Cavity reflector light injection for flat panel displays

The present invention describes a light mixing waveguide including a transparent slab waveguide having a reflectorized edge, a pair of opposing side edges adjacent to the reflectorized edge, a light transfer edge opposite the reflectorized edge, and a plurality of cavities formed inside the slab waveguide, wherein at least one of the side edges is configured to receive light from one or more light sources so that the received light is totally-internally reflected from top and bottom surfaces of the transparent slab waveguide. Interaction of the received light, the cavities, and the reflectorized edge mixes the received light prior to the light passing through the light transfer edge and into a target optical system.

TECHNICAL FIELD

The present application relates to the field of flat panel displays, and more particularly to providing a light mixing means to enhance the visual performance of transmissive flat panel displays that utilize an edge-illuminated transparent slab waveguide to provide light to the pixel shuttering mechanisms that perform image modulation on the display surface. The range of applicability of the present disclosure is not limited to direct view systems such as flat panel displays or waveguide backlights, but can also be deployed in projection-based display technologies.

BACKGROUND INFORMATION

Various flat panel display systems have been developed over the last several decades. Among them is the Time Multiplexed Optical Shutter disclosed in Selbrede U.S. Pat. No. 5,319,491 (which is incorporated in its entirety herein) and such variations as have been subsequently filed in commonly owned U.S. Pat. Nos. 7,042,618, 7,057,790, 7,218,437, 7,486,854 and U.S. Patent Publication No. 2008/0075414. The fundamental premise of such devices is that light (usually monochromatic light) is edge-injected into a transparent rectangular slab waveguide such that total internal reflection (TIR) of the injected light obtains within the waveguide, which may be mirrored on one or more of the side surfaces to insure maximum transits for rays traveling within the waveguide. The principle of operation for any of the plurality of pixels distributed across the slab waveguide involves locally, selectively, and controllably frustrating the total internal reflection of light bound within the waveguide to emit light at that pixel location. In one pixel architecture, frustration of TIR light bound within the waveguide is achieved by propelling (i.e., moving) an optically-suitable material across a microscopic gap, such that the material is at or near contact with a surface of the slab waveguide in the active position, while in the inactive position the material is sufficiently displaced from the surface of the waveguide so that light and/or evanescent coupling across the gap is negligible. The optically-suitable material, herein referred to as an “active layer”, being propelled (i.e., moved) can be an elastically deformable thin sheet (thin layer or film) of polymeric material (e.g., elastomer) with a refractive index selected to optimize the coupling of light during the contact/near-contact events. Switching the active layer between inactive and active positions can occur at very high speeds in order to permit the generation of adequate gray scale levels for multiple primary colored light (e.g., consecutive primary colored lights red-green-blue) at video frame rates in order to avoid excessive motional and color breakup artifacts while preserving smooth video generation. The flat panel display is thus comprised of a plurality of pixels, each pixel representing a discrete subsection of the display that can be individually and selectively controlled in respect to locally propelling the active layer bearing a suitable refractive index across a microscopic gap into contact or near contact with the slab waveguide. The propulsion can be achieved by the electromechanical and/or ponderomotive deformation of the thin sheet of polymeric material, said sheet being tethered at the periphery of the individual pixel geometry by standoffs that maintain the sheet in a suitable spaced-apart relation to the slab waveguide when the pixel is in the quiescent unactuated state. Application of an appropriate electrical potential across a first conductor disposed on or within the slab waveguide and a second conductor disposed on or within the active layer, causes the high-speed motion of the active layer toward the surface of the slab waveguide; actuation is deemed completed when the active layer can move no closer to the slab waveguide (either in itself, or due to physical contact with the waveguide). To facilitate light extraction, an array of micro-optical structures (of various possible geometries, such as frustums or pyramidal sections, etc.) may be optionally disposed on the waveguide-facing side of the active layer, such that pixel actuation entails contact or near-contact of these micro-optical structures with the waveguide, thus frustrating TIR light in such a way that re-direction of extracted light to the viewer is optimized. A more detailed description of micro-optical structures is disclosed in “Optical Microstructures for Light Extraction and Control” U.S. Pat. No. 7,486,854, which is incorporated herein by reference in its entirety.

Certain other display systems use similar (but not identical) principles of operation. Some utilize a backlight system where the pixels literally shutter light, usually by transverse lateral motion of an opaque MEMS-based shuttering element at each pixel parallel to the main surface (e.g., top surface) of the waveguide configured as a true backlight system proper, contra the TIR-based waveguide of Selbrede '491 which is not a true backlight given the TIR-bound condition of light traveling inside it. For a backlight system, light within the slab waveguide should not be maintained in a TIR-compliant state lest it be perpetually bound to the interior of the waveguide. Thus, the bottom surface of the waveguide can be made a scattering surface, or it can diverge from a parallel spaced-apart relation to the top surface of the waveguide, or both, to insure that light continually departs the top surface of the slab waveguide to illuminate the pixel shutter mechanisms arrayed at or above the top surface of the slab waveguide. The appeal of using a slab waveguide for transverse MEMS shutter-based systems is due to the ability to recycle unused light by configuring the waveguide-facing portions of the shutter mechanisms to be nominally reflective. Light not passing through an open shutter may then re-enter the waveguide and can be used elsewhere within the system.

In the case of devices based on Selbrede '491, in which the light sources are arrayed on one edge of the slab waveguide while the opposite end from said edge is mirrored (with either a metallic reflector disposed thereon or by imposition of a perfect dielectric mirror to gain even better reflectance), it has been determined that the luminous uniformity of the display can only be insured when the thickness of the slab waveguide is sufficiently thick. A minimum slab waveguide thickness, t, that can be utilized for the slab waveguide is a function of the length of the waveguide l, the critical angle of the waveguide θc(which is itself a function of the waveguide's refractive index), and the individual optical efficiency of a pixel on the display surface, denoted ∈. The mean free path of a given photon ensemble from origin at the light source to 99% depletion inside the waveguide is given the Greek symbol λ, which is not to be confused with the optical wavelength of that light in this context. By detuning the effective individual pixel efficiency ∈, and using the resulting average mean free path of a photon ensemble prior to 99% depletion, λ, uniformity has been demonstrated to be readily optimized when λ=3l or greater, thereby establishing a lower bound on slab thickness by the following equation:

Applying this constraint to the slab waveguide thickness enables displays based on such waveguides to achieve in excess of 60% optical efficiency (ratio of light flux input to light flux output) while simultaneously insuring far less than 1 dB variation in luminosity across the entire display surface (typically under 0.2 dB variation).

While this constraint is of minimal consequence for many applications, it does present a step backward for applications where the industry trend has been toward thinner display subsystems year after year. Thus, for a cell phone, the thickness constraint might require the waveguide to be up to 2 millimeters thick or more to insure outstanding luminous uniformity, whereas the trend in cell phone display components is for the display to be under 1 mm in total thickness. In actual fact, a waveguide thickness of 0.7 mm is desirable, given that this is a standard thickness for LCD mother glass and TFT active matrix glass. However, so thin a waveguide, by violating the thickness t constraint outlined above, runs a serious risk of suffering from debilitating nonuniformities in brightness across the display surface. The symbol t shall hereafter be denominated the minimum slab waveguide thickness that corresponds to the minimum luminous uniformity threshold limit.

Recent co-pending filings have disclosed various apodization (compensation) means in orienting and configuring the illumination means at the edge(s) of the waveguide (e.g., a varying distribution of light sources along an edge of the waveguide) to resolve luminous nonuniformity. However, the periodicity of the pixels and/or micro-optical structures disposed on the light extraction surface of the display system (e.g., top surface of the slab waveguide), in conjunction with the point source nature of the illumination means (e.g., multiple discrete LEDs), has given rise to other optically undesirable effects, such as Moiré patterns, banding, headlighting (ability to resolve the individual light sources illuminating the display system), and other light artifacts created by using discrete light sources (e.g., LEDs) to feed light to the waveguide. These light artifacts can be sufficiently severe as to create liabilities for displays that otherwise may exhibit reasonable macro-level uniformity. It is an object of the present application to address these artifacts at the illumination source by making the light entering the waveguide sufficiently diffuse (e.g., uniform) that the periodic intensity of the original light from the individual light sources can no longer be individually resolved.

SUMMARY

The problems outlined above may at least in part be solved in some embodiments of the techniques described herein. The following presents a simplified summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

The embodiments of the present disclosure provide a light mixing guide (LMG) that may sufficiently diffuse (i.e., mix) light inserted into the LMG from individual (discrete) light sources, such that the inserted light can no longer be individually resolved, referred to herein as “mixed light”, prior to injecting the mixed light into a primary waveguide (PW) of a target display system. Various embodiments of the present invention provide means to customize the light intensity output profile (e.g., linear, non-linear, etc.) that may be subsequently injected into the PW of a target display, in accordance with the particular luminosity requirements of the target display. The LMG of the present disclosure includes a transparent slab waveguide having a reflectorized edge, a pair of opposing side edges adjacent to the reflectorized edge, a light transfer edge opposite the reflectorized edge, and a plurality of hollow cavities formed inside the slab waveguide, wherein at least one of the side edges is configured to receive light from one or more light sources so that the received light is totally-internally reflected from top and bottom surfaces of the transparent slab waveguide. Interaction of the received light with one or more of the hollow cavities and the reflectorized edge mixes the received light prior to the received light passing through the light transfer edge and into a target optical system.

The foregoing has outlined rather broadly the features and technical advantages of one or more embodiments in order that the detailed description that follows may be better understood. Additional features and advantages will be described hereinafter.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments described herein. However, it will be apparent to those skilled in the art that the techniques described may be practiced without such specific details. In other instances, detailed physical features are idealized in order not to obscure the techniques described herein in unnecessary detail. For the most part, details considering geometric considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the claimed subject matter and are within the skills of persons of ordinary skill in the relevant art.

The present disclosure also provides light mixing means to insert light having a desired light intensity (light flux) profile into edge-illuminated slab waveguides, which delivers distinct advantages in display efficiency for several different species of display technology, while avoiding the kind of undesirable optical artifacts arising from the interaction of discrete light sources feeding the slab waveguide and the periodic array of optical shutters (e.g., pixels) or micro-optical structures that are often used to extract light from such displays. A light mixing means and light insertion means is described in the present disclosure to cloak discrete light sources (e.g., primary color light sources) by mixing the light from discrete light sources prior to inserting the mixed light into an edge-illuminated slab waveguide of a display. For example, the light mixing and insertion means may be employed in displays where the pixels modulate light by way of local and selective Frustration of Total Internal Reflection (FTIR) of light traveling inside the waveguide. In FTIR-based display systems, the light mixing means can prevent the aggregation of deleterious optical artifacts arising out of the interaction of non-uniform intensity light emanating from periodic discrete light sources and other optical features (e.g., light-shuttering pixels) that exhibit sufficient periodicity as to give rise to banding effects, headlighting, Moiré patterns, and/or other undesirable optical effects.

The light mixing means of the present disclosure may be employed in display systems that utilize discrete light sources (i.e., point light sources) to provide the initial light to the display. The light mixing means may enable these display systems to deliver their inherent high power efficiency with excellent luminous uniformity across the display surface (regardless of program content) without exhibiting deleterious periodicity-based optical artifacts, thereby making such display systems more competitive and successful in the marketplace. For example, the light mixing means may be employed in certain display architectures that utilize edge-illuminated slab waveguides to provide total internal reflection (TIR) light for FTIR-based displays, FTIR-based backlights for LCD panels, and FTIR-based backlights that serve as light recycling backlight subcomponents for displays that modulate light using, for example, transverse optical shutters or equivalent pixel shuttering mechanisms, to name a few, to enhance their optical performance by enhancing luminous uniformity and minimizing optical artifacts. Furthermore, the light mixing means may have utility in other optical applications beyond that of TIR-based display systems, and can be valuable as a general light mixing means as well.

The present disclosure provides a light mixing waveguide that mixes light from discrete sources (e.g., light emitting diodes (LEDs) or similar near-point light sources) in order to create an isotropic or near-isotropic a desired light flux (e.g., an isotropic light flux) that may be inserted into another optical subsystem (e.g., the backlight of a liquid crystal display, or the TIR waveguide that lies at the core of FTIR-based displays that use the principle of frustrated total internal reflection to turn pixels on and off). For the sake of illustration, the light mixing waveguide described in the present disclosure is illustrated as being utilized in conjunction with a total internal reflection (TIR) waveguide of an FTIR-based display. However, it is to be understood that restriction of the description to this specific TIR waveguide of an

FTIR-based display is not intended to restrict the range of applicability of the light mixing waveguide of the present disclosure described herein, which can be incorporated into many other optical systems that would benefit from its light mixing properties.

An exemplary FTIR-based display technology to be enhanced, with the light mixing waveguide disclosed in the present disclosure, is the current iteration of the display technology originally disclosed in U.S. Pat. No. 5,319,491, which is incorporated herein by reference in its entirety. In this illustrative display system, previously described in the Background section, pixels emit light using the principle of frustrated total internal reflection within a display architecture that leverages the principles of field sequential color generation and pulse width modulated gray scale creation. Light is edge-injected into a planar slab waveguide through at least one light injection edge and undergoes total internal reflection (TIR) within the waveguide, trapping the light inside it due to reflective coatings on the slab waveguide's edge opposite the light injection edge(s) and TIR on its upper surface, lower surface, and remaining edges. TIR light is contained within the waveguide by virtue of the waveguide having a refractive index higher than the square root of two, namely, about 1.4142) and a cladding layer (e.g., air) surrounding the slab waveguide. The TIR waveguide may be a rectangular transparent solid usually made of either glass or a suitable polymer, into which a diffuse or “mixed light” (e.g., isotropic light flux) needs to be injected at one or more of its edges (i.e., light injection edges). Distributed across the waveguide is an array of pixels that may be individually controlled to selectively emit light towards a viewer. Each of the plurality of pixels is an electrostatically controlled MEMS structure that propels (i.e., move) a relatively high refractive index thin film layer, hereafter termed the “active layer”, by controllably deforming the active layer such that at least a portion of the active layer elastically deforms and moves across a microscopic gap (e.g., typically an air-filled gap measuring between 450 to 1000 nanometers) into contact or near-contact with the upper surface of the TIR waveguide, at which point light transits across from the waveguide to the active layer either by direct contact propagation and/or by way of evanescent coupling. The active layer may optionally include an array of micro-optical structures on the waveguide-facing surface of the active layer to enhance the extraction and re-direction of TIR light from the waveguide when any of the array of pixels is actuated to an activated state.

In conjunction with this illustrative FTIR-based display system, a light mixing waveguide (LMW) may be utilized to inject “mixed light” into the slab waveguide of the illustrative display system, in order to avoid undesirable optical artifacts otherwise associated with the interaction of discrete light sources and the regular distribution (i.e., periodicity) of the pixels. The “mixed light” refers to sufficiently diffuse light that no longer exhibits the periodic light intensity emanating from the discrete light sources. Light from discrete light sources may be mixed with the LMW of the present disclosure to provide a desired light intensity profile for insertion into a primary waveguide (PW) of an optical display. The LMW may be utilized to provide the desired light intensity profile at one or more light injection edges of a primary waveguide in order to provide excellent luminous uniformity across the display surface (regardless of program content), without exhibiting deleterious periodicity-based optical artifacts, thereby making such display systems more competitive and successful in the marketplace. The utility of the light mixing waveguide is particularly beneficial to enhance luminous uniformity and minimize undesirable optical artifacts in displays having a PW thickness less than about 2 mm (e.g., 0.5 to 1.5 mm)

FIGS. 1A and 1Bare a top view schematic and a perspective view schematic, respectively, illustrating a light mixing waveguide100adjacent a primary waveguide130of a target display, for example, the TIR waveguide of the illustrative FTIR-based display system, in accordance with one embodiment of the present disclosure. The separate light mixing waveguide (LMW)100may be disposed adjacent a light injection edge132of the primary waveguide (PW)130in a spaced-apart relationship thereby forming a gap108therebetween. As previously described, the PW130may be a transparent rectangular solid designed to function as a planar waveguide in which injected light is contained until total internal reflection (TIR) is frustrated by the pixel mechanism of choice. Typically, the PW130has a thickness (z) in a range from about 0.5 mm to 4 mm.

The LMW100comprises a transparent slab waveguide101having a reflectorized edge102, a pair of opposing side edges103adjacent to the reflectorized edge102, a light transfer edge104opposite the reflectorized edge102, and a plurality of cavities105formed inside the slab waveguide101. The waveguide101may be a narrow rectangular slab comprising optical-grade glass or polymer. At least one of the side edges103is configured to receive light from one or more light sources120(i.e., discrete light sources) so that the received light is totally-internally reflected from a top surface106and a bottom surface107of the slab waveguide101. In general, all surfaces of the LMW are orthogonal or parallel to one another and mechanically smooth (to prevent undue scattering, which would result in light that violates TIR). The LMW may further comprise a reflector112to provide a reflective property to the reflective edge102to assist in directing mixed light out of the LMW through the opposite light transfer edge104and into the light injection edge132of the PW130.FIGS. 1A and 1B, then, depict the basic elements that will be further described in greater detail, and provides a reference for the various embodiments described herein.

As illustrated, the LMW100may have a height or thickness (z′) about equal to the height or thickness (z) of the PW130. In general, the thickness (z′) of the waveguide101may be equal to or less than the thickness (z) of the PW130. Although, to enhance light efficiency and luminosity in the PW130, preferably the thickness (z′) of the waveguide101is equal to or nearly equal to the thickness (z) of the PW130. It should be noted that the thickness (z′) of the waveguide101may also be thicker than the thickness (z) of the PW130, however this thickness difference may introduce some light loss when light transfers from a thicker LMW100to a thinner PW130, which detracts from the efficiency of the display system. Suitable waveguide101thickness (z′) is preferably similar to the thickness (z) of the PW130(e.g., thickness less than about 4 mm). As previously mentioned, the LMW is particularly useful to minimize optical artifacts associated with thinner primary waveguides having thicknesses less than about 2 mm. As such a preferable range of the thickness (z′) may be less than about 2 mm, and more preferably from about 0.2 to 1.5 mm.

The LMW may be mounted to the PW such that the light transfer edge104and the light injection edge132are separated by the gap108and in alignment along both their thickness (i.e., height) and length dimensions, as illustrated inFIGS. 1A and 1B. The light transfer edge104of the LMW100may be positioned or aligned adjacent the light injection edge132of the PW130such that its length dimension (x′) extends the entire length dimension (x) of the light injection edge132, thereby mating the LMW's light transfer edge104to the PW's light injection edge132. In general, the length (x′) of the waveguide101may be equal to or less than the length (x) of the PW130. However, to enhance light efficiency and luminosity in the PW130, preferably the length (x′) of the waveguide101is equal to or nearly equal to the length (x) of the PW130. It should be noted that the length (x′) of the waveguide101may also be longer than the length (x) of the PW130, however this excess length difference may introduce some light loss when light transfers from a longer LMW100to a shorter PW130, which detracts from the efficiency of the display system. In summary, matching and aligning the height (z′) and length (x′) dimensions of the light transfer edge104to the height (z) and length (x) dimensions of the light injection edge132promotes efficient light transfer. Given these criteria, the physical slab of glass or polymer forming the waveguide101could in principle be a very thin rectangle, albeit matched in thickness to the PW130(or slightly thinner than the PW) into which the LMW100will feed its diffused, fully mixed light.

Preferably, the light transfer edge104of the LMW100is separated from the light injection edge132by gap108. The distance of the gap108separating the light transfer edge104from the light injection edge132may be in a range from about 1 micron to about 100 microns. However, to minimize light leakage (i.e., light loss) as mixed light travels from the light transfer edge104to the light injection edge132, preferably the distance of the gap108is in a range from about 1 micron to 50 microns. Ideally, to minimize light loss, the gap108may be in a range from about 1 to 10 microns, however in practice this constant gap distance between the entire lengths (x′, x) of the edges104,132may be difficult to achieve due to manufacturing challenges. The gap distance108is greater than about 1 micron in order to avoid evanescent tunneling of light from the light transfer edge104to the light injection edge132. Evanescent tunneling of light would undesirably permit a significant amount of light to short circuit the mixing features (i.e., plurality of cavities105) thus not fully mixing the light prior to insertion into the PW130. Similarly, it should be noted that the light transfer edge104of the LMW100may be in contact (i.e., gap distance108is about equal to 0) with the light injection edge132of the PW130when assembled for operation, however this configuration would also undesirably permit a significant amount of light to short circuit the mixing features (i.e., plurality of cavities105) thus not fully mixing the light prior to insertion into the PW130.

In general, light from the light sources120(e.g., LEDs) is injected into the LMW100from the side edges103, wherein a gap109between the light sources120and the side edges103can insure that only TIR-compliant light enters the LMW). The side edges103where the light sources120are mounted could conceivably be much smaller in dimension than the two long edges, namely the reflectorized edge102and the light transfer edge104, although for light efficiency's sake it is usually considered proper for the light source injection surface (i.e., the surface area of the side edge103) to be at least as large as the effective active surface of the light sources themselves to avoid lossy overshoot of light. It should be noted that the light sources120are shown as unitary sources for conceptual purposes, and their precise position along edge103is a matter of design choice. More than one light source120can be arrayed on a side edge103, and the single light source120is shown for the purpose of simplifying the description. At this point, it can be appreciated that light traveling from the light sources120(which may be, for example, light emitting diodes or LEDs) can pass through the gap109into side edge103, with the resulting light now inserted into the LMW100traveling at TIR-compliant angles with respect to the top surface106and the bottom surface107of the rectangular solid slab waveguide101.

The actual light mixing may be achieved by the plurality of cavities105formed as integrated features into the physical slab waveguide101forming the LMW100. The goal of the LMW100is to mix received light therein so as to provide a desired light flux profile to the light injection edge132of the PW130via the LMW's light transfer edge104, without perturbing that light from a strict TIR regime (constraining the angles at which light rays travel within the LMW100prior to entering the PW130). Given this criterion, the light rays inserted into the LMW100from the light sources120should substantially avoid encounters that create angular deviation from TIR compliance. The plurality of cavities105serve as the core means of achieving light mixing. In one embodiment, the plurality of cavities105may be a plurality of hollow cavities that are formed in the waveguide101. The cavities105may be hollow (e.g., air-filled, vacuum) or comprise some other material (e.g., aerogel, silicone) having a refractive index lower than the refractive index of the light mixing waveguide101. One important aspect of these hollow cavities are that the cavity walls310(illustrated inFIG. 3Adiscussed below) are perpendicular to the top surface106and bottom surface107of the LMW, and thus parallel to the reflectorized edge102, the light transfer edge104, and the two side edges103where the light sources120may be mounted. Moreover, the cavity walls310are as physically smooth as possible, to reduce or substantially prevent scattering when light encounters the cavity on its journey through the LMW100. As illustrated inFIGS. 1A and 1B, a linear row of such cavities105may be distributed near the reflectorized edge102of the LMW100and parallel to that edge102, and have an inter-cavity spacing (d) distance substantially constant between adjacent cavities, in accordance with an embodiment of the present disclosure.

In various embodiments described in more detail below, the interspacing between the cavities105can be either uniform (equidistant) or graded as a function of distance from the light sources120at the side edges103. The cavities105may be a wide range of cross-sectional shapes that can be selected to optimize light mixing. Moreover, the cavities105may be through-hole cavities that extend from the top surface106to the bottom surface107of the waveguide101. Alternatively, the cavities may be wholly embedded within the waveguide101. The particular size and cross-sectional shape of the cavities may be designed to optimize the mixing process. Moreover, the geometric distribution of these cavities105inside the LMW100may range from simple linear arrays to more elaborate distributions designed to optimize the mixing process and provide the desired light flux profile that needs to be transferred to the particular PW130.

The reflector112may be disposed adjacent the surface of the reflectorized edge102and separated therefrom by a gap113, as illustrated inFIGS. 1A and 1B. Although reflector112is shown as comprising considerable thickness, this is for illustrative purposes only, and in actual fact the reflector112may be a thin sheet of aluminum or other metallic element preferably positioned in a spaced-apart relationship to the reflectorized edge102forming gap113therebetween. The reflector112may also be a substantially perfect dielectric mirror (i.e., within a selected tolerance) comprised of a series of layers bearing different thicknesses and refractive indices to create an even more efficient reflector at edge102. Alternatively, the reflector112may be disposed directly on the surface of the refelectorized edge102(not shown). In this embodiment, the reflector112may be a very thin layer of aluminum or other metallization in contact with the reflectorized edge102. Moreover, the reflector112may be a dielectric mirror in intimate contact with the reflectorized edge102. Therefore, both variations (reflector112disposed directly on the surface of edge102, or the reflector112in spaced-apart relation to the surface of edge102) jointly comprise various embodiments of this disclosure.

The principle of operation of the LMW100and the plurality of cavities105is that light can pass from the discrete light sources (e.g., LEDs) on the side edges103(most likely through a small air gap109to insure TIR compliance) into the waveguide101of the LMW100. When a light ray encounters a cavity (e.g., a hollow cavity), it will either reflect or bifurcate (reflect and refract), depending on the refractive index of the waveguide101and the angle of incidence the light ray had at the point of intersecting the cavity's surface (i.e., cavity wall310). A pure reflection occurs at angles where TIR is conserved at the cavity boundary, whereas other rays may not be TIR-compliant in the lateral dimension (albeit all rays are intended to be TIR-compliant with respect to the top surface106and the bottom surface107of the LMW100(and the upper surface and lower surface of the PM130), referred to herein as azimuthally TIR compliant. As the rays encounter more and more cavities and interact with them and the reflectorized edge102of the LMW, a thorough mixing of the light rays that pass through the light transfer edge104into the PM130will have occurred, creating a very diffuse and uniform light flux across the light transfer edge104of the LMW, in accordance with one embodiment of the present disclosure. Light that passes through the light transfer edge104of the LMW into the PM, because it is laterally TIR-noncompliant (and azimuthally TIR-compliant), will be in a highly mixed state (i.e., diffuse light) as a result of the interaction of the light source rays with the plurality of cavities and the reflectorized edge102.

FIG. 2Aprovides a close-up view of the distal end of the light mixing waveguide100, inclusive of one of the light sources120in spaced-apart relation to the light injection side edge103. The possible trajectory of one light ray, among the countless rays that are continually injected along the side edge103at many different angles and positions, is shown for illustrative purposes so that the various interactions of the ray with the cavities105and the reflectorized edge102depicted in this embodiment may be better understood. One of the rays illustrated as a ray201emitted from the light source120passes through side edge103of the waveguide101and refracts (i.e., bends) to follow the a new ray path202, the extent of refraction determined by the refractive index of the transparent material comprising the light mixing waveguide101. The ray202then encounters the reflectorized edge102and continues on as ray203until it encounters one of the hollow cavity structures105, at which point the single ray splits into two rays204,206(i.e., a reflection-refraction ray split) of potentially unequal intensities based on the laws of optics prevailing at the boundary of the cavity105(i.e., cavity wall310). Part of the original ray203travels through the cavity105as ray204, then encounters the opposite wall of the cavity to refract once again as ray205, which finally passes through the light transfer edge104in preparation for entering the light injection edge132of PW130, assuming the angle of incidence at the edge105permits such action (i.e., lateral non-TIR angle). There may be subsequent ray-splitting events at each cavity wall (i.e., boundary) encountered by the light ray during its journey, and in the interest of clarity these are not shown. Not only did ray203partially refract as ray204and205to exit the waveguide101, but the remaining energy in ray203also reflected to form a ray206which is shown as being sent back toward the reflectorized edge102(or reflector112), interacting again with said reflectorized edge102and/or reflector112to form ray207, which in turn encounters another cavity105ato refract through it as a ray208and finally ray209, which undergoes a total internal reflection event at side edge103before becoming ray210that passes through the light transfer edge104. Additional reflection-refraction ray splits, or ray-splitting events, are not shown, but these are known to occur at each boundary introduced by the cavity walls, excepting boundaries where total internal reflection occurs (namely, where the sine values of Snell's Law takes on values greater than 1, indicating that no refraction is occurring, only reflection is occurring, at the pertinent boundary, based on the intrinsic geometry and ratio of refractive indices present at the boundary). Consequently, original ray201ends up exiting the waveguide101as rays205and210, with additional rays created by ray-splitting events that are omitted fromFIG. 2aproviding even further sets of rays generated from the original ray201. This process of splitting the rays laterally by the interaction of the light rays with the cavity structures105and the reflectorized edge102and/or reflector112gives rise to exceptionally good light mixing, creating a very uniform flux of light through the surface of the light transfer edge105.

FIG. 2Bis a plan view close-up of two different light rays211,213encountering a single cavity structure105, for ease of illustration. Light ray211is incident upon the wall of cavity105at such an angle that total internal reflection occurs at the surface boundary of the cavity wall, and the reflected ray212, of identical intensity with the original ray211, results. However, an incident light ray213encounters the surface wall of cavity105at a different incidence angle than did light ray211, such that the ray splits in a reflection-refraction event at said surface, the reflected portion of the ray forming a new departing ray214, while the refracted portion of the original ray213forming a new ray215that travels through the middle of the cavity's volume, to encounter the opposite wall of the cavity at an angle that gives rise to the a ray216departing the cavity vicinity through said wall. There are additional reflection-refraction events at these cavity wall boundaries, which are omitted in the illustration for the sake of simplifying the depiction and clarifying the basic difference between the two kinds of ray interaction that are possible at a cavity structure boundary (i.e., cavity wall surface): those two ray interactions being either the spitting of the ray into a reflected and refracted ray, or sub-ray, of potentially different intensities whose intensities, when added, substantially total the original intensity of the incident wave that was split in this manner; or the incident ray may undergo total internal reflection and not split so that the original ray's energy is directed into a new reflected ray, such as incident ray211reflecting off of the wall of cavity105to form total internally reflected ray212. Note as before that reflection-refraction events can give rise to what are known as daughter rays, and the daughter rays can generate granddaughter rays upon encountering subsequent cavity walls (i.e., boundaries) due to the presence of other cavities105subsequently encountered, as well as the material comprising the light mixing waveguide.

In applications where the reflector112is not applied directly to the reflectorized surface102but rather is in a spaced-apart relation to the surface of edge102by a distance of gap113, the surface of reflectorized edge102may exhibit both total internal reflection itself in respect to light incident upon it from inside the light mixing waveguide101, or ray splitting at reflection-refraction events at the surface of edge102, such that light exiting the waveguide at edge102during such a ray-splitting event may travel across said gap113before encountering reflector112and being diverted back toward edge102for re-entry into the waveguide101. Multiple ray-splittings are possible at edge102when the reflector112is detached from the surface of edge102and put in a spaced-apart relationship thereto. However, light (e.g., laterally non-TIR compliant light) that exits the waveguide101through edge102into the gap113encounters the reflector112and returns to the waveguide101. The presence of this optional gap113, which may be filled with air, vacuum, or some other substance (e.g., aerogel, silicone) with a lower refractive index than that of the waveguide101, may reduce optical flux losses (e.g., light absorption) that often attenuate light rays incident upon metal-blased reflectors. Furthermore, the angular selectivity introduced into the optical behavior of edge102by introduction of said gap113may further enhance the mixing of light as well. Therefore, although both variations (reflector112disposed directly on edge102, or reflector112in spaced-apart relation to edge102) jointly comprise the matter covered under this disclosure, it may be preferable to minimize any light loss due to interaction of light that strikes a metallic layer (e.g., metallic reflector112), by optionally positioning the reflector112in spaced-apart relationship to the reflectorized edge102such that only non-TIR light may strike the metallic reflector112, as compared to all light (TIR and non-TIR light) when the reflector112is disposed in contact with the surface of the reflectorized edge102.

FIG. 3Aillustrates in more detail the through-hole nature of a plurality of cavities105that may be formed in the waveguide101. Moreover, the cavities105may be through-hole cavities that extend from the top surface106to the bottom surface107of the waveguide101. The interior surface of the cavities105is a cavity wall310that is sufficiently smooth so as to not cause undue scattering out of the top or bottom surfaces106,107of the waveguide101, which is perpendicular to interior cavity surface and to the cavity symmetry axis, which are themselves substantially parallel to the surface of the reflectorized edge102and the light insertion edges103. Moreover, the cavities105are not restricted to a circular cross-section (i.e., cylindrical hollow cavities passing through the waveguide101). These idealizations are used for the sake of simplifying the description and are not intended to be limiting.

Furthermore, regardless of the choice of cross-sectional geometry of the cavities105(in this instance, they are shown as circular for the sake of description), the walls310of the cavity should be parallel to waveguide surfaces of edges103,102, and104, and perpendicular to surfaces106,107. When this is the case, light entering the waveguide101from the light source(s)120does not undergo substantial perturbation to non-TIR angles with respect to surfaces106,107, but may be perturbed to non-TIR angles with respect to surfaces103,102and104after having encountered and optically interacted with any given cavity105. Furthermore, these light rays may either undergo total internal reflection at the boundary of the hollow cavity105, based on the angle of incidence upon the cavity wall310when the ray encounters it, and based on the ratio of refractive indices between the substance comprising the waveguide101and the air or vacuum or other low refractive index material or composition of materials interior to and filling the cavity105; or the ray will undergo a reflection-refraction split split, with part of the ray traveling through the cavity105and another part of the ray reflecting off the boundary of the cavity. In either case, the continual interaction of light rays with the plurality of cavities105causes the light to become further mixed and more isotropic, through the above-identified process of TIR reflection or reflection-refraction splitting, a phenomenon also known colloquially as ray-splitting. Such light will also interact with the reflector112as well as with the plurality of cavities105, so that the light distribution approaches a sufficiently mixed and isotropic form to travel through the light transfer edge104into the PW130through the light injection edge132. There will be a suitable boundary between104and132, as noted earlier, comprised either by an explicit air gap or other such gap, or a boundary demarcated by a material difference in the index of refraction between104and132if they should be abutted one to the other. Such a boundary provides the best light mixing behavior and thus constitutes a preferred embodiment, since it forces light back to the cavities105, so that only light that has been mixed by interaction with one or more of the plurality of cavities105is able to finally pass through the boundary between104and132. However, it is possible to have no appreciable boundary difference between104and132, but such an architecture will only mix approximately half of the light being injected into the light mixing waveguide from the light sources120since about half of the inserted light will entirely bypass interaction with the cavity structures105that give rise to the desired light mixing behavior.

FIG. 3Billustrates the fact that the techniques described herein are not tied to any particular cross-sectional geometry or shape for the hollow cavities that do the TIR reflection or reflection-refraction splitting, and thus achieve the desired mixing of the light being processed by the LMW101. In this instance, the cross-sectional area of the hollow cavities is a square, and the respective square column-shaped cavities302. Other possible shapes for the cross-sectional geometry for the hollow cavities are triangular, rectangular, elliptical, hexagonal, pentagonal, octagonal, or other cross-sectional shapes. Although it is expected that fabrication and manufacturing exigencies are more likely to favor circular cross-sections that provide cylindrically-shaped hollow cavities passing through the waveguide101. The cavities105may be a wide range of cross-sectional shapes and sizes that can be selected to optimize light mixing. For example, the size of the cavities105may be in a range from about 0.1 mm to about 5 mm.

FIG. 3Cillustrates that the cavity303may be completely embedded inside the waveguide101. Other configurations of cavities embedded in the waveguide101are also possible, and these alternate configurations can serve to optimize light mixing in various ways, while the actual size of the cavity is similarly optimized.

By the same token, the illustrated embodiments are not limited to having each of the cavities105,302comprising the plurality of cavities to be filled with the same material (e.g., air, vacuum, or material of specific refractive index), and it conceivable that varying the composition of the cavity's contents from one cavity to the next can accrue to the optical benefit of the mixing process by altering the optical behavior as a function of position within the light mixing waveguide101. Furthermore, the illustrated embodiments are not limited by having an isotropic material filling any given cavity, but any given cavity could have more than one refractive index material within it, arranged as desired (either with discrete boundaries between materials or a gradient from one refractive index to another), if such internal compositional variety inside the cavity provides optimal optical light mixing performance not otherwise obtainable from an isotropic interior of the cavities301.

There are alternative embodiments that angle the side edges that are used for light injection in order to alter or optimize the angular distribution of light in the waveguide401, which can be varied depending on the output requirements. For example, the rectangular shape of waveguide101inFIG. 1Amay be trapezoidal, as illustrated inFIG. 4, or formed another polygonal structure (e.g., square, pentagon, hexagon, etc.), since the light mixing will occur in that instance as well. The narrow rectangular shaped waveguide101is selected both for illustrative purposes and because it is likely to represent the lowest possible use of available space for such a LMW100.FIG. 4illustrates that side edges403may be angled (i.e., angled sides of a trapezoid), and furthermore, that the one or more light sources420may be angled adjacent the angled side edges403in order to inject light perpendicular to angled side edge403and direct the initial light from the discrete light sources201into the waveguide401and towards one or more cavities105therein, thereby increasing the mixing efficiency of the LMW400. However, it should be noted that angled light sources420need not inject light perpendicular to angled side edges403. For example, the angled light sources420depicted inFIG. 4may substitute the light sources120depicted inFIG. 1Ain order to inject light into the side edges103of rectangular waveguide101at an angle with respect to the normal perpendicular to the surface of side edge103. Injected light can be angled to improve or alter efficiency or flux distribution as light from discrete light sources120is introduced into the LMW100.

It should be noted that the efficiency of any light mixing means is also a function of light leakage, and that there are potential locations in the LMW100where leakage might occur, such as at side edges103, so that reflective means may be imposed on such surfaces, or be associated with the light sources120themselves, albeit not in such a way as to occlude any light source injecting light into the slab waveguide101, so as to contain as much light as possible inside the waveguide101so that the mixed light's primary departure route is out through the light transfer edge104and into the PW130through edge132, as illustrated inFIG. 5. In another aspect, light sources502need not be located on both edges103but can be added on only one edge103with the opposing edge being itself reflectorized by reflective element501, as illustrated inFIG. 5. In the case of mating a single light source to one of the side edges, the opposing side edge would also be a reflective surface. It is further noted, that with only one edge illuminated, the cavities may be adjusted in density, distribution, size, and interspacing, to insure an isotropic flux under such a light injection scenario.

There are also alternative embodiments that allow for the light source to be embedded into the LMW600, as illustrated inFIG. 6. Light sources620may be embedded inside the waveguide601(e.g., near the103side edges or elsewhere in the LMW) as illustrated inFIG. 6, rather than being disposed outside the LMW in spaced-apart relationship with an air gap therebetween as previously depicted with respect toFIGS. 1A and 1B. A reflective shield surrounding at least a portion of each of the embedded light sources620may be introduced (not shown) to insure TIR-compliance of such injected light being imposed, if needed.

In another aspect,FIG. 6also illustrates that reflector112may be applied directly to the surface102, rather than in a spaced-apart relation to surface102. The reflector112in contact with surface102may impart excellent reflective properties to the surface102, however there is a certain amount of light attenuation (i.e., light loss) due to light rays incident upon metal-blased reflectors. To minimize light loss, the reflector112may be in a spaced apart relationship to surface102with this optional gap113, to reduce optical flux losses that often attenuate light rays incident upon metal-blased reflectors, as previously described with respect toFIGS. 1A and 1B.

In another embodiment, the plurality of hollow cavities in the LMW are configured to provide some level of apodization, which may serve to improve luminous uniformity within the waveguide. It is understood that the LMW100is a light preprocessor and that more than one edge of the PW130may have an associated LMW100mounted to it, as display exigencies may require. As a consequence of configuring the LMW100for apodization, light flux (i.e., intensity of light) entering the PW130may no longer be spatially isotropic, but may exhibit a gradient or other desired profile. The gradient may be desirable because light injected from the LMW100into the PW130may undergo depletion as it travels through the array and encounters activated pixels. Apodization can compensate for any such depletion effects, and where it is desirable that such apodization be applied, the LMW can be configured to achieve the desired apodization by geometrically configuring the plurality of hollow cavities in a suitable manner The specific nature of the apodization gradient (linear, exponential, tuned to the pixel efficiencies present on the display surface, etc.) is adjusted to maximize uniformity in any given display setting. These represent different optimization schemas for this embodiment, and are variations intended to tune the performance of any given display system to optimize the luminous uniformity.

Referring now toFIG. 7, apodization of the light being ejected by the LMW101or701through its light transfer edge104can be achieved by such geometric strategies as defined above, by suitable selection of the position and size of the cavities so as to create more light flux in some parts of the light transfer edge104and less light flux in other parts of the light transfer edge104, which is a feature that can be called for in display systems based on frustrated TIR which tend to deplete light across the display surface; such light depletion can be compensated for by apodization of the light sources. In this instance, the apodization is applied not to discretized point sources but rather to the highly mixed flux that the waveguide701creates from the initial light injected into the LMW from the light sources (120, or720,721, and/or722, etc., as required). Therefore, apodization can be used to provide compensation of flux intensity for light entering the PW130to improve the luminous uniformity of displays that exhibit light depletion as a function of distance from the light source and the number of actuated pixels injected light encounters prior to total depletion.

The inter-cavity spacing (d) between the cavities can be either uniform (equidistant) or graded as a function of distance from the light sources120at the side edges103.FIG. 7illustrates different possible architectural arrangements in respect to both the light sources (e.g., LEDs) and their respective distribution along the edges104of the waveguide701, as well as in respect to the interspacing and spatial distribution of the cavities705,706,707,708formed inside the waveguide701. For example, the plurality of cavities105depicted inFIGS. 1A and 1Billustrate one possible embodiment that indicates the relative positions of a plurality of cavities105that are formed inside the waveguide101. The distribution of the cavities105are shown as being collinear with an equidistant inter-cavity spacing (d), and nearer reflectorized edge102(and its associated reflector112) than they are to the light transfer edge104. Alternative embodiments may use other arrangements of the plurality of cavities. Alternative embodiments are not limited to this geometric distribution of the cavities, nor are these embodiments restricted to any given quantity or size of cavities105, so long as the ray splitting gives rise to the targeted amount of light mixing. For example, a light mixing waveguide700depicted inFIG. 7illustrates a transparent waveguide701having cavities705(e.g., hollow air-filled cavities) are spaced farther apart toward the distil termini edges103, and closer together toward the center of the waveguide701. Although shown as collinear, they need not be collinear and can be distributed in any orientation that provides good light mixing. Furthermore, it is also possible to position certain cavities707in other locations within the waveguide701, which may serve to redirect light more favorably toward the reflector112and/or other cavities708.

The interior composition of the cavities705,706,707,708need not be isotropic and uniform if an anisotropy in refractive index inside the cavities leads to optical benefits in regard to light mixing, nor are the illustrated embodiments limited to having each of the cavities having the same shape, the same size, or the same interior composition and/or refractive index. Furthermore, while a preferred embodiment calls for the walls of the cavities705,706,707,708to be perpendicular to the top surface106of the waveguide701and, consequently, parallel to103and104, for the purpose of insuring that incident light rays encountering said cavities do not perturb away from angles that comply with total internal reflection within the interior of the slab waveguide701, alternative embodiments are not limited to such a geometric restriction, which may be called for in certain instances when the perturbation of rays does not present a significantly deleterious side effect in regard to system noise level or contrast ratio degradation.

FIG. 7shows a multiplicity of light sources720,721, and722, that may be used to replace the single light source120abutted on the edge104depicted inFIGS. 1A and 1B. It is possible, in some situations, for light source720to correspond to one primary color,721to another primary color, and722another primary color; or they can all be multiple-light-source systems to begin within, and arrayed along the edge(s)103due to geometric, optical, mechanical, and/or thermal considerations that dictate the use of smaller LEDs to fill edge103. It is common in the art to try to fit the insertion face of an LED to the edge of the waveguide into which its light is to be inserted, to avoid overshoot and other lossy effects. The techniques described herein are not limited to either one or three LEDs, but to any illumination sources of any kind that are arrayed along the side edges103. The examples shown in both LMWs100and700are for illustrative purposes only and are not intended to limit the all embodiments to these specifically quantified pluralities of light sources, nor are all embodiments tied to any specific location of the light sources720,721, and/or722or120along the edge(s)103.

FIG. 8illustrates an alternative embodiment wherein the air gap between the light transfer edge104and the light injection edge132of the PW130is filled with an intercalated region800having one or more specific refractive indices. Region800represents an intercalated interstitial region between the PW130and the waveguide101that may be configured with respect to its refractive index, particularly in reference to the ratio of region800refractive index to the refractive index of the waveguide101, and the ratio of region800refractive index to the refractive index of the PW130. It is to be noted that while region800can be uniform and isotropic with respect to its refractive index, this is not a restriction on all embodiments, and to illustrate the significance of this fact, the region800has been shown divided (arbitrarily, for the sake of illustration only) into three discrete regions,801,802, and803. In this example, discrete regions801and803of element800have a lower refractive index than the middle subelement802, with the result that light passing into region800from the waveguide101through light transfer edge104will have greater flux transfer into the higher refractive index region802than it will through the lower refractive index regions801and803. This difference in transfer efficiency across the boundary from edge104to the respective discrete regions of800(discrete regions801,802, and803) is premised on the different critical angles presented to light that encounters the boundary between104and800after having traveled through the waveguide101. The potential benefit of subdividing region800into discrete subregions bearing different refractive indices is that such variable refractive index regions serve to apodize and adjust the intensity of light entering the PW130based on the position along the edge132that the light enters, which can serve to improve the luminous uniformity of the light available to the primary waveguide for use in the final application (whether that be a TIR-based waveguide, a backlight for an LCD system, or other application). It should be noted that the partitioning of the region800into three discrete subregions is purely arbitrary, and that the region800can be divided into any number of regions. Alternatively, instead of discrete regions within region800, a smooth gradient of refractive index within the region800may be used to achieve apodization of light being transferred from the waveguide101into the PW130as the light passes through the light transfer edge104into region800to be potentially apodized and then passing into the PW130through the light injection edge132. Therefore, region800can bear a uniform refractive index chosen for its critical angle, it can bear different partitioned subregions with individually-tuned refractive indices, or it can be manufactured with a targeted gradient in its refractive index composition. The thickness of the region800as it fills the space between light transfer edge104and light injection edge132is arbitrary and is expected to be small for practical reasons, although in alternative embodiments any given spaced-apart relation between104and132(and consequently, any given thickness of region800, however it may be composed) may be used.

It should be understood that the general principle described herein, the use of cavities105, to achieve ray splitting to facilitate light mixing, is not only suitable for mixing primary colors (e.g., red, green, and blue, which may correspond, for illustrative purposes, to the respective light sources720,721, and722), but it may also be suitable for general optical applications where light mixing is required, including automotive applications, avionic applications, general lighting applications, display applications, etc. Its value in removing optical artifacts associated with the interaction of discretized light sources with an array of pixels or micro-optical structures is important in one specific area of art, but this does not limit the range of applicability of the techniques described herein. The specific details provided are for illustrative purposes only and should not be read as restricting the scope of the claimed subject matter.

Embodiments of the techniques described herein can enhance the luminous uniformity of display systems comprised of edge-illuminated slab waveguides that are thinner than the uniformity threshold limit (t) without incurring undesirable optical artifacts arising out of the geometric interaction of point sources with the array of pixels or micro-optical structures associated with the pixel actuation mechanism. The various embodiments described herein can be implemented on a host of display systems that could be expected to use edge-illuminated slab waveguides and/or associated light-recycling backlight subsystems or FTIR-based display technologies and would thus would be highly desirable and lead to improved image generation by system architectures based on such planar illumination architectures. These embodiments may also facilitate the mixing of primary color lights (e.g., red, green, and blue) and can also be extended to more general optical and light applications where the smooth mixing of point sources of lights (e.g., LEDs) into a uniform flux, or other desired light flux profile, passing through the surface of the light transfer edge is desirable.

It will be seen by those skilled in the art that many embodiments taking a variety of specific forms and reflecting changes, substitutions, and alternations can be made without departing from the spirit and scope of the invention. Therefore, the described embodiments illustrate but do not restrict the scope of the claims.