CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U. S. Provisional Patent Application Serial

No. 62/476,783, filed March 25, 2017, the entirety of which is incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] N/A

BACKGROUND

[0003] 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/ AMOLED s. 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.

[0004] To overcome the limitations of passive displays associated with emitted light, many passive displays are coupled to an external light source. The coupled light source may allow these otherwise passive displays to emit light and function substantially as an active display. Examples of such coupled light sources are backlights. A backlight may serve as a source of light (often a panel backlight) that is placed behind an otherwise passive display to illuminate the passive display. For example, a backlight may be coupled to an LCD or an EP display. The backlight emits light that passes through the LCD or the EP display. The light emitted is modulated by the LCD or the EP display and the modulated light is then emitted, in turn, from the LCD or the EP display. Often backlights are configured to emit white light. Color filters are then used to transform the white light into various colors used in the display. The color filters may be placed at an output of the LCD or the EP display (less common) or between the backlight and the LCD or the EP display, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] 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:

[0006] Figure 1 A illustrates a cross-sectional view of a diffraction grating in an example, according to an embodiment consistent with the principles described herein.

[0007] Figure IB illustrates a cross-sectional view of a slanted diffraction grating in an example, according to an embodiment consistent with the principles described herein.

[0008] Figure 2A illustrates a cross-sectional view of a mode-switchable backlight in an example, according to an embodiment consistent with the principles described herein.

[0009] Figure 2B illustrates a cross-sectional view of a mode-switchable backlight in another example, according to an embodiment consistent with the principles described herein.

[0010] Figure 3 illustrates a graphical representation of a combination of narrow- angle emitted light and bidirectional emitted light to provide broad-angle emitted light in an example, according to an embodiment consistent with the principles described herein. [0011] Figure 4A illustrates a cross-sectional view of a portion of a first directional backlight of a mode-switchable backlight in an example, according to an embodiment consistent with the principles described herein.

[0012] Figure 4B illustrates a cross-sectional view of a portion of a first directional backlight of a mode-switchable backlight in an example, according to another embodiment consistent with the principles described herein.

[0013] Figure 5 illustrates a cross-sectional view of a portion of a second directional backlight in an example, according to an embodiment consistent with the principles described herein.

[0014] Figure 6 illustrates a block diagram of a mode-switchable privacy display in an example, according to an embodiment consistent with the principles described herein.

[0015] Figure 7 illustrates a flow chart of a method of mode-switchable backlight operation in an example, according to an embodiment consistent with the principles described herein.

[0016] 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

[0017] Examples and embodiments in accordance with the principles described herein provide mode-switchable backlighting with application to a mode-switchable privacy display. In particular, embodiments consistent with the principles described herein provide a mode-switchable backlight configured to emit narrow-angle emitted light in a first mode and broad-angle emitted light during a second mode. In some

embodiments, the narrow-angle emitted light is configured to provide illumination exclusively to a first view zone during the first mode, while the broad-angle emitted light may be configured to provide illumination to a second view zone that includes the first view zone during the second mode. Further, the broad-angle emitted light provided in the second mode represents a combination of the narrow-angle emitted light and bidirectional emitted light having a bifurcated angular extent that is complementary to an angular range of the narrow-angle emitted light, according to various embodiments. [0018] A mode-switchable privacy display that employs the mode-switchable backlighting may be configured to provide a private displayed image during the first or 'privacy' mode, the private image being visible in the first of 'private' view zone.

Further, the mode-switchable privacy display may be configured to provide a public display image during the second or 'public' mode using the broad-angle emitted light. The public displayed image may be visible in the second or 'public' view zone that includes both the private view zone and public view zone, according to various embodiments. Uses of mode-switchable backlighting and a mode-switchable privacy display described herein include, but are not limited to, mobile telephones (e.g., smart phones), watches, tablet computes, 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.

[0019] 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.

[0020] 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.

[0021] 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 insure that total internal reflection is maintained within the plate light guide to guide light.

[0022] Herein, an 'angle-preserving scattering feature' or equivalently an 'angle- preserving scatterer' is any feature or scatterer configured to scatter light in a manner that substantially preserves in scattered light an angular spread of light incident on the feature or scatterer. In particular, by definition, an angular spread o _s of light scattered by an angle-preserving scattering feature is a function of an angular spread σ of the incident light (i.e., o _s = σ) ). In some embodiments, the angular spread o _s of the scattered light is a linear function of the angular spread or collimation factor σ of the incident light (e.g., o _s = α σ, where a is an integer). That is, the angular spread 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 o _s of the scattered light may be substantially equal to the incident light angular spread σ (e.g., o _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. In contrast, a Lambertian scatterer or a Lambertian reflector as well as a general diffuser (e.g., having or approximating Lambertian scattering) are not angle-preserving scatterers, by definition herein.

[0023] Herein, a 'polarization-preserving scattering feature' or equivalently a

'polarization-preserving scatterer' is any feature or scatterer configured to scatter light in a manner that substantially preserves in scattered light a polarization or at least a degree of polarization of the light incident on the feature or scatterer. Accordingly, a

'polarization-preserving scattering feature' is any feature or scatterer where a degree of polarization of a light incident on the feature or scatterer is substantially equal to the degree of polarization of the scattered light. Further, by definition, 'polarization- preserving scattering' is scattering (e.g., of guided light) that preserves or substantially preserves a predetermined polarization of the light being scattered. The light being scattered may be polarized light provided by a polarized light source, for example.

[0024] Herein, the term 'unilateral' as in 'unilateral scattering element,' is defined as meaning 'one-sided' or 'preferentially in one direction' correspond to a first side as opposed to another direction correspond to a second side. In particular, a backlight configured to provide or emit light in a 'unilateral direction' is defined as a backlight that emits light from a first side and not from a second side opposite the first side. For example, the unilateral direction of emitted light provided by or scattered from a backlight light may correspond to light that is preferentially directed into a first (e.g., positive) half-space, but not into the corresponding second (e.g., negative) half-space. The first half-space may be above the backlight and the second half-space may be below the backlight. As such, the backlight may emit light into a region or toward a direction that is above the backlight and emit little or no light into another region or toward another direction that is below the backlight, for example. Similarly a 'unilateral' directional scatterer such as, but not limited to, a unilateral scattering element is configured to scatter light toward and out of a first surface, but not a second surface opposite the first surface, by definition herein.

[0025] 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. Further, the diffraction grating may include a plurality of features (e.g., a plurality of grooves or ridges in a material surface) arranged in a one-dimensional (ID) array. Alternatively, the diffraction grating may comprise a two-dimensional (2D) array of features or an array of features that are defined in two dimensions. 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.

[0026] 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.

[0027] 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 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).

[0028] According to various examples described herein, a diffraction grating (e.g., a diffraction grating of a scattering 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 Θ _Μ of or provided by a locally periodic diffraction grating may be given by equation (1) as:

where λ is a wavelength of the light, m is a diffraction order, n is an index of refraction of a light guide, <i is a distance or spacing between features of the diffraction grating, θ _ι 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 _Μ 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 _Μ is provided when the diffraction order m is equal to one (i.e., m = 1).

[0029] Figure 1 A 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, Figure 1 A illustrates a light beam 50 incident on the diffraction grating 30 at an incident angle θ ₁ . The incident light beam 50 may be a beam of guided light (i.e., a guided light beam) within the light guide 40. Also illustrated in Figure 1 A 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 _Μ (or 'principal angular direction' herein) as given by equation (1). The diffraction angle Θ _Μ may correspond to a diffraction order 'w' of the diffraction grating 30, for example diffraction order m = 1 (i.e., a first diffraction order).

[0030] Herein by definition, a 'slanted' diffraction grating is a diffraction grating with diffractive features having a slant angle relative to a surface normal of a surface of the diffraction grating. According to various embodiments, a slanted diffraction grating may provide unilateral scattering by diffraction of incident light. [0031] Figure IB illustrates a cross-sectional view of a slanted diffraction grating

80 in an example, according to an embodiment consistent with the principles described herein. As illustrated, the slanted diffraction grating 80 is a binary diffraction grating located at a surface of a light guide 40, analogous to the diffraction grating 30 illustrated in Figure 1 A. However, the slanted diffraction grating 80 illustrated in Figure IB comprises diffractive features 82 having a slant angle γ relative to a surface normal (illustrated by a dashed line) along with a grating height, depth or thickness t, as illustrated. Also illustrated are the incident light beam 50 and a directional light beam 60 representing unilateral diffractive scattering of the incident light beam 50 by the slanted diffraction grating 80. Note that diffractive scattering of light in a secondary direction by the slanted diffraction grating 80 is suppressed by the unilateral diffractive scattering, according to various embodiments. In Figure IB, a 'crossed out' dashed-line arrow 90 represents suppressed diffractive scattering in the secondary direction by the slanted diffraction grating 80.

[0032] According to various embodiments, the slant angle γ of the diffractive features 82 may be selected to control a unilateral diffraction characteristic of the slanted diffraction grating 80 including a degree to which diffractive scattering in the secondary direction is suppressed. For example, the slant angle γ may be selected to be between about twenty degrees (20°) and about sixty degrees (60°) or between about thirty degrees (30°) and about fifty degrees (50°) or between about forty degrees (40°) and about fifty- five degrees (55°). A slant angle γ in a range of about 30° - 60° may provide better than about forty times (40x) suppression of the diffractive scattering in secondary direction, when compared to a unilateral direction provided by the slanted diffraction grating 80, for example. According to some embodiments, the thickness t of the diffractive features 82 may be between about one hundred nanometers (100 nm) and about four hundred nanometers (400 nm). For example, the thickness t may be between about one hundred fifty nanometers (150 nm) and about three hundred nanometers (300 nm) for grating periodicities p in a range from about 300 nm and about five hundred nanometers (500 nm).

[0033] Further, the diffractive features may be curved and may also have a predetermined orientation (e.g., 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 coupled-out 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.

[0034] Herein, a 'microprism structure' is generally defined as a structure comprising a microprism or a plurality of microprisms having an inclined sidewall(s) and configured to refractively scatter light incident on the microprism structure. If the light is incident on the microprism structure from a light guide, the microprism structure may be understood as a structure comprising a microprism or a plurality of microprisms configured to refractively couple out or scatter out light from the light guide. In some embodiments, the microprism structure may comprise an inverted microprism element. By definition herein, an 'inverted microprism element' is a microprism having a truncated conical shape with an input aperture, an inclined sidewall, and an output aperture that is larger than the input aperture. In particular, the input aperture is configured to receive light and the inclined sidewall is configured to reflect the light received through the input aperture, whereas the output aperture is configured to emit the reflected light. Thus, the input aperture is a portion of the inverted microprism element comprising an optical connection between the inverted microprism element and the light guide, and configured to receive extracted or coupled-out light from the light guide. The inclined sidewall comprises an interior surface of the inverted microprism element that is configured to reflect light. In some embodiments, the inclined sidewall may comprise a reflective layer or reflective material (e.g., a reflective material layer on an exterior surface of the sidewall). The reflective layer may be configured to provide or enhance reflection at the interior surface of the inverted microprism element. The reflected light is emitted from the output aperture of the inverted microprism element.

[0035] 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, a tapered light guide, and 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 similar collimating characteristic in one or both of two orthogonal directions that provides light collimation, according to some embodiments.

[0036] 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.

[0037] 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. [0038] Herein, a 'view zone' is defined as a region or an angular range in which a displayed image may be viewed. In particular, the displayed image may be visible within the view zone, but not visible outside of the view zone, by definition herein. Herein, a 'private display image' or a displayed image provided by a 'privacy display' is defined as an image having a restricted or limited view zone (i.e., a private view zone). In general, the private displayed image may be viewable in a limited or private view zone in front of the display and is intended for viewing only by a person(s) located within the limited or private view zone. In some embodiments, a limited or private view zone of a private displayed image or equivalently of a privacy display may be less than about forty-five degrees on either side of a normal direction of (or a direction perpendicular to) a display (i.e., view zone < ± 45°). In other embodiments, the limited or private view zone of a private displayed image or of a privacy display may be less than plus or minus thirty degrees (i.e., view zone < ± 30°) or less than less than plus or minus twenty degrees (i.e., view zone < ± 20°).

[0039] In contrast, a public displayed image or a displayed image provided by a public display has a broad-angle viewing zone and is intended for essentially unrestricted viewing. By definition herein, a 'public displayed image' or a displayed image provided by a 'public display' is an image having a view zone that is greater than the limited or private view zone of the private displayed image. In particular, a public displayed image may have a view zone (i.e., public view zone) that is greater than about forty-five degrees on either side of the normal direction of a display (i.e., view zone > ± 45°). For example, the public view zone of a public displayed image or equivalently of a public display may greater than plus or minus sixty degrees (i.e., view zone > ± 60°). In some embodiments, the public view zone may defined by 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 ± 45-65° or more).

[0040] By definition herein, the term 'narrow-angle' as used in 'narrow-angle emitted light' is light having an angular range or extent (e.g., cone angle) that is consistent with providing illumination of a private view zone or a private image that is displayed or otherwise made available within the private view zone. As such, narrow- angle emitted light may have an angular range of less than about ± 45°, or less than about ± 30°, or less than about ± 20°, by definition herein. Further, by definition, 'broad-angle emitted light' or more generally 'broad-angle' is used to refer to light having angular range or extent that is generally greater than an angular range or extent of the narrow- angle emitted light. Alternatively, 'broad-angle' refers to an angular range or extend consistent with a view zone of a public displayed image or a display configured to display a public displayed image. As such, in some embodiments, the broad-angle emitted light may have an angular range that is greater than plus and minus forty -five degrees (e.g., > ± 45°) relative to the normal direction of a display. In other embodiments, the broad-angle emitted light angular range may be greater than plus and minus fifty degrees (e.g., > ± 50°), or greater than plus and minus sixty degrees (e.g., > ± 60°), or greater than plus and minus sixty-five degrees (e.g., > ± 65°). For example, the angular range of the broad- angle emitted light may be greater than about seventy degrees on either side of the normal direction of the display (e.g., > ± 70°).

[0041] 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.

[0042] According to some embodiments of the principles described herein, a mode-switchable backlight is provided. Figure 2A illustrates a cross-sectional view of a mode-switchable backlight 100 in an example, according to an embodiment consistent with the principles described herein. Figure 2B illustrates a cross-sectional view of a mode-switchable backlight 100 in another example, according to an embodiment consistent with the principles described herein. As illustrated, the mode-switchable backlight 100 is configured to provide or emit light as narrow-angle emitted light 102 in or during a first mode (e.g., a privacy mode) and broad-angle emitted light 104 in a second mode (e.g., a public mode). In particular, Figure 2A illustrates the mode- switchable backlight 100 in the first mode with narrow-angle emitted light 102, while Figure 2B illustrates the mode-switchable backlight 100 in the second mode providing broad-angle emitted light 104. Further, a direction of the narrow-angle and broad-angle emitted light 102, 104 corresponds to a half-space above a surface of the mode-switchable backlight 100, in Figures 2A-2B.

[0043] The mode-switchable backlight 100 illustrated in Figures 2 A and 2B comprises a first directional backlight 110. The first directional backlight 110 is configured to emit or provide the narrow-angle emitted light 102 during both the first mode and the second mode of the mode-switchable backlight 100. According to various embodiments, the narrow-angle emitted light 102 has as angular range γ and may be directed into a first view zone I by the first directional backlight 110 during both the first and second modes. Moreover, the angular range γ of the narrow-angle emitted light 102 is configured to confine or substantially confine the narrow-angle emitted light 102 to the first view zone / during both modes. As such, the narrow-angle emitted light 102 may be exclusively visible within the first view zone I, according to various embodiments. In Figures 2A-2B, the narrow-angle emitted light 102 is illustrated as a plurality of arrows within the angular range γ delineated by dashed lines that correspond to the first view zone I.

[0044] As illustrated in Figures 2 A and 2B, the mode-switchable backlight 100 further comprises a second directional backlight 120. The second directional backlight 120 is configured to emit or provide bidirectional emitted light 106 exclusively during the second mode, e.g., as illustrated in Figure 2B. According to various embodiments, the bidirectional emitted light 106 has a bifurcated angular extent. Further, the bifurcated angular extent is complementary to an angular range of the narrow-angle emitted light 102. In various embodiments, a combination of the narrow-angle emitted light 102 and the bidirectional emitted light 106 during the second mode is configured to provide the broad-angle emitted light 104 having an angular range (^that is a summation of an angular range of the narrow-angle emitted light 102 and the bifurcated angular extent of the bidirectional emitted light 106. [0045] In particular, as illustrated in Figure 2B, the bidirectional emitted light 106 has a first segment 106a directed in a first direction and a second segment 106b directed in a second direction. The bifurcated angular extent of the bidirectional emitted light 106 is a combination of an angular range φ _α of the first segment 106a around or centered on the first direction and an angular range of the second segment 106b around or centered on the second direction. Further, the bidirectional emitted light 106 provided by the second directional backlight 120 is substantially excluded from the angular range γ of the narrow-angle emitted light 102, according to various embodiments. In the second mode, both the narrow-angle emitted light 102 is provide by the first directional backlight 110 and the bidirectional emitted light 106 is provided by the second directional backlight 120b. As such, the broad-angle emitted light 104 is provided by the combination of the narrow-angle emitted light 102 and the bidirectional emitted light 106, as illustrated in Figure 2B.

[0046] By definition herein, the bidirectional emitted light 106 has a bifurcated angular extent (i.e., φ _α + φ _δ ) that is 'complementary' to the angular range γ of the narrow- angle emitted light 102. That is, when the bidirectional emitted light 106 and the narrow- angle emitted light 102 are combined during the second mode, the resulting broad-angle emitted light 104 has the angular range φ, which is a sum of the angular range γ of the narrow-angle emitted light 102 and the angular extent of the bidirectional emitted light 106, i.e., φ = γ + (φ _α + φ _δ ). Put another way, the bidirectional emitted light 106 provides or essentially 'fills in' a portion of the broad-angle emitted light 104 that is not provided by the narrow-angle emitted light 102. Thus, the angular extent (φ _α + φ _b ) of the bidirectional emitted light 106 is complementary to the angular range γ of the narrow- angle emitted light 102, as illustrated in Figure 2B.

[0047] Figure 3 illustrates a graphical representation of a combination of narrow- angle emitted light 102 and bidirectional emitted light 106 to provide broad-angle emitted light 104 in an example, according to an embodiment consistent with the principles described herein. As illustrated, the narrow-angle emitted light 102 is added to the bidirectional emitted light 106 to create the broad-angle emitted light 104. Summation of the narrow-angle emitted light 102 and the bidirectional emitted light 106 is graphically represented by a plus sign '+' and an equals sign '=' in Figure 3. Note that the first segment 106a and second segment 106b of the bidirectional emitted light 106 represent regions in which the bidirectional emitted light 106 is present, while no light is present between the first and second segments 106a, 106b before the summation, as illustrated. Further, the bifurcated angular extent of the bidirectional emitted light 106 is

complementary to the angular range of the narrow-angle emitted light 102, since an angular range between the first and second segments 106a, 106b is equal to the angular range of the narrow-angle emitted light 102, as illustrated.

[0048] In some embodiments, the narrow-angle emitted light 102 is configured to provide illumination exclusively the first view zone / during the first mode. Further, the broad-angle emitted light 104 may be configured to provide illumination to the second view zone // that includes the first view zone / during the second mode, in some embodiments. Referring again to Figures 2A-2B, the mode-switchable backlight 100 may appear as illuminated to a viewer in the first view zone / during both of the first and second modes. Similarly, another viewer in the second view zone // outside of the first view zone / may perceive the mode-switchable backlight 100 as being illuminated during the second mode. However, the mode-switchable backlight 100 may appear to the other viewer in the second view zone // as being dark or not illuminated during the first mode, according to various embodiments. Thus, the first mode may be referred to as a privacy mode and the first view zone / may be a privacy view zone, since the illumination is provided only to the viewer in the first (privacy) view zone / during the first (privacy) mode, for example. On the other hand, since during the second mode both viewers may perceive the illumination, the second mode may be referred to as a public mode and the second view zone // (that includes the first view zone T) may be a public view zone, for example.

[0049] In some embodiments (e.g., as illustrated in Figures 2A-2B), the first directional backlight 1 10 of the mode-switchable backlight 100 comprises a light guide 1 12. Similarly, the second directional backlight 120b of the mode-switchable backlight 100 may comprise a light guide 122, in some embodiments (e.g., as illustrated). The light guide 1 12, 122 may be a plate light guide, according to some embodiments. The light guide 1 12, 122 is configured to guide light along a length of the light guide 1 12, 122 as guided light. For example, the light guide 1 12, 122 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 according to one or more guided modes of the light guide 112, 122, for example.

[0050] In particular, the light guide 112, 122 may be a slab or plate optical waveguide 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 using total internal reflection. According to various examples, the optically transparent material of the light guide 112, 122 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 112, 122 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 112, 122. The cladding layer may be used to further facilitate total internal reflection, according to some examples.

[0051] Further, according to some embodiments, the light guide 112, 122 is configured to guide the guided light according to total internal reflection at a non-zero propagation angle between a first surface (e.g., 'front' surface or side) and a second surface (e.g., 'back' surface or side) of the light guide 112, 122. In particular, the guided light propagates by reflecting or 'bouncing' between the first surface and the second surface of the light guide 112, 122 at the non-zero propagation angle. In some

embodiments, the guided light comprises a plurality of guided light beams of different colors of light. The light beams of the plurality of guided light beams may be guided by the light guide 112, 122 at respective ones of different color-specific, non-zero

propagation angles. Note that the non-zero propagation angle is not illustrated for simplicity of illustration.

[0052] As defined herein, a 'non-zero propagation angle' is an angle relative to a surface (e.g., the first surface or the second surface) of the light guide 112, 122. 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 112, 122, according to various

embodiments. For example, the non-zero propagation angle of the guided light 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 112, 122.

[0053] The guided light in the light guide 112, 122 may be introduced or coupled into the light guide 112, 122 at the non-zero propagation angle (e.g., about 30°-35°). For example, the guided light may be provided by a light source of the mode-switchable backlight 100. One or more of a lens, a mirror or similar reflector (e.g., a tilted collimating reflector), a diffraction grating, and a prism (not illustrated) may facilitate coupling light into an input end of the light guide 112, 122 as the guided light at the nonzero propagation angle, for example. Once coupled into the light guide 112, 122, the guided light propagates along the light guide 112, 122 in a direction that may be generally away from the input end (e.g., illustrated by bold arrows pointing along an x-axis in Figures 2A-2B).

[0054] Further, the guided light may be collimated, according to various embodiments. Herein, a 'collimated light' or '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). Further, rays of light that diverge or are scattered from the collimated light beam are not considered to be part of the collimated light beam, by definition herein. In some embodiments, the mode-switchable backlight 100 may include a collimator, such as, but not limited to, a lens, reflector or mirror, a diffraction grating, or a tapered light guide, configured to collimate the light, e.g., from a light source. In some embodiments, the light source comprises a collimator. The collimated light provided to the light guide 112, 122 is a collimated guided light. The guided light may be collimated according to or having a collimation factor σ, in various embodiments.

[0055] In some embodiments, first directional backlight 110 of the mode- switchable backlight 100 further comprises an array of unilateral scattering elements 114 spaced apart from one another along the light guide length. According to various embodiments, the array of unilateral scattering elements 114 is configured to scatter out a portion of the guided light as the narrow-angle emitted light 102 having a unilateral direction. In particular, the unilateral scattering elements 114 of the array may be separated from one another by a finite space and represent individual, distinct elements along the light guide length. That is, by definition herein, the unilateral scattering elements 114 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 unilateral scattering elements 114 of the plurality generally do not intersect, overlap or otherwise touch one another, according to some embodiments. Thus, each unilateral scattering element 114 of the unilateral scattering element array is generally distinct and separated from other ones of the unilateral scattering elements 114. Further, the unilateral direction is away from the second directional backlight 120, according to various embodiments.

[0056] According to some embodiments, the unilateral scattering elements 114 of unilateral scattering element array may be arranged in either a one-dimensional (ID) array or a two-dimensional (2D) array. For example, the unilateral scattering elements 114 may be arranged as a linear ID array. In another example, the unilateral scattering elements 114 may be arranged as a rectangular 2D array or as a circular 2D array.

Further, the array (i.e., ID 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 unilateral scattering elements 114 may be substantially uniform or constant across the array. In other examples, the inter-element distance between the unilateral scattering elements 114 may be varied one or both of across the array and along the length of the light guide 112 of the first directional backlight 110.

[0057] According to some embodiments, a unilateral scattering element 114 of the array of unilateral scattering elements 114 comprises a slanted diffraction grating. In some embodiments, all of the unilateral scattering elements 114 may be or comprise slanted diffraction gratings. The slanted diffraction grating of the unilateral scattering element 114 is configured to provide diffractive scattering in the unilateral direction and further to suppress secondary diffractive scattering, according to various embodiment.

[0058] Figure 4A illustrates a cross-sectional view of a portion of a first directional backlight 110 of a mode-switchable backlight 100 in an example, according to an embodiment consistent with the principles described herein. In particular, Figure 4A illustrates the first directional backlight 110 comprising the light guide 112 along with a unilateral scattering element 114. Further, as illustrated in Figure 4A, the unilateral scattering element 114 comprises a slanted diffraction grating 114'. Guided light 116 having a collimation factor σ, illustrated as a plurality of arrows, incident on the slanted diffraction grating 114' may be diffractively scattered out of the light guide 112 in a unilateral direction as the narrow-angle emitted light 102, as illustrated.

[0059] In some embodiments, the slanted diffraction grating 114' of the unilateral scattering element 114 may be substantially similar to the slanted diffraction grating 80 illustrated in Figure IB. For example, a slant angle of the slanted diffraction grating 114', corresponding to the slant angle γ illustrated in Figure IB, may be between about thirty degrees (30°) and about fifty degrees (50°) relative a surface normal of the light guide 112, in some embodiments. Further, the slanted diffraction grating 114' may comprise a plurality of sub-gratings, each sub-grating being a slanted diffraction grating, in some embodiments (not illustrated). As in Figure IB, the slanted diffraction grating 114' of the unilateral scattering element 114 illustrated in Figure 4 A may suppress secondary scattering in an opposite direction to the unilateral direction, e.g., illustrated by crossed- out arrows 103 in Figure 4 A.

[0060] In some embodiments, the unilateral scattering element 114 of the array of unilateral scattering elements 114 may comprise a reflection mode diffraction grating. The reflection mode diffraction grating is configured to selectively scatter out the guided light portion in the unilateral direction using a combination of diffractive scattering and reflection, according to various embodiments. In some embodiments, all of the unilateral scattering elements 114 may be or comprise reflection mode diffraction gratings. In some embodiments, the reflection mode diffraction grating comprises a diffraction grating and a layer of reflective material (e.g., a metal layer), the diffraction grating being configured to provide diffractive scattering and the reflective material layer being configured to provide reflection of diffractively scattered light to insure that the diffractive scattering produces scatter out light having the unilateral direction.

[0061] Figure 4B illustrates a cross-sectional view of a portion of a first directional backlight 110 of a mode-switchable backlight 100 in an example, according to another embodiment consistent with the principles described herein. In particular, Figure 4B illustrates the first directional backlight 110 comprising the light guide 112 along with a unilateral scattering element 114. Further, as illustrated in Figure 4B, the unilateral scattering element 114 comprises a reflection mode diffraction grating comprising a diffraction grating 114" and a reflective material layer 114"'. Guided light 116 having a collimation factor σ, illustrated as a plurality of arrows, incident on the diffraction grating 114" may be diffractively scattered out of the light guide 112 in a unilateral direction as the narrow-angle emitted light 102. Further, light that is diffractively scattered in a second direction by the diffraction grating 114" may be reflectively redirected into the unilateral direction by the reflective material layer 114"', according to various

embodiments.

[0062] In other embodiments (not illustrated), the unilateral scattering element

114 may comprise another type of unilateral scattering structure including, but not limited to, a micro-refractive element and a micro-reflective element. The micro-refractive element may comprise a microprism at a surface of the light guide 112 and configured to refractively couple the guided light portion out of the light guide 112 as the narrow-angle emitted light 102, for example. Examples of micro-reflective elements include, but are not limited to, faceted reflectors having a facet with surface angle configured to reflectively scatter out the guided light portion as the narrow-angle emitted light 102. In some embodiments, the unilateral scattering element 114 comprises an angle-preserving scatterer configured to scatter out the guided light portion having an angular range that is proportional a collimation factor cr of the guided light within the light guide 112. As such, the collimation factor cr of the guided light may determine the angular range γ of the narrow-angle emitted light 102, in some embodiments.

[0063] Referring again to Figures 2A and 2B, in some embodiments the second directional backlight 120 further comprises a directional scattering feature 124 configured to scatter out a portion or portions of the guided light within the light guide 122 as the bidirectional emitted light 106. In particular, the directional scattering feature 124 of the second directional backlight 120 may comprise a first plurality of directional scattering elements 124a and a second plurality of directional scattering elements 124b. The first plurality of directional scattering elements 124a may be configured to scatter out a portion of the guided light as the first segment 106a of the bidirectional emitted light 106 having the first direction of the bifurcated angular extent. Further, the second plurality of directional scattering elements 124b may be configured to scatter out another portion of the guided light as the second segment 106b of the bidirectional emitted light 106 having the second direction of the bifurcated angular extent. According to various embodiments, the first and second directions in combination with an angular range of the first and second segments 106a, 106b determine the bifurcated angular extent of the bidirectional emitted light 106, as described above.

[0064] According to various embodiments, any of a variety of directional scatterers may be employed as the directional scattering elements 124a, 124b. In particular, in some embodiments, a directional scattering element 124a, 124b of one or both of the first plurality of directional scattering elements 124a and the second plurality of directional scattering elements 124b may comprise a diffraction grating. A grating pitch of the diffraction grating may be used to determine a direction of light diffractively scattered out by the diffraction grating, for example. The diffraction grating may be a reflection mode diffraction grating, according to some embodiments. In other

embodiments, the diffraction grating may be a transmission mode diffraction grating and the second directional backlight 120 may further comprise a reflection layer at a surface of the light guide opposite to an emission surface from which the bidirectional emitted light 106 is provided or emitted.

[0065] In other embodiments (not illustrated), the directional scattering element

124, or equivalently directional scattering elements 124a, 124b, may comprise another type of scattering structure including, but not limited to, a micro-refractive element and a micro-reflective element. The micro-refractive element may comprise a microprism at a surface of the light guide 122 and configured to refractively couple the guided light portion out of the light guide 122 as the bidirectional emitted light 106 or equivalently as the first and second segments 106a, 106b thereof, for example. Examples of micro- reflective elements include, but are not limited to, faceted reflectors having a facet with surface angle configured to reflectively scatter out the guided light portion as the bidirectional emitted light 106. In some embodiments, the directional scattering feature 124 comprises an angle-preserving scatterer configured to scatter out the guided light portion having an angular range that is proportional a collimation factor crof the guided light within the light guide 122. As such, the collimation factor crof the guided light may determine the angular extent (i.e., angular ranges φ _α , φδ of the first and second segments 106a, 106b) of the bidirectional emitted light 106, in some embodiments.

[0066] Figure 5 illustrates a cross-sectional view of a portion of a second directional backlight 120 in an example, according to an embodiment consistent with the principles described herein. As illustrated, the second directional backlight 120 comprises the light guide 122 and a pair of directional scattering element 124a, 124b of the directional scattering feature 124. Further, the directional scattering elements 124a, 124b of the pair comprise diffraction gratings, as illustrated. A grating pitch of the diffraction grating differ from one another to provide the bidirectional emitted light 106 having the first and second directions of the bifurcated angular extent. In particular, guided light 126 having a collimation factor σ, illustrated as a plurality of arrows, incident on the diffraction gratings may be diffractively scattered out of the light guide 122 in each of the first and second directions corresponding to the first segment 106a and the second segment 106b of the bidirectional emitted light 106 as a result of the differing grating pitch of the diffraction gratings. For example, a relatively smaller grating pitch or diffractive feature spacing may provide scattered out light having the first direction, while a relatively larger grating pitch or diffractive feature spacing may provide scattered out light having the second direction, as illustrated in Figure 5.

[0067] Referring again to Figures 2 A and 2B, the mode-switchable backlight 100 may further comprise a light source 130. In particular, the light source 130 may comprise a first light source 132 optically coupled to an input of the light guide 112 of the first directional backlight 110, the first light source 132 being configured to provide the light to the light guide 112. The light source 130 may further comprise a second light source 134 optically coupled to an input of the light guide 122 of the second directional backlight 120, the second light source 134 being configured to provide the light to the light guide 122. According to various embodiments, the guided light in the light guidesl 12, 122 provided respectively by the first and second light sources 132, 134 may be collimated according to a predetermined collimation factor. In various embodiments, the light source 130 may be configured to provide light to the light guide 112 of the first directional backlight 110 using the first light source 132 during the first mode. Further, the light source 130 may be configured to provide light to both the light guide 112 of the first directional backlight 110 and the light guide 122 of the second directional backlight 120 during the second mode, according to various embodiments. In Figures 2A-2B, cross-hatching is used to indicate whether or not the first and second light sources 132, 134 are active and providing light, e.g., in the first mode (Figure 2A) and the second mode (Figure 2B), respectively.

[0068] The light source 130 including the first and second light source 132, 134 may comprise substantially any source of light (e.g., optical emitter) including, but not limited to, a light emitting diode (LED), a laser (e.g., laser diode) or a combination thereof. 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.

[0069] In some embodiments, one or both of the first light source 132 and the second light source 134 of the light source 130 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 130. The collimator is further configured to convert the substantially uncollimated light into collimated light. In particular, the collimator may provide collimated light one or both of having the non-zero propagation angle and being 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 112, 122 to propagate as guided light.

[0070] In some embodiments, the first directional backlight 110 of the mode- switchable backlight 100 may be configured to be substantially transparent to light in a direction through the light guide 112 orthogonal to (or substantially orthogonal) to a propagation direction of guided light within the light guide 112. In particular, the light guide 112 and the spaced apart unilateral scattering elements 1 14 may allow light to pass through the light guide 112 from a first (e.g., bottom) surface to a second (e.g., top) surface thereof, in some embodiments. Transparency may be facilitated, at least in part, due to both a relatively small size of the unilateral scattering elements 114 and a relative larger inter-element spacing of the unilateral scattering elements 114, in some

embodiments. Further, when slanted diffraction gratings 114' are used as the unilateral scattering elements 114, the slanted diffraction grating 114' may also be substantially transparent to light propagating orthogonal to surfaces of the light guide 112, according to some embodiments.

[0071] In some embodiments (e.g., as illustrated in Figures 2A-2B), the first directional backlight 110 is configured to provide the narrow-angle emitted light 102 from a first (e.g., top) surface. Further, the second directional backlight 120 may be adjacent to a second surface of the first directional backlight 110 opposite to the first surface. The second directional backlight 120 may be configured to provide the bidirectional emitted light 106 to the second surface of the first directional backlight 110. According to these embodiments, the first directional backlight 110 may be transparent to the bidirectional emitted light 106.

[0072] In some embodiments, a diffraction grating described herein (e.g., a slanted diffraction grating or another diffraction grating) may be a uniform diffraction grating in which a diffractive feature spacing is substantially constant or unvarying throughout the diffraction grating. In other embodiments, the diffraction grating is a 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. Further, the slant angle of the slanted diffraction grating 114' of the unilateral scattering element 114 may vary across, a length, a width, or an extent of the slanted diffraction grating 114'. Further, in some embodiment, diffractive features of diffraction gratings herein may be curved, e.g., curved grooves or ridges in a surface of a light guide.

[0073] In accordance with some embodiments of the principles described herein, a mode-switchable privacy display is provided. According to various embodiments, the mode-switchable privacy display is configured to provide a private displayed image during a first or privacy mode and to provide a public displayed image during a second or public mode. Figure 6 illustrates a block diagram of a mode-switchable privacy display 200 in an example, according to an embodiment consistent with the principles described herein. Operation of the mode-switchable privacy display 200 in the privacy mode (Mode 1) is illustrated in a left half of Figure 6, while a right half illustrates operation in the public mode (Mode 2).

[0074] According some embodiments, the private displayed image is configured to be visible in a private view zone / and the public display image is configured to be visible in a public view zone //that includes the private view zone I. In particular, a viewer 200a located in the private view zone / may be able to see both the private displayed image in the privacy mode and the public display image in the public mode. Similarly, another viewer 200b located in the public view zone II, but outside of the private view zone / may be able to see the public displayed image during the public mode, as illustrated. However, the mode-switchable privacy display 200 may appear dark and the private displayed image may not be visible to the other viewer 200b during the privacy mode, according to various embodiments. As such, the mode-switchable privacy display 200 may be selectively switched between privacy mode and public mode depending on whether the displayed image is intended to be viewed privately by the viewer 200a in the private view zone / or is meant to be viewed publically, e.g., by both the viewer 200a, 200b located substantially anywhere in the public view zone // that includes the private view zone /.

[0075] As illustrated, the mode-switchable privacy display 200 comprises a first directional backlight 210. The first directional backlight 210 is configured to provide narrow-angle emitted light 202 during both a privacy mode (Mode 1) and a public mode (Mode 2). The mode-switchable privacy display 200 further comprises a second directional backlight 220. The second directional backlight 220 is configured to provide bidirectional emitted light 204 exclusively during the public mode (Mode 2). In various embodiments, the bidirectional emitted light 204 has a bifurcated angular extent that is complementary to the narrow-angle emitted light 202. During the public mode, mode- selectable privacy display is configured to provide broad-angle emitted light comprising a combination of the narrow-angle emitted light 202 provided by the first directional backlight 210 and the bidirectional emitted light 204 provided by the second directional backlight 220, according to various embodiments.

[0076] The mode-switchable privacy display 200 illustrated in Figure 6 further comprises an array of light valves 230. The array of light valves 230 is configured to modulate the narrow-angle emitted light 202 as a private displayed image during the privacy mode. Further, the array of light valves 230 is configured to modulate the broad- angle emitted light 206 as a public displayed image during the public mode. In some embodiments (as illustrated), the first directional backlight 210 is between the second directional backlight 220 and the array of light valves 230, the first directional backlight 210 being transparent to the bidirectional emitted light 204. In various embodiments, different types of light valves may be employed as light valves 230 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. Modulated narrow- angle emitted light 202 and broad-angle emitted light 206 are illustrated using dashed line arrows to emphasize the modulation.

[0077] According to some embodiments, the first directional backlight 210 of the mode-switchable privacy display 200 may be substantially similar to the first directional backlight 110 of the mode-switchable backlight 100, described above. In particular, in some embodiments, the first directional backlight 210 may comprise a light guide configured to guide light from a light source along a length of the light guide as guided light. Further, the first directional backlight 210 may comprise an array of unilateral scattering elements configured to scatter out a portion of the guided light as the narrow- angle emitted light 202 having a unilateral direction. The unilateral direction may be toward the array of light valves 230 and away from second directional backlight 220, according to various embodiments. In some embodiments, a unilateral scattering element of the array of unilateral scattering elements may comprise one or both of a slanted diffraction grating and a reflection mode diffraction grating to provide the unilateral scattering. In other embodiments, other unilateral scattering elements may be employed. The first directional backlight 210 may further comprise the light source and more specifically a collimated light source configured to provide light to be guided as the guided light within the light guide, according to some embodiments.

In some embodiments, the second directional backlight 220 of the mode-switchable privacy display 200 may be substantially similar to the second directional backlight 120 of the above-described mode-switchable backlight 100. In particular, the second directional backlight 220 may comprise a light guide configured to guide light from a light source along a length of the light guide as guided light. Further, the second directional backlight 220 may comprise a first plurality of directional scattering elements configured to scatter out a portion of the guided light as a first segment of the

bidirectional emitted light 204 having a first direction of the bifurcated angular extent. Further, the second directional backlight 220 may comprise a second plurality of directional scattering elements configured to scatter out another portion of the guided light as a second segment of the bidirectional emitted light having a second direction of the bifurcated angular extent. According to various embodiments, the first and second directions in combination with an angular range of the first and second segments determine the bifurcated angular extent of the bidirectional emitted light 204.

[0078] In accordance with other embodiments of the principles described herein, a method of mode-switchable backlight operation is provided. Figure 7 illustrates a flow chart of a method 300 of mode-switchable backlight operation in an example, according to an embodiment consistent with the principles described herein. As illustrated in Figure 7, the method 300 of mode-switchable backlight operation comprises emitting 310 narrow-angle emitted light during both a first mode and a second mode using a first directional backlight to illuminate a first view zone. According to some embodiments, the first directional backlight may be substantially similar to the first directional backlight 110, described above with respect to the mode-switchable backlight 100. Further, the narrow angle emitted light may be substantially similar to the narrow-angle emitted light 102 provided by the first directional backlight 110.

[0079] In some embodiments, emitting 310 narrow-angle emitted light comprises guiding light along a length of a light guide as guided light. Emitting 310 narrow-angle emitted light may further comprise scattering out a portion of the guided light using an array of unilateral scattering elements spaced apart from one another along the light guide length. The array of unilateral scattering elements may be configured to scatter out the portion of the guided light as the narrow-angle emitted light having a unilateral direction, according to some embodiments. The light guide and array of unilateral scattering element may be substantially similar to the above-described light guide 112 and array of unilateral scattering elements 114, in some embodiments.

[0080] As illustrated in Figure 7, the method 300 of mode-switchable backlight operation further comprises emitting 320 bidirectional emitted light exclusively during the second mode using a second directional backlight. According to various

embodiments, the bidirectional emitted light has a bifurcated angular extent that is complementary to the narrow-angle emitted light. In some embodiments, the second directional backlight and bidirectional emitted light may be substantially similar respectively to the second directional backlight 120 and the bidirectional emitted light 106 described above with respect to the mode-switchable backlight 100. [0081] In some embodiments, emitting 320 bidirectional emitted light comprises guiding light along a length of a light guide as guided light. The light guide may be substantially similar to the light guide 122 of the second directional backlight 120, in some embodiments. In some embodiments, emitting 320 bidirectional emitted light further comprises scattering out a portion of the guided light using a first plurality of directional scattering elements configured to scatter out a portion of the guided light as a first segment of the bidirectional emitted light having a first direction of the bifurcated angular extent. Emitting 320 bidirectional emitted light further comprises scattering out another portion of the guided light using a second plurality of directional scattering elements configured to scatter out the portion of the guided light as a second segment of the bidirectional emitted light having a second direction of the bifurcated angular extent. In some embodiments, the first and second pluralities of directional scattering elements may be substantially similar to the first and second pluralities of directional scattering elements 124a, 124b, of the above-described second directional backlight 120. Likewise, the first and second segments of the bidirectional emitted light may be substantially similar to the above-described first and second segments 106a, 106b of the bidirectional emitted light 106. In particular, the first and second directions in combination with an angular range of the first and second segments determine the bifurcated angular extent of the bidirectional emitted light.

[0082] The method 300 of mode-switchable backlight operation illustrated in

Figure 7 further comprises combining 330 the narrow-angle emitted light and the bidirectional emitted light during the second mode to provide broad-angle emitted light. In various embodiments, the broad-angle emitted light illuminates a second view zone that includes the first view zone. Further, the narrow-angle emitted light exclusive illuminates the first view zone. As such, in the first mode, only the first view zone is illuminated, according to various embodiments.

[0083] In some embodiments (e.g., as illustrated in Figure 7), the method 300 of mode-switchable backlight operation further comprises modulating 340 the narrow-angle emitted light and the broad-angle emitted light using an array of light valves to display an image. The displayed image is a private displayed image during the first mode or privacy mode and a public displayed image during the second mode or public mode. Further, the private image has a viewing range limited by the angular range of the narrow-angle emitted light. That is, the private displayed image may be visible only within the first view zone. On the other hand, the public displayed image is visible in the second view zone having a wide angular range that includes the first view zone, according to various embodiments. In some embodiments, the array of light valves may be substantially similar to the array of light valves 230 described above with respect to the mode- switchable privacy display 200. Further, the private display image and public display image may be substantially similar to the private and public displayed images also described above with respect to the mode-switchable privacy display 200.

[0084] Thus, there have been described examples and embodiments of a mode- switchable backlight, a mode-switchable privacy display, and a method of mode- switchable backlight operation that employ a switchable combination of narrow-angle emitted light in a first mode and broad-angle emitted light comprising the narrow-angle emitted light and bidirectional emitted light in a second mode. 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.