Patent Publication Number: US-2022236596-A1

Title: Privacy-mode backlight, privacy display, and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation patent application of and claims priority to International Patent Application No. PCT/US2019/056402, 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. 1  illustrates a graphical representation of angular components of a directional light beam having a particular principal angular direction 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 privacy-mode backlight in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 3B  illustrates another cross-sectional view of a privacy-mode backlight in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 3C  illustrates a top view of a privacy-mode backlight in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 4  illustrates a side view of an effect of a directional optical diffuser on an image of a scattering line element in an example, according to an embodiment of the principles described herein. 
         FIG. 5A  illustrates cross-sectional view of a privacy-mode backlight in an example, according to another embodiment consistent with the principles described herein. 
         FIG. 5B  illustrates a top view of a privacy-mode backlight in an example, according to another embodiment consistent with the principles described herein. 
         FIG. 6A  illustrates a cross-sectional view of a mode-switchable display including a privacy-mode backlight in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 6B  illustrates a cross-sectional view of a mode-switchable display including a privacy-mode backlight in another example, according to an embodiment consistent with the principles described herein. 
         FIG. 7  illustrates a block diagram of a privacy display in an example, according to an embodiment consistent with the principles described herein. 
         FIG. 8  illustrates a flow chart of a method of privacy-mode backlight 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 a privacy-mode backlight. The privacy-mode backlight includes a light guide that guides light as guided light along a length of the light guide, where the guided light has a predetermined collimation factor. Moreover, the privacy-mode backlight includes a plurality of scattering line elements arranged parallel to and spaced apart from one another along the light guide length. Each of the scattering line elements is configured to scatter out through an emission surface of the light guide a portion of the guided light as emitted light, and the emitted light has an illumination beamwidth in in a direction orthogonal to the light guide length determined by the collimation factor. Furthermore, the privacy-mode backlight includes a directional optical diffuser configured to provide directional diffusion of the emitted light in a direction corresponding to the light guide length. The directional optical diffuser may be configured to provide a uniform illumination pattern of the emitted light in the direction corresponding to the light guide length. For example, a diffusion angle of the directional optical diffuser may be configured to spread out the emitted light from each of the scattering line elements to have an illumination extent at an output plane of the privacy-mode backlight that is equivalent to a distance (e.g., a center-to-center spacing) between adjacent scattering line elements of the scattering line element plurality. Consequently, the imaged scattering line elements may collectively appear to completely fill the length of the light guide with spaces between the imaged scattering line elements. 
     In some embodiments, the privacy-mode backlight is included in a display such as a privacy display configured to provide a private image to a user, where the private image is exclusively visible within an illumination beamwidth along a privacy axis of the privacy-mode backlight (along the length of the light guide). In other embodiments, the privacy-mode backlight may be part of a mode-switchable display that is configured to provide the private image during a privacy mode of the mode-switchable display and a shared image during a public mode of the mode-switchable display. In particular, the mode-switchable display may include a broad-angle backlight configured to provide broad-angle light during a shared mode, where the broad-angle light has a broad-angle illumination beamwidth enabling the user to view the shared image over a much wider angular range than that of the private image. Thus, the mode-switchable display may be configured to selectively display the private image during the private mode and the shared image during the share mode. 
     Herein, a light beam having a direction is referred to as a ‘directional light beam’ and may have 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 directional light beam. The angular component ϕ is referred to as the ‘azimuth component’ or ‘azimuth angle’ of the directional light beam. By definition, the elevation angle θ is an angle in a vertical plane (e.g., perpendicular to a plane of the display screen while the azimuth angle ϕ is an angle in a horizontal plane (e.g., parallel to the display screen plane).  FIG. 1  illustrates a graphical representation of the angular components {θ, ϕ} of a directional light beam  20  having a particular principal angular direction 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 directional light beam  20  has a central ray associated with a particular point of origin O, as illustrated. 
     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. 
     Further herein, the term ‘plate’ when applied to a light guide as in a ‘plate light guide’ is defined as a piece-wise or differentially planar layer or sheet, which is sometimes referred to as a ‘slab’ guide. In particular, a plate light guide is defined as a light guide configured to guide light in two substantially orthogonal directions bounded by a top surface and a bottom surface (i.e., opposite surfaces) of the light guide. Further, by definition herein, the top and bottom surfaces are both separated from one another and may be substantially parallel to one another in at least a differential sense. That is, within any differentially small section of the plate light guide, the top and bottom surfaces are substantially parallel or co-planar. 
     In some embodiments, the plate light guide may be substantially flat (i.e., confined to a plane) and therefore, the plate light guide is a planar 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 broadly 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 manner or a 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. 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’ in that the diffraction grating may couple 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 below a top surface of a light guide, for example. 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 plurality of diffraction gratings, 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 θ m  of or provided by a locally periodic diffraction grating may be given by equation (1) as: 
     
       
         
           
             
               
                 
                   
                     θ 
                     m 
                   
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                             θ 
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                   ( 
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     where π 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 (i.e., m=±1, ±2, . . . ). A diffraction angle θ m  of a light beam produced by the diffraction grating may be given by equation (1). First-order diffraction or more specifically a first-order diffraction angle θ m  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 or scattered-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 directional light beam  60  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. 
     While a plurality of diffraction gratings is used as an illustrative example in the discussion that follows, in some embodiments other components may be used, such as at least one of a micro-reflective element and a micro-refractive element. For example, the micro-reflective element may include a triangular-shaped mirror, a trapezoid-shaped mirror, a pyramid-shaped mirror, a rectangular-shaped mirror, a hemispherical-shaped mirror, a concave mirror and/or a convex mirror. In some embodiments, a micro-refractive element may include a triangular-shaped refractive element, a trapezoid-shaped refractive element, a pyramid-shaped refractive element, a rectangular-shaped refractive element, a hemispherical-shaped refractive element, a concave refractive element and/or a convex refractive element. 
     According to various embodiments, one or both of a principle angular direction and an angular spread of the directional light beam  60  exiting a diffraction grating  30  may be determined by a characteristic of the diffraction grating  30  including, but not limited to, a size (e.g., one or more of length, width, area, and etc.) of the diffractive grating  30  along with a ‘grating pitch’ or a diffractive feature spacing and an orientation of a diffraction grating. In some embodiments, the diffractive grating  30  or more generally a scattering element may be considered an ‘extended point light source’, i.e., a plurality of point light sources distributed across an extent of the diffraction grating  30  or scattering element, by definition herein. Further, a directional light beam produced by the diffraction grating or a scattering element has a principal angular direction given by angular components {θ, ϕ}, by definition herein, and as described above with respect to  FIG. 1 . 
     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 diffraction grating, a collimating lens, or various combinations thereof. According to various embodiments, an amount of collimation provided by the collimator may vary in a predetermined degree or amount from one embodiment to another. Further, the collimator may be configured to provide collimation in one or both of two orthogonal directions (e.g., a vertical direction and a horizontal direction). That is, the collimator may include a shape or related characteristic in one or both of two orthogonal directions that provides light collimation, according to some embodiments. 
     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. 
     Herein, an ‘angle-preserving scattering feature’ or equivalently an ‘angle-preserving scattering element’ is any feature, element, or scatterer configured to scatter light in a manner that substantially preserves in scattered light an angular spread of light incident on the feature, element, or scatterer. In particular, by definition, an angular spread σ s  of light scattered by an angle-preserving scattering feature is a function of an angular spread σ of the incident light (i.e., σ s =f(σ)). In some embodiments, the angular spread σ s  of the scattered light is a linear function of the angular spread or collimation factor σ of the incident light (e.g., σ s =a·σ, where a is a positive scale factor). That is, the angular spread σ s  of light scattered by an angle-preserving scattering feature may be substantially proportional to the angular spread or collimation factor σ of the incident light. For example, the angular spread σ s  of the scattered light may be substantially equal to the incident light angular spread σ (e.g., σ s ≈σ). A uniform diffraction grating (i.e., a diffraction grating having a substantially uniform or constant diffractive feature spacing or grating pitch) is an example of an angle-preserving scattering feature. 
     By definition, ‘broad-angle’ emitted light is defined as light having a cone angle that is greater than a cone angle of emitted light used to provide a private image or in privacy 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 be 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, ‘an element’ means one or more elements and as such, ‘the element’ means ‘the element(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 privacy-mode backlight is provided.  FIG. 3A  illustrates a cross-sectional view of a privacy-mode backlight  100  in an example, according to an embodiment consistent with the principles described herein.  FIG. 3B  illustrates another cross-sectional view of a privacy-mode backlight  100  in an example, according to an embodiment consistent with the principles described herein.  FIG. 3C  illustrates a top view of the privacy-mode backlight  100  in an example, according to an embodiment consistent with the principles described herein. In various embodiments, the privacy-mode backlight  100  is configured to emit light as emitted light  102  having a predetermined illumination beamwidth. 
     As illustrated, the privacy-mode backlight  100  comprises a light guide  110 . The light guide  110  is configured to guide light along a length of the light guide  110  as guided light  104  (i.e., a guided light beam  104 ). For example, the light guide  110  may include a material (such as a dielectric material) configured to function 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 particular, the light guide  110  may be a slab or plate 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 top surface and the bottom surface) of the light guide  110 . The cladding layer may be used to further facilitate total internal reflection, according to some examples. 
     According to various embodiments, the light guide  110  is 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’ 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 comprising different colors of light may be guided by the light guide  110  as the guided light  104  at respective ones of different color-specific, non-zero propagation angles. 
     As defined herein, a ‘non-zero propagation angle’ is an angle relative to a surface (e.g., the first surface  110 ′ or the second surface  110 ″) of the light guide  110 . Further, the non-zero propagation angle is both greater than zero and less than a critical angle of total internal reflection within the light guide  110 , according to various embodiments. For example, the non-zero propagation angle of the guided light  104  may be between about ten degrees (10°) and about fifty degrees (50°) or, in some examples, between about twenty degrees (20°) and about forty degrees (40°), or between about twenty-five degrees (25°) and about thirty-five degrees (35°). For example, the non-zero propagation angle may be about thirty degrees (30°). In other examples, the non-zero propagation angle may be about 20°, or about 25°, or about 35°. Moreover, a specific non-zero propagation angle may be chosen (e.g., arbitrarily) for a particular implementation as long as the specific non-zero propagation angle is chosen to be less than the critical angle of total internal reflection within the light guide  110 . 
     Further, the guided light  104 , or equivalently the guided light beam  104 , provided by coupling light into the light guide  110  may be a collimated light beam, according to various embodiments. Herein, a ‘collimated light’ or a ‘collimated light beam’ is generally defined as a beam of light in which rays of the light beam are substantially parallel to one another within the light beam (e.g., the guided light beam  104 ). Also, by definition herein, rays of light that diverge or are scattered from the collimated light beam are not considered to be part of the collimated light beam. In some embodiments (not illustrated) a collimator, such as a lens, diffraction grating, reflector or mirror, as described above, may be included to collimate the light, e.g., from a light source. In other embodiments, the light source itself may comprise a collimator. The collimated light provided to and guided by the light guide  110  as the guided light  104  may be a collimated guided light beam. In particular, the guided light  104  may be collimated according to or having a collimation factor σ, in various embodiments. In some embodiments, the guided light  104  has predetermined collimation factor in a width direction that is orthogonal to the light guide length. As illustrated, the width direction of the privacy-mode backlight  100  corresponds to a y-direction and the light guide length or length direction corresponds to an x-direction. 
     The privacy-mode backlight  100  illustrated in  FIGS. 3A-3C  further comprises a plurality of scattering line elements  120 . According to various embodiments, individual scattering line elements  120  of the scattering line element plurality are arranged parallel to and spaced apart from one another along the length direction (i.e., x-direction) of the light guide  110 . In particular, the scattering line elements  120  of the plurality are separated from one another by a finite (i.e., non-zero) inter-element distance or space and represent individual, distinct elements along the light guide length (i.e., x-direction, as illustrated), by definition herein. Further, the scattering line elements  120  of the plurality generally do not intersect, overlap or otherwise touch one another, according to various embodiments. As such, each scattering line element  120  of the scattering line element plurality is generally distinct and separated from other ones of the scattering line elements  120 , at least in the length or x-direction. 
     According to various embodiments, each of scattering line elements  120  of the scattering line element plurality is configured to scatter out through an emission surface of the light guide  110  (e.g., such as the first surface  110 ′) a portion of the guided light  104  as the emitted light  102 . Further, the scattering line elements  120  are configured to provide the emitted light  102  having an illumination beamwidth γ in the direction orthogonal length direction that is determined by the collimation factor σ of the guided light  104 , according to various embodiments. In  FIG. 3B , the illumination beamwidth γ in the orthogonal direction (or width direction) is depicted as being an angle in plane parallel to the y-direction. In some embodiments, the scattering line elements  120  may be substantially unidirectional scattering elements configured preferentially scatter out the guided light  104  in a direction of or toward an emission surface of the light guide  110 . For example, preferential scattering toward the emission surface is illustrated in  FIG. 3A  by arrows point toward the first surface  110 ′ of the light guide  110  by way of example and not limitation. Further, the scattering line elements  120  may be angle-preserving scatterers, where the illumination beamwidth γ of the emitted light  102  in the orthogonal or width direction is a linear function of the collimation factor σ of the guided light  104  (e.g., γ=k·σ, where k is a constant scale factor). 
     According to various embodiments, the privacy-mode backlight  100  further comprises a directional optical diffuser  130 . The directional optical diffuser is configured to provide directional diffusion of the emitted light in a direction corresponding to the light guide length. In particular, the directional optical diffuser may have a diffuser axis along the length direction of the light guide  110 , i.e., x-direction as illustrated. The directional optical diffuser  130  having the diffuser axis oriented along the length direction is configured to provide directional diffusion of the emitted light  102  in a direction corresponding to the light guide length, i.e., the x-direction, as illustrated. In various embodiments, the directional diffusion of the emitted light  102  provided by the directional optical diffuser  130  may be configured to effectively expand the apparent size of scattering line elements  120  to provide a uniform or substantially uniform illumination pattern of the emitted light  102  in the direction corresponding to the light guide length. Further, the directional optical diffuser  130  may provide substantially little or no diffusion in the width direction or y-direction, in some embodiments. For example, the directional optical diffuser  130  may be a one-dimensional (1D) optical diffuser. As such, the directional optical diffuser  130  is configured to substantially preserve the illumination beamwidth of the emitted light  102  in the orthogonal direction (i.e., width or y-direction) in order to ensure viewing privacy of the privacy-mode backlight  100 , while simultaneously providing uniform illumination in or along the length direction. 
     For example, the emitted light  102  may have the illumination beamwidth γ in the orthogonal or y-direction direction, while illumination by the emitted light  102  in the x-direction is comprises a substantially uniform illumination pattern. The uniform illumination pattern may facilitate high resolution of a display that employs the privacy-mode backlight  100  since each pixel or light valve of the display may be illuminated in a substantially uniform manner. 
     In some embodiments, a diffusion angle of the directional optical diffuser  130  is configured to effectively spread out the emitted light  102  from each of the plurality of scattering line elements  120  to have an illumination extent at an output plane of the privacy-mode backlight  100  that is equivalent to a distance between adjacent scattering line elements  120  of the scattering line element plurality. For example, the extent of the image of the scattering line elements  120  provided by the diffusion angle of the directional optical diffuser  130  may be equal to or greater than a pitch of the adjacent scattering line elements  120 . Stated differently, the diffusion angle of the directional optical diffuser may be chosen such that an image of a scattering line element  120  has an extent at the output plane that makes the plurality of scattering line elements  120  appear to be a uniform, continuous scattering element in the x-direction. As such, an effective light source provided by the privacy-mode backlight  100  along the x-direction may appear to be uniform over the length of the light guide  110  (i.e., along the x-direction). In some embodiments, the effective light source may equal to a product of the diffusion angle of the directional optical diffuser  130  and a thickness t of the light guide  110 , divided by an index of refraction of the light guide  110 . 
     For example, a size of the effective light source LS in the image of the scattering line element  120  may be given in terms of the thickness t by equation (2) as 
     
       
         
           
             
               
                 
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                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where δn x  is the diffusion angle of the directional optical diffuser  130  and n is an index of refraction of the light guide  110 . In some embodiments, the effective light source size may be equal to or even greater than a pitch or spacing between the scattering line elements  120  to insure uniform illumination in the length direction. 
       FIG. 4  illustrates a side view of an effect of a directional optical diffuser  130  on an image of a scattering line element  120  in an example, according to an embodiment of the principles described herein. In particular,  FIG. 4  illustrates several scattering line elements  120  each having a size A and a spacing or pitch p. Light  106  scattered out of the light guide  110  by the scattering line elements  120  passes through the directional optical diffuser  130  of the privacy-mode backlight  100  as emitted light  102 . The emitted light  102  passing through the directional optical diffuser  130  produces a scattering line element image  120 ′ of the scattering line elements  120  at an output plane  100 ′ of the privacy-mode backlight  100 , as illustrated. Further, as illustrated, the diffusion angle of the directional optical diffuser  130  effectively expands an extent of the scattering line element image  120 ′. In some embodiments, the diffusion angle may be chosen or predetermined to expand the extent of the scattering line element images  120 ′ sufficiently to make the scattering line elements  120  appear at the output plane  100 ′ to be substantially continuous in the x-direction, e.g., as illustrated. 
     Expanding an extent of the images of the scattering line elements  120  or equivalently spreading out the emitted light  102  from each of the plurality of scattering line elements  120  using the directional optical diffuser  130  may allow the density of the scattering line elements  120  to be reduced, while still providing uniform illumination. For example, the density of the scattering line elements  120  may be reduced to less than and in some examples, much less than, one scattering line element  120  per pixel (such as an integer fraction per pixel) of a display that employs the privacy-mode backlight  100 . This may reduce a complexity of the privacy-mode backlight  100 , which may increase the manufacturing yield and, thus, may reduce the cost of the privacy-mode backlight  100  or a display that includes the privacy-mode backlight  100 , according to some embodiments. According to some embodiments, the directional optical diffuser  130  may comprise an anisotropic light-diffusing layer or film such as, but not limited to, a holographic diffuser configured to provide anisotropic light diffusion. 
     In some embodiments, the scattering line element  120  may be a continuous or substantially continuous scattering structure along the length of the scattering line element  120  (i.e., continuous in a width or y-direction, as illustrated in  FIGS. 3B-3C ). In other embodiments, a scattering line element  120  of the scattering line element plurality may comprise an array of individual scattering elements arranged in a line along a length of the scattering line element  120  (i.e., a linear array extending in the width or the y-direction, as illustrated). In particular, adjacent individual scattering elements of the individual scattering element array may be separated from one another by a gap, in some embodiments. 
       FIG. 5A  illustrates cross-sectional view of a privacy-mode backlight  100  in an example, according to another embodiment consistent with the principles described herein.  FIG. 5B  illustrates a top view of a privacy-mode backlight  100  in an example, according to another embodiment consistent with the principles described herein. As illustrated in  FIGS. 5A-5B , the privacy-mode backlight  100  is substantially similar to the privacy-mode backlight  100  illustrated in  FIGS. 3A-3C . In particular,  FIGS. 5A-5B  illustrate the privacy-mode backlight  100  comprising the light guide  110 , the scattering line elements  120 , and the directional optical diffuser  130 . However, in the embodiment illustrated in  FIGS. 5A-5B , the scattering line elements  120  of the scattering line element plurality each comprise an array of individual scattering elements  122 . Further, the array of individual scattering elements  122  is arranged as linear array along a length of the scattering line element  120  within the light guide  110  with each individual scattering element  122  being separated from adjacent individual scattering elements by a gap. In these embodiments where individual scattering elements  122  are separated by a gap from one another, the directional optical diffuser  130  may be further configured to provide directional diffusion of the emitted light  202  in the orthogonal direction corresponding to a width direction of the light guide  110 . 
     Accordingly, the provided directional diffusion in the width direction may be configured to provide a uniform illumination pattern of the emitted light  202  in the width direction in addition to the length direction, in various embodiments. Note that a diffusion angle of the directional optical diffuser  130  in the width direction may be different from the diffusion angle in the length direction. However, the diffusion angle in the width direction may still be chosen along with a length of and a gap between the individual scattering elements  122  of the scattering line element  120  to insure viewing privacy, according to various embodiments. 
     As previously mentioned above, the scattering line elements  120  of the scattering line element plurality may comprise unidirectional scattering elements configured to preferentially scatter out the guided light  104  in a direction of the emission surface of the light guide  110 . For example, a scattering line element  120  of the scattering line element plurality may comprise a diffraction grating. The diffraction gratings may be configured to diffractively scatter out the portion of the guided light  104  from the light guide  110  as the emitted light  102  by diffractive scattering. Notably, the diffraction gratings  120  may include diffractive features comprising one or both of grooves in the second surface  110 ″ and ridges on the second surface  110 ″. Further, the grooves or ridges may be slanted to provide unidirectional scattering, for example. 
     In other embodiments, the scattering line elements  120  may include reflective islands (localized reflectors) aligned with the diffraction gratings  120  adjacent to scattering line elements  120  opposite to the emission surface (i.e., the first surface  110 ′). For example, a reflective island may be aligned with and have an extent or a size corresponding to an extent or a size of a diffraction grating of the scattering line element  120 . More generally, the reflective island may be patterned in a manner corresponding to the scattering line element  120 . The reflective islands may comprise a reflective material configured to reflectively redirect light scattered by the scattering line element  120  in an incorrect direction (i.e., away from the emission surface) into a direction corresponding to a direction of the emitted light  102 . In these embodiments, the scattering line element  120  comprising a diffraction grating and the reflective island may represent a reflection mode diffraction grating. In other embodiments, such as when a reflective island is not employed, the scattering line elements  120  may comprise a transmission mode diffraction grating defined or implemented on a surface or within the light guide  110 . 
     In some embodiments, the reflective island of the scattering line elements  120  may include a metal (e.g., gold, aluminum, silver, etc.) or a polymer-metal combination (e.g., an aluminum polymer film), or even a dielectric layer (e.g., silicon nitride or titanium oxide) configured as a reflector. Moreover, in some embodiments, the reflective islands may be separated from the scattering line element  120 , e.g., by an air gap or by a gap filled with a dielectric material. 
     In some embodiments where the scattering line elements  120  comprise a diffraction grating, the diffraction grating may include a plurality of diffractive features spaced apart from one another by a diffractive feature spacing (which is sometimes referred to as a ‘grating spacing’) or a diffractive feature or grating pitch configured to provide diffractive coupling out of the guided light portion. According to various embodiments, the spacing or grating pitch of the diffractive features in the diffraction grating may be sub-wavelength (i.e., less than a wavelength of the guided light). Note that the diffraction grating may include a plurality of different grating spacings (e.g., two or more grating spacings) or a variable grating spacing or pitch to diffractively scatter out the guided light portion. 
     According to some embodiments, the diffractive features of the diffraction grating may comprise one or both of grooves and ridges that are spaced apart from one another. The grooves or the ridges may comprise a material of the light guide  110 , e.g., may be formed in a surface of the light guide  110 . In another example, the grooves or the ridges may be formed from a material other than the light guide material, e.g., a film or a layer of another material on a surface of the light guide  110 . Note that grating characteristics (such as grating pitch, groove depth, ridge height, etc.) and/or a density of diffraction gratings along the x-direction may be used to compensate for a change in optical intensity of the guided light  104  within the light guide  110  as a function of propagation distance, according to some embodiments. 
     In some embodiments, the diffraction grating of the scattering line element  120  may be a uniform diffraction grating in which the diffractive feature spacing is substantially constant or unvarying throughout the diffraction grating. In other embodiments, the diffraction grating may comprise a variable or chirped diffraction grating. By definition, the ‘chirped’ diffraction grating is a diffraction grating exhibiting or having a diffraction spacing of the diffractive features (i.e., the grating pitch) that varies across an extent or length of the chirped diffraction grating. In some embodiments, the chirped diffraction grating may have or exhibit a chirp of the diffractive feature spacing that varies linearly with distance. As such, the chirped diffraction grating is a ‘linearly chirped’ diffraction grating, by definition. In other embodiments, the chirped diffraction grating may exhibit a non-linear chirp of the diffractive feature spacing. Various non-linear chirps may be used including, but not limited to, an exponential chirp, a logarithmic chirp or a chirp that varies in another, substantially non-uniform or random but still monotonic manner. Non-monotonic chirps such as, but not limited to, a sinusoidal chirp or a triangle or sawtooth chirp, may also be employed. Combinations of any of these types of chirps may also be employed. Note that, in some embodiments described herein, the emitted light  102  may include an effect of refraction due to the portion of the guided light  104  exiting the light guide  110  at the first surface  110 ′. 
     While the preceding discussion described the plurality of scattering line elements  120  as or comprising diffraction gratings  120 , in other embodiments a wide variety of optical components may be used as scattering line elements  120  to scatter out the emitted light  102 . For example, the scattering line elements  120  may comprise micro-reflective elements that are configured to reflectively scatter out the portion of the guided light  104 . In another example, the scattering line elements  120  may comprise micro-refractive elements that are configured to refractively scatter out the portion of the guided light  104  as the emitted light  102 . For example, the micro-reflective elements may include a triangular-shaped mirror, a trapezoid-shaped mirror, a pyramid-shaped mirror, a rectangular-shaped mirror, a hemispherical-shaped mirror, a concave mirror and/or a convex mirror. Note that these micro-reflective and micro-refractive elements may be located on the second surface  110 ″, on the first surface  110 ′, or between the first surface  110 ′ and the second surface  110 ″ of the light guide  110 , according to various embodiments. Furthermore, an optical feature of the scattering line element  120  may be a ‘positive feature’ that protrudes out a surface, or it may be a ‘negative feature’ that is recessed into a surface. 
     According to some embodiments, the privacy-mode backlight  100  may be used as a backlight in a mode-switchable display  100   a .  FIG. 6A  illustrates a cross-sectional view of a mode-switchable display  100   a  including a privacy-mode backlight  100  in an example, according to an embodiment consistent with the principles described herein.  FIG. 6B  illustrates a cross-sectional view of a mode-switchable display  100   a  including a privacy-mode backlight  100  in another example, according to an embodiment consistent with the principles described herein. In particular,  FIG. 6A  illustrates the mode-switchable display  100   a  in or during a privacy mode, while  FIG. 6B  illustrates the mode-switchable display  100   a  in or during a broad-angle or shared mode. According to various embodiments, the mode-switchable display  100   a  is configured to selectively display a private image during the privacy mode and a shared image during the shared mode. 
     As illustrated, the mode-switchable display  100   a  comprises the privacy-mode backlight  100  configured to provide emitted light  102  that, in turn, may be modulated to provide images having a narrow illumination beamwidth or equivalently a narrow viewing angle in or during the privacy mode of the mode-switchable display  100   a . In particular, in the privacy mode, the provided guided light in the light guide  110  of the privacy-mode backlight  100  may be scattered out and directed away from mode-switchable display  100   a  by the scattering line elements  120 . The emitted light  102  from scattering line elements  120  may then be modulated using an array of light valves  140  (described below) of the mode-switchable display  100   a , to facilitate the display of the private image having the narrow illumination beamwidth or narrow viewing angle (e.g., γ). The private image may be visible only within the narrower illumination beamwidth or narrow viewing angle and therefore may allow a user of the mode-switchable display  100   a  to more securely view the private image, according to various embodiments. 
     Alternatively, in the shared mode, the mode-switchable display  100   a  may dynamically switch (e.g., may be switched on demand) to providing the shared image having a wide or broad-angle illumination beamwidth or equivalently a broad-angle viewing angle (e.g., φ). In particular, the broad-angle illumination beamwidth during the shared mode is greater than, and in some embodiments substantially greater than, the narrow illumination beamwidth of the privacy mode. For example, the broad-angle illumination beamwidth of the shared mode may be greater than about twenty degrees (e.g., &gt;±20°), while the narrow illumination beamwidth of the privacy mode may be less than about twenty degrees (e.g., &lt;±20°), for example. In another example, the broad-angle illumination beamwidth of the shared mode may be greater than about sixty degrees (e.g., &gt;±60°), or greater than about forty degrees (e.g., &gt;±40°), or greater than about 30 degrees (e.g., &gt;±30°), while the narrow illumination beamwidth of the privacy mode may be less than about thirty degrees (e.g., &lt;±30°), or less than about twenty degrees (e.g., &lt;±20°), or less than about ten degrees (e.g., &lt;±10°), respectively. In some embodiments, the narrow illumination beamwidth or viewing angle may be less than about one half (½) of the broad-angle illumination beamwidth, or less than about one quarter (¼) of the broad-angle illumination beamwidth, or even less. 
     To provide the broad-angle illumination beamwidth or viewing angle, the mode-switchable display  100   a  further comprises a broad-angle backlight  150 , as illustrated in Figured  6 A- 6 B. According to various embodiments, the broad-angle backlight  150  is configured to provide broad-angle light  152  during the shared mode, i.e., emitted light having an illumination beamwidth that corresponds to the broad-angle illumination beamwidth or viewing angle of the mode-switchable display  100   a  during the shared mode. As illustrated, the broad-angle backlight  150  may be adjacent to a side (i.e., the second surface  110 ″) of the privacy-mode backlight  100  opposite to a side adjacent to the light valve array. According to various embodiments, the light guide  110  and the scattering line elements  120  may be configured to be transparent to the broad-angle light  152  provided by the broad-angle backlight  150  during the shared mode. According to various embodiments, the broad-angle backlight  150  may comprise substantially any planar light source configured to provide broad-angle illumination including a backlight that includes a light guide and broad-angle scattering element. Further, the shared image may have substantially the same brightness and resolution as the private image, in some embodiments. 
     As illustrated in  FIGS. 6A-6B , the mode-switchable display  100   a  further comprises the array of light valves  140 . The array of light valves  140  is configured to modulate light to provide an image that is displayed by the mode-switchable display  100   a . In particular, the light valve array configured to modulate the broad-angle light  152  to provide the shared image during the share mode of the mode-switchable display  100   a , as illustrated. Further, as noted above, the array of light valves  140  is configured to modulate the emitted light  102  provided by the privacy-mode backlight  100  as a private image during the privacy mode of the mode-switchable display  100   a . In various embodiments, any of a variety of 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. 
     According to some embodiments, a size of the scattering line element  120  in the length or x-direction of the light guide  110  is comparable to a size of a light valve  140 . Herein, the ‘size’ may be defined in any of a variety of manners to include, but not be limited to, a length, a width or an area. For example, the size of a light valve  140  may be a length thereof and the comparable size of the scattering line element  120  may also be a length of the scattering line element  120 . In another example, the size may refer to an area such that an area of the scattering line element  120  may be comparable to an area of the light valve  140 . 
     In some embodiments, a size of the scattering line element  120  is comparable to the light valve size such that the diffraction grating size is between about fifty percent (50%) and about two hundred percent (200%) of the light valve size. In other examples, the scattering line element size is in a range that is greater than about sixty percent (60%) of the light valve size, or greater than about seventy percent (70%) of the light valve size, or greater than about eighty percent (80%) of the light valve size, or greater than about ninety percent (90%) of the light valve size, and that is less than about one hundred eighty percent (180%) of the light valve size, or less than about one hundred sixty percent (160%) of the light valve size, or less than about one hundred forty (140%) of the light valve size, or less than about one hundred twenty percent (120%) of the light valve size. For example, by ‘comparable size’, the scattering line element size may be between about seventy-five percent (75%) and about one hundred fifty (150%) of the light valve size. In another example, the scattering line element may be comparable in size to the light valve size, where the scattering line element size is between about one hundred twenty-five percent (125%) and about eighty-five percent (85%) of the light valve size. According to some embodiments, the comparable sizes of the scattering line element  120  and the light valve  140  may be chosen to reduce, or in some examples to minimize, dark zones between scattering line elements  120  of the mode-switchable display  100   a . Moreover, the comparable sizes of the scattering line element  120  and the light valve  140  may be chosen to reduce, and in some examples to minimize, Moire associated with the mode-switchable display  100   a , e.g., the scattering line element size may be about equal to the light valve size. 
     Referring again to  FIGS. 3A and 3C , the privacy-mode backlight  100  may further comprise a light source  160 . According to various embodiments, the light source  160  is configured to provide the light to be guided within light guide  110 . In particular, the light source  160  may be located adjacent to an entrance surface or end (input end) of the light guide  110 . In various embodiments, the light source  160  may comprise substantially any source of light (e.g., optical emitter) including, but not limited to, an LED, a laser (e.g., laser diode) or a combination thereof. In some embodiments, the light source  160  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  160  may be a substantially broadband light source configured to provide substantially broadband or polychromatic light. For example, the light source  160  may provide white light. In some embodiments, the light source  160  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, the light source  160  may further comprise a collimator. The collimator may be configured to receive substantially uncollimated light from one or more of the optical emitters of the light source  160 . The collimator is further configured to convert the substantially uncollimated light into collimated light. In particular, the collimator may provide collimated light that is collimated according to a predetermined collimation factor, according to some embodiments. Moreover, when optical emitters of different colors are employed, the collimator may be configured to provide the collimated light having one or both of different, color-specific, non-zero propagation angles and having different color-specific collimation factors. The collimator is further configured to communicate the collimated light beam to the light guide  110  to propagate as the guided light  104 , described above. 
     In accordance with some embodiments of the principles described herein, a privacy display is provided.  FIG. 7  illustrates a block diagram of a privacy display  200  in an example, according to an embodiment consistent with the principles described herein. According to various embodiments, the privacy display  200  is configured to display a private image having a narrow view angle. In particular, modulated emitted light  202  from the privacy display  200  provides the private image in a restricted angular range (i.e., narrow view angle) that may be viewable by a user  200   a , but not by others, e.g., that are outside the restricted angular range or private view zone. The modulated emitted light  202  is illustrated as dashed arrows emanating from the privacy display  200  in  FIG. 7  to emphasize the modulation thereof by way of example and not limitation. 
     As illustrated, the privacy display  200  comprises a plurality of scattering line elements  220  distributed along a length of a light guide  210 . The scattering line element plurality is configured to scatter out guided light from the light guide  210  as emitted light having a predetermined illumination beamwidth in a direction orthogonal to the light guide length. In some embodiments, the light guide  210  may be substantially similar to the light guide  110 , described above with respect to the privacy-mode backlight  100 . For example, the light guide may be configured to guide light as a guided light beam, according to total internal reflection, in various embodiments. Further, the light guide  210  may be a plate light guide configured to guide light from a light-input edge thereof. Further, the plurality of scattering line elements  220  may be substantially similar to the scattering line elements  120  of the above-described privacy-mode backlight  100 . 
     In particular, the guided light may be collimated according to a collimation factor. Further, the predetermined illumination beamwidth may be determined by the collimation factor of the guided light, in some embodiments. More particularly, the collimation factor of the guided light may be specifically selected to achieve the predetermined illumination beamwidth. In some embodiments, scattering line elements  220  of the scattering line element plurality may comprise scattering elements configured to preferentially scatter out the guided light in a direction of an emission surface of the light guide  210 . As such, the scattering line elements may be unidirectional scattering line elements. For example the scattering line elements may include a reflector or a reflective island, as described above. 
     In various embodiments, scattering line elements  220  may comprise a diffraction grating or a plurality of diffraction gratings configured to provide the emitted light  202 . In particular, the diffraction grating may be configured to diffractively scatter out a portion of the guided light from the light guide  210  as the emitted light  202 . In some embodiments, the diffraction grating may be substantially similar to the diffraction grating of the scattering line element  120 , described above. In other embodiments, the scattering line elements  220  may comprise other scattering elements including, but not limited to micro-reflective elements and micro-refractive elements, as described above with respect to the scattering line element  120  of the privacy-mode backlight  100 . 
     The privacy display  200  illustrated in  FIG. 7  further comprises an array of light valves  230 . The array of light valves  230  is configured to modulate the emitted light  202  to provide the private image. In some embodiments, the array of light valves  230  may be substantially similar to the array of light valves  140 , as described above with respect to the privacy-mode backlight  100 . 
     According to some embodiments, a size of a scattering line element  220  of the scattering line element plurality in a length direction along the light guide  210  is comparable to a size of a light valve of the array of light valves  230 . For example, the size of the scattering line element may be greater than one half of the light valve size and less than twice the light valve size, in some embodiments. 
     According to various embodiments, the privacy display  200  further comprises a directional optical diffuser  240 , as illustrated in  FIG. 7 . The directional optical diffuser is located between the light guide  210  and the array of light valves  230 , as illustrated in  FIG. 7 . In various embodiments, the directional optical diffuser is configured to provide directional diffusion of the emitted light  202  in a direction corresponding to the light guide length. In some embodiments, the directional optical diffuser  240  may be substantially similar to the directional optical diffuser  130  of the above-described privacy-mode backlight  100 . In particular, the directional diffusion of the directional optical diffuser  240  may provide uniform illumination of the light valve array in the light guide length direction, according to various embodiments. 
     In some of these embodiments (not illustrated in  FIG. 7 ), the privacy display  200  may further comprise a light source. The light source may be configured to provide the light to the light guide  210  collimated according to a collimation factor to provide a predetermined angular spread of the guided light within the light guide  210 , for example. According to some embodiments, the light source may be substantially similar to the light source  160 , described above. 
     In some embodiments (e.g., as illustrated in  FIG. 7 ), the privacy display  200  may further comprise a broad-angle backlight  250  located adjacent to a side of the light guide  210  opposite to a side adjacent to the light valve array. In some embodiments, the broad-angle backlight  250  may be substantially similar to the broad-angle backlight  150  of the above-described mode-selectable display  100   a  and the privacy display  200  may be or operate as a mode-selectable display. In particular, the broad-angle backlight  250  may be configured to provide broad-angle light  252  during a shared mode of the privacy display  200 . Further, the light valve array may be configured to modulate the broad-angle light  252  to provide a shared image during the shared mode. According to various embodiments, the shared image may be viewed by a plurality of users  200   b  in various different locations within a shared view zone having a broad-angle or relatively unrestricted angular range, as illustrated in  FIG. 7  by way of example and not limitation. When operated as a mode-selectable display, the light guide  210  and scattering line elements  220  of the privacy display  200  may be configured to be transparent to the broad-angle light  252 . In addition, the privacy display  200  may be mode-switchable between the shared mode to provide the shared image and a privacy mode to provide the private image, according to some embodiments. 
     In accordance with other embodiments of the principles described herein, a method of privacy-mode backlight operation is provided.  FIG. 8  illustrates a flow chart of a method  300  of privacy-mode backlight operation in an example, according to an embodiment consistent with the principles described herein. As illustrated in  FIG. 8 , the method  300  of privacy-mode backlight operation comprises guiding  310  the light in a light guide as collimated guided light having a predetermined collimation factor. According to some embodiments, the light guide may be substantially similar to the light guide  110  described above with respect to the privacy-mode backlight  100 . In particular, the light may be guided according to total internal reflection within the light guide, according to various embodiments. 
     According to various embodiments, the method  300  of privacy-mode backlight operation further comprises scattering out  320  a portion of the collimated guided light from the light guide as emitted light having an illumination beamwidth. Scattering out the collimated guided light portion uses a plurality of scattering line elements spaced apart from one another along a length of the light guide, in various embodiments. In some embodiments, the light guide and scattering line elements may be substantially similar respectively to the light guide  110  and scattering line elements  120 , described above with respect to the privacy-mode backlight  100 . In some embodiments, the scattering line elements of the scattering line element plurality may comprise unidirectional scattering elements that preferentially scatter out the collimated guided light in a direction of emitted light from the light guide. Further, according to various embodiments, the illumination beamwidth of the emitted light scattered out by the scattering line element plurality is determined by the collimation factor, the illumination beamwidth being in a direction orthogonal to the light guide length. 
     The method  300  of privacy-mode backlight operation illustrated in  FIG. 8  further comprises diffusing  330  the emitted light in a direction corresponding to the light guide length using a directional optical diffuser. In some embodiments, the directional optical diffuser may be substantially similar to the directional optical diffuser  130  of the privacy-mode backlight  100 , as described above. In some embodiments, a diffusion angle of the directional optical diffuser spreads out the emitted light from scattering line elements of the scattering line element plurality to provide an illumination extent equivalent to a distance between adjacent scattering line elements of the scattering line element plurality. In some embodiments, a size of a scattering line element of the scattering line element plurality may be comparable to a size of a light valve of a light valve array used to modulate the emitted light as a private image displayed by a privacy display. 
     In some embodiments (not illustrated), the method  300  of privacy-mode backlight operation further comprises modulating the emitted light to display a private image using an array of light valves. The plurality of light valves may be substantially similar to the array of light valves  140  described above with respect to the privacy-mode backlight  100 . 
     In some embodiments (not illustrated), the method  300  of privacy-mode backlight operation may further comprise providing light to the light guide using a light source. The provided light may be collimated according to a predetermined collimation factor. According to some embodiments, the light source may be substantially similar to the light source  160 , described above. 
     Thus, there have been described examples and embodiments of a privacy-mode backlight, a privacy display, and a method of privacy-mode backlight operation that employ line scattering elements and a directional diffuser. 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.