HK1242427B - Unidirectional grating-based backlighting employing a reflective island - Google Patents

Description

Unidirectional grating-based backlight using reflective islands

CROSS-REFERENCE TO RELATED APPLICATIONS

not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

not applicable.

Background Art

Electronic displays are nearly ubiquitous media for conveying information to users of a wide variety of devices and products. The most common electronic displays include cathode ray tubes (CRTs), plasma display panels (PDPs), liquid crystal displays (LCDs), electroluminescent displays (EL), organic light-emitting diode (OLED) and active-matrix OLED (AMOLED) displays, electrophoretic displays (EPs), and various displays that employ electromechanical or current light modulation (e.g., digital micromirror devices, electrowetting displays, etc.). Generally, electronic displays can 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). 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 EPs. Passive displays often have attractive performance characteristics, including but not limited to inherently low power consumption, but their lack of ability to emit light can limit their use in many practical applications.

In order to overcome the limitations of passive displays associated with emitting light, many passive displays are coupled to external light sources. The coupled light source can allow these originally passive displays to emit light and essentially function as active displays. An example of such a coupled light source is a backlight. A backlight is a light source (usually a panel light source) that is placed behind the originally passive display to illuminate the passive display. For example, a backlight can be coupled to an LCD or EP display. The backlight emits light that passes through the LCD or EP display screen. The emitted light is modulated by the LCD or EP display, and the modulated light is then emitted from the LCD or EP display. The backlight is typically configured to emit white light. Color filters are then used to convert the white light into the various colors used in the display. For example, a color filter can be placed at the output of the LCD or EP display (less common) or between the backlight and the LCD or EP display.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features of examples and embodiments according to the principles described herein may be more readily understood by reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like structural elements, and wherein:

FIG. 1 illustrates a cross-sectional view of a diffraction grating in an example according to an embodiment consistent with the principles described herein.

2A shows a cross-sectional view of a unidirectional grating-based backlight in an example of an embodiment consistent with the principles described herein.

2B shows a cross-sectional view of a unidirectional grating-based backlight in an example according to another embodiment consistent with the principles described herein.

2C shows a perspective view of a unidirectional grating-based backlight in an example according to an embodiment consistent with the principles described herein.

3 shows a cross-sectional view of a portion of a unidirectional grating-based backlight depicting geometry associated with facilitating propagation of a guided light beam, in an example of an embodiment consistent with the principles described herein.

FIG4 shows a block diagram of an electronic display in an example according to an embodiment consistent with the principles described herein.

5 shows a top view of an array of reflective islands in an example, according to an embodiment consistent with the principles described herein.

6 shows a flow chart of a method of electronic display operation in an example of an embodiment consistent with the principles described herein.

Certain examples and embodiments have other features in addition to and instead of the features shown in the above-mentioned reference figures. These and other features are described in detail below with reference to the above-mentioned reference figures.

DETAILED DESCRIPTION

Embodiments according to the principles described herein provide electronic display backlights that utilize reflective redirection of secondary light beams. Specifically, as described above, a unidirectional backlight for an electronic display utilizes a diffraction grating to couple light out of a light guide and directs the coupled-out light as a primary light beam into a viewing direction of the electronic display. Additionally, reflective islands within the light guide are utilized to reflectively redirect the diffraction-generated secondary light beams from the backlight into the viewing direction. In some embodiments, the primary light beam and the reflectively redirected secondary light beams combine to produce a light beam that is brighter (i.e., has a higher intensity) than either the primary light beam or the secondary light beams themselves. For example, by enhancing the light intensity of the emitted light beam, the electronic display can exhibit improved efficiency.

According to some embodiments, the outcoupled light comprising a primary light beam and reflected redirected secondary light beams forms a plurality of light beams directed in a viewing direction. Furthermore, according to various embodiments of the principles described herein, the beam of outcoupled light formed by the combination of the primary light beam and the reflected redirected secondary light beam can have a primary angular direction that is different from other outcoupled primary and secondary light beam pairs. In some embodiments, the plurality of light beams comprising primary and secondary light beam pairs form or provide a light field in the viewing direction. In some embodiments, primary and secondary light beam pairs having different primary angular directions (also referred to as "the differently directed" light beams or light beam pairs) can be employed to display three-dimensional (3-D) information. For example, the differently directed primary and secondary light beam pairs can be modulated and used as pixels of a "glasses free" 3-D electronic display.

As used herein, a "light guide" is defined as a structure that uses total internal reflection to guide light within a structure. In particular, a light guide can include a core that is substantially transparent at the operating wavelength of the light guide. In various examples, a "light guide" generally refers to a dielectric light waveguide that uses total internal reflection to guide light at an interface between the dielectric material of the light guide and the material or medium surrounding the light guide. By definition, a condition for total internal reflection is that the refractive index of the light guide is greater than the refractive index of the surrounding medium adjacent to the surface of the light guide material. In some examples, in addition to or in lieu of the above-mentioned refractive index difference, the light guide can include a coating to further promote total internal reflection. For example, the coating can be a reflective coating. According to various examples, the light guide can be any of several types of light guides, including but not limited to one or both of a plate or guide plate and a guide bar.

Furthermore, herein, when applied to a light guide, the term "plate" as in "plate light guide" is defined as a segmented or differentially planar layer or sheet. In particular, a plate light guide is defined as a light guide configured to guide light in two substantially orthogonal directions defined by the top and bottom surfaces (i.e., opposing surfaces) of the light guide. Furthermore, according to the definitions herein, the top and bottom surfaces are both separate from each other and may be substantially parallel to each other, at least in terms of their differences. That is, within any region of the plate light guide where the differences are small, the top and bottom surfaces are substantially parallel or coplanar. In some examples, the plate light guide may be substantially flat (e.g., defined as a plane), and thus the plate light guide is a planar light guide. In other examples, 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 plate light guide. However, in various examples, any curvature has a sufficiently large radius of curvature to ensure that total internal reflection is maintained in the plate light guide to guide the light.

According to various examples described herein, a diffraction grating (e.g., a multibeam diffraction grating) can be used to scatter or couple light out of a light guide (e.g., a plate light guide) as a "primary" beam. Typically, the diffraction grating also generates a "secondary" beam in addition to the primary beam. According to various examples, when the primary beam is directed or coupled out of the light guide, the diffraction-generated secondary beams are typically directed back into the light guide by a diffraction grating located on or within the light guide. In particular, the diffraction angle θ _m provided by or provided by a locally periodic transmission diffraction grating at the surface of the light guide can be given by equation (1):

Where λ is the wavelength of the light, m is the diffraction order, d is the distance between the features of the diffraction grating, _θi is the angle of incidence of the light on the diffraction grating, and n is the refractive index of the material on the side of the diffraction grating (i.e., the "light-incident" side or the light-guiding side) from which the light is incident on the diffraction grating. For simplicity, equation (1) assumes that the refractive index of the side of the diffraction grating on the light-incident side or the light-guiding side has a refractive index of 1. In general, the diffraction order m is given by an integer that can be positive or negative.

According to various examples, the diffraction angle _θm of the primary light beam generated by the diffraction grating can be given by equation (1), where the diffraction order is positive (e.g., m>0), while the diffraction angle _θm of the secondary light beam can have a negative diffraction order (e.g., m<0). Thus, according to the definitions herein, a "primary light beam" can be defined as a light beam generated by diffraction having a positive diffraction order. Furthermore, a "secondary light beam" can be defined as a light beam generated by diffraction having a negative diffraction order.

FIG1 shows a cross-sectional view of a diffraction grating 10 in an example according to an embodiment consistent with the principles described herein. For example, the diffraction grating 10 can be on the surface of a light guide. Additionally, FIG1 shows a light beam 20 incident on the diffraction grating 10 at an angle of incidence _θi. A primary light beam 30, resulting from diffraction by the diffraction grating 10 and having a diffraction angle _θm (or principal angular direction), and a secondary light beam 40, resulting from diffraction by the diffraction grating 10 and having a corresponding (albeit negative) diffraction angle θ _-m , are shown, both given by equation (1). As shown, the primary light beam 30 corresponds to a diffraction order "m," while the secondary light beam 40 has a corresponding negative diffraction order "-m."

As used herein, a "diffraction grating" is generally defined as a plurality of features (i.e., diffraction features) arranged to provide diffraction of light incident on the diffraction grating. In some examples, the plurality of features can be arranged in a periodic or quasi-periodic manner. For example, a diffraction grating can include a plurality of features arranged in a one-dimensional (1-D) array (e.g., a plurality of grooves in a material surface). In other examples, a diffraction grating can be a two-dimensional (2-D) array of features. For example, a diffraction grating can be a 2-D array of protrusions or holes on a material surface.

Thus, according to the definition herein, a "diffraction grating" is a structure that provides diffraction of light incident on the diffraction grating. If light is incident on the diffraction grating from a light guide, the diffraction or diffractive scattering provided can result in and is therefore referred to as "diffractive coupling", because the diffraction grating can couple light out of the light guide by diffraction. The diffraction grating also redirects or changes the angle of light by diffraction (i.e., the diffraction angle). In particular, as a result of diffraction, the light leaving the diffraction grating (i.e., the diffracted light of the primary and secondary beams) typically has a propagation direction that is different from the propagation direction of the light incident on the diffraction grating (i.e., the incident light). Changing the propagation direction of light by diffraction is referred to herein as "diffractive redirection". Thus, a diffraction grating can be understood as a structure comprising diffractive features that diffractively redirect light incident on the diffraction grating and, if the light is incident from a light guide, the diffraction grating can also diffractively couple light out of the light guide. (eg as in the case of a primary beam), and diffractively generates corresponding light (eg as in the case of a secondary beam) which is directed into the light guide.

Furthermore, as defined herein, the features of a diffraction grating are referred to as "diffractive features" and may be one or more in, in, or on a surface (or a boundary between two materials). For example, the surface may be the surface of a plate light guide. The diffractive features may include any of a variety of structures that diffract light, including but not limited to one or more of grooves, ridges, holes, and protrusions at, in, or on the surface. For example, a diffraction grating may include a plurality of parallel grooves in the surface of a material. In another example, a diffraction grating may include a plurality of parallel ridges rising from the surface of a material. The diffractive features (e.g., grooves, ridges, holes, protrusions, 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 curve, a rectangular profile (e.g., a binary diffraction grating), a triangular profile, and a sawtooth profile (e.g., a blazed grating).

As defined herein, a "multibeam diffraction grating" is a diffraction grating that generates diffracted redirected light (e.g., diffracted outcoupled light) comprising a plurality of primary beams. Furthermore, as defined herein, the plurality of primary beams generated by the multibeam diffraction grating have mutually different principal angular directions. A multibeam diffraction grating may also diffractively generate a plurality of secondary beams. The secondary beams generated by the multibeam diffraction grating generally correspond to the primary beams and have correspondingly different principal angular directions. In particular, as defined herein, one of the plurality of primary (or secondary) beams has a predetermined principal angular direction that is different from another of the plurality of primary (or secondary) beams due to diffraction of incident light by the multibeam diffraction grating. For example, the plurality of primary beams may include eight beams having eight different principal angular directions. For example, the combined eight beams (i.e., the plurality of beams) may represent a light field. Furthermore, there may be a set of eight secondary beams generated by the multibeam diffraction grating, wherein the eight secondary beams also have eight different principal angular directions. Additionally, the secondary beams can correspond to beams in the plurality of primary beams (i.e., have associated primary angular directions), and the secondary beams (when reflectively redirected as described below) can be combined with the corresponding primary beams as part of or augment the light field. According to various examples, the different primary angular directions of the various primary and secondary beams are determined by a combination of the orientation or rotation of the diffraction features of the multibeam diffraction grating at the origin of the respective beams relative to the propagation direction of light incident on the multibeam diffraction grating, and the grating pitch or spacing.

According to various embodiments described herein, a diffraction grating (e.g., a multibeam diffraction grating) is employed to generate outcoupled light that represents pixels of an electronic display. In particular, primary light beams generated by the diffraction grating by diffraction coupling light out of a light guide can represent or correspond to pixels of the electronic display. Additionally, reflective redirection of secondary light beams generated by diffraction can also contribute to electronic display pixels. In particular, the light guide and diffraction grating (i.e., a multibeam diffraction grating) can be part of a backlight for an electronic display or used with an electronic display, such as, but not limited to, a "glasses-free" three-dimensional (3-D) electronic display (e.g., also known as a multi-view or "holographic" electronic display or an autostereoscopic display). Thus, the differently oriented light beams generated by diffraction from the light guide using the multibeam diffraction grating can be or represent "pixels" of the 3-D electronic display.

As used herein, a "light source" is defined as a source of light (e.g., a device or apparatus that generates and emits light). For example, a light source can be a light emitting diode (LED) that emits light when activated. As used herein, a light source can be substantially any light source or light emitting source, including but not limited to light emitting diodes (LEDs), lasers, organic light emitting diodes (OLEDs), polymer light emitting diodes, plasma-based light emitters, fluorescent lamps, incandescent lamps, and virtually any other light source. The light generated by a light source can have a color (i.e., can include light of a specific wavelength) or can be a range of wavelengths (e.g., white light).

In addition, as used herein, the article "a" is intended to have its ordinary meaning in the patent art, i.e., "one or more". For example, "a grating" refers to one or more gratings, and therefore, "grating" refers to "grating(s)" in this article. In addition, "top", "bottom", "above", "below", "up", "down", "front", "back", "first", "second", "left" or "right" mentioned herein are not used as limitations in this article. In this article, unless otherwise expressly stated, the term "about" when applied to a value generally means within the tolerance range of the equipment used to produce the value, or in some examples, means plus or minus 10%, or plus or minus 5%, or plus or minus 1%. In addition, the term "substantially" used herein means, for example, most, or almost all, or all, or an amount in the range of about 51% to about 100%. In addition, the examples herein are intended to be illustrative only and are for discussion purposes only and not for limitation.

In accordance with some embodiments of the principles described herein, a unidirectional grating-based backlight is provided. Figure 2A shows a cross-sectional view of a unidirectional grating-based backlight 100 in accordance with an example of an embodiment consistent with the principles described herein. Figure 2B shows a cross-sectional view of a unidirectional grating-based backlight 100 in accordance with an example of another embodiment consistent with the principles described herein. Figure 2C shows a perspective view of a unidirectional grating-based backlight 100 in accordance with an example of an embodiment consistent with the principles described herein. In accordance with various embodiments, the reflective redirection of the diffraction-generated secondary beam increases or adds to the intensity of the emitted beam (e.g., the light field) to increase the brightness of the unidirectional grating-based backlight 100. In accordance with various embodiments, the increased brightness can improve the efficiency of the unidirectional grating-based backlight 100.

For example, as shown in Figures 2A-2C, light diffracted outcoupled from a unidirectional grating-based backlight 100 can be used to form or provide multiple primary light beams directed away from the surface of the unidirectional grating-based backlight 100 to form a light field. The diffracted outcoupled light is part of the guided light 104 within the unidirectional grating-based backlight 100. The diffraction of the light that provides diffraction coupling also diffracts to produce secondary light beams. According to various embodiments, the reflective redirection of the secondary light beams can add to or increase the light intensity of the primary light beam 102.

In particular, the primary beam 102 can be combined with the reflectively redirected secondary beam 106 (as indicated by the dashed arrow) to form or provide a light field of the unidirectional grating-based backlight 100. Furthermore, according to some embodiments, the beam 102 and the corresponding reflectively redirected secondary beam 106 provided by the unidirectional grating-based backlight 100 can be configured to have a primary angular direction that is different from other primary beams 102 and other reflectively redirected secondary beams 106, respectively. In some examples, the primary beam 102 and the reflectively redirected secondary beam 106 can have a predetermined direction (primary angular direction) and a relatively narrow angular spread within the light field.

In some embodiments, the unidirectional grating-based backlight 100 can be a light source or "backlight" for an electronic display. Specifically, according to some embodiments, the electronic display can be a so-called "glasses-free" three-dimensional (3-D) electronic display (e.g., a multi-view display or an autostereoscopic display), in which the various light beams 102, 106 correspond to or represent pixels associated with different "views" of the 3-D display. The increased light intensity of the light generated by the unidirectional grating-based backlight 100 can increase the brightness of the electronic display (e.g., a 3-D electronic display). For example, the primary angular direction of the primary light beam 102 can be substantially similar relative to the primary angular direction of the reflectively redirected secondary light beam 106. Thus, the primary light beam 102 and the corresponding reflectively redirected secondary light beam 106 can be substantially co-directed or have substantially the same primary angular direction, and further, for example, the primary angular direction can correspond to the angular direction of a particular view of the 3-D electronic display. As a result, according to some examples, the combination of the primary and secondary light beams 102, 106 can represent or correspond to pixels (or equivalently, views) of the 3-D display. Furthermore, for example, a pixel corresponding to the combination of the primary and secondary beams 102 , 106 will be brighter than a pixel including only the primary beam 102 .

In some embodiments, the combination of primary and secondary light beams 102, 106 can be modulated (e.g., by a light valve as described below). For example, modulating different sets of combined light beams 102, 106 at different angular directions away from the unidirectional grating-based backlight 100 can be particularly useful for dynamic 3-D electronic display applications. That is, different sets of modulated light beams 102, 106 directed toward a particular viewing direction can represent dynamic pixels of a 3-D electronic display corresponding to that particular viewing direction.

As shown in Figures 2A-2C, the unidirectional grating-based backlight 100 includes a light guide 110. In particular, according to some embodiments, the light guide 110 can be a plate light guide 110. The light guide 110 is configured to guide light from a light source (not shown in Figures 2A-2C) as guided light 104. For example, the light guide 110 can include a dielectric material configured as an optical waveguide. The dielectric material can have a first refractive index that is greater than a second refractive index of a medium surrounding the dielectric optical waveguide. For example, the difference in refractive index is configured to promote total internal reflection of the guided light 104 according to one or more guided modes of the light guide 110.

In some embodiments, light from a light source is guided as a light beam 104 along the length of the light guide 110. Furthermore, the light guide 110 can be configured to guide the light (i.e., the guided light beam 104) at a non-zero propagation angle. For example, the guided light beam 104 can be guided (e.g., as a light beam) at a non-zero propagation angle within the light guide 110 using total internal reflection.

A non-zero propagation angle is defined herein as an angle relative to a surface (e.g., a top surface or a bottom surface) of the light guide 110. In some examples, the non-zero propagation angle of the guided light beam 104 can be between about ten (10) degrees and about fifty (50) degrees, or in some examples, between about twenty (20) degrees and about forty (40) degrees, or between about twenty-five (25) degrees and about thirty-five (35) degrees. For example, the non-zero propagation angle can be about thirty (30) degrees. In other examples, the non-zero propagation angle can be about 20 degrees, or about 25 degrees, or about 35 degrees.

In some examples, light from a light source is introduced or coupled into the light guide 110 at a non-zero propagation angle (e.g., approximately 30-35 degrees). One or more of a lens, a mirror, or similar reflector (e.g., a tilted collimating reflector) and a prism (not shown) can assist in coupling the light into the input end of the light guide 110 at a non-zero propagation angle. Once coupled into the light guide 110, the guided light beam 104 propagates along the light guide 110 in a direction generally away from the input end (e.g., as shown along the x-axis in Figures 2A-2B). In addition, the guided light beam 104 is reflected or "bouncing" between the top and bottom surfaces of the light guide 110 at the non-zero propagation angle (e.g., as shown, the rays of the guided light beam 104 are represented by the extended angled arrows).

According to some examples, guided light beam 104 generated by coupling light into light guide 110 can be a collimated light beam. In particular, by "collimated light beam" is meant that the light rays within guided light beam 104 are substantially parallel to one another within guided light beam 104. Light rays that diverge or scatter from the collimated light beam of guided light beam 104 are not considered part of the collimated light beam as defined herein. For example, collimation of the light used to generate collimated guided light beam 104 can be provided by a lens or a reflector (e.g., a tilted collimating reflector, etc.) used to couple the light into light guide 110.

In some examples, the light guide 110 (e.g., as a plate light guide 110) can be a sheet or plate-like optical waveguide comprising an extended substantially flat sheet of optically transparent dielectric material. The substantially flat sheet of dielectric material is configured to guide the guided light beam 104 using total internal reflection. According to various examples, the optically transparent material of the light guide 110 can include or be made of any of a variety of dielectric materials, including, but not limited to, one or more of various types of glass (e.g., quartz 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 can also include a cladding layer (not shown) on at least a portion of a surface (e.g., one or both of the top and bottom surfaces) of the light guide 110. According to some examples, the cladding layer can be used to further promote total internal reflection.

According to various embodiments, the unidirectional grating-based backlight 100 further includes a diffraction grating 120. In some examples, the diffraction grating 120 can be located at a surface (e.g., a front surface or a top surface) of the light guide 110, for example, as shown in Figures 2A-2B. In other examples (not shown), the diffraction grating 120 can be located within the light guide 110. The diffraction grating 120 is configured to diffractively scatter or couple out a portion of the guided light beam 104 as a primary light beam 102. The diffraction grating 120 is further configured to direct the primary light beam 102 away from the light guide surface in a predetermined primary angular direction. The primary angular direction of the primary light beam 102 has an elevation angle and an azimuth angle. Furthermore, according to various examples, the diffraction grating 120 is configured to diffractively generate a secondary light beam from another portion of the guided light beam 104, as described below. The diffraction-generated secondary beam may be directed into the light guide 110 with a negative principal angular direction corresponding to the predetermined principal angular direction of the primary beam 102 (eg, opposite to the outcoupled light guide 110 ).

According to various embodiments, the diffraction grating 120 includes a plurality of diffraction features 122 configured to provide diffraction. The provided diffraction is responsible for diffractionally coupling a portion of the guided light beam 104 out of the light guide 110 as the main light beam 102. For example, the diffraction grating 120 may include one or both of grooves in the surface of the light guide 110 or ridges protruding from the surface of the light guide 110, which serve as the diffraction features 122. The grooves and ridges may be arranged parallel or substantially parallel to each other and, at least at some point, perpendicular to the propagation direction of the guided light beam 104 coupled out by the diffraction grating 120.

In some examples, the grooves or ridges can be etched, ground, or molded into or applied to a surface. Thus, the material of the diffraction grating 120 can include the material of the light guide 110. For example, as shown in FIG2A , the diffraction grating 120 includes substantially parallel grooves formed in the surface of the light guide 110. In FIG2B , the diffraction grating 120 includes substantially parallel ridges protruding from the surface of the light guide. In other examples (not shown), the diffraction grating 120 can include a film or layer applied or affixed to the surface of the light guide.

When the diffraction grating 120 is a member of a plurality of diffraction gratings 120, the plurality of diffraction gratings 120 can be arranged at, on, or in a surface of the light guide 110 in various configurations, according to various examples. For example, the plurality of diffraction gratings 120 can be arranged in columns and rows across the surface of the light guide (e.g., as an array). In another example, the plurality of diffraction gratings 120 can be arranged in groups (e.g., three gratings in a group, each grating in the group associated with a different color of light), and the groups can be arranged in rows and columns. In yet another example, the plurality of diffraction gratings 120 can be substantially randomly distributed across the surface of the light guide 110.

According to some embodiments, the diffraction grating 120 is or includes a multibeam diffraction grating 120. According to various embodiments, the multibeam diffraction grating 120 is configured to couple a portion of the guided light beam 104 out of the light guide 110 by or using diffractive coupling (e.g., also referred to as "diffractive scattering"). For example, a portion of the guided light beam 104 can be diffractively coupled out by the multibeam diffraction grating 120 through a light guide surface (e.g., through a top surface of the light guide 110) as a plurality of main light beams 102 (e.g., as shown in Figures 2A and 2B). Furthermore, the plurality of main light beams 102 coupled out by the multibeam diffraction grating 120 are directed away from the light guide surface by the multibeam diffraction grating 120. According to various embodiments, a main light beam 102 in the plurality of main light beams has a main angular direction that is different from other main light beams 102 in the plurality of main light beams. According to various embodiments, the multiple main light beams 102 coupled out by the multi-beam diffraction grating 120 together form a light field of the unidirectional grating-based backlight 100 .

Furthermore, the multi-beam diffraction grating 120 may generate a plurality of secondary beams due to diffraction of another portion of the guided light beam 104. Typically, the diffraction-generated secondary beams are initially directed away from the multi-beam diffraction grating 120 and are directed into the light guide 110 at a primary angular direction that is different from that of the other secondary beams in the plurality of secondary beams. The primary angular directions of the diffraction-generated secondary beams have respective elevation and azimuth angles. In particular, the elevation angle of the primary angular direction of a particular secondary beam is substantially equal in magnitude to the elevation angle of the primary angular direction of the corresponding primary beam 102 in the plurality of primary beams, but of opposite sign. Furthermore, the azimuth angle of the primary angular direction of a particular secondary beam may be substantially equal to the azimuth angle of the primary angular direction of the corresponding primary beam (e.g., see FIG. 1 ). For example, a primary beam 102 having an elevation angle of sixty degrees (60°) and an azimuth angle of ten degrees (10°) may have a corresponding diffraction-generated secondary beam having an elevation angle of negative sixty degrees (-60°) and an azimuth angle of ten degrees (10°).

According to various examples, the multi-beam diffraction grating 120 can include a chirped diffraction grating 120. By definition, a "chirped" diffraction grating 120 is a diffraction grating that exhibits or has a diffraction spacing of a diffraction feature that varies over the extent or length of the chirped diffraction grating 120, for example, as shown in Figures 2A and 2B. The varying diffraction spacing is referred to herein as "chirp." Thus, the guided light beam 104 diffractively coupled from the light guide 110 exits or is emitted from the chirped diffraction grating 120 as a primary light beam 102, which passes through the chirped diffraction grating 120 at different diffraction angles corresponding to different origins. Similarly, the secondary light beams generated by diffraction also exit the chirped diffraction grating 120 at different diffraction angles corresponding to different origins. Due to the predefined chirp, the chirped diffraction grating 120 ensures predetermined different main angular directions of the outcoupled main beam 102 and of the diffracted secondary beams.

In Figures 2A-2C, the multibeam diffraction grating 120 is a chirped diffraction grating 120. In particular, as shown, at a first end of the multibeam diffraction grating 120 (e.g., near the light source) the diffraction features 122 are closer together than at the second end. In addition, the diffraction spacing d of the diffraction features 122 is shown to vary from the first end to the second end. In some examples, the chirped diffraction grating 120 can have or exhibit a chirp in the diffraction spacing d that varies linearly with distance (e.g., see Figures 2A-2C). Therefore, the chirped diffraction grating 120, as shown, can be referred to as a "linearly chirped" diffraction grating.

In another example (not shown), the chirped diffraction grating 120 can exhibit a nonlinear chirp of the diffraction spacing d. Various nonlinear chirps that can be used to implement the chirped diffraction grating 120 include, but are not limited to, exponential chirps, logarithmic chirs, or chirps that vary in another substantially non-uniform or random, but still monotonic, manner. Non-monotonic chirps can also be used, such as, but not limited to, sinusoidal chirps or triangular or sawtooth chirps. Combinations of any of these types of chirps can also be employed.

In some examples, when the guided light beam 104 propagates in the light guide 110 in a direction from the first end of the multibeam diffraction grating 120 to the second end of the multibeam diffraction grating 120 (e.g., as shown in FIG2A ), the primary light beam 102 generated by coupling light out of the light guide 110 using the multibeam diffraction grating 120 may diverge (i.e., become a diverging light beam 102). Alternatively, according to other examples (not shown), a converging primary light beam 102 may be generated when the guided light beam 104 propagates in the opposite direction in the light guide 110, i.e., from the second end to the first end of the multibeam diffraction grating 120. Similarly, the diffraction-generated secondary light beams (not shown in FIG2A-2C ) may be respectively substantially divergent or substantially converging (although in a direction substantially entering the light guide 110).

Referring to FIG2C , a multibeam diffraction grating 120 includes curved and chirped diffractive features 122 (e.g., grooves or ridges) at, in, or on the surface of a light guide 110. A guided light beam 104 has an incident direction relative to the multibeam diffraction grating 120 and the light guide 110, as indicated by the thick arrow labeled 104 in FIG2C . A plurality of coupled-out or emitted main light beams 102 are also shown directed away from the multibeam diffraction grating 120 at the surface of the light guide 110. The main light beams 102 are shown emitted at a plurality of predetermined different principal angular directions. In particular, as shown, the predetermined different principal angular directions of the emitted main light beams 102 differ in both azimuth and elevation (e.g., to shape a light field). According to various examples, both the predetermined chirp of the diffractive features 122 and the curvature of the diffractive features 122 can contribute to the predetermined different principal angular directions of the emitted main light beams 102.

For example, due to the curvature, the diffractive features 122 within the multibeam diffraction grating 120 can have varying orientations relative to the incident direction of the guided light beam 104. In particular, the orientation of the diffractive features 122 within the multibeam diffraction grating 120 at a first point or location can be different from the orientations of the diffractive features 122 at other points or locations. According to some examples, for the outcoupled or emitted light beam 102, the azimuthal component φ of the primary angular direction {θ, φ} of the primary light beam 102 (and the azimuthal components of the secondary light beams) can be determined or correspond to the azimuthal orientation angle φ _f of the diffractive features 122 at the origin of the light beam 102 (i.e., at the point where the incident guided light 104 is outcoupled). Thus, the various orientations of the diffractive features 122 within the multibeam diffraction grating 120 produce different primary light beams 102 and corresponding secondary light beams having different primary angular directions {θ, φ}, at least according to their respective azimuthal components φ.

In particular, at different points along the curve of the diffraction feature 122, the "underlying diffraction grating" of the multibeam diffraction grating 120 associated with the curved diffraction feature 122 has different azimuthal orientation angles φ _f . Thus, the curve at a given point along the curved diffraction feature 122 has a particular azimuthal orientation angle φ _f that is typically different from the azimuthal orientation angle φ f that the curve has at another point along the curved diffraction feature 122. Furthermore, the particular azimuthal orientation angle φ _f results in a corresponding azimuthal component _φ of the primary angular direction {θ, φ} of the primary light beam 102 emitted from the given point. In some examples, the curve of the diffraction feature (e.g., groove, ridge, etc.) can represent a portion of a circle. The circle can be coplanar with the lightguide surface. In other examples, the curve can represent a portion of an ellipse or another curved shape, e.g., a portion coplanar with the lightguide surface.

In other examples, the multi-beam diffraction grating 120 can include diffractive features 122 that are "piecewise" curved. Specifically, while the diffractive features may not describe a substantially smooth or continuous curve per se, the diffractive features can still face different angles relative to the incident direction of the guided light beam 104 at different points along the diffractive features within the multi-beam diffraction grating 120. For example, the diffractive features 122 can be grooves that include a plurality of substantially straight segments, each segment having a different orientation than adjacent segments. According to various examples, the different angles of the segments together can approximate a curve (e.g., a segment of a circle). In other examples, the diffractive features 122 can simply have different orientations relative to the incident direction of the guided light beam 104 at different locations within the multi-beam diffraction grating 120 that do not approximate a particular curve (e.g., a circle or ellipse).

2A and 2B , the unidirectional grating-based backlight 100 further includes reflective islands 130. According to various embodiments, the reflective islands are located between the front and rear surfaces (i.e., opposing surfaces) of the light guide 110. In some embodiments, the reflective islands 130 are both located within the light guide 110 (i.e., between the front and rear surfaces) and are significantly spaced apart from the front and rear surfaces.

According to various embodiments, the reflective islands 130 are configured to reflectively redirect the secondary beams generated by diffraction of the diffraction grating 120. In particular, the reflective islands 130 are configured to reflectively redirect the diffraction-generated secondary beams in the direction of or corresponding to the outcoupled primary beam 102. The reflective redirection by the reflective islands 130 results in or produces reflectively redirected secondary beams 106 that can exit the light guide 110 (e.g., through the diffraction grating 120), as shown in Figures 2A-2B , which are distinguished from the primary beam 102 using dashed lines.

In some embodiments, reflective islands 130 are metal islands 130 that include a reflective metal layer. The metal layer can include a reflective or "polished" metal, such as, but not limited to, silver, gold, aluminum, nickel, chromium, or various combinations or alloys thereof. In other examples, reflective islands 130 can include additional reflective island structures or layers, including, but not limited to, Bragg reflector islands, or more specifically, distributed Bragg reflector (DBR) islands. According to some embodiments, the metal layer or other reflective island structure can be deposited (e.g., using vacuum deposition) or otherwise disposed on a layer of light guide 110. Additional material of light guide 110 can then be added (e.g., deposited, laminated, etc.) on top of the deposited metal island layer or other reflective island structure to position reflective islands 130 between the front and back surfaces within light guide 110.

As defined herein, reflective islands 130 are discrete reflective structures or layers having a length and width that are less than the length and width of light guide 110. In particular, as the term "island" is used, reflective islands 130 are not continuous films or layers within light guide 110, e.g., relative to the direction of propagation of guided light beam 104. Instead, reflective islands 130 have a finite length and a finite width that is less than, and in some examples, significantly less than, one or both of the length and width of light guide 110. In some embodiments, the scale or size of reflective islands 130 is approximately equal to the scale or size of diffraction grating 120. In some embodiments, reflective islands 130 can be laterally aligned with diffraction grating 120. For example, as illustrated in Figures 2A and 2B, light guide 110 can be a plate light guide 110 having diffraction grating 120 on a front surface and reflective islands 130 located below (e.g., spaced apart), with the reflective islands located in a plane parallel to the front surface and laterally aligned with diffraction grating 120. For example, as shown, the diffraction grating 120 and the reflective islands 130 can be vertically aligned or “vertically stacked.” Furthermore, as shown in Figures 2A-2B, as an example, the reflective islands 130 can be approximately the same size as the overlying diffraction grating 120.

According to some embodiments, the distance (e.g., vertical distance or separation distance) between the diffraction grating 120 and the reflective islands 130 is selected to facilitate propagation of the guided light beam 104 within the light guide 110. For example, when the reflective island size is approximately equal to the diffraction grating size, and the guided light beam 104 is configured to propagate at a non-zero propagation angle γ, the separation distance h between the diffraction grating 120 and the reflective islands 130 can be selected to be approximately equal to half the grating pitch P multiplied by a tangent to the non-zero propagation angle γ. Specifically, the separation distance h can be given by equation (2):

The grating pitch P is the lateral (eg, horizontal) spacing between a diffraction grating 120 and an adjacent (eg, preceding) diffraction grating 120 .

FIG3 shows a cross-sectional view of a portion of a unidirectional grating-based backlight in an example of an embodiment consistent with the principles described herein, depicting the geometry associated with facilitating propagation of a guided light beam. Specifically, FIG3 shows a portion of a pair of diffraction gratings 120 at the front surface 110′ of the light guide 110 and a portion of a pair of reflective islands 130 aligned below each diffraction grating 120, as shown. Also shown in FIG3 are elongated arrows bounded by a pair of dashed lines, which are guided light beams 104 having a non-zero propagation angle γ. The dashed lines on either side of the elongated arrows show the extent or beamwidth W of the guided light beam 104. By selecting the separation distance h between the diffraction grating 120 and the associated reflective island 130 using the grating pitch P according to equation (2), the beamwidth W of the guided light beam 104, defined by the space between the two reflective islands 130, can be wide enough to fully illuminate the diffraction grating 120, as shown. Also shown in FIG3 are reflected redirected secondary beams 106 originating from the surfaces of the reflective islands 130. In particular, the reflected redirected secondary beam 106 is a secondary beam 106' generated by diffraction reflected onto the reflective island surface. For simplicity and clarity, the primary beam 102 is not shown in FIG3.

In accordance with some embodiments, the unidirectional grating-based backlight 100 may further include a light source (not shown in Figures 2A-2C and 3). The light source may be configured to provide light that is a guided light beam 104 when coupled into the light guide 110. In various embodiments, the light source may be substantially any light source, including but not limited to the light sources listed above, such as one or more of a light emitting diode (LED), a fluorescent lamp, and a laser. In some examples, the light source may produce substantially monochromatic light having a narrowband spectrum represented by a particular color. In other examples, the light provided by the light source has a substantially broad spectrum. For example, the light produced by the light source may be white light, and the light source may be fluorescent.

Some embodiments according to the principles described herein provide an electronic display. In various embodiments, the electronic display is configured to emit modulated light beams that serve as pixels of the electronic display. Furthermore, in various examples, the emitted modulated light beams can be preferentially directed toward a viewing direction of the electronic display as a plurality of light beams in different directions. In some examples, the electronic display is a three-dimensional (3-D) electronic display (e.g., a glasses-free 3-D electronic display). According to various examples, different ones of the modulated light beams in different directions can correspond to different “views” associated with the 3-D display. For example, the different views can provide a “glasses-free” (e.g., autostereoscopic) representation of information displayed by the 3-D display.

FIG4 shows a block diagram of an electronic display 200 in an example according to an embodiment consistent with the principles described herein. Specifically, the electronic display 200 shown in FIG4 is a 3-D electronic display 200 (e.g., a "glasses-free" 3-D electronic display) that is configured to emit modulated light beams 202 representing pixels corresponding to different views of the 3-D electronic display 200. By way of example and not limitation, the emitted, modulated light beams 202 shown in FIG4 are diverging (e.g., as opposed to converging).

The 3-D electronic display 200 shown in FIG4 includes a plate light guide 210 for guiding light. The light guided in the plate light guide 210 becomes the light source of the modulated light beam 202 emitted by the 3-D electronic display 200. According to some examples, the plate light guide 210 can be substantially similar to the light guide 110 described above for the unidirectional grating-based backlight 100. For example, the plate light guide 210 can be a flat plate light waveguide, which is a planar sheet of dielectric material configured to guide light by total internal reflection. The guided light can be guided as a light beam with a non-zero propagation angle. Furthermore, according to some embodiments, the guided light beam can be a collimated light beam.

The 3-D electronic display 200 shown in FIG4 also includes a multi-beam diffraction grating array 220. As described above, in some examples, the multi-beam diffraction grating 220 can be substantially similar to the multi-beam diffraction grating 120 of the unidirectional grating-based backlight 100. In particular, the multi-beam diffraction grating 220 of the array is configured to couple out a portion of the guided light as a plurality of main beams 204. Furthermore, the multi-beam diffraction grating 220 is configured to direct the main beams 204 into a corresponding plurality of different main angular directions to form a light field.

Furthermore, in some embodiments, the multi-beam diffraction grating array 220 may include chirped diffraction gratings. In some examples, the diffractive features (e.g., grooves, ridges, etc.) of the multi-beam diffraction grating 220 are curved diffractive features. For example, the curved diffractive features may include curved (i.e., continuously curved or segmented) ridges or grooves, and the spacing between the curved diffractive features may vary as a function of the distance between the multi-beam diffraction gratings 220 in the array.

As shown in FIG4 , the 3-D electronic display 200 further includes an array of reflective islands 230. The reflective islands 230 are located within the plate light guide 210. In particular, according to some embodiments, the reflective islands 230 may be located between and spaced apart from the front and rear surfaces of the plate light guide 210. Furthermore, the array of reflective islands 230 is co-located or aligned (e.g., vertically stacked) with the multibeam diffraction grating array 220, such that each multibeam diffraction grating 220 has a corresponding reflective island 230. Each reflective island 230 is configured to reflectively redirect diffracted secondary beams from the corresponding multibeam diffraction grating 220. Furthermore, the reflective islands 230 reflectively redirect the diffracted secondary beams. In turn, the multibeam diffraction grating 220 is configured to also direct the reflected redirected secondary beams out of the plate light guide 210 in the directions of a plurality of outcoupled primary beams. As a result, according to various embodiments, the resulting light field includes a primary beam 204 and reflectively redirected secondary beams 206. In some embodiments, the primary light beam 204 and the corresponding reflected redirected secondary light beam 206 are substantially co-oriented (eg, have similar primary angular directions) within the light field.

In some embodiments, the reflective islands 230 of the array can be substantially similar to the reflective islands 130 described above with respect to the unidirectional grating-based backlight 100. For example, the reflective islands 230 can comprise metal islands. Furthermore, the reflective islands can be laterally (e.g., horizontally) aligned with the corresponding multibeam diffraction grating 220. In some embodiments, the size or scale of the reflective islands 230 can be substantially similar to the size or scale of the corresponding multibeam diffraction grating 220. Furthermore, the spacing between the multibeam diffraction grating 220 and the aligned reflective islands 230 can be given by equation (2) above.

In some embodiments, the reflectivity of the reflective islands 230 is modulated as a function of distance along the array. For example, the reflectivity of the reflective islands 230 can be modulated to gradually increase the reflectivity of individual reflective islands 230 along the length of the array of reflective islands 230. For example, increasing the reflectivity can be employed to compensate for a loss in intensity of the guided light beam as a function of distance in the light guide plate 210.

FIG5 illustrates a top view of an array of reflective islands 230 in accordance with an embodiment consistent with the principles described herein. As shown in FIG5 , the reflectivity of the reflective islands 230 is modulated by gaps 232 as a function of distance (e.g., from left to right in FIG5 ). Specifically, the reflective islands 230 in the reflective island array include reflective strips 234 (e.g., strips of reflective material or metal) separated by gaps 232 (i.e., without the presence of reflective material or metal). The reflectivity of a selected reflective island 230 is determined and, therefore, modulated by the width of the gaps 232 relative to the width of the reflective strips 234 in the corresponding reflective island 230 in the direction of propagation of the guided light (bold arrow 104). For example, a reflective island 230 that includes more reflective (e.g., metal) strips 234 than gaps 232 will have a greater reflectivity than other reflective islands 230 in the array that include fewer reflective strips 234 than gaps 232. FIG5 illustrates the increase in reflectivity with distance in the propagation direction 104 as the width of the reflective strips 234 relative to the gaps 232 increases. In another example (not shown), the reflectivity can be modulated by varying the number of gaps and strips, respectively. In yet another example, reflectivity modulation can be provided by varying the thickness or density of the reflective material layer of the reflective islands 230 as a function of distance along the array (e.g., similar to how a half-silvered mirror is formed). Any of a variety of other methods of modulating reflectivity can also be employed, including, for example, but not limited to, varying the number of layers of DBR-based mirror islands.

Referring again to FIG. 4 , the 3-D electronic display 200 further includes a light valve array 240. According to various embodiments, the light valve array 240 includes a plurality of light valves configured to modulate the primary light beam 204 and the reflected, redirected secondary light beams 206 coupled from the plate light guide 210. In particular, the light valves of the light valve array 240 modulate the combined primary light beam 204 and the reflected, redirected secondary light beams 206 to provide modulated light beams 202. The modulated light beams 202 represent pixels of the 3-D electronic display 200. Furthermore, different modulated light beams 202 can correspond to different views of the 3-D electronic display. In various examples, different types of light valves can be employed in the light valve array 240, including, but not limited to, one or more of liquid crystal (LC) light valves and electrophoretic light valves. By way of example, dashed lines are used in FIG. 4 to emphasize the modulation of the modulated light beams 202.

In some examples (e.g., as shown in FIG. 4 ), the 3-D electronic display 200 further includes a light source 250. The light source 250 is configured to provide light propagating in the plate light guide 210 as guided light. In particular, according to some examples, the guided light is light from the light source 250 coupled to an edge of the plate light guide 210. In some examples, the light source 250 is substantially similar to the light source described above with respect to the unidirectional grating-based backlight 100. For example, the light source 250 can include LEDs of a specific color (e.g., red, green, blue) to provide monochromatic light or a broadband light source, such as, but not limited to, fluorescent light, to provide broadband light (e.g., white light).

In accordance with some embodiments of the principles described herein, a method of electronic display operation is provided. FIG6 shows a flow chart of a method 300 of electronic display operation in an example of an embodiment consistent with the principles described herein. As shown in FIG6 , the method 300 of electronic display operation includes guiding 310 light in a light guide. In some embodiments, the light guide and the guided light can be substantially similar to the light guide 110 and the guided light beam 104 described above with respect to the unidirectional grating-based backlight 100. In particular, in some embodiments, the light guide can guide 310 the guided light as a light beam (e.g., a collimated, straight light beam) based on total internal reflection. For example, the light beam can be guided 310 at a non-zero propagation angle. Furthermore, in some embodiments, the light guide can be a substantially planar dielectric light waveguide (e.g., a plate light guide).

The method 300 of operating an electronic display further includes diffractively coupling out 320 portions of the guided light using a plurality of diffraction gratings. In some embodiments, the diffraction gratings are multi-beam diffraction gratings, and the portions of the guided light are diffractively coupled out 320 using the multi-beam diffraction gratings to produce a plurality of primary light beams directed out and away from the lightguide surface. In particular, according to some embodiments, the primary light beams can be directed away from the lightguide surface at different primary angular directions to form a light field. In some examples, the plurality of primary light beams are substantially similar to the plurality of primary light beams 102, 204 described above with respect to the unidirectional grating-based backlight 100 and the 3-D electronic display 200.

According to various examples, the multi-beam diffraction grating is located on the surface of the light guide. For example, the multi-beam diffraction grating can be formed in the surface of the light guide as grooves, ridges, etc. In other examples, the multi-beam diffraction grating can include a film on the surface of the light guide. In some examples, the diffraction grating, and more specifically, the multi-beam diffraction grating, is substantially similar to the multi-beam diffraction grating 120 described above with respect to the unidirectional grating-based backlight 100. In other examples, the diffraction grating is located elsewhere, including but not limited to within the light guide. According to some embodiments, the main beams forming the light field can correspond to pixels of an electronic display. In particular, multiple main beams can correspond to pixels of different views of a three-dimensional (3-D) electronic display.

As shown in FIG6 , the method 300 of electronic display operation further includes reflectively redirecting 330 secondary beams in the direction of the coupled or emitted plurality of primary beams. For example, the secondary beams of the reflectively redirected 330 are directed out of the light guide (i.e., emitted from the light guide) and can be combined with the primary beams to join the formed light field (e.g., to increase the intensity of the light field). According to various embodiments, the reflectively redirecting 330 secondary beams is performed using reflective islands. According to various embodiments, the secondary beams are diffracted from another portion of the guided light and are directed toward the reflective islands by the multi-beam diffraction grating.

In some embodiments, the reflective islands can be substantially similar to the reflective islands 130 described above with respect to the unidirectional grating-based backlight 100. In particular, according to some embodiments, the reflective islands are located in the light guide between the front and rear surfaces of the light guide and spaced apart from the front and rear surfaces. Furthermore, for example, the reflective islands can be metal islands comprising a reflective metal layer. Furthermore, the reflective islands can be members of an array of reflective islands that are spaced apart from and laterally aligned (e.g., vertically stacked) with respect to the array of multi-beam diffraction gratings. In some examples, as described above, the array of reflective islands can have a modulated reflectivity of the reflective islands as a function of distance in the light guide in the direction of propagation of the guided light. Furthermore, the sub-beams of the reflective redirection 330 can be substantially similar to the reflective redirected sub-beams 106, 206 described above with respect to the unidirectional grating-based backlight 100 and the 3-D electronic display 200.

In some examples, the method 300 of operating an electronic display further includes modulating 340 the emitted primary light beams and the reflected redirected 330 light beams using a plurality of light valves. In particular, a light field formed by the plurality of emitted primary light beams, including a plurality of substantially combined emitted secondary light beams, is modulated 340 by passing through or otherwise interacting with the corresponding plurality of light valves. According to some embodiments, the modulated 340 primary and secondary light beams in the formed light field can form pixels of an electronic display (e.g., a 3-D electronic display). For example, the modulated 340 primary and secondary light beams of the formed light field can provide a plurality of different views of a 3-D electronic display (e.g., a glasses-free 3-D electronic display).

In some examples, the plurality of light valves used to modulate 340 the primary and secondary light beams are substantially similar to the light valve array 240 described above with respect to the 3-D electronic display 200. For example, the light valves can include liquid crystal light valves. In another example, the light valves can be another type of light valve, including but not limited to one or both of an electrowetting light valve and an electrophoretic light valve, or a combination of a liquid crystal light valve or other light valve types.

Thus, examples of unidirectional grating-based backlights, 3-D electronic displays, and methods of electronic display operation that employ reflective redirection of diffraction-generated secondary beams have been described. It should be understood that the above examples are merely illustrative of some of the many specific examples of the principles described herein. Clearly, those skilled in the art can readily devise numerous other arrangements without departing from the scope of the appended claims.

Claims (18)

1. A unidirectional grating-based backlight, comprising: An optical guide, configured to guide a beam of light at a non-zero propagation angle; A diffraction grating is provided on the surface of the light guide, the diffraction grating being configured to diffractically couple a portion of the guided light beam as a main beam, and to orient the main beam away from the light guide surface at a predetermined principal angle direction; the diffraction grating is also configured to diffractically generate a secondary beam and orient the secondary beam into the light guide; and A reflective island within the light guide, between the light guide surface and the opposing surface of the light guide, the reflective island being configured to reflectively redirect the secondary beam out of the light guide in the direction of the primary beam. Wherein, the size of the reflection island is equal to the size of the diffraction grating, and the distance between the diffraction grating and the reflection island is equal to half the grating spacing multiplied by the tangent of the non-zero propagation angle of the guided beam, and the grating spacing is the lateral spacing between the diffraction grating and the adjacent diffraction grating at the light guide surface. 2. The unidirectional grating-based backlight as claimed in claim 1, wherein the diffraction grating includes a multi-beam diffraction grating configured to couple a portion of the guided beam as a plurality of master beams, wherein the master beams among the plurality of master beams have different principal angle directions. 3. The unidirectional grating-based backlight as described in claim 2, wherein the multi-beam diffraction grating includes a chirped diffraction grating. 4. The unidirectional grating-based backlight as claimed in claim 2, wherein the multibeam diffraction grating comprises one of curved grooves and curved ridges spaced apart from each other. 5. The unidirectional grating-based backlight as claimed in claim 2, wherein the different principal angular directions of the main beam are configured to form a light field, the light field being configured to provide pixels corresponding to different views of a three-dimensional (3-D) electronic display. 6. The unidirectional grating-based backlight as described in claim 1, wherein the reflective island is a metal island comprising a reflective metal layer. 7. The unidirectional grating-based backlight as claimed in claim 1, wherein the light guide is a plate light guide, and wherein the reflection island is aligned with the diffraction grating. 8. An electronic display comprising the unidirectional grating-based backlight of claim 1, wherein the pixels of the electronic display comprise the main beam combined with the secondary beam that is reflected and redirected. 9. The electronic display of claim 8, further comprising a light valve for modulating the combined main beam and the reflected and redirected secondary beam, the light valve being adjacent to the light guide surface including the diffraction grating. 10. A three-dimensional (3D) electronic display, comprising: Plate light guide for guiding light; An array of multibeam diffraction gratings, wherein the multibeam diffraction gratings of the grating array are configured to diffractically couple a portion of the guided light in the plate light guide as a plurality of main beams, the plurality of main beams being oriented in a plurality of corresponding different principal angle directions to form a light field, and the multibeam diffraction gratings are further configured to diffractically generate a plurality of secondary beams and direct the plurality of secondary beams into the plate light guide. An array of reflective islands within the plate light guide, aligned with the multi-beam diffraction grating array, wherein each reflective island in the array is configured to reflectively redirect the plurality of secondary beams to the plurality of primary beams from the aligned multi-beam diffraction grating of the grating array; and An array of light valves is configured to modulate the main beam and the reflected and redirected secondary beam, the modulated beam representing pixels corresponding to different views of the 3D electronic display. Wherein, the size of the reflection island in the reflection island array is equal to the size of the multi-beam diffraction grating in the grating array, and the distance between the multi-beam diffraction grating and the reflection island is equal to half of the grating spacing multiplied by the tangent of the non-zero propagation angle of the guided light, and the grating spacing is the lateral spacing between the multi-beam diffraction grating and the adjacent multi-beam diffraction grating at the light guide surface. 11. The 3-D electronic display of claim 10, wherein the multi-beam diffraction grating of the grating array comprises a chirped diffraction grating having curved diffraction features. 12. The 3-D electronic display of claim 10, wherein the plate light guide is configured to guide light as a collimated beam at a non-zero propagation angle. 13. The 3-D electronic display of claim 10, wherein the reflective island comprises a metal island. 14. The 3-D electronic display of claim 10, wherein the reflectivity of the reflective islands is modulated as a function of distance along the island array. 15. The 3-D electronic display of claim 14, wherein the reflective islands in the island array comprise reflective strips separated by gaps, the reflectivity of the reflective islands being modulated by the width of the gaps relative to the width of the reflective strips. 16. The 3-D electronic display of claim 10, wherein the light valve array comprises a plurality of liquid crystal light valves. 17. A method for operating an electronic display, the method comprising: Guide the light beam into the optical guide at a non-zero propagation angle; A portion of the guided light is diffracted using a multi-beam diffraction grating to generate multiple principal beams that are far from the surface of the light guide in different principal angular directions, thereby forming an optical field; and Using spaced-apart reflective islands located between the front and rear surfaces of the light guide, secondary beams are diffractically redirected away from the light guide in the direction of the plurality of primary beams. These secondary beams are generated diffractically and directed toward the reflective islands through the multi-beam diffraction grating. Wherein, the size of the reflection island is equal to the size of the multi-beam diffraction grating, and the distance between the multi-beam diffraction grating and the reflection island is equal to half the grating spacing multiplied by the tangent of the non-zero propagation angle of the guided light, and the grating spacing is the lateral spacing between the multi-beam diffraction grating and the adjacent multi-beam diffraction grating at the light guide surface. 18. The method of operating an electronic display according to claim 17, further comprising modulating the main beam and the reflected and redirected secondary beam using a plurality of light valves, wherein the modulated main beam and the secondary beam form pixels corresponding to different views of the three-dimensional 3-D electronic display.