Optical systems and head-mounted display devices

Abstract

Some embodiments of the present application disclose an optical system and a head-mounted display device. In the lens group of the optical system, the lens group is composed of a third lens, a second lens, and a first lens installed sequentially along the propagation direction of the incident light, and the lens group has two adjacent Fresnel planes, and the imaging spot size of the optical system is smaller than 50 μm. The embodiments of the present application provide a design solution for a short-focus, high-resolution direct-transmission optical path structure, which can be applied to a head-mounted display device and has excellent imaging effects.

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 202111446819.7 and title of invention 'Optical system and head-mounted display device', the entire contents of which are incorporated by reference into this application.

[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, which places 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 is positioned far from the display screen, which increases the size of the VR device, hindering product miniaturization and potentially resulting in a poor user experience. Additionally, there are solutions currently available for VR devices that use a folded optical path; while this solution enables miniaturization and lightweight design, it presents issues such as high manufacturing costs, low light efficiency (<25%), and ghosting.

[0005] Therefore, it is very necessary to proceed with a new design for the existing display optical system. 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 lens group consisting of a third lens, a second lens, and a first lens installed sequentially along the propagation direction of incident light, and the lens group has two adjacently installed Fresnel planes, and the imaging spot size of the optical system is smaller than 50 μm.

[0008] Optionally, a first gap (T1) is installed between the first lens and the second lens, and A second gap (T2) is installed between the third lens and the second lens, and the second gap (T2) is larger than the first gap (T1). The first lens, the second lens, and the third lens are located on the same optical axis.

[0009] Optionally, the refractive power of the first lens and the second lens are both positive.

[0010] Optionally, the two adjacent surfaces of the first lens and the second lens are Fresnel surfaces.

[0011] Optionally, the first lens includes a first surface and a second surface, and the second lens includes a third surface and a fourth surface, and The second surface and the third surface are installed adjacent to each other and are both Fresnel surfaces, and The first surface and the fourth surface are both aspherical.

[0012] Optionally, the third lens includes a fifth surface and a sixth surface, and The fifth surface is installed adjacent to the fourth surface, and at least one of the fifth surface and the sixth surface is a non-spherical surface.

[0013] Optionally, the refractive power of the third lens is negative.

[0014] 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.

[0015] 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.

[0016] Optionally, the effective focal length (f) of the lens group satisfies 15mm ≤ f ≤ 25mm.

[0017] According to another aspect of the present application, a head-mounted display device is provided.

[0018] The head-mounted display device described above includes any of the aforementioned optical systems. Effects of the invention

[0019] In the solution provided by the embodiment of the present application, by using three lenses to form a lens group (3P structure), designing two adjacent Fresnel planes in the optical path structure, and combining lenses close to the light incident side, low chromatic dispersion and a short focal length of the optical path can be achieved, and the formed optical system can have high light efficiency and achieve high-resolution imaging. The embodiment of the present application proposes a design solution for a short focal length, high-resolution direct-transmission optical path structure, and since the formed optical system can be applied to a head-mounted display device (e.g., a VR device), it helps to achieve miniaturization and weight reduction of the head-mounted display device.

[0020] 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

[0021] In order to more clearly explain the embodiments of the present application or the technical details of the prior art, the attached 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 that a person skilled in the art can obtain other drawings 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 curvature and distortion of the optical system according to an embodiment of the present application. FIG. 5 is a diagram of the chromatic dispersion of an optical system according to an embodiment of the present application. FIG. 6 is a schematic diagram of the imaging principle of an optical system according to Embodiment 2 of the present application. FIG. 7 is a spot diagram of an optical system according to Embodiment 2 of the present application. FIG. 8 is a top plane curvature and distortion diagram of an optical system according to Example 2 of the present application. FIG. 9 is a chromatic dispersion diagram of an optical system according to Example 2 of the present application. Specific details for implementing the invention

[0022] Hereinafter, technical details regarding the embodiments of the present application will be described with reference to the drawings of the embodiments of the present application; however, it should be understood that the above-described embodiments are merely some of the embodiments of the present application and do not represent all embodiments. All other embodiments that a person skilled in the art could obtain without creative effort based on the embodiments of the present application also fall within the scope of protection of the present application.

[0023] In the following description of at least one exemplary embodiment, the following description is essentially merely illustrative and is by no means intended to limit the present application and its application or use.

[0024] 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.

[0025] 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.

[0026] It should be noted that similar symbols and characters indicate similar items in subsequent drawings, so once an item is defined in one drawing, no further discussion is necessary in subsequent drawings.

[0027] 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 and FIGS. 2.

[0028] According to one aspect of the embodiments of the present application, an optical system is provided. The optical system is a single-focus, high-resolution direct-transmission optical system and is 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.

[0029] An optical system according to one embodiment of the present application includes a lens group as shown in FIGS. 1 and 2, wherein the lens group consists of a third lens (4), a second lens (3), and a first lens (1) installed sequentially, and the lens group has two adjacent Fresnel planes, and the imaging spot size of the optical system is smaller than 50 μm.

[0030] The optical system according to the embodiment of the present application is a direct-transmission type optical path structure design, and the optical path structure design is relatively simple.

[0031] It should be noted that the above optical system may further include a display screen (1). The display screen (1) may be used to emit a light beam of an optical path structure, i.e., the incident light described above.

[0032] That is, in the optical system solution according to the embodiment of the present application, a lens group having three lenses is designed, and the lens group is installed, for example, on the light-emitting side of a display screen (1), specifically in the direction of propagation of light emitted by the display screen (1), and is used to induce incident light on a person's eye (5) for imaging.

[0033] The imaging spot size of the optical system of the embodiment of the present application is smaller than 50 μm, and since the spot size value is relatively small, the imaging quality of the entire optical system is better (in fact, the smaller the spot size value, the better the imaging effect of the optical system).

[0034] As a further step, for example, the spot size is smaller than 2 display pixels. Ultimately, this contributes to implementing magnified display of display optical systems with a pixel per inch (PPI) of 1000 or more.

[0035] In the solution according to the embodiment of the present application, a lens group (3P structure) is formed using three lenses, and two adjacent Fresnel planes are designed in the optical path structure, and a lens close to the light incident side is combined, thereby enabling low chromatic dispersion and short focal length of the optical path, and the formed optical system can achieve high-resolution imaging with high optical efficiency.

[0036] The embodiments of the present application propose a design solution for a short-focus, high-resolution direct-transmission optical path structure, and the formed optical system can be applied, for example, to a head-mounted display device (e.g., a VR device), and help to achieve miniaturization and weight reduction of the head-mounted display device.

[0037] In addition, the optical system solution according to the embodiment of the present application is lower than existing folded optical path solutions in terms of processing difficulty and manufacturing cost.

[0038] 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 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, defects associated with using a folded optical path can be improved, processing difficulty and production costs are relatively low, and the direct-transmission optical structure is simpler than that of a folded optical path.

[0039] As illustrated in FIGS. 1 and 2, the optical system of an embodiment of the present application further comprises, for example, a display screen (1), said display screen (1) having, for example, a high PPI (e.g., 1.4 inches, pixel size 24 μm). Based on this, neither a conventional single-piece lens (1P) structure nor a double-piece lens (2P) structure is sufficient to resolve this type of display screen. The reasons are as follows.

[0040] A single-piece lens (1P) has only optimization for the plane-shaped degrees of freedom of two faces, so its focusing ability is limited, it cannot correct aberrations or chromatic differences, and the pixel size (i.e., spot size) that can be seen in the entire field of view is about 80μm to 100μm, and more importantly, it cannot achieve the purpose of a single focal point.

[0041] The dual-piece lens (2P) can achieve a single focal point by increasing the degree of freedom when optimizing the surface shape of the lens surface, but there is still a limit to the resolution, so the size of the pixels that can be resolved in the entire field of view (i.e., the imaging spot size) is about 60μm to 80μm, so the imaging effect is not good.

[0042] However, the solution provided by the embodiment of the present application employs a 3-piece lens (3P) structure to further improve pixel resolution and correct color difference to some extent. The imaging spot size of the optical system is smaller than 50 μm. The lens group composed of the 3-piece lens (3P) structure has a short focal length. Two adjacent Fresnel planes are designed considering the air gap of the optical path structure and the short focal length. In addition, color difference removal is performed using the third lens (4) in consideration of the effect of color difference.

[0043] In some examples of the present application, as shown in FIGS. 1 and 2, a first gap (T1) is installed between the first lens (2) and the second lens (3), and the second lens (4) is installed between the third lens (4) and the second lens (3), and the second gap (T2) is larger than the first gap (T1), and the first lens (2), the second lens (3), and the third lens (4) are located on the same optical axis.

[0044] In an embodiment of the present application, a narrow air gap is installed between the first lens (2) and the second lens (3), and at the same time, a narrow air gap is also installed between the second lens (3) and the third lens (4). Optimizing the design of the air gap between each lens contributes to the miniaturization of the entire optical system.

[0045] Optionally, the first gap (T1) is set to 0.2mm ≤ T1 ≤ 1mm, the second gap (T2) is set to 1mm ≤ T2 ≤ 3mm, and the second gap (T2) is larger than the first gap (T1).

[0046] As can be seen from the aforementioned gap size, a narrow air gap of appropriate size is formed between the first lens (2) and the second lens (3), and between the second lens (3) and the third lens (4), respectively, so that the optical system formed can better realize ultra-short focus and high resolution.

[0047] In addition, after reasonably installing the spacing of each lens in the lens group, it is also necessary to consider the size of the gap between the third lens (4) and the display screen (1).

[0048] For example, a third gap (T3) is installed between the third lens (4) and the display screen (1).

[0049] Optionally, the third interval (T3) is set to 5mm ≤ T3 ≤ 15mm.

[0050] In some examples of the present application, as shown in FIGS. 1 and 2, the refractive power of the first lens (2) and the second lens (3) are both positive.

[0051] That is, the first lens (2) and the second lens (3) are both designed as positive lenses, for example.

[0052] In some embodiments of the present application, as shown in FIGS. 1 and FIGS. 2, two adjacent surfaces of the first lens (2) and the second lens (3) are Fresnel surfaces.

[0053] By designing two adjacent Fresnel planes in the light path structure, it helps to reduce stray light.

[0054] In some examples of the present application, as shown in FIGS. 1 and 2, the first lens (2) includes a first surface (21) and a second surface (22), and the second lens (3) includes a third surface (31) and a fourth surface (32), wherein the second surface (22) and the third surface (31) are installed adjacent to each other and are both Fresnel surfaces, and the first surface (21) and the fourth surface (32) are both aspherical surfaces.

[0055] As shown in FIGS. 1 and 2, the optical system according to the embodiment of the present application has the first surface (21) of the first lens (2) facing directly to the human eye (5) and is installed as an aspherical surface (e.g., a convex surface), and the second surface (22) of the first lens (2) is installed as a Fresnel surface, so that the first lens (2) (positive lens) forms a combination of two surface shapes, an aspherical surface and a Fresnel surface.

[0056] Optionally, an anti-reflective coating (AR) is coated on the first surface (21) and the second surface (22) of the first lens (2), respectively.

[0057] After coating each of the first lens (2) with an anti-reflection film, the reflected light is reduced by the anti-reflection film, thereby increasing the transmittance of light from both sides of the first lens (2).

[0058] Optionally, in addition to the coated anti-reflective film, a hardened film may also be coated on the first surface (21) of the first lens (2).

[0059] The reason is that the first surface (21) of the first lens (2) faces outward and needs to be protected from damage caused by scratches, collisions, etc., and the lifespan of the first lens (2) can be improved by coating a hardening film. By coating a hardening film on the first surface (21), that is, by applying a hardening treatment to the first surface (21), the hardness, strength, etc. of the first surface (21) can be improved. This is useful for extending the lifespan of the entire optical system.

[0060] Of course, the embodiments of the present application are not limited to coating a hardened film on the first surface (21) of the first lens (2), and a hardened film may also be coated on the second surface (22) of the first lens (2), and those skilled in the art may flexibly adjust according to specific requirements, and this is not specifically limited in the present application.

[0061] In addition, in an embodiment of the present application, the first lens (2) has the following parameters.

[0062] For example, the absolute value of the radius (R1) of the first surface (21) of the first lens (2) satisfies 35mm ≤ R1 ≤ 65mm, the absolute value of the radius (R2) of the second surface (22) of the first lens (2) satisfies 20mm ≤ R2 ≤ 40mm, and the absolute value of the conic coefficient (K1) of the first surface (21) and the second surface (22) of the first lens (2) satisfies K1 ≤ 20.

[0063] Among them, the surface design of the first surface (21) and the second surface (22) is different.

[0064] Specifically, the first surface (21) facing outward is designed as an aspherical surface (e.g., a convex surface), and the second surface (22) is designed as a Fresnel surface; by applying the first lens (2), formed by a combination of the Fresnel surface and the aspherical surface, to the optical path structure, the effects of a short focal length and high resolution can be obtained.

[0065] In an embodiment of the present application, after optimizing the surface shape of the first lens (2), considering the difficulty and cost of processing, it is more preferable to design the value of the conic constant, i.e., K1, of the first lens (2) at [-10, 10], and for the radius of the Fresnel surface of the first lens (2) to be R>23mm.

[0066] In some embodiments of the present application, the planar combination shape of the second lens (3) and the first lens (2) may be the same, and a narrow air gap is maintained between them.

[0067] For example, the third surface (31) of the second lens (3) is a Fresnel surface, and the fourth surface (32) of the second lens (3) is installed as an aspherical surface (e.g., a convex surface).

[0068] The second lens (3) (positive lens) is positioned between the first lens (2) and the third lens (4), and the second lens (3) is positioned closer to the first lens (2), and can form a positive lens group in combination with the first lens (2), and both can provide greater refractive power for the entire optical path structure.

[0069] Optionally, the anti-reflective film is also coated on the third surface (31) and the fourth surface (32).

[0070] The reflected light is reduced by the above anti-reflection film, and the transmittance of light is increased on both sides of the second lens (3).

[0071] In addition, in an embodiment of the present application, the second lens (3) has the following parameters.

[0072] In some examples of the present application, the absolute value of the radius (R3) of the third surface (31) of the second lens (3) satisfies 20mm ≤ R3 ≤ 40mm, the absolute value of the radius (R4) of the fourth surface (32) of the second lens (3) satisfies 30mm ≤ R4 ≤ 60mm, and the absolute value of the conic coefficient (K2) of the third surface (31) and the fourth surface (32) of the second lens (3) satisfies K2 ≤ 20.

[0073] In the embodiment of the present application, after optimizing the surface shape of the second lens (3), considering the difficulty and cost of processing, the conic coefficient, i.e., the K2 value of the second lens (3), is designed to be [-10, 10], and it is more preferable that the radius of the Fresnel surface of the second lens (3) is greater than 23 mm. This is basically the same as the first lens (2).

[0074] It is necessary to explain that both the first lens (2) and the second lens (3) 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 the cutting tool breaking (this is because processing the sawtooth shape is difficult, and the smaller the acute angle of the sawtooth shape, the more difficult the processing of the bevel angle and the operation become). Therefore, the conic coefficient K value is set within the range [-10, 10], and the R value of the Fresnel surface of each lens is set to 23mm or more.

[0075] In the solution of the embodiment of the present application, the first lens (2) and the second lens (3), that is, the two positive lenses, both use a combination of an aspherical surface (e.g., a convex surface) and a Fresnel surface, and by selecting and combining materials according to different refractive indices and Abbe numbers, low chromatic dispersion and short focal length of the optical path structure are realized.

[0076] In some examples of the present application, as shown in FIGS. 1 and 2, the third lens (4) includes a fifth surface (41) and a sixth surface (42), the fifth surface (41) is adjacent to the fourth surface (32), and at least one of the fifth surface (41) and the sixth surface (42) is aspherical.

[0077] For example, the refractive power of the third lens (4) is negative.

[0078] The third lens (4) is a negative lens that is thin in the center and thick at the periphery, and has the ability to emit light. The third lens (4) can be used to remove color difference in the entire light path structure.

[0079] The third lens (4) may be, for example, a double-sided 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).

[0080] More preferably, the fifth surface (41) is installed as a flat surface, and the sixth surface (42) is installed as a concave surface. That is, in the optical path structure, the surface adjacent to the fourth surface (32) is flat, and the surface adjacent to the display screen (1) is concave.

[0081] In addition, in an embodiment of the present application, the third lens (4) has the following parameters.

[0082] For example, the absolute value of the radius R5 of the fifth surface (41) of the third lens (4) satisfies R5≥30mm, the absolute value of the radius (R6) of the sixth surface (42) of the third lens (4) satisfies 30mm≤R6≤60mm, and the absolute value of the conic coefficient K3 of the fifth surface (41) and the sixth surface (42) of the third lens (4) satisfies K3≤10.

[0083] Optionally, the fifth surface (41) and the sixth surface (42) are both coated with an anti-reflective coating (AR).

[0084] After coating each of the two sides of the third lens (4) with an anti-reflection film, the reflected light can be reduced by the anti-reflection film, thereby increasing the light transmittance on both sides of the third lens (4).

[0085] In a specific example of the present application, as illustrated in FIG. 1, the optical system comprises a display screen (1) and a lens group consisting of the first lens (2), the second lens (3), and the third lens (4). The display screen (1) can emit light rays as a display light source, which can be incident on the lens group as incident light. Both the first lens (2) and the second lens (3) are positive lenses in the form of a combination of an aspherical surface and a Fresnel surface, and the third lens (4) is a negative lens. Each surface of the three lenses is treated with an anti-reflective coating, and furthermore, the first surface (21) of the first lens (2) is treated with a hardened film coating and an anti-reflective coating. Based on this, the process proceeds as illustrated in FIG. 2.

[0086] The imaging beam (light) emitted by the display screen (1) passes through the sixth surface (42) of the third lens (4), which is coated with an anti-reflective film, and enters the interior of the third lens (4). The propagating light that has passed through the third lens (4) is diverged and enters the second lens (3). Since the second lens (3) is also a positive lens with anti-reflective films coated on both sides, the light is focused through the second lens (3) and enters the first lens (2). The first lens (2) is still a focusing positive lens, and the light propagates through the first lens (2) and enters the human eye (5) to create an image. In the entire optical system, there is no optical path folding, and since an anti-reflective film is coated on the surface of each lens, the light transmission efficiency is high.

[0087] In some examples of the present application, the first lens (2) and the second lens (3) are made of the same material, both being COP material, and the third lens (4) is made of OKP material or EP material.

[0088] COP, OKP, and EP materials are all light-transmitting resin materials, so by manufacturing lenses using these materials, the weight of the lens group can be reduced and lightweighting can be achieved.

[0089] In material selection, each lens is designed to optimize its composition by combining materials with high refractive indices and high or low Abbe numbers, taking into account single focal length and chromatic aberration.

[0090] In addition, the materials of the first lens (2), the second lens (3), and the third lens (4) can be reasonably selected by a person skilled in the art according to actual needs and are not limited to the types of materials described above.

[0091] In some examples of the present application, the center thickness value (h1) of the first lens (2) is 2mm ≤ h1 ≤ 4mm, the center thickness value (h2) of the second lens (3) is 3mm ≤ h2 ≤ 5mm, and the center thickness value (h3) of the third lens (4) is 2mm ≤ h3 ≤ 4mm.

[0092] In some examples of the present application, the effective focal length (f1) of the first lens (2) is 20mm ≤ f1 ≤ 40mm, the effective focal length (f2) of the second lens (3) is 20mm ≤ f2 ≤ 40mm, and the effective focal length (f3) of the third lens (4) is -75mm ≤ f3 ≤ -35mm.

[0093] The effective focal length (f1) of the first lens (2) and the effective focal length (f2) of the second lens (3) are both greater than the effective focal length (f) of the lens group.

[0094] The sum of the effective focal length (f1) of the first lens (2) and the effective focal length (f2) of the second lens (3) is greater than the absolute value of the effective focal length (f3) of the third lens (4).

[0095] The effective focal length (f) of the above lens group satisfies the relationship 15mm ≤ f ≤ 25mm.

[0096] The present application provides a single-focus optical system. It is a direct-transmission optical system capable of high-resolution imaging, with no optical path folding in the entire optical system.

[0097] 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 to match the 1.4-inch display screen (1). (2) The distortion is less than 34.6%, and the curvature of the upper surface is less than 0.5mm. (3) The color difference is less than 178 µm. The virtual image distance is 1500 mm. (4) Spot sizes of 0~1.0F are all smaller than 2 display pixels and provide clear imaging in the visible light band (450nm~630nm). (5) The effective focal length of the system is 19.2 mm. Example 1

[0098] Example 1 provides an optical system and adopts the structural parameters of the optical system according to Table 1.

[0099] 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. The above-described aspheric coefficient can satisfy the following equation. (1) ; 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 i is the coefficient of each higher-order term, and 2i is the order of the aspherical high-order term.

[0100] In the solution design of the present application, the gentleness of the curvature of the upper surface is taken into account, and the higher-order spherical coefficient reaches up to the 6th order. The performance of the optical system of Example 1 of the present application is reflected by the following parameters.

[0101] 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 44μm, which is a relatively small value and helps improve imaging quality.

[0102] 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 34.6%. 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 178nm. Example 2

[0103] Example 2 provides an optical system and adopts the structural parameters of the optical system according to Table 2, and the optical system is as illustrated in FIG. 6. Table 2 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.

[0104] The performance of the optical system of Example 2 of the present application is reflected by the following parameters.

[0105] As shown in Fig. 7, the spot size is maximum at a maximum field of view of 0.9F, and the maximum value is smaller than 48μm, which is a relatively small value and helps improve imaging quality.

[0106] As shown in Fig. 8, 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 35%.

[0107] As shown in Fig. 9, the maximum chromatic dispersion of RGB is at the maximum viewing position, and the total RGB is 450nm to 610nm, and the LCA is 170nm.

[0108] An embodiment of the present application provides a single-focus, high-resolution direct-transmission optical system that does not include a folded optical path. (1) Two positive lenses and one negative lens are combined to form a 3-piece lens group (3P structure), and the surface shape of the two positive lenses is optimized to design an aspherical + Fresnel surface, and the two Fresnel surfaces are installed adjacent to each other to obtain ultra-short focal length. (2) In the above lens group, high resolution and low chromatic dispersion effects can also be obtained by using a combination of two positive lenses and one negative lens. (3) The selection and combination of materials with different refractive indices and Abbe numbers helps to achieve low chromatic dispersion of the optical path. (4) The entire optical system is a direct-transmission type optical structure, and the cost and difficulty of processing are relatively low.

[0109] According to another aspect of the present application, a head-mounted display device is provided.

[0110] The head-mounted display device includes an optical system described in any one of the above.

[0111] The head-mounted display device mentioned above is, for example, a VR device.

[0112] The embodiments described above have focused on explaining the differences between various embodiments, and better embodiments may be formed by combining the different optimized features among the various embodiments, provided that the features are not contradictory. For the sake of brevity of the specification, repetitive descriptions are not provided herein.

[0113] Although some specific embodiments of this application have been described in detail by way of example, those skilled in the art should understand that said 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 said embodiments may be made without departing from the scope and spirit of this application. The scope of the application is defined by the appended claims. Explanation of the symbols 1. Display screen; 2. First lens; 3. Second lens; 4. Third lens; 21. Page 1; 22. Page 2; 31. Page 3; 32. Page 4; 41. Page 5; 42. Page 6

Claims

Claim 1 An optical system comprises a lens group consisting of a third lens (4), a second lens (3), and a first lens (2) installed sequentially along the propagation direction of incident light, wherein the first lens (2) includes a first surface (21) and a second surface (22), and the second lens (3) includes a third surface (31) and a fourth surface (32), wherein the second surface (22) and the third surface (31) are installed adjacent to each other and are both Fresnel surfaces, wherein the imaging spot size of the optical system is smaller than 50 μm, wherein the absolute value of the radius R1 of the first surface (21) satisfies 35 mm ≤ R1 ≤ 65 mm, wherein the absolute value of the radius R2 of the second surface (22) satisfies 20 mm ≤ R2 ≤ 40 mm, and wherein the absolute value of the conic coefficient K1 of the first surface (21) and the second surface (22) satisfies K1 ≤ 20, and wherein An optical system characterized in that the absolute value of the radius R3 of the third surface (31) satisfies 20mm ≤ R3 ≤ 40mm, the absolute value of the radius R4 of the fourth surface (32) satisfies 30mm ≤ R4 ≤ 60mm, and the absolute value of the conic coefficient K2 of the third surface (31) and the fourth surface (32) satisfies K2 ≤ 20. Claim 2 An optical system according to claim 1, wherein a first gap T1 is installed between the first lens (2) and the second lens (3), and the first gap T1 is installed such that 0.2 mm ≤ T1 ≤ 1 mm; a second gap T2 is installed between the third lens (4) and the second lens (3), and the second gap T2 is installed such that 1 mm ≤ T2 ≤ 3 mm; the second gap T2 is larger than the first gap T1; and the first lens (2), the second lens (3), and the third lens (4) are located on the same optical axis. Claim 3 An optical system according to claim 1, characterized in that the refractive power of the first lens (2) and the second lens (3), which have Fresnel surfaces installed adjacent to each other, are both positive, and the refractive power of the third lens (4) is negative. Claim 4 An optical system according to claim 1, characterized in that the center thickness value h1 of the first lens (2) is 2mm ≤ h1 ≤ 4mm, the center thickness value h2 of the second lens (3) is 3mm ≤ h2 ≤ 5mm, and the center thickness value h3 of the third lens (4) is 2mm ≤ h3 ≤ 4mm. Claim 5 An optical system characterized in that, in claim 1, the first surface (21) and the fourth surface (32) are both aspherical, and the first lens (2) and the second lens (3) both use a combination of aspherical and Fresnel surfaces. Claim 6 An optical system according to claim 5, wherein the third lens (4) comprises a fifth surface (41) and a sixth surface (42), the fifth surface (41) is installed adjacent to the fourth surface (32), at least one of the fifth surface (41) and the sixth surface (42) is aspherical, the absolute value of the radius R5 of the fifth surface (41) satisfies R5 ≥ 30 mm, the absolute value of the radius R6 of the sixth surface (42) satisfies 30 mm ≤ R6 ≤ 60 mm, and the absolute value of the conic coefficient K3 of the fifth surface (41) and the sixth surface (42) satisfies K3 ≤ 10. Claim 7 An optical system according to claim 1, wherein among the first lens (2) and the second lens (3), the value of the conic coefficient K is set in the range [-10, 10], and the radius value of the Fresnel surface of each lens is 23 mm or more. Claim 8 An optical system according to claim 1, wherein the first lens (2) and the second lens (3) are made of the same material, both being COP material, and the third lens (4) is made of OKP material or EP material. Claim 9 An optical system according to claim 1, characterized in that the effective focal length (f1) of the first lens (2) is 20mm ≤ f1 ≤ 40mm, the effective focal length (f2) of the second lens (3) is 20mm ≤ f2 ≤ 40mm, and the effective focal length (f3) of the third lens (4) is -75mm ≤ f3 ≤ -35mm. Claim 10 An optical system according to claim 1, characterized in that the effective focal length (f) of the lens group satisfies 15mm ≤ f ≤ 25mm. Claim 11 A head-mounted display device characterized by including an optical system described in any one of claims 1 to 10.

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