FIELD OF THE INVENTION

The present invention generally relates to the field of lighting. In particular, the present invention relates to a light-emitting diode (LED) based light-emitting device. BACKGROUND OF THE INVENTION

Light-emitting diodes intended for indication purposes have been used for a long time, but high-brightness LEDs, e.g. LEDs having a brightness that is high enough to enable general illumination of various locations such as rooms, have in a short period of time caused a significant growth in the LED and lighting applications market. High-brightness LEDs are generally associated with a small size, a relatively high efficacy (and associated low temperature), a relatively long lifetime, a wide color gamut and ease of control.

Naturally, such LEDs are of importance to lighting designers in developing new lighting applications. Such LEDs may also be utilized in replacing conventional light generation devices, such as filamented light bulbs or halogen lamps. Such LEDs are also generally capable of emitting light of various colors. Thus, as the performance of LEDs improves and the costs of LEDs decreases, LEDs are expected to a significant degree replace conventional light sources such as incandescent lamps and fluorescent tubes.

In particular in the field of beam shaping, new options of so-called secondary optics such as lenses and/or mirror have become available where optical elements are positioned close to the LED for converting the emission pattern of the LED, which often is Lambertian, into another, desired pattern, such as a narrow cone-shape pattern. In general, such secondary optics are constituted by combinations of one or more LEDs and a separate optical beam-shaping element. Thus, two or more components in general have to be positioned in a desired position relatively each other in order to achieve the desired beam- shaping and be maintained in the desired position, accurately and securely. Such solutions may hence pose mechanical difficulties with regards to assembly and/or use.

In general, available secondary optics is relatively large (i.e. takes up a considerable amount of space), especially in comparison with the light-emitting area of a LED. A typical diameter of secondary optics may range from about 20 mm to about 50 mm, and the thickness of secondary optics may typically range from about 10 mm to about 20 mm, to be compared with the light-emitting area of a LED that is typically about lxl mm ² . Thus, such secondary optics are in general relatively bulky and/or obtrusive. SUMMARY OF THE INVENTION

It is with respect to the above considerations and others that the present invention has been made. In particular, the inventor has realized that it would be desirable to achieve a light-emitting device where one or more LEDs and additional optics may be provided in an integrated solution that may relatively easily be assembled and maintained in a desired position relatively each other. Furthermore, the inventor has realized that it would be desirable to achieve a light-emitting device where one or more LEDs and additional optics may provided in an integrated solution that is relatively thin and compact in comparison with known devices.

To better address one or more of these concerns, a light-emitting device having the features defined in the independent claim is provided. Further advantageous embodiments of the present invention are defined in the dependent claims.

According to a first aspect of the present invention, there is provided a light- emitting device comprising at least one light guide, an optical coupler and at least one LED. The optical coupler is adapted to optically couple the at least one LED to at least one input surface portion of the at least one light guide. The optical coupler comprises at least one light-angle selecting transflector, adapted to at least partially reflect light incident on the at least one light-angle selecting transflector within a predetermined angle interval with respect to a surface normal of the at least one LED and at least partially transmit light incident on the at least one light-angle selecting transflector outside the predetermined angle interval. The at least one light guide comprises at least one outcoupling element adapted to couple out light from an output surface portion of the light guide, such that at least some of the light outputted from the output surface portion has an intensity having a predetermined angular distribution with respect to a surface normal of the at least one LED.

In other words, the present invention is based on at least one LED optically coupled to a light guide by means of an optical coupler acting as an angular filter for light emitted from the at least one LED and being incident on the optical coupler. The inventor has realized that by such a configuration the at least one light guide may be made very thin, typically down to about 30% of the side of the LED light-emitting surface (e.g., about 0.3 mm for a LED having a light-emitting area of about lxl mm ² ) without substantial light losses, as described further in the following. This is in contrast to conventional secondary optics, which typically has a thickness that ranges from about 10 mm to about 20 mm.

By means of such an optical coupler a relatively thin and compact assembly of a LED light redirection structure combined with one or more LEDs can be realized. By providing the light guide with, or shaping the light guide so that it comprises, at least one optical element adapted to couple out light from an output surface portion of the light guide, at least some of the light outputted from the output surface portion of the light guide may have an intensity having a predetermined angular distribution with respect to a surface normal of the at least one LED.

In the context of some embodiments of the present invention, by a surface normal of a LED it may be meant a normal of a surface portion of the light-emitting surface of the LED.

The predetermined angle interval may be such that light coupled into the at least one light guide satisfies a total internal reflection (TIR) condition at an output surface portion of the light guide. In other words, the multilayer angular filter may reflect light that will not fulfill the TIR condition in the light guide and transmit light that fulfils the TIR condition in the light guide, thereby enabling achieving a relatively high optical efficiency (that is, the ratio of the luminous flux outputted from the light-emitting device and the initial amount of the installed luminous flux) for the light-emitting device. The latter light may then propagate through the light guide via TIR and be coupled out by means of at least one outcoupling element in the light guide, the at least one outcoupling element being adapted to couple out light from an output surface portion of the light guide.

The overall shape of the at least one light guide may for example be substantially circularly disk-shaped, though the present invention is not limited to such a choice. Rather, the overall shape of the light guide may be chosen on the basis of user, capacity, application and/or design requirements.

A light guide comprising a substantially circular disk-shaped light-guide structure may be advantageous in that such a geometrical arrangement may facilitate achieving a light-emitting device configured to emit light having an intensity that has a predetermined angular distribution with respect to a surface normal of the at least one LED.

Some or all of the components of the light-emitting device may be coupled together by means of (optical) index matching adhesive, such as index matching silicone adhesive, which may provide optical contact, e.g. promoting light extraction from the LED, eliminating optical Fresnel losses, etc., and at the same time provide mechanical stability of the assembly of components of the light-emitting device.

Such a configuration may provide an integrated solution for a LED light redirection structure, wherein the at least one LED and additional optics for light redirection may be relatively easily and accurately assembled, and the relative positions of the at least one LED and the additional optics may be accurately and securely maintained. In this manner, a compact and thin LED redirection structure for beam shaping of at least one single color LED may be achieved. Such a configuration may be utilized to provide unobtrusive illumination to various locations such as rooms.

The at least one LED may be configured to emit either red, green or blue light.

Optical coupling referred to in the foregoing and in the following with reference to some embodiments of the present invention may be realized in a number of ways, including bonded and non-bonded configurations. Optical coupling between elements or components may for example be achieved by means of a suitable adhesive, a (thin) optically conducting layer arranged between the elements or components that are to be optically coupled to each other, etc. Each such arrangement may have an appropriate index of refraction for index matching the elements or components that are to be optically coupled together.

The at least one outcoupling element may for example comprise at least one facet surface arranged at an angle of about 62° to about 66° with respect to a surface normal of the at least one LED, or arranged at an angle of about 24° to about 28° with respect to a tangent to a surface of the at least one LED, or arranged such that an angle between a surface normal of the at least one facet surface and a surface normal of the at least one LED is between about 24° to about 28°.

Such a configuration may enable achieving a light output from the output surface portion of the light-emitting device that has an intensity having an angular

distribution with respect to the surface normal substantially situated about the surface normal. The intensity distribution may be such that the intensity at the full width at half maximum (FWHM) of a cross-sectional light intensity profile in a plane defined by the normal and the output surface portion is relatively high. At the same time, the FWHM may be relatively low (i.e. the intensity angular distribution may be situated substantially about the surface normal). In other words, light output from the output surface portion of the light-emitting device may be substantially collimated along the direction of the surface normal. Alternatively or optionally, the at least one outcoupling element may comprise at least one facet surface arranged at an angle of about 58° to about 62° with respect to a surface normal of the at least one LED, or arranged at an angle of about 28° to about 32° with respect to a tangent to a surface of the at least one LED, or arranged such that an angle between a surface normal of the at least one facet surface and a surface normal of the at least one LED is between about 28° to about 32°.

In other words, the at least one outcoupling element may comprise a facet surface comprising two facet surface portions arranged at an angle of about 62° to about 66° and of about 58° to about 62°, respectively, with respect to a surface normal of the at least one LED (or arranged at an angle of about 24° to about 28° and of about 28° to about 32°, respectively, with respect to a tangent to a surface of the at least one LED, or arranged such that an angle between a surface normal of the at least one facet surface and a surface normal of the at least one LED is between about 24° to about 28° and between about 28° to about 32°, respectively). Such a configuration may enable achieving a light output from the output surface portion of the light-emitting device that has an intensity having an angular

distribution with respect to the surface normal substantially situated about the surface normal. Compared to a single-portion facet surface configuration such as described in the foregoing, such a configuration may enable achieving an intensity distribution such that the intensity at the FWHM of the cross-sectional light intensity profile is even higher.

Alternatively or optionally, the at least one outcoupling element may comprise at least one facet surface arranged at an angle of about 28° to about 32° with respect to a surface normal of the at least one LED, or arranged at an angle of about 58° to about 62° with respect to a tangent to a surface of the at least one LED or arranged such that an angle between a surface normal of the at least one facet surface and a surface normal of the at least one LED is between about 58° to about 62°.

Compared to the facet surface configurations described in the foregoing, such a configuration may achieve an intensity distribution such that the FWHM of the cross- sectional light intensity profile is lower.

With respect to some embodiments of the present invention, the at least one facet surface may comprise a specularly reflecting surface such as a mirror.

In the context of some embodiments of the present invention and with reference to a tangent to a surface of a LED, the surface of the LED may be a surface portion of the light-emitting surface of the LED. The at least one light guide may comprise a substantially flat, wedge-shaped light-guiding structure that is tapered from an output surface portion to the at least one input surface portion at angle of about 1° to about 9° with respect to one of the output surface portion and the at least one input surface portion. In other words, the at least one light guide may comprise a wedge-shaped light-guiding structure having a wedge angle of about 1° to about 9°.

Such a configuration may enable achieving the outcoupling of light from output surface portion of the light-emitting device to be in a relatively narrow angular range.

In some applications the wedge angle may be between about 1° and about 3°. The light-emitting device may comprise at least one optical redirection structure arranged adjacent to, or optically coupled to, the output surface portion of the at least one light guide. The at least one optical redirection structure may be adapted to redirect light such that at least some of the light outputted from the at least one optical redirection structure has an intensity having a predetermined angular distribution with respect to a surface normal of the at least one LED.

In other words, the optical redirection structure may be utilized to collimate the light outputted from the output surface portion of the light-emitting device, for example substantially along the surface normal.

Such an optical redirection structure may be adapted either to redirect received light in a predefined direction or to redirect light in a more diffuse (random) direction. Such an optical redirection structure may for example be integrated in the light guide or be optically coupled to the light guide.

The arrangement of a first component or element adjacent to a second component or element as referred to in the foregoing and in the following with reference to some embodiments of the present invention means that the first component and the second component, or surface portions of the first and second component, respectively, are not in direct contact with each other but separated from each other by a suitable material or medium, e.g. by a slit of air. Such a separation may be small in comparison with dimensions of the first and/or the second component.

The at least one optical redirection structure may comprise a plurality of micro -prismatic optical elements. At least one micro -prismatic optical element may comprise at least one substantially flat facet surface, each of the at least one substantially flat facet surface being arranged at a predetermined angle with respect to a surface normal of the at least one LED. By the design of the configuration and/or shape of the micro -prismatic optical element(s), such a configuration may enable achieving a light output from the output surface portion of the light-emitting device such that the outputted light has an intensity having a desired angular distribution with respect to the surface normal, for example on the basis of user, design and/or application requirements.

For example, according to a specific configuration the at least one micro- prismatic optical element may comprise two flat facet surfaces arranged at an angle of about - 22° to about -19° and about 33° to about 36°, respectively, with respect to the surface normal of the at least one LED.

The at least one outcoupling element may for example comprise at least one first parabolic facet surface being at least partially reflecting and arranged such that a focal point of the at least one first parabolic facet is located at a distance from the at least one light guide substantially equal to the thickness of the at least one light guide in a direction parallel to a surface normal of the at least one LED.

Such a configuration may enable achieving a light output from the output surface portion of the light-emitting device that has an intensity having an angular

distribution with respect to the surface normal substantially situated about the surface normal. The intensity distribution may be such that the intensity at the FWHM of a cross-sectional light intensity profile in a plane defined by the normal and the output surface portion is relatively high. At the same time, the FWHM may be relatively low (i.e. the intensity angular distribution may be situated substantially about the surface normal). In other words, light output from the output surface portion of the light-emitting device may be substantially collimated along the direction of the surface normal.

Alternatively or optionally, the at least one outcoupling element may comprise at least one second parabolic facet surface being at least partially reflecting and arranged such that a focal point of the at least one second parabolic facet is located at a surface portion of the at least one LED.

In other words, the at least one outcoupling element may comprise a dual parabolic facet surface comprising two parabolically shaped surface portions being at least partially reflecting. Such a segmented parabolic facet surface configuration may enable achieving a light output from the output surface portion of the light-emitting device that has an intensity having an angular distribution with respect to the surface normal substantially situated about the surface normal. Compared to a single-portion parabolic facet surface configuration such as described in the foregoing, such a configuration may enable achieving an intensity distribution such that the intensity at the FWHM of the cross-sectional light intensity profile is even higher.

The configuration described in the immediate foregoing is based on that a first portion of the angular distribution of light entering the input surface portion of the light guide, the first portion of the angular distribution of light comprising relatively large angles with respect to a surface normal of the at least one LED, is focused by means of the first parabolic facet surface (or a facet surface having a simpler geometrical shape) and the remaining portion of the angular distribution of light entering the input surface portion of the light guide is focused by means of the second parabolic facet surface (or a facet surface having a simpler geometrical shape).

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

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 A is a schematic view of a portion of a light-emitting device according to an exemplifying embodiment of the present invention;

Fig. IB is a graph of the transmissivity of a light-angle selective transflector in accordance with an exemplifying embodiment of the present invention, as a function of the angle of incidence of light incident on the light-angle selective transflector;

Fig. 1C is a graph illustrating a working principle in accordance with an exemplifying embodiment of the present invention;

Figs. 2-4, 5 A, 5B, 6 and 7A are schematic side views of light-emitting devices according to exemplifying embodiments of the present invention; and

Fig. 7B is a schematic side view of a micro -prismatic optical element in accordance with an exemplifying embodiment of the present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplifying embodiments of the invention are shown. This 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 be thorough and fully convey the scope of the invention to those skilled in the art. Furthermore, like numbers refer to like or similar elements or components throughout.

Referring now to Fig. 1 A, there is shown a schematic view of a portion of a light-emitting device 100. The light-emitting device 100 comprises a LED 101 coupled to an input surface portion 102a of a light guide 102 by means of an optical coupler, generally referenced by the dashed rectangle indicated by the reference numeral 103, which optical coupler 103 comprises at least one light-angle selecting transflector 104, for example constituted by a plurality of dielectric layers such as indicated in Fig. 1A. As further indicated in Fig. 1A, Fig. 1A shows a portion of the light guide 102 and a portion of the light- angle selecting transflector 104. The light-angle selective transflector 104 and the LED 101 may be coupled together by means of a layer 105 of index matching adhesive, such as index matching silicone adhesive, comprised in the optical coupler 103.

The light-angle selecting transflector 104 is adapted to at least partially reflect light incident on the light-angle selecting transflector 104 (of which light some light ray paths are shown in Fig. 1 A) within a predetermined angle interval with respect to a surface normal of the LED 101 and at least partially transmit light incident on the light-angle selecting transflector 104 outside the predetermined angle interval. With a surface normal of the LED 101 it is here meant a normal to a light-emitting surface of the LED 101 where the light- emitting surface faces the optical coupler 103. In other words, the light-angle selective transflector 104 of the optical coupler 103 may act as an angular filter, reflecting light rays having a small angle of incidence, below some predetermined angle of incidence, back into the LED 101, whereas light rays having a large angle of incidence, above the predetermined angle of incidence, may be transmitted into the light guide 102.

The LED 101 may be mounted on a support 106, for example comprising a printed circuit board (PCB) or the like.

The predetermined angle interval may be such that light coupled into the at least one light guide 102 satisfies a total internal reflection (TIR) condition at an output surface portion 102b of the light guide 102. In other words, the multilayer angular filter 104 may reflect light that will not fulfill the TIR condition in the light guide 102, and transmit light that fulfils the TIR condition in the light guide 102. The latter light may then propagate through the light guide 102 via TIR and be coupled out by means of at least one outcoupling element (not shown in Fig. 1 A, see Figs. 2-4, 5A, 5B, 6 and 7A) comprised in the light guide 102, the at least one outcou ling element being adapted to couple out light from an output surface portion 102b of the light guide 102.

Referring now to Fig. IB, there is shown a exemplifying diagram of the transmissivity of the multilayer angular filter 104 as a function of the angle of incidence Θ of light incident on the multilayer angular filter 104. As described in the foregoing, light having an angle of incidence Θ below some predetermined angle ( _C is reflected and light having an angle of incidence Θ above ( _C is transmitted through the multilayer angular filter 104. The particular value of ( _C shown in Fig. IB is by way of example only.

With further reference to Fig. 1 A, there is in general a minimum thickness requirement that the light guide 102 needs to fulfill in order to prevent the light entering the light guide 102 from reflecting from a top surface of the light guide 102 (the top surface generally facing the input surface portion 102a of the light guide 102) back into the LED 101. For example, according to a first geometrical estimate the minimum thickness for a LED having a light-emitting area of about lxl mm ² would be about 0.79 mm.

Referring now to Fig. 1C, there is shown a graph for illustrating the efficiency of incoupling of light generated by a LED to the light guide via a light-angle selective transflector (such as a dielectric multilayer angular filter) as a function of the thickness d of the light guide. More precisely, the graph in Fig. 1C shows the relative light flux Φ leaving the optical element (that is, the ratio of the light flux leaving the optical element and the total light flux from the light-emitting area of the LED) versus the thickness d of the light guide. The graph shown in Fig. 1C was obtained by means of optical modeling, with the

assumptions that the light-emitting area of the LED was lxl mm ² and the reflectivity of the LED was 65%. As can be seen in Fig. 1C, the thickness d of the light guide may be reduced substantially below the first estimate 0.79 mm without significant efficiency reductions.

With reference to Fig. 2, there is shown a schematic side view of a light- emitting device 200 according to an exemplifying embodiment of the present invention. The light-emitting device 200 comprises a LED 201 optically coupled to an input surface portion 202a of a light guide 202 via a light-angle selective transflector 203 adapted to at least partially reflect light incident on the light-angle selecting transflector 203 within a

predetermined angle interval with respect to a surface normal of the LED 201 and at least partially transmit light incident on the at least one light-angle selecting transflector 203 outside the predetermined angle interval. The LED 201 may for example be coupled to the light-angle selecting transflector 203 via a layer 204 of index matching adhesive, such as index matching silicon adhesive.

With further reference to Fig. 2, the light guide 202 comprises one or more outcoupling elements 205 comprised by facet surfaces arranged at an angle φ of about 62° to about 66° with respect to a surface normal of the LED 201. Each of the facet surfaces of the outcoupling elements 205 may for example comprise a specularly reflecting mirror.

Alternatively or optionally, φ may be between about 58° and about 62°. Alternatively or optionally, φ may be between about 28° and about 32°. Each outcoupling element 205 may be adapted to couple out light from an output surface portion 202b of the light guide 202 such that at least a portion of light outputted from the output surface portion 202b has an intensity having a predetermined angular distribution with respect to a surface normal of the LED 201.

Referring now to Fig. 3, there is shown a schematic side view of a light- emitting device 300 according to an exemplifying embodiment of the present invention. The light-emitting device 300 comprises a LED 301 optically coupled to an input surface portion 302a of a light guide 302 via a light-angle selective transflector 303 adapted to at least partially reflect light incident on the light-angle selecting transflector 303 within a predetermined angle interval with respect to a surface normal of the LED 301 and at least partially transmit light incident on the at least one light-angle selecting transflector 303 outside the predetermined angle interval.

The LED 301 may for example be coupled to the light-angle selecting transflector 303 via a layer 304 of index matching adhesive, such as index matching silicon adhesive.

With further reference to Fig. 3, the light guide 302 comprises one or more outcoupling elements 305 comprised by facet surfaces, each facet surface comprising two facet surface portions 305a, 305b arranged at an angle φ of about 62° to about 66° with respect to a surface normal of the LED 301 and an angle ψ ₂ of about 58° to about 62° with respect to a surface normal of the LED 301. Each of the facet surface portions 305a, 305b of the outcoupling elements 305 may for example comprise a specularly reflecting mirror.

Each outcoupling element 305 may be adapted to couple out light from an output surface portion 302b of the light guide 302 such that at least a portion of light outputted from the output surface portion 302b has an intensity having a predetermined angular distribution with respect to a surface normal of the LED 301. Referring now to Fig. 4, there is shown a schematic side view of a light- emitting device 400 according to an exemplifying embodiment of the present invention. The light-emitting device 400 comprises a LED 401 optically coupled to an input surface portion 402a of a light guide 402 via a light-angle selective transflector 403 adapted to at least partially reflect light incident on the light-angle selecting transflector 403 within a predetermined angle interval with respect to a surface normal of the LED 401 and at least partially transmit light incident on the at least one light-angle selecting transflector 403 outside the predetermined angle interval.

The LED 401 may for example be coupled to the light-angle selecting transflector 403 via a layer of index matching adhesive, such as index matching silicon adhesive (not shown in Fig. 4, see, e.g., Fig. 3).

With further reference to Fig. 4, the light guide 402 comprises one or more outcoupling elements 404, each outcoupling element 404 comprising a parabolic facet surface being at least partially reflecting.

As indicated in Fig. 4, the parabolically shaped facet surfaces of the outcoupling elements 404 may be configured such that a focal point of at least one parabolic facet surface is located at a distance from the light guide 402 that is substantially equal to the thickness of the light guide 402 in a direction parallel to a surface normal of the LED 401. By such a configuration, a mirror image 405 of the LED 401 may be formed at a distance from the light guide 402 that is substantially equal to the thickness of the light guide 402 in a direction parallel to a surface normal of the LED 401. The mirror image 405 of the LED 401 should preferably be located substantially in the focal point of the at least one parabolic facet surface.

Each outcoupling element 404 may be adapted to couple out light from an output surface portion 402b of the light guide 402 such that at least a portion of light outputted from the output surface portion 402b has an intensity having a predetermined angular distribution with respect to a surface normal of the LED 401.

Referring now to Fig. 5A, there is shown a schematic side view of a light- emitting device 500 according to an exemplifying embodiment of the present invention. The light-emitting device 500 comprises a LED 501 optically coupled to an input surface portion 502a of a light guide 502 via a light-angle selective transflector 503 adapted to at least partially reflect light incident on the light-angle selecting transflector 503 within a predetermined angle interval with respect to a surface normal of the LED 501 and at least partially transmit light incident on the at least one light-angle selecting trans flector 503 outside the predetermined angle interval.

The LED 501 may for example be coupled to the light-angle selecting trans flector 503 via a layer of index matching adhesive, such as index matching silicon adhesive (not shown in Fig. 5A, see, e.g., Fig. 3).

With further reference to Fig. 5A, the light guide 502 comprises one or more outcoupling elements 504, wherein each outcoupling element 504 comprises a parabolic facet surface comprising a first (bottom) and a second (top) facet surface portion 504a, 504b, respectively, each having a parabolic shape. The parabolically shaped facet surface portions 504a, 504b may be at least partially reflecting.

In accordance with the embodiment depicted in Fig. 5A, at least one first facet surface portion 504a may be configured such that its focal point is located substantially at the light-emitting surface of the LED 501. At least one second facet surface portion 504b may be configured such that its focal point is located substantially at the position of the mirror image 505 of the LED 501, similar to the configuration of the light-emitting device 400 described with reference to Fig. 4. The mirror image 505 of the LED 501 may be located at a distance from the light guide 502 that is substantially equal to the thickness of the light guide 502 in a direction parallel to a surface normal of the LED 501.

Each outcoupling element 504 may be adapted to couple out light from an output surface portion 502b of the light guide 502 such that at least a portion of light outputted from the output surface portion 502b has an intensity having a predetermined angular distribution with respect to a surface normal of the LED 501.

Referring to Fig. 5B, there is shown a schematic side view of a light-emitting device 500 according to an exemplifying embodiment of the present invention. Fig. 5B illustrates by way of example an exemplifying geometrical configuration of the facet surface portions 504a, 504b.

The configurations described with reference to Figs. 5A and 5B are based on that a first portion of the angular distribution of light entering the input surface portion 502a of the light guide 502, the first portion of the angular distribution of light comprising relatively large angles with respect to a surface normal of the LED 501, is focused by means of the first parabolic facet surface portion 504a (or a facet surface portion having a simpler geometrical shape) and the remaining portion of the angular distribution of light entering the input surface portion 502a of the light guide 502 is focused by means of the second parabolic facet surface portion 504b (or a facet surface having a simpler geometrical shape). Referring now to Fig. 6, there is shown a schematic side view of a light- emitting device 600 according to an exemplifying embodiment of the present invention. The light-emitting device 600 comprises a LED 601 optically coupled to an input surface portion 602a of a light guide 602 via a light-angle selective transflector 603 adapted to at least partially reflect light incident on the light-angle selecting transflector 603 within a predetermined angle interval with respect to a surface normal of the LED 601 and at least partially transmit light incident on the at least one light-angle selecting transflector 603 outside the predetermined angle interval.

The LED 601 may for example be coupled to the light-angle selecting transflector 603 via a layer of index matching adhesive, such as index matching silicon adhesive (not shown in Fig. 6, see, e.g., Fig. 3).

In accordance with the embodiment depicted in Fig. 6, the light guide 602 comprises a substantially flat, wedge-shaped light guiding structure, wherein the wedge- shaped light guide is tapered from an output surface portion 602b to the input surface portion 602a at an angle φ of about 1° to about 9° with respect to one or both of the output surface portion 602b and the input surface portion 602a. Depending on application requirements and/or user needs, the wedge angle φ may be between about 1° to about 3°.

Referring now to Fig. 7 A, there is shown a schematic side view of a light- emitting device 700 according to an exemplifying embodiment of the present invention. The light-emitting device 700 comprises a LED 701 optically coupled to an input surface portion 702a of a light guide 702 via a light-angle selective transflector 703 adapted to at least partially reflect light incident on the light-angle selecting transflector 703 within a predetermined angle interval with respect to a surface normal of the LED 701 and at least partially transmit light incident on the at least one light-angle selecting transflector 703 outside the predetermined angle interval.

The LED 701 may for example be coupled to the light-angle selecting transflector 703 via a layer of index matching adhesive, such as index matching silicon adhesive (not shown in Fig. 7A, see, e.g., Fig. 3).

In accordance with the embodiment depicted in Fig. 7A, the light guide 702 may comprise a substantially flat, wedge-shaped light guiding structure, wherein the wedge- shaped light guide is tapered from an output surface portion 702b to the input surface portion 702a at an angle φ of about 1° to about 9° with respect to one or both of the output surface portion 702b and the input surface portion 702a. Depending on application requirements and/or user needs, the wedge angle φ may be between about 1° to about 3°. With further reference to Fig. 7 A, the light-emitting device 700 may comprise an optical redirection structure 704 arranged adjacent to, or being optically coupled to, the output surface portion 702b of the light guide 702.

The optical redirection structure 704 may be adapted to redirect light such that at least some of the light outputted from the optical redirection structure 704 has an intensity having a predetermined angular distribution with respect to a surface normal of the LED 701.

In accordance with the embodiment depicted in Fig. 7A, the optical redirection structure 704 may comprise a plurality of micro -prismatic optical elements 704a (of which only some are indicated by reference numerals in Fig. 7A), or teeth structures.

Each micro -prismatic optical element 704a may comprise at least one substantially flat facet surface arranged at a predetermined angle with respect to a surface normal of the LED 701. By the particular choice of micro -prismatic optical element configuration, the light-emitting device 700 may be configured so that at least some of the light outputted from the optical redirection structure 704 has an intensity having a desired angular distribution with respect to a surface normal of the LED 701.

With reference to Fig. 7B, there is shown a micro -prismatic optical element 704a of an optical redirection structure in accordance with an exemplifying embodiment of the present invention. The micro -prismatic optical element 704a exhibits an asymmetric teeth structure. More precisely, the micro -prismatic optical element 704a comprises two flat facet surfaces 705, 706 arranged at an angle φ of about -22° to about -19° and an angle ψ ₂ of about 33° to about 36°, respectively, with respect to the surface normal n of the LED (the LED not shown in Fig. 7B, see Fig. 7A).

In conclusion, it is disclosed a light-emitting device comprising at least one light guide, an optical coupler and at least one LED. The optical coupler is adapted to optically couple the at least one LED to at least one input surface portion of the at least one light guide. The optical coupler may comprise at least one light-angle selecting transflector. The at least one light guide comprises at least one outcoupling element adapted to couple out light from the light guide such that at least some of the light outputted from the light guide has an intensity having a predetermined angular distribution with respect to a surface normal of the at least one LED. By such a configuration, one or more LEDs and additional optics may be provided in an integrated solution that may relatively easily be assembled and maintained in a desired position relatively each other, and which integrated solution may be relatively thin and compact in comparison with known devices. While the 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 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 measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.