Projection device

Abstract

A projection device comprises an imaging module and a light guide plate. The imaging module is used for generating parallel beams; the light guide plate comprises a light-in surface, a light-out surface and a plurality of transflective interfaces, the light-in surface is coupled to the imaging module, the light-in surface and the light-out surface form a first oblique angle, the transflective interfaces are arranged in the light guide plate, the transflective interfaces and the light-out surface form a second oblique angle, and the transflective interfaces are sequentially arranged along an optical axis parallel to the light-out surface.

Description

Technical Field

[0001] This application relates to an optical device, and more particularly to an optical projection device. Background Technology

[0002] Traditional Augmented Reality Head-Up Displays (AR HUDs) have a significant problem: the virtual image produced by the imaging components is relatively close, creating a large discrepancy between the distance of the image and the distance of objects in the driver's environment. When the driver switches focus, their eyes must frequently adjust between the close-up virtual image and distant objects, leading to eye strain. This frequent focus adjustment can cause discomfort, especially during long drives, and may even cause the driver's attention to drift away from external objects, affecting driving safety. Therefore, effectively improving the visual difference between the virtual image distance and the distance of objects in the environment has become a major challenge in the development of AR HUD technology. Summary of the Invention

[0003] In view of this, the applicant proposes a projection device comprising an imaging module and a light guide plate. The imaging module is used to generate a parallel light beam. The light guide plate includes an incident surface, an exiting surface, and a plurality of transflective interfaces. The incident surface is coupled to the imaging module and forms a first oblique angle with the exiting surface. The plurality of transflective interfaces are disposed within the light guide plate and form a second oblique angle with the exiting surface, and are arranged sequentially along an optical axis parallel to the exiting surface.

[0004] In one embodiment of the projection device of this application, a reflective plate is included, which is disposed above the light-emitting surface of the light guide plate and forms a third oblique angle with the light-emitting surface.

[0005] In one embodiment of the projection device of this application, the first oblique angle is less than 90 degrees and greater than the critical angle for total internal reflection of the parallel beam at the light guide plate.

[0006] In one embodiment of the projection device of this application, the light guide plate is composed of multiple light guide media, and each of the multiple transflective interfaces is a coated surface between the various light guide media.

[0007] In one embodiment of the projection device of this application, the light guide plate includes a cavity and a reflective surface, the light emitting surface of the light guide plate is a transmissive-reflective surface, the cavity is located between the transmissive-reflective surface and the reflective surface, and the plurality of transmissive-reflective interfaces are a plurality of transmissive-reflective mirrors within the cavity.

[0008] In one embodiment of the projection device of this application, the spacing between each of the plurality of transflective interfaces along the optical axis is equal.

[0009] In one embodiment of the projection device of this application, the plurality of transflective interfaces includes N transflective interfaces, and the reflectivity of each transflective interface is expressed as:

[0010]

[0011] Where R is the reflectivity, and i is the order in which the distance between each of the transflective and reflective interfaces and the incident surface is closest.

[0012] In one embodiment of the projection device of this application, the imaging module includes a display screen and a lens group. The display screen is used to generate a diverging light beam, and the lens group is used to convert the diverging light beam into the parallel light beam. The lens group sequentially includes a first biconvex lens, a second biconvex lens, a first biconcave lens, a second biconcave lens, a concave-convex lens, and a third biconvex lens.

[0013] In one embodiment of the projection device of this application, the imaging module includes a prism, which is disposed between the display screen and the lens group, and the display screen is coupled to the light-incident surface of the prism.

[0014] In one embodiment of the projection device of this application, the imaging module includes a plurality of displays, a light combining component, and a lens group. Each of the displays is used to generate a diverging monochromatic light beam. The light combining component is disposed between the plurality of displays and the lens group, and each of the displays is coupled to a different light-incident surface of the light combining component. The light combining component is used to mix the plurality of diverging monochromatic light beams to generate a diverging combined light beam. The lens group is used to convert the diverging combined light beam into the parallel light beam. Attached Figure Description

[0015] Figure 1 This is a diagram illustrating the current usage status of existing projection equipment.

[0016] Figure 2 This is a schematic diagram of a projection device according to some embodiments of this application.

[0017] Figure 3A This is a schematic diagram of a light guide plate according to some embodiments of this application.

[0018] Figure 3B This is a schematic diagram of a light guide plate according to other embodiments of this application.

[0019] Figure 4 This is a schematic diagram of an imaging module according to the first embodiment of this application.

[0020] Figure 5A This is a modulation conversion function diagram of an imaging module according to some embodiments of this application.

[0021] Figure 5B These are field curvature diagrams of imaging modules according to some embodiments of this application.

[0022] Figure 5C These are distortion images of imaging modules according to some embodiments of this application.

[0023] Figure 6 This is a schematic diagram of an imaging module according to a second embodiment of this application.

[0024] Figure 7 This is a schematic diagram of an imaging module according to a third embodiment of this application.

[0025] In the attached figures, the following reference numerals are used:

[0026] 10: Projection device

[0027] 1: Imaging module

[0028] 11: Display screen

[0029] 12: Lens Group

[0030] 121: First biconvex lens

[0031] 122: Second biconvex lens

[0032] 123: First biconcave lens

[0033] 124: Second biconcave lens

[0034] 125: Concave and convex lenses

[0035] 126: Third biconvex lens

[0036] 13: Prism

[0037] 131: Surface receiving light

[0038] 14: Photosynthetic Components

[0039] 141: Optical combining prism

[0040] 142: Coated surface

[0041] 143: Filter

[0042] 15: Protection board

[0043] 2: Light guide plate

[0044] 21: Light-receiving surface

[0045] 22: Exposed surface

[0046] 23: Transparent Interface

[0047] 231: Transparent and Reflective Lens

[0048] 24: Light guiding medium

[0049] 25: Coated surface

[0050] 26: Cavity

[0051] 27: Reflective surface

[0052] 28: See the reverse side

[0053] 3: Transparent plate

[0054] 91: Projection equipment

[0055] 92:Virtual image

[0056] Ld: Diverging Beam

[0057] Ld1: Red diverging beam

[0058] Ld2: Green diverging beam

[0059] Ld3: Blue diverging beam

[0060] Lp: parallel beam

[0061] Le: External beam

[0062] θ1: First oblique angle

[0063] θ2: Second oblique angle

[0064] θ3: Third oblique angle

[0065] S1~S16: Surface

[0066] X1: Optical axis

[0067] d: Spacing Detailed Implementation

[0068] Figure 1 This is a diagram illustrating the current usage status of the projection equipment. Please refer to it. Figure 1 The projection device 91 is positioned in front of the vehicle's windshield, and its projected light beam is reflected by the windshield to the driver. The reflected light beam from the windshield forms a virtual image 92 behind the mirror, with the image distance equivalent to the distance between the projection device 91 and the windshield. When the driver observes the virtual image 92, they must be within a specific viewing angle range to receive the reflected light beam from the windshield.

[0069] Figure 2 These are schematic diagrams of projection devices according to some embodiments of this application; Figure 3A These are schematic diagrams of light guide plates according to some embodiments of this application, please refer to them as well. Figure 2 and Figure 3AThe projection device 10 includes an imaging module 1 and a light guide plate 2, with the light incident surface 21 of the light guide plate 2 coupled to the imaging module 1. In this embodiment, the imaging module 1 generates a parallel beam Lp, which enters the light guide plate 2 through the light incident surface 21 and partially penetrates each transflection / reflection port 23 within the light guide plate 2, and is partially reflected. Finally, all the reflected parallel beams Lp are projected onto the observer. The projection device 10 can be placed in front of partially translucent glass, such as, but not limited to, a vehicle's windshield. In this embodiment, as... Figure 2 As shown, the projection device 10 includes an imaging module 1, a light guide plate 2, and a reflective plate 3. The reflective plate 3 is disposed above the light-emitting surface 22 of the light guide plate 2, and the reflective plate 3 forms a third oblique angle θ3 with the light-emitting surface 22 of the light guide plate 2. In this way, the parallel light beam Lp from the light-emitting surface 22 of the light guide plate 2 is partially reflected to the observer by the reflective plate 3. Furthermore, the observer can also receive an external light beam Le through the reflective plate 3, thus simultaneously observing the image formed by the parallel light beam Lp and the external light beam Le.

[0070] like Figure 3A As shown, the light guide plate 2 includes an incident surface 21, an exiting surface 22, and multiple transflective interfaces 23. In this embodiment, the incident surface 21 is disposed on the side of the light guide plate 2, and the incident surface 21 and the exiting surface 22 form a first oblique angle θ1. Therefore, when a parallel beam Lp passes orthogonally through the incident surface 21, the incident angle and reflection angle of the parallel beam Lp at the exiting surface 22 are also the first oblique angle θ1. The larger the first oblique angle θ1, the more transflective interfaces 23 the parallel beam Lp can pass through, thus widening the observable range of the image; the smaller the first oblique angle θ1, the higher the proportion of light reflected by each transflective interface 23 from the parallel beam Lp, thus increasing the visibility of the image. Multiple transflective interfaces 23 are disposed within the light guide plate 2 and arranged sequentially along the optical axis X1, wherein the direction of the optical axis X1 corresponds to the projection component of the traveling direction of the parallel beam Lp onto the exiting surface 22. In this embodiment, the light guide plate 2 includes five transflective interfaces 23 arranged sequentially along the optical axis X1. Figure 3A They are arranged in order from left to right.

[0071] Each reflective interface 23 forms a second oblique angle θ2 with the light-emitting surface 22. When the parallel beam Lp passes through the reflective interface 23, partial reflection and partial transmission occur. The larger the second oblique angle θ2, the more reflective interfaces 23 the parallel beam Lp can pass through, thus widening the observable range of the image; the smaller the second oblique angle θ2, the higher the proportion of light reflected by each reflective interface 23 from the parallel beam Lp, thus increasing the visibility of the image. In this embodiment, the incident angle of the parallel beam Lp at the first reflective interface 23 is equivalent to the first oblique angle θ1 minus the second oblique angle θ2. In some embodiments, the first oblique angle θ1 and the second oblique angle θ2 are not equal to avoid the parallel beam Lp passing orthogonally through one or more reflective interfaces 23 without reflection. In some embodiments, twice the sum of the second oblique angle θ2 and the third oblique angle θ3 minus the first oblique angle θ1 equals 90 degrees (i.e., 2*(θ2+θ3)-θ1=90°), so that the reflected beam of the reflective plate 3 is parallel to the light guide plate 2, allowing the observer to observe the projected image to the maximum extent from a level viewing angle.

[0072] In some embodiments, the light guide plate 2 is composed of multiple light guiding media 24, and multiple transflective interfaces 23 are the coated surfaces 25 between the light guiding media 24. In this embodiment, the light guide plate 2 includes six groups of light guiding media 24 and five coated surfaces 25 between the light guiding media 24. The material of the light guiding media 24 may be, but is not limited to, polymethyl methacrylate (PMMA), polycarbonate (PC), polystyrene (PS), quartz glass (SiO2), multimode or single-mode optical fiber, fluoride glass, polymer optical fiber (POF), sapphire crystal (Al2O3), or lithium niobate (LiNbO3). In some embodiments, the first oblique angle θ1 is less than 90 degrees and greater than the critical angle θc for total internal reflection of the parallel beam Lp at the light guide plate 2. The critical angle θc for total internal reflection of the parallel beam Lp needs to take into account the refractive index n1 of the light guide medium 24 and the refractive index n2 of the external medium (assuming it is air, then the refractive index n2 is 1), which can be expressed as the following formula:

[0073]

[0074] Therefore, when the first oblique angle θ1 is greater than the critical angle for total internal reflection of the parallel beam Lp at the light guide plate 2, the parallel beam Lp can travel along the optical axis X1 within the light guide plate 2 to the maximum extent without undergoing refraction during reflection at the light emitting surface 22, thus avoiding energy loss. This allows the beam to pass through the most transflection / reflection interfaces 23, resulting in the widest possible image viewing range. In some embodiments, the surface of the light emitting surface 22 can also be coated to form a coated surface 25 with a refractive index n2. In this embodiment, the critical angle for total internal reflection of the parallel beam Lp takes into account both the refractive index of the light guide medium 24 and the coating of the light emitting surface 22.

[0075] In some embodiments, it is assumed that the light guide plate 2 includes N transflection / reflection interfaces 23, and the reflectivity of each transflection / reflection interface 23 can be expressed by the following formula:

[0076]

[0077] Where R is the reflectivity, and i is the order in which the light-transmitting and reflective interfaces 23 are closest to the light-incident surface 21 of the light guide plate 2. For example, such as Figure 3A As shown, the light guide plate 2 in this embodiment includes five reflective interfaces 23. The rightmost reflective interface 23 is numbered 1 (i.e., i=1), the next rightmost reflective interface 23 is numbered 2 (i.e., i=2), and the leftmost reflective interface 23 is numbered 5 (i.e., i=5). Therefore, the reflectivity of each reflective interface 23 can be expressed as follows:

[0078] R1 = 0.2

[0079]

[0080]

[0081]

[0082]

[0083] Therefore, when light passes through each reflective interface 23, the reflected light energy is the same, ensuring that the visibility of the image seen by the observer is the same at different viewing angles. In some embodiments, when the reflective interface 23 is a coated surface 25 between light guiding media 24, the refractive index of each coated material gradually increases in sequence; or, the number of interference coating layers on each coated surface 25 gradually increases in sequence.

[0084] In some embodiments, the spacing d between the plurality of transflective interfaces 23 along the optical axis X1 is equal. More specifically, as... Figure 3AAs shown, the five reflective interfaces 23 of the light guide plate 2 are all spaced by a distance d. This makes the distance between the reflected light generated by the parallel beam Lp when it passes through these reflective interfaces 23 the same, thus producing an image that is evenly distributed in space. This prevents the image from suddenly disappearing or becoming blurry when the observer adjusts the viewing angle.

[0085] Figure 3B This is a schematic diagram of a light guide plate according to other embodiments of this application. Please refer to... Figure 3B In this embodiment, the light guide plate 2 includes a cavity 26 and a reflective surface 27, and the light-emitting surface 22 of the light guide plate 2 is a transmissive-reflective surface 28. The cavity 26 is located between the transmissive-reflective surface 28 and the reflective surface 27, and the cavity 26 contains a plurality of transmissive-reflective mirrors 231. The cavity 26 can be a vacuum or filled with other media, such as air. The material of the reflective surface 27 can be, but is not limited to, aluminum, silver, gold, silicon dioxide (SiO2), titanium dioxide (TiO2), magnesium fluoride (MgF2), aluminum oxide, silicon nitride, polycarbonate, or polytetrafluoroethylene (PTFE). Similar to... Figure 3A In this embodiment, the light-incident surface 21 of the light guide plate 2 is disposed on the side of the light guide plate 2, and the light-incident surface 21 forms a first oblique angle θ1 with the transmissive surface 28. Therefore, when a parallel beam Lp passes orthogonally through the light-incident surface 21, the incident angle and reflection angle of the parallel beam Lp at the transmissive surface 28 are also the first oblique angle θ1. Transmissive mirrors 231 are arranged sequentially along an optical axis X1 parallel to the transmissive surface 28, and each transmissive mirror 231 forms a second oblique angle θ2 with the transmissive surface 28. In some embodiments, the spacing d between the plurality of transmissive mirrors 231 along the optical axis X1 is equal.

[0086] Figure 4 This is a schematic diagram of the imaging module according to the first embodiment of this application. Please refer to... Figure 4In this embodiment, the imaging module 1 includes a display screen 11 and a lens group 12. The display screen 11 is disposed at one end of the lens group 12, and the light guide plate 2 is disposed at the other end of the lens group 12. Here, the display screen 11 is used to generate a diverging light beam Ld. The display screen 11 can be, but is not limited to, a liquid crystal display (TFT LCD), an organic light-emitting diode (OLED), a micro LED, or an LED. Although the display screen 11 acts as a planar light source under ideal conditions, in reality, the display screen 11 generates light rays in other directions in addition to those orthogonal to the display screen 11, thus forming a diverging light beam Ld. Furthermore, in some embodiments, the size of the display screen 11 does not match the size of the light incident surface 21 of the light guide plate 2, thus requiring adjustment of the image size generated by the display screen 11.

[0087] Lens group 12 is used to convert the diverging light beam Ld into a parallel light beam Lp, so that the focal length of the virtual image presented to the observer by the projection device 10 is at infinity, thereby reducing the visual difference between the virtual image distance and objects in the external environment. On the other hand, lens group 12 can also be used to reduce the image generated by the display screen 11 to produce a high-resolution image that can be projected through the light guide plate 2. The lens material of lens group 12 can be quartz or plastic. In this embodiment, as... Figure 4 As shown, the lens group 12, from left to right, includes a first biconvex lens 121, a second biconvex lens 122, a first biconcave lens 123, a second biconcave lens 124, a concave-convex lens 125, and a third biconvex lens 126. In this embodiment, the parameters of the lens group 12 are presented in Table 1 (attached at the end of the document). Specifically, the first biconvex lens 121 includes a light-emitting surface S1 and a light-incident surface S2; the second biconvex lens 122 includes a light-emitting surface S3 and a light-incident surface S4; the first biconcave lens 123 includes a light-emitting surface S5 and a light-incident surface S6; the second biconcave lens 124 includes a light-emitting surface S7 and a light-incident surface S8; the concave-convex lens 125 includes a light-emitting surface S9 and a light-incident surface S10; and the third biconvex lens 126 includes a light-emitting surface S11 and a light-incident surface S12. In this embodiment, the concave-convex lens 125 and the third biconvex lens 126 converge the divergent beam Ld generated by the display screen 11 to reduce the cross-sectional area of ​​the beam. The first biconcave lens 123 and the second biconcave lens 124 work together with the former two to eliminate image chromatic aberration. The first biconvex lens 121 and the second biconvex lens 122 gradually adjust the divergent beam Ld into a parallel beam Lp.

[0088] In some embodiments, the imaging module 1 includes a display screen 11, a lens group 12, and a prism 13, with the prism 13 disposed between the display screen 11 and the lens group 12. The display screen 11 is coupled to the light-incident surface 131 of the prism 13, such as... Figure 4 As shown, the display screen 11 is coupled to the light-incident surface 131 of the prism 13 via a protective plate 15. The prism 13 increases the distance between the display screen 11 and the lens group 12, allowing the image generated by the display screen 11 to be completely projected onto the surface S12 of the biconvex lens and converged, and preventing light from experiencing reflection loss or severe dispersion due to excessive refractive index differences when passing through the surface S12. The protective plate 15 prevents friction between the display screen 11 and the prism 13 due to vibration. In some embodiments, the protective plate 15 and the display screen 11, and / or the protective plate 15 and the prism 13, can be bonded together using optical adhesive. In this embodiment, the parameters of the lens group 12 are presented in Table 2 (attached at the end of the document).

[0089] Figure 5A This is a modulation conversion function diagram of an imaging module according to some embodiments of this application. Please refer to... Figure 5A . Figure 5A The horizontal axis represents the spatial frequency of the projected image from imaging module 1; the vertical axis represents the optical conversion function coefficients. Figure 5A Solid lines represent the modulation transfer function (MTF) of light rays at different object heights (0.00mm, 0.80mm, 2.40mm, 4.00mm, 5.60mm, 7.20mm, 7.62mm) on the meridional focal plane of the image displayed on screen 11; dashed lines represent the MTF of light rays at different object heights (0.00mm, 0.80mm, 2.40mm, 4.00mm, 5.60mm, 7.20mm, 7.62mm) on the sagittal focal plane. The MTF represents the contrast retention capability of imaging module 1 at different spatial frequencies; the MTF values ​​of each curve decrease as the spatial frequency increases. Figure 5A This demonstrates that for images with an object height of 5.6 mm or more at the meridional focal plane, the MTF of imaging module 1 rapidly drops below 0.7 within a spatial frequency range of 8.0 cycles / mm, while the MTF performance of other curves decreases slowly as the spatial frequency increases.

[0090] Figure 5B These are field curvature diagrams of imaging modules according to some embodiments of this application; Figure 5C These are distortion diagrams of imaging modules based on some embodiments of this application; please refer to them as well. Figure 5B and Figure 5C . Figure 5B The horizontal axis represents the offset of the imaging focal point from the paraxial focal plane; the vertical axis represents the offset of the imaging focal point along the Y-axis. Figure 2 The field of view (vertical direction) Figure 5BSolid lines represent the focal shift of light rays of different wavelengths (455nm, 528nm, 620nm) at the meridional focal plane; dashed lines represent the focal shift of light rays of different wavelengths (455nm, 528nm, 620nm) at the sagittal focal plane. Figure 5C The horizontal axis represents the percentage of distortion; the vertical axis represents the field of view at the imaging focal point along the Y-axis. In this embodiment, the maximum field of view is 18 degrees, the meridional field curvature and sagittal field curvature are both less than 0.15 mm, and the maximum distortion is within -1.2%.

[0091] Figure 6 This is a schematic diagram of the imaging module according to the second embodiment of this application. Please refer to... Figure 6 In some embodiments, the imaging module 1 includes multiple displays 11, a beam combining assembly 14, and a lens group 12. Each display 11 is used to generate a diverging monochromatic light beam. The beam combining assembly 14 is disposed between the multiple displays 11 and the lens group 12, and each display 11 is coupled to a different light-incident surface of the beam combining assembly 14. In this embodiment, the imaging module 1 includes three displays 11: a red light display 11 (generating a red diverging light beam Ld1), a green light display 11 (generating a green diverging light beam Ld2), and a blue light display 11 (generating a blue diverging light beam Ld3). In this embodiment, the beam combining assembly 14 is a beam combining prism 141, which can be composed of multiple sub-prisms, each sub-prism having a coated surface 142. This coated surface 142 has a filtering effect, allowing light of a specific wavelength to pass through while reflecting other wavelengths of light. For example, Figure 6 The coated surface 142 of the beam combining prism 141 allows the green diverging beam Ld2 to pass directly through, while reflecting the red diverging beam Ld1 and the blue diverging beam Ld3. The beam combining prism 141 mixes all monochromatic diverging beams Ld to produce a mixed-color diverging beam Ld, and the lens group 12 then converts the mixed-color diverging beam Ld into a parallel beam Lp. In this embodiment, the imaging module 1 displays different colors of light through multiple displays 11, allowing for more detailed representation of each color. In some embodiments, the refractive index of the beam combining prism 141 can be matched to the lens of the lens group 12 or the protective plate 15 of the display screen 11. In some embodiments, the display screen 11 and the beam combining prism 141, and / or the protective plate 15 and the beam combining prism 141, can be bonded together using optical adhesive.

[0092] Figure 7 This is a schematic diagram of an imaging module according to a third embodiment of this application. Please refer to... Figure 7In this embodiment, the imaging module 1 includes three displays 11, and the light combining assembly 14 is composed of multiple filters 143. Similar to the coated surface 142 of the light combining prism 141 in the second embodiment, the filters 143 in this embodiment have a filtering effect, allowing light of a specific wavelength to pass through while reflecting other wavelengths. Compared to the light combining prism 141, the light combining assembly 14 using filters 143 allows for lower component and manufacturing costs.

[0093] The proportions, structures, dimensions, and other features shown in the drawings of this application are only for illustrating the embodiments described in this application, to facilitate reading and understanding of this application by those skilled in the art, and are not intended to limit the scope of the claims of this application. Furthermore, any modifications, alterations, or adjustments to the content described in the foregoing embodiments, without affecting the purpose and effect of the invention, shall fall within the scope of the claims asserted in this application.

[0094] Table 1. Parameters of Lens Group 12 according to some embodiments of this application

[0095]

[0096] Table 2. Parameter table of the protective plate 15 and prism 13 according to some embodiments of this application

[0097]

[0098]

Claims

1. A projection device, characterized by The image module comprises: a light guide plate comprising an incident light surface, an emergent light surface and a plurality of transmissive-refractive interfaces, the incident light surface being coupled to the image module and the incident light surface and the emergent light surface forming a first oblique angle, the plurality of transmissive-refractive interfaces being arranged in the light guide plate and the plurality of transmissive-refractive interfaces and the emergent light surface forming a second oblique angle and being sequentially arranged along an optical axis parallel to the emergent light surface. The transmissive-refractive plate is arranged above the emergent light surface of the light guide plate and forms a third oblique angle with the emergent light surface.

2. The projection apparatus of claim 1, wherein The first oblique angle is less than 90 degrees and greater than the critical angle of total reflection of the parallel light beam in the light guide plate.

3. The projection apparatus of claim 1, wherein The light guide plate is composed of a plurality of light guide media, and each of the plurality of transmissive-refractive interfaces is a coated surface between each of the light guide media.

4. The projection apparatus of claim 1, wherein The light guide plate comprises a cavity and a reflective surface, the emergent light surface of the light guide plate is a transmissive-refractive surface, the cavity is between the transmissive-refractive surface and the reflective surface, and the plurality of transmissive-refractive interfaces are a plurality of transmissive-refractive lenses in the cavity.

5. The projection apparatus of claim 1, wherein The spacing of each of the plurality of transmissive-refractive interfaces along the optical axis is equal.

6. The projection apparatus of claim 1, wherein The plurality of transmissive-refractive interfaces comprises N transmissive-refractive interfaces, and the reflectivity of each of the transmissive-refractive interfaces is represented as:

7. The projection apparatus of claim 1, wherein wherein R is the reflectivity, and i is the order of the interval distance of each of the transmissive-refractive interfaces from the incident light surface. The image module comprises a display screen and a lens group, the display screen is used to generate a divergent light beam, the lens group is used to convert the divergent light beam into the parallel light beam, and the lens group sequentially comprises a first double convex lens, a second double convex lens, a first double concave lens, a second double concave lens, a meniscus lens, and a third double convex lens.

8. The projection apparatus of claim 1, wherein The image module comprises a prism arranged between the display screen and the lens group, and the display screen is coupled to the incident light surface of the prism.

9. The projection apparatus of claim 8, wherein The image module comprises a plurality of display screens, a light combining assembly, and a lens group, each of the display screens is used to generate a divergent monochromatic light beam, the light combining assembly is arranged between the plurality of display screens and the lens group, and each of the display screens is coupled to a different incident light surface of the light combining assembly, the light combining assembly is used to mix a plurality of the divergent monochromatic light beams to generate a divergent combined light beam, and the lens group is used to convert the divergent combined light beam into the parallel light beam.

10. The projection apparatus of claim 1, wherein ​

Images