Patent Description:
To overcome various potential applicability limitations of passive displays associated with light emission, 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.

<CIT> describes a lightguide which includes an ultra thin lightguide layer that can be used for backlighting and frontlighting displays. <CIT> describes an indirect lighting arrangement comprising at least one cylindrical waveguide. At least one end of the cylindrical waveguide is designed to couple light from a first light source into the cylindrical waveguide, and the lateral surface of the cylindrical wave guide has a coupling section with a holographic optical element which is designed to emit light from the cylindrical waveguide into a plate-shaped waveguide.

The invention is defined by the appended independent claims with certain embodiments defined in the appended dependent claims.

Embodiments in accordance with the invention a bar collimator comprising the features recited in claim <NUM> or a method comprising the subject-matter recited in claim <NUM>. In particular, a bar collimator is provided that includes a light guide configured to receive light at an end of the light guide and to guide the received light along a length of the light guide as guided light. The bar collimator further includes a diffraction grating disposed on a side of the light guide. According to various embodiments, the diffraction grating is configured to diffractively couple out a portion of the guided light and to direct the coupled-out portion toward an input of a backlight as a substantially collimated beam of light. The collimated beam of light or 'collimated light' has an extent corresponding to a length of the backlight input. The collimated beam may provide an illumination source of the backlight, according to some embodiments.

According to various embodiments, light from a light source (e.g., a plurality of LEDs) may be coupled into a bar collimator for collimation. According to some embodiments, the collimated light from the bar collimator may be coupled into a light guide of a backlight used in an electronic display. For example, the backlight may be a grating-based backlight including, but not limited to, a multibeam diffraction grating-based backlight. In some embodiments, the electronic display may be a three-dimensional (3D) or multiview electronic display used to display 3D information, e.g., as a 3D or multiview image. For example, the electronic display may be an autostereoscopic or `glasses free' 3D electronic display.

In particular, a 3D electronic display may employ a grating-based backlight to provide illumination of a 3D or multiview image being displayed by the 3D electronic display. For example, the grating-based backlight may comprise a plurality of diffraction gratings configured to provide coupled-out light beams corresponding to pixels of the 3D electronic display (or equivalently of the 3D image). In various embodiments, the coupled-out light beams may have different principal angular directions (also referred to as 'the differently directed light beams') from one another. According to some embodiments, these differently directed light beams produced by the diffraction-grating based backlight may be modulated and serve as 3D pixels corresponding to 3D views of the 3D electronic display used to display the 3D information. In these embodiments, the light collimation provided by the bar collimator may be used to produce collimated light that is substantially uniform (i.e., without striping) within the diffraction grating-based backlight.

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 bar guide and a strip guide.

Further herein, the term 'bar' when applied to a light guide as in a `bar collimator' is defined as a three-dimensional column, which is sometimes referred to as a 'bar' guide. In particular, a bar collimator is defined as a light guide configured to guide light along a length bounded by a pair of opposing surfaces aligned in two substantially orthogonal directions (top, bottom, and two sides). According to various embodiments, top and bottom surfaces of the bar collimator light guide are substantially parallel to one another in at least a differential sense. Similarly, two other generally opposing sides are also substantially parallel to one another in at least a differential sense, according to various embodiments. That is, within any differentially small region or length of the bar collimator, opposing surfaces (e.g., top and bottom, a pair of sides, etc.) are substantially parallel to one another. In some embodiments, a bar collimator may be a substantially rectangular column having a length along which a top and a bottom are substantially parallel to one another and two sides also substantially parallel to one another, as discussed above.

According to various embodiments described herein, a diffraction grating may be employed to scatter or couple light out of a light guide (e.g., a bar collimator) as a light beam. Herein, a 'diffraction grating' is generally defined as a plurality of features (i.e., diffractive features) arranged to provide diffraction of light incident on the diffraction grating. In some examples, the plurality of features may be arranged in a periodic or quasi-periodic manner. For example, the plurality of features (e.g., a plurality of grooves in a material surface) of the diffraction grating may be arranged in a one-dimensional (<NUM>-D) array. In other examples, the diffraction grating may be a two-dimensional (<NUM>-D) array of features. The diffraction grating may be a <NUM>-D array of bumps on or holes in a material surface, for example.

As such, and by definition herein, the 'diffraction grating' is a structure that provides diffraction of light incident on the diffraction grating. If the light is incident on the diffraction grating from a light guide, the provided diffraction or diffractive scattering may result in, and thus be referred to as, 'diffractive coupling' in that the diffraction grating may couple light out of the light guide by diffraction. The diffraction grating also redirects or changes an angle of the light by diffraction (i.e., at a diffractive angle). In particular, as a result of diffraction, light leaving the diffraction grating (i.e., diffracted light) 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 light guide.

Further, by definition herein, the features of a diffraction grating are referred to as 'diffractive features' and may be one or more of at, in and on a surface (i.e., wherein a 'surface' refers to a boundary between two materials). The surface may be a surface of a bar collimator, 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, and these structures may be one or more of at, in and on the surface. For example, the diffraction grating may include a plurality of parallel grooves in a material surface. In another example, the diffraction grating may include a plurality of parallel ridges rising out of the material surface. If the diffraction grating comprises parallel grooves, parallel ridges, etc. at a side surface, the diffraction grating comprises 'vertical' diffractive features that are parallel to one another (i.e., parallel vertical diffractive features), by definition herein. The diffractive features (whether 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).

Herein, a `light source' is defined as a source of light (e.g., an apparatus or device that emits light). For example, the light source may be a light emitting diode (LED) that emits light when activated. A light source herein may be substantially any source of light or 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 a light source may have a color or may include a particular wavelength of light. As such, a 'plurality of light sources of different colors' is explicitly defined herein as a set or group of light sources in which at least one of the light sources produces light having a color, or equivalently a wavelength, that differs from a color or wavelength of light produced by at least one other light source of the light source plurality. Moreover, the `plurality of light sources of different colors' may include more than one light source of the same or substantially similar color as long as at least two light sources of the plurality of light sources are different color light sources (i.e., produce a color of light that is different between the at least two light sources). Hence, by definition herein, a plurality of light sources of different colors may include a first light source that produces a first color of light and a second light source that produces a second color of light, where the second color differs from the first color.

Further, as used herein, the article 'a' is intended to have its ordinary meaning in the patent arts, namely 'one or more'. For example, `a grating' means one or more gratings and as such, 'the grating' means 'the grating(s)' herein. Also, any reference herein to 'top', 'bottom', 'upper', 'lower', 'up', 'down', 'front', back', 'first', 'second', 'left' or 'right' is not intended to be a limitation herein. Herein, the term 'about' when applied to a value generally means within the tolerance range of the equipment used to produce the value, or may mean plus or minus <NUM>%, or plus or minus <NUM>%, or plus or minus <NUM>%, unless otherwise expressly specified. Further, the terms 'substantially' and `about,' as used herein, mean a majority, or almost all, or all, or an amount within a range of about <NUM>% to about <NUM>%. Moreover, examples herein are intended to be illustrative only and are presented for discussion purposes and not by way of limitation.

<FIG> illustrates a side view schematic diagram of a backlight system <NUM> according to an embodiment consistent with the principles described herein. In <FIG>, backlight system <NUM> may include a bar collimator <NUM> disposed proximal a backlight <NUM>. The bar collimator <NUM> comprises a light guide <NUM> and a diffraction grating <NUM> disposed on a side of the light guide <NUM>. Further, diffraction grating <NUM> extends in the y-direction in a three-dimensional frame, e.g., as illustrated. In some embodiments (e.g., as illustrated), the diffraction grating <NUM> is on a side of the light guide <NUM> adjacent an input side <NUM> of the backlight <NUM> (i.e., a 'backlight-adjacent' side). In other embodiments, the diffraction grating <NUM> may be disposed on a side of the bar collimator <NUM> or equivalently on a side of the light guide <NUM> that is opposite the side adjacent to input side <NUM> of the backlight <NUM>.

In certain embodiments, the light guide <NUM> of the bar collimator <NUM> is configured to receive light at an end (<FIG> at <NUM>, <NUM>) of the light guide <NUM>. The end may be substantially orthogonal to the side upon which the diffraction grating <NUM> is disposed, as illustrated. The light beam may be received from a light source <NUM> or a plurality of light sources, e.g., light sources <NUM>, <NUM>. The light guide <NUM> is further configured to guide the received light along a length (from end-to-end) of the light guide <NUM> as guided light <NUM>. The diffraction grating <NUM> of the bar collimator <NUM> is configured to diffractively couple out a portion of the guided light <NUM> and to direct the coupled-out portion of guided light <NUM> toward the input side <NUM> of the backlight <NUM> as a beam of collimated light <NUM>. The collimated light <NUM> provides an illumination source for the backlight <NUM> and further has an extent corresponding to a length of the input side <NUM>, according to the claimed invention. In some embodiments, the backlight <NUM> also comprises a diffraction grating <NUM> to provide projected light <NUM> from the backlight <NUM> to illuminate a display, such as a 3D display or the like. For example, diffraction grating <NUM> may extend along an x-direction of the three-dimensional frame.

According to various embodiments, the light guide <NUM> is configured to guide the guided light <NUM> using total internal reflection. For example, the light guide <NUM> may include a dielectric material configured as an optical waveguide, the dielectric material having a refractive index that is greater than a refractive index of a medium surrounding the optical waveguide. A difference between refractive indices of the dielectric material and the surrounding medium facilitates total internal reflection of the guided light <NUM> within the bar collimator <NUM> according to one or more guided modes thereof. A non-zero propagation angle of the guided light <NUM> within the light guide <NUM> may correspond to an angle that is less than a critical angle for total internal reflection, according to various examples.

In some examples, the light guide <NUM> may be a bar-shaped, column optical waveguide. The bar-shaped, column optical waveguide is a rectangular, bar-shaped column as illustrated in <FIG> and <FIG>. The substantially rectangular bar-shaped column of dielectric material is configured to guide the guided light <NUM> using total internal reflection. The optically transparent material of the light guide <NUM> 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 <NUM> may further include a cladding layer on at least a portion of a surface (e.g., the top surface and/or the bottom surface) of the light guide <NUM> (not illustrated). The cladding layer may be used to further facilitate total internal reflection, according to some examples.

Once introduced into the light guide <NUM>, the guided light <NUM> propagates along the light guide <NUM> in a direction that is generally away from an input end(s) <NUM>, <NUM> of light guide <NUM>. In <FIG>, propagation of the guided light <NUM> is illustrated as an arrow pointing along the y-direction and representing a propagating optical beam within the light guide <NUM>. The propagating optical beam may represent one or more of the optical modes of the light guide <NUM>, for example. The propagating optical beam of the guided light <NUM> generally propagates by 'bouncing' or reflecting off of the walls (top, bottom and sides) of the light guide <NUM> at an interface between the material (e.g., dielectric) of the light guide <NUM> and the surrounding medium due to total internal reflection, according to various examples. Bouncing or reflecting of the guided light <NUM> is not explicitly illustrated for simplicity of illustration.

<FIG> illustrates a top view schematic diagram of the backlight system <NUM> of <FIG> in an example, according to an embodiment consistent with the principles described herein. In <FIG>, the backlight system <NUM> may also include a 'first' light source <NUM>. In some embodiments the backlight system <NUM> further includes another or 'second' light source <NUM>. The second light source <NUM> may be included to provide additional light and thus increase an intensity of light provided to and totally internally reflected (i.e., guided light intensity) within the bar collimator <NUM> as the guided light <NUM>. In some embodiments, one or both of these light sources <NUM>, <NUM> may comprise a light emitting diode (LED) such as, but not limited to, a white LED, disposed adjacent and proximal to the bar collimator <NUM> including the light guide <NUM>. For example, the first light source <NUM> may be disposed adjacent a first end at <NUM> of the bar collimator <NUM>, e.g., as shown in <FIG>. Further, the second light source <NUM>, when present, may be disposed adjacent a second end at <NUM> of the bar collimator <NUM>, e.g., as is also shown in <FIG>. Such a configuration may allow the bar collimator <NUM> and light guide <NUM> to totally internally reflect the emitted light from one or the other or both light sources <NUM>, <NUM> within the light guide <NUM>. The guided light <NUM> within the light guide <NUM> may be diffractively coupled out as collimated light <NUM> via diffraction grating <NUM> of the bar collimator <NUM> and into the backlight <NUM> at input side <NUM>, as shown in <FIG>. In certain embodiments, the input side <NUM> extends the length of bar collimator <NUM> and diffraction grating <NUM> (e.g., in the y-direction, as illustrated).

<FIG> illustrates a top view schematic diagram of a bar collimator <NUM> in an embodiment consistent with the principles described herein. In <FIG>, bar collimator <NUM> comprises a light guide <NUM> and a diffraction grating <NUM> disposed along a length of the bar collimator <NUM> (e.g., in the y-direction, as illustrated). The diffraction grating <NUM> is configured to diffractively couple out light as the collimated light <NUM> of the bar collimator <NUM> in the x-direction towards the backlight <NUM>, as discussed above.

According to some examples, the diffraction grating <NUM> may include a chirped diffraction grating. By definition, the 'chirped' diffraction grating is a diffraction grating exhibiting or having a diffraction grating pitch or spacing of the diffractive features that varies across an extent or length of the chirped diffraction grating. Herein, the varying diffraction spacing is referred to as a 'chirp'. As a result of the chirp, the guided light <NUM> that is diffractively coupled out of the light guide <NUM> exits or is emitted from the chirped diffraction grating as the collimated light <NUM> beam at different diffraction angles corresponding to different points of origin across the chirped diffraction grating.

In some examples, the chirped diffraction grating may have or exhibit a chirp of the diffractive spacing that varies linearly with distance. As such, the chirped diffraction grating may be referred to as a 'linearly chirped' diffraction grating. In another example, the chirped diffraction grating may exhibit a non-linear chirp of the diffractive spacing. Various non-linear chirps that may be used to realize the chirped diffraction grating include, but are not limited to, an exponential chirp, a logarithmic chirp or a chirp that varies in another, substantially non-uniform or random but still substantially 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.

In some embodiments (e.g. as illustrated in <FIG>), the diffraction grating <NUM> is disposed on a side of the light guide <NUM> adjacent to the backlight <NUM>. In these embodiments, the diffraction grating <NUM> may comprise a transmission mode diffraction grating. The diffraction grating <NUM> comprising the transmission mode diffraction grating is configured, by definition herein, to diffractively couple out a portion of the guided light <NUM> directly through the side of the light guide <NUM> adjacent to the backlight <NUM> (i.e., backlight-adjacent side).

<FIG> illustrates a cross sectional view of a portion of a bar collimator <NUM> in an example, according to an embodiment consistent with the principles described herein. <FIG> illustrates a cross sectional view of a portion of a bar collimator <NUM> in an example, according to another embodiment consistent with the principles described herein. In particular, both <FIG> illustrate a portion of a bar collimator <NUM> that includes a transmission mode diffraction grating <NUM>' on a light guide surface (e.g., as illustrated in <FIG>). As illustrated, the transmission mode diffraction grating <NUM>' is located on the backlight-adjacent surface of the light guide <NUM>. Note, the backlight (e.g., backlight <NUM> of <FIG> and <FIG>) is omitted in <FIG>, but would be located below the bar collimator <NUM> where it to be illustrated.

In particular, as illustrated in <FIG>, the transmission mode diffraction grating <NUM>' includes grooves (i.e., diffractive features) formed in a surface <NUM> of a side of the light guide <NUM>. The side surface <NUM> is the backlight-adjacent side of the light guide <NUM>, as illustrated in <FIG>. For example, the light guide <NUM> may include a glass or a plastic/polymer sheet with grooves formed in the backlight-adjacent side thereof.

<FIG> illustrates a transmission mode diffraction grating <NUM>' that includes ridges (i.e., diffractive features) of a grating material <NUM> on the backlight-adjacent side surface <NUM> of the light guide <NUM>. Etching or molding a deposited layer of the grating material <NUM>, for example, may produce the ridges. In some examples, the grating material <NUM> that makes up the ridges illustrated in <FIG> may include a material that is substantially similar to a material of the light guide <NUM>. In other examples, the grating material <NUM> may differ from the material of the light guide <NUM>. For example, the light guide <NUM> may include a glass or a plastic/polymer material and the grating material <NUM> may comprise a material such as, but not limited to, silicon nitride. In <FIG>, the grating material <NUM> is also optically transparent, according to some embodiments.

In other embodiments, the diffraction grating <NUM> may be disposed on a side of the light guide <NUM> opposite to the backlight-adjacent side surface <NUM> (or equivalently the backlight-adjacent side). In these embodiments, the diffraction grating <NUM> may configured as a reflection mode diffraction grating and thus be referred to as a 'reflective' diffraction grating <NUM>". As a reflection mode diffraction grating, the reflective diffraction grating <NUM>" is configured to diffractively redirect a portion of the guided light <NUM> and reflect the diffractively redirected portion through the light guide <NUM> and out of the backlight-adjacent side surface <NUM> toward the backlight <NUM>. As such, the guided light portion is diffractively coupled out by both diffractive redirection and reflection using the reflective diffraction grating <NUM>".

<FIG> illustrates a cross sectional view of a portion of a bar collimator <NUM> in an example, according to another embodiment consistent with the principles described herein. <FIG> illustrates a cross sectional view of a portion of a bar collimator <NUM> in an example, according to yet another embodiment consistent with the principles described herein. In particular, both <FIG> illustrate a portion of the bar collimator <NUM> that includes a reflective diffraction grating <NUM>" configured as a reflection mode diffraction grating. As illustrated, the reflective diffraction grating <NUM>" is at or on a surface <NUM> of the light guide <NUM> opposite the backlight-adjacent side surface <NUM>. Note, the backlight (e.g., the backlight <NUM> of <FIG> and <FIG>) is omitted in <FIG>, but would be located below the illustrated bar collimator <NUM> where it to be illustrated.

In <FIG>, the reflective diffraction grating <NUM>" includes grooves (diffractive features) formed in the surface <NUM> of the light guide <NUM> to reflectively diffract and redirect a portion of the guided light <NUM> back through the light guide <NUM> and out of the backlight-adjacent side surface <NUM>. As illustrated, the grooves are filled with and further backed by a reflective material layer <NUM> comprising a metal or similar reflective material to provide additional reflection and improve a diffractive efficiency, for example. In other words, the reflective diffraction grating <NUM>" includes the reflective material layer <NUM>, as illustrated. In other examples (not illustrated), the grooves may be filled with a grating material (e.g., silicon nitride) and then backed or substantially covered by the reflective material layer <NUM>.

<FIG> illustrates a reflective diffraction grating <NUM>" that includes ridges (diffractive features) formed of a grating material <NUM> on the surface <NUM> of the light guide <NUM> to create the reflection mode diffraction grating. The ridges may be etched from a layer of the grating material <NUM> (e.g., silicon nitride, for example. In some examples (e.g., as illustrated), the reflective diffraction grating <NUM>" is backed by the reflective material layer <NUM> to substantially cover the ridges of the reflective diffraction grating <NUM>" to provide increased reflection and improve the diffractive efficiency, for example.

<FIG> illustrates a schematic view of a diffraction grating <NUM> in an example, according to an embodiment consistent with the principles described herein. In <FIG>, diffraction grating <NUM> disposed on a surface of the light guide <NUM>. The structure of the diffraction grating <NUM> may include grating properties along a y-direction of the bar collimator <NUM> such as a groove width <NUM> of a groove <NUM> disposed between a pair of ridges <NUM> of the diffraction grating <NUM> along with a ridge width <NUM> of a ridge <NUM> of the pair. Additional grating properties include, but are not limited to, a grating depth <NUM>, a grating period <NUM>, and grating duty cycle. The `grating duty cycle' may be defined as the ratio of ridge width <NUM> of the ridges <NUM> to groove width <NUM> of the grooves <NUM>. In some embodiments, these elements may be varied to provide a non-uniform pitch and to vary diffraction angles.

In some embodiments (not illustrated), grating depth <NUM> (e.g., groove depth) may change or be varied along the y-direction to vary diffractive strength. Thus, in certain embodiments, not only may the diffraction grating <NUM> have a chirp or other feature spacing variation along the length of the diffraction grating <NUM> to optimize or control a shape of the out-coupled or beam of collimated light <NUM>, but also one or both of the diffraction grating duty cycle and grating depth <NUM> may be varied along the y-direction to further control or adjust characteristics of the light beam that is coupled out. In particular, changing 'diffractive strength' (i.e., how strong the coupled-out portion is at any point along the grating) using grating depth <NUM> may be used to adjust for a decrease in an intensity of the guided light <NUM> propagating within the light guide <NUM> of the bar collimator <NUM> as a function of propagation distance.

In certain embodiments, (see <FIG>) diffraction grating <NUM> may extract light from the bar collimator <NUM> without changing light distribution. Instead only the light propagation direction may be changed, e.g., from the y-direction to the x-direction as illustrate in <FIG> (i.e., at <NUM> and <NUM>, respectively. For example, the internally reflected or guided light <NUM> in bar collimator <NUM> may include substantially a <NUM>° cone while propagating within the light guide <NUM> of the bar collimator <NUM>. When the guided light portion is diffractively coupled out via the diffraction grating <NUM>, only one side or about half of the <NUM>° cone (i.e., about ±<NUM>°) will interact with the diffraction grating structural side of the bar collimator <NUM>. The light that is diffractively coupled out changes direction but may maintain the light distribution of the substantially <NUM>° cone of light, for example. Thus, the light coupled out from the bar collimator <NUM> will be about half of the substantially <NUM>° cone in this example or about <NUM>°.

<FIG> illustrates a flowchart of a method <NUM> of collimating light to provide backlight illumination in an example, according to an embodiment consistent with the principles described herein. In <FIG>, method <NUM> of collimating light comprises: <NUM> receiving the light from the light source(s) into a light guide; <NUM> guiding the received light in a direction along a length of the light guide; <NUM> diffractively coupling out a portion of the guided light as collimated light using a diffraction grating. The method <NUM> further comprises <NUM> receiving the collimated light from the diffraction grating into a backlight.

In some embodiments (not illustrated), a lens may be included in the backlight system, e.g., the backlight system <NUM> illustrated in <FIG> and <FIG>. The lens may be disposed between the light source(s) <NUM>, <NUM> and the light guide. The lens may be configured to assist in focusing the emitted light from the light source(s) <NUM>, <NUM>, for example. In other embodiments (not illustrated), a lens may be disposed along the length of the light guide <NUM> between the backlight <NUM> and the light guide. This lens may be configured to assist in focusing the emitted light (i.e., collimated light <NUM>) from the light guide <NUM>, for example.

Claim 1:
A backlight system, comprising:
a bar collimator (<NUM>), the bar collimator comprising:
a light guide (<NUM>) configured to receive light at an end of the light guide and to guide the received light along a length of the light guide as guided light (<NUM>); and
a diffraction grating (<NUM>) disposed along a side of the light guide;
a light source (<NUM>) configured to provide light to the light guide of the bar collimator, the light source being adjacent to an end of the light guide of the bar collimator; and
a backlight (<NUM>) adjacent to the light guide of the bar collimator,
wherein the diffraction grating is configured to diffractively couple out a portion of the guided light and to direct the coupled-out portion toward an input of the backlight (<NUM>) as a collimated beam of light (<NUM>),
wherein the collimated beam of light has an extent corresponding to a length of the backlight input the backlight being configured to receive the collimated light from the diffraction grating of the bar collimator,
characterized in that:
a) the diffraction grating comprises a reflection mode diffraction grating disposed on a side of the light guide opposite to a side adjacent to the backlight;
or in that
b) the diffraction grating comprises a chirped diffraction grating,
or in that
c) the diffraction grating comprises one or both of a duty cycle and a grating depth configured to vary along a length of the diffraction grating.