WO2013012564A2

WO2013012564A2 - Coextruded optical sheet

Info

Publication number: WO2013012564A2
Application number: PCT/US2012/045506
Authority: WO
Inventors: Moris Amon; Fumitomo Hide; Alexey TITOV
Original assignee: Rambus Inc.
Priority date: 2011-07-20
Filing date: 2012-07-05
Publication date: 2013-01-24
Also published as: TW201304943A; WO2013012564A3

Abstract

A coextruded optical sheet includes layers of materials having different optical properties. The materials vary in relative amounts in a transverse direction of the optical sheet, orthogonal to a machine direction in which the optical sheet is extruded. The different optical properties are due to different concentrations of light-redirecting elements in the materials, for example, by using light-redirecting elements, such as light-redirecting particles, in one of the materials and not in the other of the materials. The optical sheet may be coextruded from a suitable extrusion die, from supplies of the materials.

Description

COEXTRUDED OPTICAL SHEET

BACKGROUND

[0001] Light guides used in edge lit lighting applications are normally formed by forming or placing features on the surface of the light guide that frustrates total internal reflection of light within the light guide, causing light to be emitted from the light guide. To control the intensity profile of the emitted light, the density or dimensions of the features are typically varied at different positions on the surface of the light guide.

[0002] An alternative technological approach to cause light to be emitted from a light guide is to use light-scattering particles dispersed in the volume of the light guide.

Applications of such technology currently exist but lack the means to control the spatial distribution of the particles, and as a result, the distribution of the particles within the volume of the light guide is generally uniform and the intensity profile of the emitted light cannot be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] The annexed drawings are not necessarily to scale.

[0004] Fig. 1 is a perspective view of an embodiment of a coextruded optical sheet.

[0005] Fig. 2 is a cross-sectional view of the optical sheet of Fig. 1 along section 2— 2 of Fig. 1.

[0006] Fig. 3 is a perspective view of an embodiment of a coextruded light guide.

[0007] Fig. 4 is a plan view of the passages part of an extrusion die for making a coextruded optical sheet.

[0008] Fig. 5A is a perspective view of an embodiment of an extrusion die for making a coextruded optical.

[0009] Fig. 5B is a cross-sectional view of part of the extrusion die of Fig. 5 A along the section line 5B-5B of Fig. 5A.

[0010] Figs. 6-9 are cross-sectional views of other embodiments of coextruded optical sheets.

DETAILED DESCRIPTION

[0011] A coextruded optical sheet comprises layers having respective optical properties, the respective optical properties varying in a transverse direction orthogonal to a machine direction in which the optical sheet is extruded. The different respective optical properties are due to different concentrations of light-redirecting elements, such as light-redirecting particles or light-redirecting voids, dispersed in the layers and variation in relative thicknesses of the respective layers in the transverse direction. An example of layers with different concentrations of light-redirecting elements is a first layer having no light- redirecting elements and a second layer having light-redirecting elements. An example of light-redirecting particles is spheroidal particles having a refractive index different from the surrounding medium. In another example, the light-redirecting particles are elongate and are aligned in the machine direction. In yet another example, the light-redirecting particles have various shapes. The optical sheet is coextruded through a suitable extrusion die.

[0012] Fig. 1 shows a coextruded optical sheet 10 that has opposed, planar major surfaces 12 and 14. The sheet 10 includes two layers (not shown in Fig. 1) that have different respective optical properties. The optical properties include light-redirecting properties.

[0013] The optical sheet 10 is produced by extruding in a machine direction 20. In a plane parallel to the major surfaces 12 and 14 of the optical sheet 10, a transverse direction 22 is orthogonal to the machine direction 20. The layers have respective thicknesses that vary in the transverse direction 22 but are constant in the machine direction 20 so that the materials of the layers vary in relative amount in the transverse direction 22.

[0014] Each layer comprises an optically transmissive material such as a polymer or glass formed into a continuous phase. These continuous-phase materials may be the same for both layers.

[0015] Fig. 2 is a cross-sectional view showing an example of the optical sheet 10 taken in a plane orthogonal to the machine direction 20 (Fig. 1). In the example shown, the optical sheet 10 has two layers 32 and 34extruded from polymers that are optically transmissive. In one embodiment, both layers 32 and 34 comprise the same polymer in the respective continuous phases. In another embodiment, the layers 32 and 34 comprise different polymers in the respective continuous phases. Examples of suitable polymers include acrylic polymers such as poly(methyl methacrylate) (PMMA), acrylic co-polymers of monomers such as methyl acrylate, butyl acrylate, and methyl methacrylate, polycarbonate, polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), and silicone polymers such as polydimethylsiloxane (PDMS).

[0016] The layers 32 and 34 have thicknesses that vary in the transverse direction 22 (Figs. 1 and 2). The thicknesses vary from a first edge surface 46 to a second edge surface 48. In the illustrated embodiment the layers 32 and 34 have a continuous variation in respective thicknesses in the transverse direction 22. Furthermore, in the illustrated embodiment the first layer 32 increases in thickness non-linearly with distance from the first edge surface 46. The first layer 32 has a nonzero thickness at both of the edge surfaces 46 and 48. The first layer 32 has a minimum thickness at the first edge surface 46, where the second layer 34 has a maximum thickness. The first layer 32 has a maximum thickness at the second edge surface 48, where the second layer 34 has a minimum thickness. The second layer 34 decreases in thickness complementarily to the increasing thickness of the first layer 32 in the transverse direction 22. Therefore, in the illustrated embodiment, the optical sheet 10 has a constant thickness in the transverse direction 22. In an alternative embodiment, the thickness of the optical sheet 10 varies in the transverse direction 22. In a further embodiment, the major surfaces 12 and 14 are non-planar.

[0017] At least one of the layers 32 and 34 includes light-redirecting elements 52 dispersed therein. As shown, the light-redirecting elements 52 are only in the first layer 32, with the second layer 34 containing no optically-active concentration of light-redirecting elements. However in another embodiment the layers 32 and 34 both contain light- redirecting elements, such as at different concentrations and/or with different light-redirecting properties.

[0018] The light-redirecting elements 52 are uniformly distributed within the layer 32. The light-redirecting elements 52 scatter the light that impinges thereon. In one embodiment, the light-redirecting elements are solid light-redirecting particles. The particles may comprise metal oxides (e.g., silica, alumina, titanium dioxide) or metals (e.g., aluminum ). In another embodiment, the light-redirecting particles are made of an optically-transmissive polymer having a refractive index different from the surrounding continuous phase. The polymer particles may be for example solidified droplets of a higher melting point polymer, such as PBT in poly(methyl acrylate) (PMA) or a polyamide in PMMA. Typical sizes of the particles are smaller than the thickness of the thinnest region of the layer that contains the particles. In one embodiment, particles have dimensions smaller than one tenth of the thickness of the thinnest region of the layer containing the particles. Furthermore, the light- redirecting particles may be similar or be different in size and shape.

[0019] In some embodiments, the light redirecting particles are particles of a wavelength- shifting material that absorbs light and re-emits light at one or more wavelengths different from that of the absorbed light. Examples of a wavelength-shifting material include a phosphor material, a luminescent material, a luminescent nanomaterial such as a quantum dot material, a conjugated polymer material, an organic fluorescent dye, and an organic phosphorescent dye.

[0020] In another embodiment the light-redirecting elements 52 are light-redirecting voids within the layer 32. One example of a method of introducing the voids into an extruded material is to inject a supercritical fluid, such as liquid carbon dioxide, into the continuous phase before extrusion. Another example of introducing the voids into an extruded material is to add a chemical foaming agent to the continuous phase prior to extrusion.

[0021] The optical sheet 10, in which the layers 32 and 34 have thicknesses that vary in the transverse direction 22, has a local average concentration of the light-redirecting elements 52 that also varies in the transverse direction 22. The local average concentration of the light-redirecting elements is determined in the direction of the thickness of the optical sheet 10 from the major surface 12 to the major surface 14. At any given location x in the transverse direction 22, the local average concentration C_x of the light redirecting elements 52 is equal to the weighted sum of the concentration Ci of the light redirecting elements in the layer 32 at the location and the concentration C2 of the light redirecting elements (if any) in the layer 34 at the location. The concentrations are weighted in accordance with the thicknesses ti, t₂ of the respective layers at the location. The local average concentration is given by C_x = (Citi + C2t2)/(ti + 1₂). In the example illustrated in Fig. 2, the local average concentration of the light-redirecting elements is less at the first edge surface 46 than at the second edge surface 48. In this example, the local average concentration varies continuously from a minimum value near the first edge surface 46 to a maximum value near the second edge 48.

[0022] Fig. 3 shows an example of a lighting assembly 115 in which the optical sheet 10 or part thereof is used as a light guide 100. The light guide 100 is edge lit by a light source 1 10. Light 102 enters the light guide 100 through a light input edge 104, and propagates through the light guide 100 in the transverse direction 22 toward a second edge 106. The light 102 propagates in the light guide 100 by total internal reflection between opposed major surfaces 112 and 1 14. The light input edge 104 of the light guide 100 corresponds to the first edge surface 46 (Fig. 2) of the optical sheet 10 shown in Fig. 1.

[0023] The light source 110 can be any type of light source. Examples of types of light sources include lasers, incandescent light sources, gas discharge lamps, arc lamps, compact fluorescent lamps, halogen lamps, and solid state light emitting devices, such as light emitting diodes (LEDs), laser diodes, and organic LEDs (OLEDs).

[0024] In this embodiment, the light-redirecting elements 52 act as light-extracting elements that extract the light 102 from the light guide 100 through the major surfaces 1 12,

114 of the light guide 100. A reflector (not shown) may be located adjacent, e.g., the major surface 1 12 to reflect light extracted through the major surface 1 12 back through the light guide 100 and out the other major surface 114 to provide light output from only the major surface 1 14.

[0025] The local intensity of the light extracted from a region of the light guide 100 depends on the local average concentration of the light-redirecting elements 52 in the region and the intensity of light passing through the region. According to an embodiment, the light guide 100 has a local average concentration of the light-redirecting elements 52 that varies in the transverse direction 22 and is less at the light input edge 104 than at the second edge 106. Other possible characteristics of the light guide 100 are discussed above with regard to the optical sheet 10 shown in Figs. 1 and 2.

[0026] According to an embodiment, the light guide 100 has layers that vary in thickness in the transverse direction 22. In one embodiment, the variation in thickness is configured to provide a nominally uniform intensity profile of the extracted light in which the intensity varies by no more than +/- 10 % from a mean intensity value. The intensity profile characterizes the variation of intensity with position in the major surfaces 1 12, 1 14 through which the light is extracted from the light guide 100. In another embodiment, the intensity profile has a maximum intensity of the extracted light in a region between the light input edge 104 and the second edge 106, such as in a region mid way between the light input edge 104 and the second edge 106.

[0027] In the illustrated embodiment, the major surfaces of the light guide 100 are planar. In another embodiment the major surfaces are non-planar. In yet another embodiment, the light guide 100 varies in thickness, such as decreases in thickness, with distance from the light input edge 104.

[0028] Some embodiments of the lighting assembly 115 are used for general lighting. Other embodiments of the lighting assembly 115 are used as a backlight for a liquid-crystal or other type of display (not shown). Other applications are possible. The lighting assembly

1 15 additionally includes a power supply (not shown) to provide power to the light source 110 and may additionally include a suitable heatsink (not shown) to remove heat from the light source.

[0029] According to another embodiment the optical sheet 10 is used as a diffuser having spatially-varying diffusive properties.

[0030] Fig. 4 shows one possible arrangement for a manifold 120 of an extrusion die for extruding one layer of the optical sheet 10 (Fig. 1). A molten material supply 121 supplies molten material to a central channel 122. In an example, the molten material supply 121 is provided by providing a continuous-phase polymer, providing a light-redirecting polymer, and mixing the continuous-phase polymer and the light-redirecting polymer to produce the molten material supply 121. The central channel 122 splits off into side channels 124 and 126, which spread the molten material in the transverse direction 22. The molten material is then extruded in the machine direction 20. The asymmetric flow rate through the channels 124, 126 represented by arrows 128 in Fig. 4 is obtained by configuring the side channels 124, 126 to have a varying cross-section in the transverse direction 22. The portion of the manifold with larger cross section (and, hence, the higher exit flow rate) corresponds to the thicker portion of the optical sheet layer in the transverse direction 22. The arrangement illustrated in Fig. 4 is for one layer; the manifold for the additional layer is configured to have a different variation of cross-section in the transverse direction 22 to obtain the optical sheet 10 with approximately constant thickness in the transverse direction 22.

[0031] Fig. 5A shows an extrusion die 150 for coextruding the optical sheet 10. A first molten material supply 141 provides a first molten material to a first manifold 140 in the extrusion die 150. According to a first embodiment, the first molten material comprises a polymer that forms a continuous phase and light-redirecting particles dispersed therein. In an embodiment in which the light-redirecting particles are elongate, the elongate light- redirecting particles tend to align in the machine direction in the optical sheet 10 (FIG. l). An example of an elongate light-redirecting particle is a glass fiber fragment.

[0032] A second molten material supply 161 provides a second molten material to a second manifold 160 of the extrusion die 150. In this embodiment, the second molten material comprises the same polymer as the first molten material for the continuous phase, but has no operably effective concentration of light-redirecting particles. In another embodiment, the second material includes second light-redirecting particles, different from the first light-redirecting particles, such that the second material has a light-redirecting property different from the light-redirecting property of the first material. In another embodiment the second material has a concentration of light-redirecting particles different from the concentration of light-redirecting particles in the first material.

[0033] An elongate passage 145 receives the first molten material from the first manifold 140. An elongate passage 165 receives the second molten material from the second manifold 160. The passages 145 and 165 merge at a junction 170 to form a third passage 175 that is elongate in the machine direction 20. The third passage 175 terminates at the extrusion die exit 154, from which the first and second molten materials are coextruded in the machine direction 20 to form the optical sheet 10 (Fig. 1) as a multi-layer optical sheet.

[0034] The passages 145, 165, and 175 are defined by die members 182, 184, and 186. The middle die member 184 constitutes a common wall that defines in part both the first passage 145 and the second passage 165, while separating the passages 145 and 165 from one another. The first end die member 182 has a major surface 192 that faces the middle die member 184, with the first passage 145 being located between the major surface 192 and the middle die member 184. The second end die member 186 has a major surface 196 that faces the middle die member 184, with the second passage 165 being located between the major surface 196 and the middle die member 184. In other embodiments, the middle die member 184 is omitted and instead the passages 145 and 165 have individual walls that merge at the junction 170.

[0035] With reference now in addition to Fig. 5B, in the first embodiment, the first passage 145 has a cross-sectional dimension that varies in the transverse direction 22, to vary the amount of the first molten material extruded, and hence the thickness of the first layer, in the transverse direction 22 with layer thickness that decreases with increasing distance from an end surface 198 of the middle die member 184. The cross-sectional dimension is in a direction orthogonal to major surfaces 200 and 202 of the passage 175. The second passage 165 has a cross-sectional dimension that varies in the transverse direction 22, to vary the amount of the second molten material extruded, and hence the thickness of the second layer, in the transverse direction 22 with layer thickness that increases with distance from the end surface 198 of the middle member 184. The combined cross-sectional dimension of the first passage 145 and the second passage 165 is constant in the transverse direction 22. The third passage 175 has constant cross-sectional dimension in the transverse direction. In another embodiment, both of the passages 145 and 165 have cross-sectional dimensions that vary in other ways in the transverse direction 22 to define the thickness respective layers of the optical sheet 10. [0036] Fig. 6 shows an example of an optical sheet 240 that has a first layer 242 and a second layer 244. The first layer 242 contains light-redirecting elements 252 in a continuous phase. The second layer 244 contains a lower concentration of light-redirecting elements than the first layer 242. In an example the second layer 244 contains no optically-active concentration of light-redirecting elements. The second layer 244 has a thickness that changes in the transverse direction 22. However, unlike the optical sheet 10 (Fig. 2), the second layer 244 of the optical sheet 240 has a nonzero thickness at both ends 256 and 258 of the optical sheet 240, and the first layer 242 includes the major surface 12 and the second layer 244 includes the major surface 14.

[0037] Fig. 7 shows an example of an optical sheet 260 having four layers 262-268. Multiple layers allow the manufacture of thicker optical sheets. Multiple layers additionally or alternatively allow the manufacture of optical sheets with different concentration gradients by using different combinations of dies. The first layer 262 and the third layer 266 are layers that contain light-redirecting elements 272 in a continuous phase. In the illustrated embodiment, the first layer 262 and the third layer 266 are layers of the same material, having the same light-redirecting elements 272 and the same continuous phase. The second layer 264 and the fourth layer 268 contain a lower concentration of light-redirecting elements than the layers 262 and 266. In an example, the layers 262 and 266 contain no optically-active concentration of light-redirecting elements. Apart from the additional layers the optical sheet 260 has similar characteristics to other optical sheets described herein.

[0038] Fig. 8 shows an example of another optical sheet 280 that includes six layers 282- 292. The first layer 282, the third layer 286, and the fifth layer 290 are layers that contain light-redirecting elements 296 in a continuous phase. The second layer 284, the fourth layer 288, and the sixth layer 292 contain a lower concentration of light-redirecting elements than the layers 282, 286, and 290. In an example, the layers 282, 286, and 290 contain no optically-active concentration of light-redirecting elements. Apart from the additional layers the optical sheet 280 has similar characteristics to other optical sheets described herein.

[0039] Fig. 9 shows an example of another optical sheet 300 that includes three layers 310-330. The second layer 320 contains light-redirecting elements 340. The first layer 310 and the third layer 330 contain no optically-active concentration of light-redirecting elements, or contain a lower concentration of light-redirecting elements than the layer 320. In this example, the second layer 320 containing the light-redirecting elements 340 is located symmetrically with respect to a plane 350 located mid-way between, and parallel to, the major surfaces 360, 370 of the optical sheet 300. The second layer 320 increases in thickness symmetrically relative to the plane 350 with increasing distance from the edge 354. The layers 310 and 330 have a combined thickness that changes with distance from an edge 354 complementarily to the changing thickness of the second layer 320 with distance from the edge 354. In an embodiment where the optical sheet 300 is used as a light guide, the edge 354 is a light input edge for inputting light to be output through both major surfaces 360 and 370 of the optical sheet 300.

[0040] Optical sheets having three or more layers can be made using the process described above using additional manifolds through which additional molten materials flow. All manifolds supply respective molten materials into respective passages of an extrusion die in which the passages converge to a common passage that terminates at the extrusion die exit 154 (Fig. 5 A). The molten materials are coextruded from the extrusion die in the machine direction 20 to form the multi-layer optical sheet.

[0041] In this disclosure, the phrase "one of followed by a list is intended to mean the elements of the list in the alternative. For example, "one of A, B and C" means A or B or C. The phrase "at least one of followed by a list is intended to mean one or more of the elements of the list in the alterative. For example, "at least one of A, B and C" means A or B or C or (A and B) or (A and C) or (B and C) or (A and B and C).

[0042] Other alternatives and variations are possible with regard to the above-described devices and/or methods. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the above-described devices and/or methods. In addition, while a particular feature of may have been described above with respect to only one or more of several above-described devices and/or methods, such feature may be combined with one or more other features of the other above-described devices and/or methods, as may be desired and advantageous for any given or particular situation.

Claims

CLAIMS What is claimed is:

1. A coextruded optical sheet having opposed major surfaces, the sheet comprising layers comprising respective materials having different concentrations of light-redirecting elements;

wherein:

the sheet is coextruded in a machine direction, and has a transverse direction orthogonal to the machine direction and parallel to the major surfaces; and

the materials vary in relative amount along the transverse direction.

2. The optical sheet of claim 1, wherein the materials are constant in relative amount along the machine direction.

3. The optical sheet of claim 1, wherein:

each of the materials comprises a respective continuous phase; and

at least one of the materials comprises a respective concentration of the light- redirecting elements within the respective continuous phase.

4. The optical sheet of claim 3, wherein the continuous phases of the materials are optically transmissive.

5. The optical sheet of claim 3, wherein the continuous phase of at least one of the materials comprises a polymer.

6. The optical sheet of claim 3, wherein the continuous phases of the materials comprise the same polymer.

7. The optical sheet of claim 3, wherein the continuous phases of the materials comprise different polymers.

8. The optical sheet of claim 3, wherein the continuous phase of at least one of the materials comprises glass.

9. The optical sheet of claim I, wherein the light-redirecting elements are light- redirecting particles.

10. The optical sheet of claim 9, wherein the light-redirecting particles are optically transmissive.

1 1. The optical sheet of claim 9, wherein the light-redirecting particles are spheroidal.

12. The optical sheet of claim 9, wherein the light-redirecting particles are elongate.

13. The optical sheet of claim 9, wherein the light-redirecting particles comprise glass fiber fragments.

14. The optical sheet of claim 9, wherein the light-redirecting particles vary in one or both of size and shape.

15. The optical sheet of claim 9, wherein the light-redirecting particles are particles of a wavelength-shifting material.

16. The optical sheet of claim 9, wherein the light-redirecting particles are luminescent particles.

17. The optical sheet of claim I, wherein the light-redirecting elements are light- redirecting voids.

18. The optical sheet of claim 1, wherein one of the materials contains no optically- active concentration of light-redirecting elements.

19. The optical sheet of claim 1, wherein the light-redirecting elements are uniformly distributed in at least one of the materials.

20. The optical sheet of claim 1, wherein a local average concentration of light- redirecting elements varies in the transverse direction.

21. The optical sheet of claim 1, wherein the layers are more than two in number.

22. The optical sheet of claim 20, wherein the two of the layers are of the same material.

23. The optical sheet of claim 1, wherein the layers vary continuously in thickness in the transverse direction.

24. The optical sheet of claim 23, wherein:

the optical sheet has opposed edge surfaces separated in the transverse direction; and at one of the edge surfaces, one of the layers has a maximum thickness and the other of the layers has a minimum thickness.

25. The optical sheet of claim 1, wherein:

the optical sheet has opposed edge surfaces separated in the transverse direction; and the materials of the layers each comprise a respective continuous phase, and the material of a first of the layers comprises a greater concentration of light-redirecting elements dispersed in the continuous phase thereof than the material of a second of the layers.

26. The optical sheet of claim 25, wherein the second of the layers decreases in thickness complementarily to the increasing thickness of the first of the layers.

27. The optical sheet of claim 25, wherein a third of the layers comprises a material having a concentration of light-redirecting elements less than that of the material of the first of the layers and the first of the layers is between the second of the layers and the third of the layers.

28. The optical sheet of claim 27, wherein, with respect to a plane mid-way between, and parallel to, the major surfaces of the optical sheet, the first of the layers symmetrically increases in thickness with distance from one of the edge surfaces of the optical sheet.

29. The optical sheet of claim 27, wherein the second of the layers and the third of the layers have a combined thickness that decreases with distance from the one of the edge surfaces complementarily to the increasing thickness of the first of the layers.

30. The optical sheet of claim 25, wherein the optical sheet has thickness that decreases with distance from one of the edge surfaces of the optical sheet.

31. The optical sheet of claim 30, wherein the first of the layers increases in thickness non-linearly with distance from the one of the edge surfaces.

32. The optical sheet of claim 25, wherein the material of the second of the layers contains no optically-active concentration of light-redirecting elements.

33. The optical sheet of claim 25, wherein the first of the layers increases in thickness non-linearly with distance from one of the edge surfaces of the optical sheet.

34. The optical sheet of claim 25, wherein the first of the layers has a non-zero thickness at one of the edge surfaces of the optical sheet.

35. The optical sheet of claim 1, wherein the optical sheet has a constant thickness at least in the transverse direction.

36. The optical sheet of claim 1, wherein the major surfaces are non-planar at least in the transverse direction.

37. The optical sheet of claim 1, wherein:

the optical sheet is a light guide configured for receiving light from a light source; the light propagates in the transverse direction through the optical sheet by total internal reflection; and the light-redirecting elements function as light-extracting elements to extract the light from the light guide through the major surfaces.

38. The optical sheet of claim 37, wherein:

the optical sheet has opposed edge surfaces separated in the transverse direction; each of the materials comprises a respective continuous phase;

each of the materials comprises a different respective concentration of light- redirecting elements within the respective continuous phase; and

a local average of the concentration of the light-redirecting elements is less at one edge surface than at the other edge surface.

39. The optical sheet of claim 38, wherein the local average concentration of the light-redirecting elements increases continuously from a minimum value near the one edge surface to a maximum value near the other edge surface.

40. The optical sheet of claim 37, wherein the one of the edge surfaces of the optical sheet provides a light input edge of the light guide.

41. The optical sheet of claim 40, wherein the local average concentration of the light-redirecting elements increases with distance from the light input edge to extract light from the light guide with an intensity profile that varies by no more than +/- 10% from a mean intensity value.

42. The optical sheet of claim 40, wherein a first of the layers increases in thickness with distance from the light input edge to extract light from the light guide with an intensity profile that varies by no more than +/- 10% from a mean intensity value.

43. The optical sheet of claim 42, wherein the intensity profile has a maximum partway between the edge surfaces.

44. The optical sheet of claim 1, wherein the optical sheet is a diffuser.

45. A light guide for emitting light with a defined intensity profile, the light guide comprising:

layers each comprising a respective continuous phase, a first of the layers comprising a greater concentration of light-redirecting elements dispersed in a continuous phase thereof than a second of the layers;

wherein:

the light guide has a light input edge and a second edge, opposite the light input edge;

the light-redirecting elements function as light-extracting elements to extract light received at the light input edge from the light guide; and

the first of the layers increases in thickness with distance from the light input edge, and the second of the layers decreases in thickness with distance from the light input edge.

46. The light guide of claim 45, wherein the light guide comprises opposed major surfaces that are non-planar at least in the direction from the light input edge to the second edge.

47. The light guide of claim 45, wherein the light-redirecting elements are light- redirecting particles.

48. The light guide of claim 47, wherein the light-redirecting particles are optically transmissive.

49. The light guide of claim 47, wherein the light-redirecting particles are spheroidal.

50. The light guide of claim 45, wherein at least one of the continuous phases comprises a polymer.

51. The light guide of claim 45, wherein at least one of the continuous phases comprises glass.

52. The light guide of claim 45, wherein the light-redirecting elements have a local average concentration that increases with distance from the light input edge to extract the light from the light guide with an intensity profile that varies by no more than +/- 10% from a mean intensity value.

53. The light guide of claim 52, wherein the light guide has thickness that decreases with distance from the light input edge.

54. The light guide of claim 53, wherein the first of the layers increases in thickness non-linearly with distance from the light input edge.

55. The light guide of claim 45, wherein the first of the layers increases in thickness with distance from the light input edge to extract the light from the light guide with an intensity profile that varies by no more than +/- 10% from a mean intensity value.

56. The light guide of claim 55, wherein the intensity profile has a maximum partway between the light input edge and the second edge.

57. The light guide of claim 45, wherein the second of the layers decreases in thickness complementarily to the increasing thickness of the first of the layers.

58. The light guide of claim 45, wherein a third of the layers comprises a material having a concentration of light-redirecting elements less than that of the material of the first of the layers and the first of the layers is between the second of the layers and the third of the layers.

59. The light guide of claim 58, wherein, with respect to a plane mid-way between, and parallel to, the major surfaces of the optical sheet, the first of the layers symmetrically increases in thickness with distance from the light input edge.

60. The light guide of claim 58, wherein the second of the layers and the third of the layers have a combined thickness that decreases with distance from the light input edge complementarily to the increasing thickness of the first of the layers.

61. The light guide of claim 45, wherein the material of the second of the layers contains no optically-active concentration of light-redirecting elements.

62. The light guide of claim 45, wherein the first of the layers increases in thickness non-linearly with distance from the light input edge.

63. The light guide of claim 45, wherein the first of the layers has a non-zero thickness at the light input edge.

64. A method of making an optical sheet having opposed major surfaces, the method comprising:

providing a first material comprising light-redirecting elements;

providing a second material; and

coextruding the first material and the second material in a machine direction to form the optical sheet, the materials being coextruded such that the relative amount of the materials varies in a transverse direction that is orthogonal to the machine direction and parallel the major surfaces to vary a local average concentration of light-redirecting elements along the transverse direction.

65. The method of making an optical sheet of claim 64, wherein the providing the first material comprises:

providing a continuous-phase polymer;

providing a light-redirecting polymer; and

mixing the continuous-phase polymer and the light-redirecting polymer to form the first material in which particles of the light-redirecting polymer are dispersed in the continuous-phase polymer.

66. The method of claim 65, wherein:

the second material has second light-redirecting elements therein; and

the materials have different light-redirecting properties.

67. The method of claim 66, wherein the second light-redirecting elements have a second concentration different from the first concentration.

68. The method of claim 66, wherein the second light-redirecting elements are of a type different from the light-redirecting elements of the first material.

69. The method of claim 64, wherein:

the method additionally comprises providing an extrusion die defining passages, the passages comprising a first passage and a second passage that merge into a third passage; and the coextruding comprises concurrently extruding the first material through the first passage and the third passage of the extrusion die and the second material through the second passage and the third passage of the extrusion die, the first material and the second material combining in the third passage to form the optical sheet as a multi-layer optical sheet.

70. The method of claim 69, wherein at least one of the passages has a cross-sectional profile in a plane orthogonal to the machine direction, the cross-sectional profile varying in the transverse direction.

71. An extrusion die for coextruding a multi-layer optical sheet, the die comprising a body comprising an elongate first extrusion passage, an elongate second extrusion passage and a third extrusion passage elongate in a machine direction defined therein, the first extrusion passage and the second extrusion passage merging into the third extrusion passage at a junction, wherein:

the third extrusion passage has opposed major surfaces and is characterized by a transverse direction orthogonal to the major surfaces and the machine direction; and

the first extrusion passage has a cross-sectional dimension at the junction, in a direction orthogonal to the major surfaces of the third extrusion passage that varies in the transverse direction.

72. The extrusion die of claim 71, wherein the second extrusion passage has a cross- sectional dimension at the junction, in a direction orthogonal to the major surfaces of the third extrusion passage that varies in the transverse direction complementarily to the variation of the cross-sectional dimension of the first extrusion passage.

73. The extrusion die of claim 71, wherein the extrusion die is for extruding a material comprising a greater concentration of light-redirecting elements through the first extrusion passage than through the second extrusion passage.