Patent Publication Number: US-2022229303-A1

Title: Display device module and head-mounted display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/CN2020/119721, filed on Sep. 30, 2020, which claims priority to Chinese Patent Application No. 201910960169.4, filed on Oct. 10, 2019, and Chinese Patent Application No. 201910960170.7, filed on Oct. 10, 2019. All of the aforementioned patent applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     This application relates to the optical field, and in particular, to a display device module and a head-mounted display device. 
     BACKGROUND 
     A head-mounted display device may enhance or expand, by using additional information generated by a computer, a real-world scene seen by a user. This greatly changes a manner in which a human being interacts with the computer or the external world. The device uses a combination of a plurality of technologies in different research fields, and is quickly applied to fields such as entertainment, scientific research, simulation training, and telemedicine. 
     In a head-mounted display device based on an augmented reality (augmented reality, AR) technology and a mixed reality (mixed reality, MR) technology, a virtual image displayed on a display panel needs to be first processed by a display device module, and then superposed with a real image to be presented to the user. Because the head-mounted display device is worn on the head of the user, to reduce pressure on the head of the user, a compact design is further required for the head-mounted display device. 
     SUMMARY 
     Embodiments of this application provide a head-mounted display device, so that a thickness of a display device module in the head-mounted display device in a viewing direction of human eyes is reduced, and a compact design of the head-mounted display device is implemented. 
     According to a first aspect, this application provides a display device module, including a display panel, a non-coaxial optical component, a first optical element, and a second optical element, where the non-coaxial optical component includes an incident surface and an emergent surface, and the incident surface of the non-coaxial optical component faces the display panel, so that light emitted by the display panel is capable of being transmitted through the incident surface and transmitted from the emergent surface; 
     the first optical element is configured to reflect, to the second optical element, the light transmitted from the emergent surface, where an included angle is provided between the first optical element and the second optical element; and 
     the second optical element is configured to reflect, back to the first optical element, the light reflected by the first optical element, so that the light is transmitted from the first optical element. 
     The display device module may further include a fastening system. The fastening system is configured to fasten the display panel, the non-coaxial optical component, the first optical element, and the second optical element. The second optical element is configured to reflect, back to the first optical element, the light reflected by the first optical element, so that the light is transmitted from the first optical element to the human eyes. The second optical element is further configured to transmit ambient light. 
     Optionally, in an optional design of the first aspect, the non-coaxial optical component further includes at least one reflective surface; and 
     the at least one reflective surface is disposed between the incident surface and the emergent surface, so that the light transmitted through the incident surface is capable of being transmitted from the emergent surface through reflection by the at least one reflective surface. 
     Optionally, in an optional design of the first aspect, the first optical element and the second optical element are sheet-like optical structures. 
     Optionally, in an optional design of the first aspect, the first optical element and the second optical element are plate-like optical structures or optical structures with at least one bent side. 
     Optionally, in an optional design of the first aspect, an opening direction of the included angle formed between the first optical element and the second optical element faces the emergent surface of the non-coaxial optical component. 
     Optionally, in an optional design of the first aspect, the non-coaxial optical component is located on a same side of the first optical element as the second optical element. 
     Optionally, in an optional design of the first aspect, the incident surface of the non-coaxial optical component faces the display panel, so that all light emitted by the display panel is capable of being transmitted through the incident surface and transmitted from the emergent surface. 
     Optionally, in an optional design of the first aspect, the first optical element includes a first end and a second end, the first end is an end that is of the first optical element and that is close to the non-coaxial optical component, the second end is an end that is of the first optical element and that is away from the non-coaxial optical component, and a distance between the first end and the human eyes is less than a distance between the second end and the human eyes. 
     Optionally, in an optional design of the first aspect, the second optical element includes a third end and a fourth end, the third end is an end that is of the second optical element and that is close to the non-coaxial optical component, the fourth end is an end that is of the second optical element and that is away from the non-coaxial optical component, and a distance between the third end and the human eyes is greater than a distance between the fourth end and the human eyes. 
     Optionally, in an optional design of the first aspect, the display device module further includes a compensation prism. 
     The compensation prism is disposed between the display panel and the incident surface of the non-coaxial optical component; or 
     the compensation prism is disposed between the emergent surface of the non-coaxial optical component and the first optical element. 
     Optionally, in an optional design of the first aspect, materials of the compensation prism and the non-coaxial optical component are different, and dispersion coefficients of the compensation prism and the non-coaxial optical component are different. 
     Optionally, in an optional design of the first aspect, materials of the compensation prism and the non-coaxial optical component are different, and refractive indexes of the compensation prism and the non-coaxial optical component are different. 
     Optionally, in an optional design of the first aspect, a part of reflective film is disposed on a surface that is of the first optical element and that faces the second optical element. 
     Optionally, in an optional design of the first aspect, a first polarization beam splitter is disposed on the surface that is of the first optical element and that faces the second optical element, and a first phase retarder is disposed on a surface that is of the first polarization beam splitter and that faces the second optical element. 
     Optionally, in an optional design of the first aspect, a second phase retarder is further disposed between the display panel and the first optical element. 
     Optionally, in an optional design of the first aspect, a second phase retarder is further disposed on a first reflective surface. 
     Optionally, in an optional design of the first aspect, a second phase retarder is further disposed on a second reflective surface. 
     Optionally, in an optional design of the first aspect, a second phase retarder is further disposed on the emergent surface. 
     Optionally, in an optional design of the first aspect, geometric shapes, relative positions, and materials of the incident surface, the at least one reflective surface, and the emergent surface meet a preset relationship, so that light that is emitted by the display panel and that is in a same field of view intersects outside the emergent surface to form a linear image. 
     Optionally, in an optional design of the first aspect, based on a focal power parameter of the second reflective surface, the light that is emitted by the display panel and that is in the same field of view converges and intersects in a first direction to form the linear image, where the first direction is perpendicular to a plane formed by a horizontal viewing direction and a direction in which a line between the two eyes is located when a head-mounted display device is worn. 
     Optionally, in an optional design of the first aspect, the first optical element is asymmetric in the first direction. 
     Optionally, in an optional design of the first aspect, the second optical element is asymmetric in the first direction. 
     Optionally, in an optional design of the first aspect, a part of reflective film is disposed on at least one surface of the second optical element. 
     According to a second aspect, this application further provides a head-mounted display device, including a left-eye display and a right-eye display, where the left-eye display and the right-eye display include the display device module according to any one of the optional designs of the first aspect. 
     This embodiment of this application provides the head-mounted display device, including the display panel, the non-coaxial optical component, the first optical element, and the second optical element. The non-coaxial optical component includes the incident surface and the emergent surface, and the incident surface of the non-coaxial optical component faces the display panel, so that light emitted by the display panel is capable of being transmitted through the incident surface and transmitted from the emergent surface. The first optical element is configured to reflect, to the second optical element, the light transmitted from the emergent surface, where an included angle is provided between the first optical element and the second optical element. The second optical element is configured to reflect, back to the first optical element, the light reflected by the first optical element, so that the light is transmitted from the first optical element to the human eyes. Compared with a manner in which physical glass or plastic is used for an optical structure in front of the eyes, and therefore a compensation prism needs to be added to correct distorted imaging of an external scene, in this embodiment of this application, because an angle is provided between the first optical element and the second optical element, in other words, a gap is left between the first optical element and the second optical element, natural ambient light propagating to the human eyes is not refracted, imaging of the external scene is not distorted, and no additional compensation prism needs to be added. Therefore, a thickness of the display module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. 
     According to a third aspect, this application provides a display device module, including a display panel, a freeform prism, a first optical element, and a second optical element, where 
     the freeform prism includes an incident surface, at least one reflective surface, and an emergent surface, and the incident surface of the freeform prism faces the display panel, so that light emitted by the display panel is capable of being transmitted through the incident surface and transmitted from the emergent surface through reflection by the at least one reflective surface; 
     the first optical element is configured to reflect, to the second optical element, the light transmitted from the emergent surface, where an included angle is provided between the first optical element and the second optical element; and 
     the second optical element is configured to reflect, back to the first optical element, the light reflected by the first optical element, so that the light is transmitted from the first optical element. 
     Optionally, in an optional design of the third aspect, the first optical element and the second optical element are sheet-like optical structures. 
     Optionally, in an optional design of the third aspect, the first optical element and the second optical element are plate-like optical structures or optical structures with at least one bent side. 
     Optionally, an opening direction of the included angle formed between the first optical element and the second optical element faces the emergent surface of the freeform prism. 
     Optionally, in an optional design of the third aspect, the freeform prism is located on a same side of the first optical element as the second optical element. 
     Optionally, in an optional design of the third aspect, the display device module further includes a compensation prism; and 
     the compensation prism is disposed between the display panel and the incident surface of the freeform prism; or 
     the compensation prism is disposed between the emergent surface of the freeform prism and the first optical element. 
     Optionally, in an optional design of the third aspect, materials of the compensation prism and the freeform prism are different, and dispersion coefficients of the compensation prism and the freeform prism are different. 
     Optionally, in an optional design of the third aspect, materials of the compensation prism and the freeform prism are different, and refractive indexes of the compensation prism and the freeform prism are different. 
     Optionally, in an optional design of the third aspect, a part of reflective film is disposed on a surface that is of the first optical element and that faces the second optical element. 
     Optionally, in an optional design of the third aspect, a first polarization beam splitter is disposed on the surface that is of the first optical element and that faces the second optical element, and a first phase retarder is disposed on a surface that is of the first polarization beam splitter and that faces the second optical element. 
     Optionally, in an optional design of the third aspect, a second phase retarder is further disposed between the display panel and the first optical element. 
     Optionally, in an optional design of the third aspect, a second phase retarder is further disposed on the reflective surface. 
     Optionally, in an optional design of the third aspect, a second phase retarder is further disposed on the emergent surface. 
     Optionally, in an optional design of the third aspect, geometric shapes, relative positions, and materials of the incident surface, the at least one reflective surface, and the emergent surface meet a preset relationship, so that light that is emitted by the display panel and that is in a same field of view intersects outside the emergent surface to form a linear image. 
     Optionally, in an optional design of the third aspect, the light that is emitted by the display panel and that is in the same field of view converges and intersects in a first direction to form the linear image, where the first direction is perpendicular to a plane formed by a horizontal viewing direction and a direction in which a line between the two eyes is located when a head-mounted display device is worn. 
     Optionally, in an optional design of the third aspect, the first optical element is asymmetric in the first direction. 
     Optionally, in an optional design of the third aspect, the second optical element is asymmetric in the first direction. 
     Optionally, in an optional design of the third aspect, a part of reflective film is disposed on at least one surface of the second optical element. 
     According to a fourth aspect, this application further provides a head-mounted display device, including a left-eye display and a right-eye display, where the left-eye display and the right-eye display include the display device module according to any one of the optional designs of the third aspect. 
     According to a fifth aspect, this application further provides a head-mounted display device, where the head-mounted display device is an augmented reality (AR) device or a mixed reality (MR) device, and includes a left-eye display module, a right-eye display module, a middle cover, and legs, where 
     the middle cover is configured to fasten the left-eye display module, the right-eye display module, and the legs, and the left-eye display module or the right-eye display module includes the display device module according to any one of the optional designs of the first aspect and any one of the optional designs of the third aspect. 
     This embodiment of this application provides the head-mounted display device, including the display panel, the freeform prism, the first optical element, and the second optical element. The freeform prism includes the incident surface, the at least one reflective surface, and the emergent surface, and the incident surface of the freeform prism faces the display panel, so that light emitted by the display panel is capable of being transmitted through the incident surface and transmitted from the emergent surface through reflection by the at least one reflective surface. The first optical element is configured to reflect, to the second optical element, the light transmitted from the emergent surface, where an included angle is provided between the first optical element and the second optical element. The second optical element is configured to reflect, back to the first optical element, the light reflected by the first optical element, so that the light is transmitted from the first optical element. Compared with a manner in which physical glass or plastic is used for an optical structure in front of the eyes, and therefore a compensation prism needs to be added to correct distorted imaging of an external scene, in this embodiment of this application, because an angle is provided between the first optical element and the second optical element, in other words, a gap is left between the first optical element and the second optical element, natural ambient light propagating to the human eyes is not refracted, imaging of the external scene is not distorted, and no additional compensation prism needs to be added. Therefore, a thickness of the display module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of a head-mounted display device worn by a user; 
         FIG. 2  is a schematic diagram of a head-mounted display device; 
         FIG. 3 a    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 3 b    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 3 c    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 3 d    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 3 e    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 3 f    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 4 a    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 4 b    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 5  is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 6 a    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 6 b    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 6 c    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 6 d    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 6 e    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 6 f    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 6 g    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 7 a    is a schematic structural diagram of a display device module according to an embodiment of this application; 
         FIG. 7 b    is a schematic structural diagram of a display device module according to an embodiment of this application; and 
         FIG. 8  is a schematic structural diagram of a display device module according to an embodiment of this application; 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes the embodiments of this application with reference to the accompanying drawings. It is clear that the described embodiments are merely some but not all of the embodiments of this application. A person of ordinary skill in the art may learn that the technical solutions provided in the embodiments of this application are also applicable to a similar technical problem as a technology evolves and a new scenario emerges. 
     In this specification, claims, and accompanying drawings of this application, the terms “first”, “second”, and the like are intended to distinguish similar objects but do not necessarily indicate a specific order or sequence. It should be understood that the data termed in such a way interchangeable in proper circumstance so that the embodiments described herein can be implemented in other orders than the order illustrated or described herein. In addition, the terms “include”, “contain” and any other variants mean to cover the non-exclusive inclusion, for example, a process, method, system, product, or device that includes a list of steps or modules is not necessarily limited to the modules, but may include other modules not expressly listed or inherent to such a process, method, product, or device. Names or numbers of steps in this application do not mean that the steps in the method procedure need to be performed in a chronological/logical order indicated by the names or numbers. An execution sequence of the steps in the procedure that have been named or numbered may be changed based on technical objectives to be implemented, provided that a same or similar technical effect can be achieved. 
       FIG. 1  is a schematic diagram of a head-mounted display device  100  worn by a user  102 . The head-mounted display device  100  may be configured to display an augmented reality image and a physical object in a background scene in the real world. The head-mounted display device  100  may include a frame  104  for positioning the device at a target viewing position relative to the eyes of the user  102 . 
       FIG. 2  is a schematic diagram of the head-mounted display device  100  in  FIG. 1 . As shown in  FIG. 2 , the perspective head-mounted display device  100  includes a right-eye display  200   a  and a left-eye display  200   b . Each perspective display (the right-eye display  200   a  or the left-eye display  200   b ) may be configured to display a virtual image to a user and allow the user to view an environment in the real world. For example, each perspective display may include a display device configured to emit display light to the eyes of the user, where the light passes through an optical structure, and the display device may further allow ambient light in the real world to reach the eyes of the user. In addition,  FIG. 2  schematically shows a microphone  202  that can be configured to output acoustic information to the user. Such acoustic information may be in any suitable form, and includes but is not limited to voice output that is in any suitable language (which is, for example, selected by the user) and that is generated by a computer, a tone or other sound that is not specially used for any language, and/or any other suitable sound. In some embodiments, another type of output may be provided by the head-mounted display device  100 , for example, tactile/touch output. 
     The left-eye display  200   b  and the right-eye display  200   a  may be positioned at a viewing position relative to the eyes by using a fastening mechanism such as one or more frames  104 . For example, as shown in  FIG. 2 , the frame  104  may be supported by the ear of the user by using an earpiece  206  and supported by the nose of the user by using a bridge  208 , to reduce sliding of the frame  104 . It should be understood that the supports (for example, the earpiece  206 , a nose pad, and the bridge  208 ) shown in  FIG. 2  are essentially examples, and the perspective display of the perspective head-mounted display device may be positioned at the viewing position by using any suitable mechanism. For example, an additional support may be used, and/or one or more of the supports shown in  FIG. 2  may be removed, replaced, and/or expanded, to position the perspective display at the viewing position. In addition, the perspective display may be positioned at the viewing position by using a mechanism other than a support that is physically in contact with the user. This is not limited in this application. 
       FIG. 3 a    is a schematic structural diagram of a display device module according to an embodiment of this application. Specifically, the display device module shown in  FIG. 3 a    may be a part of the left-eye display  200   b  and the right-eye display  200   a  in  FIG. 2 . As shown in  FIG. 3 a   , an example in which a non-coaxial optical component is a freeform prism is used, and the display device module provided in this application may include: 
     a display panel  301 , a non-coaxial optical component  302 , a first optical element  303 , a second optical element  304 , and a fastening system. It should be understood that the non-coaxial optical component in this embodiment may also be referred to as a non-coaxial optical system. 
     In this application, an example in which the non-coaxial optical component  302  includes a first reflective surface  3025  and a second reflective surface  3021  is used for description. This is not limited in this application. In this embodiment, that the quantity of reflective surfaces  3021  is 2 is merely used as an example for description. 
     The non-coaxial optical component  302  includes an incident surface  3022 , the first reflective surface  3025 , the second reflective surface  3021 , and an emergent surface  3024 . 
     The non-coaxial optical component  302  includes the incident surface  3022 , the first reflective surface  3025 , the second reflective surface  3021 , and the emergent surface  3024 . The incident surface  3022  of the non-coaxial optical component  302  faces the display panel  301 , so that light emitted by the display panel  301  is capable of being transmitted through the incident surface  3022  and transmitted from the emergent surface  3024  through reflection between the first reflective surface  3025  and the second reflective surface  3021 . 
     In this embodiment of this application, the display panel  301  may include any suitable component configured to generate an image for display, and the component includes but is not limited to a microdisplay and one or more light sources. 
     Optionally, in some embodiments, the display panel  301  may include a reflective microdisplay such as a liquid crystal on silicon (liquid crystal on silicon, LCOS) display. In other embodiments, the display panel  301  may include an self-luminous microdisplay such as an organic light-emitting diode (organic light-emitting diode, OLED) array display type, an inorganic light-emitting diode (inorganic light-emitting diode, ILED) array display type, and/or any other suitable microdisplay. The display panel  301  may include one or more light sources such as an RGB LED array, one or more white LEDs (which have, for example, a color filter apparatus), and/or any suitable light source structure for illumination. 
     In this embodiment of this application, the display panel  301  may display an image, and therefore may be used as a light source to emit light. 
     Optionally, in this embodiment of this application, as shown in  FIG. 3 b   , a distance L 8  between the display panel  301  and the incident surface  3022  of the non-coaxial optical component  302  may be within a range of 0.1 mm to 1 mm. 
     In this embodiment of this application, the non-coaxial optical component  302  may include at least three surfaces, and each surface may be but is not limited to a non-planar and non-spherical surface. For example, the non-coaxial optical component  302  may include the first reflective surface  3025 , the second reflective surface  3021 , the incident surface  3022 , and the emergent surface  3024 . The emergent surface  3024  is a lower end face of the non-coaxial optical component  302 , the emergent surface  3024  may be a non-rotational symmetric transmittance surface, and an edge of the emergent surface  3024  is bent from air towards the non-coaxial optical component. In this way, an angle of incidence of main light can be reduced, thereby facilitating aberration correction. The incident surface  3022  is an upper end face of the non-coaxial optical component  302 , the incident surface  3022  may be a non-rotational symmetric transmittance surface, and an edge of the incident surface  3022  may be bent towards the non-coaxial optical component  302  in a horizontal direction, to provide focal power for imaging in the horizontal direction. 
     It should be noted that the horizontal direction is a horizontal direction existing when a user wears a head-mounted display device. 
     In this embodiment of this application, the second reflective surface  3021  may be an end face of the non-coaxial optical component  302 . When the user wears the head-mounted display device, the second reflective surface  3021  faces away from the human eye, and is an outer surface of the non-coaxial optical component  302 . The second reflective surface  3021  may be a non-rotational symmetric reflective surface. In this embodiment of this application, light from the display panel  301  travels along a display optical path, enters the non-coaxial optical component after the light is transmitted through the incident surface  3022 , is reflected between the first reflective surface  3025  and the second reflective surface  3021 , and is finally transmitted from the emergent surface  3024  through reflection by the second reflective surface  3021 . 
     It should be noted that, in this embodiment of this application, the first reflective surface  3025  and the second reflective surface  3021  of the non-coaxial optical component  302  each may be coated with a reflective film. In this case, the first reflective surface  3025  and the second reflective surface  3021  of the non-coaxial optical component  302  may reflect all incident light. 
     It should be noted that, in this embodiment of this application, the non-coaxial optical component may include a plurality of reflective surfaces. After the light emitted by the display panel  301  is transmitted through the incident surface  3022  of the non-coaxial optical component  302 , the light enters the non-coaxial optical component  302 , and is transmitted out of the non-coaxial optical component  302  from the emergent surface  3024  through folding by the plurality of reflective surfaces. A quantity of reflective surfaces is not limited in this application. In this embodiment of this application, optionally, an edge of the second reflective surface  3021  may be bent towards the non-coaxial optical component  302  in a plumb direction, to provide positive focal power for secondary imaging in the plumb direction. 
     It should be noted that the plumb direction is a plumb direction existing when the user wears the head-mounted display device. 
     Optionally, in this embodiment of this application, as shown in  FIG. 3 b   , a width L 7  of the non-coaxial optical component  302  in a horizontal viewing direction of the human eyes may be within a range of 3 mm to 15 mm. 
     In this embodiment of this application, when the user correctly wears the head-mounted display device, the light from the display panel  301  may travel along the display optical path, and sequentially pass through the non-coaxial optical component  302 , the first optical element  303 , the second optical element  304 , and the first optical element  303 , to reach the human eyes. 
     Specifically, the first optical element  303  may be configured to reflect the light to the second optical element  304 . The second optical element  304  is configured to reflect, to the first optical element  303 , the light reflected by the first optical element  303 . The first optical element  303  is configured to transmit the light reflected by the second optical element  304 . 
     In this embodiment of this application, the first optical element  303  may include a first surface and a second surface, the first surface is a surface facing the second optical element  304 , and the second surface is a surface facing away from the second optical element  304 . The second optical element  304  may include a third surface and a fourth surface, the third surface is a surface facing the first optical element  303 , and the fourth surface is a surface facing away from the first optical element  303 . 
     The first surface of the first optical element  303  may be configured to reflect light to the third surface of the second optical element  304 . The third surface of the second optical element  304  is configured to reflect, to the first surface of the first optical element  303 , the light reflected by the first optical element  303 . The first surface of the first optical element  303  is further configured to transmit the light reflected by the second optical element  304 , so that the light is transmitted from the second surface of the first optical element  303 . 
     In this embodiment of this application, when the user wears the head-mounted display device, the second optical element  304  may be further configured to transmit natural ambient light, so that the natural light is emitted to the human eyes. 
     Specifically, the fourth surface of the second optical element  304  may be configured to transmit the natural ambient light. 
     In this embodiment of this application, an angle may be provided between the first optical element  303  and the second optical element  304 , in other words, a gap may be left between the first optical element  303  and the second optical element  304 . 
     Optionally, in this embodiment of this application, the first optical element  303  and the second optical element  304  may be optical structures with at least one bent side. As shown in  FIG. 3 e   , one side of the first optical element  303  is bent towards the second optical element  304 , and two opposite sides of the second optical element  304  are bent towards the first optical element  303 . It should be noted that bending regions on the first optical element  303  and the second optical element  304  may be any one or more sides of the first optical element  303  and the second optical element  304 . This is not limited in this application. 
     In this embodiment of this application, the first optical element may include a first end and a second end, the first end is an end that is of the first optical element and that is close to the non-coaxial optical component, the second end is an end that is of the first optical element and that is away from the non-coaxial optical component, and a distance between the first end and the human eyes is less than a distance between the second end and the human eyes. The second optical element includes a third end and a fourth end, the third end is an end that is of the second optical element and that is close to the non-coaxial optical component, the fourth end is an end that is of the second optical element and that is away from the non-coaxial optical component, and a distance between the third end and the human eyes is greater than a distance between the fourth end and the human eyes. In other words, there is an angle between the first optical element and the second optical element, and the angle faces the non-coaxial optical component above. In addition, a side that is of the first optical element and that is close to the non-coaxial optical component is closer to the human eyes than a side far away from the non-coaxial optical component, and a side that is of the second optical element and that is close to the non-coaxial optical component is farther away from the human eyes than a side far away from the non-coaxial optical component. Compared with a manner in which physical glass or plastic is used for an optical structure in front of the eyes, and therefore a compensation prism needs to be added to correct distorted imaging of an external scene, in this embodiment of this application, because a gap is left between the first optical element  303  and the second optical element  304 , natural ambient light propagating to the human eyes is not refracted, imaging of the external scene is not distorted, and no additional compensation prism needs to be added. Therefore, a thickness of the display device module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. Specifically, in this embodiment of this application, under a condition of a same design parameter (for example, a field of view of 40 degrees), the thickness of the display device module in the head-mounted display device provided in this application may be at least 5 mm less than a thickness of a display device module in a head-mounted display device in a conventional technology. In this embodiment, the thickness of the display device module may be understood as a longest distance that is between a side, of the display device module, close to the human eyes and a side away from the human eyes and that exists when the user wears the head-mounted display device. 
     In this embodiment of this application, as shown in  FIG. 3 a   , there may be an angle between the first optical element  303  and the second optical element  304 . Specifically, when the user wears the head-mounted display device, a position for disposing the first optical element  303  in space may be nearly vertical. There is an angle between the first optical element  303  and the second optical element  304 . A distance L 2  that is between a side of the second optical element  304  and a side of the first optical element  303  and that exists at a lower vertical position is less than a distance L 1  that is between a side of the second optical element  304  and a side of the first optical element  303  and that exists at an upper vertical position. 
     It should be noted that a vertical direction may be a vertical direction existing when the user wears the head-mounted display device. 
     Optionally, as shown in  FIG. 3 a   , in this embodiment of this application, an opening direction of the included angle that may exist between the first optical element  303  and the second optical element  304  may face the emergent surface  3024  of the non-coaxial optical component  302 . 
     In an embodiment, the top of the first optical element and the top of the second optical element may be extended to wrap the display panel and the non-coaxial optical component. In this case, when the user wears the head-mounted display device, a distance that is between the second optical element  304  and the first optical element  303  and that exists at the lower vertical position may be greater than a distance that is between the second optical element  304  and the first optical element  303  and that exists at the upper vertical position. In this embodiment, the first optical element  303  may include a first region that is used to reflect, to the second optical element  304 , light reflected by the reflective surface of the non-coaxial optical component  302 , and a second region that may transmit light reflected by the second optical element  304 . Correspondingly, the second optical element  304  may include a third region that is used to reflect, to the first optical element  303 , the light reflected by the first optical element  303 . In this case, when the user wears the head-mounted display device, a distance L 2  that is between the third region of the second optical element  304  and the second region and that exists at the lower vertical position is less than a distance L 1  that is between the third region and the first region and that exists at the upper vertical position. 
     In an embodiment, as shown in  FIG. 3 a   , the non-coaxial optical component  302  is located in the opening direction of the angle formed between the first optical element  303  and the second optical element  304 , and the non-coaxial optical component  302  may be located on a same side of the first optical element  303  as the second optical element  304 . It should be noted that the emergent surface of the non-coaxial optical component is located on a same side of the first optical element as the second optical element. 
     It should be noted that, in this embodiment, the first optical element and the second optical element may be non-flat sheet-like optical structures, and surface equations of the first optical element  303  and the second optical element  304  may be represented by using the following equation that constitutes no limitation: 
     
       
         
           
             
               z 
               = 
               
                 
                   
                     cr 
                     2 
                   
                   
                     1 
                     + 
                     
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               1 
                               + 
                               k 
                             
                             ) 
                           
                           ⁢ 
                           
                             c 
                             2 
                           
                           ⁢ 
                           
                             r 
                             2 
                           
                         
                       
                     
                   
                 
                 + 
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     N 
                   
                   ⁢ 
                   
                     
                       A 
                       i 
                     
                     ⁢ 
                     
                       
                         E 
                         i 
                       
                       ⁡ 
                       
                         ( 
                         
                           x 
                           , 
                           y 
                         
                         ) 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     where 
     c is a surface radius, k is a surface conic coefficient, A i  is a polynomial coefficient of i th  term, N may be a positive integer, E is an additional polynomial of i th  term, a specific form may be: E 1 =x, E 2 =y, E 3 =x 2 , E 4 =xy, E 5 =y 2 , E 6 =x 3 , E 7 =x 2 y, E 8 =xy 2 , E 9 =y 3 , . . . , where x and y are surface local coordinates, and r=x 2 +y 2 . 
     It should be noted that the foregoing surface equation is merely an example. This is not limited in this application. 
     Optionally, in this embodiment of this application, as shown in  FIG. 3 b   , a thickness L 4  of the first optical element  303  may be within a range of 0.1 mm to 3 mm, and a thickness L 3  of the second optical element  304  may be within a range of 0.1 mm to 3 mm. 
     Optionally, in this embodiment of this application, as shown in  FIG. 3 b   , when the user wears the head-mounted display device, a distance L 6  between the human eyes and a lens of the head-mounted display device may be within a range of 8 mm to 30 mm. It should be noted that a thickness of the lens of the head-mounted display device in this application may change correspondingly with different design parameters of the module. For example, the thickness of the lens of the head-mounted display device may change at least between 3 mm and 15 mm with different field of view parameters. Optionally, in this embodiment of this application, each element included in the display device module may be fastened at a corresponding position by disposing a fastening element. 
     In this embodiment, the display device module further includes the fastening system. The fastening system may include a housing, a bearing surface, a connecting piece, a V-shaped groove, and another mechanical structure, or may include a material used for fastening or connection. 
     In this embodiment, each element included in the display device module may be fastened at a corresponding position by disposing the fastening system. For example, in this embodiment of this application, a side that is of the display panel  301  and that faces away from a light emitting surface may be a bearing surface, and the display panel  301  may be fastened to a housing of the head-mounted display device by using the bearing surface. 
     Optionally, in this embodiment of this application, the non-coaxial optical component  302  may be fastened to the display panel through mechanical connection. 
     Optionally, in this embodiment of this application, the first optical element  303  may be connected to the non-coaxial optical component  302  through positioning by using a V-shaped groove, or the first optical element  303  may be fastened to the non-coaxial optical component  302  by using a bearing surface that is extended outside a clear aperture region. 
     Optionally, in this embodiment of this application, the second optical element  304  may be connected to the non-coaxial optical component  302  through positioning by using a V-shaped groove, or the second optical element  304  may be fastened to the non-coaxial optical component  302  by using a bearing surface that is extended outside a clear aperture region. 
     It should be noted that the foregoing manner of fastening the optical element is merely an example, and does not constitute a limitation on this application. 
     As shown in  FIG. 3 f   , the non-coaxial optical component may alternatively include two lenses (a lens  1  and a lens  2 ) and a reflective surface. The lens  1  includes the incident surface  3022 , and the lens  2  includes the emergent surface  3024 . 
     It should be noted that the foregoing non-coaxial optical component is merely an example, and a specific structure of the non-coaxial optical component is not limited in this application. 
     This embodiment of this application provides the head-mounted display device, including the display panel, the non-coaxial optical component, the first optical element, and the second optical element. The non-coaxial optical component includes the incident surface and the emergent surface, and the incident surface of the non-coaxial optical component faces the display panel, so that light emitted by the display panel is capable of being transmitted through the incident surface and transmitted from the emergent surface. The first optical element is configured to reflect the light transmitted from the emergent surface, where an included angle is provided between the first optical element and the second optical element. The second optical element is configured to reflect, back to the first optical element, the light reflected by the first optical element, so that the light is transmitted from the first optical element. Compared with a manner in which physical glass or plastic is used for an optical structure in front of the eyes, and therefore a compensation prism needs to be added to correct distorted imaging of an external scene, in this embodiment of this application, because an angle is provided between the first optical element and the second optical element, in other words, a gap is left between the first optical element and the second optical element, natural ambient light propagating to the human eyes is not refracted, imaging of the external scene is not distorted, and no additional compensation prism needs to be added. Therefore, a thickness of the display module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. 
     Optionally, referring to  FIG. 3 c   , in this embodiment of this application, light that is emitted by the display panel  301  and that is in a same field of view may converge in a first direction, to form a linear image  305  on an optical path between the second reflective surface  3021  and the first optical element  303 . The first direction is perpendicular to a plane formed by a horizontal viewing direction and a direction in which a line between the two eyes is located when the head-mounted display device is worn. 
     It should be noted that the light in the same field of view in this application may be understood as light emitted by a same light emitting point on the display panel  301 . 
     Further, a linear image may be formed on an optical path between the emergent surface  3024  and the first optical element  303 . 
     The first direction in this embodiment of this application may be a vertically downward (or referred to as the plumb direction) direction (references may be made to a first direction shown in  FIG. 3 e   ) existing when the user wears the head-mounted display device. Light reflected by the second reflective surface  3021  converges on a plumb surface and does not converge in the horizontal direction, to form a plurality of linear images on the optical path between the second reflective surface  3021  and the first optical element  303 . Each linear image is formed through convergence of light corresponding to a same field of view. 
     In this embodiment of this application, geometric shapes, relative positions, and used materials of the at least one reflective surface, the incident surface, and the emergent surface (for example, the first reflective surface  3025 , the second reflective surface  3021 , the incident surface  3022 , and the emergent surface  3024 ) may be changed, so that the non-coaxial optical component  302  generates different focal power in the first direction and a second direction (references may be made to a second direction shown in  FIG. 3 e   ) for incident light of the display panel  301 . In this way, after passing through the non-coaxial optical component  302 , light from any point on the display panel  301  intersects in the first direction, and does not intersect in the second direction at any position from the first optical element  303  to the reflective surface  3021 . 
     It should be noted that the first direction and the second direction in this embodiment of this application are directions perpendicular to each other. The first direction is a vertically downward (or referred to as the plumb direction) direction existing when the user wears the head-mounted display device, and the second direction is a direction in which a line between two lenses is located when the user wears the head-mounted display device. 
     In this embodiment of this application, the first optical element  303  may be asymmetric in the plumb direction, the second optical element  304  may be asymmetric in the plumb direction, and the second optical element may be symmetric in the horizontal direction. Because the light reflected by the reflective surface  3021  converges in the first direction, a “sag” facing the second optical element exists in lowermost light in the light reflected by the second reflective surface  3021 . As shown in  FIG. 3 d   , an upper edge of the second optical element  304  may be closer to the first optical element  303  when the light reflected by the reflective surface  3021  is not blocked, so that the gap between the first optical element  303  and the second optical element  304  is reduced. Therefore, a thickness of the display device module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. 
     Optionally, in this embodiment of this application, the display device module may further include a compensation prism  401 . The compensation prism  401  is disposed on an optical path between the display panel  301  and the first optical element  303 , and materials of the compensation prism  401  and the non-coaxial optical component  302  are different. 
       FIG. 4 a    is a schematic structural diagram of a display device module according to an embodiment of this application. As shown in  FIG. 4 a   , the compensation prism  401  having a different material from the non-coaxial optical component  302  may be disposed on the emergent surface  3024  of the non-coaxial optical component  302  to correct chromatic aberration. A refractive index of the compensation prism  401  is different from a refractive index of the non-coaxial optical component  302 . For example, a material with a low refractive index may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a high refractive index may be selected for the compensation prism  401 . Alternatively, a material with a high refractive index may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a low refractive index may be selected for the compensation prism  401 . 
     Optionally, a dispersion coefficient of the compensation prism  401  is different from a dispersion coefficient of the non-coaxial optical component  302 . For example, a material with a low dispersion coefficient may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a high dispersion coefficient may be selected for the compensation prism  401 . Alternatively, a material with a high dispersion coefficient may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a low dispersion coefficient may be selected for the compensation prism  401 . 
     Optionally, in another embodiment,  FIG. 4 b    is a schematic structural diagram of a display device module according to an embodiment of this application. As shown in  FIG. 4 b   , the compensation prism  401  having a different material from the non-coaxial optical component  302  may be disposed on the incident surface  3022  of the non-coaxial optical component  302  to correct chromatic aberration. A refractive index of the compensation prism  401  is different from a refractive index of the non-coaxial optical component  302 . For example, a material with a low refractive index may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a high refractive index may be selected for the compensation prism  401 . Alternatively, a material with a high refractive index may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a low refractive index may be selected for the compensation prism  401 . 
     Optionally, a dispersion coefficient of the compensation prism  401  is different from a dispersion coefficient of the non-coaxial optical component  302 . For example, a material with a low dispersion coefficient may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a high dispersion coefficient may be selected for the compensation prism  401 . Alternatively, a material with a high dispersion coefficient may be selected for the non-coaxial optical component  302 , and correspondingly, a material with a low dispersion coefficient may be selected for the compensation prism  401 . 
     Optionally, in this embodiment of this application, the compensation prism  401  may be attached to the light emitting surface of the display panel  301 , attached to the incident surface of the non-coaxial optical component  302 , attached to the reflective surface  3021  of the non-coaxial optical component  302 , or attached to the emergent surface of the non-coaxial optical component  302 . 
     Optionally, in this embodiment of this application, the compensation prism  401  may be fastened to the housing of the head-mounted display device by using a bearing surface, fastened to the display panel through mechanical connection, or connected to the non-coaxial optical component through positioning by using a V-shaped groove. 
     It should be noted that the foregoing manner of fastening the compensation prism  401  is merely an example. This is not limited in this application. 
       FIG. 5  is a schematic structural diagram of a display device module according to an embodiment of this application. As shown in  FIG. 5 , the display device module further includes a first polarization beam splitter  501 , a first phase retarder  502 , and a second phase retarder  503 . 
     The first polarization beam splitter  501  is disposed on the surface that is of the first optical element  303  and that faces the second optical element  304 , the first phase retarder  502  is disposed on a surface that is of the first polarization beam splitter  501  and that faces the second optical element  304 , and the second phase retarder  503  is disposed on the optical path between the display panel  301  and the first optical element  303 . 
     In this embodiment of this application, the first phase retarder  502  and the second phase retarder  503  may be quarter-wave plates. 
     Optionally, in this embodiment of this application, the second phase retarder  503  may be disposed between the display panel  301  and the incident surface  3022  of the non-coaxial optical component  302 . 
     Optionally, in an embodiment, the second phase retarder  503  (the quarter-wave plate) may be attached to the light emitting surface of the display panel  301 . In this embodiment, light emitted by the display panel  301  passes through the second phase retarder  503  (the quarter-wave plate), and becomes a type of circularly polarized light, for example, left-handed circularly polarized light. The first polarization beam splitter  501  is disposed on the surface that is of the first optical element  303  and that faces the second optical element  304 , and the first phase retarder  502  is disposed on the surface that is of the first polarization beam splitter  501  and that faces the second optical element  304 . The first polarization beam splitter  501  may reflect light in a polarization state and transmit light in another polarization state, for example, reflect S polarized light and transmit P polarized light (or reflect P polarized light and transmit S polarized light). An example in which the first polarization beam splitter  501  may reflect the S polarized light and transmit the P polarized light is used for description in this application. 
     When light passes through the first phase retarder  502  (the quarter-wave plate) for the first time, the light becomes a type of linearly polarized light, for example, S polarized light. When the light reaches the first polarization beam splitter  501 , the light is reflected, then passes through the first phase retarder  502  (the quarter-wave plate) again, and becomes circularly polarized light, for example, left-handed polarized light. When the light reaches the second optical element  304 , the light is reflected by the second optical element  304 , and a circularly polarization state is changed, for example, the light is changed to right-handed polarized light. The light passes through the first phase retarder  502  (the quarter-wave plate) again, and becomes linearly polarized light, for example, P polarized light. The light passes through the first polarization beam splitter  501  again. In this case, a polarization state of the light is different from a polarization state existing when the light (the light reflected by the second reflective surface  3021  of the non-coaxial optical component  302 ) is incident for the first time. The light is transmitted when passing through the first polarization beam splitter  501 . 
     It is clearly that, only a combination form of the polarization beam splitter and the quarter-wave plate is described in the foregoing embodiment. Another form should also fall within the protection scope of this patent. For example, the polarization beam splitter and the quarter-wave plate are placed at another position on the optical path, or the polarization beam splitter and the quarter-wave plate are separately placed at different positions on the optical path when a prism having a different material is added in front of or behind the non-coaxial optical component. This is not limited in this application. For example, the second phase retarder may be further disposed between the display panel and the first optical element, the second phase retarder may be further disposed on the first reflective surface, the second phase retarder may be further disposed on the second reflective surface, or the second phase retarder may be further disposed on the emergent surface. 
       FIG. 6 a    is a schematic structural diagram of a display device module according to an embodiment of this application. Specifically, the display device module shown in  FIG. 6 a    may be a part of the left-eye display  200   b  and the right-eye display  200   a  in  FIG. 2 . As shown in  FIG. 6 a   , the display device module provided in this application may include: 
     a display panel  301 , a freeform prism  302 , a first optical element  303 , and a second optical element  304 . 
     The freeform prism  302  includes an incident surface  3022 , a reflective surface  3021 , and an emergent surface  3024 . It should be noted that a quantity of reflective surfaces  3021  may be greater than one. This is not limited in this application. In this embodiment, that the quantity of reflective surfaces  3021  is 1 is merely used as an example for description. 
     The incident surface  3022  of the freeform prism  302  faces the display panel  301 , so that light emitted by the display panel  301  is capable of being transmitted through the incident surface  3022  and transmitted from the emergent surface  3024  through reflection by the reflective surface  3021 . 
     In this embodiment of this application, the display panel  301  may include any suitable component configured to generate an image for display, and the component includes but is not limited to a microdisplay and one or more light sources. 
     Optionally, in some embodiments, the display panel  301  may include a reflective microdisplay such as a liquid crystal on silicon (liquid crystal on silicon, LCOS) display. In other embodiments, the display panel  301  may include an self-luminous microdisplay such as an organic light-emitting diode (organic light-emitting diode, OLED) array display type, an inorganic light-emitting diode (inorganic light-emitting diode, ILED) array display type, and/or any other suitable microdisplay. The display panel  301  may include one or more light sources such as an RGB LED array, one or more white LEDs (which have, for example, a color filter apparatus), and/or any suitable light source structure for illumination. 
     In this embodiment of this application, the display panel  301  may display an image, and therefore may be used as a light source to emit light. 
     Optionally, in this embodiment of this application, as shown in  FIG. 6 b   , a distance L 8  between the display panel  301  and the incident surface  3022  of the freeform prism  302  may be within a range of 0.1 mm to 1 mm. 
     In this embodiment of this application, the freeform prism  302  may include at least three surfaces, and each surface may be but is not limited to a non-planar and non-spherical surface. For example, the freeform prism  302  may include the reflective surface  3021 , the incident surface  3022 , and the emergent surface  3024 . The emergent surface  3024  is a lower end face of the freeform prism  302 , the emergent surface  3024  may be a non-rotational symmetric transmittance surface, and an edge of the emergent surface  3024  is bent from air towards the freeform prism. In this way, an angle of incidence of a chief ray can be reduced, thereby facilitating aberration correction. The incident surface  3022  is an upper end face of the freeform prism  302 , the incident surface  3022  may be a non-rotational symmetric transmittance surface, and an edge of the incident surface  3022  may be bent towards the freeform prism  302  in a horizontal direction, to provide focal power for imaging in the horizontal direction. It should be noted that the horizontal direction is a horizontal direction existing when a user wears a head-mounted display device. 
     In this embodiment of this application, the reflective surface  3021  is an end face of the freeform prism  302 . When the user wears the head-mounted display device, the reflective surface  3021  faces away from the human eye, and is an outer surface of the freeform prism  302 . The reflective surface  3021  may be non-rotational symmetric reflective surface. In this embodiment of this application, light from the display panel  301  travels along a display optical path, enters the freeform prism after the light is transmitted through the incident surface  3022 , and is transmitted from the emergent surface  3024  through directional reflection by the reflective surface  3021 . 
     It should be noted that in this embodiment of this application, the reflective surface  3021  of the freeform prism  302  may be coated with a reflective film. In this case, the reflective surface  3021  of the freeform prism  302  may reflect all light transmitted through the incident surface  3022 . 
     It should be noted that, in this embodiment of this application, the freeform prism may include a plurality of reflective surfaces. After the light emitted by the display panel  301  is transmitted through the incident surface  3022  of the freeform prism  302 , the light enters the freeform prism  302 , and is transmitted out of the freeform prism  302  from the emergent surface  3024  through folding by the plurality of reflective surfaces. A quantity of reflective surfaces is not limited in this application. In this embodiment of this application, optionally, an edge of the reflective surface  3021  may be bent towards the freeform prism  302  in a plumb direction, to provide positive focal power for secondary imaging in the plumb direction. It should be noted that the plumb direction is a plumb direction existing when the user wears the head-mounted display device. 
     For example, the quantity of reflective surfaces is 2.  FIG. 6 f    and  FIG. 6 g    show a structure existing when the freeform prism includes two reflective surfaces. As shown in  FIG. 6 f    and  FIG. 6 g   , the freeform prism  302  includes a reflective surface  3021  and a reflective surface  3025 . After light is transmitted through the incident surface  3022  of the freeform prism  302 , the light enters the freeform prism  302 , is reflected by the reflective surface  3025  to the reflective surface  3021 , and is transmitted out of the freeform prism  302  from the emergent surface  3024  through reflection by the reflective surface  3021 . 
     With an automatic optimization algorithm of a computer, the freeform prism may form a linear image surface by setting a proper boundary condition and merit function. A specific process is as follows: 
     1. Reverse design is performed by using reversibility of light, and parameters of the first optical element and the second optical element are set, so that a planar lightwave at an exit pupil position forms the linear image surface near the emergent surface  3024 . 
     2. Reverse design is performed by using reversibility of light, and a curvature of a position that is on the emergent surface  3024  and that is close to the linear image surface is set to be similar to a curvature of the linear image surface, to correct a field curvature (similar to a function of a field lens in a camera lens). 
     3. Reverse design is performed by using reversibility of light, and the reflective surface  3021  is close to an intersection point of chief rays in all fields of view, to correct aberration of a system diaphragm near the system diaphragm. 
     4. Reverse design is performed by using reversibility of light, and the reflective surface  3025  is configured to fold the optical path. 
     5. Reverse design is performed by using reversibility of light, the boundary condition is set, and when an exit pupil center is used as a coordinate origin, 
     
       
         
           
             
               
                 
                   
                     
                       K 
                       1 
                     
                     ⁢ 
                     
                       Z 
                       2 
                     
                   
                   + 
                   
                     C 
                     1 
                   
                   - 
                   
                     Y 
                     2 
                   
                 
                 
                   
                     1 
                     + 
                     
                       K 
                       1 
                       2 
                     
                   
                 
               
               &lt; 
               0 
             
             , 
           
         
       
     
     where K 1  and C 1  are a slope and an intercept of upper-edge light in an upper-edge field of view before the light is reflected by the reflective surface  3021 , and Y 2  and Z 2  are coordinates of an intersection between the upper-edge light in the upper-edge field of view and the emergent surface. 
     6. Reverse design is performed by using reversibility of light, the boundary condition is set, and when the exit pupil center is used as the coordinate origin, 
     
       
         
           
             
               
                 
                   
                     
                       K 
                       3 
                     
                     ⁢ 
                     
                       Z 
                       4 
                     
                   
                   + 
                   
                     C 
                     3 
                   
                   - 
                   
                     Y 
                     4 
                   
                 
                 
                   
                     1 
                     + 
                     
                       K 
                       3 
                       2 
                     
                   
                 
               
               &gt; 
               0 
             
             , 
           
         
       
     
     where K 3  and C 3  are a slope and an intercept of lower-edge light in a lower-edge field of view before the light is reflected by the reflective surface  3021 , and Y 4  and Z 4  are coordinates of an intersection between the lower-edge light in the lower-edge field of view and the incident surface. 
     7. Reverse design is performed by using reversibility of light, the merit function is set, and the automatic optimization algorithm (for example, a least square algorithm) is run by using a PSF of each field of view at a display screen position as the merit function, to obtain the final freeform prism. 
     Optionally, in this embodiment of this application, as shown in  FIG. 6 b   , a width L 7  of the freeform prism  302  in a horizontal viewing direction of the human eyes may be within a range of 3 mm to 15 mm. 
     In this embodiment of this application, when the user wears the head-mounted display device, the light from the display panel  301  may travel along the display optical path, and sequentially pass through the freeform prism  302 , the first optical element  303 , the second optical element  304 , and the first optical element  303 , to reach the human eyes. 
     Specifically, the first optical element  303  may be configured to reflect the light to the second optical element  304 . The second optical element  304  is configured to reflect, to the first optical element  303 , the light reflected by the first optical element  303 . The first optical element  303  is configured to transmit the light reflected by the second optical element  304 . 
     In this embodiment of this application, the first optical element  303  may include a first surface and a second surface, the first surface is a surface facing the second optical element  304 , and the second surface is a surface facing away from the second optical element  304 . The second optical element  304  may include a third surface and a fourth surface, the third surface is a surface facing the first optical element  303 , and the fourth surface is a surface facing away from the first optical element  303 . 
     The first surface of the first optical element  303  may be configured to reflect light to the third surface of the second optical element  304 . The third surface of the second optical element  304  is configured to reflect, to the first surface of the first optical element  303 , the light reflected by the first optical element  303 . The first surface of the first optical element  303  is further configured to transmit the light reflected by the second optical element  304 , so that the light is transmitted from the second surface of the first optical element  303 . 
     In this embodiment of this application, when the user wears the head-mounted display device, the second optical element  304  may be further configured to transmit natural ambient light, so that the natural light is emitted to the human eyes. Specifically, the fourth surface of the second optical element  304  may be configured to transmit the natural ambient light. 
     In this embodiment of this application, an angle may be provided between the first optical element  303  and the second optical element  304 , in other words, a gap may be left between the first optical element  303  and the second optical element  304 . 
     Optionally, in this embodiment of this application, the first optical element  303  and the second optical element  304  may be optical structures with at least one bent side. As shown in  FIG. 6 e   , one side of the first optical element  303  is bent towards the second optical element  304 , and two opposite sides of the second optical element  304  are bent towards the first optical element  303 . It should be noted that bending regions on the first optical element  303  and the second optical element  304  may be any one or more sides of the first optical element  303  and the second optical element  304 . This is not limited in this application. 
     Compared with a manner in which physical glass or plastic is used for an optical structure in front of the eyes, and therefore a compensation prism needs to be added to correct distorted imaging of an external scene, in this embodiment of this application, because a gap is left between the first optical element  303  and the second optical element  304 , natural ambient light propagating to the human eyes is not refracted, imaging of the external scene is not distorted, and no additional compensation prism needs to be added. Therefore, a thickness of the display module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. Specifically, in this embodiment of this application, under a condition of a same design parameter (for example, a field of view of 40 degrees), the thickness of the display device module in the head-mounted display device provided in this application may be at least 5 mm less than a thickness of a display device module in a head-mounted display device in a conventional technology. In this embodiment, the thickness of the display device module may be understood as a longest distance that is between a side, of the display device module, close to the human eyes and a side away from the human eyes and that exists when the user wears the head-mounted display device. 
     In this embodiment of this application, as shown in  FIG. 6 a   , there may be an angle between the first optical element  303  and the second optical element  304 . Specifically, when the user wears the head-mounted display device, a position for disposing the first optical element  303  in space may be nearly vertical. There is an angle between the first optical element  303  and the second optical element  304 . A distance L 2  that is between a side of the second optical element  304  and a side of the first optical element  303  and that exists at a lower vertical position is less than a distance L 1  that is between a side of the second optical element  304  and a side of the first optical element  303  and that exists at an upper vertical position. It should be noted that a vertical direction may be a vertical direction existing when the user wears the head-mounted display device. 
     Optionally, as shown in  FIG. 6 a   , in this embodiment of this application, an opening direction of the included angle that may exist between the first optical element  303  and the second optical element  304  may face the emergent surface  3024  of the freeform prism  302 . 
     In an embodiment, the top of the first optical element and the top of the second optical element may be extended to wrap the display panel and the freeform prism. In this case, when the user wears the head-mounted display device, a distance that is between the second optical element  304  and the first optical element  303  and that exists at the lower vertical position may be greater than a distance that is between the second optical element  304  and the first optical element  303  and that exists at the upper vertical position. In this embodiment, the first optical element  303  may include a first region that is used to reflect, to the second optical element  304 , light reflected by the reflective surface of the freeform prism  302 , and a second region that may transmit light reflected by the second optical element  304 . Correspondingly, the second optical element  304  may include a third region that is used to reflect, to the first optical element  303 , the light reflected by the first optical element  303 . In this case, when the user wears the head-mounted display device, a distance L 2  that is between the third region of the second optical element  304  and the second region and that exists at the lower vertical position is less than a distance L 1  that is between the third region and the first region and that exists at the upper vertical position. 
     In an embodiment, as shown in  FIG. 6 a   , the freeform prism  302  is located in the opening direction of the angle formed between the first optical element  303  and the second optical element  304 , and the freeform prism  302  may be located on a same side of the first optical element  303  as the second optical element  304 . It should be noted that the emergent surface of the freeform prism may be alternatively located on a same side of the first optical element as the second optical element. 
     It should be noted that, in this embodiment, the first optical element and the second optical element may be non-flat sheet-like optical structures, and surface equations of the first optical element  303  and the second optical element  304  may be represented by using the following equation that constitutes no limitation: 
     
       
         
           
             
               z 
               = 
               
                 
                   
                     cr 
                     2 
                   
                   
                     1 
                     + 
                     
                       
                         1 
                         - 
                         
                           
                             ( 
                             
                               1 
                               + 
                               k 
                             
                             ) 
                           
                           ⁢ 
                           
                             c 
                             2 
                           
                           ⁢ 
                           
                             r 
                             2 
                           
                         
                       
                     
                   
                 
                 + 
                 
                   
                     ∑ 
                     
                       i 
                       = 
                       1 
                     
                     N 
                   
                   ⁢ 
                   
                     
                       A 
                       i 
                     
                     ⁢ 
                     
                       
                         E 
                         i 
                       
                       ⁡ 
                       
                         ( 
                         
                           x 
                           , 
                           y 
                         
                         ) 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
     where 
     c is a surface radius, k is a surface conic coefficient, A i  is a polynomial coefficient of i th  term, N may be a positive integer, E is an additional polynomial of i th  term, and a specific form may be: E 1 =x, E 2 =y, E 3 =x 2 , E 4 =xy, E 5 =y 2 , E 6 =x 3 , E 7 =x 2 y, E 8 =xy 2 , E 9 =y 3 , . . . , where x and y are surface local coordinates, and r=x 2 +y 2 . 
     It should be noted that the foregoing surface equation is merely an example. This is not limited in this application. 
     Optionally, in this embodiment of this application, alternatively, the first optical element and the second optical element may be flat plates. This is not limited in this application. 
     Optionally, in this embodiment of this application, as shown in  FIG. 6 b   , a thickness L 4  of the first optical element  303  may be within a range of 0.1 mm to 3 mm, and a thickness L 3  of the second optical element  304  may be within a range of 0.1 mm to 3 mm. 
     Optionally, in this embodiment of this application, as shown in  FIG. 6 b   , when the user wears the head-mounted display device, a distance L 6  between the human eyes and a lens of the head-mounted display device may be within a range of 8 mm to 30 mm. It should be noted that a thickness of the lens of the head-mounted display device in this application may change correspondingly with different design parameters of the module. For example, the thickness of the lens of the head-mounted display device may change at least between 3 mm and 15 mm with different field of view parameters. 
     Optionally, in this embodiment of this application, each element included in the display device module may be fastened at a corresponding position by disposing a fastening element. 
     For example, in this embodiment of this application, a side that is of the display panel  301  and that faces away from a light emitting surface may be a bearing surface, and the display panel  301  may be fastened to a housing of the head-mounted display device by using the bearing surface. 
     Optionally, in this embodiment of this application, the freeform prism  302  may be fastened to the display panel through mechanical connection. 
     Optionally, in this embodiment of this application, the first optical element  303  may be connected to the freeform prism  302  through positioning by using a V-shaped groove, or the first optical element  303  may be fastened to the freeform prism  302  by using a bearing surface that is extended outside a clear aperture region. 
     Optionally, in this embodiment of this application, the second optical element  304  may be connected to the freeform prism  302  through positioning by using a V-shaped groove, or the second optical element  304  may be fastened to the freeform prism  302  by using a bearing surface that is extended outside a clear aperture region. 
     It should be noted that the foregoing manner of fastening the optical element is merely an example, and does not constitute a limitation on this application. 
     This embodiment of this application provides the head-mounted display device, including the display panel  301 , the freeform prism  302 , the first optical element  303 , and the second optical element  304 . The freeform prism  302  includes the incident surface  3022 , the at least one reflective surface  3021 , and the emergent surface  3024 , and the incident surface  3022  of the freeform prism  302  faces the display panel  301 , so that light emitted by the display panel  301  is transmitted through the incident surface  3022  and transmitted out of the freeform prism  302  from the emergent surface  3024  through reflection by the at least one reflective surface  3021 . The first optical element  303  is configured to receive and reflect the light transmitted from the emergent surface  3024 , where an included angle is provided between the first optical element  303  and the second optical element  304 . The second optical element  304  is configured to reflect, back to the first optical element  303 , the light reflected by the first optical element  303 , so that the light is transmitted from the first optical element  303 . Compared with a manner in which physical glass or plastic is used for an optical structure in front of the eyes, and therefore a compensation prism needs to be added to correct distorted imaging of an external scene, in this embodiment of this application, because a gap is left between the first optical element  303  and the second optical element  304 , natural ambient light propagating to the human eyes is not refracted, imaging of the external scene is not distorted, and no additional compensation prism needs to be added. Therefore, a thickness of the display module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. 
     Optionally, referring to  FIG. 6 c   , in this embodiment of this application, light that is emitted by the display panel  301  and that is in a same field of view may converge in a first direction, to form a linear image  305  on an optical path between the reflective surface  3021  and the first optical element  303 . The first direction is perpendicular to a plane formed by a horizontal viewing direction and a direction in which a line between the two eyes is located when the head-mounted display device is worn. 
     It should be noted that the light in the same field of view in this application may be understood as light emitted by a same light emitting point on the display panel  301 . 
     Further, a linear image may be formed on an optical path between the emergent surface  3024  and the first optical element  303 . 
     The first direction in this embodiment of this application may be a vertically downward (or referred to as the plumb direction) direction (references may be made to a first direction shown in  FIG. 6 e   ) existing when the user wears the head-mounted display device. Light reflected by the reflective surface  3021  converges on a plumb surface and does not converge in the horizontal direction, to form a plurality of linear images on the optical path between the reflective surface  3021  and the first optical element  303 . Each linear image is formed through convergence and intersection of light corresponding to a same field of view. 
     In this embodiment of this application, geometric shapes, relative positions, and used materials of the at least one reflective surface, the incident surface, and the emergent surface may be changed, so that the freeform prism  302  generates different focal power in the first direction and a second direction (references may be made to a second direction shown in  FIG. 6 e   ) for incident light of the display panel  301 . In this way, after passing through the freeform prism  302 , light from any point on the display panel  301  intersects in the first direction, and does not intersect in the second direction at any position from the first optical element  303  to the reflective surface  3021 . 
     It should be noted that the first direction and the second direction in this embodiment of this application are directions perpendicular to each other. The first direction is a vertically downward (or referred to as the plumb direction) direction existing when the user wears the head-mounted display device, and the second direction is a direction in which a line between two lenses is located when the user wears the head-mounted display device. 
     In this embodiment of this application, the first optical element  303  may be asymmetric in the plumb direction, the second optical element  304  may be asymmetric in the plumb direction, and the second optical element may be symmetric in the horizontal direction. Because the light reflected by the reflective surface  3021  converges in the first direction, a “sag” facing the second optical element exists in lowermost light in the light reflected by the reflective surface. As shown in  FIG. 6 d   , an upper edge of the second optical element  304  may be closer to the first optical element  303  when the light reflected by the reflective surface  3021  is not blocked, so that the gap between the first optical element  303  and the second optical element  304  is reduced. Therefore, a thickness of the display device module in the head-mounted display device in a viewing direction of the human eyes is reduced, and a compact design of the head-mounted display device is implemented. 
     Optionally, in this embodiment of this application, the display device module may further include a compensation prism  401 . The compensation prism  401  is disposed on an optical path between the display panel  301  and the first optical element  303 , and materials of the compensation prism  401  and the freeform prism  302  are different. 
       FIG. 7 a    is a schematic structural diagram of a display device module according to an embodiment of this application. As shown in  FIG. 7 a   , the compensation prism  401  having a different material from the freeform prism  302  may be disposed on the emergent surface  3024  of the freeform prism  302  to correct chromatic aberration. A refractive index of the compensation prism  401  is different from a refractive index of the freeform prism  302 . For example, a material with a low refractive index may be selected for the freeform prism  302 , and correspondingly, a material with a high refractive index may be selected for the compensation prism  401 . Alternatively, a material with a high refractive index may be selected for the freeform prism  302 , and correspondingly, a material with a low refractive index may be selected for the compensation prism  401 . 
     Optionally, a dispersion coefficient of the compensation prism  401  is different from a dispersion coefficient of the freeform prism  302 . For example, a material with a low dispersion coefficient may be selected for the freeform prism  302 , and correspondingly, a material with a high dispersion coefficient may be selected for the compensation prism  401 . Alternatively, a material with a high dispersion coefficient may be selected for the freeform prism  302 , and correspondingly, a material with a low dispersion coefficient may be selected for the compensation prism  401 . 
     Optionally, in another embodiment,  FIG. 7 b    is a schematic structural diagram of a display device module according to an embodiment of this application. As shown in  FIG. 7 b   , the compensation prism  401  having a different material from the freeform prism  302  may be disposed on the incident surface  3022  of the freeform prism  302  to correct chromatic aberration. A refractive index of the compensation prism  401  is different from a refractive index of the freeform prism  302 . For example, a material with a low refractive index may be selected for the freeform prism  302 , and correspondingly, a material with a high refractive index may be selected for the compensation prism  401 . Alternatively, a material with a high refractive index may be selected for the freeform prism  302 , and correspondingly, a material with a low refractive index may be selected for the compensation prism  401 . 
     Optionally, a dispersion coefficient of the compensation prism  401  is different from a dispersion coefficient of the freeform prism  302 . For example, a material with a low dispersion coefficient may be selected for the freeform prism  302 , and correspondingly, a material with a high dispersion coefficient may be selected for the compensation prism  401 . Alternatively, a material with a high dispersion coefficient may be selected for the freeform prism  302 , and correspondingly, a material with a low dispersion coefficient may be selected for the compensation prism  401 . 
     Optionally, in this embodiment of this application, the compensation prism  401  may be attached to the light emitting surface of the display panel  301 , attached to the incident surface of the freeform prism  302 , attached to the reflective surface  3021  of the freeform prism  302 , or attached to the emergent surface of the freeform prism  302 . 
     Optionally, in this embodiment of this application, the compensation prism  401  may be fastened to the housing of the head-mounted display device by using a bearing surface, fastened to the display panel through mechanical connection, or connected to the freeform prism through positioning by using a V-shaped groove. 
     It should be noted that the foregoing manner of fastening the compensation prism  401  is merely an example. This is not limited in this application. 
     It should be understood that, provided that there is no technical contradiction, for the related descriptions of the display device module provided in  FIG. 6 a    to  FIG. 7 b   , refer to the descriptions of the display device module in  FIG. 3 a    to  FIG. 5  and the related embodiments. For example, the display device module provided in  FIG. 6 a    to  FIG. 7 b    may also include the fastening system in  FIG. 3 a    to  FIG. 5  and the related embodiments. 
       FIG. 8  is a schematic structural diagram of a display device module according to an embodiment of this application. As shown in  FIG. 8 , the display device module further includes a first polarization beam splitter  501 , a first phase retarder  502 , and a second phase retarder  503 . 
     The first polarization beam splitter  501  is disposed on the surface that is of the first optical element  303  and that faces the second optical element  304 , the first phase retarder  502  is disposed on a surface that is of the first polarization beam splitter  501  and that faces the second optical element  304 , and the second phase retarder  503  is disposed on the optical path between the display panel  301  and the first optical element  303 . 
     In this embodiment of this application, the first phase retarder  502  and the second phase retarder  503  may be quarter-wave plates. 
     Optionally, in this embodiment of this application, the second phase retarder  503  may be disposed between the display panel  301  and the incident surface  3022  of the freeform prism  302 . 
     Optionally, in an embodiment, the second phase retarder  503  (the quarter-wave plate) may be attached to the light emitting surface of the display panel  301 . In this embodiment, light emitted by the display panel  301  passes through the second phase retarder  503  (the quarter-wave plate), and becomes a type of circularly polarized light, for example, left-handed circularly polarized light. The first polarization beam splitter  501  is disposed on the surface that is of the first optical element  303  and that faces the second optical element  304 , and the first phase retarder  502  is disposed on the surface that is of the first polarization beam splitter  501  and that faces the second optical element  304 . The first polarization beam splitter  501  may reflect light in a polarization state and transmit light in another polarization state, for example, reflect S polarized light and transmit P polarized light (or reflect P polarized light and transmit S polarized light). An example in which the first polarization beam splitter  501  may reflect the S polarized light and transmit the P polarized light is used for description in this application. 
     When light passes through the first phase retarder  502  (the quarter-wave plate) for the first time, the light becomes a type of linearly polarized light, for example, S polarized light. When the light reaches the first polarization beam splitter  501 , the light is reflected, then passes through the first phase retarder  502  (the quarter-wave plate) again, and becomes circularly polarized light, for example, left-handed polarized light. When the light reaches the second optical element  304 , the light is reflected by the second optical element  304 , and a circularly polarization state is changed, for example, the light is changed to right-handed polarized light. The light passes through the first phase retarder  502  (the quarter-wave plate) again, and becomes linearly polarized light, for example, P polarized light. The light passes through the first polarization beam splitter  501  again. In this case, a polarization state of the light is different from a polarization state existing when the light (the light reflected by the reflective surface  3021  of the freeform prism  302 ) is incident for the first time. The light is transmitted when passing through the first polarization beam splitter  501 . 
     It is clearly that, only a combination form of the polarization beam splitter and the quarter-wave plate is described in the foregoing embodiment. Another form should also fall within the protection scope of this patent. For example, the polarization beam splitter and the quarter-wave plate are placed at another position on the optical path, or the polarization beam splitter and the quarter-wave plate are separately placed at different positions on the optical path when a prism having a different material is added in front of or behind the freeform prism. This is not limited in this application. For example, the second phase retarder may be further disposed between the display panel and the first optical element, the second phase retarder may be further disposed on the reflective surface, or the second phase retarder may be further disposed on the emergent surface. 
     It may be clearly understood by a person skilled in the art that, for the purpose of convenient and brief description, for a detailed working process of the foregoing system, apparatus, and unit, refer to a corresponding process in the foregoing method embodiments, and details are not described herein again. 
     In the several embodiments provided in this application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described apparatus embodiment is merely an example. For example, the unit division is merely logical function division and may be other division in actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms. 
     The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed on a plurality of network units. Some or all of the units may be selected based on actual requirements to achieve the objectives of the solutions of the embodiments. 
     In addition, functional units in the embodiments of this application may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units are integrated into one unit. The integrated unit may be implemented in a form of hardware, or may be implemented in a form of a software functional unit. 
     When the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, the integrated unit may be stored in a computer-readable storage medium. Based on such an understanding, the technical solutions of this application essentially, or the part contributing to the prior art, or all or some of the technical solutions may be implemented in the form of a software product. The software product is stored in a storage medium and includes several instructions for instructing a computer device (which may be a personal computer, a server, or a network device) to perform all or some of the steps of the methods described in the embodiments of this application shown in  FIG. 2 . The foregoing storage medium includes: any medium that can store program code, such as a USB flash drive, a removable hard disk, a read-only memory (read-only memory, ROM), a random access memory (random access memory, RAM), a magnetic disk, or an optical disc. 
     In conclusion, the foregoing embodiments are merely intended for describing the technical solutions of this application, but not for limiting this application. Although this application is described in detail with reference to the foregoing embodiments, persons of ordinary skill in the art should understand that they may still make modifications to the technical solutions described in the foregoing embodiments or make equivalent replacements to some technical features thereof, without departing from the scope of the technical solutions of the embodiments of this application.