WO2013050853A2

WO2013050853A2 - Method of making a lightguide

Info

Publication number: WO2013050853A2
Application number: PCT/IB2012/001951
Authority: WO
Inventors: Paul Blair; Joe Hsieh; Navin Suyal
Original assignee: Exxelis Global Limited
Priority date: 2011-10-04
Filing date: 2012-10-02
Publication date: 2013-04-11
Also published as: GB201117041D0; GB2495477A; WO2013050853A3

Abstract

A method of making a Iightguide, comprising the steps of (a) providing a Iightguide film having upper and lower surfaces allowing substantial total internal reflection of light therebetween; (b) applying a viscous and curable material at least to the lower surface in a pattern; (c) applying a surface texture to the material; and (d) curing the material, the resulting surface textured pattern on the base of the Iightguide being capable of extracting light from the Iightguide is disclosed.

Description

METHOD OF MAKING A LIGHTGUIDE

Background to the Invention

[0001] This invention relates to a method of making a lightguide.

[0002] A lightguide is a film that provides for substantial total internal reflection of light between an upper and lower surface. The light is injected at the edge of the film, perpendicular to the upper and lower surfaces, by one or more optical sources. The lightguide can be used as a backlight when a textured profile is included on the upper and lower surfaces, but preferably the lower surface only, so as to scatter the light out of the lightguide. The textured surface relies on refraction and/ or reflection to redirect the light out of the lightguide.

[0003] Lightguides are commonly deployed within the backlights found in display systems, such as in a liquid crystal display (LCD) panels; in advertising panels; in general niumination applications; or in information displays, for example automotive dashboards, control panels on domestic appliances and machine status indicators. In some cases illumination is required from the majority of the area of the backlight, whereas in other cases illumination is only required from specific areas of the backlight.

[0004] One known lightguide manufacturing method uses white scattering dots generally applied by screen printing on to the lightguide surface as disclosed in US-4630895-A and US-5673128-A. There is no control of the scattering angle with this approach (i.e., the drops are flat with low contact angles due to the limited surface energy difference between the optical substrates and the resins forming the white dots) and the particulate additives used to enhance scattering (typically T1O2) possess a significant optical absorption coefficient in the visible range, which leads to a gradual yellowing of the light as the distance from the light source increases.

[0005] Another manufacturing method in the prior art is the injectio moulding of lightguides as disclosed in US-6480307-B1 and US-6623667-B2. The mould is machined using a flycutting, turning, milling, grinding, etching, or similar, process. A material is melted, added to the mould, allowed to solidify (or potentially cured) and removed. The mould machining processes are not suited to cases where the scattering (i.e., machined) area is considerably smaller than the lightguide area, and especially cases where the shape of the scattering area is complex and selective in nature, such as is the case for an information display. Further, injection moulding is not suited to rapid manufacturing and has poor economies of scale in low volume manufacturing.

[0006] A further known manufacturing method is the hot or UV embossing of microstructures as disclosed in US-7252428-B2 and US-7543974-B2. Hot, or UV- cured, embossing of lightguides is typically a reel-to-reel process and the pattern to be embossed is formed as a surface profile or texture in a drum. The surface profile is created by diamond turning, or similar, and the profile can vary along the length of the drum. In UV-cured embossing, typically the entire lower surface of the lightguide is coated in the uncured resin. This leads to considerable material wastage when the scattering region is a small fraction of the total lightguide area. Further, the area of the lightguide outside the scattering region must be optically smooth and coplanar with the lightguide surfaces to avoid unwanted light extraction. A further disadvantage of reel-to-reel embossing is that when the surface profile is required to vary along the circumference of the drum, the size of the lightguide is then limited by the drum's circumference. Where the surface profile is required to vary around the circumference of the drum gravure-printing techniques are available, but this process shares the disadvantages of injection-moulding mould manufacturing. Where the process is embossing by stamping, the process again shares the drawbacks of injection- moulding.

Summary of the Invention

[0007] It is an aim of the invention to provide a, lightguide manufacturing process that allows for efficient usage of materials, is adapted for rapid and low- to-high volume manufacturing, and results in a thin, bright and efficient backlight.

[0008] The present invention provides a method of making a lightguide, comprising the steps of (a) providing a lightguide film having upper and lower surfaces allowing substantial total internal reflection of light therebetween; (b) applying a viscous and curable material to the lower surface in a pattern; (c) applying a surface texture to the material; and (d) curing the material, the resulting surface textured pattern on the base of the lightguide being capable of extracting light from the lightguide.

[0009] The material applied in step (b) may comprise a curable resin. It may include scattering nanoparticles, e.g. of T1O2, to promote scattering. It may include a UV-degradation iming additive such as UV absorber or an active component such as phosphor. It may be applied in a high resolution printing process, for example inkjet printing or screen printing. The quantity of material applied may vary over the surface. In particular, the material may be applied only to a part of the lightguide from which light is to be extracted.

[0010] Prior to step .(b), a surface modifying agent, e.g. a polymer or oligomer, may be applied to the surface. The role of such a surface modifying agent may be planarization, alteration of surface energy, or modification or matching of refractive index.

[0011] The texture applied in step (c) may be arranged to cooperate with the pattern of material to enhance light scattering. The texture may be uniform over the surface, or may vary thereover, to efficiently generate uniform niumination, or to generate a pattern of varying illumination (as the relationship will be more complex). The texture may comprise a randomly roughened profile or periodic geometric microstructures, for example prisms, pyramids, cones and/ or microlense5. [0012] Step (c) may for example comprise reel-to-reel embossing or stamping embossing.

[0013] In order to provide a lightguide having two opposed textured surfaces, steps (b), (c) and (d) can be performed on the upper surface as well as the lower surface, with the optional features mentioned above.

[0014] The invention also provides a backlight comprising an opaque facia including a plurality of transparent windows; a lightguide made according to the method described above, providing illumination solely in defined areas to be iUuminated; and light sources arranged at the periphery of the lightguide.

[0015] The backlight may include an opaque facia having windows corresponding to said defined areas, which may amount to less than 50% of the area of the lightguide and may define text and/ or icons. The facia may be of non- rectangular shape. A reflecting film may be arranged behind the lightguide.

Brief Description of the Drawings

[0016] The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

[0017] Figure 1 is a schematic exploded view of a backlight unit incorporating a lightguide made according to the invention;

[0018] Figures 2a to 2c schematically show a method according to an embodiment of the invention;

[0019] Figures 3a to 3c show resin images produced according to the invention;

[0020] Figures 4a and 4b show a varying resin distribution embodiment of the inventive method;

[0021] Figures 5a and 5b show an alternative embodiment;

[0022] Figure 6 is an exploded view of a backlight according to an embodiment of the invention; and [0023] Figures 7a and 7b are side views of respective embodiments of the backlight.

Detailed Description of Particular Embodiments

[0024] Figure 1 is an exploded diagram illustrating an exemplary layout of the backlight consistent with the present invention. The backlight unit 2 includes a lightguide film 1, having upper and lower surfaces capable of guiding light through total internal reflection; an optional optical reflector 3 below the lower surface of the lightguide, an optional diffuser film 4 above the lower surface of lightguide 1 and an optional brightness enhancing film 5 above the optional diffuser film 4. The lightguide 1 includes light sources 6 optically coupled to the film that inject light into the film. The optical sources 6 are distributed as required around the lightguide 1 according to user constraints. Examples of light sources include light-emitting diodes (LED) of any type: top- and edge- LEDs, organic LED (OLEDs), or other point sources of light, including light sources delivered via optical fibre and lasers. The textured profile on the lower surface scatters light out of the lightguide towards the optional diffuser 4. Light scattered towards the base reflector 3 is reflected through the lightguide 1 towards the optional diffuser 4. The optional diffuser film 4 acts to improve the uniformity of the illumination from the lightguide 1. The optional brightness enhancing film 5 acts to improve the on-axis brightness of the illumination from the lightguide 1. In this way the backlight unit 2 is used to illuminate a display system, such as in a liquid crystal display (LCD) panel; in an advertising panel; or in a general lighting application; or in an information display, for example in an automotive dashboard, or a control and status panel in a domestic appliance or industrial machine. In some cases illumination is required from the majority of the area of the backlight, whereas in other cases illumination is only required from specific areas of the backlight.

[0025] Figure 2 is a schematic representation of the method for making a lightguide. In Figure 2a, the lightguide 1 has an upper surface 7 and a lower surface 8 that are substantially parallel (in a further embodiment there is a angle between the upper surface 7 and a lower surface 8 to form a wedged lightguide) and provide for total internal reflection of light between the upper surface 7 and the lower surface 8. Examples of suitable lightguide materials include polycarbonate (PC), polyethylene terephthalate (PET) and polymethyl methacrylate (PMMA), cyclic olefin copolymer (COC) and cyclic olefin polymers (COP). More generally, the lightguide film will be a material that is optically transparent in the visible spectrum, having a thickness, generally between 0.050 mm to 10 mm, more particularly between 0.125mm and 5mm.

[0026] In Figure 2b, the lightguide 1 lower surface 8 is coated in a viscous and curable polymer resin 9. The coating process is preferably a high resolution printing process, examples of which include inkjet printing and screen printing, but could be any deposition process that is compatible with the polymer resin and capable of achieving a drop size on the printed surface in the range of 10 micrometers to 500 micrometers diameter, more particularly in the range of 50 to 200 micrometers diameter. Further the high resolution printing process allows for control of the height of the deposited resin. Figure 2b shows a resin distribution 9 where there is a variation in resin height. Also included in Figure 2b is an illustrative example of a surface textured stamp 10. In this illustration, the stamp 10 has a periodic microstructured surface texture 11 that is transferred into the resin distribution 9. The cured polymer resin has a refractive index preferably similar to, or slightly higher than, the lightguide 1. In Figure 2c the embossing process creates a variable height microstructured surface texture 12 in the lightguide 1. The microstructured surface texture 12 extracts light preferably uniformly over the lightguide area, but the distribution of light could take any desired form.

[0027] The printed resin distribution alone is not able to efficiently extract light from the lightguide. The substrate material and deposited resin have similar material properties and the rninimum surface-energy profile is ill-suited for light extraction. The printed resin distribution is designed in conjunction with the surface texture to ensure that the required illumination profile is generated by the lightguide. Commercial design software, such as LightTools developed by Optical Research Associates, can be used to optimise both resin distribution and surface texture to achieve a uniform luminance across the lightguide area, or a more complex iUumination distribution.

[0028] Printing techniques capable of patterning the lightguide include inkjet printing and screen printing, and preferably include piezoelectric drop-on- demand inkjet technology. Generally, the printing process controls the volume of resin deposited and a translatio stage controls the two-dimensional distribution. More generally, these functions are combined into a commercially available inkjet printer, such as those available from EPSON Ltd. One versed in the state-of-the- art would understand that different spatially varying resin distributions can be printed. Examples in Figure 3 include grey-scale and half-tone images. In Figure 3a the resin distribution 13 on the lightguide 1 is a grey-scale image. In this example, the resin thickness is continuously varying (illustrated in the resin distribution 9 of Figure 2b) and is controlled by the printing process and the properties of the materials. Further, the deployment of raster image processor (RIP) software as the interface between the design software and printer leads to a half-tone image 14 as shown in Figure 3b. In this example the resin distribution is largely of constant height and software varies the filling density of the image. In Figure 3c a more complex resin image 15 includes the grey-scale printing of geometrical shapes 16 and text 17 such as might be required to Hluminate a company logo, icon, or other pattern. The graduated patterns (from light to dark) in the geometrical shapes 16 and text 17 are illustrative of increasing resin deposition volume (or height) and accordingly an increase in light scattering further from the light source 6.

[0029] As illustrated in Figure 4a, the resin distribution can be formed by varying the density distribution of a two-dimensional array of similar droplets 18 such as may be formed by screen printing, or by printing via direct control with a piezoelectric drop-on-demand inkjet printing head, such as those commercially available from XAAR PLC. To generate uniform illumination the density of droplets 18 increases in a controlled manner as a function of the distance to the light source 6. Also included in Figure 4a is an illustrative example of an embossing drum 19. The drum 19 has a surface texture 20, in this illustration a periodic microstructure, which is transferred into the distribution of resin droplets 18. In Figure 4b the introduction of the surface texture process creates a microstructured surface 21 that varies in density.

[0030] In another embodiment, as illustrated in Figure 5a, the lightguide 1 upper surface 7 and lower surface 8 are both coated in a viscous and curable polymer resin 22. The coating process is preferably a high resolution printing process, examples of which includes inkjet printing and screen printing, but could be any deposition process that is compatible with the polymer resin and capable of achieving a drop size on the printed surface in the range of 10 micrometers to 500 micrometers diameter, more particularly in the range of 50 to 200 micrometers diameter. Also the high resolution printing process allows for control of the height of the deposited resin. Figure 5a shows a resin distribution 22 where there is a variation in resin height. The surface texture 11, which in this illustration is a periodic microstructure, present in the stamp 10, is transferred into both resin distributions 22. In Figure 5b the surface texturing process creates a variable height microstructured surface 23 on the upper surface 7 and lower surface 8. The microstructured surface texture 23 extracts light preferably uniformly over the lightguide area, but the distribution of light could take any desired form. In this embodiment the microstructured surface texture 23 can extract light through both the upper surface 7 and lower surface 8, or by deploying an optional optical reflector 3 below the lower surface 8 of lightguide 1 the microstructured surface texture 23 can extract light through the upper surface 7 only. The two texturing processes can be carried out simultaneously or sequentially. [0031] Where the height of the resin is varied, the luminance at different parts of the lightguide depends on how much resin is available to fill the applied surface texture. The shape of the surface texture can be random and non-repeating - such as would be formed when sand blasting or chemical etching a metal embossing master. When the surface texture is a periodic pattern, the shape of the repeating microstructure can be, but is not limited to, prisms, pyramids, cones or microlenses. In the case of periodic microstructures, the geometry of the microstructure can be optimised to maximise luminance in conjunction with the distribution of the light sources. Optionally, the spatial distribution of the random or periodic or other microstructures can be uniform or varied. In one embodiment, a flat metal (or other suitable material) shim is patterned and wrapped around an embossing drum. Alternatively the drum can itself be selectively patterned using, any of the known methods in the art. These and other techniques would be understood by those skilled in the art. In this way the degree of light extraction is determined by the distribution of the resin deposited by the high-resolution printing process and the distribution (and shape) of the surface texture applied to the patterned resin.

[0032] The surface texture can also be optimised to maximise luminance in conjunction with any additional optical films, such as, but not limited to, diffuser films and brightness enhancing films. The diffuser film 4 diffuses light. Generally, the diffuser film has a thickness of less than 1 mm, specifically less than or equal to 0.5 mm. As used herein, the terms "diffuse" or "diffusing" are intended to include light scattering or diffusion by reflection, refraction or diffraction from surface textures and/ or particles, and so forth. The brightness enhancing film 5 provides a high light collimation capability to give high lurniriance performance. The brightness enhancing film is typically a prism film, which can include a polymer base with a coating layer having a prismatic texture. The brightness enhancing film is commercially available from multiple sources such as Exxelis Ltd. In a further embodiment the backlight unit 2 includes a diffuser film 4 and brightness enhancing film 5 combined into a single film, such multi-functional films are also commercially available from multiple sources such as Exxelis Ltd.

Ϊ0033] In another embodiment the scattering efficiency of the lightguide is further improved through the introduction of nano- and micro-particles into the resin matrix; examples of particle additives include, but are not limited to, titanium- oxide (TiC ), Alumina (AI2O3), Silica (S1O2), ITO/ ATO or polymeric particles such as PMMA, polyimide and polycarbonate as well as combinations of the foregoing materials. In this embodiment the resin is preferably screen printed, but generally is deposited using any high resolution printing process that is compatible with a particulate resin. A higher scattering efficiency allows for a reduction in material where the scattering process is shared between the structure of the profile and optical diffusion in the volume of the profile. In an alternative embodiment, nano- and micro-particles dispersed in the resin can also comprise optical phosphors that emit light in the visible spectrum. Such phosphors can be deployed to add a colour cast to the emitted light, or to convert UV light from UV LEDs to visible light through the absorption of UV and re-emission in visible, such as yellow, light. Further, certain type of UV absorbers (such as UVA's and HALS) can also be included in the resin to prevent the yellowing of the lightguide. The introduction of nano- and micro-particles into the resin matrix is facilitated by the manufacturing process of the present invention, but incompatible with processes that rely on the structuring of bulk material such as injection moulding and hot-embossing.

[0034] The scattering efficiency of the lightguide is a function of the optical sources, the lightguide material, the resin and the surface texture. The volume distribution of resin can be optionally controlled by the properties of the resin and the surface of the substrate. The viscosity and surface energy of the resin may be tailored to promote the resolution, maximum deposited height and ease of printing. In a further embodiment, a surface treatment in the form of a layer of polymer or oligomer applied to the substrate prior to the application of the resin may also be used to promote the resolution, maximum deposited height and ease of printing through the control of the viscosity and surface tension. Such surface treatments can be applied using a reel-to-reel process, by spraying or any other process known in the state of the art. For alteration of the surface energy a resin formulation (such as that used above for application of the coating) can be modified using modifiers such as a silicone polyacrylate resign e.g. Tego ® Protect 5001 from Evonik Tego Chemie GmbH or an aliphatic urethane tetraacrylate oligomer e.g. Ebecryl ® 8100 from Cytec Surface Specialities Inc, and optionally with common organic solvents. The typical concentration of Tego Protect 5001 was in the range of 1% to 10% by weight, while Ebecryl 8100 was used in a concentration of around 30-70% by weight.

[0035] The manufacturing process of the invention combines the resin depositing and texturing steps and optionally the other steps described above in a single process system. The preferential process for depositing resin is inkjet printing. The printing head can be positioned prior to the texturing stage to rrurdmise the time between deposition and embossing thus irrinnrrising resin spread, loss of resolution and mamtaimng the deposited resin height.

[0036] In known lightguides the microstructured features cover a substantial portion of the lightguide. An aspect of the present invention is a lightguide manufactured using the process described above and deployed in a backlight where the area covered by the microstructured texture is considerably smaller than the area of the lightguide film. In these cases the iUurriination is only required from specific areas of the lightguide. For example, the microstructured texture can be a small icon or piece of text where the area of the text is considerably smaller than the area of the lightguide. It is a particular advantage of the process described above that the microstructure texture, which is required to vary in density, height, or in some other geometrical parameter, can be formed at a high resolution and efficiently in the specific small fraction of the lightguide area. It is a further advantage of the process described above that the high resolution patterning of the microstructure texture/ and especially text, allows for efficient scattering features with line widths of less than 500 micrometers and particularly less than 250 micrometers.

[0037] A backlight unit, consistent with the present invention, includes a lightguide of the present invention, an optical reflector below the lower surface of lightguide, an optional diffuser film above the lower surface of lightguide, an optional brightness enhancing film above the optional diffuser film and, in this embodiment, an opaque facia with transparent windows above the optional brightness enhancing film. The lightguide includes light sources optically coupled to the film that inject light into the film. The microstructured features scatter light out of the lightguide towards to the facia. Light scattered towards the base reflector is reflected towards the facia. The optional diffuser film acts to improve the uniformity of the illumination from the lightguide. The optional brightness enhancing film acts to improve the on-axis brightness of the illumination from the lightguide. In this example, the facia is optically opaque and includes apertures (voids or optically transparent areas) some of which correspond with the microstructured areas of the lightguide. In this embodiment the lightguide provides illumination through the apertures in the opaque facia, the opaque facia otherwise prevents stray light from reaching the viewer. The transparent areas in the facia may be in the form of icons, logos, text or other patterns or geometrical shapes. Alternatively in a different embodiment, the backlight unit does not include a facia and the illumination from the lightguide is directly viewed. In another different embodiment, the backlight unit does not include a facia and the illumination from the lightguide may provide iUumination direct to the viewer, or to a display system, such as one or more liquid crystal display (LCD) panels placed over the backlight unit.

[0038] An exploded view of a backlight unit 24 in accordance with an aspect of the present invention is shown in Figure 6. The backlight unit includes a lightguide film 25, formed from an upper and lower surface capable of guiding light through total interned reflection; an optical reflector 26 below the lower surface of lightguide, an optional diffuser film 27 above the lower surface of lightguide, an optional brightness enhancing film 28 above the optional diffuser film 27 and an opaque facia 29 above the optional brightness enhancing film 28. The facia 29 in this example is optically opaque and includes apertures (voids or optically transparent areas) 30 that correspond with the microstructured area 33 of the lightguide. The transparent areas 30 may be in the form of icons, text, logos or other patterns and geometrical shapes. The facia 29 may also include apertures 31 that have no corresponding microstructured area in the lightguide 25. These apertures 31 may be present in every film in the backlight unit 24 and may serve, for example, as alignment aids during the backlight assembly. The facia 29 may include further apertures 32 that have no corresponding microstructured area in the lightguide 25. These apertures 32 may be present in every film in the backlight unit and are illuminated areas of the backlight unit 24 that are not illuminated by the lightguide 25. In the exemplary drawing of Figure 6 these apertures 32 are iUurninated by further illumination sources (not shown) under the optical reflector 26. Thus the backlight unit facia 29 may contain multiple apertures of differing purpose, only some of which are iUurninated, and only some of which are illurninated by the lightguide 25. In another embodiment the optional facia is not required and the apertures 30 that correspond with the microstructured area 33 of the lightguide may provide Ulumination direct to the viewer, or illuminate a display system, such as a liquid crystal display (LCD) placed over the backlight unit.

[0039] The lightguide 25 in this example has a complex, non-rectilinear shape and includes light sources 34 optically coupled to the film that inject light into the film. The optical sources 34 are distributed as required around the lightguide 25 according to user constraints, and the layout of the facia aperture(s), whether illuminated by this backlight or not. Examples of light sources include light- emitting diodes (LED), top- and edge- LEDs, organic LED (OLEDs), or other point sources of light, including light sources using optical fibre or laser delivery. The microstructured features 33, which only cover a small fraction of the total lightguide area, scatter light out of the lightguide towards the facia 29, light scattered towards the base reflector 26 is reflected towards to facia 29. The optional diffuser film 27 acts to improve the uniformity of the illumination from the lightguide 25. The optional brightness enhancing film 28 acts to improve the on-axis brightness of the illumination from the lightguide 25.

[0040] A side view of the backlight unit 24 in accordance with the embodiment in Figure 6 is shown in Figure 7. In Figure 7a the backlight unit includes a lightguide film 25, an optical reflector 26 and an opaque facia 29 with optically transparent apertures 30 above the microstructured area 37 of the lightguide 25. The lightguide includes light sources 34 optically coupled to the film that inject light into the film. In Figure 7 an illustrative light ray 35 is guided within the lightguide through total internal reflection. A different illustrative ray 36 is shown incident on the microstructured area 37. In this example the ray 36 is shown scattered out of the lightguide 25 via reflection. The ray 36 passes through the optically transparent aperture 30 in the opaque facia 29. A further ray 38 is shown incident on the microstructured area 37. In this example the ray 38 is shown scattered out of the lightguide 25 via refraction and is incident on the optical reflector 26. After reflection, the ray 38 is incident on the opaque facia 29 and absorbed. In Figure 7b the backlight unit includes a lightguide film 25, an optical reflector 26, an optional diffuser film 27, an optional brightness enhancing film 28 above the optional diffuser film 27 and an opaque facia 29 with optically transparent windows 30 above the optional brightness enhancing film 28. The lightguide includes light sources 34 optically coupled to the film that inject light into the film. An illustrative ray 39 is shown incident on the microstructured area 37. In this example the ray 39 is shown scattered out of the lightguide 25 via reflection. The ray 39 passes through the diffuser film 27 and is randomly scattered towards the brightness enhancing film 28 according to the diffusion strength of the diffuser film. The brightness enhancing film 28 acts to collimate the light. The ray 39 passes through the optically transparent aperture 30 in the opaque facia 29. To illustrate the advantage of the diffuser film 27 a further ray 40 is shown incident on the microstructured area 37. In this example the ray 37 is shown scattered out of the lightguide 25 via refraction and is incident on the diffuser film 27. In Figure 7a in the absence of the diffuser film 27 the ray 38 is incident on the opaque facia 29. In Figure 7b the ray 40 passes through the diffuser film 27 and is randomly scattered towards the brightness enhancing film 28 according to the diffusion strength of the diffuser. The brightness enhancing film 28 acts to collimate the light. The ray 40 passes through the optically transparent aperture 30 in the opaque facia 29.

Claims

1. A method of making a lightguide, comprising the steps of (a) providing a lightguide film having upper and lower surfaces allowing substantial total internal reflection of light therebetween; (b) applying a viscous and curable material at least to the lower surface in a pattern; (c) applying a surface texture to the material; and (d) curing the material, the resulting surface textured pattern on the base of the lightguide being capable of extracting light from the lightguide.

2. A method according to claim 1, wherein the material applied in step (b) comprises a resin.

3. A method according to claim 1 or 2, wherein the material applied in step (b) includes scattering nanoparticles to promote scattering.

4. A method according to claim 1, 2 or 3, wherein the material applied in step (b) includes a UV-degradan^n-liming additive.

5. A method according to any preceding claim, wherein step (b) comprises a high resolution printing process.

6. A method according to claim 5, wherein step (b) comprises inkjet printing.

7. A method according to claim 5, wherein step (b) comprises screen printing.

8. A method according to any preceding claim, wherein the quantity of material applied in step (b) varies over the surface.

9. A method according to claim 8, wherein in step (b) the material is applied only to a part of the lightguide from which light is to be extracted.

10. A method according to any preceding claim, wherein prior to step (b), a surface modifying agent, e.g. a polymer or oligomer, is applied to the surface.

11. A method according to any preceding claim, wherein the texture applied in step (c) is arranged to cooperate with the pattern of material to enhance light scattering.

12. A method according to any preceding claim, wherein the texture is uniform over the surface.

13. A method according to any one of claims 1 to 11, wherein the texture varies over the surface.

14. A method according to any preceding claim, wherein the texture comprises a randomly roughened profile.

15. A method according to any preceding claim, wherein the texture comprises periodic geometric microstructures, for example prisms, pyramids, cones and/ or microlenses.

16. A method according to any preceding claim, wherein step (c) comprises reel-to-reel embossing.

17. A method according to any preceding claim, wherein step (c) comprises stamping embossing.

18. A method according to any preceding claim, wherein steps (b), (c) and (d) are performed on both upper and lower surfaces of the lightguide film.

19. A method according to any preceding claim wherein the surface textured pattern has line widths of less than 500 micrometers and in particular less than 250 micrometers.

20. A backlight comprising an opaque facia including a plurality of transparent windows; a lightguide made according to the method of any preceding claim, providing illumination solely in defined areas; and light sources arranged at the periphery of the lightguide.

21. A backlight according to claim 19, including an opaque facia having apertures corresponding to said defined areas.

22. A backlight according to any one of claim 20, wherein the facia is of non- rectangular shape.

23. A backlight according to claim 19, 20 or 21, wherein said defined areas amount to less than 50% of the area of the lightguide.

24. A backlight according to any one of claims 19 to 22, wherein said defined areas define text and/ or icons.

25. A backlight according to any one of claims 19 to 23, including a reflecting film arranged behind the lightguide.