WO2014033576A1

WO2014033576A1 - Illumination device based on light guide with light diffusing particles

Info

Publication number: WO2014033576A1
Application number: PCT/IB2013/056581
Authority: WO
Inventors: Maarten Marinus Johannes Wilhelmus Van Herpen; Tim Dekker; Bram KNAAPEN; Anthonie Hendrik Bergman
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-08-31
Filing date: 2013-08-12
Publication date: 2014-03-06

Abstract

An illumination device (1) and a luminaire are disclosed. The illumination device (1) comprises a primary light guide (2) with embedded light scattering and/or reflecting particles (5) and a secondary light guide (7) arranged to receive light from light emitting elements (6) and to couple the received light to the primary light guide (2). The secondary light guide (7) may be arranged such that light out-coupled via the light out- coupling surface (9) of the secondary light guide (8) complies with a selected criterion for spatial uniformity of over the light out-coupling surface (9) of the secondary light guide (7), e.g. such that the intensity of light coupled out the secondary light guide (7) has an increased spatial uniformity with respect to the spatial uniformity of the intensity of light received from the light emitting elements (6). The illumination device (1) provides uniform lighting and reduced occurrence of bright light spots in the primary light guide (2) that are uncomfortable for a viewer of the illumination device (1).

Description

Illumination device based on light guide with light diffusing particles

FIELD OF THE INVENTION

The present invention relates to an illumination device comprising a primary light guide with embedded light scattering and/or reflecting particles, at least one light emitting element, and a secondary light guide.

BACKGROUND OF THE INVENTION

Illumination devices comprising a light source coupled with a light guide sheet or plate, which is able to propagate light internally, redirect and out-couple the light from its surface, provide for illuminating surfaces such as shelves, interior panels, signs and posters.

An example of a light guide for use in such an illumination device is the ACRYLITE® EndLighten sheet from Evonik Industries. It comprises a sheet of a light conducting acrylic material in which light diffusing particles are embedded. The acrylic sheet accepts light from a light source through its end surfaces, from where the light propagates within the sheet by means of internal reflection. The light diffusing particles embedded in the sheet redirect the travelling light such that that at least some of it exits the surface of the sheet, thereby giving the sheet its illuminating properties.

In order to light the entire surface of such a light guide, especially if the light guide is relatively large, a light source providing light having a relatively high brightness is normally required. The brighter the light, the further into the light guide the light is able to travel. Often light is also coupled into the light guide from more than one direction, in order to provide more light into the light guide and to achieve more uniform lighting output from the light guide.

The use of high brightness light sources, such as light-emitting diodes (LEDs) with a high brightness, however can result in bright light spots being visible to a user or viewer at the periphery of the light guide, i.e. along the edges of the light guide where the LEDs are positioned. These bright light spots arise due to the fact that some of the light that is generated by the light sources is not captured into the light guide. Light that escapes capture into the light guide instead passes directly from the light source to the viewer. Such bright light spots at the periphery of the light guide can be very uncomfortable for the viewer. CN201044028 Y discloses a backlight module comprising a light guide plate having a light emitting surface, a light guide rod which is extended from one side of the light guide plate, and a high-power LED module. The light guide rod is extended from one side of the light guide plate. The light provided by the high-power LED module enters a light inlet of the light guide rod, where it further enters a total reflection path arranged in the light guide rod. A part of the light is introduced into the light guide plate and another part advances in spiral rotation and the light is gradually introduced into the light guide plate.

SUMMARY OF THE INVENTION

In view of the above discussion, a concern of the present invention is to alleviate or even eliminate the problem of having bright light spots at the periphery of the light guide that are visible to a user, and to provide an illumination device with relatively few or even no such bright light spots visible to the user.

To address at least one of this concern and other concerns, an illumination device in accordance with the independent claim is provided. Preferred embodiments are defined by the dependent claims.

According to a first aspect of the present invention, there is provided an illumination device comprising:

at least one light emitting element;

a primary light guide comprising embedded light scattering and/or reflecting particles and a light in-coupling surface adapted to couple light impinging on the light in-coupling surface into the primary light guide; and

a secondary light guide comprising a light in-coupling surface adapted to couple light impinging on the light in-coupling surface into the secondary light guide and a light out-coupling surface adapted to couple light out of the secondary light guide. The secondary light guide is arranged to receive light emitted by the at least one light emitting element via the light in-coupling surface of the secondary light guide, and is arranged relatively to the primary light guide, or vice versa, such that at least some of the light that is coupled out from the light out-coupling surface of the secondary light guide impinges on the in-coupling surface of the primary light guide.

The secondary light guide may be arranged for propagating and redirecting light received in the secondary light guide, and the light out-coupling surface of the secondary light guide may possibly be arranged relatively to the light in-coupling surface of the secondary light guide, or vice versa, such that light out-coupled via the light out-coupling surface of the secondary light guide complies with a selected criterion for spatial uniformity over the light out-coupling surface of the secondary light guide.

In the context of the present application, spatial uniformity of light as used herein may be with respect to the intensity and/or brightness of the light.

The selected criterion for spatial uniformity of light out-coupled via the light out-coupling surface of the secondary light guide over the light out-coupling surface of the secondary light guide may for example be with respect to the spatial uniformity of light being in-coupled via the light in-coupling surface of the secondary light guide. The secondary light guide may be configured to modify the received light such that light coupled out from the light out-coupling surface of the secondary light guide has an increased spatial uniformity over the light out-coupling surface of the secondary light guide with respect to spatial uniformity of light over the light in-coupling surface of the secondary light guide.

Hence, compliance with a selected criterion for spatial uniformity of light out- coupled via the light out-coupling surface of the secondary light guide over the light out- coupling surface of the secondary light guide may be achieved based on the spatial uniformity of light being in-coupled via the light in-coupling surface of the secondary light guide.

The selected criterion for spatial uniformity of light out-coupled via the light out-coupling surface of the secondary light guide over the light out-coupling surface of the secondary light guide may for example be a desired or required magnitude or measure of spatial uniformity of light out-coupled via the light out-coupling surface of the secondary light guide over the light out-coupling surface of the secondary light guide.

The light emitting element or elements emit light during use. The emitted light enters the secondary light guide and is coupled from the secondary light guide into the primary light guide. The primary light guide accepts light from the secondary light guide through or via at least one light in-coupling surface, from where the light propagates within the primary light guide by means of total internal reflection. The light scattering and/or reflecting particles embedded in the light guide redirect the light propagating within the light guide unit such that at least some of it may exit a surface, e.g. light out-coupling surface, of the primary light guide, thereby giving the primary light guide at least some of its

illuminating properties.

The secondary light guide may be arranged for propagating and redirecting light within the secondary light guide, for example depending on the angle of incidence of the light rays impinging on the light in-coupling surface of the secondary light guide, and/or possibly arranged such that the light out-coupling surface of the secondary light guide is arranged relatively to the light in-coupling surface of the secondary light guide, or vice versa, such that light out-coupled via the light out-coupling surface of the secondary light guide complies with, or at least has an increased compliance with, a selected criterion for spatial uniformity over the light out-coupling surface of the secondary light guide. For example, the secondary light guide may be configured to modify the light received from the light emitting element such that light coupled out from the out-coupling surface of the secondary light guide has an increased spatial uniformity with respect to the spatial uniformity of the light that was coupled into the in-coupling surface of the secondary light guide. In other words, the secondary light guide may be arranged to receive light emitted from the light emitting element, to redistribute the light within the secondary light guide so as to achieve a relatively high spatial uniformity over the light out-coupling surface of the secondary light guide, and to further couple the redistributed light into the primary light guide. The redistributed light out-coupled from the secondary light guide and in-coupled into the primary light guide may have an increased spatial uniformity over the light out-coupling surface of the secondary light guide compared to the light emitted from the light emitting element over the light in-coupling surface of the secondary light guide.

The light emitted by a light emitting element such as a LED or another pointlike light source is often relatively non-uniform. The secondary light guide is arranged to spread and/or redistribute light emitted by the at least one light emitting element such that the light that out-coupled from the light out-coupling surface of the secondary light guide and subsequently coupled into the primary light guide is relatively uniform. The secondary light guide may also or alternatively be arranged to modify, spread, redirect and/or redistribute the light received from the at least one light emitting element, such that the light that impinges on the light in-coupling surface of the primary light guide has an increased spatial uniformity across the extension of the light in-coupling surface of the primary light guide, with respect to the light that would have impinged on the light in-coupling surface of the primary light guide if the light emitted by the at least one light emitting element was directly coupled into the primary light guide.

The secondary light guide may be arranged to modify, spread, redirect and/or redistribute the light coupled into the secondary light guide such that the light that subsequently is coupled into the primary light guide is spatially uniform across the in- coupling surface of the primary light guide, i.e. such that the brightness and/or intensity of the light is substantially the same or even the same along the extension of the light in- coupling surface of the primary light guide. Alternatively, the secondary light guide may be arranged to modify, spread, redirect and/or redistribute the light coupled into the secondary light guide such that the brightness and/or intensity of the light that is subsequently coupled into the primary light guide varies smoothly along the extension of the light in-coupling surface of the primary light guide, but such that it is more spatially uniform than light emitted by the light emitting elements.

The modified, spread, redirected and/or redistributed light coupled out from the light out-coupling surface of the secondary light guide is subsequently coupled into the primary light guide via its light in-coupling surface. Since the light coupled into the primary light guide has a relatively high uniformity, the occurrence of bright light spots appearing at the edge of the primary light guide may be reduced or even eliminated.

The dimensions, e.g. the length, thickness and/or width, of the light out- coupling surface of the secondary light guide are advantageously approximately equal to or smaller than the dimensions of the light in-coupling surface of the primary light guide.

Thereby, the light out-coupled from the secondary light guide is more efficiently captured and coupled into the primary light guide. This further contributes to decreased occurrence of bright light spots along the edge of the primary light guide, since less light is not in-coupled into the primary light guide.

The illumination device may comprise more than one secondary light guide. Hence, in different embodiments the illumination device comprises two or more secondary light guides. The secondary light guide may have various forms, such as a plate, a rod or a fiber. In one embodiment, the secondary light guide is an optical fiber. The secondary light guide is preferably substantially void or free of light scattering and/or reflecting particles, or even completely void or free of scattering and/or reflecting particles.

The primary light guide is arranged to enable propagation of light coupled into it by means of total internal reflection (TIR). It comprises a material through which light can propagate, in which light scattering and/or reflecting particles are embedded. The material in which the light scattering and/or reflecting particles are embedded is preferably a transparent material. The term "transparency", as referred to herein, is the physical property of allowing light to pass through the material without being scattered. In different embodiments, the primary light guide comprises a material selected from polymethylmethacrylate) (PMMA), polycarbonate, glass and/or silicon rubber. PMMA is sometimes called acrylic glass. A primary light guide may comprise more than one of these materials. For example, the primary light guide may comprise PMMA, polycarbonate, glass and/or silicon rubber. The primary light guide may have various forms, such as a plate, a rod or a fiber. The shapes of the primary light guide may be substantially regular or irregular. At least a portion of the outer surface of the primary light guide, e.g. the out-coupling surface, may be smooth. In other example, at least a portion of the outer surface of the primary light guide is rough, i.e. not smooth. Preferably the outer surface is smooth, such that a minimum of light is out-coupled due to irregularities on the outer surface. However, a rough outer surface may be used in cases where light output through the outer surface is desired. The primary light guide may have a rectangular, triangular or circular shape or have any other regular or irregular shape. In one embodiment, the primary light guide is an optical fiber comprising light scattering and/or reflecting particles. The primary light guide comprises light scattering and/or reflecting particles which are embedded into the material of the light guide that enables light propagation therein.

The illumination device of the present invention comprises at least one light emitting element. A plurality of, i.e. two or more, light emitting elements may be used in order to increase the amount of light that is coupled into the primary light guide and thereby make the primary light guide brighter and more uniformly lighted. A plurality of light emitting elements may in particular be used in illumination devices comprising a relatively large primary light guide. The light emitting elements may in principle be any kind of element that is able to generate and emit light. For example, the light emitting elements may comprise light emitting diodes, LEDs. RGB LEDs are advantageously used to enable dynamic color light output from the illumination device. The at least one light emitting element in an illumination device according to the present invention may be of the same type or different types.

In one embodiment the light out-coupling surface of the secondary light guide is arranged adjacent to, but not in optical contact with, the light in-coupling surface of the primary light guide. With "optical contact" it is in this context meant that the out-coupling surface of the secondary light guide and the in-coupling surface of the primary light guide are joined together without an air gap in-between. According to the embodiment mentioned immediately above, there is thus an air gap between the light out-coupling surface of the secondary light guide and the light in-coupling surface of the primary light guide. The purpose of the air gap is to cause light to travel through the secondary light guide with total internal reflection. In case there would be no gap between the primary and secondary light guide, light might not remain in the secondary light guide, but immediately travel into the primary light guide. The secondary light guide may be arranged to spread, redirect and/or redistribute the light by means and configurations known within the art. For example, the secondary light guide may comprise internal light reflecting and/or scattering elements or surfaces that redirect and/or redistribute the light before it is out-coupled from the secondary light guide.

In one embodiment, the secondary light guide is wedge shaped. A base of the wedge comprises the light in-coupling surface and an interior surface of the wedge comprises a light reflecting surface arranged to redirect and/or redistribute the light emitted from the at least one light emitting element. An advantage of a wedge shaped or triangular secondary light guide is that the light out-coupled from it and into the primary light guide may be highly uniform. The interior surface of the wedge may comprise a surface of the wedge having a surface normal directed into the secondary light guide.

In another embodiment, the secondary light guide is arranged to enable light propagation by means of total internal reflection within the secondary light guide, and the light out-coupling surface of the secondary light guide comprises a plurality of light out- coupling structures arranged on the light out-coupling surface of said secondary light guide. Each light out-coupling structure of the plurality of light out-coupling structures is adapted to couple out light impinging on the light out-coupling structure from the secondary light guide. The light out-coupling structures may for instance comprise or be constituted by paint dots or scratches on the outer surface of the secondary light guide This embodiment has the advantage that light can be out-coupled from the secondary light guide at the positions where out-coupling is desired or required.

In one embodiment, the light out-coupling structures may be spatially distributed on the out-coupling surface of the secondary light guide such that light coupled out from the light out-coupling surface of the secondary light guide has a selected spatial uniformity over the light out-coupling surface. A spatial uniformity that is selected such that the light is evenly out-coupled across the light out-coupling surface of the secondary light guide enables uniform lighting of a primary light guide having a uniform shape such as square, i.e. where the distance that the light travels in the primary light guide is substantially the same or the same across the primary light guide.

In another embodiment, the light out-coupling structures are arranged such that the intensity of light out-coupled from the secondary light guide over its light out- coupling surface varies with the distance from the at least one light emitting element to the point of light out-coupling. For example, the variation of the intensity of light out-coupled from the secondary light guide over its light out-coupling surface with the distance from the at least one light emitting element to the point of light out-coupling may be dependent on the shape of the primary light guide. This enables uniform lighting of a primary light guide having a shape in which the distance that the light travels in the primary light guide varies across the primary light guide, for instance where the primary light guide is triangular or has an irregular shape. Accordingly, the number of light out-coupling structures per unit area may vary over the light out-coupling surface of the secondary light guide with the width of the light out-coupling surface of the primary light guide at the point of incidence, i.e. the width of the light out-coupling surface of the primary light guide at the point where the light from the secondary light guide is coupled into the primary light guide.

In one embodiment, the secondary light guide is configured to collimate received light such that light coupled out from the light out-coupling surface of the secondary light guide has an increased degree of collimation with respect to the degree of collimation of light impinging on the light in-coupling surface of the secondary light guide. Such a secondary light guide not only spatially modifies, redirects and/or redistributes the light emitted by the at least one light emitting element, but also collimates the light. By

collimating the light more light may be coupled into the primary light guide, making the illumination device brighter and more efficient.

A flat collimator is advantageously used to match a flat or plate shaped primary light guide. Examples of flat collimators include the flat collimating LED

waveguides described in patent documents US2011096570 Al, US2011085332 Al and US2011063855 Al . A flat collimator may for instance be arranged to collimate light in a first direction by use of reflective surfaces having a collimating angle, and to collimate light in a second direction, which is perpendicular to the first direction, by use of grooved surfaces that are substantially perpendicular to the reflective surfaces.

In one embodiment, the primary light guide and/or the secondary light guide comprises an optical fiber. The optical fiber of the primary light guide comprises embedded light scattering and/or reflecting particles, while the optical fiber of the secondary light guide preferably is substantially void or free of embedded light scattering and/or reflecting particles or even completely void or free of embedded light scattering and/or reflecting particles.

An illumination device according to the present invention may be used for illuminating shelves, interior panels, thin profile signs and poster panels, etc. The illumination device may advantageously be comprised in a luminaire, such as a consumer luminaire used for general lighting of a space, such as a home.

According to a second aspect of the present invention, there is provided a luminaire comprising an illumination device according to the present invention.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments.

It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the invention will be described below with reference to the accompanying drawings, wherein:

Fig.1 depicts an embodiment of an illumination device according to the present invention.

Fig. 2 depicts another embodiment of an illumination device according the present invention, comprising a wedge shaped secondary light guide.

Fig. 3 depicts a further embodiment of an illumination device according to the present invention, comprising a secondary light guide with light out-coupling structures.

Fig. 4 depicts a secondary light guide with light out-coupling structures in accordance with an embodiment of the present invention.

Fig. 5 depicts an embodiment of an illumination device according to the present invention, comprising a triangular primary light guide and a secondary light guide with light out-coupling structures.

Fig. 6 depicts an embodiment of an illumination device according to the present invention, in which the secondary light guide acts as a collimator.

As illustrated in the figures, the sizes of certain structures are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of

embodiments of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the invention to those skilled in the art. Furthermore, like numbers refer to the same or similar elements or components throughout.

Fig. 1 shows an illumination device 1 comprising a primary light guide 2, secondary light guides 7 and light emitting elements 6. The light emitting elements 6 are arranged to couple light 10 into the secondary light guides 7. The secondary light guides 7 are arranged for propagating and redirecting light received in the respective secondary light guides 7 depending on the angle of incidence of the light rays impinging on respective light in-coupling surfaces 8 of the secondary light guides 7. The respective light out-coupling surfaces 9 of the secondary light guides 7 are also arranged relatively to the respective light in-coupling surfaces 8 of the secondary light guides 7. By these arrangements the secondary light guide 7 is arranged such that the light out-coupled via the light out-coupling surface 9 of the secondary light guide 7 complies with a selected criterion for spatial uniformity of over the light out-coupling surface 9 of the secondary light guide 7. Subsequently, the light out- coupled from the secondary light guides 7 is coupled into the primary light guide 2 as illustrated in Fig. 1. The primary light guide 2 receives the light 11 from the secondary light guides 7 through its light in-coupling surfaces 3. Only one of the light in-coupling surfaces 3 of the primary light guide is indicated by reference numeral in Fig. 1. The primary light guide 2 comprises embedded light scattering and/or reflecting particles that facilitate or enable out- coupling of light traveling or propagating within the primary light guide 2 as output light 12.

The light emitting elements 6 are in principle any kind of elements that are able to generate and emit light. Preferably the light emitting elements 6 are light emitting diodes, LEDs, such as RGB LEDs. The light emitting elements 6 may be of the same type or different types.

The primary light guide 2 is a waveguide which is arranged to receive input light through or via a light in-coupling surface 3 and to out-couple the light through or via a light out-coupling surface 4. In the embodiment shown in Fig. 1, the primary light guide 2 is substantially plate shaped, having edge surfaces along its edges, as well as a top surface and a bottom surface. The top and bottom surfaces are parallel. The light in-coupling surface 3 is arranged on at least one of the edge surfaces and is perpendicular to the top and bottom surfaces. The light out-coupling surface 4 is arranged on the top surface. In other

embodiments, light out-coupling surfaces 4 may be arranged both on the top and the bottom surfaces of the primary light guide 2. In most embodiments a light out-coupling surface 4 is arranged on both the top and bottom surfaces, since light that is scattered or reflected by the light scattering and/or reflecting particles 5 embedded in the primary light guide 2 generally travels in all directions. Thus, in order to have an out-coupling surface 4 on only the top surface, additional optics are needed to redirect light towards the top surface. For example, a reflective layer may be arranged on the bottom surface, redirecting light towards the top surface.

The primary light guide 2 is arranged to enable propagation of light coupled into it by means of the principle of total internal reflection (TIR). The primary light guide 2 comprises a material through which light can propagate, preferably a transparent material. Examples of such materials include transparent acrylic materials such as

polymethylmethacrylate) (PMMA), polycarbonate, glass and silicon rubber. Embedded in the transparent material are light scattering and/or reflecting particles 5. The light scattering and/or reflecting particles 5 redirects at least some of the light beams that impinge upon them towards the light out-coupling surface 4, at an angle of incidence that is smaller than the critical angle for TIR, thus enabling or facilitating the light beam to be out-coupled from the light out-coupling surface 4 of the primary light guide 2.

The illumination device 1 shown in Fig. 1 comprises two secondary light guides 7. Each of the secondary light guides 7 comprises a light in-coupling surface 8 arranged to receive light 10 emitted from one or more light emitting elements 7, and a light out-coupling surface 9 arranged to out-couple light 11 which is subsequently coupled into the primary light guide 2. According to the embodiment depicted in Fig. 1, neither of the secondary light guides 7 comprises embedded light scattering and/or reflecting particles such as present in the primary light guide 2.

Each of the secondary light guides 7 is arranged to receive light 10 from the light emitting elements 6 via the respective light in-coupling surfaces 8, and to modify, redirect, spread and/or redistribute the received light such that the light that subsequently is coupled into the primary light guide 2 is spatially uniform across the in-coupling surface 3 of the primary light guide 2, i.e. such that the brightness and/or intensity of the light is substantially the same or even the same along the extension of the light in-coupling surface 3 of the primary light guide 2. Alternatively, each of the secondary light guides 7 may be arranged to modify, redirect, spread and/or redistribute the light coupled into the secondary light guides 7 via the respective light in-coupling surfaces 8 such that the light that subsequently is coupled into the primary light guide 2 is such that the brightness and/or intensity of the light varies smoothly along the extension of the light in-coupling surface 3 of the primary light guide 2 but such that the light is more spatially uniform than light emitted by the light emitting elements 6.

Since the light coupled into the primary light guide 2 has a relatively high uniformity, the occurrence of bright light spots appearing along the edges of the primary light guide 2 close to the light in-coupling surfaces 3 may be reduced or even eliminated.

Each of the secondary light guides 7 may be arranged and/or shaped in various ways appreciated by a person skilled the art. The secondary light guides 7 shown in Fig.1 are constituted by rectangular or rod shaped light guides.

Fig. 2 shows another embodiment of the illumination device, including a wedge shaped or triangular secondary light guide 7. The base of the wedge comprises a light in-coupling surface 8 and is arranged proximate the light emitting element 6, while the tip of the wedge is arranged distant from the light emitting element 6 as illustrated in Fig. 2. The wedge shaped secondary light guide 7 has an interior light reflecting surface 14 arranged to redirect and/or redistribute light that is coupled into the secondary light guide via the light in- coupling surface 8. Light 10 from the light emitting elements 6 enters the in-coupling surface 8 of the secondary light guide 7 and is reflected by the reflecting surface 14 at an angle such that it is out-coupled through or via the light out-coupling surface 9. As illustrated in Fig. 2, there is an angle a between the light reflecting surface 14 and the light out-coupling surface 9. An advantage of a wedge shaped or triangular secondary light guide 7 is that the light out- coupled from the secondary light guide and coupled into the primary light guide 2 may be highly uniform. That is, the spatial distribution of the brightness and/or intensity of the light that is out-coupled from the secondary light guide 7 may be highly uniform across the out- coupling surface 9.

Fig. 3 shows a schematic, cross-sectional top view of an illumination device 1 according to an embodiment of the present invention, comprising a secondary light guide 7 with out-coupling structures 15. The secondary light guide 7 according to this embodiment comprises a light guide in which light can travel by means of TIR, schematically illustrated in Fig. 3 by the black arrows within the secondary light guide 7. The light guide 7 comprises a transparent material, for example a transparent acrylic material such as

polymethylmethacrylate) (PMMA), polycarbonate, glass or silicon rubber. Light out- coupling structures 15 are provided on the light out-coupling surface 9 of the secondary light guide 7. The light out-coupling structures 15 are arranged to out-couple light rays that impinge upon them, such that the thereby out-coupled light 11 is coupled into the primary light guide 2. The light out-coupling structures 15 may for instance comprise or be constituted by paint dots and/or scratches on the light out-coupling surface 9 of the secondary light guide 7. The light out-coupling structures 15 are arranged on the light out-coupling surface 9 at positions where extraction, i.e. out-coupling, of light is desired. An advantage of this embodiment is that a desired pattern by which light is extracted from the light out- coupling surface 9 can be arranged by arranging the out-coupling structures 15 on the light out-coupling surface 9 of the secondary light guide 7 in a corresponding pattern, as is schematically shown in Fig. 4.

According to the embodiment depicted in Fig. 4, light out-coupling structures 15 are spatially distributed on the light out-coupling surface 9 of the secondary light guide 7 such that light 11 coupled out from the light out-coupling surface 9 of the secondary light guide 7 has a selected spatial uniformity over the light out-coupling surface 9, for example such that the out-coupled light 11 is more spatially uniform than the in-coupled light 10 from the light emitting elements 6a, 6b, 6c. The light out-coupling structures 15 may for instance be evenly distributed over the light out-coupling surface 9 such that the brightness and/or intensity of the light 11 that is out-coupled from the secondary light guide 7, and which light 11 subsequently can be coupled into to the primary light guide (not shown in Fig. 4) becomes evenly distributed over the light out-coupling surface 9. Alternatively, the light out-coupling structures 15 may be spatially distributed on the light out-coupling surface 9 of the secondary light guide 7 such that the light that subsequently is coupled into the primary light guide is such that the brightness and/or intensity of the light 11 varies smoothly along the extension of the light in-coupling surface of the primary light guide, but such that the light 11 is more spatially uniform than light emitted by the light emitting elements 6a, 6b, 6c. For example, the pattern by which light is extracted from the light out-coupling surface 9 may be adapted to the shape of the primary light guide 2, such that more bright and/or intense light is coupled into the primary light guide at positions of the primary light guide 2 where the light out- coupling surface 4 of the primary light guide 2 is wider, i.e. at positions or locations on the light in-coupling surface 3 of the primary light guide 2 where the light can propagate far or even the furthest into the primary light guide 2. In other words, the number of light out- coupling structures 15 per unit area on the light out-coupling surface 9 may vary over the light out-coupling surface 9 with the width of the light out-coupling surface 4 of the primary light guide 2 at the point of incidence.

An example is shown in Fig. 5, showing an illumination device 1 with a triangular primary light guide 2. The width of the light out-coupling surface 4 of the primary light guide 2 varies with the position on the light in-coupling surface 3 of the primary light guide 2. The width wi at the one of the ends of the light out-coupling surface 3 is for example shorter than the width w₂ at the middle of the light out-coupling surface 3. The light 11 that is coupled into the middle of the light in-coupling surface 3 for example needs to travel further into the primary light guide 2 than the light that is coupled into the ends of the light in- coupling surface 3. A larger number of light out-coupling structures 15 per unit area are arranged at the middle of the out-coupling surface 9 of the secondary light guide 7 than at the ends, such that more light is coupled into the middle of the light in-coupling surface 3 of the primary light guide 2 than at the ends of the light in-coupling surface 3 of the primary light guide 2. The pattern of out-coupling structures 15 can be calculated or determined to ensure a high uniformity of light over the entire light out-coupling surface 4 of the primary light guide 2, depending on the shape of the out-coupling surface 4 of the primary light guide 2.

Fig. 6 shows an embodiment of an illumination device 1 in which the secondary light guide 7 also acts as a collimator. The secondary light guide 7 is configured to collimate received light such that light coupled out from the light out-coupling surface 9 of the secondary light guide 7 has an increased degree of collimation with respect to the degree of collimation of light impinging on the light in-coupling surface 8 of the secondary light guide 7. Such a secondary light guide 7 not only spatially modifies, redirects and/or redistributes the light 10 emitted by the light emitting element 6, but also collimates the light. By collimating the light 10, more light is coupled into the primary light guide 2, making the illumination device 1 brighter and more efficient.

In one embodiment of the present invention the illumination device 1 comprises optical fibers as primary and secondary light guides 2, 7. In such an embodiment, a first optical fiber, as secondary light guide 7, is used to couple light into a second optical fiber, as primary light guide 2. The second optical fiber, i.e. the primary light guide 2, comprises embedded light scattering and/or reflecting particles.

In conclusion, an illumination device and a luminaire are disclosed. The illumination device comprises a primary light guide with embedded light scattering and/or reflecting particles and a secondary light guide arranged to receive light from light emitting elements and to couple the received light to the primary light guide. The secondary light guide may be arranged such that light out-coupled via the light out-coupling surface of the secondary light guide complies with a selected criterion for spatial uniformity of over the light out-coupling surface of the secondary light guide, e.g. such that the intensity of light coupled out the secondary light guide has an increased spatial uniformity with respect to the spatial uniformity of the intensity of light received from the light emitting elements. The illumination device provides uniform lighting and reduced occurrence of bright light spots in the primary light guide that are uncomfortable for a viewer of the illumination device.

While the present invention has been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An illumination device (1) comprising:

at least one light emitting element (6);

a primary light guide (2) comprising embedded light scattering and/or light reflecting particles (5) and a primary light in-coupling surface (3) adapted to couple light impinging on the primary light in-coupling surface into the primary light guide; and

a secondary light guide (7) comprising a secondary light in-coupling surface (8) adapted to couple light impinging on the secondary light in-coupling surface into the secondary light guide, and a secondary light out-coupling surface (9) adapted to couple light out of the secondary light guide,

wherein the secondary light guide is arranged to receive light emitted by the at least one light emitting element via the secondary light in-coupling surface, and

wherein the secondary light guide is arranged relative to the primary light guide so that at least some of the light that is coupled out from the secondary light out- coupling surface impinges on the primary in-coupling surface.

2. The illumination device according to claim 1, wherein the secondary light guide is arranged for propagating and redirecting light received in the secondary light guide, and wherein the secondary light out-coupling surface is arranged relative to the secondary light in-coupling surface so that light that is coupled out via the secondary light out-coupling surface complies with a selected criterion for spatial uniformity over the secondary light out- coupling surface.

3. An illumination device according to claim 1 or 2, wherein the secondary light guide is configured to modify received light such that the intensity and/or brightness of the light coupled out from the secondary light out-coupling surface has an increased spatial uniformity over the secondary light out-coupling surface with respect to the spatial uniformity of the intensity and/or brightness of light over the secondary light in-coupling surface.

4. The illumination device according to any one of claims 1 to 3, wherein the secondary out-coupling surface is arranged adjacent to, but not in optical contact with, the primary light in-coupling surface.

5. The illumination device according to any of claims 1 to 4, wherein the secondary light guide is substantially void of light scattering and/or light reflecting particles.

6. The illumination device according to any of the previous claims, wherein the secondary light guide is wedge-shaped, a base of the wedge comprising the secondary light in-coupling surface and an interior surface of the wedge comprising a light reflecting surface (14) arranged to redirect and/or redistribute the light emitted from the light emitting element.

7. The illumination device according to any of the previous claims, wherein the secondary light guide is arranged to enable light propagation by means of total internal reflection within the secondary light guide, and wherein the secondary light out-coupling surface comprises a plurality of light out-coupling structures (15).

8. The illumination device according to claim 7, wherein the light out-coupling structures are spatially distributed on the secondary light out-coupling surface such that light coupled out from the secondary light out-coupling surface has a selected spatial uniformity over the secondary light out-coupling surface.

9. The illumination device according to claim 7 or 8, wherein the light out- coupling structures are arranged such that intensity of light out-coupled from the secondary light guide over the secondary light out-coupling surface varies with the distance from the at least one light emitting element to the point of light out-coupling.

10. The illumination device according to claim 9, wherein the variation of the intensity of light out-coupled from the secondary light guide over the secondary light out- coupling surface with the distance from the at least one light emitting element to the point of light out-coupling is dependent on the shape of the primary light guide.

11. The illumination device according to any one of claims 7 to 10 wherein the number of light out-coupling structures per unit area on the secondary light out-coupling surface varies over the secondary light out-coupling surface with the width (wi , w₂) of the primary light out-coupling surface at the point of incidence.

12. The illumination device according any of claims 1 to 11, wherein the secondary light guide is configured to collimate received light such that light coupled out from the secondary light out-coupling surface has an increased degree of collimation with respect to the degree of collimation of light impinging on the secondary light in-coupling surface.

13. The illumination device according to any of claims 1 to 12, wherein the primary light guide and/or the secondary light guide comprises an optical fiber.

A luminaire comprising an illumination device (1) according to any of claims