Fresnel lens and image observation device

Abstract

To provide a Fresnel lens that can prevent flare.SOLUTION: A Fresnel lens that can prevent flare is provided. A plurality of prisms (11) forming the Fresnel lens (10) each have an effective surface (Es) and a non-effective surface (Ns). In each prism (11), the non-effective surface (Ns) has a first non-effective surface (Ns1), and a second non-effective surface (Ns2) closer to an apex (11c) of the prism (11) than the first non-effective surface (Ns1). The first non-effective surface is inclined with respect to an optical axis (Z1).SELECTED DRAWING: Figure 5A

Description

[Technical Field]

[0001] The present invention relates to a Fresnel lens and an image observation device. [Background technology]

[0002] A Fresnel lens is composed of multiple prisms arranged from the optical axis toward the outer periphery of the lens. A Fresnel lens is disclosed in Patent Document 1 listed below. Each prism has a substantially triangular cross section and has an effective surface and an ineffective surface. The effective surface faces the outer periphery of the Fresnel lens and is inclined with respect to the optical axis. The effective surface corresponds to the lens surface of a convex lens having desired optical performance divided into multiple annular regions. The ineffective surface faces the optical axis and is parallel to the optical axis. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 138480 Summary of the Invention [Problem to be solved by the invention]

[0004] In an image observation device that allows viewing of moving images through a Fresnel lens, the light of the image reaches not only the effective surface of the prism but also the ineffective surface. This light may be reflected by the ineffective surface and reach the observer's eye. Light that is reflected by the ineffective surface and reaches the observer's eye causes flare. In other words, light appears to spread around the light-emitting point in the image due to the light reflected by the ineffective surface. [Means for solving the problem]

[0005] One example of a Fresnel lens proposed in this disclosure has a plurality of prisms arranged from the optical axis toward the periphery. Each of the plurality of prisms has an effective surface facing the periphery of the Fresnel lens and tilted with respect to the optical axis, and an ineffective surface facing the optical axis, with the apex of the prism between the effective surface and the ineffective surface. In at least one of the plurality of prisms, the ineffective surface has a first surface and a second surface closer to the apex of the prism than the first surface, and at least the first surface is tilted with respect to the optical axis. This Fresnel lens can suppress flare.

[0006] An example of an image observation device proposed in the present disclosure includes the Fresnel lens and a display element, and this image observation device can suppress flare.

[0007] In this Fresnel lens, the non-effective surface may be bent at the boundary between the first surface and the second surface. Alternatively, the non-effective surface may be curved as a whole, with one part functioning as the first surface and another part functioning as the second surface. In this Fresnel lens, both the first surface and the second surface may be inclined with respect to the optical axis. In this case, the angles of the first surface and the second surface with respect to the optical axis may be the same. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing a cross section of a Fresnel lens. [Figure 2A] FIG. 2 is an enlarged cross-sectional view of a Fresnel lens proposed in the present disclosure. [Figure 2B] 10A and 10B are diagrams for explaining the mechanism by which flare occurs due to an ineffective surface. [Figure 3] 10 is a diagram for explaining the relationship between the distance from the optical axis to the prism and the angle of the non-effective surface. FIG. [Figure 4] 10 is a diagram for explaining the relationship between the distance from the optical axis to the prism and the angle of the non-effective surface. FIG. [Figure 5A]FIG. 2 is an enlarged cross-sectional view of the apex of a prism having a second non-effective surface. [Figure 5B] FIG. 2 is an enlarged cross-sectional view showing the apex of a prism that does not have a second ineffective surface. [Figure 6] FIG. 1 is a diagram showing a head-mounted display as an example of an image observation device. DETAILED DESCRIPTION OF THE INVENTION

[0009] The Fresnel lens proposed in this disclosure will be described below. In this specification, a Fresnel lens 10 shown in FIG. 1 and the like will be described as an example of the Fresnel lens proposed in this disclosure.

[0010] The Fresnel lens 10 is mounted on an image observation device having a display element. For example, the Fresnel lens 10 is used as an eyepiece optical system in a head-mounted display 100 (see FIG. 6). The head-mounted display 100 has a display element ID arranged in front of the eye of a user (observer). The display element ID is, for example, a liquid crystal display device, an organic EL (electroluminescence) display device, or a micro LED display device. When the Fresnel lens 10 is mounted on the head-mounted display 100, two Fresnel lenses 10 are arranged side by side on the left and right. In addition to the Fresnel lens 10, the head-mounted display 100 may also use another Fresnel lens located in the direction of the optical axis Z1 relative to the Fresnel lens 10. The image observation device in which the Fresnel lens 10 is mounted is not limited to the head-mounted display 100, and may be applied to, for example, an electronic viewfinder of a camera.

[0011] As shown in Fig. 1, multiple prisms 11 are arranged on a plane intersecting with optical axis Z1 (for example, a plane perpendicular to optical axis Z1, a concave surface, a convex surface, etc.). In Fig. 1, Fresnel lens 10 is arranged so that prisms 11 protrude toward display element ID. Unlike Fig. 1, Fresnel lens 10 may be arranged so that prisms 11 protrude toward the viewer's pupil.

[0012] When the Fresnel lens 10 is viewed in the direction of the optical axis Z1, each prism 11 has an annular shape centered on the optical axis Z1. When the Fresnel lens 10 is viewed in the direction of the optical axis Z1, the multiple prisms 11 are arranged radially (in a direction perpendicular to the optical axis Z1). Each prism 11 has an effective surface Es and a non-effective surface Ns. Each prism 11 also has an apex 11c between the effective surface Es and the non-effective surface Ns. The effective surface Es faces the outer periphery of the Fresnel lens 10 and is inclined with respect to the optical axis Z1. That is, the effective surface Es is inclined so that the distance from the optical axis Z1 to the normal De of the effective surface Es increases toward the front side of the Fresnel lens 10 (the display element ID side in FIG. 1). The effective surface Es corresponds to a lens surface of a convex lens having the optical performance required for an image observation device, divided into multiple annular regions. The divided lens surfaces are arranged along a plane that intersects with the optical axis Z1 (for example, a plane perpendicular to the optical axis Z1, a concave surface, or a convex surface) to form the Fresnel lens 10. The non-effective surface Ns is a surface that faces the optical axis Z1. In other words, the normal Dn of the non-effective surface Ns extends toward the optical axis Z1.

[0013] FIG. 2A is a diagram showing a cross section of a Fresnel lens 10 proposed in this disclosure. As shown in this figure, in each prism 11, the non-effective surface Ns has a slope that is inclined with respect to the optical axis Z1. The non-effective surface Ns is inclined so that the distance from the optical axis Z1 to the non-effective surface Ns increases toward the apex 11c. (The distance between the non-effective surface Ns and the optical axis Z1 is the distance in a direction perpendicular to the optical axis Z1.) This inclination of the non-effective surface Ns can suppress the occurrence of flare. Preferably, the non-effective surface Ns of all prisms 11 has a slope, but the non-effective surface Ns of only some of the prisms 11 may have a slope.

[0014] FIG. 2B is a diagram illustrating the mechanism by which flare due to the non-effective surface occurs. This diagram shows a Fresnel lens 90, which, unlike Fresnel lens 10, has a non-effective surface Ns parallel to the optical axis Z1. When an observer looks at a light-emitting point Lp of a display element ID, light L1 emitted from the light-emitting point Lp is refracted by the effective surface Es of a prism 91A and reaches the observer's pupil. In addition to light L1, light is also emitted from the light-emitting point Lp and reaches the non-effective surface Ns. In the example shown, light L2 is reflected by the non-effective surface Ns of a prism 91B located outside of prism 91A and reaches the observer's pupil. Light L3 enters prism 91C from the effective surface Es of prism 91C located inside prism 91A, reflects off the non-effective surface Ns, and reaches the observer's pupil. Thus, when an observer looks at light-emitting point Lp of display element ID, not only is light L1 refracted by the effective surface Es and reaches the observer's pupil, but also light L2 and L3 is generated that is reflected by the non-effective surface Ns and reaches the observer's pupil. This light L2 and L3 is the cause of flare. Below, the light that heads toward the non-effective surface Ns, as represented by light L2 and L3, is referred to as "non-effective light," and the light that is incident on the effective surface Es, as represented by light L1, is referred to as "effective light."

[0015] As shown in FIG. 2A, in the Fresnel lens 10, when an observer looks at the light-emitting point Lp of the display element ID, effective light L1 emitted from the light-emitting point Lp is refracted by the effective surface Es of the prism 11A and reaches the observer's pupil. The light-emitting point Lp also generates the aforementioned non-effective light L2 and L3. (The direction in which the non-effective light L2 and L3 shown in FIG. 2A emerge from the light-emitting point Lp is the same as the direction in which the non-effective light L2 and L3 shown in FIG. 2B emerge from the light-emitting point Lp.) However, in the Fresnel lens 10, the non-effective surface Ns is inclined with respect to the optical axis Z1, and an angle θn is maintained between the non-effective surface Ns and the direction of the optical axis Z1. Therefore, although the non-effective light L2 is totally reflected by the non-effective surface Ns of the prism 11B located outside the prism 11A, it misses the position of the observer's pupil. Furthermore, because the non-effective surface Ns is inclined with respect to the optical axis Z1, the non-effective light L3 also misses the position of the observer's pupil. In other words, the angle θn is designed in each prism 11 so that the non-useful light L2 and L3 deviates from the position of the observer's pupil, thereby making it possible to suppress the occurrence of flare.

[0016] The angle θn between the non-effective surface Ns and the optical axis Z1 is larger than the draft angle. A "draft angle" is a slope formed on the surface of a mold to facilitate separation of the molded product (Fresnel lens 10) from the mold when molding the Fresnel lens 10 using the mold, and this slope is inclined with respect to the separation direction between the mold and the molded product. The draft angle is, for example, 1 to 5 degrees. In contrast, the angle θn between the non-effective surface Ns and the direction of the optical axis Z1 is, for example, larger than 5 degrees. The angle θn is preferably larger than 15 degrees. The angle θn is more preferably larger than 20 degrees. The angle θn is even more preferably larger than 30 degrees. Moreover, the angle θn is smaller than 40 degrees. More preferably, the angle θn is smaller than 35 degrees.

[0017] The angle θn of the non-effective surface Ns may change depending on the distance from the optical axis Z1 to the prism 11. This makes it possible to more effectively prevent the non-effective light L2 and L3 from reaching the viewer's pupil. Depending on the angle of the effective surface Es (the optical characteristics of the Fresnel lens 10), for example, the angle θn may increase as the distance from the optical axis Z1 to the prism 11 increases. Conversely, depending on the angle of the effective surface Es, the angle θn may decrease as the distance from the optical axis Z1 to the prism 11 increases.

[0018] 3 and 4 are diagrams for explaining the relationship between the distance from the optical axis Z1 to the prism and the angle θn.

[0019] FIG. 3A shows a prism 91 close to the optical axis Z1 (a prism whose non-effective surface Ns is not inclined), and FIG. 3B shows a prism 91 located at a relatively large distance from the optical axis Z1. As described above, the multiple effective surfaces Es constituting a Fresnel lens correspond to the divided lens surfaces of a convex lens. Therefore, as shown in FIG. 3A, the inclination of the effective surface Es is relatively gentle at positions close to the optical axis Z1, and as shown in FIG. 3B, the inclination of the effective surface Es is relatively steep at positions far from the optical axis Z1. Therefore, the angle of incidence θe1·θe2 of the non-effective light L3 to the non-effective surface Ns is smaller in the prism 91 far from the optical axis Z1 (FIG. 3B) than in the prism 91 close to the optical axis Z1 (FIG. 3A). (That is, the incident angle θe2 shown in FIG. 3(b) is smaller than the incident angle θe1 ​​shown in FIG. 3(a).) Therefore, at positions close to the optical axis Z1, even if the angle θn between the non-effective surface Ns and the direction of the optical axis Z1 is relatively small, it is possible to prevent the non-effective light L3 from reaching the non-effective surface Ns. In this way, since the incident angles θe1·θe2 of the non-effective light L3 with respect to the non-effective surface Ns vary depending on the distance from the optical axis Z1, it is desirable to also change the angle θn of the non-effective surface Ns depending on the distance from the optical axis Z1.

[0020] FIG. 4 shows non-useful light L21, L22, L23, and L24 emitted from light-emitting points Lp1 and Lp2. The non-useful light L21 and L22 emitted from light-emitting point Lp1 close to the optical axis Z1 reach the Fresnel lens 10. In contrast, a portion of the non-useful light (light L24) emitted from light-emitting point Lp2 far from the optical axis Z1 passes through the outside of the Fresnel lens 10. Therefore, the closer to the outer periphery of the Fresnel lens 10, the less likely the non-useful light L2 (see FIG. 2A) is to reach the observer's pupil. Therefore, even if the angle θn between the non-useful surface Ns and the direction of the optical axis Z1 is small at the outer periphery of the Fresnel lens 10, it is possible to prevent the non-useful light L2 (see FIG. 2A) from reaching the observer's pupil. Therefore, at the outer periphery of the Fresnel lens 10, the angle θn of the non-useful surface Ns may be relatively small.

[0021] Fig. 5A is an enlarged cross-sectional view of the apex 11c of the prism 11. As shown in Fig. 5A, the prism 11 has a curved surface Cs at the apex 11c. Forming such a curved surface Cs at the apex 11c makes it easier to mold the Fresnel lens 10.

[0022] The non-effective surface Ns of the prism 11 includes a first non-effective surface Ns1 that is relatively far from the apex 11c and a second non-effective surface Ns2 that is relatively close to the apex 11c. The first non-effective surface Ns1 and the second non-effective surface Ns2 are aligned in the direction of the optical axis Z1. The angle θn described above is maintained between the first non-effective surface Ns1 and the direction of the optical axis Z1. As described with reference to FIGS. 3 and 4, the angle θn of the first non-effective surface Ns1 may change depending on the distance from the optical axis Z1. The angle between the second non-effective surface Ns2 and the direction of the optical axis Z1 is smaller than the angle θn between the first non-effective surface Ns1 and the direction of the optical axis Z1.

[0023] As shown in FIG. 5A, a portion of the light emitted from the light-emitting point Lp1 (light L4 in FIG. 5A) is refracted by the curved surface Cs and enters the prism 11. The second non-effective surface Ns2 prevents this light L4 from reaching the viewer's pupil. FIG. 5B shows a prism 11 that does not have such a second non-effective surface Ns2. In this figure, the position of the light-emitting point Lp1 and the direction of the light L4 emitted from the light-emitting point Lp1 are the same as those in FIG. 5A. As shown in FIG. 5B, if the non-effective surface Ns does not have the second non-effective surface Ns2 and is entirely tilted with respect to the optical axis Z1, light L4 is generated that is refracted by the curved surface Cs and travels along the non-effective surface Ns. If this light L4 reaches the viewer's pupil, it will cause flare. In contrast, in the prism 11 shown in FIG. 5A, the second non-effective surface Ns2 is formed on the non-effective surface Ns, so that the second non-effective surface Ns2 blocks the light L4 and prevents the light L4 from reaching the viewer's pupil.

[0024] In addition, in the prism 11 shown in Figure 5A, light L5 emitted from the light emitting point Lp2 and reflected by the outer surface of the second non-effective surface Ns2 can be blocked by the first non-effective surface Ns1, preventing the light L5 from reaching the observer's pupil.

[0025] The second non-effective surface Ns2 is, for example, parallel to the optical axis Z1. The second non-effective surface Ns2 may be inclined with respect to the optical axis Z1. In this case, the angle between the second non-effective surface Ns2 and the optical axis Z1 may be the same as the above-mentioned draft slope or may be larger than the draft slope.

[0026] As shown in FIG. 5A, the distance H4 (the distance along the optical axis Z1) from the apex 11c of the prism 11 (the uppermost end of the prism 11) to the boundary M between the second non-effective surface Ns2 and the first non-effective surface Ns1 is greater than the diameter of the curved surface Cs. By designing the distance H4 in this manner, in other words, by designing the size of the second non-effective surface Ns2 in this manner, the second non-effective surface Ns2 can function effectively with respect to light refracted by the curved surface Cs. In this description, the "diameter of the curved surface Cs" refers to the diameter of an imaginary circle R (see FIG. 5A) that is tangent to the inner surface of the curved surface Cs. Circle R is the circle with the largest diameter among the imaginary circles that are tangent to the inner surface of the curved surface Cs. The diameter of the curved surface Cs is, for example, 3 μm or more and 20 μm or less. Therefore, when the diameter of the curved surface Cs is greater than 3 μm, the distance H4 from the apex 11c to the boundary M between the second ineffective surface Ns2 and the first ineffective surface Ns1 is greater than 3 μm, for example.

[0027] Furthermore, the distance H4 from the apex 11c of the prism 11 to the boundary M between the second non-effective surface Ns2 and the first non-effective surface Ns1 is smaller than half the height H5 (see FIG. 2A) of the prism 11. By designing the distance H4 in this way, the size of the first non-effective surface Ns1 can be ensured. As a result, non-effective light L2 and L3 (see FIG. 3) can be prevented from reaching the observer's pupil. In this description, the height H5 of the prism 11 is the distance (distance in the direction along the optical axis Z1) from the boundary between the non-effective surface Ns and the effective surface Es of two adjacent prisms 11 to the apex 11c.

[0028] As described above, each of the multiple prisms 11 has an effective surface Es that faces outward from the Fresnel lens 10 and is inclined with respect to the optical axis Z1, and an ineffective surface Ns that faces the optical axis Z1, with the apex 11c of the prism 11 being located between the effective surface Es and the ineffective surface Ns. In each prism 11, the ineffective surface Ns has a first ineffective surface Ns1 and a second ineffective surface Ns2 that is closer to the apex 11c of the prism 11 than the first ineffective surface Ns1. The first ineffective surface is inclined with respect to the optical axis Z1. This structure makes it possible to prevent ineffective light L2 and L3 from reaching the viewer's pupil and causing flare.

[0029] The Fresnel lens proposed in this disclosure is not limited to the example of the Fresnel lens 10 described above.

[0030] For example, the angle between the second non-effective surface Ns2 and the direction of the optical axis Z1 may be the same as the angle between the first non-effective surface Ns1 and the direction of the optical axis Z1. In other words, the non-effective surface Ns may be a flat plane.

[0031] In yet another example, the non-effective surface Ns may not be curved at the boundary M between the first non-effective surface Ns1 and the second non-effective surface Ns2. In other words, the non-effective surface Ns may be curved as a whole, and the first non-effective surface Ns1 and the second non-effective surface Ns2 may be smoothly connected.

[0032] In still another example, the non-effective surface Ns may have, in addition to the first non-effective surface Ns1 and the second non-effective surface Ns2, a third non-effective surface and a fourth non-effective surface that are angled differently from the first non-effective surface Ns1 and the second non-effective surface Ns2.

Claims

1. A Fresnel lens having a plurality of prisms arranged from the optical axis toward the periphery, each of the plurality of prisms has an effective surface facing the outer periphery of the Fresnel lens and inclined with respect to the optical axis, and an ineffective surface facing the optical axis, and has an apex of the prism between the effective surface and the ineffective surface; In at least one of the plurality of prisms, the non-effective surface has a first surface and a second surface closer to the apex of the prism than the first surface, at least the first surface is inclined with respect to the optical axis, and an angle between the second surface and the optical axis is smaller than an angle between the first surface and the optical axis; the at least one prism has a curved surface at the apex; The distance from the apex to the boundary between the second surface and the first surface is greater than the diameter of the curved surface. Fresnel lens.

2. The diameter of the curved surface is 3 μm or more and 20 μm or less, the distance from the apex to the boundary between the second surface and the first surface is greater than 3 μm; 2. The Fresnel lens according to claim 1.

3. each of the plurality of prisms has the first surface; The angle of the first surface with respect to the optical axis varies depending on the distance of the first surface from the optical axis.

2. The Fresnel lens according to claim 1.

4. The angle between the first surface and the optical axis direction is larger than the draft angle of the Fresnel lens.

2. The Fresnel lens according to claim 1.

5. The angle between the first surface and the direction of the optical axis is greater than 5 degrees and is equal to or smaller than 40 degrees.

2. The Fresnel lens according to claim 1.

6. 10. An image viewing device comprising the Fresnel lens of claim 1 and a display element.

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