PATENT DOCUMENT

Publication Number: US-11719947-B1
Application Number: US-202016871063-A
Country: US
Kind Code: B1

Title: Prism beam expander

Abstract:
An optical component includes a block of a transparent material, having a trapezoidal cross-section defined by first and second parallel, rectangular faces on mutually-opposing sides of the block and third and fourth faces oriented diagonally at opposing ends of the first and second faces. One or more planar, partially-reflecting layers extend within the block between the third and fourth faces in an orientation parallel to the first and second faces.

Claims:
The invention claimed is: 
     
       1. An optical component, comprising:
 a block of a transparent material, having a trapezoidal cross-section defined by first and second parallel, rectangular faces on mutually-opposing sides of the block and third and fourth faces oriented diagonally at opposing ends of the first and second faces; and 
 one or more planar, partially-reflecting layers extending within the block between the third and fourth faces in an orientation parallel to the first and second faces. 
 
     
     
       2. The optical component according to  claim 1 , wherein the trapezoidal cross-section comprises a parallelogram. 
     
     
       3. The optical component according to  claim 1 , wherein the trapezoidal cross-section comprises an isosceles trapezoid. 
     
     
       4. The optical component according to  claim 1 , and comprising an anti-reflective coating on the third and fourth faces. 
     
     
       5. The optical component according to  claim 1 , wherein the first and second faces are configured to reflect internally rays of light that impinge on the first and second faces after passing through the partially-reflecting layers. 
     
     
       6. The optical component according to  claim 5 , and comprising a reflective coating on the first and second faces. 
     
     
       7. The optical component according to  claim 1 , wherein the one or more planar, partially-reflecting layers comprise multiple planar, partially-reflecting layers, which are parallel to and spaced apart between the first and second faces. 
     
     
       8. The optical component according to  claim 6 , wherein a spacing between the partially-reflecting layers does not exceed one-fifth of a length of the second face. 
     
     
       9. The optical component according to  claim 1 , wherein a height of the block between the first and second faces does not exceed one fifth of a length of the second face. 
     
     
       10. Image projection apparatus, comprising:
 an optical pupil expander, comprising:
 a block of a transparent material, having a trapezoidal cross-section defined by first and second parallel, rectangular faces on mutually-opposing sides of the block and third and fourth faces oriented diagonally at opposing ends of the first and second faces; and 
 one or more planar, partially-reflecting layers extending within the block between the third and fourth faces in an orientation parallel to the first and second faces; and 
 
 an image projector, which is configured to project a beam of light onto the third face of the block with a given input beam width, at an angle selected so that the beam is reflected internally within the block while being partially reflected and partially transmitted multiple times by the one or more planar, partially-reflecting layer, and exits through the fourth face with an output beam width at least twice the input beam width. 
 
     
     
       11. The apparatus according to  claim 10 , wherein the optical pupil expander is configured to homogenize an intensity of the light in the output beam. 
     
     
       12. The apparatus according to  claim 10 , wherein the angle is selected so that the beam is reflected from the first and second faces of the block by total internal reflection. 
     
     
       13. A method for image projection, comprising:
 providing an optical pupil expander, comprising:
 a block of a transparent material, having a trapezoidal cross-section defined by first and second parallel, rectangular faces on mutually-opposing sides of the block and third and fourth faces oriented diagonally at opposing ends of the first and second faces; and 
 one or more planar, partially-reflecting layers extending within the block between the third and fourth faces in an orientation parallel to the first and second faces; and 
 
 projecting a beam of light onto the third face of the block with a given input beam width, at an angle selected so that the beam is reflected internally within the block while being partially reflected and partially transmitted multiple times by the one or more planar, partially-reflecting layer, and exits through the fourth face with an output beam width at least twice the input beam width. 
 
     
     
       14. The method according to  claim 13 , wherein projecting the beam of light comprises homogenizing an intensity of the light in the output beam. 
     
     
       15. The method according to  claim 13 , wherein projecting the beam of light comprises selecting the angle so that the beam is reflected from the first and second faces of the block by total internal reflection. 
     
     
       16. The method according to  claim 13 , wherein the trapezoidal cross-section comprises a parallelogram. 
     
     
       17. The method according to  claim 13 , wherein the trapezoidal cross-section comprises an isosceles trapezoid. 
     
     
       18. The method according to  claim 13 , wherein the one or more planar, partially-reflecting layers comprise multiple planar, partially-reflecting layers, which are parallel to and spaced apart between the first and second faces, wherein a spacing between the partially-reflecting layers does not exceed one-fifth of a length of the second face. 
     
     
       19. The method according to  claim 13 , wherein a height of the block between the first and second faces does not exceed one fifth of a length of the second face. 
     
     
       20. The method according to  claim 13 , wherein providing the optical pupil expander comprises providing a plurality of sub-blocks of the transparent material, coating a surface of at least one of the sub-blocks with the one or more planar, partially-reflecting layers, and cementing the sub-blocks together to form the block of the transparent material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/868,933, filed Jun. 30, 2019, which is incorporate herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to optical components, and particularly to beam expanders and homogenizers. 
     BACKGROUND 
     Optical beam expanders for expanding and homogenizing a narrow input beam in one dimension are commonly required in optical apparatuses, such as certain types of projection systems. The expansion may be achieved by splitting and replicating the narrow input beam several times, and the homogenization is achieved by controlling the ratios into which the beam is split. 
     SUMMARY 
     Embodiments of the present invention provide improved optical components and systems, as well as methods for their manufacture and use. 
     There is therefore provided, in accordance with an embodiment of the invention, an optical component, including a block of a transparent material, having a trapezoidal cross-section defined by first and second parallel, rectangular faces on mutually-opposing sides of the block and third and fourth faces oriented diagonally at opposing ends of the first and second faces. One or more planar, partially-reflecting layers extend within the block between the third and fourth faces in an orientation parallel to the first and second faces. 
     In some embodiments, the trapezoidal cross-section includes a parallelogram. Alternatively, the trapezoidal cross-section includes an isosceles trapezoid. 
     In a disclosed embodiment, the optical component includes an anti-reflective coating on the third and fourth faces. 
     Typically, the first and second faces are configured to reflect internally rays of light that impinge on the first and second faces after passing through the partially-reflecting layers. In a disclosed embodiment, the optical component includes a reflective coating on the first and second faces. 
     In some embodiments, the one or more planar, partially-reflecting layers include multiple planar, partially-reflecting layers, which are parallel to and spaced apart between the first and second faces. In a disclosed embodiment, a spacing between the partially-reflecting layers does not exceed one-fifth of a length of the second face. Additionally or alternatively a height of the block between the first and second faces does not exceed one fifth of a length of the second face. 
     There is also provided, in accordance with an embodiment of the invention, image projection apparatus, including an optical pupil expander, which includes a block of a transparent material, having a trapezoidal cross-section defined by first and second parallel, rectangular faces on mutually-opposing sides of the block and third and fourth faces oriented diagonally at opposing ends of the first and second faces. One or more planar, partially-reflecting layers extend within the block between the third and fourth faces in an orientation parallel to the first and second faces. An image projector is configured to project a beam of light onto the third face of the block with a given input beam width, at an angle selected so that the beam is reflected internally within the block while being partially reflected and partially transmitted multiple times by the one or more planar, partially-reflecting layer, and exits through the fourth face with an output beam width at least twice the input beam width. 
     In some embodiments, the optical pupil expander is configured to homogenize an intensity of the light in the output beam. 
     Typically, the angle is selected so that the beam is reflected from the first and second faces of the block by total internal reflection. 
     There is additionally provided, in accordance with an embodiment of the invention, a method for image projection, which includes providing an optical pupil expander, including a block of a transparent material, having a trapezoidal cross-section defined by first and second parallel, rectangular faces on mutually-opposing sides of the block and third and fourth faces oriented diagonally at opposing ends of the first and second faces, with one or more planar, partially-reflecting layers extending within the block between the third and fourth faces in an orientation parallel to the first and second faces. A beam of light is projected onto the third face of the block with a given input beam width, at an angle selected so that the beam is reflected internally within the block while being partially reflected and partially transmitted multiple times by the one or more planar, partially-reflecting layer, and exits through the fourth face with an output beam width at least twice the input beam width. 
     In a disclosed embodiment, providing the optical pupil expander includes providing a plurality of sub-blocks of the transparent material, coating a surface of at least one of the sub-blocks with the one or more planar, partially-reflecting layers, and cementing the sub-blocks together to form the block of the transparent material. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic pictorial illustration of an image projection apparatus, in accordance with an embodiment of the invention; 
         FIGS.  2 ,  3 ,  4  and  5    are schematic sectional views of beam expanders, in accordance with embodiments of the invention; and 
         FIGS.  6 A,  6 B,  6 C,  6 D,  6 E and  6 F  are schematic illustrations of ray paths in a beam expander for various angles of incidence, along with associated spatial irradiance distributions of an output beam from the beam expander, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Optical beams that are expanded by a beam expander in one dimension often suffer from a low degree of homogenization, due to the design and structure of the expander. There is a need to achieve both a substantial beam expansion in one dimension and a high degree of homogenization of the expanded beam in order to satisfy the needs of applications such as in projection systems. 
     The embodiments of the present invention that are described herein address this need by means of an optical component functioning as a one-dimensional (1D) beam expander that expands an input beam through a large number of splits and reflections before emitting an expanded and homogenized exit beam. In the disclosed embodiments, the beam expander comprises a block of optically transparent dielectric material, such as glass, that transmits the majority of the optical power. This is accomplished in the wavelength range of interest, such as the visible range. (The terms “optical rays,” “optical radiation,” and “light,” as used in the present description and in the claims, refer generally to any and all of visible, infrared, and ultraviolet radiation.) 
     The shape of the block is typically polyhedral, i.e., a three dimensional solid with flat polygonal faces. In the described embodiments, the shape of the block is hexahedral, but other suitable shapes may alternatively be used. Two opposing faces of the block, referred to as the first and second faces, are parallel to one another. The third and fourth faces of the block are oriented diagonally at opposing ends of the first and second faces. The beam expander comprises one or more planar, partially-reflecting layers extending within the block between the third and fourth faces in an orientation parallel to the first and second faces. In practice, the block is typically fabricated by coating component sub-blocks with thin-film coatings configured for partial reflection, and then cementing the component sub-blocks to each other to form the complete block with internal coatings. 
     The fifth and sixth faces, opposing each other, are generally parallel to each other and perpendicular to the other faces. However, the orientation of the fifth and sixth faces may deviate from the above in order to accommodate requirements such as mechanical mounting of the block. The numbering of the faces as first, second, and so on, is arbitrary and chosen for convenience only. 
     In some embodiments, the block has a trapezoidal cross-section, as defined by the first, second, third and fourth faces. The choice of a specific cross-section is dictated by the requirements for the angle of incidence and the angle of exit of the beams that are respectively incident to and emitted by the block. In one embodiment, the cross-section is an isosceles trapezoid, wherein the dihedral angle between the third face and the first face is equal to the dihedral angle between the fourth face and the first face. In another embodiment, the cross-section is a parallelogram, wherein the dihedral angle between the third face and the first face is supplementary to the dihedral angle between the fourth face and the first face, i.e., the sum of the two dihedral angles is 180 degrees. In this case, the block comprises a parallelepiped. 
     A narrow light beam is projected onto the third face, entering the block. (“Narrow” refers here to the diameter of the beam as compared to the lateral dimensions of the third face. Furthermore, in the present description the term “ray” is used to denote optical rays, and “beam” is used to denote an optical beam, which may be represented by a collection of rays.) The optical power contained in the rays is partially reflected and transmitted multiple times by the partially-reflecting layer or layers within the block, thus splitting each ray repeatedly. When the rays strike the first or second face, they are internally reflected within the block. By spacing the third and fourth faces substantially farther apart than the spacings between the partially-reflecting layers within the block, a large number of parallel rays are eventually generated and exit through the fourth face as an expanded beam. This expanded output beam extends across the fourth face with a transverse dimension that is much larger than the diameter of the input beam, for example with a width at least twice that of the input beam, and with roughly homogeneous intensity across the output beam. Thus, beam expansion is accomplished. 
     The first and second faces may be uncoated, when the internal angles of the beams within the block are beyond the critical angle for total internal reflection (TIR), or coated with a reflective coating, such as aluminum. The third and fourth faces may be coated by an anti-reflective coating in order to reduce reflection losses of the light entering into and exiting from the block. 
     System Description 
       FIG.  1    is a schematic pictorial illustration of an image projection apparatus  20 , in accordance with an embodiment of the invention. Apparatus  20  may be used, for example, as part of a virtual reality (VR) or augmented reality (AR) system, as well as in other image projection applications. This apparatus is shown here as a non-limiting example of the use of a 1D beam expander as provided by embodiments of the present invention. Other uses of such a beam expander will be apparent to those skilled in the art after reading the present description and are considered to be within the scope of the invention. 
     Apparatus  20  is based on a typical format of eyeglasses. For the sake of simplicity, only the left side of apparatus  20  (as referenced to an observer wearing the system) is shown. The right side is a mirror image of the left side. Alternatively, the right side may comprise only a lens with or without optical power, but without a display. 
     Apparatus  20  comprises a suitably modified eyeglass frame  22 , a 1D optical collimation assembly  24 , a scanning mirror assembly  26 , and a 1D pupil expander  28 . A controller  30  is coupled to collimation assembly  24  and to scanning mirror assembly  26 . Collimation assembly  24  comprises an emitter array  34 , collimation optics  36 , and a 1D beam expander  38 . Collimation optics  36  are rotationally symmetrical and have their apertures shaped to form rectangles due to the narrow horizontal dimension of emitter array  34 , which results in a narrow horizontal field-of-view. Emitter array  34  comprises, for example, an array of vertical-cavity surface-emitting lasers (VCSELs) or micro-light-emitting diodes (micro-LEDs), which emit respective beams toward optics  36 . 
     Scanning mirror assembly  26  comprises an elongated rectangular first scanning mirror  40  and a mirror actuator  42 , such as a galvanometer, with an axis of rotation of the scanning mirror assembly parallel to the Y-axis of a Cartesian coordinate system  32 , corresponding to the vertical axis of the eyeglasses. (Cartesian coordinate system  32  is used for the sake of clarity and convenience only. Other coordinate systems may be alternatively used.) The dimensions of scanning mirror  40  are, for example, 20 mm×5 mm, wherein the long dimension, along the Y-axis, which is the axis of rotation, is determined by the dimension of pupil expander  28  in the Y-direction. Minimizing the short dimension (nominally along the X-direction), perpendicular to the axis of rotation, enables high-speed scanning and makes it possible to integrate assembly  26  unobtrusively into the eyeglass frame of apparatus  20 . Alternatively, scanning mirror assembly  26  may comprise a long rotating polygon with an electric motor drive. 
     Pupil expander  28  comprises, for example, a waveguide, a surface grating, or a holographic element. Pupil expanders of these sorts are known in the art of AR displays, for example, and their details are beyond the scope of the present description. 
     Prism Designs 
       FIG.  2    is a schematic sectional view of 1D beam expander  38 , in accordance with an embodiment of the invention. Beam expander  38  comprises a block  122  of optically transparent material, such as glass. Block  122  comprises a first face  124 , a second face  126 , a third face  128 , and a fourth face  130 , all perpendicular to the plane of  FIG.  2   , and all having rectangular shapes. Block  122  further comprises fifth and sixth faces (not shown) that are parallel to the plane of  FIG.  2   . 
     In this embodiment, first and second faces  124  and  126  are mutually parallel, as are third and fourth faces  128  and  130 . A cross-section  132  of block  122 , perpendicular to faces  124 ,  126 ,  128 , and  130 , is a parallelogram, and the block comprises a parallelepiped. (In optical nomenclature, a block of this shape is commonly termed, in deviation from the geometrical definition, a “rhomb” or a “rhomboid”, even when the lengths of first and second faces  124  and  126  in the plane of the figure are different from those of third and fourth faces  128  and  130 .) 
     Planar, partially-reflecting layers  134  extend within block  122  between third and fourth faces  128  and  130  in an orientation parallel to first and second faces  124  and  126 . Partially-reflecting layers  134  commonly have a ratio of reflectance to transmittance (R/T-ratio) that is optimized to achieve uniform homogenization over the expanded beam. The R/T-ratio may be, for example, 50:50, 60:40, or any other ratio up to 90:10. 
     Beam expander  38  may in an alternative embodiment be designed to be used in conjunction with emitter array  34  comprising a polarized light source. In this case, the R/T-ratio for partially-reflecting layers  134  may be optimized for the specific polarization of the source. 
     Alternatively, when emitter array  34  comprises a non-polarized light source and a non-polarizing spatial modulator, such as a digital micro-mirror device (DMD), partially-reflecting layers  134  may be optimized for both s- and p-polarizations. 
     Block  122  is fabricated from component sub-blocks  122   a - d , after coating partially-reflecting layers  134  over those faces of the sub-blocks that will become internal faces in the final assembled block. Partially-reflecting layers  134  typically comprise single or multi-layer thin films of dielectric and/or metallic materials. 
     An input beam  136  of width W in , wherein W in  is measured in the plane of the figure (i.e. the Y-Z-plane), impinges on third face  128 . Input beam  136  is represented schematically by optical rays  138   a - c . In the example shown in  FIG.  1   , input beam  136  is a narrow, collimated beam that is directed by collimation optics  36  into beam expander  38 . A part of input beam  136 , depicted by rays  138   b - c , impinges on a partially-reflecting layer  134   a . Another part of the beam, depicted by ray  138   a , impinges on second face  126  at an angle of incidence θ i . After reflecting from second face  126  as a ray  140   a , it impinges on a partially-reflecting layer  134   a . Each time a ray impinges on one of partially-reflecting layers  134 , it is split into child rays, which are transmitted and reflected rays, as is shown schematically in  FIG.  2   . After a number of reflections and transmissions, some of the children rays impinge on first face  124 , as shown schematically by a ray  140   b , and are reflected from the first face back into block  122 . Rays that impinge on fourth face  130  exit as an expanded output beam  142  of width W out  (in the plane of the figure, i.e., the Y-Z-plane), schematically represented by rays  144 . 
     To ensure the homogeneity of output beam  142 , it is important that the rays of input beam  136  undergo a large number of reflections from partially-reflecting layers  134 . For this reason, the spacing h between adjacent partially-reflecting layers, as well as the spacing between the partially-reflecting layers and faces  124  and  126 , is typically much smaller than the length L of first and second faces  124  and  126 . For example, the ratio of L to h may advantageously be between 3 and 20, although other ratios may be used. 
     Furthermore, in order to ensure sufficient expansion of input beam  136 , i.e., a high ratio of W out :W in , the height H of block  122 , meaning the distance between the first and second faces, is advantageously made much larger than W in . A typical ratio of H:W in  is between 3 and 10, although other ratios may be used. 
     The spacings between adjacent partially-reflecting layers  134 , such as h in  FIG.  2   , may be either equal or unequal. Depending on the ratio of L:h, partially-reflecting layers  134  may or may not extend all the way through block  122  from third face  128  to fourth face  130 . Specifically, a high L:h ratio allows partially-reflecting layers  134  not to extend all the way through block  122 . 
     As each ray is split at each partially-reflecting layer  134 , the optical flux carried by the split rays is reduced, as shown schematically by the thickness of the rays in  FIG.  2   . For the sake of clarity, not all rays that are produced by reflections and transmissions from initial rays  138   a - c  are shown in  FIG.  2   . 
     Third and fourth faces  128  and  130  are desirably coated with an anti-reflective coating in order to reduce reflection losses of beams entering into and exiting from beam expander  38 . First and second faces  124  and  126  may be left uncoated, provided that the angle of incidence θ i  exceeds the critical angle θ c  for TIR. For cases in which e does not exceed the critical angle θ c  (and optionally, even when e exceeds the critical angle), first and second faces  124  and  126  may be coated with a reflective coating, such as aluminum. Anti-reflective coatings and reflective coatings may be similarly applied in the embodiments described below. 
       FIG.  3    is a schematic sectional view of a beam expander  150 , in accordance with another embodiment of the invention. Beam expander  150  is similar to beam expander  38 , and the same labels are used for both beam expanders. In  FIG.  3   , input beam  136  is represented by ten rays, whose splits into child rays within beam expander  150  are all shown. In addition to output beam  142 ,  FIG.  3    also shows a second output beam  152  exiting through first face  124 , due to rays within block  122  that have been reflected by TIR on fourth face  130 . In  FIG.  3   , as well as in subsequent  FIGS.  4 ,  5 , and  6 A- 6 F , the illustrated ray paths are the results of a simulation by an optical ray trace program. 
       FIG.  4    is a schematic sectional view of a beam expander  160 , in accordance with yet another embodiment of the invention. The cross-section of beam expander  160  is an isosceles trapezoid. However, both functionally and in terms of its parts, beam expander  160  is similar to beam expander  150 , and the same labels as in beam expander  150  are used to denote its parts (except that in beam expander  160 , the sub-blocks making up block  122  are of unequal sizes). The different tilt of fourth face  130 , as compared to beam expander  150 , causes output beam  142  to exit at a different angle. The lengths of first face  124  and the second face  126  in the plane of  FIG.  4   , L 1  and L 2 , respectively, are now unequal. In optical nomenclature, a prism of this shape is commonly termed a “Dove-prism.” 
       FIG.  5    is a schematic sectional view of a beam expander  200 , in accordance with still another embodiment of the invention. Like beam expander  160 , beam expander  200  has the shape of a Dove prism. In the present embodiment, the length L 2  of second face  126  is 90 mm, the length L 4  of fourth face  130  is 12 mm, the angle α between the fourth face and the second face is 40 degrees, and the width W in  of input beam  136  is 1 mm. All of the dimensions of beam expander  200  may be varied according to performance requirements. The 40 degree angle is selected in order to maintain a total-internal-reflection state for the rays propagating down the axis of the Dove prism. 
     In this example, the aspect ratio L 2 :H is about 10. Alternatively, the aspect ratio may vary between 5 and 15, or may assume even larger or smaller values. Due to the high aspect ratio L 2 :H of the illustrated embodiment, a large number of internal reflections, and hence ray splittings and generations of child rays, are achieved even with the relatively low number of three partially-reflecting layers  134  in beam expander  160 . Partially-reflecting layers  134  do not extend all the way to third and fourth faces  128  and  130 . Partially-reflecting layers  134  are located at unequal spacings from each other, as well as from first and second faces  124  and  126 . 
     An inset  202  shows a simulated spatial distribution  204  of the irradiance of exit beam  42  in the plane of fourth face  130 . Distribution  204  shows a high degree of homogeneity (uniformity) of output beam  142 , as well as the large extent its 1D expansion. 
       FIGS.  6 A- 6 F  are schematic illustrations of ray paths in beam expander  200  for various angles of incidence, along with the associated spatial irradiance distributions of output beam  142 , in accordance with an embodiment of the invention. 
     Each of the six  FIGS.  6 A- 6 F  shows in its upper part beam expander  200  with simulated rays, and in its lower part an inset  202  containing simulated spatial distribution  204  of the irradiance of exit beam  142  in the plane of fourth face  130 , as in  FIG.  5   . For the sake of clarity, only some of the labels are marked in  FIG.  6 A , and no labels are marked in  FIGS.  6 B- 6 F . The text above beam expander  200  in each of the six figures indicates the angle of incidence of input beam  136 , wherein the angle is taken as positive in a counter-clockwise direction from a normal to third face  128 . These angles may represent the expanded range of angles for light reflected from scanning mirror  40  in  FIG.  1   . Below inset  202 , in each of the six figures, the efficiency of beam expander  200  for the respective input beam is noted, meaning the ratio between the total optical flux of output beam  142  and the total flux of input beam  136 . 
     A high degree of uniformity and an efficiency of 100%, is observed for angles of incidence of −25 degrees and −15 degrees ( FIGS.  6 A- 6 B ). However, at a −5 degree angle of incidence ( FIG.  6 C ) the efficiency is 100%, but the uniformity has deteriorated significantly. At higher angles of incidence (+5 degrees and above), the uniformity improves, but efficiency decreases to 81%, 75% and 67% for angles of +5 degrees ( FIG.  6 D ), +15 degrees ( FIG.  6 E ), and +25 degrees ( FIG.  6 F ), respectively. The decreased efficiency results from a leakage of rays through other faces than fourth face  130 , as shown in the respective figures. For angles of incidence that are +5 degrees and above, the efficiency can be increased by orienting fourth face  130  at a steeper angle with respect to first and second faces  124  and  126 . 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Metadata:
Filing Date: 20200511
Publication Date: 20230808
Grant Date: 20230808
Priority Date: 20190630
Inventors: HAJATI, ARMAN
HANSOTTE, Eric J.
UPTON, Robert S.
Assignee: APPLE INC
CPC Classifications: [{"code": "G02B27/0983", "inventive": true, "first": true, "tree": "[]"}, {"code": "B23K26/0732", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0927", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0983", "inventive": true, "first": true, "tree": "[]"}, {"code": "B23K26/0732", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0927", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0983", "inventive": true, "first": true, "tree": "[]"}, {"code": "G02B27/0927", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0081", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B27/0172", "inventive": true, "first": false, "tree": "[]"}, {"code": "G02B2027/0125", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 87522354