WO2008110659A1

WO2008110659A1 - Light distributing device for providing a color effect

Info

Publication number: WO2008110659A1
Application number: PCT/FI2007/050133
Authority: WO
Inventors: Tapani Levola; Pasi Saarikko; Pauli Laitinen
Original assignee: Nokia Corporation
Priority date: 2007-03-14
Filing date: 2007-03-14
Publication date: 2008-09-18

Abstract

A light guide (100) comprises a waveguiding substrate (7), an input surface (10) to couple multi-color light (B0) into said substrate (7) to form a multi-color light beam (WL0) propagating in said substrate (7), a diffractive grating (50) to diffract light of said multi-color light beam (WL0) in order to provide a first diffracted light beam (BL1) and a second diffracted light beam (RGL0) waveguided in said substrate (7) in different directions, said first diffracted light beam (BL1) having a different color than said second diffracted light beam (RGL0), and an out-coupling element (21) to couple light of said first diffracted light beam (BL1) out of said substrate (7). The grating period (d) of the grating (50) may be e.g. in the range of 120 to 250 nm. The combination of the light guide (100) and a white light-emitting diode (90) may be adapted to provide colorful back or front-lighting for a key set (300).

Description

LIGHT DISTRIBUTING DEVICE FOR PROVIDING A COLOR EFFECT

FIELD OF THE INVENTION

The present invention relates to distributing light by using a light guide.

BACKGROUND

Planar waveguides are cost-effective devices to provide lighting for e.g. liquid crystal displays or keysets. Light which is initially provided e.g. by an external light emitting diode (LED) may be distributed to a larger area by means of a planar waveguide. The use of thin planar waveguides may facilitate reducing size, weight and manufacturing costs of a portable device.

US patent 6,598,987 discloses a light guide which is used for illuminating a user interface of an electronic device. The light guide is formed of a thin film which comprises input and output diffraction gratings.

SUMMARY

The object of the present invention is to provide a light distributing device. The object of the present invention is also to provide a method for distributing light.

According to a first aspect of the invention, there is provided a light guide according to claim 1.

According to a second aspect of the invention, there is provided a combination of a light guide and a light source according to claim 12.

According to a third aspect of the invention, there is provided a device comprising a light guide and a key set according to claim 14.

According to a fourth aspect of the invention, there is provided a method for distributing light according to claim 16. According to a fifth aspect of the invention, there is provided a light distributing means according to claim 18.

The dependent claims define further embodiments of the invention.

A light guide comprises a waveguiding substrate, a sub-wavelength grating and an out-coupling element. Multi-color light provided by a light source is coupled into the substrate through an input surface. The sub-wavelength grating is adapted to separate at least one color component from the multi-color light propagating in the light guide. The separated color component is coupled out of the light guide by the out- coupling element to provide a visual color effect.

Multi-color light comprises herein also white light, as white light may comprise e.g. red, green and blue components, or a continuous band of wavelengths.

The color components of in-coupled light may be separated from each other and directed wavelength-selectively towards different out- coupling elements by the same grating. The grating may also be used to direct and/or concentrate colored light into a predetermined out- coupling element

Diffracted light is not coupled out of the light guide by the sub- wavelength grating to a significant extent. Therefore the color of the light coupled out of the substrate by the out-coupling elements may be substantially independent of the viewing angle, i.e. independent of the orientation of the light guide with respect to a viewer.

The use of colored dyes or colored filters may be avoided by using the sub-wavelength grating. This facilitates manufacturing of the light guide. This may also facilitate recycling as the light guide may be implemented by using only one material.

In an embodiment, several colors may be provided by using only one light source. In particular, a single white light-emitting diode may be used to simultaneously provide a white light-emitting region, a blue light-emitting region, a yellow light-emitting region, a green light- emitting region and/or a red light-emitting region, said regions being spatially separate.

In an embodiment, a thin keypad and/or a thin display may be implemented. The light guide may be used to provide front light and/or back light for a key set and/or a display.

The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings in which

Fig. 1 shows, in a three dimensional view, angles associated with diffraction of light rays from a diffraction grating,

Fig. 2 shows diffraction efficiency of a sub-wavelength grating as a function of wavelength of light diffracted in the reflective diffraction order -1 ,

Fig. 3 shows, in a top view, a light guide comprising out-coupling elements and a sub-wavelength grating to separate color components of a multi-color light beam,

Fig. 4 shows, in a side view, coupling of multi-color light into a light guide through a side face of the light guide,

Fig. 5 shows, in a top view, a light guide comprising out-coupling elements to provide several different colors, Fig. 6 shows, in a top view, a light guide comprising a sub- wavelength grating, said grating comprising curved diffractive features to focus or collimate light towards a predetermined area,

Fig. 7 shows, in a side view, coupling of multi-color light into a light guide through a planar surface of the light guide,

Fig. 8 shows, in a three dimensional view, a device comprising a light guide to provide illumination for a key set, and

Fig. 9 shows, in a three dimensional view, a light guide comprising a waveguiding ridge.

DETAILED DESCRIPTION

Referring to Fig. 1 , a light ray LR|_N may impinge on a diffraction grating 50 in order to provide one or more diffracted light rays LR_D|_FF. The direction of the impinging light ray LR|_N may be defined by the input zenith angle Oj and the input azimuthal angle q>|. The input zenith angle Oj is the angle between the direction of the incident light ray LR|_N and the normal NR1 of the grating 50. The input azimuthal angle ψj is the angle between a direction SG and the projection PO of the impinging light ray LR_!N on the grating 50. The direction SG is perpendicular to the diffractive features 5 of the grating, and the direction SG is in the plane of the grating. The output zenith angle θ_q is the wavelength- dependent angle between the diffracted light ray LR_D|_FF and the normal NR1. The output azimuthal angle φ_q is the angle between a direction SG and the projection of the diffracted light ray LR_D|_FF on the grating 50 (not shown in Fig. 1).

The grating 50 comprises diffractive features 5 which have a grating period d at least in one direction. The grating 50 is implemented on an interface 6 which is between a substantially transparent substrate 7 and a second medium 4. The substrate 7 has a refractive index U₁. The second medium 4 may have a refractive index n₂ such that n₂ < n-i, in order to implement a waveguiding substrate 7. The substrate 7 may be e.g. polymethylmethacrylate (PMMA) having a refractive index of 1.49, polyvinylidene chloride, polysulfone resin, or glass. The second medium 4 may be e.g. air having refractive index of 1.00. The second medium 4 may also be e.g. fluoropolymer having a refractive index in the range of 1.30 to 1.40, e.g. polytetrafluoroethylene, fluorinated ethylene propylene, or perfluoroalkoxy.

The direction SY is perpendicular to the direction SX, and the direction SZ is perpendicular to the directions SX and SY. The grating 50 is in a plane defined by the directions SX and SY. The direction SG may deviate from the direction SX.

The grating equation (1) in its general form describes the direction of the diffracted light ray LR_D|_FF as a function of the direction of the impinging ray LR|_N and the diffraction order q:

k²n₂ ² UΑ² {θ_q ) = k²n² sin² (θ_t ) cos² (φ_t ) + [k²n² sin(0,- ) sinfø ) + 2πqldj

(1 )

k denotes the magnitude of the wave vector, k is equal to 2π/λ₀. λ₀ denotes the wavelength of light in vacuum, n-i denotes the refractive index on the side of the impinging ray LR|_N n₂ denotes the refractive index on the side of the diffracted ray LR_D|_FF.- q denotes the order of diffraction, d denotes the grating period of the grating 50.

When the direction SG is parallel to the direction SX, then the wave vector of the diffracted ray LR_D|_FF has a component α_q in the direction SX, a component β_q in the direction SY, and a component γ_q in the direction SZ. The magnitudes of said components may be expressed as:

a_q = kn₂ sin(0_? ) cos(^ ) (2) β_a = kn₂ Un{θ_a)ύn{φ_cl) (3) χ_q = kn₂ cos(θ_q) (4)

The following diffraction equations may be written for the components in the directions SX, SY and SZ:

kn₂ sin(#₀ ) cos(#>_o ) = kri_j sin(^ ) costø ) + -^- (5)

^H d

kn₂ ύn{θ_q ) ύn{φ_q ) = krij sm(θ_t ) sintø ) (6)

føz₂ cos(0₉ ) = ± ^fc²«| - a_q - β_q (7)

When the azimuthal angle ψ_\ is substantially equal to zero, equations (5) to (7) may be reduced into the form:

fc«2 cos((9_? ) = ± -^k²H₂ - a_q (10)

When the diffracted ray LR_D|_FF propagates in the same medium as the impinging ray LR_!N then eq. (8) is further reduced into the "familiar" form of the grating equation:

It may be desired that light is not coupled out of the waveguiding substrate 7 by the grating 50. This object may be attained by selecting the grating period d of the grating 50 to be smaller than or equal to a maximum grating period d_m. Setting q=-1 and θ_q = -90° in eq. (5) gives:

Eq. (12) corresponds to an out-coupled ray diffracted in the order -1 propagating along the interface 6. The maximum grating period d_m may be solved from eq. (12):

d_m = ^ (13)

The output azimuthal angle φ_q may be solved from eq. (6) by setting θ_q = -90°. λ_b is the shortest wavelength that is desired be confined within the substrate 7.

The wavelength λ_b may be selected according to the spectrum of the multi-color light impinging on the grating 50. The wavelength λ_b may be selected to be e.g. substantially equal to 400 nm in order to correspond to the limit of the wavelengths visible to the human eye. However, the sensitivity of the human eye to the wavelengths in the range of 400 to 450 nm is so low, that it may be acceptable that light having wavelength shorter than 450 nm is partially coupled out of the substrate 7. Thus, the wavelength λ_b may be selected to be e.g. substantially equal to 450 nm. Yet, if the light source provides wavelengths only in a predetermined range, e.g. in the range of 500 to 700 nm, the wavelength λ_b may be selected to be substantially equal to the shorter limit of said range, i.e. 500 nm.

For example, when θj = 45°, φι = 0°, n_λ = 1.49, n₂=1.00, and λ_b= 450 nm, then the maximum grating period d_m given by eg. (13) is 219 nm. Said refractive indices n-i and n₂ correspond to an interface 6 between polymethylmethacrylate and air.

When considering diffraction back into the substrate 7 in the order -1 , the maximum cutoff wavelength λ_Cuτ_,M corresponding to the maximum grating period d_m may be solved from eq. (12) by setting n₂ ⁼n₁: ^λcm_,M = (sin(^) COS(^₁-) + cos(φ_q ))njd_m (14)

The output azimuthal angle φ_q may be solved e.g. from eqs. (5) to (7).

When θj = 45°, φ, = 0°, n_ή = 1.49, n₂=1.00, and d_m = 219 nm, then the maximum cutoff wavelength λ_Cuτ_,M given by eq. (14) is 557 nm.

The cutoff wavelength λ_Cuτ may be selected to be shorter than or equal to the maximum value λ_Cuτ_,M- Reflected diffraction in the order -1 ceases to exist when the wavelength λ₀ of the impinging ray LR|_N is greater than a cutoff wavelength λ_Cuτ- Light which has a wavelength λ₀ longer than the cutoff wavelength λ_Cuτ is diffracted in the zeroth order only back into the substrate 7. Light which has a wavelength λ₀ shorter than the cutoff wavelength λ_Cuτ may be diffracted in the orders -1 and 0 back into the substrate 7.

The grating period d of the grating 50 may be shorter than maximum grating period d_m given by eq. (13). A grating period d corresponding to a desired cutoff wavelength λ_Cuτ may be solved from eq. (15):

d = ^ (15)

HJ (sin(0_j ) COStø; ) + COS(^ ))

The wavelength range of visible light is typically considered to be from 400 nm to 760 nm. The desired cutoff wavelength λ_Cuτ of the grating may be selected to be e.g. in the range of 440 to 640 nm in order to separate at least one color component from multi-color light.

If θ| = 45°, φι = 0°, and n-i = 1.49, then the grating period d of the grating 50 may be selected to be approximately equal to λ_Cuτ/2.5 Thus, the grating period d may be e.g. in the range of 170 to 250 nm in order to provide a cutoff wavelength λ_Cuτ which is in the range of 440 to 640 nm. The grating period d corresponding to the 440 nm cutoff limit λ_Cuτ may be even smaller for an input azimuth angle φ_\ deviating from zero, for a large input zenith angle θj and/or for a substrate having a high refractive index. For example, if θj = 70°, ψ_\ = 0°, and n-i = 1.88, then the grating period d of the grating 50 may be selected to be approximately equal to λ_Cuτ/3.6. Thus, the grating period d may be e.g. as low as 120 nm in order to provide a cutoff wavelength λ_Cuτ of 440 nm.

Thus, the grating period d of a sub-wavelength grating 50 may be e.g. in the range of 120 to 250 nm. The sub-wavelength grating 50 may act as a low-pass filter for light diffracted in the zeroth order. Wavelengths shorter than λ_Cuτ may be removed to a considerable extent from the light diffracted in the zeroth order. The sub-wavelength grating 50 may act as a high-pass filter for light diffracted in the order -1 , respectively. Wavelengths longer than λ_Cuτ are substantially removed from the light diffracted in the order -1. Thus, the diffraction grating 50 may act as a filter to separate one or more color components of multi-color light.

Multi-color means herein that the light comprises at least a first color component and a second color component such that their separation is at least 20 nm. In particular, the multi-color light may be substantially white light. White light may have e.g. red, green and blue components provided by one or more emitters. White light, e.g. blackbody radiation, may also have a substantially continuous spectrum, but also in that case it comprises an infinite number of spectral components which are separated by more than 20 nm from each other.

The cutoff wavelength λ_Cuτ of the grating 50 may be selected to be between wavelengths of said color components. A desired cutoff wavelength λ_Cuτ may be implemented, respectively, by selecting the input zenith angle θj, the input azimuthal angle φι, the grating period d, and the refractive index n-i of the substrate 7 according to eq. (16):

X_cuτ = (sintø ) cosfø ) + cos(^ ))n_jd (16)

A light beam impinging on the grating 50 may be diverging instead of being a collimated beam. To the first approximation, the situation may be governed by considering the diffraction of a single impinging light ray LR|_N which propagates in the direction of said light beam. The direction of a light beam means the average direction of light rays constituting said light beam.

For an individual light ray LR|_N or for a collimated light beam, substantially all optical power diffracted in the order -1 is at wavelengths shorter than the cutoff wavelength λ_Cuτ- For a diverging beam a fraction of optical power diffracted in the order -1 may also be at wavelengths longer than the cutoff wavelength λ_Cuτ- However, even in that case the majority of the optical power diffracted in the order -1 is at wavelengths shorter than said cutoff wavelength λ_Cuτ-

Fig. 2 shows, by way of example, diffraction efficiency Eff of the diffraction order -1 as a function of wavelength. The light ray LR_!N impinges on a binary surface relief grating 50 implemented on a PMMA substrate 7. The second medium is air. The impinging light comes from the substrate side and is diffracted back into said substrate 7. The grating period d of the grating 50 is 200 nm and the light impinges on the grating at an input zenith angle θj of 55 degrees and at an input azimuthal angle φι of 20°. It may be noticed that the cutoff wavelength λ_Cuτ for said arrangement is approximately 520 nm. The diffraction efficiency Eff means the ratio of diffracted optical power to the optical power impinging on the grating at a predetermined wavelength.

Referring to Fig. 3, a light beam BO provided by a light source 90 may be coupled into a light guide 100 through an input surface 10 to form an in-coupled multi-color light beam WLO. Light rays of the multi-color beam WLO impinge on the grating 50, and the diffracted rays provide a first diffracted beam BL1 and a second diffracted beam RGLO. The first diffracted beam comprises light rays diffracted in the diffraction order -1 and the second diffracted beam comprises light rays diffracted in the zeroth diffraction order. The multi-color beam WLO may be e.g. white light, the first diffracted beam BL1 may be e.g. blue light, and the second diffracted beam RGLO may comprise the remaining red and green components of the white light. The light of the beam RGLO may appear yellow. The light beam BO may be provided e.g. by a white light-emitting diode (LED). The light source 90 may be a combination of a first LED having a first color and a second LED having a second color. The light source 90 may also be an incandescent, fluorescent or gas discharge lamp.

The first diffracted beam BL1 propagates in a different direction than the second diffracted beam RGLO. In particular, the projection of the first diffracted beam BL1 on the SX-SY-plane may have a different direction than the projection of the second diffracted beam RGLO. An angle δ between said projections may be e.g. in the range of 90 to 180 degrees. In particular, the angle δ may be in the range of 120 to 160 degrees. The angle δ may be or even in the range of 160 to 180 degrees to direct the first diffracted beam BL1 substantially backwards with respect to the multi-color beam WLO.

For comparison, the direction of a light beam may also be changed by using e.g. die-cut openings which reflect light by total internal reflection. However, a prerequisite for total internal reflection is that the solid-gas interface is substantially inclined with respect to the impinging light beam, which limits the applicability of said openings to direct light backwards with respect to the impinging light beam.

The diffracted beams may be spatially separate. Consequently, the out-coupling element 21 may couple light of the first diffracted beam BL1 out of the plane of the substrate 7 such that the light of the second diffracted beam RGL1 is substantially not coupled out by said element 21. The second out-coupling element 22 may couple light of the second diffracted beam RGL1 out of the plane of the substrate 7 such that the light of the first diffracted beam RGL1 is substantially not coupled out by said element 22.

In order to implement e.g. a light distributing device 100 to illuminate a keyset and/or display (Fig. 8), the width b1 and/or the length a1 of the planar waveguide may be e.g. in the range of 5 to 100 mm. The sum of the areas of the out-coupling elements 21 , 22 may be e.g. greater than 5 % of the one-sided area of the substrate 7. Referring to Fig. 4, the light guide 100 comprises a waveguiding substrate 7, which may comprise two substantially planar and substantially parallel surfaces. Waveguided light is confined within the substrate 7 by total internal reflections (TIR).

The light BO of the light source 90 may be coupled into the light guide 100 through a side face. The input surface 10 may be substantially smooth, it may comprise a grating, or it may comprise an array of prisms. The prisms or the grating may be adapted to collimate a diverging beam BO provided by a light source 90. The input surface 10 may be substantially perpendicular to the planar surfaces 6a, 6b of the substrate 7.

The absolute thickness hi of a substantially planar substrate 7 may be e.g. in the range of 0.2 to 0.5 mm. In order to implement light and/or flexible structures, the thickness hi may be in the range of 0.1 to 0.2 mm. In order to implement very light and/or flexible structures, the thickness hi may be in the range of 0.05 to 0.1 mm.

The substrate 7 may be of substantially transparent material, e.g. polycarbonate, polymethylmethacrylate (PMMA) or acrylic. The substrate 7 may be perfectly planar or slightly bent, e.g. cylindrically or spherically bent. The substrate 7 may be of stiff material or of flexible material.

Light of the first diffracted beam BL1 may be coupled out of the substrate by a first out-coupling element 21 , and/or light of the second diffracted beam RGLO may be coupled out of the light guide 100 by a second out-coupling element 22. The first out-coupling element 21 may provide a beam B1 and the second out-coupling element 22 may provide a beam B2. The beams B1 and B2 have different colors. The out-coupled beams B1 and B2 may be viewed by a human viewer (not shown).

An out-coupling element 21 , 22 may be e.g. a diffractive grating, a prism, an array of prisms, a diffusing surface, or a mirror embossed, molded or attached on, or embedded in the substrate 7. The out- coupling element 21 , 22 may also be a substantially transparent object which is in contact with the surface of the substrate 7 causing local frustration of total internal reflection. The diffraction gratings of out- coupling elements 21 , 22 may have a grating constant d selected e.g. from the range of 0.2 - 4 μm.

The grating 50 and/or the out-coupling elements 21 , 22 may be implemented on the same planar surface 6a, 6b of the light guide 100. The grating 50 may be e.g. a surface relief grating. The surface relief grating 50 may be slanted.

The light beam B1 and/or the light beam B2 may also be transmitted through the substrate 7 (not shown in Fig. 4).

The grating 50 and/or the out-coupling elements 21 , 22 may also be implemented on different planar surfaces 6a, 6b of the light guide 100 (not shown). The grating 50 and/or the out-coupling elements 21 , 22 may also be embedded in the substrate 7 (not shown).

Referring to Fig. 5, the light guide 100 may comprise a grating 50 to separate a blue component BL1 from white light WLO. The cutoff wavelength λ_Cuτ of the grating 50 may be adapted to be e.g. 490 nm. The blue light BL1 may be coupled out of the substrate 7 by an out- coupling element 21. The remaining red and green components RGLO may be coupled out of the substrate 7 by an out-coupling element 22. Light provided by the out-coupling element 22 may appear yellow.

The light guide 100 may further comprise one or more out-coupling elements 20 to couple undiffracted white light WLO out of the plane of the substrate 7.

The light guide 100 may further comprise a second grating 51 to separate a blue light BL1 from the white light WLO. Said blue light may be directed towards the out-coupling element 21 to further increase the intensity of blue light provided by said element 21. The cutoff wavelength λ_Cuτ of the grating 51 may be adapted to be e.g. in the range of 485 to 500 nm. In particular, the cutoff wavelength λ_Cuτ of said grating 51 may be substantially equal to 490 nm.

The red and green light RGLO remaining after the second grating 51 may be separated into a green component GL1 and a red component

RLO by a third grating 52. The green light GL1 may be coupled out of the substrate 7 by an out-coupling element 23, and the red component

RLO may be coupled out of the substrate 7 by an out-coupling element

24. The cutoff wavelength λ_Cuτ of the grating 52 may be adapted to be e.g. in the range of 565 to 625 nm. In particular, the cutoff wavelength λ_Cuτ of said grating 52 may be substantially equal to 590 nm.

When determining the maximum grating period d_m of the third grating 52, the shortest wavelength λ_B inserted in eq. (13) may be substantially equal to the cutoff wavelength λ_Cuτ of the second grating 51.

Referring to Fig. 6, the grating 50 may comprise curved diffractive features 5 to concentrate light diffracted in the diffraction order -1 towards a predetermined out-coupling element 21. In particular, said grating 50 may be used to increase the intensity provided by the out- coupling element 21 when the multi-color beam WLO is diverging. Said grating 50 may provide a beam BL1 which is collimated, focused, or diverging.

Referring to Fig. 7, the input surface 10 may also be on the planar surface 6a or 6b of the substrate 7, instead of being on the side face 3. The input surface 10 may be e.g. a grating or an array of prisms.

Referring to Fig. 8, a device 900 may comprise a light guide 100 to provide illumination for a key set 300. An out-coupling element may be adapted to illuminate a pattern, e.g. a number or a symbol, which is associated with a function of a switch of the key set 300. The keyset

300 may be a keypad or a keyboard. The light guide 100 may comprise a plurality of out-coupling elements 20, 21 , 22. The location of the out- coupling elements 20, 21 , 22 may be associated with the locations of the switches or proximity sensors of the key set 300. Touch-sensitive elements or switches of the key set 300 may be positioned under the back side of light guide 100, as shown in Fig. 8, if the light guide 100 is at least partially flexible. A set of proximity sensors may be positioned under the light guide 100. Alternatively, at least partially transparent touch-sensitive elements, switches and or proximity sensors may be positioned on the top of the light guide 100 (not shown in Fig. 8). The light guide 100 may be an integrated part of an illuminated key set 300.

The device 900 may further comprise a battery, data processing and/or telecommunications module 500. The device 900 may further comprise a display 400. The device 900 may be portable. The device 900 may comprise telecommunications capabilities. The device 900 may be e.g. a mobile phone, and/or a computer.

Yet, the device 900 may be a personal digital assistant (PDA), a communicator, a navigation instrument, a digital camera, a video recording/playback device, an electronic wallet, an electronic ticket, an audio recording/playback device, a game device, a measuring instrument, and/or a controller for a machine.

The light guide 100 may comprise one or more openings or indentations 42 to reduce coupling of light from a first area of the substrate 7 to a second area of the substrate, for example in order to prevent leaking of yellow light to the white-emitting out-coupling elements 20. The openings 42 may be implemented e.g. by die-cutting.

One or more light guides 100 may be used to provide front and/or back lighting to e.g. a liquid crystal (LCD) display 400 or a MEMS display (Micro-Electro-Mechanical System).

Referring to Fig. 9, the light guide 100 may further comprise one or more waveguiding ridges 8 to facilitate coupling of light into the light guide 100 through an input end of said ridge and/or in order to distribute light to desired portions 7a, 7b substrate 7. The height of said ridge 8 may be greater than the thickness hi of the substrate portions 7a, 7b. One or more of the portions 7a, 7b may be substantially planar. The ridge 8 may be straight, curved, bifurcated and/or tapered. One or more gratings 51 , 52 may be implemented on the ridge 8 and/or on the planar portions 7a, 7b. For example, white light WLO coupled into the ridge 8 through the input surface 10 impinges on a first grating 51 and is split into blue light BL1 and red-green light RGLO. The blue light BL1 is coupled out of the substrate 7 by an out-coupling element 21. The red-green light RGLO waveguided in the ridge 8 is distributed into the planar portions 7a, 7b through a common portion between the ridge and said portions 7a, 7b. The red-green light RGLO may be coupled out of the substrate 7 by out-coupling elements 22. A part of the red-green light RGLO may impinge on a second grating 52 which splits the red- green light RGLO into a red light RLO and green light GL1. The green light GL1 may be coupled out by an out-coupling element 23 and the red light RLO may be coupled out by an out-coupling element 24.

The light guide 100 may comprise one or more protrusions (not shown) to protect the out-coupling elements 20, 21 , 22.

A waveguiding core of the light guide 100, and in particular one or more of its planar surfaces may be covered with a cladding layer which has lower refractive index than said core. The cladding may comprise e.g. fluoropolymer, in particular polytetrafluoroethylene.

The light guide 100 may also be used to provide light-emitting signs. The signs may be e.g. extremely lightweight "fasten seatbelt" signs for airplanes, or luminous highway traffic signs. In other words, a light- emitting sign may comprise a light guide 100, wherein the visual appearance of a light emitting portion or portions may be defined by the perimeter of one or more out-coupling elements 20, 21 , 22, or by a mask superposed over one or more out-coupling elements.

For the person skilled in the art, it will be clear that modifications and variations of the devices and method according to the present invention are perceivable. All drawings are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

1. A device (100) comprising:

- a waveguiding substrate (7), - an input surface (10) to couple multi-color light (BO) into said substrate (7) to form a multi-color light beam (WLO) propagating in said substrate (7),

- a diffractive grating (50) to diffract light of said multi-color light beam (WLO) in order to provide a first diffracted light beam (BL1) and a second diffracted light beam (RGLO) waveguided in said substrate (7) in different directions such that said first diffracted light beam (BL1) has a different color than said second diffracted light beam (RGLO), and

- an out-coupling element (21) to couple light of said first diffracted light beam (BL1 ) out of said substrate (7).

2. The device (100) of claim 1 wherein the grating period (d) of said grating (50) is in the range of 120 to 250 nm.

3. The device (100) according to claim 1 or 2 comprising a second out- coupling element (22) to couple light of said second diffracted light beam (RGLO) out of said substrate (7).

4. The device (100) according to any of the claims 1 to 3 comprising a further out-coupling element (20) to couple undiffracted light of said multi-color light beam (WLO) out of said substrate (7).

5. The device (100) according to any of the claims 1 to 4 wherein said grating (50) is a surface relief grating implemented on a surface of said substrate (7).

6. The device (100) according to any of the claims 1 to 5 wherein said substrate (7) is substantially planar.

7. The device (100) according to any of the claims 1 to 6 further comprising a waveguiding ridge (8) to distribute light into a substantially planar portion (7a, 7b) of said substrate (7).

8. The device (100) according to any of the claims 1 to 7 wherein said second diffracted light beam (RGLO) is adapted to be diffracted in the reflective order zero, and said first diffracted light beam (BL1) is adapted to be diffracted in the reflective order minus one.

9. The device (100) according to claim 8 wherein the grating period d of said grating (50) has been selected to correspond to a predetermined cutoff wavelength λ_Cuτ according to an equation

d = ^λcuτ

U₁ (sintø ) cosfø ) + cosfø₉ ))

where n-i is the refractive index of said substrate (7), n₂ is the refractive index of said second medium (4), θj is the zenith angle of a light ray (LR|_N) impinging on said grating (50), ψ_\ is the azimuth angle of said impinging light ray (LR|_N), and φ_q is the azimuth angle of a corresponding diffracted light ray (LR_D|_FF) diffracted in the zenith angle of -90 degrees in the diffraction order of -1 , said impinging light ray (LR|_N) having the same direction as said multi-color light beam (WLO), the majority of the optical power of said first diffracted light beam (BL1) being at wavelengths shorter than said cutoff wavelength λ_Cuτ-

10. The device (100) of any of the claims 1 to 9 wherein the grating period (d) of said grating (50) has been selected to substantially prevent out-coupling of visible light through said grating (50) at wavelengths longer than or equal to a predetermined wavelength limit (λ_B).

11. The device (100) of claim 10 wherein said grating (50) has been implemented on an interface (6) between said substrate (7) and a second medium (4), the grating period d of said grating (50) fulfilling a condition

d ≤

H₁ sin(#_j ) cos(<pi ) + n₂ cos(p^ ) where λ_b is said predetermined wavelength limit, n-i is the refractive index of said substrate (7), n₂ is the refractive index of said second medium (4), θj is the zenith angle of a light ray (LR|_N) impinging on said grating (50), ψj is the azimuth angle of said impinging light ray (LR_!N), and φ_q is the azimuth angle of a corresponding diffracted light ray (LR_D|_FF) diffracted in the zenith angle of -90 degrees in the diffraction order of -1 , said impinging light ray (LR|_N) having the same direction as said multi-color light beam (WLO).

12. A device (100) comprising:

- one or more light sources (90) to provide multi-color light (BO),

- a substantially planar waveguiding substrate (7),

- an input surface (10) to couple said multi-color light (BO) into said substrate (7) to form a multi-color light beam (WLO) propagating in said substrate (7),

- a diffractive grating (50) to diffract light of said multi-color light beam (WLO) in order to provide a first diffracted light beam (BL1) and a second diffracted light beam (RGLO) waveguided in said substrate (7) in different directions, said first diffracted light beam (BL1) having a different color than said second diffracted light beam (RGLO), and

13. The device (100) of claim 12 wherein the grating period (d) of said grating (50) is in the range of 120 to 250 nm.

14. A device (100) comprising:

- a key set (300),

- one or more light sources (90) to provide multi-color light (BO), - a substantially planar waveguiding substrate (7),

- an out-coupling element (21) to couple light of said first diffracted light beam (BL1 ) out of said substrate (7) to provide illumination for said key set (300).

15. The device (100) of claim 14 wherein the grating period (d) of said grating (50) is in the range of 120 to 250 nm.

16. A method for distributing light by using a substantially planar waveguiding substrate (7), a grating (50) and an out-coupling element (21), said method comprising:

- coupling multi-color light (BO) into said substrate (7) to form a multicolor light beam (WLO) propagating in said substrate (7), - diffracting light of said multi-color light beam (WLO) by said grating (50) to provide a first diffracted light beam (BL1 ) and a second diffracted light beam (RGLO) waveguided in said substrate (7) in different directions, said first diffracted light beam (BL1) having a different color than said second diffracted light beam (RGLO), and - coupling light of said first diffracted light beam (BL1) out of said substrate (7) by said out-coupling element (21).

17. The method of claim 16 wherein the grating period (d) of said grating (50) is in the range of 120 to 250 nm.

18. A light distributing means (100) comprising:

- a waveguiding means (7) for waveguiding light,

- an input means (10) for coupling multi-color light (BO) into said waveguiding means (7) to form a multi-color light beam (WLO) propagating in said waveguiding means (7),

- a diffractive means (50) for diffracting light of said multi-color light beam (WLO) in order to provide a first diffracted light beam (BL1 ) and a second diffracted light beam (RGLO) waveguided in said waveguiding means (7) in different directions such that said first diffracted light beam (BL1 ) has a different color than said second diffracted light beam (RGLO), and - an out-coupling means (21) for coupling light of said first diffracted light beam (BL1) out of said waveguiding means (7)

19. The light distributing means (100) of claim 18 wherein the grating period (d) of said diffractive means (50) is in the range of 120 to 250 nm.