Optical systems and head-mounted display devices

Abstract

Some embodiments of the present application disclose an optical system and a head-mounted display device. The optical system includes a third lens, a second lens, and a first lens installed sequentially along the propagation direction of incident light, and the optical system has two adjacently installed Fresnel surfaces, at least one of which is a curved base. The embodiments of the present application provide a short-focus and lightweight optical structure design solution. Among these, the use of a Fresnel surface with a curved base can improve imaging quality to some extent compared to a flat base and can reduce the TTL value and the total weight of the optical system.

Description

Technology Field

[0001] This application claims priority to a Chinese patent application filed with the Chinese Intellectual Property Office on November 30, 2021, with application number 202111449752.2 and title of invention 'Optical system and head-mounted display device', and incorporates the entire contents of the same into this application by reference.

[0002] This application relates to the field of optical imaging technology, and more specifically to optical systems and head-mounted display devices. Background Technology

[0003] Recently, Augmented Reality (AR) and Virtual Reality (VR) technologies are being applied to smart wearable devices and are developing rapidly. The core components of both Augmented Reality and Virtual Reality technologies are display optical systems. The quality of the display effects of the display optical system directly determines the quality of the smart wearable device. Currently, smart wearable devices must still meet the requirements for miniaturization and lightweighting, and this is placing corresponding demands on display optical systems as well.

[0004] VR devices can be cited as an example of existing related technologies. Most conventional VR devices utilize display optical systems formed by combining a single lens and a display screen. However, due to optical path imaging requirements, the lens must be positioned far from the display screen, which increases the size of the VR device and hinders product miniaturization. Furthermore, conventional optical structures generally carry a relatively large weight, which may result in a poor user experience when worn. Additionally, while there are solutions currently available for VR devices that use folded optical paths, this approach enables miniaturization and weight reduction; however, it suffers from high production costs, low light efficiency (<25%), and ghosting issues.

[0005] Therefore, it is very necessary to proceed with a new design for existing display optical systems. The problem to be solved

[0006] The purpose of this application is to provide a new technical solution for optical systems and head-mounted display devices. means of solving the problem

[0007] According to one aspect of the present application, an optical system is provided. The optical system comprises a third lens, a second lens, and a first lens installed sequentially along the direction of propagation of incident light, and the optical system has two adjacently installed Fresnel surfaces, at least one of which is a curved base.

[0008] Optionally, the refractive power of the first lens and the second lens are both positive, and the refractive power of the third lens is negative.

[0009] Optionally, two adjacent surfaces of the first lens and the second lens are Fresnel surfaces, and at least one of the Fresnel surfaces is a curved base.

[0010] Optionally, the first lens comprises a first surface and a second surface, and the second lens comprises a third surface and a fourth surface, wherein the second surface and the third surface are installed adjacent to each other and are both Fresnel surfaces, at least one of which is a curved base, and the first surface and the fourth surface are both aspherical.

[0011] Optionally, a first gap (T1) is installed between the first lens and the second lens, and the first gap (T1) is set to 0.2mm ≤ T1 ≤ 1mm, and a second gap (T2) is installed between the second lens and the third lens, and the second gap (T2) is set to 1mm ≤ T2 ≤ 3mm, and the first lens, the second lens, and the third lens are located on the same optical axis.

[0012] Optionally, the third lens comprises a fifth surface and a sixth surface, and at least one of the fifth surface and the sixth surface is aspherical.

[0013] Optionally, the first lens and the second lens are both made of the same material, specifically COP material, and the third lens is made of OKP material or EP material.

[0014] Optionally, the effective focal length (f1) of the first lens is 20mm ≤ f1 ≤ 40mm, the effective focal length (f2) of the second lens is 20mm ≤ f2 ≤ 40mm, and the effective focal length (f3) of the third lens is -75mm ≤ f3 ≤ -35mm.

[0015] Optionally, the effective focal length (f) of the optical system satisfies 15mm ≤ f ≤ 25mm.

[0016] According to another aspect of the present application, a head-mounted display device is provided.

[0017] The head-mounted display device described above includes any of the aforementioned optical systems. Effects of the invention

[0018] Embodiments of the present application provide a design solution for a short-focus and lightweight optical structure. The solution comprises a lens group composed of multiple (e.g., three) lenses and designs two adjacent Fresnel planes in the optical path structure, wherein at least one Fresnel plane is designed with a curved base. Since the lens can be made thinner and lighter by designing with a curved base, this design can reduce the total weight of the optical system and the total optical length (TTL) of the entire optical system, while simultaneously improving imaging quality. The formed optical system can be applied to head-mounted display devices (e.g., VR devices), and thus helps realize the trend of miniaturization and lightweighting of head-mounted display devices.

[0019] Other features and advantages of the present application will become apparent from the following detailed description of exemplary embodiments of the present application with reference to the accompanying drawings. Brief explanation of the drawing

[0020] In order to more clearly explain the embodiments of the present application or the technical solutions of the prior art, the accompanying drawings necessary for describing the embodiments or prior art are briefly introduced below. The drawings in the following description are merely a part of the drawings of the present application, and it is obvious to those skilled in the art that other drawings can be obtained from the provided drawings without creative effort. FIG. 1 is a schematic diagram of the structure of an optical system according to an embodiment of the present application. FIG. 2 is a schematic diagram of the imaging principle of an optical system according to an embodiment of the present application. FIG. 3 is a spot diagram of an optical system according to Embodiment 1 of the present application. FIG. 4 is a diagram of the surface curvature and distortion of an optical system according to Embodiment 1 of the present application. FIG. 5 is a diagram of the chromatic dispersion of an optical system according to Embodiment 1 of the present application. FIG. 6 is a spot diagram of an optical system according to Example 2 of the present application. FIG. 7 is a diagram of the field plane curvature and distortion of an optical system according to Example 2 of the present application. FIG. 8 is a diagram of the chromatic dispersion of an optical system according to Example 2 of the present application. FIG. 9 is a spot diagram of an optical system according to Example 3 of the present application. FIG. 10 is a diagram of the field plane curvature and distortion of an optical system according to Example 3 of the present application. FIG. 11 is a diagram of the chromatic dispersion of an optical system according to Example 3 of the present application. Specific details for implementing the invention

[0021] Hereinafter, technical solutions of the embodiments of the present application will be described with reference to the drawings of the embodiments of the present application; however, the embodiments described above are merely some of the embodiments of the present application and are not all of them. Based on the embodiments of the present application, all other embodiments obtained by a person skilled in the art without creative effort fall within the scope of protection of the present application.

[0022] The following description of at least one exemplary embodiment is by nature merely illustrative and is by no means intended to limit its application or use.

[0023] Techniques, methods, and devices known to a person of ordinary skill in the relevant technical field may not be described in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification.

[0024] In all examples described and discussed herein, all specific values ​​are to be interpreted merely as exemplary and not as restrictive. Accordingly, other cases of exemplary embodiments may have different values.

[0025] It should be noted that similar symbols and characters indicate similar items in subsequent drawings, and that once an item is defined in one drawing, no further discussion is necessary in subsequent drawings.

[0026] Hereinafter, an optical system and a head-mounted display device according to an embodiment of the present application will be described in detail with reference to FIGS. 1 to 11.

[0027] According to one aspect of an embodiment of the present application, an optical system is provided. The optical system is a short-focus, lightweight, and high-resolution direct-transmission optical system suitable for application to electronic devices such as head-mounted displays (HMDs), such as VR devices (VR glasses or VR helmets, etc.). Thus, it has good application prospects.

[0028] An optical system according to one embodiment of the present application includes a third lens (3), a second lens (2), and a first lens (1) installed sequentially along the propagation direction of incident light, as shown in FIGS. 1 and 2, and the optical system has two adjacently installed Fresnel surfaces, at least one of which is a curved base.

[0029] The optical system according to the embodiment of the present application is designed as a direct-transmission optical path structure, and since the optical path structure is relatively simple, it can be easily manufactured.

[0030] It should be noted that the above optical system may further include a display screen (display) (4).

[0031] The above display screen (4) can be used to emit a light beam in an optical path structure. That is, it provides incident light to an optical system.

[0032] That is, the optical system solution according to the embodiment of the present application designs and applies a lens group including, for example, three optical lenses. As shown in FIGS. 1 and 2, the lens group is positioned at the light-emitting side of the display screen (4), specifically in the direction of propagation of incident light emitted by the display screen (4), and can be used to project incident light onto a person's eye (5) to perform imaging, thereby realizing the imaging function of the optical system.

[0033] Because the spot size of the optical system provided by the embodiment of the present application is relatively small (the spot size value is about 50 μm), the imaging quality of the entire optical system is relatively good.

[0034] An optical system according to an embodiment of the present application has two adjacently installed Fresnel planes, and at least one Fresnel plane is a curved base. Compared to a conventional solution that employs only a Fresnel plane with a flat base, the use of a Fresnel plane with a curved base increases the degree of freedom in designing the optical path structure, thereby providing a basis for further improving imaging quality. This helps to form a high-resolution and short-focus optical structure design solution.

[0035] In the optical system according to the embodiment of the present application, since a curved base is used for at least one Fresnel surface, the imaging quality can be improved by approximately 25% (35μm vs. 48μm) compared to a conventional flat base Fresnel surface, the total optical track length (TTL) can be reduced by approximately 1mm, and the total weight of the entire optical system can be reduced by approximately 8.2%. As a result, the formed optical system has the characteristics of lightweight and miniaturization, while simultaneously having excellent imaging quality.

[0036] Embodiments of the present application propose a design solution for a single-focus and lightweight optical structure. A lens group is formed with multiple lenses (e.g., three optical lenses), and two adjacent Fresnel planes are designed within the optical path structure, wherein at least one Fresnel plane is a curved base. Since the use of a Fresnel plane with a curved base can make the lens thinner and lighter, this design can reduce the total weight of the optical system and the total optical length (TTL) of the entire optical system, while simultaneously improving imaging quality.

[0037] In summary, the solution according to the embodiment of the present application resolves the problems associated with conventional combination solutions of a monolithic lens and a display screen, where the lens is positioned relatively far from the display screen, resulting in a larger VR device size, which is disadvantageous for product miniaturization, and low light efficiency. At the same time, it can also improve upon the drawbacks of using a folded optical path, resulting in relatively low processing difficulty and production costs, and the direct-transmission optical structure is also simpler than that of a folded optical path.

[0038] As illustrated in FIGS. 1 and 2, the optical system of an embodiment of the present application further comprises, for example, a display screen (4), said display screen (4) having, for example, a high PPI (e.g., 1.4 inches, pixel size 24 μm). In this case, neither the conventional single lens (1P) structure nor the dual lens (2P) structure is sufficient to resolve this type of display screen. The reason is as follows.

[0039] A single lens (1P) can only be optimized for two surface plane degrees of freedom, so the focusing function is limited, aberrations or chromatic differences cannot be corrected, and the pixel size (i.e., imaging spot size) that can be seen in the entire field of view is about 80μm to 100μm, and more importantly, the purpose of a single focal point cannot be achieved.

[0040] Although the dual lens (2P) can achieve a single focal point by increasing the degree of freedom for optimizing the lens surface shape, there is still a limit to the resolution, so the pixel size (i.e., imaging spot size) that can be resolved in the entire field of view is about 60μm to 80μm.

[0041] The optical lens group structure employed in the embodiment of the present application can further improve resolution and correct color difference to some extent, forms a direct transmission type short-focus optical path structure, and the imaging spot size of the entire optical system is relatively small, and the spot size value is about 50 μm, so the imaging effect is good.

[0042] In some examples of the present application, as shown in FIGS. 1 and 2, the refractive power of the first lens (1) and the second lens (2) are both positive, and the refractive power of the third lens (3) is negative.

[0043] That is, in the entire optical system, the first lens (1) and the second lens (2) are, for example, both positive lenses, and the third lens (3) is, for example, a negative lens, so two positive lenses are combined with one negative lens to form a direct transmission type single focal length, high-resolution optical path structure.

[0044] It can be seen that the first lens (1) and the second lens (2) are combined with their respective Fresnel surfaces to provide a relatively large focal power, and at the same time, color difference removal can be performed by using a negative lens, i.e., the third lens, in consideration of the effect of color difference, so that a low chromatic dispersion effect of the optical path structure can be realized by adding a negative lens.

[0045] In some examples of the present application, as shown in FIGS. 1 and 2, two adjacent surfaces of the first lens (1) and the second lens (2) are Fresnel surfaces, and at least one of the Fresnel surfaces is a curved base.

[0046] It can be understood that in the overall optical path structure, at least one of two adjacent Fresnel planes is a curved base.

[0047] More preferably, both adjacent Fresnel surfaces are designed with a curved base. As a result, the first lens (1) and the second lens (2) can be made lighter and thinner, and the weight and TTL value of the optical system can be further reduced and the imaging quality can be improved.

[0048] Of course, one Fresnel surface may serve as a curved base and the other as a flat base, and the two may be installed adjacent to each other. Those skilled in the art may flexibly select this according to specific circumstances, and this application is not specifically limited herein.

[0049] In a specific embodiment of the present application, as shown in FIG. 1, the first lens (1) includes a first surface (11) and a second surface (12), and the second lens (2) includes a third surface (21) and a fourth surface (22). The second surface (12) and the third surface (21) are installed adjacent to each other and are both Fresnel surfaces, and at least one of the Fresnel surfaces is a curved base. The first surface (11) and the fourth surface (22) are both non-spherical surfaces.

[0050] Further, the first surface (11) and the fourth surface (22) are both convex surfaces.

[0051] Optionally, as illustrated in FIG. 1, in the optical system provided in the embodiment of the present application, the first surface (11) of the first lens (1) faces directly with the human eye (5), and the first surface (11) is coated with an anti-reflective film and a hardened film, and the second surface (12) of the first lens (1) and the third surface (21) of the second lens (2) are installed adjacent to each other, and the second surface (12) is coated with an anti-reflective film.

[0052] In the case of the first lens (1) above, the surface design of the first surface (11) and the second surface (12) is different. Since the first surface (11) is installed as an aspherical (convex) surface and the second surface (12) is installed as a Fresnel surface with a curved base, the first lens (1) forms a combination of two types of surface designs, an aspherical surface and a Fresnel surface with a curved base, thereby obtaining the effects of a short focal point and high resolution.

[0053] Of course, the second surface (12) may be a Fresnel surface of a flat base.

[0054] In an embodiment of the present application, at least one anti-reflective coating (AR) is designed to be coated on the first surface (11) and the second surface (12) of the first lens (1), respectively. This is because, after coating both sides of the first lens (1) with an anti-reflective coating, reflected light is reduced by the anti-reflective coating, thereby increasing the light transmittance on each surface of the positive lens.

[0055] For example, in addition to the anti-reflective film (AR) being coated on the first surface (11) of the first lens (1), a hardened film may also be coated on the first surface (11). This is because the first surface (11) of the first lens (1) faces outward and needs to be protected from damage caused by scratches, collisions, etc., and the lifespan of the first lens (1) can be improved by coating it with a hardened film. By coating the hardened film on the first surface (11), that is, by applying a hardening treatment to the first surface (11), the hardness, strength, etc. of the first surface (11) can be improved. This is useful for extending the lifespan of the entire optical system.

[0056] Of course, the solution of the present application is not limited to coating a hardened film only on the first surface (11) of the first lens (1), and a hardened film may also be coated on the second surface (12) of the first lens (1), and a person skilled in the art may flexibly adjust it according to specific requirements, and this is not specifically limited in the present application.

[0057] In addition, in an embodiment of the present application, the first lens (1) has the following parameters.

[0058] For example, the absolute value of the radius (R1) of the first surface (11) of the first lens (1) satisfies 45mm ≤ Abs (R1) ≤ 65mm, the absolute value of the curved base radius (R0) of the second surface (12) of the first lens (1) satisfies 40mm ≤ Abs (R0) ≤ 200mm, the absolute value of the radius (R2) of the second surface (12) satisfies 20mm ≤ Abs (R2) ≤ 40mm, and the absolute value of the conic coefficient (K1) of the first surface (11) and the second surface (12) of the first lens (1) satisfies Abs (K1) ≤ 10.

[0059] In an embodiment of the present application, after optimizing the surface shape of the first lens (1), considering the difficulty and cost of processing, it is preferable to set the value of the conic constant, i.e., K1, of the first lens (1) at [-10, 10], and to set the radius value of the Fresnel surface (i.e., the second surface (12)) of the curved base of the first lens (1) to be greater than 23 mm.

[0060] In some embodiments of the present application, the surface combination of the second lens (2) and the first lens (1) may be the same, and as shown in FIGS. 1 and 2, the third surface (21) of the second lens (2) is a Fresnel surface (e.g., a Fresnel surface of a curved base), and the fourth surface (22) of the second lens (2) is installed as an aspherical surface (e.g., a convex surface).

[0061] It is necessary to explain that when the Fresnel surface (i.e., the second surface (12)) of the first lens (1) is a curved base, the Fresnel surface (i.e., the third surface (21)) of the second lens (2) may be a curved base or a flat base.

[0062] Optionally, an anti-reflective coating (AR) is also applied to the third surface (21) and the fourth surface (22). By reducing reflected light through the anti-reflective coating, the light transmittance on both sides of the second lens (2) can be increased.

[0063] In addition, in an embodiment of the present application, the second lens (2) has the following parameters.

[0064] In some embodiments of the present application, the absolute value of the radius (R0') of the curved base of the third surface (21) satisfies 60mm ≤ Abs(R0') ≤ 170mm, the absolute value of the radius (R3) of the third surface (21) satisfies 20mm ≤ Abs(R3) ≤ 40mm, the absolute value of the radius (R4) of the fourth surface (22) satisfies 60mm ≤ Abs(R4) ≤ 80mm, and the absolute value of the conic coefficient (K2) of the third surface (21) and the fourth surface (22) of the second lens (2) satisfies Abs(K2) ≤ 10.

[0065] The surface combination of the second lens (2) is the same as the surface combination of the first lens (1).

[0066] After optimizing the surface shape of the second lens (2), considering the difficulty and cost of processing, it is preferable to set the value of the conic coefficient, i.e., K2, of the second lens (2) at [-10, 10], and to set the radius of the Fresnel surface (third surface (21)) of the curved base of the second lens (2) to be greater than 23 mm.

[0067] It is necessary to explain that both the first lens (1) and the second lens (2) have Fresnel surfaces. Considering the processing of the lens surface shape, it is necessary to set the surface shape parameters within a certain range. Otherwise, there is a risk of low processing accuracy or blade breakage (this is because processing the sawtooth shape is difficult, and the smaller the acute angle of the sawtooth shape, the more difficult the processing angle and operation become). Therefore, the value of the conic coefficient K is set to the range [-10, 10], and the R value of the Fresnel surface is set to 23mm or more.

[0068] For example, the first lens (1) and the second lens (2) both use a combination method of an aspherical and a curved base Fresnel surface, and furthermore, by selecting and adjusting the material according to different refractive indices and Abbe numbers, low chromatic dispersion and short focal length of the optical path structure can be realized.

[0069] In some examples of the present application, as shown in FIGS. 1 and 2, the third lens (3) includes a fifth surface (31) and a sixth surface (32), and at least one of the fifth surface (31) and the sixth surface (32) is aspherical.

[0070] That is, in the design of the solution of the present application, the third lens is designed as a negative lens that is thin in the center and thick at the periphery, and has the ability to emit light. The third lens (3) can be used to eliminate color difference in the entire light path structure.

[0071] The third lens may be, for example, a double concave lens (i.e., both sides are concave) or a flat concave lens (i.e., one side is concave and the other side is flat).

[0072] More preferably, the fifth surface (31) is configured as a flat surface, and the sixth surface (32) is configured as a concave surface. That is, in the optical path structure, the surface adjacent to the fourth surface (22) is flat, and the surface adjacent to the display screen (4) is a concave surface.

[0073] Optionally, both the fifth surface (31) and the sixth surface (32) are coated with an anti-reflective coating (AR).

[0074] After the two surfaces of the third lens (3) are each coated with an anti-reflective film, the reflected light can be reduced by the anti-reflective film, thereby increasing the light transmittance on the two surfaces of the third lens (3).

[0075] The lens group and the combination of the lens planes of the present application effectively realize high resolution and low chromatic dispersion characteristics.

[0076] In addition, in an embodiment of the present application, the third lens (3) has the following parameters.

[0077] The absolute value of the radius (R5) of the fifth surface (31) of the third lens (3) satisfies Abs(R5)≥200mm, the absolute value of the radius (R6) of the sixth surface (32) of the third lens (3) satisfies 30mm≤Abs(R6)≤60mm, and the absolute value of the conic coefficient (K3) of the fifth surface (31) and the sixth surface (32) of the third lens (3) satisfies Abs(K3)≤10.

[0078] In a specific example of the present application, as illustrated in FIG. 1, the optical system may include a display screen (4), a first lens (1), a second lens (2), and a third lens (3), and the display screen (4) is used as a display light source. The first lens (1) and the second lens (2) are optical elements formed by Fresnel surfaces with an aspherical and curved base, and the third lens (3) on the side closer to the display screen (4) is installed as a negative lens. An anti-reflection coating (AR) treatment is performed on the surface of each lens, and a cured film coating + anti-reflection coating treatment is performed on the first surface (11) of the first lens (1). Based on this, the process proceeds as illustrated in FIG. 2.

[0079] The light beam emitted by the display screen (4) passes through the sixth surface (32) of the third lens (3), which is coated with an anti-reflective film, and enters the interior of the third lens (3). The propagating light beam that passes through the third lens (3) is then emitted and enters the second lens (2). Both sides of the second lens (2) are also coated with an anti-reflective film, and the light beam passes through the second lens (2), is focused, and then enters the first lens (1). The first lens (1) is a focusing positive lens, and the light beam is transmitted through the first lens (1) and then enters the human eye (5) to create an image. There is no optical path folding in the entire optical system, and the surface of each lens is coated with an anti-reflective film, so the light propagation efficiency is high.

[0080] In an embodiment of the present application, a first gap (T1) is installed between the first lens (1) and the second lens (2), and a second gap (T2) is installed between the third lens (3) and the second lens (2), and the second gap (T2) is larger than the first gap (T1).

[0081] In some examples of the present application, a first gap (T1) is installed between the first lens (1) and the second lens (2), and the first gap (T1) is set to 0.2 mm ≤ T1 ≤ 1 mm, and a second gap (T2) is installed between the second lens (2) and the third lens (3), and the second gap (T2) is set to 1 mm ≤ T2 ≤ 3 mm, and the first lens (1), the second lens (2), and the third lens (3) are located on the same optical axis.

[0082] In the solution provided by the embodiment of the present application, a narrow air gap is installed between the first lens (1) and the second lens (2), and at the same time, a narrow air gap is also installed between the second lens (2) and the third lens (3). The solution of the present application contributes to the miniaturization of the entire optical system by optimizing the design of the air gap between each lens.

[0083] In addition, if a display screen (4) is installed in the optical system, after reasonably installing the spacing between each lens, it is also necessary to consider the size of the gap between the third lens (3) and the display screen (4).

[0084] For example, the third gap (T3) is installed between the third lens (3) and the display screen (4).

[0085] Optionally, the third interval (T3) is set to 5mm ≤ T3 ≤ 15mm.

[0086] In some examples of the present application, the first lens (1) and the second lens (2) are made of the same material and are both COP material, and the third lens is OKP material or EP material.

[0087] COP, OKP, and EP materials are all transparent resin materials, and since they are lightweight, using these materials to manufacture lenses can reduce the weight of the lens group and achieve lightweighting.

[0088] In terms of material selection, each lens is designed to optimize its structure by combining materials with high refractive indices and low Abbe numbers, taking into account single focal length and chromatic aberration.

[0089] The materials of the first lens (1), the second lens (2), and the third lens (3) can be reasonably selected by a person skilled in the art according to actual demand and are not limited to the materials described above.

[0090] In some examples of the present application, the center thickness value (h1) of the first lens (1) is 2mm ≤ h1 ≤ 4mm, the center thickness value (h2) of the second lens (2) is 3mm ≤ h2 ≤ 5mm, and the center thickness value (h3) of the third lens (3) is 2mm ≤ h3 ≤ 4mm.

[0091] Since the thickness of each lens is not excessive, it is also advantageous for reducing the weight of the entire optical path structure.

[0092] In some examples of the present application, the effective focal length (f1) of the first lens (1) is 20mm ≤ f1 ≤ 40mm, and The effective focal length (f2) of the second lens (2) is 20mm ≤ f2 ≤ 40mm, and The effective focal length (f3) of the third lens (3) is -75mm ≤ f3 ≤ -35mm.

[0093] In the present application, it is necessary to explain that the effective focal length (f1) of the first lens (1) and the effective focal length (f2) of the second lens (2) are both greater than the effective focal length (f) of the optical system. The sum of the effective focal length (f1) of the first lens (1) and the effective focal length (f2) of the second lens (2) is greater than the absolute value of the effective focal length (f) of the optical system.

[0094] In some examples of the present application, the effective focal length (f) of the optical system satisfies 15mm ≤ f ≤ 25mm.

[0095] The present application provides a single-focus optical system. It is a direct-transmission optical system capable of high-quality imaging and free from optical path folding in the entire optical system.

[0096] The following is an example of application of the solution according to an embodiment of the present application. (1) A 90-degree viewing angle was achieved by matching the 1.4-inch display screen (4). (2) The distortion is less than 34.6%, and the curvature of the upper surface is less than 0.6mm. (3) The color difference is less than 158 μm. The virtual image distance is 1500 mm. (4) The spot size of the entire field of view is smaller than 35 μm, and clear imaging is achieved in the visible light band (450 nm to 630 nm). (5) The effective focal length of the optical system is 18.4 mm. Example 1

[0097] Example 1 provides an optical system and adopts the structural parameters of the optical system according to Table 1.

[0098] Table 1 lists the optical surface numbers (Surface) sequentially numbered from the human eye (5) (aperture) to the display screen (1), the curvature (C) of each optical surface on the optical axis, the distance (T) between each optical surface and the next optical surface on the optical axis from the human eye (5) (aperture) to the display screen (4), and the even-order aspherical coefficients α2, α3, and α4, respectively.

[0099] Among them, the aspherical coefficient can satisfy the following equation. (1)

[0100] In Equation (1), z is a coordinate along the optical axis direction, Y is a radial coordinate with a lens length unit as the unit, C is the curvature (1 / R), K is the conic coefficient, and α iis the coefficient of each higher-order term, and 2i is the order of the aspherical high-order term. In the solution design of the present application, considering the gentleness of the curvature of the image plane, the spherical coefficient of the higher-order term reaches up to the 6th order. 0101] The performance of the optical system of Example 1 of the present application is reflected by the following parameters.

[0102] As shown in Fig. 3, the spot size is maximum at a maximum field of view of 1.0F, and the maximum value is smaller than 35μm.

[0103] As shown in Fig. 4, the RGB wavelengths of the T&S direction surface curvature are all smaller than 0.5 mm, and the maximum distortion at the maximum field of view is smaller than 34.5%.

[0104] As shown in Fig. 5, the maximum RGB chromatic dispersion is the maximum viewing position, the total RGB is 450nm to 610nm, and the LCA is 158nm. Example 2

[0105] Example 2 provides an optical system that employs the structural parameters of the optical system according to Table 2.

[0106] The surface parameters for achieving optimized imaging quality are as follows. The second surface (12) of the first lens (1) is a Fresnel surface of a curved base, and the radius value is 40 mm. If it is smaller than 40 mm, the imaging quality is degraded.

[0107] When the radius value of the second surface (12) of the first lens (1) is 40mm, the radius of the curved base of the corresponding third surface (21) (Fresnel surface of the curved base) of the second lens (2) is 170mm.

[0108] The quality of the optical system performance of Example 2 of the present application is reflected by the following parameters.

[0109] As shown in Fig. 6, the spot size is maximum at a maximum field of view of 1.0F, and the maximum value is smaller than 30μm.

[0110] As shown in Fig. 7, the RGB wavelengths of the T&S direction surface curvature are all smaller than 0.5 mm, and the maximum distortion at the maximum field of view is smaller than 31%.

[0111] As shown in Fig. 8, the maximum RGB chromatic dispersion is the maximum viewing position, the total RGB is 450nm to 630nm, and the LCA is 160nm. Example 3

[0112] Example 3 provides an optical system that employs the structural parameters of the optical system according to Table 3.

[0113] The surface parameters for achieving optimized imaging quality are as follows.

[0114] The second surface (12) of the first lens (1) is a Fresnel surface with a curved base, and its radius value is 200 mm. If it is larger than 200 mm, the imaging quality is degraded.

[0115] When the radius value of the second surface (12) of the first lens (1) is 40mm, the radius of the curved base of the corresponding third surface (21) (Fresnel surface of the curved base) of the second lens (2) is 62mm.

[0116] The quality of the optical system performance of Example 3 of the present application is reflected by the following parameters.

[0117] As shown in Fig. 9, the spot size is maximum at a maximum field of view of 0.9F, and the maximum value is smaller than 31μm.

[0118] As shown in Fig. 10, the RGB wavelengths of the T&S direction surface curvature are all smaller than 0.5 mm, and the maximum distortion at the maximum field of view is smaller than 38%.

[0119] As shown in Fig. 11, the maximum RGB chromatic dispersion is the maximum viewing position, the total RGB is 450nm to 630nm, and the LCA is 140nm.

[0120] An embodiment of the present application provides a single-focus, high-definition direct-transmission optical system that does not include a folded optical path. (1) Ultra-short focus is achieved by combining three lenses (or more). (2) By using a combination of "aspherical + curved base Fresnel surface" surfaces, a narrow air gap is realized, and furthermore, a short TTL is realized. (3) Low chromatic dispersion of the optical path is achieved by selecting and combining different refractive indices and Abbe numbers. (4) High resolution and low chromatic dispersion are achieved through the combination of "double convex lens + concave lens".

[0121] According to another aspect of the present application, a head-mounted display device is provided.

[0122] The head-mounted display device described above includes any of the aforementioned optical systems.

[0123] The head-mounted display device mentioned above is, for example, a VR device.

[0124] The embodiments described above have focused on explaining the differences between various embodiments; however, better embodiments can be formed by combining the different optimization features among the various embodiments, provided that the different optimization features among them do not contradict one another. For the sake of brevity, repetitive descriptions are omitted.

[0125] Although some specific embodiments of this application have been described in detail by way of example, those skilled in the art should understand that such embodiments are for illustrative purposes only and are not intended to limit the scope of this application. Those skilled in the art will understand that modifications to such embodiments may be made without departing from the scope and spirit of this application. The scope of the application is limited by the appended claims. Explanation of the symbols 1. First lens, 2. Second lens, 3. Third lens, 4. Display screen, 5. Human eye, 11. Page 1, 12. Page 2, 21. Page 3, 22. Page 4, 31. Page 5, 32. Page 6.

Claims

Claim 1 In an optical system, a third lens (3), a second lens (2), and a first lens (1) are sequentially installed along the propagation direction of incident light, wherein the first lens (1) includes a first surface (11) and a second surface (12) installed as a Fresnel surface, and the second lens (2) includes a third surface (21) and a fourth surface (22), wherein the third surface (21) is installed as a Fresnel surface adjacent to the second surface (12), and at least one of the Fresnel surfaces of the second surface (12) and the third surface (21) is a curved base, and the refractive power of the first lens (1) and the second lens (2) having adjacent Fresnel surfaces is positive, and the third lens (3) includes a fifth surface (31) and a sixth surface (32), wherein at least one of the fifth surface (31) and the sixth surface (32) is an aspherical surface, and the radius of the fifth surface (31) An optical system characterized in that the absolute value of R5 satisfies Abs(R5)≥200mm, the absolute value of the radius R6 of the 6th surface (32) satisfies 30mm≤Abs(R6)≤60mm, and the absolute value of the conic coefficient K3 of the 5th surface (31) and the 6th surface (32) satisfies Abs(K3)≤10. Claim 2 An optical system according to claim 1, wherein the third lens (3) is installed adjacent to the second lens (2) having the Fresnel plane on the incident light side, and the refractive power of the third lens (3) is negative. Claim 3 An optical system according to claim 1, wherein the absolute value of the radius R1 of the first surface (11) satisfies 45mm ≤ Abs(R1) ≤ 65mm, the absolute value of the curved base radius R0 of the second surface (12) satisfies 40mm ≤ Abs(R0) ≤ 200mm, the absolute value of the radius R2 of the second surface (12) satisfies 20mm ≤ Abs(R2) ≤ 40mm, the absolute value of the conic coefficient K1 of the first surface (11) and the second surface (12) satisfies Abs(K1) ≤ 10, and the first surface (11) is an aspherical surface. Claim 4 An optical system characterized in that, in paragraph 3, the absolute value of the radius R0' of the curved base of the third surface (21) satisfies 60mm ≤ Abs(R0') ≤ 170mm, the absolute value of the radius R3 of the third surface (21) satisfies 20mm ≤ Abs(R3) ≤ 40mm, the absolute value of the radius R4 of the fourth surface (22) satisfies 60mm ≤ Abs(R4) ≤ 80mm, the absolute value of the conic coefficient K2 of the third surface (21) and the fourth surface (22) satisfies Abs(K2) ≤ 10, and the fourth surface (22) is an aspherical surface. Claim 5 An optical system according to claim 1, wherein a first gap (T1) is installed between the first lens (1) and the second lens (2), and the first gap (T1) is set to 0.2mm ≤ T1 ≤ 1mm, a second gap (T2) is installed between the second lens (2) and the third lens (3), and the second gap (T2) is set to 1mm ≤ T2 ≤ 3mm, and the first lens (1), the second lens (2), and the third lens (3) are located on the same optical axis. Claim 6 An optical system characterized in that, in claim 1, the first lens (1) and the second lens (2) are both made of the same material and are both COP material, and the third lens (3) is made of OKP material or EP material. Claim 7 An optical system according to claim 1, characterized in that the effective focal length (f1) of the first lens (1) is 20mm ≤ f1 ≤ 40mm, the effective focal length (f2) of the second lens (2) is 20mm ≤ f2 ≤ 40mm, and the effective focal length (f3) of the third lens (3) is -75mm ≤ f3 ≤ -35mm. Claim 8 An optical system according to claim 1, characterized in that the effective focal length (f) of the optical system satisfies 15mm ≤ f ≤ 25mm. Claim 9 A head-mounted display device characterized by including an optical system described in any one of claims 1 to 8. Claim 10 delete

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