is reflected and becomes circularly polarized light in the reverse direction, returning to the second quarter-wave plate 22. The circularly polarized light that has returned to the second quarter-wave plate 22 is converted by the second quarter-wave plate 22 into linearly polarized light having a polarization direction perpendicular to the polarization direction when it first passed through the second reflective polarizer 12, and then enters the second reflective polarizer 12. This linearly polarized light is reflected by the polarization selectivity of the second reflective polarizer 12. The light reflected by the second reflective polarizer 12 is converted into circularly polarized light by the second quarter-wave plate 22, and then enters and transmits through the half mirror HM. Hereinafter, for convenience, the light traveling along this path will be referred to as "light on path 2."

[0091] The light of path 1 and the light of path 2 converge at the half mirror HM. The circularly polarized light converged at the half mirror HM is converted by the first quarter-wave plate 21 into linearly polarized light having a polarization direction perpendicular to the polarization direction when the light first passed through the second reflective polarizer 12, and then enters the first reflective polarizer 11. This linearly polarized light passes through the first reflective polarizer 11 due to its polarization selectivity and is guided to the pupil plane EP. Therefore, the optical system according to this embodiment improves the light efficiency of the optical system, and can increase the light efficiency by up to 50%. At the same time, power consumption in the image display element can be reduced.

[0092] On the other hand, conventional optical systems of this type have a low light efficiency of 25% or less, and therefore require increased brightness of the display surface to obtain a bright image at the pupil plane. Here, we briefly explain this conventional optical system. Conventional optical systems generally include, in order from the pupil plane side to the display surface side, a reflective polarizer, a first quarter-wave plate, a lens with refractive power, a half mirror, and a second quarter-wave plate. In this optical system, light emitted from the display surface passes through the second quarter-wave plate, the half mirror, the lens, and the first quarter-wave plate, is reflected by the reflective polarizer, and then re-enters the half mirror. The light incident on the half mirror is reflected by the half mirror, passes through the first quarter-wave plate and the reflective polarizer, and reaches the pupil plane. Because light enters the half mirror twice along this light path, the amount of light ultimately reaching the pupil plane from the display surface is less than 25%. This means that in the conventional optical system, in order to obtain the same level of brightness at the pupil plane as the optical system of this embodiment, it is necessary to increase the brightness of the display surface, which means that the power consumption of the image display element will increase.

[0093] In this regard, the optical system according to this embodiment actively utilizes the light emitted from the display surface IMG and reflected by the half mirror HM as light for path 2, thereby achieving a higher light efficiency than ever before.

[0094] The optical system of this embodiment provides desirable effects by satisfying the following conditional expressions (1) to (25). (1)-1.25 <r3 / r6<-0.75 (2) 0.75 <f2 / f3<1.25 (3)2 <r3 / f<8 (4)-1.00 <r2 / r3<-0.25 (5)-3.00 <r2 / |r8|<-0.02 (6)-0.85 <r2 / f1<-0.25 (7)-8 <r6 / f<-2 (8)4 <f2 / f<15 (9)39<νd1<73 (10)39<νd3<73 (11)-5 <r2 / f<-1 (12)0.4 <r3 / |r7|<6.0 (13) 0.5<|r7| / f<7.0 (14) 13<|r7| / T3<50 (15)3<|r7| / D4<37 (16) 0.5<|r8| / f<105.0 (17) 0.1<|r8 / f4|<11.5 (18)4<|r8| / D4<540 (19) 1.0<(D² / f²)×100<4.5 (20) 1.0<(D3 / f3)×100<4.5 (21)0.25 <f1 / f2<1.00 (22)0.20 <f1 / |f4|<1.05 (23)0.45 <f2 / |f4|<1.65 (24)4 <f3 / f<15 (25)4<|f4| / f<14 however, D2: Thickness of the second lens L2 on the optical axis X D3: Thickness of the third lens L3 on the optical axis X D4: Thickness of the fourth lens L4 on the optical axis X T3: the distance on the optical axis X from the surface of the third lens L3 on the display surface side to the surface of the fourth lens L4 on the pupil plane side νd1: Abbe number of the first lens L1 for the d line νd3: Abbe number for the d line of the third lens L3 f: focal length of the entire optical system f1: focal length of the first lens L1 f2: Focal length of the second lens L2 f3: focal length of the third lens element L3 f4: focal length of the fourth lens L4 r2: paraxial curvature radius of the display surface side surface of the first lens L1 r3: paraxial curvature radius of the pupil-side surface of the second lens L2 r6: paraxial curvature radius of the display surface side surface of the third lens L3 r7: paraxial curvature radius of the pupil-side surface of the fourth lens L4 r8: paraxial curvature radius of the display surface side of the fourth lens L4

[0095] It is not necessary to satisfy all of the above conditional expressions, and by satisfying each conditional expression individually, it is possible to obtain the effects corresponding to each conditional expression.

[0096] Furthermore, the optical system of this embodiment will achieve more desirable effects if it satisfies the following conditional expressions (1a) to (25a). (1a)-1.10 <r3 / r6<-0.85 (2a) 0.87 <f2 / f3<1.12 (3a)3.5 <r3 / f<6.5 (4a)-0.8 <r2 / r3<-0.4 (5a)-2.500 <r2 / |r8|<-0.025 (6a)-0.7 <r2 / f1<-0.4 (7a)-6.5 <r6 / f<-3.5 (8a)6 <f2 / f<12 (9a)47<νd1<64 (10a)47<νd3<64 (11a)-4.0 <r2 / f<-1.7 (12a)0.7 <r3 / |r7|<5.0 (13a)1<|r7| / f<6 (14a)20<|r7| / T3<41 (15a)4.5<|r7| / D4<30.5 (16a) 1.2<|r8| / f<86.0 (17a) 0.15<|r8 / f4|<9.40 (18a)6<|r8| / D4<450 (19a) 1.75<(D2 / f2)×100<3.70 (20a) 1.75<(D3 / f3)×100<3.70 (21a)0.35 <f1 / f2<0.80 (22a)0.35 <f1 / |f4|<0.90 (23a)0.65 <f2 / |f4|<1.35 (24a)6 <f3 / f<12 (25a) 6.5<|f4| / f<11.5 However, the symbols in each conditional expression are the same as those explained in the previous paragraph. Note that for conditional expressions (1a) to (25a), the lower limit or upper limit of the corresponding conditional expression (1) to (25) may be applied.

[0097] In this embodiment, the aspherical shape adopted for the aspherical lens surface is expressed by Equation 1, where Z is the axis in the optical axis direction, H is the height in the direction perpendicular to the optical axis, R is the paraxial radius of curvature, k is the conic coefficient, and An is the n-th order aspherical coefficient.

[0098]

number

[0099] Next, examples of the optical system according to this embodiment will be shown. In each example, f represents the focal length of the entire optical system, Fno represents the F-number, ω represents the half angle of view, ih represents the maximum image height, and TTL represents the total optical length. Here, the total optical length is the distance on the optical axis from the pupil plane to the display surface.

[0100] Additionally, i denotes the surface number counted from the pupil surface side, r denotes the paraxial radius of curvature, d denotes the distance between lens surfaces on the optical axis (surface spacing), Nd denotes the refractive index of the d-line (reference wavelength), and νd denotes the Abbe number for the d-line. Aspherical surfaces are indicated by adding an asterisk (*) after the surface number i.

[0101] In the optical systems of each embodiment, the distance on the optical axis between the pupil plane EP, which is the eyepoint, and the lens surface closest to the pupil plane is called the pupil distance. In evaluating aberrations, there is a one-to-one correspondence between the aberration of a light ray that reaches the pupil plane EP when a light emitting point is provided on the display surface side and the aberration of a light ray that reaches the display surface IMG when a light emitting point is provided on the pupil plane EP side. For this reason, in each embodiment, the aberration of the light ray that reaches the display surface IMG is evaluated.

[0102] Example 1

[0103] The basic lens data is shown in Table 1 below.

[0104] [Table 1]

[0105] The optical system of Example 1 satisfies conditional expressions (1) to (25) as shown in Table 3.

[0106] FIG. 3 shows the spherical aberration (mm), astigmatism (mm), and distortion (%) for the optical system of Example 1. The spherical aberration diagram shows the amount of aberration for each wavelength of the F-line (486 nm), d-line (588 nm), and C-line (656 nm). The astigmatism diagram also shows the amount of aberration for the d-line at the sagittal image plane S (solid line) and the amount of aberration for the d-line at the tangential image plane T (dashed line) (the same applies to FIG. 6). As shown in FIG. 3, each aberration is well corrected.

[0107] Example 2

[0108] The basic lens data is shown in Table 2 below.

[0109] [Table 2]

[0110] The optical system of Example 2 satisfies conditional expressions (1) to (25) as shown in Table 3.

[0111] Fig. 6 shows the spherical aberration (mm), astigmatism (mm), and distortion (%) for the optical system of Example 2. As shown in Fig. 6, each aberration is well corrected.

[0112] Table 3 shows the values ​​of conditional expressions (1) to (25) in the optical systems of Examples 1 and 2.

[0113] [Table 3] [Industrial Applicability]

[0114] When the optical system according to the present invention is applied to an image display device, it is possible to contribute to the miniaturization of the image display device and the improvement of the light quantity efficiency, and also to achieve high performance. [Explanation of symbols]

[0115] EP pupil plane L1 First lens 11 First reflective polarizer 21 First quarter-wave plate L2 Second lens HM Half Mirror L3 Third lens 22 Second quarter-wave plate 12 Second reflective polarizer L4 4th lens IMG display surface

Claims

1. From the pupil plane to the display plane, a first lens having a positive refractive power; a first reflective polarizer; and a second lens having a positive refractive power; Half mirror and a third lens having a positive refractive power; a second reflective polarizer; and A fourth lens is provided, a first quarter-wave plate disposed between the pupil plane and the half mirror; a second quarter-wave plate disposed between the half mirror and the display surface; the first lens has a convex surface on the display surface side in a paraxial direction, An optical system characterized by satisfying the following conditional expressions (1) and (2): (1) -1.25<r3 / r6<-0.75 (2) 0.75<f2 / f3<1.25 however, r3: paraxial radius of curvature of the pupil plane side surface of the second lens, r6: paraxial radius of curvature of the display surface side surface of the third lens, f2: focal length of the second lens, f3: focal length of the third lens, Let's say.

2. 2. The optical system according to claim 1, wherein the following conditional expression (3) is satisfied: (3) 2<r3 / f<8 r3: paraxial radius of curvature of the pupil plane side surface of the second lens, f: focal length of the entire optical system, Let's say.

3. 2. The optical system according to claim 1, wherein the following conditional expression (4) is satisfied: (4) -1.00<r2 / r3<-0.25 however, r2: paraxial radius of curvature of the display surface side surface of the first lens, r3: paraxial radius of curvature of the pupil plane side surface of the second lens, Let's say.

4. 2. The optical system according to claim 1, wherein the following conditional expression (5) is satisfied: (5) -3.00<r2 / |r8|<-0.02 r2: paraxial radius of curvature of the display surface side surface of the first lens, r8: paraxial radius of curvature of the display surface side surface of the fourth lens, Let's say.

5. 2. The optical system according to claim 1, wherein the following conditional expression (6) is satisfied: (6) -0.85<r2 / f1<-0.25 r2: paraxial radius of curvature of the display surface side surface of the first lens, f1: focal length of the first lens, Let's say.

6. 2. The optical system according to claim 1, wherein the following conditional expression (7) is satisfied: (7)-8<r6 / f<-2 r6: paraxial radius of curvature of the display surface side surface of the third lens, f: focal length of the entire optical system, Let's say.

7. 2. The optical system according to claim 1, wherein the following conditional expression (8) is satisfied: (8) 4<f2 / f<15 f2: focal length of the second lens, f: focal length of the entire optical system, Let's say.

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