Patent Publication Number: US-2022236473-A1

Title: Multibeam backlight, multiview display, and method with diffraction grating filling fraction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation patent application of and claims priority to International Patent Application No. PCT/US2019/056401, filed Oct. 15, 2019, the contents of which are incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND 
     Electronic displays are a nearly ubiquitous medium for communicating information to users of a wide variety of devices and products. Most commonly employed electronic displays include the cathode ray tube (CRT), plasma display panels (PDP), liquid crystal displays (LCD), electroluminescent displays (EL), organic light emitting diode (OLED) and active matrix OLEDs (AMOLED) displays, electrophoretic displays (EP) and various displays that employ electromechanical or electrofluidic light modulation (e.g., digital micromirror devices, electrowetting displays, etc.). Generally, electronic displays may be categorized as either active displays (i.e., displays that emit light) or passive displays (i.e., displays that modulate light provided by another source). Among the most obvious examples of active displays are CRTs, PDPs and OLEDs/AMOLEDs. Displays that are typically classified as passive when considering emitted light are LCDs and EP displays. Passive displays, while often exhibiting attractive performance characteristics including, but not limited to, inherently low power consumption, may find somewhat limited use in many practical applications given the lack of an ability to emit light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features of examples and embodiments in accordance with the principles described herein may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, where like reference numerals designate like structural elements, and in which: 
         FIG. 1A  illustrates a perspective view of a multiview display in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 1B  illustrates a graphical representation of the angular components of a light beam having a particular principal angular direction corresponding to a view direction of a multiview display in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 2  illustrates a cross-sectional view of a diffraction grating in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 3A  illustrates a cross-sectional view of a multibeam backlight in an example, according an embodiment consistent with the principles described herein. 
         FIG. 3B  illustrates a perspective view of a multibeam backlight in an example, according an embodiment consistent with the principles described herein. 
         FIG. 4A  illustrates a plan view of a diffraction grating having a filling fraction in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 4B  illustrates a plan view of another diffraction grating having a filling fraction in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 4C  illustrates a perspective view of a diffraction grating having a filling fraction in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 5A  illustrates a plan view of a diffraction grating having a filling fraction in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 5B  illustrates a plan view of a diffraction grating having a filling fraction in an example, according to another embodiment consistent with the principles described herein. 
         FIG. 6A  illustrates a cross sectional view of a multibeam element having a reflective island in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 6B  illustrates a plan view of a multibeam element having a reflective island in an example, according to another embodiment of the principles described herein. 
         FIG. 7  illustrates a block diagram of a multiview display in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 8  illustrates a flowchart of a method of multiview display operation in an example, according to an embodiment consistent with the principles described herein. 
     
    
    
     Certain examples and embodiments have other features that are one of in addition to and in lieu of the features illustrated in the above-referenced figures. These and other features are detailed below with reference to the above-referenced figures. 
     DETAILED DESCRIPTION 
     Examples and embodiments in accordance with the principles described herein provide backlighting that provides diffractive efficiency control of a diffraction grating using a filling fraction of diffractive features, with application to electronic displays. In various embodiments consistent with the principles herein, a multibeam backlight employing a plurality of multibeam elements having a diffraction grating configured to provide directional light beams is provided. The diffraction grating comprises diffractive features and filling features in various embodiments. The filling features are positioned and oriented to interrupt the diffractive features to establish the filling fraction as a ratio of an area of the diffractive features relative to an area of the filling features within the diffraction grating. The filling fraction controls a diffractive efficiency of the diffraction grating. Uses of the backlighting and various backlit displays described herein may include, but are not limited to, mobile telephones (e.g., smart phones), watches, tablet computers, mobile computers (e.g., laptop computers), personal computers and computer monitors, automobile display consoles, cameras displays, and various other mobile as well as substantially non-mobile display applications and devices. 
     Herein, a ‘multiview display’ is defined as an electronic display or display system configured to provide different views of a multiview image in different view directions.  FIG. 1A  illustrates a perspective view of a multiview display  10  in an example, according to an embodiment consistent with the principles described herein. As illustrated in  FIG. 1A , the multiview display  10  comprises a screen  12  to display a multiview image to be viewed. The multiview display  10  provides different views  14  of the multiview image in different view directions  16  relative to the screen  12 . The view directions  16  are illustrated as arrows extending from the screen  12  in various different principal angular directions; the different views  14  are illustrated as shaded polygonal boxes at the termination of the arrows (i.e., depicting the view directions  16 ); and only four views  14  and four view directions  16  are illustrated, all by way of example and not limitation. Note that while the different views  14  are illustrated in  FIG. 1A  as being above the screen, the views  14  actually appear on or in a vicinity of the screen  12  when the multiview image is displayed on the multiview display  10 . Depicting the views  14  above the screen  12  is only for simplicity of illustration and is meant to represent viewing the multiview display  10  from a respective one of the view directions  16  corresponding to a particular view  14 . 
     A view direction or equivalently a light beam having a direction corresponding to a view direction of a multiview display generally has a principal angular direction given by angular components {θ, φ}, by definition herein. The angular component θ is referred to herein as the ‘elevation component’ or ‘elevation angle’ of the light beam. The angular component φ is referred to as the ‘azimuth component’ or ‘azimuth angle’ of the light beam. By definition, the elevation angle θ is an angle in a vertical plane (e.g., perpendicular to a plane of the multiview display screen) while the azimuth angle φ is an angle in a horizontal plane (e.g., parallel to the multiview display screen plane). 
       FIG. 1B  illustrates a graphical representation of the angular components {θ, φ} of a light beam  20  having a particular principal angular direction corresponding to a view direction (e.g., view direction  16  in  FIG. 1A ) of a multiview display in an example, according to an embodiment consistent with the principles described herein. In addition, the light beam  20  is emitted or emanates from a particular point, by definition herein. That is, by definition, the light beam  20  has a central ray associated with a particular point of origin within the multiview display.  FIG. 1B  also illustrates the light beam (or view direction) point of origin O. 
     Further herein, the term ‘multiview’ as used in the terms ‘multiview image’ and ‘multiview display’ is defined as a plurality of views representing different perspectives or including angular disparity between views of the view plurality. In addition, herein the term ‘multiview’ explicitly includes more than two different views (i.e., a minimum of three views and generally more than three views), by definition herein. As such, ‘multiview display’ as employed herein is explicitly distinguished from a stereoscopic display that includes only two different views to represent a scene or an image. Note however, while multiview images and multiview displays include more than two views, by definition herein, multiview images may be viewed (e.g., on a multiview display) as a stereoscopic pair of images by selecting only two of the multiview views to view at a time (e.g., one view per eye). 
     By definition herein, a ‘multibeam element’ is a structure or element of a backlight or a display that produces light that includes a plurality of directional light beams. Directional light beams of the plurality of directional light beams (or ‘directional light beam plurality’) produced by a multibeam element have different principal angular directions from one another, by definition herein. In particular, by definition, a directional light beam of the directional light beam plurality has a predetermined principal angular direction that is different from another directional light beam of the directional light beam plurality. According to some embodiments, a size of the multibeam element may be comparable to a size of a light valve used in a display that is associated with the multibeam element (e.g., a multiview display). In particular, the multibeam element size may be between about one half and about two times the light valve size, in some embodiments. In some embodiments, a multibeam element may provide polarization-selective scattering. 
     According to various embodiments, the directional light beam plurality may represent a light field. For example, the directional light beam plurality may be confined to a substantially conical region of space or have a predetermined angular spread that includes the different principal angular directions of the light beams in the light beam plurality. As such, the predetermined angular spread of the directional light beams in combination (i.e., the directional light beam plurality) may represent the light field. 
     According to various embodiments, the different principal angular directions of the various directional light beams in the directional light beam plurality are determined by a characteristic including, but not limited to, a size (e.g., one or more of length, width, area, and etc.) of the multibeam element along with other characteristics. For example, in a diffractive multibeam element, a ‘grating pitch’ or a diffractive feature spacing and an orientation of a diffraction grating within diffractive multibeam element may be characteristics that determine, at least in part, the different principal angular directions of the various directional light beams. In some embodiments, the multibeam element may be considered an ‘extended point light source’, i.e., a plurality of point light sources distributed across an extent of the multibeam element, by definition herein. Further, a directional light beam produced by the multibeam element may have a principal angular direction given by angular components {θ, φ}, as described below with respect to  FIG. 1B . 
     Herein, a ‘light guide’ is defined as a structure that guides light within the structure using total internal reflection. In particular, the light guide may include a core that is substantially transparent at an operational wavelength of the light guide. In various examples, the term ‘light guide’ generally refers to a dielectric optical waveguide that employs total internal reflection to guide light at an interface between a dielectric material of the light guide and a material or medium that surrounds that light guide. By definition, a condition for total internal reflection is that a refractive index of the light guide is greater than a refractive index of a surrounding medium adjacent to a surface of the light guide material. In some embodiments, the light guide may include a coating in addition to or instead of the aforementioned refractive index difference to further facilitate the total internal reflection. The coating may be a reflective coating, for example. The light guide may be any of several light guides including, but not limited to, one or both of a plate or slab guide and a strip guide. 
     In some embodiments, the light guide may be substantially flat (i.e., confined to a plane) and therefore, the light guide is a planar or plate light guide. In other embodiments, the plate light guide may be curved in one or two orthogonal dimensions. For example, the plate light guide may be curved in a single dimension to form a cylindrical shaped plate light guide. However, any curvature has a radius of curvature sufficiently large to ensure that total internal reflection is maintained within the plate light guide to guide light. 
     Herein, a ‘diffraction grating’ is generally defined as a plurality of features (i.e., diffractive features) arranged to provide diffraction of light incident on the diffraction grating. In some examples, the plurality of features may be arranged in a periodic or quasi-periodic manner. In other examples, the diffraction grating may be a mixed-period diffraction grating that includes a plurality of diffraction gratings, each diffraction grating of the plurality having a different periodic arrangement of features. Further, the diffraction grating may include a plurality of diffractive features (e.g., a plurality of grooves or ridges in a material surface) arranged in a one-dimensional (1D) array. In other examples, the diffraction grating may be a two-dimensional (2D) array of diffractive features. The diffraction grating may be a 2D array of bumps on or holes in a material surface, for example. In some examples, the diffraction grating may be substantially periodic in a first direction or dimension and substantially aperiodic (e.g., constant, random, etc.) in another direction across or along the diffraction grating. 
     As such, and by definition herein, the ‘diffraction grating’ is a structure that provides diffraction of light incident on the diffraction grating. If the light is incident on the diffraction grating from a light guide, the provided diffraction or diffractive scattering may result in, and thus be referred to as, ‘diffractive coupling’ or ‘diffractive scattering’ in that the diffraction grating may couple or scatter light out of the light guide by diffraction. The diffraction grating also redirects or changes an angle of the light by diffraction (i.e., at a diffractive angle). In particular, as a result of diffraction, light leaving the diffraction grating generally has a different propagation direction than a propagation direction of the light incident on the diffraction grating (i.e., incident light). The change in the propagation direction of the light by diffraction is referred to as ‘diffractive redirection’ herein. Hence, the diffraction grating may be understood to be a structure including diffractive features that diffractively redirects light incident on the diffraction grating and, if the light is incident from a light guide, the diffraction grating may also diffractively couple out the light from the light guide. 
     Further, by definition herein, the features of a diffraction grating are referred to as ‘diffractive features’ and may be one or more of at, in and on a material surface (i.e., a boundary between two materials). The surface may be a top surface or bottom surface of a light guide, for example. In other examples, the surface may be internal to the light guide. The diffractive features may include any of a variety of structures that diffract light including, but not limited to, one or more of grooves, ridges, holes and bumps at, in or on the surface. For example, the diffraction grating may include a plurality of substantially parallel grooves in the material surface. In another example, the diffraction grating may include a plurality of parallel ridges rising out of the material surface. The diffractive features (e.g., grooves, ridges, holes, bumps, etc.) may have any of a variety of cross sectional shapes or profiles that provide diffraction including, but not limited to, one or more of a sinusoidal profile, a rectangular profile (e.g., a binary diffraction grating), a triangular profile and a saw tooth profile (e.g., a blazed grating). 
     According to various examples described herein, a diffraction grating (e.g., a diffraction grating of a multibeam element, as described below) may be employed to diffractively scatter or couple light out of a light guide (e.g., a plate light guide) as a light beam. In particular, a diffraction angle of or provided by a locally periodic diffraction grating may be given by equation (1) as: 
     
       
         
           
             
               
                 
                   
                     θ 
                     m 
                   
                   = 
                   
                     
                       sin 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           n 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             i 
                           
                         
                         - 
                         
                           
                             m 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             λ 
                           
                           d 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where λ is a wavelength of the light, m is a diffraction order, n is an index of refraction of a light guide, d is a distance or spacing between features of the diffraction grating, θ i  is an angle of incidence of light on the diffraction grating. For simplicity, equation (1) assumes that the diffraction grating is adjacent to a surface of the light guide and a refractive index of a material outside of the light guide is equal to one (i.e., n out =1). In general, the diffraction order m is given by an integer. A diffraction angle θ m  of a light beam produced by the diffraction grating may be given by equation (1) where the diffraction order is positive (e.g., m&gt;0). For example, first-order diffraction is provided when the diffraction order m is equal to one (i.e., m=1). 
       FIG. 2  illustrates a cross sectional view of a diffraction grating  30  in an example, according to an embodiment consistent with the principles described herein. For example, the diffraction grating  30  may be located on a surface of a light guide  40 . In addition,  FIG. 2  illustrates a light beam  50  incident on the diffraction grating  30  at an incident angle θ i . The light beam  50  is a guided light beam within the light guide  40 . Also illustrated in  FIG. 2  is a directional light beam  60  diffractively produced and coupled-out by the diffraction grating  30  as a result of diffraction of the incident light beam  50 . The directional light beam  60  has a diffraction angle θ m  (or ‘principal angular direction’ herein) as given by equation (1). The diffraction angle θ m  may correspond to a diffraction order ‘m’ of the diffraction grating  30 , for example. Further, the diffractive features may be curved and may also have a predetermined orientation (e.g., a slant or a rotation) relative to a propagation direction of light, according to some embodiments. One or both of the curve of the diffractive features and the orientation of the diffractive features may be configured to control a direction of light coupled-out by the diffraction grating, for example. For example, a principal angular direction of the directional light may be a function of an angle of the diffractive feature at a point at which the light is incident on the diffraction grating relative to a propagation direction of the incident light. 
     Herein a ‘collimator’ is defined as substantially any optical device or apparatus that is configured to collimate light. For example, a collimator may include, but is not limited to, a collimating mirror or reflector, a collimating lens, a diffraction grating, or various combinations thereof. Herein, a ‘collimation factor,’ denoted σ, is defined as a degree to which light is collimated. In particular, a collimation factor defines an angular spread of light rays within a collimated beam of light, by definition herein. For example, a collimation factor σ may specify that a majority of light rays in a beam of collimated light is within a particular angular spread (e.g., +/−σ degrees about a central or principal angular direction of the collimated light beam). The light rays of the collimated light beam may have a Gaussian distribution in terms of angle and the angular spread may be an angle determined at one-half of a peak intensity of the collimated light beam, according to some examples. 
     Herein, a ‘light source’ is defined as a source of light (e.g., an optical emitter configured to produce and emit light). For example, the light source may comprise an optical emitter such as a light emitting diode (LED) that emits light when activated or turned on. In particular, herein the light source may be substantially any source of light or comprise substantially any optical emitter including, but not limited to, one or more of a light emitting diode (LED), a laser, an organic light emitting diode (OLED), a polymer light emitting diode, a plasma-based optical emitter, a fluorescent lamp, an incandescent lamp, and virtually any other source of light. The light produced by the light source may have a color (i.e., may include a particular wavelength of light), or may be a range of wavelengths (e.g., white light). In some embodiments, the light source may comprise a plurality of optical emitters. For example, the light source may include a set or group of optical emitters in which at least one of the optical emitters produces light having a color, or equivalently a wavelength, that differs from a color or wavelength of light produced by at least one other optical emitter of the set or group. The different colors may include primary colors (e.g., red, green, blue) for example. 
     By definition, ‘broad-angle’ emitted light is defined as light having a cone angle that is greater than a cone angle of the view of a multiview image or multiview display. In particular, in some embodiments, the broad-angle emitted light may have a cone angle that is greater than about twenty degrees (e.g., &gt;±20°). In other embodiments, the broad-angle emitted light cone angle may be greater than about thirty degrees (e.g., &gt;±30°), or greater than about forty degrees (e.g., &gt;±40°), or greater than about fifty degrees (e.g., &gt;±50°). For example, the cone angle of the broad-angle emitted light may be greater than about sixty degrees (e.g., &gt;±60°). 
     In some embodiments, the broad-angle emitted light cone angle may defined to be about the same as a viewing angle of an LCD computer monitor, an LCD tablet, an LCD television, or a similar digital display device meant for broad-angle viewing (e.g., about ±40-65°). In other embodiments, broad-angle emitted light may also be characterized or described as diffuse light, substantially diffuse light, non-directional light (i.e., lacking any specific or defined directionality), or as light having a single or substantially uniform direction. 
     Further, as used herein, the article ‘a’ is intended to have its ordinary meaning in the patent arts, namely ‘one or more’. For example, ‘a diffraction grating’ means one or more diffraction grating and as such, ‘the diffraction grating’ means ‘diffraction grating(s)’ herein. Also, any reference herein to ‘top’, ‘bottom’, ‘upper’, ‘lower’, ‘up’, ‘down’, ‘front’, back’, ‘first’, ‘second’, ‘left’ or ‘right’ is not intended to be a limitation herein. Herein, the term ‘about’ when applied to a value generally means within the tolerance range of the equipment used to produce the value, or may mean plus or minus 10%, or plus or minus 5%, or plus or minus 1%, unless otherwise expressly specified. Further, the term ‘substantially’ as used herein means a majority, or almost all, or all, or an amount within a range of about 51% to about 100%. Moreover, examples herein are intended to be illustrative only and are presented for discussion purposes and not by way of limitation. 
     According to some embodiments of the principles described herein, a multibeam backlight is provided.  FIG. 3A  illustrates a cross sectional view of a multibeam backlight  100  in an example, according to an embodiment consistent with the principles described herein.  FIG. 3B  illustrates a perspective view of the multibeam backlight  100  in an example, according to an embodiment consistent with the principles described herein. The perspective view in  FIG. 3B  is illustrated with a partial cut-away to facilitate discussion herein. 
     As illustrated, the multibeam backlight  100  comprises a light guide  110 . The light guide  110  is configured to guide light along a length of the light guide as guided light  104  (i.e., a guided light beam  104 ). For example, the light guide  110  may include a dielectric material configured as an optical waveguide. The dielectric material may have a first refractive index that is greater than a second refractive index of a medium surrounding the dielectric optical waveguide. The difference in refractive indices is configured to facilitate total internal reflection of the guided light  104  according to one or more guided modes of the light guide  110 , for example. 
     In some embodiments, the light guide  110  may be a slab or plate of an optical waveguide (i.e., a plate light guide) comprising an extended, substantially planar sheet of optically transparent, dielectric material. The substantially planar sheet of dielectric material is configured to guide the guided light  104  using total internal reflection. According to various examples, the optically transparent material of the light guide  110  may include or be made up of any of a variety of dielectric materials including, but not limited to, one or more of various types of glass (e.g., silica glass, alkali-aluminosilicate glass, borosilicate glass, etc.) and substantially optically transparent plastics or polymers (e.g., poly(methyl methacrylate) or ‘acrylic glass’, polycarbonate, etc.). In some examples, the light guide  110  may further include a cladding layer (not illustrated) on at least a portion of a surface (e.g., one or both of the first surface and the second surface) of the light guide  110 . The cladding layer may be used to further facilitate total internal reflection, according to some examples. 
     Further, according to some embodiments, the light guide  110  may be configured to guide the guided light  104  according to total internal reflection at a non-zero propagation angle between a first surface  110 ′ (e.g., front or top surface or side) and a second surface  110 ″ (e.g., back or bottom surface or side) of the light guide  110 . In particular, the guided light  104  propagates by reflecting or ‘bouncing’ between the first surface  110 ′ and the second surface  110 ″ of the light guide  110  at the non-zero propagation angle. In some embodiments, a plurality of guided light beams  104  comprising different colors of light may be guided by the light guide  110  at respective ones of different color-specific, non-zero propagation angles. Note, the non-zero propagation angle is not illustrated in  FIG. 3A  for simplicity of illustration. However, a bold arrow depicting a propagation direction  103  illustrates a general propagation direction of the guided light  104  along the light guide length (e.g., in an x-direction) in  FIG. 3A . 
     As illustrated in  FIGS. 3A-3B , the multibeam backlight  100  further comprises a plurality of multibeam elements  120  spaced apart from one another along the light guide length. In particular, the multibeam elements  120  of the multibeam element plurality are separated from one another by a finite space and represent individual, distinct elements along the light guide length. That is, by definition herein, the multibeam elements  120  of the multibeam element plurality are spaced apart from one another according to a finite (i.e., non-zero) inter-element distance (e.g., a finite center-to-center distance). Further, the multibeam elements  120  of the multibeam element plurality generally do not intersect, overlap or otherwise touch one another, according to some embodiments. That is, each multibeam element  120  of the multibeam element plurality is generally distinct and separated from other ones of the multibeam elements  120 . 
     According to some embodiments, the multibeam elements  120  of the multibeam element plurality may be arranged in either a one-dimensional (1D) array or a two-dimensional (2D) array. For example, the multibeam elements  120  may be arranged as a linear 1D array. In another example, the multibeam elements  120  may be arranged as a rectangular 2D array or as a circular 2D array. Further, the array (i.e., 1D or 2D array) may be a regular or uniform array, in some examples. In particular, an inter-element distance (e.g., center-to-center distance or spacing) between the multibeam elements  120  may be substantially uniform or constant across the array. In other examples, the inter-element distance between the multibeam elements  120  may be varied one or both of across the array and along the length of the light guide  110 . 
     According to various embodiments, a multibeam element  120  of the multibeam element plurality comprises a diffraction grating configured to diffractively scatter a portion of the guided light  104  out of the light guide  110  as directional light beams  102 . As such, the multibeam element  120  may be referred to as a ‘diffractive multibeam element.’ The directional light beams  102  scattered out by the multibeam element  120  have different directions corresponding to different view directions of a multiview display associated with the multibeam backlight  100 , according to various embodiments.  FIGS. 3A and 3B  illustrate the directional light beams  102  as a plurality of diverging arrows depicted as being directed way from the first (or front) surface  110 ′ of the light guide  110 . Light emitted by the multibeam backlight  100  as emitted light comprises the directional light beams  102  that are diffractively scattered out by the multibeam elements  120 . 
     According to various embodiments, a filling fraction of diffractive features within the diffraction grating is configured to control a diffractive scattering efficiency of the multibeam element  120 . Herein, a ‘filling fraction’ is defined as a percentage of an area of the diffraction grating that is filled with diffractive features. Equivalently, the ‘filling fraction’ may also be defined as a ratio of the area that includes diffractive features to an area that does not include diffractive features. For example, in some embodiments (described below), the diffraction grating may comprise filling features that interrupt the diffractive features. Accordingly, the ‘filling fraction’ may also be defined as a percentage of the diffraction grating area that includes of diffractive features as opposed to filling features, or equivalently, a ratio of the area that includes of diffractive features to an area that comprises filling features. 
       FIG. 4A  illustrates a plan view of a diffraction grating  125  having a filling fraction in an example, according to an embodiment consistent with the principles described herein. In particular, the diffraction grating  125  may be a diffraction grating of a multibeam element  120  of the multibeam backlight  100 . As illustrated, the diffraction grating  125  comprises diffractive features  126 . The diffractive features  126  are configured to diffractively redirect light incident on the diffraction grating  125  and may diffractively scatter light out of the light guide  110  by diffraction when the diffraction grating  125  is incorporated in a multibeam element  120 , according to various embodiments. 
     The diffraction grating  125  illustrated in  FIG. 4A  further comprises filling features  127  configured to provide a filling fraction of the diffraction features  126  or equivalently of the diffraction grating  125 . According to various embodiments, the filling features  127  are configured to be optically inert to light incident on the diffraction grating  125  along the propagation direction of the guided light (e.g., guided light  104 ). As such, the filling features  127  may provide little to no diffraction of the incident guided light. The filling fraction of the diffractive features  126  of  FIG. 4A  is the percentage of the diffraction grating  125  occupied by the diffractive features  126 , or the ratio of the diffraction features  126  to filling features  127 . 
       FIG. 4B  illustrates a plan view of another diffraction grating  125  having a filling fraction in an example, according to an embodiment consistent with the principles described herein. Again, the diffraction grating  125  illustrated in  FIG. 4B  may be a diffraction grating of a multibeam element  120  of the multibeam backlight  100 , in some embodiments. As illustrated in  FIG. 4B , the diffraction grating  125  comprises both diffractive features  126  and filling features  127  configured to provide the filling fraction. Further, an area occupied by the diffractive features  126  relative to an area having the filling features  127  is smaller than that of the diffraction grating  125  illustrated in  FIG. 4A . As a result, the filling fraction of the diffraction grating  125  of  FIG. 4B  is less than that of  FIG. 4A . With a lower filling fraction, the diffraction grating  125  of  FIG. 4B  may have a lower diffractive efficiency, or equivalently, may provide less diffraction of light incident on the diffraction grating  125  per unit area than the diffraction grating  125  of  FIG. 4A , for example. 
       FIG. 4C  illustrates a perspective view of a diffraction grating  125  having a filling fraction in an example, according to an embodiment consistent with the principles described herein. For example, the diffraction grating  125  illustrated in  FIG. 4C  may represent a perspective view of the diffraction grating  125  illustrated in  FIG. 4A . In particular, the diffraction grating  125  of  FIG. 4C  has both diffractive features  126  and filling features  127 . Further, the diffraction grating  125  is located on a surface of the light guide  110 , as illustrated. 
     In some embodiments, the diffractive features  126  of the diffraction grating  125  have an orientation that is orthogonal to or at least substantially orthogonal to a propagation direction of the guided light. Referring to  FIGS. 4A-4C , the diffractive features  126  are depicted as being generally oriented along ay-direction and orthogonal to the propagation direction  103  of the guided light  104  (as shown by an arrow), which is illustrated in the x-direction. Further, the filling features  127  of the diffraction gratings  125  illustrated in  FIGS. 4A-4C  have an orientation parallel to or substantially parallel to the propagation direction  103  and thus are oriented along the x-direction in the embodiments illustrated in  FIGS. 4A-4C . As a result, the filling features  127  intersect and interrupt the diffractive features  126  of the diffraction grating  125  to establish the filling fraction of the diffraction grating  125 , as illustrated. 
     In some embodiments, both the diffractive features  126  and filling features  127  of the diffraction grating  125  may comprise ridges on a surface of the light guide  110 . In other embodiments, both the diffractive features  126  and the filling features  127  of the diffraction grating  125  may comprise grooves in the light guide surface. 
     For example,  FIGS. 4A-4C  illustrate the diffractive features  126  and filling features  127  as ridges on the surface of the light guide  110 , where the ridges are depicted using a crosshatched area in  FIGS. 4A and 4B , for example. As illustrated, the filling features  127  are oriented to intercept and interrupt the diffractive features  126 , as previously described, at an intersection between a ridge of a diffractive feature  126  and a ridge of a filling feature  127 . In particular, the intersection between the diffractive feature  126  and the filling feature  127  is configured to introduce a gap in the diffractive feature  126  that interrupts the diffractive feature  126 . The gap in the ridge of the diffractive feature  126  represents a reduction of an area or length of diffractive features  126  that reduces the diffractive efficiency of the diffraction grating  125  according to the filling fraction. 
       FIG. 5A  illustrates a diffraction grating  125  having a filling fraction in an example, according to another embodiment consistent with the principles described herein.  FIG. 5B  illustrates a diffraction grating  125  having a filling fraction in an example, according to another embodiment consistent with the principles described herein. In particular, in  FIGS. 5A and 5B , diffractive features  126  and filling features  127  of the diffraction grating  125  comprise grooves in the light guide surface. As with ridges of  FIGS. 4A-4C , the grooves representing the diffractive features  126  illustrated in  FIGS. 5A-5B  are oriented orthogonal to the propagation direction  103  of the guided light  104 , while grooves representing the filling features  127  are oriented parallel to the propagation direction  103  of the guided light  104 . Also, as was the case in  FIGS. 4A-4C , the grooves of the filling features  127  illustrated in  FIGS. 5A-5B  are configured to intersect and interrupt the grooves representing the diffractive features  126  of the diffraction grating  125 . In particular, an intersection between a groove of the diffractive features  126  and a groove of the filling features  127  illustrated in  FIGS. 5A-5B  is configured to introduce a gap in the diffractive feature  126  that interrupts the diffractive feature  126 . The gap in the groove of the diffractive features  126  represents a reduction of a length or an area of diffractive features  126  that diminishes the diffractive efficiency of the diffraction grating  125 , accordingly. 
     In some embodiments, the filling fraction may be configured to increase as a function of distance along the length of the light guide  110 . The increase in the filling fraction may provide a corresponding increase in diffractive scattering efficiency of multibeam elements  120  of the multibeam element plurality. The increase in diffractive scattering efficiency may be configured to compensate for a concomitant reduction in an intensity of guided light within the light guide  110  along the light guide length, in some embodiments. In other embodiments, the filling fraction of the diffractive features  126  may be configured to follow other functions of distance. For example, the filling fraction may be configured to decrease as a function of distance along the light guide length. In some embodiments, the filling fraction may be configured to increase up to the certain point of the light guide length, and then decrease for the remainder of the light guide length. The filling fraction may also be configured to vary along the light guide length various functions of distance. For example, the filling fraction may have configured to vary linearly, logarithmically, or vary as a sinusoidal wave as a function of the light guide length. 
     In some embodiments, the diffraction grating  125  may further comprise a reflective material layer, or more particularly, a reflective island comprising a reflective material or reflective material layer. A reflective material of the reflective material layer or reflective island may comprise substantially any reflective material or reflective material layer including, but not limited to, a reflective metal (e.g., aluminum, silver, gold, etc.) or a reflective polymer (e.g., an aluminum polymer composite) as well as various reflective films, e.g., an enhanced specular reflector (ESR) film such as Vikuiti™ ESR, manufactured by 3M corporation, St. Paul, Minn. In some embodiments, the filling fraction may be either provided or augmented by the reflective material layer or reflective island. 
       FIG. 6A  illustrates a cross sectional view of a multibeam element  120  having a reflective island  129  in an example, according to an embodiment consistent with the principles described herein.  FIG. 6B  illustrates a plan view of a multibeam element  120  having a reflective island  129  in an example, according to another embodiment of the principles described herein. In particular, the multibeam element  120  illustrated in  FIGS. 6A-6B  comprises a diffraction grating  125  at a surface of the light guide  110  and a reflective island  129  adjacent to the light guide surface. Also illustrated are diffractive features  126  of the diffraction grating  125 , by way of example and not limitation. According to various embodiments, the reflective island  129  of the multibeam element  120  comprises a reflective material or reflective material layer and is configured to redirect a portion of the diffractively scattered light in a direction of the directional light beams  102 . In some embodiments, the reflective island  129  may have an extent corresponding to an extent of the diffraction grating  125 . 
     As illustrated in  FIG. 6B , the reflective island  129  may comprise openings  129 ′ in a reflective material of the reflective island  129 . A ratio of an area of the reflective island  129  to an area of the openings  129 ′ within the diffraction grating  125  may define or correspond to the filling fraction, in some embodiments. For example, as illustrated  FIG. 6B , the filling fraction of the diffraction grating  125  may be about fifty percent (50%). Correspondingly, a ratio of an area of the reflective material to an area of the openings  129 ′ within the diffraction grating  125  is about 50% (e.g., equivalently, the reflective island  129  covers about half of the multibeam element  120 ). In other embodiments (not illustrated), the reflective material may reside within the ridges or grooves that provide the diffractive features and filling features of the diffraction grating  125 . The reflective material being within the grooves or ridges may enhance a performance of the diffractive features, for example. 
     Referring back to  FIG. 3A , the multibeam backlight  100  may further comprise a light source  130 . According to various embodiments, the light source  130  is configured to provide the light to be guided within light guide  110 . In particular, the light source  130  may be located adjacent to an entrance surface or end (input end) of the light guide  110 . In various embodiments, the light source  130  may comprise substantially any source of light (e.g., optical emitter) including, but not limited to, one or more light emitting diodes (LEDs) or a laser (e.g., laser diode). In some embodiments, the light source  130  may comprise an optical emitter configured produce a substantially monochromatic light having a narrowband spectrum denoted by a particular color. In particular, the color of the monochromatic light may be a primary color of a particular color space or color model (e.g., a red-green-blue (RGB) color model). In other examples, the light source  130  may be a substantially broadband light source configured to provide substantially broadband or polychromatic light. For example, the light source  130  may provide white light. In some embodiments, the light source  130  may comprise a plurality of different optical emitters configured to provide different colors of light. The different optical emitters may be configured to provide light having different, color-specific, non-zero propagation angles of the guided light corresponding to each of the different colors of light. 
     In some embodiments comprising a light source  130  as described above, the filling fraction of diffractive features of the diffraction grating  125  may be configured to control the diffractive scattering efficiency of the multibeam element as a function of a distance from the light source  130  along the light guide  110 . For example, the filling fraction may be configured to increase as a function of distance from the light source  130  along the length of the light guide  110 , the increase in the filling fraction providing a corresponding increase in diffractive scattering efficiency of multibeam elements  120  of the multibeam element plurality to compensate for a reduction in an intensity of guided light within the light guide along the light guide length, in some embodiments. 
       FIG. 3A  further illustrates an array of light valves  140 . As illustrated, the array of light valves  140  is configured to modulate the directional light beams  102  of the directional light beam plurality. In various embodiments, different types of light valves may be employed as the light valves  140  of the light valve array including, but not limited to, one or more of liquid crystal light valves, electrophoretic light valves, and light valves based on electrowetting. 
     The array of light valves  140  may be part of a multiview display that employs the multibeam backlight  100 , for example, and is illustrated in  FIGS. 3A and 3B  along with the multibeam backlight  100  for the purpose of facilitating discussion herein. As such, principal angular directions of the directional light beams  102  correspond to view directions of the multiview display. Further, in some embodiments a size of the multibeam element  120  may be between about twenty-five percent (25%) and about two hundred percent (200%) of a size of a light valve  140  of the light valve array. In other embodiments, the multibeam element size may be between about fifty percent (50%) and about one hundred fifty percent (150%) of the light valve size. For example, the multibeam element size and the light valve size may be substantially equal in size. 
     In accordance with some embodiments of the principles described herein, a multiview display is provided.  FIG. 7  illustrates a block diagram of a multiview display  200  in an example, according to an embodiment consistent with the principles described herein. As illustrated, the multiview display  200  comprises a light guide  210  configured to guide light along the light guide. In some embodiments, the light guide  210  may be substantially similar to the light guide  110  of the multibeam backlight  100 , previously described. As such, the light guide  210  may be configured to guide the guided light using total internal reflection. Further, the guided light may be guided at a non-zero propagation angle by or within the light guide  210 . In some embodiments, the guided light may be collimated or may be a collimated light beam. In particular, the guided light may be collimated according to or having a collimation factor σ, in some embodiments. 
     The multiview display  200  further comprises an array of multibeam elements  220  spaced apart from one another along the light guide  210 . The array of multibeam elements  220  is configured to scatter out guided light from the light guide  210  as directional light beams  202  having directions corresponding to view directions of the multiview display  200 . The multibeam elements  220  of the multibeam element array may be located on surface of or within the light guide  210 , according to various embodiments. In some embodiments, a multibeam element  220  of the multibeam element array may be substantially similar to the multibeam element  120  of the multibeam backlight  100 , described above. In particular, the multibeam element  220  of the multibeam element array comprises a diffraction grating having diffractive features and filling features. 
     The multiview display  200  further comprises an array of light valves  230 . The array of light valves  230  is configured to modulate the directional light beams  202  to provide a multiview image. In some embodiments, the array of light valves  230  may be substantially similar to the array of light valves  140  described above with respect to the multibeam backlight  100 . For example, the array of light valves  230  may employ any of a variety of different types of light valves including, but not limited to, one or more of liquid crystal light valves, electrophoretic light valves, and light valves based on electrowetting. 
     According to various embodiments, a filling fraction of the diffractive features relative to the filling features within the diffraction grating of the multibeam elements  220  is configured to control a diffractive scattering efficiency of the multibeam element  220 . The filling fraction may be defined as a percentage of an area of the diffraction grating that is filled with diffractive features, or as a ratio of areas of diffractive features to areas of filling features within a diffraction grating. The diffractive efficiency of the multibeam element may increase concomitant with the filling fraction of the diffractive features, as described above with respect to the multibeam backlight  100 . 
     In some embodiments, a size of the multibeam element  220  of the multibeam element array is comparable to a size of a light valve  230  of the light valve array. In some embodiments, the size of the multibeam emitter is comparable to the light valve size such that the multibeam element size is between about one quarter and about two times of the light valve size. In other embodiments, the multibeam element size may be between about fifty percent (50%) and about two hundred percent (200%) of the light valve size. The correspondence between the multibeam element size and the light valve size may be configured to minimize or even eliminate Moiré or similar effects, for example. 
     In some embodiments, the filling features are arranged within the diffraction grating of the multibeam element  220  parallel to a propagation direction of the guided light. In this orientation, the filling features are configured to intersect and interrupt the diffractive features of the diffraction grating to establish the filling fraction as a ratio of an area of the diffractive features relative to an area of the filling features within the diffraction grating, as previously described with respect to the multibeam backlight  100 . In some embodiments, both the diffractive features and the filling features comprise one of grooves in a surface of the light guide  210  and ridges on the surface of the light guide  210 . 
     In some embodiments, the multibeam element  220  of the multibeam element array further comprises a reflective material layer configured to reflect diffractively scattered light in a direction of the directional light beams  202 . The reflective material layer may comprise a reflective material substantially similar to the reflective material described above with respect to the multibeam backlight  100 . In some embodiments, the reflective material layer may be located in the grooves of the diffractive features and the filling features. In some embodiments, the reflective material layer may be located between the ridges of the diffractive features and the filling features. In some embodiments, the reflective material layer may comprise a reflective island and may include openings, as is described above. 
     As illustrated in  FIG. 7 , the multiview display  200  may further comprise a broad-angle backlight  240  adjacent to the light guide  210 . The broad-angle backlight  240  illustrated  FIG. 7  is adjacent to a side of the light guide  210  opposite to the light valve array. In particular, as illustrated, the broad-angle backlight  240  is adjacent to a bottom surface of the light guide  210 . The broad-angle backlight  240  is configured to provide broad-angle light as broad-angle emitted light  242  during a two-dimensional (2D) mode of the multiview display  200 . Further, the light valve array may be configured to modulate the broad-angle emitted light as a 2D image during the 2D mode. 
     According to some embodiments, the light guide  210  and the array of multibeam elements  220  may be configured to be optically transparent to the broad-angle emitted light  242  provided by the adjacent broad-angle backlight  240 . Thus, broad-angle emitted light  242  may be configured to pass through a thickness of light guide  210 . The broad-angle emitted light  242  from the broad-angle backlight  240  is therefore received through the bottom surface of the light guide  210 , transmitted through a thickness of the light guide  210 , and emitted from the array of light valves  230 . Because the light guide  210  is optically transparent to the broad-angle light, the broad-angle emitted light  242  is not substantially affected by the light guide  210 . 
     The multiview display  200  of  FIG. 7  may selectively operate in either a two-dimensional (2D) mode or a multiview mode. In the 2D mode, the multiview display  200  is configured to emit the broad-angle emitted light  242  provided by the broad-angle backlight  240 . In the multiview mode, the multiview display  200  is configured to emit the directional light beams  202  provided by the light guide  210  and multibeam elements  220 , as previously described. The combination of the light guide  210  and broad-angle backlight  240  may be used in mode switchable (2D/multiview) display, for example. 
     According to some embodiments of the principles described herein, a method of multiview display operation is provided.  FIG. 8  illustrates a flowchart of the method  300  of multiview display operation in an example, according to an embodiment consistent with the principles described herein. As illustrated, the method  300  comprises guiding  310  light along a light guide as guided light. In some embodiments, the light guide may be substantially similar to the light guide  110  described above with respect to the multibeam backlight  100 . For example, the guided light is guided and propagates along the light guide using total internal reflection within the light guide. In some embodiments, the guided light may be guided at a non-zero propagation angle within the light guide. Further, the guided light may be collimated according to a collimation factor, in some embodiments. 
     The method  300  of multiview display operation illustrated in  FIG. 8  further comprises scattering out  320  a portion of the guided light as directional light beams using a multibeam element of a plurality of multibeam elements arranged along the light guide. In some embodiments, the multibeam element may be substantially similar the multibeam element  120  of the above-described multibeam backlight  100 . In particular, the multibeam element of the plurality of multibeam elements comprises a diffracting grating having diffractive features and filling features that interrupt the diffractive features, according to some embodiments. 
     As illustrated in  FIG. 8 , the method  300  of multiview display operation further comprises modulating  330  the directional light beams using an array of light valves to provide a multiview image. In some embodiments, the array of light valves may be substantially similar to the array of light valves  140  described above with respect to the multibeam backlight  100 . In some embodiments, a size of the multibeam element is comparable to a size of a light valve of the array of light valves. 
     According to various embodiments, a filling fraction of the diffractive features relative to the filling features within the diffraction grating controls a diffractive scattering efficiency of the multibeam element as a function of distance along the light guide. In some embodiments, the filling fraction is configured to increase as a function of distance along the length of the light guide, causing the diffractive scattering efficiency to increase as a function of the same distance. 
     In some embodiments, the filling features are parallel to a propagation direction of the guided light within the light guide. In some embodiments, the diffraction features are oriented perpendicularly to the propagation direction of the guided light, and gaps formed in the diffractive features by the filling features reduce an area or length of diffractive features to decrease a diffractive efficiency of the diffraction grating. Both the diffractive features and the filling features may comprise one of grooves in a surface of the light guide and ridges on the surface of the light guide, in some embodiments. In some embodiments, a reflective material layer may be located in the grooves of the diffractive features and the filling features or between the ridges of the diffractive features and the filling features. In other embodiments, the reflective material layer may be located adjacent to, but separated from, the grooves or ridges. Openings in the reflective material layer may provide or augment the filling fraction, according to some embodiments. In some embodiments, an extent of the reflective material layer may be comparable to a size or extent of the diffraction grating of the multibeam element. As such, the reflective material layer may be a reflective island, in some embodiments. 
     Thus, there have been described examples and embodiments of a multibeam backlight, a multiview display, and a method of multiview display operation that employ a filling fraction of diffractive features within a diffraction grating to control a diffractive scattering efficiency of multibeam elements comprising the diffraction grating. It should be understood that the above-described examples are merely illustrative of some of the many specific examples that represent the principles described herein. Clearly, those skilled in the art can readily devise numerous other arrangements without departing from the scope as defined by the following claims.