Optical systems for displays

Abstract

The optical system (100, 111, 125, 126, 127, 128) includes an aperture-expanding optical element (LOE) (106) having main surfaces (103a, 103b) separated by a first thickness T1. The LOE (106) includes a redirection configuration for progressively redirecting the light within the LOE and coupling out the light toward the viewer. The coupling-in device includes a coupled optical element (CLE) (104) having mutually parallel surfaces separated by a second thickness T2 which is less than half the first thickness TL. The CLE (104) is bonded to the main surface (103a) at an interface (105) provided with a beam-splitter coating having at least 50% reflectivity. The coupling-in device also includes an input coupler deployed to couple light corresponding to a collimated image into the CLE.

Description

Technical Field

[0001] The present invention relates to an optical system for a display, and more particularly to a configuration for image injection into a light guide optical element having optical aperture expansion.

Background Art

[0002] A light guide-based display typically employs a light guide in the form of a slab having mutually parallel front and rear surfaces to direct an image in front of a user's eye and couple it out towards the eye for viewing. In some cases, the light guide can achieve one-dimensional or two-dimensional optical aperture expansion by gradually redirecting light within the light guide and / or within the coupling-out process. The gradual redirection of light is typically performed by a set of embedded partial reflectors or by a diffractive optical element.

[0003] Coupling an image into a light guide presents a design challenge. Optimal image uniformity is achieved when the image light "fills" the thickness of the light guide, i.e., when all the light rays of the image and its reflections are present at every point within the thickness of the light guide. This requires a relatively large projector and coupling configuration, which contradicts the practical goal of minimizing the size and weight of the system as much as possible.

Summary of the Invention

[0004] The present invention is an optical system.

[0005] According to the teaching of one embodiment of the present invention, an optical system is provided, the optical system being an aperture-expanding light guide optical element (LOE) having a pair of mutually parallel main surfaces separated by a first thickness, the LOE supporting the propagation of light by internal reflection at the main surfaces, the LOE having (i) a first redirection configuration deployed within a first region of the LOE for progressively redirecting light propagating in a first in-plane direction toward a second region of the LOE toward a second in-plane direction, and (ii) a second redirection configuration deployed within a second region of the LOE for progressively redirecting light propagating in a second in-plane direction toward the outward direction of the LOE for viewing by a viewer, the aperture-expanding light guide optical element being provided, the optical system being an aperture-expanding light guide optical element (LOE) having a pair of mutually parallel main surfaces separated by a first thickness, the LOE supporting the propagation of light by internal reflection at the main surfaces, the LOE having (i) a first redirection configuration deployed within a first region of the LOE for progressive redirection of light propagating in a second in-plane direction toward a second region of the LOE toward a second in-plane direction, and (ii) a second redirection configuration deployed within a second region of the LOE for viewing by a viewer, The coupling-in device includes (b) a photo-optical element (LOE), (i) a coupled light guide element (CLE) having a pair of mutually parallel surfaces separated by a second thickness of less than half a first thickness, wherein the parallel surfaces of the CLE have an area of ​​less than 10 percent of the area of ​​the main surface of the LOE, and one of the surfaces of the CLE is joined to one of the main surfaces of the LOE at an interface, and at least a portion of the interface is provided with a beam splitter coating having a reflectivity of at least 50%, and (ii) an input coupler deployed to couple light corresponding to a collimated image into the CLE.

[0006] According to further features of one embodiment of the present invention, the input coupler is a coupling prism that presents an input surface that is substantially perpendicular to the principal ray of the collimated image coupled into the CLE.

[0007] According to further features of one embodiment of the present invention, a first reversal configuration includes a first set of mutually parallel internal partial reflective surfaces that are not parallel to the main surface, and a second reversal configuration includes a second set of mutually parallel internal partial reflective surfaces that are angled obliquely to the main surface.

[0008] According to further features of one embodiment of the present invention, the CLE further comprises a preliminary aperture expansion device.

[0009] According to further features of one embodiment of the present invention, a preliminary aperture expansion device includes a set of mutually parallel internal partial reflective surfaces within the CLE for progressively redirecting light coupled by an input coupler.

[0010] According to further features of one embodiment of the present invention, the input coupler is deployed such that the principal rays of the collimated image coupled into the CLE propagate in a first in-plane direction after a single reflection from one of the internal partial reflection surfaces of the CLE.

[0011] According to a further feature of one embodiment of the present invention, the input coupler is deployed such that the principal rays of the collimated image coupled into the CLE propagate in the first in-plane direction after being reflected twice from the internal partial reflective surface of the CLE.

[0012] According to further features of one embodiment of the present invention, the preliminary aperture expansion type device comprises a diffractive optical element associated with the CLE.

[0013] According to a further feature of one embodiment of the present invention, the input coupler is deployed such that the principal rays of the collimated image coupled into the CLE propagate in a first in-plane direction after being redirected twice by diffraction in the diffractive optical element, thereby canceling out the chromatic dispersion generated by the first diffraction in the diffractive optical element.

[0014] According to further features of one embodiment of the present invention, the first reversal configuration of the LOE is a diffractive optical element configured to match the diffractive optical element of the CLE and cancel out the chromatic dispersion generated by the diffractive optical element of the CLE.

[0015] According to further features of one embodiment of the present invention, a portion of the interface between the CLE and LOE, under a preliminary aperture-expanding device, is provided with a highly reflective coating.

[0016] According to further features of one embodiment of the present invention, the input coupler is a first diffractive optical element associated with the surface of the CLE, and the second reversal configuration is a second diffractive optical element configured to match the first diffractive optical element and cancel out the chromatic dispersion generated by the first diffractive optical element.

[0017] According to further features of one embodiment of the present invention, the beam splitter coating has a reflectivity of 55% to 95%, and in a particularly preferred case, 65% to 90%.

[0018] According to further features of one embodiment of the present invention, the beam splitter coating has a reflectivity that progressively decreases along a first in-plane direction.

[0019] According to further features of one embodiment of the present invention, the second thickness is 20% to 40% of the first thickness.

[0020] The present invention is described herein merely as an example with reference to the accompanying drawings. [Brief explanation of the drawing]

[0021] [Figure 1A] This is a schematic isometric view of an optical system constructed and operable according to the teachings of the present invention, employing a coupled optical guide (CLE) for introducing a collimated image to the observer's eye as a light guide optical element (LOE). [Figure 1B] Figure 1A is a schematic plan view of the optical system. [Figure 2A] Figure 1A illustrates a first modified embodiment of the optical system of the present invention, and is similar to Figure 1A, employing a CLE with a preliminary aperture expansion achieved by an even number of reflections. [Figure 2B] A first modified embodiment of the optical system of the present invention is illustrated in a figure similar to Figure 1B, which employs a CLE with a preliminary aperture expansion achieved by an even number of reflections. [Figure 3A]A figure similar to FIG. 1A, which illustrates a second modified embodiment of the optical system of the present invention, and employs a CLE having a preliminary aperture expansion achieved by an odd number of reflections. [Figure 3B] A figure similar to FIG. 1B, which illustrates a second modified embodiment of the optical system of the present invention, and employs a CLE having a preliminary aperture expansion achieved by an odd number of reflections. [Figure 4A] A schematic isometric view similar to FIG. 3A, which illustrates a modified embodiment where the CLE is subdivided into two parts for continuous aperture expansion and is coupled to the LOE. [Figure 4B] A schematic side view of FIG. 4A. [Figure 5A] A figure similar to FIG. 4A, which illustrates a further modified embodiment of the optical system of the present invention, and employs a pair of optically matched diffractive optical elements (DOEs), one within the CLE and one within the LOE. [Figure 5B] A figure similar to FIG. 4B, which illustrates a further modified embodiment of the optical system of the present invention, and employs a pair of optically matched diffractive optical elements (DOEs), one within the CLE and one within the LOE. [Figure 6] A schematic isometric view similar to FIG. 4A, which illustrates a further modified embodiment of the optical system of the present invention, where the coupling-in of light into the CLE and the coupling-out of light from the LOE are achieved by a pair of optically matched DOEs.

Embodiments for Carrying Out the Invention

[0022] The present invention is an optical system and a corresponding method for manufacturing the optical system.

[0023] The principle and operation of the optical system according to the present invention, as well as the corresponding method, can be better understood by referring to the drawings and the accompanying specification. ​​Referring here to the drawings, Figures 1A to 6 schematically illustrate various embodiments of optical systems that can be constructed and operated in accordance with the teachings of the present invention. Generally, the optical system includes an aperture-expanding optical element (LOE) 106 having a pair of mutually parallel main surfaces 103a, 103b separated by a first thickness T1, which supports the propagation of light by internal reflection at the main surfaces. The LOE 106 includes a first redirection configuration deployed within a first region 107 of the LOE for progressively redirecting light propagating in a first in-plane direction D1 to propagate in a second in-plane direction D2 toward a second region 109 of the LOE, and a second redirection configuration deployed within a second region 109 of the LOE for progressively redirecting light propagating in a second in-plane direction D2 toward the outward direction (direction D3) of the LOE for viewing by an observer.

[0025] A coupling-in device is deployed for merging a projected image from a projector (not shown) into the LOE 106. The coupling-in device includes a pair of mutually parallel surfaces having a second thickness T2 that is less than half of a first thickness T1 and, in particular, especially preferred cases, 20% to 40% of the first thickness T1. The parallel surfaces of the CLE 104 have an area of ​​less than 20 percent, preferably less than 10 percent, of the area of ​​the main surfaces 103a, 103b. One of the parallel surfaces of the CLE 104 is bonded to one of the main surfaces 103a of the LOE 106 at an interface 105, and at least a portion of it is provided with a beam splitter coating having a reflectivity of at least 50%. The coupling-in device also includes an input coupler deployed to couple light corresponding to the collimated image into the CLE. In a first set of particularly preferred embodiments illustrated in Figures 1A to 5B, the input coupler for the input principal ray D0a is a coupling prism that presents an input surface substantially perpendicular to the principal ray of the collimated image coupled to the CLE. Alternatively, this prism may be a reflective prism, for example, coated with a 100% reflector, for coupling with the principal ray D0b, which strikes from the opposite side of the light guide 106, in the input image. In the following discussion, for the sake of brevity of presentation, only the input direction D0a options will be referred to.

[0026] The basic functionality of this aspect of the present invention can be understood by referring to Figures 1A and 1B, which illustrate an optical system 100 constructed and operable according to a first embodiment of the present invention. The general propagation of the beam (corresponding to the principal rays of the collimated image and showing the "in-plane component" within the photoguide) is shown as block arrows in Figure 1A, while specific rays are shown as black arrows. In the plan view of Figure 1B, this distinction is not visible. The light collides onto a prism 102a that couples the light into the CLE 104. Because the CLE 104 is thinner than the LOE 106, the coupling-in interface can be smaller, allowing the use of a smaller projector (not shown) than otherwise required for the LOE. The partial reflection interface 105 between CLE104 and LOE106 is a partial reflector, and as light is induced within CLE104, it causes a gradual "leak" of light from CLE104 to LOE106, thereby resulting in relatively uniform illumination across the thickness of LOE106, effectively expanding the aperture along the thickness of the light guide and helping to "fill" the thickness of the LOE with the projected image.

[0027] In the preferred embodiment illustrated herein, the first redirection configuration is implemented as a first set of mutually parallel internal partial reflective surfaces 108 that are not parallel to the main surface 103a, and the second redirection configuration is implemented as a second set of mutually parallel internal partial reflective surfaces 110 that are angled obliquely to the main surface 103a. The partial reflective surfaces 108 progressively reflect and redirect guided light propagating in the first in-plane direction D1 within the first region 107 so as to propagate toward the second region 109, thereby achieving a first dimension of aperture expansion along direction D1. The partial reflective surfaces 110 progressively combine image light from the light guide within the second region 109 for user viewing, thereby achieving a second dimension of aperture expansion along direction D2.

[0028] In addition to reducing the thickness of the entrance pupil, the coupling-in configuration described herein is also advantageous due to its ease of manufacture. The coupling-in configuration is a small assembly that is bonded to the main surface of the light guide, an easily accessible mounting surface, and does not require high-precision alignment.

[0029] CLE104 is preferably deployed on an area of ​​LOE106 located outside or at least peripherally to the user's viewing area, so that the edges of CLE104 do not obstruct the user's field of view. The relatively small size of CLE104 compared to LOE106 covers less than 20%, preferably less than 10%, of the LOE area, facilitating the inconspicuous deployment of the CLE. When peripherally located within the viewing area, the beam splitter coating at interface 105 may be implemented using a multilayer dielectric coating, which has angle-dependent reflectivity, is transparent (transmittance greater than 90%) at small incident angles, and has the desired reflectivity as defined below for larger angles related to image propagation within the LOE. When CLE104 is outside the viewing area, a simple dielectric or metallic partially reflective coating may be used. CLE104 is preferably a “slab” type light guide, meaning that both its in-plane dimensions are at least an order of magnitude larger than its thickness.

[0030] The reflectivity of the beam splitter coating at interface 105 is preferably selected as a function of the angle range of the injected image and the thickness and length of CLE 104, so that a significant portion of the image light intensity undergoes multiple internal reflections within CLE 104 while a considerable portion of the image light intensity gradually "leaks" into LOE 106, resulting in most of the image light intensity leaking out of CLE 104 by the edges of the CLE. Preferred reflectivity is typically at least 50%, more preferably 55% to 95%, and most preferably 65% ​​to 90%. In some cases, the beam splitter coating has a reflectivity that progressively decreases along the first in-plane direction, for example, as two or more strips of reflective coatings having different reflectivity.

[0031] Now, looking at Figures 2A and 2B, these illustrate a further optical system, generally designated 111, which is constructed and operable according to a further embodiment of the present invention. Optical system 111 is substantially similar in structure and function to optical system 100, having similarly labeled equivalent elements. Optical system 111 differs from optical system 100 in that the CLE 104 includes a preliminary aperture expansion device, which is implemented here as a set of mutually parallel internal partial reflective surfaces 112 within the CLE 104 to progressively redirect the light coupled in by the input coupler. This allows for the use of an entrance pupil defined by prism 102b, which is smaller than that of prism 102a in the previous embodiment.

[0032] In the embodiment illustrated herein, the image is injected with an in-plane component parallel to direction D1, and the redirected rays entering LOE 106 are reflected twice (or several other even numbers) from the surface 112 of CLE 104 before propagating in the first in-plane direction D1. Optionally, at least a portion of the interface 105 below the region of the partially reflective surface 112 may have a high reflectivity similar to that discussed below with reference to Figures 4A and 4B, so as to minimize light loss after odd-numbered reflections in the region of the partially reflective surface 112 and to restrict coupling out to the light guide 106 to the region of CLE 104 beyond surface 112. Additionally or alternatively, efficiency may be enhanced by providing a first surface 112a having high reflectivity, positioned so that incoupled light does not pass through the high-reflectivity surface, but all odd-numbered reflected light reaches the first surface and is returned toward direction D1.

[0033] The effect of the preliminary aperture-expanding device is to achieve an initial widening of the image entry pupil width, thereby facilitating the use of a narrower entry pupil to the prism 102b and a correspondingly smaller image projector. Additionally or alternatively, this preliminary in-plane expansion can relax the design requirements for the first aperture-expanding device, for example, by allowing the use of a larger spacing between consecutive partial reflective surfaces 108. In all other respects, the structure and function of the optical system 111 are similar to those of the optical system 100 and will be understood by referring to the above description.

[0034] Now, looking to Figures 3A and 3B, a further optical system, generally designated 125, is shown, constructed and operable according to a further embodiment of the present invention. Optical system 125 has substantially the same structure and function as optical systems 100 and 111, having similarly labeled equivalent elements. Similar to optical system 111, the CLE 104 of optical system 125 includes a preliminary aperture expansion device implemented as a set of mutually parallel internal partial reflecting surfaces 112 for progressively redirecting light coupled in by an input coupler. However, in this case, the image is injected through a prism 102c having an in-plane component not parallel to direction D1, and the redirected rays entering the LOE 106 propagate in the first in-plane direction D1 after being reflected once (or another odd number of times) from the surfaces 112 of the CLE 104. In this scenario, efficiency can be enhanced by providing a continuously increasing reflectivity of the surfaces 112 along the optical path.

[0035] In this embodiment, the injection direction of the image light is illustrated here as being perpendicular to the first in-plane direction D1, with a partially reflective surface 112 at a 45-degree angle to D1. However, other injection angles and corresponding orientations of the surface 112 can be selected.

[0036] Surface 112 may be perpendicular (orthogonal) to the main surface of the LOE, or it may be angled. Optionally, a pair of angled surfaces 112 may be used such that the injected image spans a first range of angles, and the reflected image (after one or another odd number of reflections) spans a second range of angles. In this case, the partially reflective coating of interface 105 may be selectively angled to be partially reflective in the second range of angles, while enabling coupling from the reflected image and having a higher reflectivity in the first range of angles, and minimizing the loss of light that does not contribute to the output image.

[0037] Referring here to Figures 4A and 4B, these illustrate embodiments of optical systems 125 and 126 having a modified interface for enhanced efficiency. Specifically, in this case, a portion 105a of the interface between CLE 104 and LOE 106 under a preliminary aperture-expanding device is provided with a highly reflective coating, i.e., a reflectivity of over 90%, preferably at least over 95%, while the aforementioned beam-splitting coating is applied beyond the preliminary aperture-expanding device (e.g., surface 112) into a second portion 105b of the interface. This helps to ensure that light propagating in an undesirable direction (e.g., after zero or even reflections in the case of Figure 4A) is not coupled to the light guide 106, but instead is restricted to CLE 104 until it encounters an additional partially reflective surface 112, and thus increases the amount of light that is ultimately correctly directed. Light that has undergone an odd number of reflections propagates in the desired direction D1, passes through the second portion of CLE104, where it is progressively coupled out through the second portion 105b of the interface, as described above. The highly reflective coating preferably also extends beneath the coupling prism 102c. This approach is equally applicable to embodiments using the geometric shapes of Figures 2A and 2B, where the internal partial reflective surface 112 may also be localized in the first portion of CLE104 having the highly reflective interface, and the coupled output occurs in a separate second portion of CLE104.

[0038] In all other respects, the structure and operation of optical system 126 are equivalent to those of optical system 125 and will be understood by referring to the corresponding descriptions above.

[0039] The present invention has so far been described with reference to optical systems in which the redirection of light in LOE106 and any preliminary aperture expansion in CLE104 are all achieved by reflective elements. However, it should be noted that one or more of the redirection components may be alternatively implemented using diffractive optical elements such as surface gratings, volume gratings, or holographic elements, which are known in the art. Typically, to avoid the effects of significant dispersion that are characteristic of diffractive optical elements, the system design should preferably employ two equal, but opposite, redirections performed by the diffractive elements such that the second redirection compensates for the effects of dispersion introduced by the first redirection.

[0040] One such embodiment, which can be understood by referring to Figure 2A, would replace the set of partially reflective surfaces 112 within the CLE 104 with a diffractive optical element associated with the CLE. Such an embodiment should be configured such that the principal rays of the collimated image coupled into the CLE propagate in the first in-plane direction after being redirected twice by diffraction in the diffractive optical element, so as to cancel out the chromatic dispersion produced by the first diffraction in the diffractive optical element. All other aspects of this design are described above with reference to Figures 2A and 2B.

[0041] An alternative approach illustrated in Figures 5A and 5B provides an optical system 127 that is geometrically similar to the optical system 126 in Figures 4A and 4B, but employs a diffractive optical element 130a associated with the CLE 104 for preliminary aperture expansion within the CLE 104, and a second diffractive optical element 130b as the first reorientation configuration of the LOE 106. The DOE 130b is configured to match the DOE 130a so as to cancel out the chromatic dispersion generated by the diffractive optical element 130a of the CLE 104. In this embodiment, the direction of light injection into the CLE 104 should be parallel to the second in-plane direction D2 so that after two reorientations, the light leaves the DOE 130b along the in-plane direction D2 again. Coupling of the image light into the CLE 104 is done via a prism 102c, and coupling out from the image toward the user's eye is achieved by partially reflecting off the surface 110, as described above with reference to optical systems 125 and 126.

[0042] Now, turning to Figure 6, this shows a further optical system, generally designated 128, illustrating an alternative diffraction (or mixed “hybrid”) embodiment. In this case, the input coupler is implemented as a first diffractive optical element 132a associated with the surface of the CLE 104, and the second reversal configuration is a second diffractive optical element 132b configured to match the first diffractive optical element to cancel out the chromatic dispersion produced by the first diffractive optical element. In this case, the image injection is typically substantially perpendicular to the principal surfaces of the CLE and LOE, where the principal ray is shown as D0c.

[0043] Further embodiments, not shown, may implement a perfect diffraction embodiment using two pairs of diffraction elements to cancel out the effects of their respective dispersions, by combining the diffraction elements 130a and 130b of Figure 5A with the diffraction elements 132a and 132b of Figure 6.

[0044] In all of the above cases, the LOE106 is schematically illustrated as a rectangular element. In a typical and practical embodiment, the LOE is molded to fit into a suitable support structure, such as an eyeglass frame, that supports the LOE when properly aligned when facing the user's eye. The display device may typically include two such optical devices, each supplied with a collimated image from a miniature image projector, and also include onboard components such as processing components, power supplies, and various communication subsystems, all of which are required for each application and are generally known in the art. These additional components are not part of the present invention and are therefore not described in detail here.

[0045] The above description is intended to serve as an example only, and it will be understood that many other embodiments are possible within the scope of the invention as defined in the attached claims.

Claims

1. An optical system, (a) An aperture-expanding optical element (LOE) having a pair of mutually parallel main surfaces separated by a first thickness, wherein the LOE supports the propagation of light by internal reflection at the main surfaces, and the LOE (i) A first direction-changing configuration deployed within the first region of the LOE for gradually changing the direction of light propagating in the first in-plane direction and propagating it toward the second region of the LOE in the second in-plane direction, (ii) An aperture-expanding light guide optical element (LOE) having a second direction-changing configuration deployed within the second region of the LOE for gradually changing the direction of light propagating in the second in-plane direction toward the outward direction of the LOE for viewing by a viewer, (b) A coupling-in device, (i) A coupled light guide element (CLE) having a pair of mutually parallel surfaces separated by a second thickness which is less than half the first thickness, wherein the parallel surfaces of the CLE have an area which is less than 10 percent of the area which is the area which is the area which is the area which is the main surface of the LOE, and one of the surfaces of the CLE is bonded to one of the main surfaces of the LOE at an interface, and at least a portion of the interface is provided with a beam splitter coating which has a reflectivity of at least 50%, (ii) An optical system comprising a coupling-in device, which includes an input coupler deployed to couple light corresponding to a collimated image into the CLE.

2. The optical system according to claim 1, wherein the input coupler is a coupling prism that presents an input surface substantially perpendicular to the principal ray of the collimated image coupled into the CLE.

3. The optical system according to claim 1, wherein the first reversal configuration includes a first set of mutually parallel internal partial reflective surfaces that are not parallel to the main surface, and the second reversal configuration includes a second set of mutually parallel internal partial reflective surfaces that are angled obliquely to the main surface.

4. The optical system according to claim 1, wherein the CLE further comprises a preliminary aperture expansion device.

5. The optical system according to claim 4, wherein the preliminary aperture expansion device includes a set of mutually parallel internal partial reflective surfaces within the CLE for progressively redirecting the light coupled by the input coupler.

6. The optical system according to claim 5, wherein the input coupler is deployed such that the principal rays of the collimated image coupled into the CLE propagate in the first in-plane direction after a single reflection from one of the internal partial reflective surfaces of the CLE.

7. The optical system according to claim 5, wherein the input coupler is deployed such that the principal rays of the collimated image coupled into the CLE propagate in the first in-plane direction after being reflected twice from the internal partial reflective surface of the CLE.

8. The optical system according to claim 4, wherein the preliminary aperture expansion device comprises a diffractive optical element associated with the CLE.

9. The optical system according to claim 8, wherein the input coupler is deployed such that the principal rays of the collimated image coupled into the CLE are reoriented twice by diffraction in the diffractive optical element, and then propagate in the first in-plane direction to cancel out the color dispersion generated by the first diffraction in the diffractive optical element.

10. The optical system according to claim 8, wherein the first direction-changing configuration of the LOE is a diffractive optical element configured to match the diffractive optical element of the CLE and cancel out the color dispersion generated by the diffractive optical element of the CLE.

11. The optical system according to claim 4, wherein a highly reflective coating is provided on a portion of the interface between the CLE and the LOE, located below the preliminary aperture expansion device.

12. The optical system according to claim 1, wherein the input coupler is a first diffractive optical element associated with the surface of the CLE, and the second direction reversal configuration is a second diffractive optical element configured to match the first diffractive optical element and cancel out the color dispersion generated by the first diffractive optical element.

13. The optical system according to claim 1, wherein the beam splitter coating has a reflectivity of 55% to 95%.

14. The optical system according to claim 1, wherein the beam splitter coating has a reflectivity of 65% to 90%.

15. The optical system according to claim 1, wherein the beam splitter coating has a reflectivity that gradually decreases along the first in-plane direction.

16. The optical system according to claim 1, wherein the second thickness is 20% to 40% of the first thickness.

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