WO2008059319A1

WO2008059319A1 - Spectrum-specific out-coupling of light

Info

Publication number: WO2008059319A1
Application number: PCT/IB2006/054318
Authority: WO
Inventors: Jyrki Kimmel; Pasi Saarikko; Tapani Levola
Original assignee: Nokia Corporation
Priority date: 2006-11-17
Filing date: 2006-11-17
Publication date: 2008-05-22

Abstract

This invention relates to an apparatus, comprising an interface configured to receive light with a first spectrum and light with a second spectrum; a medium in which the light with the first spectrum and the light with the second spectrum can propagate; and out-coupling units configured to separately couple out the light with the first spectrum and the light with the second spectrum from the medium at predetermined positions. The invention further relates to a method, comprising receiving light with a first spectrum and light with a second spectrum in a medium in which the light with the first spectrum and the light with the second spectrum can propagate; and separately coupling out the light with the first spectrum and the light with the second spectrum from the medium at predetermined positions via out-coupling units.

Description

Spectrum-Specific Out-Coupling of Light

FIELD OF THE INVENTION

This invention relates to an apparatus and a method in the context of spectrum-specific out-coupling of light.

BACKGROUND OF THE INVENTION

Out-coupling of light is for instance performed in backlights that are deployed in Liquid Crystal Displays (LCDs) and other light-modulating displays which may or may not be polarizing. In LCDs, a matrix of picture elements (pixels) is set up by means of a plurality of cells, wherein each cell, for instance a twisted nematic cell, acts as a controllable polarisation filter and thus can be electrically controlled to either block or let pass polarized light impinging on the cells from a backlight, which may for instance be an illuminated light guide plate. Therein, light from the backlight may for instance be polarized by a polarization filter. Light being passed by the cells can be perceived by a viewer, whereas blocked light is not perceived by the viewer. In this way, a black-and-white picture can be presented by the LCD.

United States Patent Application US 2005/0052732 Al discloses a light guide plate of rectangular cross- section suited for use as backlight in an LCD. Light is fed into the light guide plate from a light source via a light incidence surface of the light guide plate. To achieve that light is emitted from the light emitting surface, which is orthogonally positioned with respect to the light incidence surface, diffraction grating units are used on the light emitting surface.

Extending LCD capabilities to coloured presentation of pictures can for instance be achieved by using three or more interlaced pixel matrices, for instance one pixel matrix for the generation of red pixels, one pixel matrix for the generation of blue pixels, and one pixel matrix for the generation of green pixels. These three interlaced pixel matrices can be imagined as a single pixel matrix, which comprises a plurality of pixel groups, each pixel group comprising a red, green and blue pixel. As a backlight structure for this pixel matrix, usually a white backlight with a colour filter attached thereon is used, wherein this colour filter comprises small filter elements for letting pass either red, green and blue light and wherein these filter elements are arranged in a mosaic-like structure according to the desired pixel groups of the pixel matrix.

Adding colour filters to an LCD reduces efficiency, since each filter element lets pass only a small amount of the spectrum of white light that is impinging on the filter element. Furthermore, colour purity and gamut of the LCD then highly depend on the filter elements.

SUMMARY OF THE INVENTION

It is thus, inter alia, a general object of the present invention to provide an apparatus and a method that overcome some or all of the above-mentioned problems.

An apparatus is proposed, comprising an interface configured to receive light with a first spectrum and light with a second spectrum; a medium in which the light with the first spectrum and the light with the second spectrum can propagate; and out-coupling units configured to separately couple out the light with the first spectrum and the light with the second spectrum from the medium at predetermined positions.

Furthermore, an apparatus is proposed, the apparatus comprising means for receiving light with a first spectrum and light with a second spectrum; a medium in which the light with the first spectrum and the light with the second spectrum can propagate; and means for separately coupling out the light with the first spectrum and the light with the second spectrum from the medium at predetermined positions.

Finally, a method is proposed, the method comprising receiving light with a first spectrum and light with a second spectrum in a medium in which the light with the first spectrum and the light with the second spectrum can propagate; and separately coupling out the light with the first spectrum and the light with the second spectrum from the medium at predetermined positions via out- coupling units.

Therein, the light with the first spectrum and the light with the second spectrum may for instance be defined by respective spectra of contiguous or non-contiguous wavelengths in the visible and/or non-visible range. In case of contiguous spectra, the spectra may range from narrow to broad, may overlap or not, and may have centre wavelengths that range from being close to each other to being far from each other.

The light of the first spectrum and the light of the second spectrum may for instance be understood as light of a first colour and light of a second colour, respectively. For instance, the light with the first spectrum and light with the second spectrum may be one of red light with a wavelength of 630 nm, green light with a wavelength of 530 nm, and blue light with a wavelength of 460 nm.

The interface receives the light with the first spectrum and the light with the second spectrum. Therein, said interface may be configured to receive the light of the first spectrum and the light of the second spectrum jointly. This may for instance be accomplished by receiving the light with the first spectrum and the light with the second spectrum from a single light source. Alternatively, said interface may be configured to receive the light of the first spectrum and the light of the second spectrum separately. This may for instance be accomplished by receiving the light of the first spectrum and the light of the second spectrum from two different light sources. Therein, the interface may for instance be a surface of the medium. Equally well, the interface may comprise further in-coupling units for jointly or separately coupling the light with the first spectrum and the light with the second spectrum into the medium, such as diffraction gratings or other in-coupling units.

The medium may consist of one material or of a plurality of different materials, wherein the materials may have the same or different optical characteristics, such as for instance refractive indices in order to achieve total reflection at interfaces or surfaces of the medium. The medium may for instance be a waveguide. The out-coupling units are configured to couple the light with the first spectrum and the light with the second spectrum out separately. To this end, the out-coupling units may comprise one or more out-coupling units that only couple out the light with the first spectrum, and one or more out-coupling units that only couple out the light with the second spectrum. Therein, said out- coupling may be understood as a complete or partial out- coupling, for instance only certain wavelengths of the light with the first spectrum and only certain wavelengths of the light of the second spectrum may be coupled out. Furthermore, the light with the first spectrum and the light with the second spectrum are coupled out at predetermined positions, i.e. there are predetermined positions where only light with the first spectrum is coupled out and predetermined positions where only light with the second spectrum is coupled out. The predetermined positions may for instance be arranged in a geometrical structure.

Thus light of different spectra is received in a medium, propagated through the medium and separately coupled out of the medium at predetermined positions. At the predetermined positions, thus light of a specific spectrum, e.g. a specific colour, is available even without requiring colour filters, increasing efficiency and reducing both dimensions and costs of the apparatus.

The out-coupling units may comprise at least one diffraction grating for coupling out the light with the first spectrum, and at least one diffraction grating for coupling out the light with the second spectrum. In particular, the out-coupling units may comprise a plurality of diffraction gratings for coupling out the light with the first spectrum, and a plurality of out- coupling units for coupling out the light with the second spectrum. Therein, the diffraction gratings may be adapted to the spectrum of the light with the first spectrum and the spectrum of the light with the second spectrum, respectively, to allow efficient out-coupling with small coupling losses. In case of contiguous spectra, the diffraction gratings may for instance be adapted to the centre wavelength of the spectra. Furthermore, the diffraction gratings may be configured to couple out highly polarized light. The diffraction gratings may for instance be formed in or on the medium, for instance as embossed grooves. The grating elements of the diffraction gratings may have rectangular cross- section or any other form of cross-section.

The out-coupling units may be arranged according to a predetermined geometry to allow to couple out the light with the first spectrum and the light with the second spectrum at the predetermined positions.

The predetermined positions may substantially be determined by pixel positions of a pixel matrix. The pixel matrix may for instance be a pixel matrix of a display, e.g. an LCD. The out-coupling units then may be configured to separately couple out light with the first spectrum, e.g. light of a first colour, and light with the second spectrum, e.g. light of a second colour, at certain pixel positions in the pixel matrix. This may for instance allow to construct a two-coloured display without requiring colour filters, since light of the first and second colour may be separately received in the medium, propagates in the medium and then is separately coupled out by the out-coupling units. The out-coupling units may have at least one of a collimating property, an angle-restricting property and a polarization control property. Implementing these properties in the out-coupling units may allow dispensing with further optical means that may add size and complexity to the apparatus. For instance, if polarization control is implemented in the out-coupling units, an additional polarization filter may no longer be required.

The apparatus according to the present invention may further comprise separate light sources for generating the light with the first spectrum and the light with the second spectrum. The light sources may for instance be light sources having narrow wavelength band, for instance Light Emitting Diodes (LEDs) or semiconductor lasers or frequency-doubled solid-state lasers. The apparatus according to the first exemplary embodiment of the present invention then may allow confining, via the interface, the medium and the out-coupling units, light of a specific spectrum as originating from the light sources to predetermined positions. Alternatively, said apparatus may comprise a joint light source for generating the light with the first spectrum and the light with the second spectrum. The light source for separately or jointly generating the light of the first spectrum and the light of the second spectrum may be integrated into said apparatus, for instance, if said apparatus is a backlight medium, said light source (s) may be integrated into said backlight medium.

The apparatus according to the present invention may for instance be a backlight for a colour display or a part thereof. The apparatus according to the present invention may also be a module for a device with a colour display. Therein, the colour display may be an LCD display, or an electrowetting, electrochromic or electrophoretic display or any other type of light-modulating display which may or may not be polarizing. The apparatus according the present invention may also be configured to work as an illuminated keymat, keypad or keyboard or to illuminate icons or signs. The apparatus may be a part of a portable electronic device.

According to a first exemplary embodiment of the present invention, the interface comprises at least one diffraction grating for coupling the received light with the first spectrum into the medium, and at least one diffraction grating for coupling the received light with the second spectrum into the medium. Therein, the in- coupling of the light with the first spectrum and the light with the second spectrum may be performed separately or jointly. Therein, the diffraction gratings may be optimized for the spectrum of the respective light that is to be coupled into the medium. This allows to reduce the coupling losses. Alternatively, other coupling methods such as butt-coupling, or prism-coupling may be deployed.

This first exemplary embodiment of the present invention may further comprise distribution units configured to distribute the light with the first spectrum and the light with the second spectrum within the medium. Therein, distribution may be performed jointly or separately for the light with the first spectrum and the light with the second spectrum, respectively. The distribution units may for instance distribute the light received via the interface and coupled into the medium via the diffraction gratings. Therein, different distribution units may exist for distribution of the light with the first spectrum and the light with the second spectrum, and the distribution units may be adapted to the light with the first spectrum and to the light with the second spectrum, respectively. The out- coupling units then may receive the light distributed by the distribution units for separate out-coupling.

In this first exemplary embodiment of the present invention, the distribution units may have fan-out gratings for distributing the light with the first spectrum and the light with the second spectrum.

According to a second exemplary embodiment of the present invention, the interface is configured to receive the light with the first spectrum and the light with the second spectrum in substantially orthogonal directions. For instance, if the medium is a waveguide of rectangular cross-section, the light with the first spectrum and the light with the second spectrum may be received via surfaces of the waveguide that are orthogonal to each other .

In this second exemplary embodiment of the present invention, an orientation of the out-coupling unit for coupling out the light with the first spectrum may be substantially orthogonal to an orientation of the out- coupling unit for coupling out the light with the second spectrum. Separate out-coupling thus may be achieved by exploiting orthogonal propagation directions of light of different spectra within the medium. Therein, said out- coupling units may for instance be diffraction gratings. According to a third exemplary embodiment of the present invention, the medium comprises a substrate and a coating. The coating may for instance be arranged on, below, within or around the substrate. The substrate and the coating may for instance have different optical characteristics, such as different refraction indices. Therein, the medium may be configured in a way that total internal reflection of light may be achieved at either the interface between the substrate and the coating or at the outer surface of the coating. The interface at which the total internal reflection occurs may be controlled with the aid of the incidence angle of light that impinges on the medium. The light with the first spectrum and the light of the second spectrum may each consist of separate wavelength bands. These bands may interact with the interface structure and the outer surface structures of the medium in different ways. For example, a shorter wavelength part of the light with the first spectrum or the light with the second spectrum may be coupled out at the interface between substrate and coating, but not a longer wavelength part. The longer wavelength part may be coupled out from the structures at the outer surface of the medium. Therefore each colour, first or second, may be split in shorter and longer wavelength bands, that have different out-coupling characteristics.

In this third exemplary embodiment of the present invention, the medium may be configured in a way that propagation of the light with the first spectrum through an interface between the substrate and the coating is possible, that total reflection of the light with the first spectrum at the outer surface of the coating is supported and that total internal reflection of the light with the second spectrum at the interface between the substrate and the coating is supported.

In this third exemplary embodiment of the present invention, the out-coupling units may comprise at least one out-coupling unit that is configured to couple out the light with the first spectrum and that is substantially arranged on the outer surface of the coating, and at least one out-coupling unit that is configured to couple out the light with the second spectrum and that is substantially arranged on the interface between the substrate and the coating. Therein, said out-coupling unit arranged on the outer surface of the coating may couple out light of longer wavelengths, and said out-coupling unit arranged on the interface between the substrate and the coating may couple out light of shorter wavelengths. The actual out-coupling may be of transmissive or reflective type, and may be a partial or complete out-coupling.

According to a fourth exemplary embodiment of the present invention, the interface is configured to receive light with the first spectrum, the light with the second spectrum and light with a third spectrum; the light with the first spectrum, the light with the second spectrum and the light with the third spectrum can propagate in the medium; and the out-coupling units are configured to separately couple out the light with the first spectrum, the light with the second spectrum and the light with the third spectrum at predetermined positions. Therein, said reception of said light with said first, second and third spectrum may be a joint or separated reception. In this fourth exemplary embodiment of the present invention, the medium may comprise a substrate and a coating.

In this fourth exemplary embodiment of the present invention, the medium may furthermore be configured in a way that propagation of the light with the first spectrum and the light with the second spectrum through the interface between the substrate and the coating is possible, that total reflection of the light with the first spectrum and the light with the second spectrum at the outer surface of the coating is supported, and that total internal reflection of the light with the third spectrum at the interface between the substrate and the coating is possible. In this way, the light with the third spectrum is essentially kept within the substrate.

In this fourth exemplary embodiment of the present invention, the out-coupling units may furthermore comprise at least one out-coupling unit that is configured to couple out the light with the first spectrum and that is substantially arranged on the outer surface of the coating, at least one out-coupling unit that is configured to couple out the light with the second spectrum and that is substantially arranged on the outer surface of the coating, and at least one out- coupling unit that is configured to couple out the light with the third spectrum and that is substantially arranged on the interface between the substrate and the coating.

In this fourth exemplary embodiment of the present invention, the at least one out-coupling unit configured to couple out the light with the first spectrum, the at least one out-coupling unit configured to couple out the light with the second spectrum and the at least one out- coupling unit configured to couple out the light with the third spectrum may furthermore be diffraction gratings. Each of the diffraction gratings may particularly be adapted to the wavelength of the light of a specific spectrum which is to be coupled out by the respective diffraction grating.

In this fourth exemplary embodiment of the present invention, the diffraction grating for the light with the first spectrum and the diffraction grating for the light with the third spectrum may have a first orientation, the diffraction grating for the light with the second spectrum may have a second orientation, and the first and the second orientation may be substantially orthogonal to each other. This may particularly contribute to separate out-coupling of the light with the first spectrum and the light with the second spectrum.

In this fourth exemplary embodiment of the present invention, the interface for receiving the light with the first spectrum, the light with the second spectrum and the light with the third spectrum may be configured to receive the light with the second spectrum in a direction that is orthogonal to a direction in which the light with the first spectrum and the light with the third spectrum are received. This may particularly contribute to separate out-coupling of the light with the first spectrum and the light with the second spectrum.

In this fourth exemplary embodiment of the present invention, the diffraction grating for the light with the third spectrum may be configured to not allow diffracted orders for the light with the first spectrum. This may prevent coupling out of light with the first spectrum from the diffraction grating for the light with the third spectrum.

In this fourth exemplary embodiment of the present invention, the light with the first spectrum may be red light, the light with the second spectrum may be green light, and the light with the third spectrum may be blue light.

It should be noted that the above description of the present invention and its exemplary embodiments applies to both the apparatus and the method according to the present invention. Furthermore, it should be noted that all features described above with respect to specific embodiments equally well apply to the other embodiments and can be combined with the features of the other embodiments .

BRIEF DESCRIPTION OF THE FIGURES In the figures show:

Fig. 1: a flowchart of a method according to an exemplary embodiment of the present invention; Fig. 2a: a top view of a backlight structure according to an embodiment of the present invention; Fig. 2b: a top view of a backlight structure according to a further embodiment of the present invention; Fig. 3: a cross-sectional view of an apparatus according to a further embodiment of the present invention; Fig. 4a: a more detailed cross-sectional view of the apparatus according to Fig. 3 for transmissive out-coupling; Fig. 4b: a more detailed cross-sectional view of the apparatus according to Fig. 3 for reflective out-coupling;

Fig. 5: an illustration of the orientation of the out- coupling diffraction gratings of the apparatus according to Figs. 3, 4a and 4b; Fig. 6a: a grating element of an exemplary diffraction grating for out-coupling red light; Fig. 6b: a grating element of an exemplary diffraction grating for out-coupling green light; Fig. 6c: a grating element of an exemplary diffraction grating for out-coupling blue light; Fig. 7a: a graph showing the out-coupling efficiency η for red light at a diffraction grating according to Fig. 6a; Fig. 7b: a graph showing the out-coupling efficiency η for green light at a diffraction grating according to Fig. 6b; and Fig. 7c: a graph showing the out-coupling efficiency η for blue light at a diffraction grating according to Fig. 6c.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 depicts a flowchart 100 of a method according to an exemplary embodiment of the present invention. In a first step 100, red, green and blue light is received in a medium, for instance in a backlight structure or a waveguide, in which the red, green and blue light can propagate. Therein, said red, green and blue light may be received separately or jointly. In a second step 101, the red, green and blue light are coupled out of the medium at predetermined positions via a out-coupling units.

Fig. 2a illustrates, in top view, a backlight structure 200 according to an embodiment of the present invention. This backlight structure 200 may for instance be used to illuminate a Liquid Crystal Display (LCD) from behind. With this backlight structure, in particular the method steps of the flowchart 100 of Fig. 1 can be performed.

The backlight structure 200 is composed of two superposed rectangular plates 201 and 202. The plates can have different shapes, thicknesses and refractive indices, and may be laminated as well. In the top view of Fig. 2a, bottom plate 202 is only partially visible. Bottom plate 202 comprises a high efficiency diffraction grating 202-1 serving as an interface for receiving red light and for coupling red light into bottom plate 202, in particular towards fan-out grating 202-2. Therein, red light may for instance be received from a light source arranged substantially behind bottom plate 202. This light source may for instance be a light source having a narrow wavelength such as a Light Emitting Diode (LED) or a laser (e.g. a semiconductor laser or a frequency-doubled solid-state laser) . To achieve high efficiency in- coupling, diffraction grating 202-1 may for instance be particularly adapted to the wavelength of the red light as output by the light source. Fan-out grating 202-2 is deployed to distribute the red light received from diffraction grating 202-1 within bottom plate 202. Bottom plate 202 further comprises a plurality of high- efficiency diffraction gratings 202-3 for coupling out the red light received form fan-out grating 202-2. In Fig. 2a, diffraction gratings 202-3 are located below the top plate 201 and thus are depicted in dashed lines.

Out-coupling diffraction gratings 202-3 are arranged in a geometrical structure that reflects the arrangement of red pixels in a pixel matrix of an LCD for which backlight structure 200 serves as a backlight. One benefit of this is that light is out-coupled into the transmissive area of the pixel only, avoiding black matrix and pixel transistor areas where light otherwise would be absorbed. The distribution of light at the surface of the backlight structure 200 can be tailored by modifying the grating profiles, the grating area and the grating height to achieve a desired level of uniformity.

Of course, alternatively, a stripe structure, a delta or hexagonal structure or any other geometrical structure could have been chosen in order to optimally adapt backlight structure 200 to the LCD.

Bottom plate 202 is thus configured to receive red light, to propagate red light and to couple out red light at positions that correspond to colour pixels of a display. Provided that the beam divergence is small enough, backlight structure 200 thus can dispense with colour filters .

In a similar manner, high efficiency in-coupling diffraction gratings 201-la, fan-out gratings 201-2a and out-coupling diffraction gratings 201-3a for green light and high efficiency in-coupling diffraction gratings 201- Ib, fan-out gratings 201-2b and out-coupling diffraction gratings 201-3b for blue light are provided in backlight structure 200. However, for both green and blue light, the aforementioned gratings are located on one plate, i.e. top plate 201. Therein, in-coupling diffraction gratings 201-la and fan-out gratings 201-2a are positioned at the left of top plate 201, and in-coupling diffraction gratings 201-lb and fan-out grating 201-2b are positioned at the right of top plate 201. The out- coupling diffraction gratings 201-3a for green light and 201-3b for blue light are geometrically arranged to match the green and blue pixels of the pixel matrix of the LCD that is to be illuminated by backlight structure 200, respectively.

From the above description of backlight structure 200, it is thus readily clear that, in the present embodiment, red, green and blue light is separately received in backlight structure 200 (for instance from substantially monochromatic light sources) , propagated therein and separately coupled out at predetermined positions. Thus only the desired primary colour is coupled out at a given pixel. Due to the separate and colour-specific in- coupling, propagation and out-coupling of coloured light, no colour filters may be required. This contributes to both the efficiency and the thinness of the backlight structure and reduces costs. Furthermore, colour performance (e.g. in terms of colour purity) of the light that is coupled out by the out-coupling diffraction gratings is improved as compared to white light that is filtered with a colour filter.

The out-coupling gratings 202-3, 201-3a and 201-3b could furthermore be designed to have a collimating property, an angle-restricting property, or a polarization control property, to name but a few possibilities. In case of the out-coupling gratings having a polarization control property, in addition to colour filters, then also one polarizer could be dispensed with, so that another 0.1- 0.4 mm in thickness could be saved.

In the exemplary embodiment of Fig. 2a, a plane backlight structure was chosen. Alternatively, a bent or curved backlight structure with separate in- and out-coupling of coloured light could be designed as well.

Furthermore, instead of the in-coupling diffraction gratings 201-1, 202-la and 202-lb, equally well a butt- coupling structure, a molded plastic optical coupling structure or a prism-coupling structure, to name but a few, could be deployed.

Instead of out-coupling diffraction gratings 202-3, 201- 3a and 201-3b, equally well other wavelength-specific structures could be deployed for separately coupling out red, green and blue light.

In the exemplary embodiment of Fig. 2a, red, green and blue light were exemplarily coupled in and out of the backlight structure 200. It is readily understood that, equally well, only one or two different types of light or more than three different types of light could be used, and that, of course, the choice of light is not limited to red, green and blue light. It may even be advantageous to use light in the non-visible spectrum, e.g. to use fluorescence or similar effects.

Fig. 2b illustrates, in top view, a backlight structure 203 according to a further embodiment of the present invention. This backlight structure 203 may for instance be used to illuminate a Liquid Crystal Display (LCD) from behind. With this backlight structure, in particular the method steps of the flowchart 100 of Fig. 1 can be performed.

In contrast to backlight structure 200 of Fig. 2a, backlight structure 203 of Fig. 2b consists of a single plate, which is configured to receive and propagate light of red colour 204 (depicted in dashed lines) , light of blue colour 205 (depicted in solid lines) and light of green colour 206 (depicted in dash-dotted lines). Said light may be jointly or separately received via one or more interfaces of said backlight structure 203. As illustrated in Fig. 2b, the light of red 204 and blue colour 205 have been received in a direction that is orthogonal to a direction in which green light 206 has been received, in order to decouple the light of green colour 206 from the light of red 204 and blue colour 205. Backlight structure 203 comprises a plurality of out- coupling units arranged in groups of out-coupling units, wherein each group comprises one out-coupling unit 207 for light of red colour 204, one out-coupling unit 208 for light of blue colour 205, and one out-coupling unit 209 for light of green colour 209. In Fig. 2b, only nine of such groups are exemplarily depicted. Therein, said light of said red 204, blue 205 and green colour 206 are coupled out separately from each other. Said out-coupling units 207-209 are geometrically arranged to match the pixel positions of a display. Said out-coupling units 207-209 may for instance be diffraction gratings respectively adapted to couple out the light of red, blue and green colour. Furthermore, for instance, the single plate may optionally include a coating layer, i.e. means for separating red/blue out-coupling. Fig. 3 is a cross-sectional view of an apparatus 300 according to a further embodiment of the present invention. Apparatus 300 may for instance be used as a backlight for a Liquid Crystal Display (LCD) or as an illumination for a keymat or keyboard. Apparatus 300 is particularly suited for performing the method steps of the flowchart 100 of Fig. 1, i.e. to separately receive red, green and blue light in a medium, and to separately couple out the red, green and blue light from the medium at predetermined positions.

Apparatus 300 comprises a waveguide 305 with out-coupling diffraction gratings 303a, 303b and 303c for red (wavelength 630 nm) , green (wavelength 530 nm) and blue light (wavelength 460 nm) , respectively. The diffraction gratings may for instance be embossed on the surface of waveguide 305 or otherwise formed in or attached to the surface of waveguide 305. Therein, directly forming the diffraction gratings on the surface of waveguide 305 yields a particularly cost-efficient structure. If the apparatus 300 is deployed as a backlight for an LCD, a plurality of the diffraction gratings 303a, 303b and 303c for out-coupling of red, green and blue light may for instance be arranged in a geometrical structure to match respective pixels of a pixel matrix of a colour LCD.

Via an interface, e.g. the air-waveguide interface, waveguide 305 receives red light 302a from light source 301a, green light 302b from light source 301b and blue light 302c from light source 301c. Therein, the light sources 301a, 301b and 301c may for instance be Light Emitting Diodes (LEDs) or lasers, such as for instance semiconductor, frequency-doubled or pumped solid-state lasers. In this configuration, red light 302a and blue light 302c are received in waveguide 305 in orthogonal direction with respect to green light 302b, i.e. red light 302a and blue light 302c are received in a direction that is parallel to the plane in which the cross-sectional view of Fig. 3 is lying, and green light 302b is received in a direction that is parallel to the normal vector of the plane and that is illustrated as an "X" within a circle.

Therein, each light field is assumed to propagate inside the waveguide 305 at an angle that supports total internal reflection, i.e. red light 302a, green light 302b and blue light 302c are reflected from top and bottom surfaces of the waveguide 305 thus supporting guided mode propagation.

Via out-coupling diffraction gratings 304a for red light, 304b for green light and 304c for blue light, red light 304a, green light 304b and blue light 304c is coupled out of waveguide 305. The orientation of out-coupling gratings 304a, 304b and 304c is depicted in Fig. 5. It is readily visible that diffraction grating 303a for red light and diffraction grating for blue light 303c have the same orientation, whereas diffraction grating 303b for green light has an orthogonal orientation.

Figs. 4a and 4b show more detailed cross-sectional views of the waveguide 305 of apparatus 300 according to Fig. 3 for transmissive out-coupling (Fig. 4a) and reflective out-coupling (Fig. 4b) . Therein, reference numbers already used in Fig. 3 are maintained. Furthermore, the orientation of the diffraction gratings 303a, 303b and 303c as illustrated in Fig. 5 also applies to Figs. 4a and 4b. Waveguide 305 according to Fig. 4a consists of a substrate 305-1 and a coating layer 305-2. The output coupling grating 303c for blue light 302c is formed on the waveguide substrate 305-2, while the output coupling grating 303b for green light 302b (illustrated as an "X" within a circle) and the output coupling grating 303a for red light 302a are formed on top of the coating layer 305-2.

The refractive index of the coating layer 305-2 is selected to be smaller than that of the substrate 305-1, and the incidence angle of blue light 302c is chosen high enough to support total internal reflection of blue light at the interface between substrate 305-1 and coating layer 305-2. This effectively isolates the blue light 302c from the diffraction gratings 303a and 303b for red and green light thus enabling output coupling of blue light 302c to be obtained from diffraction grating 303c only. Moreover, the thickness of coating layer 305-2 is selected to be small enough to enable decoupling of out- coupled blue light 304c from the diffraction gratings 303a and 303b for red and green light.

To prevent output coupling of red light from the diffraction grating 303c for blue light, the grating period of diffraction grating 303c is selected to be small enough to function as an effective index medium for long wavelength (red) light. In other words, grating 303c for blue light does not support any diffracted modes for red light, thus preventing the red light 302a from being coupled out of the waveguide by the diffraction grating 303c for blue light. To prevent green light from being output coupled by the diffraction grating 303c for blue light, green light 302b is coupled into the waveguide 305 in an orthogonal direction with respect to the blue light 302c. Since the green light 302b is now propagating in parallel to the groove direction of the diffraction grating 303c for blue light (see Fig. 5), it is effectively not coupled out by diffraction grating 303c.

Output coupling gratings 303b and 303a for green and red light are formed on top of the coating layer 305-2, and the incidence angles of the respective light are selected to be small enough to enable propagation through the interface between substrate 305-1 and coating layer 305- 2, but high enough to support total internal reflection at the interface between the coating layer 305-2 and the medium adjoining to the coating layer 205-2, for instance air .

As depicted in Fig. 5, the groove direction of the grating 303b for green light is selected to be orthogonal with respect to the groove direction of the gratings 303a and 303c for red and blue light. This enables green light 302b to be coupled out of the waveguide 305 with diffraction grating 303b, while red light 302a is not affected by it. On the other hand, the orthogonal groove direction decouples green light 302b from interaction with the diffraction grating 303a for red light.

The grating configuration as described above enables blue light 302c, green light 302b and red light 302a to be coupled out of the waveguide 305 by the gratings 303c for blue light, 303b for green light and 303a for red light, respectively, yielding out-coupled blue light 304c, out- coupled green light 304b and out-coupled red light 304a. Cross talk of the output coupling is effectively eliminated with the use of the coating layer 305-2 and crossed grating configurations.

Fig. 4b depicts the waveguide 305 of Fig. 3 for the case of reflective out-coupling. With respect to the transmissive out-coupling configuration of Fig. 4a, it is readily seen that, in Fig. 4b, the coating layer 305-2 is formed below the substrate 305-1, so that the out-coupled red light 304a, green light 304b and blue light 304c propagates through the substrate 305-1 before leaving the waveguide 305. However, for the remaining features, the description of the transmissive out-coupling configuration according to Fig. 4a also applies to the reflective out-coupling configuration according to Fig. 4b.

In the following, an implementation example for the apparatus 300 of Fig. 3 will be described with reference to Figs. 6a-6c and Figs. Ia-Ic.

In this implementation example, substrate 305-1 is made of polycarbonate, and the coating layer 305-2 consists of Teflon (AF1600), with a refractive index of U₁=I.34. The three used types of out-coupling diffraction gratings for red, green and blue light are respectively partially depicted in Figs. 6a-6c. Therein, it is readily understood that the structure and dimensions of the out- coupling gratings are of exemplary nature only and are not intended to restrict the out-coupling gratings according to the present invention in any way. Fig. 6a depicts one out of a plurality of grating elements of diffraction grating 303a (see Figs. 3 and 5) for out-coupling red light. This diffraction grating is formed on top of the coating layer 305-1, which has a thickness of 0.4 μm. The grating element has a height of 0.28 μm and a width of 0.315 μm. Within the grating, this grating element periodically occurs every 0.45 μm. In other words, the grating comprises a plurality of grating elements of width 0.315 μm separated by air-filled grooves of 0.135 μm.

Fig. 6b depicts one out of a plurality of grating elements of diffraction grating 303b (see Figs. 3 and 5) for out-coupling green light. This diffraction grating is formed on top of the coating layer 305-1, which has a thickness of 0.4 μm. The grating element has a height of 0.35 μm and a width of 0.16 μm. Within the grating, this grating element periodically occurs every 0.4 μm. In other words, the grating comprises a plurality of grating elements of width 0.16 μm separated by air-filled grooves of 0.24 μm.

Fig. 6c depicts one out of a plurality of grating elements of diffraction grating 303c (see Figs. 3 and 5) for out-coupling blue light. This diffraction grating is formed in coating layer 305-2 at the interface between coating layer 305-2 and substrate 305-1. The coating layer 305-2, once again, has a thickness of 0.4 μm. At the bottom of the coating layer 305-2, a plurality of grating elements is formed. One of these grating elements is depicted in Fig. 6c, with a height of 0.2 μm and a width of 0.132 μm. Within the grating, this grating element occurs periodically every 0.22 μm. The grooves between the periodically occurring grating elements are filled with coating layer material as well. Figs. Ia-Ic illustrate the out-coupling efficiency η, i.e. the ratio of out-coupled light with respect to the light propagating through waveguide 300 (see Fig. 3), respectively for red light (wavelength λ=630 nm) , green light (wavelength λ=530 nm) and blue light (wavelength λ=460 nm) as a function of the incidence angle θ, i.e. the angle under which light is entering waveguide 300. Only propagation modes T_i™, T_o™, T_i™ and T₀ ^Tm are depicted, since further propagation modes only achieved negligible out-coupling efficiencies.

Fig. 7a depicts the out-coupling efficiency η for red light at the diffraction grating 303a for red light (see Figs. 3 and 6a) . From Fig. 7a, it can readily be seen that, for incidence angles θ between 35° and 50°, an out- coupling efficiency for the T_i^TE mode above 0.07 can be achieved. Furthermore, for the range between 40° and 50°, the out-coupling efficiency of all other modes but mode T- i™ approaches zero, so that the out-coupled red light is highly polarized. For this range of incidence angles, the out-coupling efficiency for all modes of green and blue light is found to approach zero (not depicted in Fig. 7a) .

Fig. 7b depicts the out-coupling efficiency η for green light at the diffraction grating 303b for green light (see Figs. 3 and 6b) . From Fig. 7b, it can readily be seen that, for incidence angles θ between 40° and 50°, an out-coupling efficiency above 0.05 can be achieved for the T-i™ mode, and that this mode is the only propagation mode left, so that highly polarized green light is coupled out from the diffraction grating. Furthermore, for this range of incidence angles, the out-coupling efficiency for both red and blue light is found to approach zero (not depicted in Fig. 7b) .

Finally, Fig. 7c depicts the out-coupling efficiency η for blue light at the diffraction grating 303c for blue light (see Figs. 3 and 6c) . From Fig. 7c, it can readily be seen that, for incidence angles θ between 55° and 70°, an out-coupling efficiency above 0.05 can be achieved for the T-i™ mode, and that this mode is the only propagation mode, so that highly polarized blue light is coupled out from the diffraction grating. Furthermore, for this range of incidence angles, the out-coupling efficiency for both red and green light is found to approach zero (not depicted in Fig. 7c) .

From the above description, it should have become apparent that the apparatus 300 of Fig. 3 allows to output different colours in spatially separated regions (e.g. different pixel positions of an LCD if apparatus 300 is used as a backlight) allowing for a significant improvement in the overall lighting efficiency for LCD backlights. Colour selection contrast can be very high, thus allowing a separate colour filter to be removed from an LCD display module (and thus to reduce the thickness of a display module) . Furthermore, with a suitable grating design, the output coupled light can be made highly polarized, allowing removal of one polarizer from an LCD module and yielding a factor-2 improvement in overall light efficiency (and a further reduction in display module thickness) . When deployed in the context of keymat illumination, the apparatus 300 of Fig. 3 enables multiple graphical icons to be implemented on a single waveguide keymat. The icons can be switched on/off separately by changing the illumination wavelength and/or direction .

The invention has been described above by means of exemplary embodiments. It should be noted that there are alternative ways and variations which are obvious to a skilled person in the art and can be implemented without deviating from the scope and spirit of the appended claims .

Claims

1. An apparatus, comprising an interface configured to receive light with a first spectrum and light with a second spectrum; a medium in which the light with the first spectrum and the light with the second spectrum can propagate; and out-coupling units configured to separately couple out the light with the first spectrum and the light with the second spectrum from the medium at predetermined positions.

2. The apparatus according to claim 1, wherein the out- coupling units comprise at least one diffraction grating for coupling out the light with the first spectrum, and at least one diffraction grating for coupling out the light with the second spectrum.

3. The apparatus according to any of the preceding claims, wherein the out-coupling units are arranged according to a predetermined geometry to allow to couple out the light with the first spectrum and the light with the second spectrum at the predetermined positions .

4. The apparatus according to any of the preceding claims, wherein the predetermined positions are substantially determined by pixel positions of a pixel matrix.

5. The apparatus according to any of the preceding claims, wherein the out-coupling units have at least one of a collimating property, an angle-restricting property and a polarization control property.

6. The apparatus according to any of the preceding claims, further comprising separate light sources for generating the light with the first spectrum and the light with the second spectrum.

7. The apparatus according to any of the preceding claims, wherein the apparatus is a backlight for a colour display or a part thereof.

8. The apparatus according to any of the preceding claims, wherein the apparatus is a module for a device with a colour display.

9. The apparatus according to any of the claims 7-8, wherein the colour display is an LCD display.

10. The apparatus according to any of the claims 1-7, wherein the apparatus is configured to work as illuminated keymat.

11. The apparatus according to any of the preceding claims, wherein the apparatus is part of a portable electronic device.

12. The apparatus according to any of the preceding claims, wherein the interface comprises at least one diffraction grating for coupling the received light with the first spectrum into the medium, and at least one diffraction grating for coupling the received light with the second spectrum into the medium.

13. The apparatus according to any of the preceding claims, further comprising distribution units configured to distribute the light with the first spectrum and the light with the second spectrum within the medium.

14. The apparatus according to claim 13, wherein the distribution units comprise at least one fan-out grating for distributing the light with the first spectrum, and at least one fan-out grating for distributing the light with the second spectrum.

15. The apparatus according to any of the preceding claims, wherein the interface is configured to receive the light with the first spectrum and the light with the second spectrum in substantially orthogonal directions.

16. The apparatus according to claim 15, wherein an orientation of the out-coupling unit for coupling out the light with the first spectrum is substantially orthogonal to an orientation of the out-coupling unit for coupling out the light with the second spectrum.

17. The apparatus according to any of the preceding claims, wherein the medium comprises a substrate and a coating.

18. The apparatus according to claim 17, wherein the medium is configured in a way that propagation of the light with the first spectrum through an interface between the substrate and the coating is possible, that total reflection of the light with the first spectrum at the outer surface of the coating is supported and that total internal reflection of the light with the second spectrum at the interface between the substrate and the coating is supported.

19. The apparatus according to any of the claims 17-18, wherein the out-coupling units comprise at least one out-coupling unit that is configured to couple out the light with the first spectrum and that is substantially arranged on the outer surface of the coating, and at least one out-coupling unit that is configured to couple out the light with the second spectrum and that is substantially arranged on the interface between the substrate and the coating.

20. The apparatus according to any of the preceding claims, wherein the interface is configured to receive light with the first spectrum, the light with the second spectrum and light with a third spectrum, wherein the light with the first spectrum, the light with the second spectrum and the light with the third spectrum can propagate in the medium; and wherein the out-coupling units are configured to separately couple out the light with the first spectrum, the light with the second spectrum and the light with the third spectrum at predetermined positions.

21. The apparatus according to claim 20, wherein the medium comprises a substrate and a coating.

22. The apparatus according to claim 21, wherein the medium is configured in a way that propagation of the light with the first spectrum and the light with the second spectrum through an interface between the substrate and the coating is possible, that total reflection of the light with the first spectrum and the light with the second spectrum at the outer surface of the coating is supported, and that total internal reflection of the light with the third spectrum at the interface between the substrate and the coating is possible.

23. The apparatus according to claim 22, wherein the out- coupling units comprise at least one out-coupling unit that is configured to couple out the light with the first spectrum and that is substantially arranged on the outer surface of the coating, at least one out-coupling unit that is configured to couple out the light with the second spectrum and that is substantially arranged on the outer surface of the coating, and at least one out-coupling unit that is configured to couple out the light with the third spectrum and that is substantially arranged on the interface between the substrate and the coating.

24. The apparatus according to claim 23, wherein the at least one out-coupling unit configured to couple out the light with the first spectrum, the at least one out-coupling unit configured to couple out the light with the second spectrum and the at least one out- coupling unit configured to couple out the light with the third spectrum are diffraction gratings.

25. The apparatus according to claim 24, wherein the diffraction grating for the light with the first spectrum and the diffraction grating for the light with the third spectrum have a first orientation, wherein the diffraction grating for the light with the second spectrum has a second orientation, and wherein the first and the second orientation are substantially orthogonal to each other.

26. The apparatus according to claim 25, wherein the interface for receiving the light with the first spectrum, the light with the second spectrum and the light with the third spectrum is configured to receive the light with the second spectrum in a direction that is orthogonal to a direction in which the light with the first spectrum and the light with the third spectrum are received.

27. The apparatus according to claims 26, wherein the grating for the light with the third spectrum is configured to not allow diffracted orders for the light with the first spectrum.

28. The apparatus according to any of the claims 20-27, wherein the light with the first spectrum is red light, wherein the light with the second spectrum is green light, and wherein the light with the third spectrum is blue light.

29. A method, comprising receiving light with a first spectrum and light with a second spectrum in a medium in which the light with the first spectrum and the light with the second spectrum can propagate; and separately coupling out the light with the first spectrum and the light with the second spectrum from the medium at predetermined positions via out- coupling units.

30. The method according to claim 29, wherein the receiving comprises coupling the light with the first spectrum and the light with the second spectrum into the medium by means of diffraction gratings.

31. The method according to any of the claims 29-30, further comprising distributing the light with the first spectrum and the light with the second spectrum within the medium with fan-out gratings.

32. The method according to any of the claims 29-31, wherein diffraction gratings are used for coupling out the light with the first spectrum and the light with the second spectrum from the medium.

33. The method according to any of the claims 29-32, wherein the predetermined positions are substantially determined by pixel positions of a pixel matrix.

34. The method according to any of the claims 29-33, wherein the light with the first spectrum and the light with the second spectrum are received in orthogonal directions.

35. The method according to claim 34, wherein the light with the first spectrum and the light with the second spectrum are coupled out from the medium by diffraction gratings, and wherein an orientation of a diffraction grating for coupling out the light with the first spectrum is substantially orthogonal to an orientation of a diffraction grating for coupling out the light with the second spectrum.

36. The apparatus according to any of the claims 29-35, wherein the medium comprises a substrate and a coating substantially arranged on the substrate.

37. The method according to claim 36, wherein propagation of the light with the first spectrum through the interface between the substrate and the coating is supported, wherein total reflection of the light with the first spectrum at the outer surface of the coating is supported, and wherein total internal reflection of the light with the second spectrum at the interface between the substrate and the coating is supported.

38. The method according to any of the claims 36-37, wherein the light with the first spectrum is substantially coupled out on the outer surface of the coating, and wherein the light with the second spectrum is substantially coupled out on the interface between the substrate and the coating.

39. The method according to any of the claims 29-38, wherein the light with the first spectrum, light with the second spectrum and light with a third spectrum are received in the medium, wherein the light with the first spectrum, the light with the second spectrum and the light with the third spectrum can propagate in the medium; and wherein the light with the first spectrum, the light with the second spectrum and the light with the third spectrum are separately coupled out from the medium at predetermined positions.

40. The method according to any of the claims 29-39, wherein the out-coupling units are arranged according to a predetermined geometry to allow to couple out the light with the first spectrum and the light with the second spectrum at the predetermined positions.

41. The method according to any of the claims 29-40, wherein the predetermined positions are substantially determined by pixel positions of a pixel matrix.