WEDGE LIGHT EXTRACTOR WITH RISERS

Abstract: A wedge-shaped light extractor (20) having a light-redirecting surface comprising a plurality of projections (22a, 22b, 22c). The projections (22a, 22b, 22c) include risers (23) that improve extraction efficiency of the facets (25), thereby facilitating efficient use of available light from a light source.

Description: WEDGE LIGHT EXTRACTOR WITH RISERS Technical Field This invention relates to electronic displays, and to point to line, line to area or point to area converters for illuminating displays. Background Art A variety of devices have been proposed for illuminating electronic displays. These devices include backlighting panels, front lighting panels, concentrators, reflectors, structured-surface films and other optical devices for redirecting, collimating, distributing or otherwise manipulating light. Efficient use of the light is particularly important in battery powered electronic displays such as those used in cell phones, personal digital assistants and laptop computers. U.S. Patent No. 4,751,615 (Abrams) shows a tapering wedge transparent page lighting device. U.S. Patent No. 4,811,507 (Blanchet) shows a front light illuminating panel having light-emitting striations whose depth increases with distance from the light source. U.S. Patent Nos. 5,005,108 (Pristash et al.); 5,050,946 (Hathaway et al.) and 5,594,830 (Winston et al.) show structured-surface panel illuminators whose panel depth varies along the length of the panel. U.S. Patent Nos. 4,799,137 (Aho); 4,874,228 (Aho et al.); 5,054,885 (Melby); and 5,190,370 (Miller et al.) show various light fixtures having inclined or curved prismatic structured surface films. U.S. Patent Nos. 5,359,691 and 5,608,837 (both to Tai et al.); U.S. Patent No. 5,485,354 (Ciupke et al.); and U.S. Patent Nos. 5,608,550 and 5,894,539 (both to Epstein et al.) show structured-surface backlighting or front lighting devices having triangular microprisms or microgrooves separated by flat land areas or lands. European Patent Application EP 0 802 446 Al (Seiko Epson) shows a structured-surface backlighting device having rectangular projections separated by lands. U.S. Patent No. 5,671,994 (Tai et al) shows a structured-surface front lighting device having trapezoidal projections separated by lands. U.S. Patent No. 5,396,350 (Beeson et al.); U.S. Patent Nos. 5,428,468 and 5,555,109 (both to Zimmerman et al.) and U.S. Patent No. 5,555,329 (Kuper et al.) show various structured-surface backlighting devices having an array of microprisms with tilted sidewalls. U.S. Patent Nos. 5,506,929, 5,668,913 and 5,835,661 (all to Tai et al.) and U.S. Patent No. 5,613,751 (Parker et al.) show light-expanding systems for converting a point light source into a linear or planar light beam. Summary of Invention Although a number of backlight and front light guide devices have been proposed, there is an ongoing need for more efficient designs and for reduced power consumption. Many current designs do not use all of the light supplied by the light source. If such unused light could be channeled to the display, then power consumption could be further reduced and display brightness could be increased. Several of the above-mentioned designs have a more or less constant thickness and a generally flat structured surface. These designs may fail to extract light that travels to the end of the device without striking (and thereby being reflected from or extracted by) the structured surface. A wedge-shaped design can reduce the amount of (and thus the loss of) light that travels the length of the device without being reflected or extracted. However, some current wedge designs are difficult to manufacture. For example, if the device tapers to a thin edge at its distal end, the edge can be difficult to mold and may fracture or craze in use. If instead a mirrored surface is employed at the distal end, some light will be lost due to inefficient reflection at the mirror, and other light will be lost due to reduced efficiency of extraction for reflected light returning towards the input end of the device. Addition of a mirror can also increase manufacturing costs or lower manufacturing yields. The efficiency and evenness of light extraction by wedge-shaped backlight or front light guide devices generally is governed in part by the gross overall dimensions and shape of the wedge, and by the fine-featured (usually microscopic) dimensions and shapes of any structured-surface features on the device. For example, the design shown in the above- mentioned Hathaway et al. patent has an inclined structured-surface face in the form of a series of descending steps. Each step has a flat land area and an inclined facet. Light typically is extracted from such a device at the facets. Light typically is redistributed within such a device by total internal reflection or "TIR" of light striking the lands or by TIR of light striking an opposing surface of the device. The reflected light travels toward the narrow or distal end of the device and may eventually emerge from the device by extraction at a facet. However, some of the reflected light will be lost at the distal end of the device and thus is not available for display illumination. Several of the other constant thickness or wedge-shaped backlight or light guide devices mentioned above can also be regarded as having facets and lands, although other terms may have been used to describe the parts of these devices. In general, the overall shape of the backlight or light guide and the geometry of the structured surface will affect the ratio of extracted light to reflected light for these devices as well. We have found that by using a wedge design having a structured surface that includes risers which serve to increase the portion of the facet area available for light extraction, we can obtain improvements in uniformity of illumination, control over light extraction, efficiency, or in more than one or even all of these factors. The present invention provides, in one aspect, a light extraction device having: a) a light input end; b) a generally planar light output surface that can deliver light to a display; and c) a structured light-redirecting surface located generally opposite the light output surface; wherein the light output surface and light-redirecting surface define a generally wedge- shaped profile that decreases in thickness from the light input end towards the center of the device; the light-redirecting surface reflects light towards the light output surface and thence to the display; and the light-redirecting surface comprises a plurality of projections comprising a riser segment, plateau segment, and facet segment, with land segments flanking the projections. Without risers, the amount of extracted light will be determined by the orientation and size of the facets and lands. When risers are added to the design, the size of each facet (and thus the facet area available for extraction) is in effect increased. The overall wedge- shaped extractor profile permits all or a significant portion of the total facet area to participate in extraction. Thus, inclusion of a riser and use of an overall wedge shape enables alteration of the extractor design to increase light extraction to a display. This can enable improvements in the overall size and efficiency of the device and can make better use of the available light. The present invention also provides point to line converters, backlights and front light guides comprising the above-described wedge-shaped light extraction device, and displays containing such extraction devices. Brief Description of the Drawing Fig. la is a schematic view of a front light guide. Fig. lb is a schematic view of a backlight. Fig. 2 is a magnified cross-sectional side view of a light extraction device of the invention. Fig. 3 is a magnified cross-sectional side view of a light extraction device of the invention, showing descriptive angle β. Fig. 4 is a magnified cross-sectional side view of a light extraction device of the invention, showing descriptive angles p, θ, φ, and γ and descriptive heights r t , and . Fig. 5 is a perspective view of another light extraction device of the invention. Fig. 6 is a perspective view of yet another light extraction device of the invention. Fig. 7 is a perspective view of a further light extraction device of the invention. Fig. 8 is a magnified view of the device of Fig. 7, showing the paths taken by two light rays. Fig. 9 is a cross-sectional view of an additional light extraction device of the invention. Detailed Description The wedge-shaped light extraction devices of the invention provide point to line, line to area or point to area light conversion that collimates or redirects light from a light source. The extraction device (which for brevity will sometimes be referred to as the extractor) can be used to illuminate a display. If the light source is a point light source, then typically the source has a relatively small cross-sectional area compared to the cross- sectional area of the display (e.g., on the order of less than 1/100 of the area of the display). If the light source is an elongated (e.g., a line) light source, then typically the source has a rectangular cross-sectional area whose long dimension is approximately ten or more times its short dimension, and which may approximate a short or long dimension of the face of the display. Some of the extractor's components and their relationship to one another can conveniently be described by comparison to a reference plane. Usually the light output surface of the extractor is planar or nearly planar. For purposes of this invention, the reference plane will be taken to be the plane formed by (or closely approximating) the light output surface of the extractor. Referring now to Figure la, a front light guide and display generally identified as 10a is shown in schematic form. Front light guide 11a lies above display 12a. Light ray 16a enters light guide 11a through light input surface 13a. Light ray 16a strikes light- redirecting surface 14a on a light-reflecting facet segment (not shown), passes downward as reflected light ray 17a towards display 12a, and exits light guide 11a through light output surface 15a. Upon striking display 12a (and assuming that display 12a is suitably modulated to reflect light), light ray 17a is reflected upward as light ray 18a towards viewer 19a. Light ray 18a passes through surface 15a (which, although identified as a light output surface, also serves in this front light guide design as a surface through which reflected light ray 18a can reenter the light guide), passes through light guide 11a and exits through light-redirecting surface 14a at a land, facet or plateau segment (not shown). Referring to Fig. lb, a backlight and display generally identified as 10a is shown in schematic form. Front light guide lib lies below display 12b. Light ray 16b enters backlight lib through light input surface 13b. Upon striking light-redirecting surface 14b at a facet segment (not shown), light ray 16b is reflected upwards as light ray 17b. Light ray 17b exits backlight lib through light output surface 15b and (assuming that display 12a is suitably modulated to transmit light) passes through display 12b towards viewer 19b. Referring to Fig. 2, front light guide 20 of the invention has a generally wedge- shaped cross section that diminishes in thickness from left to right in the figure. Light output surface 21 is located generally opposite a light-redirecting surface having a plurality of projections 22a, 22b and 22c and intervening lands such as lands 26 and 27. Projection 22b has riser 23 adjoining plateau 24, which in turn adjoins facet 25. Lands 26 and 27 flank projection 22b. For purposes of discussion, projections such as projection 22b can be referred to as "generally trapezoidal", even though the projection has only three sides and even though a quadrilateral formed by drawing an imaginary line to complete the fourth side of the projection might not have two parallel sides. Fig. 3 shows front light guide 30 of the invention and an imaginary line 36 constructed by drawing a line through vertices 35a through 35e formed at the respective intersections of risers 33a through 33e with plateaus 34a through 34e of projections 32a through 32e. Imaginary line 36 forms an angle β (beta) with a line 37 drawn through the plane formed by generally planar light output surface 31. Referring to Fig. 4, front light guide 40 of the invention has light output surface 41 and projection 42. Again for purposes of discussion, projection 42 can be referred to as "generally trapezoidal", even though it has only three sides and even though a quadrilateral formed by drawing an imaginary line to complete the fourth side of projection 42 might not have two parallel sides. Imaginary lines 48a and 48b are drawn parallel to light output surface 41 through the respective upper and lower vertices of riser 43. Riser 43 has height rj j , and forms angle p (rho) with imaginary line 48b (and, if projected, with surface 41). Plateau 44 slopes downward toward light output surface 41, forming angle θ (theta) with imaginary line 48a (and, if projected, with surface 41). Facet 45 has height f^ measured between imaginary lines 48c and 48d which are drawn parallel to surface 41 through the respective lower and upper vertices of facet 45. Facet 45 slopes downward toward light output surface 41, forming angle φ (phi) with imaginary line 48d (and, if projected, with surface 41). Lands 46 and 47 flank projection 42. Land 47 slopes upward away from light output surface 41, forming angle γ (gamma) with imaginary line 48c (and, if projected, with surface 41). Fig. 5 shows a point to line converter 51 of the invention incorporating a wedge- shaped extractor. For purposes of illustration, surface structures such as projections 52a, 52b and 52c are shown in exaggerated scale. Preferably such projections would be much smaller as described in more detail below. Light rays emitted from point light source 50 enter light input surface 54 of beam collector 55 and travel down the length of light pipe 56. Light rays striking facets such as 55a, 55b, 55c and 55d will be extracted by TIR through light output surface 58. Light striking lands such as 57a, 57b, 57c and 57d will be reflected by TIR within light pipe 56 towards surface 58 and distal end 59 of point to line converter 56. Upon striking surface 58, these latter light rays will be further reflected by TIR from surface 58 towards other projections (not labeled) in light pipe 56. Substantially all the light rays will eventually exit light pipe 56 through generally planar light output surface 58 and will form a generally evenly distributed rectangular light beam. Fig. 6 shows a point to line converter 61 of the invention having two light sources 60a and 60b. As with the converter of Fig. 5, the projections in the converter of Fig. 6 are shown in exaggerated scale. Light rays from light sources 60a and 60b enter input surfaces 64a and 64b of beam collectors 65a and 65b and travel down light pipe 66. The thickness of light pipe 66 decreases along the length of the device, reaching a minimum at narrow portion 67. Light is extracted or reflected from projections such as 62a through 62d, eventually exiting light pipe 66 through light output surface 68 as a generally evenly distributed rectangular light beam. Fig. 7 shows a front light guide and display assembly 70 of the invention. Light guide 71 lies atop display 79, separated slightly by optional air gap 78. Light enters light guide 71 through light input surface 76 and travels along the guide to strike facets on projections such as 72a through 72d. Fig. 8 shows a magnified view of a portion of the front light guide 71 of Fig. 7. Light ray 81a strikes facet 75 and is reflected downward as light ray 81b through output surface 78 towards display 79. Upon striking display 79 (and assuming that display 79 is suitably modulated), light ray 81b will be reflected upwards as light ray 81c, reentering light guide 71 through surface 78 and then exiting light guide 71 through land 76. Light ray 83a, which approaches the structured surface of light guide 71 along a different path than light ray 81a, strikes land 77 and is reflected as light ray 83b towards surface 78 and distal end (not shown) of light guide 71. Owing to the slight upward tilt of land 77, ray 83a will strike land 77 (and reflected ray 83b will strike surface 78) at a higher angle of incidence than would be the case if land 77 were parallel to surface 78. For the same reason, reflected ray 83b will strike surface 78 at a point located closer to the distal end of light guide 71 than would be the case if land 77 were parallel to surface 78. This aids in the improved efficiency of extraction of light from light guide 71, by increasing the angle of incidence for light eventually reaching the facets. Fig. 9 shows the light path traveled by two light rays 91a and 93a traveling through a backlight 94 of the invention. Light ray 91a strikes land 96 and is reflected towards the distal end (not shown) of backlight 91 as light ray 91b. Light ray 93a strikes facet 95 and is reflected upwards towards display 99 as light ray 93b. If display 99 is suitably modulated, then light wave 93b will pass through display 99 towards a viewer (not shown). Overall, the light extractor has a wedge-shaped profile. The vertices between the risers and plateaus can if desired lie along a straight line. If the vertices lie along a straight line, and if the light output surface is designated as a reference plane, and if an imaginary straight line is drawn through the vertices between risers and plateaus, then the angle β between the reference plane and the imaginary line preferably is about 0.1 to about 6 degrees, more preferably about 0.4 to about 4 degrees. In one embodiment of the invention having riser-plateau vertices that lie along a straight line, the facet heights are constant along the length of the extractor. In another such embodiment, the facet heights diminish from the input end of the extractor towards the distal or narrow end. In a further embodiment, the facet heights diminish from the input end of the extractor towards the distal or narrow end, the plateau segment lengths are constant along the length of the extractor, and the riser heights diminish from the distal end of the extractor towards the input end by an amount sufficient so that the riser-plateau vertices lie along a straight line. The vertices between risers and plateaus can if desired lie along a curved path. In one such embodiment, the facet heights diminish from the input end of the extractor towards the distal or narrow end and the land segment lengths remain constant along the length of the extractor. In another such embodiment, the facet heights are constant and the land lengths diminish from the input end of the extractor towards the distal or narrow end. In any event, the angle β for an extractor whose riser plateau vertices lie along a curved path should be approximated by calculating the arctangent of the (difference between the input end and distal end thicknesses of the extractor, divided by the length of the extractor), as shown in the following formula: β = arc tan((input end thickness-distal end thickness)/extractor length). The individual projections need not all be identical and need not all have the same angular orientation, shape or dimensions. However, for ease of manufacturing, generally it will be preferred to form projections whose riser, plateau and facet segments have the same angular orientation and segment length from projection to projection. The land segments also can if desired be similar to one another in angular orientation and segment lengths. Preferably however the projections are spaced at a relatively coarser pitch near the light input end of the extractor, and at a relatively finer pitch towards the narrowest portion (as in the narrow central isthmus of the point to line converter of Fig. 6) or distal end (as in end opposite the light input end in the extractors of Figs. 5, 7 and 9) of the extractor. This change in spacing can conveniently be accomplished by progressively decreasing the length of the land segments from the input end to the distal end (or narrowest portion) of the extractor. Each riser segment preferably is planar although other shapes such as convex or concave shapes can be used if desired. The risers do not ordinarily reflect or transmit light rays from inside the light extractor, and thus can have a less than optically smooth surface. However, care should be taken to avoid excessive riser surface roughness. That will help avoid ghosting and other visual artifacts that may arise due to backscattering of light within the light extractor and consequent weak transmission of backscattered light through the risers. The riser heights r h preferably are about 0.001 to about 0.5 mm, more preferably about 0.002 to about 0.02 mm. The angle p between the reference plane and a riser preferably is about 90 to about 45 degrees, more preferably about 85 to about 65 degrees. Usually a portion of each plateau segment will lie in the shadow cast by light rays striking the adjoining riser and thus will not participate in light reflection or extraction. In a front light guide construction, the unshadowed portions of the plateaus can serve as upper viewing ports for extracted light. Preferably, the plateaus are planar although they can have other shapes such as convex or concave shapes if desired. Preferably however the plateaus have flat, optically smooth surfaces so as not to induce distortion in the extracted light. The plateau segment lengths preferably are about 0.001 to about 1.5 mm, more preferably about 0.004 to about 0.12 mm. The angle θ between the reference plane and a plateau preferably is about 0 to about 10 degrees, more preferably about 0 to about 2 degrees, with the plateau segments most preferably being parallel to the reference plane (or in other words, θ most preferably is zero). The facet or ramp segments direct light toward the display whereupon it can be extracted (sometimes following one or more further reflections) and suitably modulated by the display. Preferably, the facets are planar although they can have other shapes such as convex or concave shapes if desired. Preferably the facets have flat, optically smooth surfaces so as not to induce scattering of the reflected light. The facet heights ft preferably are greater than the riser heights, and more preferably are about 0.001 to about 1.5 mm, most preferably about 0.002 to about 0.12 mm. The angle φ between the reference plane and a facet preferably is about 20 to about 50 degrees, more preferably about 25 to about 45 degrees. The land segments of the light-redirecting surface reflect light by TIR toward the narrow portion or distal end of the extractor. The lands can also serve as a lower viewing port for extracted light. Preferably, the lands are planar although they can have other shapes such as convex or concave shapes if desired. Preferably the lands have flat, optically smooth surfaces so as not to induce scattering of the reflected light. As noted above, the land segments preferably decrease in length along the length of the extractor. A preferred pitch is from about 0.06 to about 12 projections per mm at the light input end to about 250 to about 1 projection per mm at the distal end or narrowest portion of the extractor. A preferred land segment length is about 0.003 to about 15 mm, more preferably about 0.003 to about 1.2 mm. The angle γ between the reference plane and a land preferably is about 0 to about 5 degrees, more preferably about 0.5 to about 2 degrees. Most preferably, the lands are tilted in the opposite direction from the wedge. When so tilted, light that comes from the input end of the extractor and strikes the lands will tend subsequently to strike the facets at a higher angle of incidence, thereby increasing extraction efficiency. The projections can extend across the full width of the light-redirecting surface (as is shown, for example, in Fig. 7) or can be in the form of shorter, less than full width segments, which can be aligned with one another in rows and columns or staggered from row to row. Rows of projections can be arranged in parallel to the light input surface or at an angle with respect to that surface. Preferably, the projections extend across the full width of the light-redirecting surface and are generally parallel to the light input surface. The light input surface and light output surface can be planar or have a convex or concave curvature. When the light source is a point source, the light input surface preferably has a convex curvature. If the incoming light is not evenly distributed along the width of the input surface, then lenslets, prisms, a roughened surface or other microstructures can be added to the light input surface to more evenly distribute the light. The light output surface preferably has an optically smooth finish, in order to minimize transmission losses, undesired scattering and distortion. The light extractor can have any desired overall size and thickness but preferably it is as thin as possible, e.g., 5 mm or less at the thickest portion of the extractor. The light extractor can be square, rectangular, oval or any other desired shape when viewed from the intended display observation point. The size and shape of the extractor usually will be dictated by the size and shape of the desired display device and by whether the extractor is intended to function as a point to line, line to area or point to area converter. The light extractors of the invention are particularly useful as point to area or line to area front light guides for subminiature or miniature display devices illuminated with light emitting diodes (LEDs) powered by small batteries. Suitable display devices include color or monochrome LCD devices for cell phones, pagers, personal digital assistants, clocks, watches, calculators, laptop computers, vehicular displays and the like. The display can also be a fixed graphic such as a poster or sign, or a variable-appearance substrate such as "Gyricon" electronic display material (under development by Xerox Corporation). In addition to LEDs, other suitable illumination sources include fluorescent or incandescent lamps, electroluminescent lights and the like. The light extractors of the invention can be fabricated from a wide variety of optically suitable materials including polycarbonate; polyacrylates such as polymethyl methacrylate; polystyrene and glass, with high refractive index plastics such as polycarbonate being preferred. The light extractors preferably are made by molding, embossing, curing or otherwise forming an injection moldable resin against a lathe-turned tool or other formed surface, made of metal or other durable material that bears a negative replica of the desired structured surface. Methods for making such formed surfaces and for molding, embossing or curing the extractor will be familiar to those skilled in the art. Individual light extractor designs can if desired be evaluated without the need for actual extractor fabrication, by using suitable ray-tracing modeling software such as "ASAP" from Breault Research Organization, Inc., "Code V" and "Light Tools" from Optical Research Associates, "Optica" from Wolfram Research, Inc. and "ZEMAX" from Focus Software, Inc.. The invention is further illustrated in the following non-limiting examples. Examples Two generally planar and three generally wedge-shaped extractors were simulated using CAD software and evaluated using ASAP ray tracing software. This approach gives generally good agreement with observed actual results without the need to mold and fabricate prototypes. Each extractor would be made from 1.58 refractive index polycarbonate plastic. All would have a 30 mm width, 40 mm length, and 1 mm thick input aperture. The distal ends of Extractors 1 and 2 would be 1 mm thick. The distal ends of Extractors 3, 4 and 5 would be 0.2 mm thick. The light sources were assumed to be rectangular, 30 mm long, 1 mm high, and with a Lambertian light distribution. The general shapes of the steps or projections in Extractors 1 through 5 are shown in Figs. 10a through lOe, respectively. The dashed lines in Figs. 10a through lOe are drawn parallel to the reference plane. Extractors 4 and 5 would be extractors of the invention. The angles and dimensions of the segments for each extractor are listed below in Table I. For Extractors 1 through 4, the lands (and plateaus if present) would be horizontal (γ is zero). For Extractor 5, the lands would tilt slightly upward at an angle γ of 0.75 degrees. For all extractors, the risers (if present) would be tilted at an angle p of 90 degrees. The facets in each extractor would be tilted at an angle φ of 30 degrees. For Extractors 1 through 3, the riser height and facet height values shown in Table I would apply to the entire extractor. For Extractors 4 and 5, Table I shows the height of the facet nearest the input end of the extractor. The facet heights for Extractors 4 and 5 would decrease from the input end towards the distal end of each extractor. For Extractor 5, Table I shows the height of the riser nearest the distal end of the extractor. The riser heights for Extractor 5 would decrease from the distal end towards the input end of the extractor. These variations in facet or riser height permit the riser-plateau vertices of Extractors 4 and 5 to lie along a straight line. The pitch between projections was varied for each extractor in order to optimize predicted light output to a display to within about ±10% over the area of the extractor's light output surface. The predicted light extraction efficiency for each extractor was then calculated and recorded. Extractors 1 and 3 through 5 were assumed to have 200 steps or projections along the length of the extractor. Extractor 2 was assumed to have 400 projections along the length of the extractor. This provided an increase in efficiency compared to Extractor 2, but uniformity of light output decreased and the large number of projections would make Extractor 2 more difficult to manufacture. TABLE I Riser Plateau Facet Total No. Starting Minimum Extractor Height, Length, Height, of Pitch, ΔPitch, Pitch, Extracto No. β θ φ mm mm mm Projections mm mm mm Efficienc 1 0° 0° 90° 0° 30° 0 0..000055 0 0..0011776688 0 0..000055 200 0.30 1.00363 0.06 27% x lO '3 0° 0° 90° 0° 30° 0 0..000055 0 0..0011776688 0 0..000055 400 0.175 4.151 0.06 42% x lO "4 3 1.45° 0° NA 0° 30° NNAA NNAA 00..000044 200 0.21832 0 0.21832 40% 4 1.45° 0° 90° 0° 30° 00..000055 00..0011776688 00..001133 200 0.4 2.077855 0.06 56% x l0 "J 1.45° 0.75° 90° 0° 30° 0.0046 0.01768 0.0138 200 0.44 2.56236 0.06 59% x lO "3 Notes: "NA" = Not Applicable As shown in Table I, the extractors of the invention (Extractor 4 and Extractor 5) had a considerably better overall efficiency than extractors without risers (Extractor 1 and Extractor 2). The extractors of the invention also had considerably better overall efficiency than a step- wedge style extractor (Extractor 3). This improved efficiency should help provide significantly increased battery life in a battery-powered display. Further increases in efficiency may be available via further adjustment of the land tilt angle γ but at a potential decrease in overall uniformity of illumination. Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not limited to the illustrative embodiments set forth above.

Claims: Claims 1. A light extraction device having: a) a light input end; b) a generally planar light output surface that can deliver light to a display; and c) a structured light-redirecting surface located generally opposite the light output surface; wherein the light output surface and light-redirecting surface define a generally wedge- shaped profile that decreases in thickness from the light input end towards the center of the device; the light-redirecting surface reflects light towards the light output surface and thence to the display; and the light-redirecting surface comprises a plurality of projections comprising a riser segment, plateau segment, and facet segment, with land segments flanking the projections. 2. A light extraction device according to claim 1, wherein the presence of the riser segments increases the extraction of light from the device by the facet segments. 3. A light extraction device according to claim 1, wherein variation in the height of the riser segments will alter the ratio of light that is reflected by the facet segments to light that is reflected by the land segments. 4. A light extraction device according to claim 1, wherein if an imaginary line is drawn from the vertex between a land segment and an adjoining riser segment on a projection to the vertex between a facet segment on that projection and an adjoining land segment, in order to form an imaginary quadrilateral, the quadrilateral is generally trapezoidal in cross-section. 5. A light extraction device according to claim 1, wherein the device decreases in thickness from the light input end along the entire length of the device. 6. A light extraction device according to claim 1, wherein if an imaginary line is drawn through the vertices between riser segments and adjoining plateau segments, and the light output surface is designated as a reference plane, then the angle β between the reference plane and the imaginary line is about 0.4 to about 4 degrees. 7. A light extraction device according to claim 6, wherein the angle p between the reference plane and a riser segment is about 85 to about 65 degrees, the angle θ between the reference plane and a plateau segment is about 0 about 2 degrees and the angle φ between the reference plane and a facet segment is about 20 to about 60 degrees. 8. A light extraction device according to claim 6, wherein the angle γ between the reference plane and a land segment is about 0.5 to about 2 degrees. 9. A light extraction device according to claim 6, wherein the land segments and the imaginary line are tilted in opposite directions with respect to the reference plane. 10. A light extraction device according to claim 6, wherein the vertices lie along a straight line. 11. A light extraction device according to claim 6, wherein the vertices lie along a curved line. 12. A light extraction device according to claim 1, wherein the lengths of the land segments diminish from the input end of the extractor towards the narrowest portion or distal end of the extractor. 13. A light extraction device according to claim 1, wherein the projections are spaced at a relatively coarser pitch near the light input end of the extractor, and at a relatively finer pitch towards the narrowest portion or distal end of the extractor. 14. A light extraction device according to claim 1, wherein the riser segment heights r h are about 0.001 to about 0.5 mm, the plateau segment lengths are about 0.002 to about 1.5 mm, the facet segment heights ft are about 0.001 to about 1.5 mm and the land segment lengths are about 0.003 to about 15 mm. 15. A light extraction device according to claim 14, wherein the facet segment heights are greater than the riser segment heights. 16. A light extraction device according to claim 1, wherein the projections extend across the full width of the light-redirecting surface and are generally parallel to the light input surface. 17. A display device comprising a point or elongated light source optically coupled to a light extraction device according to claim 1, the extraction device being optically coupled to a light valve that modulates light from the light extraction device. 18. A display device according to claim 17, wherein the light extraction device is a front light guide. 19. A display device according to claim 17, wherein the light extraction device is a backlight. 20. A display device according to claim 17, wherein the device comprises a cellular telephone, pager, personal digital assistant, clock, watch, calculator, laptop computer or transportation vehicle.

Inventors: BOYD GARY T, MILLER RICHARD A, WHITNEY LELAND R, BOYD, GARY, T., MILLER, RICHARD, A., WHITNEY, LELAND, R.
Applicants: 3M INNOVATIVE PROPERTIES CO, 3M INNOVATIVE PROPERTIES COMPANY
Application year: 2001
Main classification: F21V8
IPC codes: F21V8, F21Y101, G02B5, G02B6, G02F1