WO2010109381A1

WO2010109381A1 - Lighting device

Info

Publication number: WO2010109381A1
Application number: PCT/IB2010/051175
Authority: WO
Inventors: Michel C. J. M. Vissenberg; Lieven R. R. Desmet; Willem L. Ijzerman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-03-24
Filing date: 2010-03-18
Publication date: 2010-09-30

Abstract

This invention relates to a lighting device (1) comprising a light source (3), a light guide (5), an output light beam forming element (7), arranged to form light exiting from the light guide into an output light beam, and a reflective element (15). The light guide has a rear surface (9), a front surface (11), and a light entrance surface. At least a portion of the rear surface is inclined relative to the front surface. The reflective element is arranged at said inclined portion of the rear surface, such that light which has entered the light guide through the light entrance surface is directed out through the front surface. The light entrance surface is provided with output light beam broadening grooves extending substantially perpendicular to the front surface.

Description

Lighting device

FIELD OF THE INVENTION

The present invention relates to a lighting device comprising a light guide, a light source, a reflective element, and an output light beam forming element. Further, the present invention relates to a method of manufacturing such a lighting device.

BACKGROUND OF THE INVENTION

A lighting device of the above-mentioned basic structure is disclosed in EP 0 760 962 Bl, and more particularly in Fig. 14 therein. The rear surface of the light guide is inclined relative to the front surface, and the reflective element is arranged at the inclined surface. The light guide has an entry-surface constituted by the wall of the centre optical cavity. The output light beam forming element is embodied by a redirection plate, which is arranged in front of the front surface of the light guide. Light rays propagate outward from the light source, partly forward to exit the lighting device directly through the redirection plate, and partly to enter the light guide through the entry surface, to be reflected once or more by the front and rear surfaces, and then to exit the lighting device through the front surface and through the redirection plate. For the purposes of this application, the light beam properties of interest are beam width and glare. More particularly, it is desired to design a lighting device having a high intensity output light beam within a predefined viewing angle 2xα, where α is the polar angle measured from the centre axis of the lighting device, and the beam, and a low intensity of light output outside of the viewing angle, in order not to cause excessive glare. However, it is also desired to be able to present lighting devices having different beam widths as appropriate for different applications of the lighting devices. Generally, broadening the beam means increasing the intensity also at viewing angles in excess of the beam width. A term commonly used within this field is "tail intensity", i.e. the level of intensity at the tail of a distribution graph showing intensity versus angle. A commonly used glare norm says that the light intensity should be lower than 1 kCd/m² at α=65 degrees.

In order to lower the costs of manufacture it would be desirable to be able to offer a number of different light properties, as regards beam width, while keeping the number of different parts low. Preferably, lighting devices providing different types of beams should share a relative large number of parts.

SUMMARY OF THE INVENTION It is an object of the present invention to enable the manufacture of different lighting device models as regards beam width while keeping the ratio of different parts and similar parts low.

This object is achieved by a lighting device according to the present invention as defined in claim 1. The invention is based on an insight that it is possible to alter the light output characteristics of the lighting device by amending the geometrical properties of the light entrance surface without causing excessive glare at large viewing angles.

Thus, in accordance with an aspect of the present invention, there is provided a lighting device comprising a light source, a light guide, an output light beam forming element, arranged to form light exiting from the light guide into an output light beam, and a reflective element. The light guide has a rear surface, a front surface, and a light entrance surface, which is arranged to receive light from the light source. At least a portion of the rear surface is inclined relative to the front surface. The reflective element is arranged at said inclined portion of the rear surface, such that light which has entered the light guide through the light entrance surface is directed out through the front surface. The light entrance surface is provided with output light beam broadening grooves extending substantially perpendicular to the front surface.

In accordance with an embodiment of the lighting device, the grooves are each defined by at least two surface portions arranged at an angle to each other. Thereby the forming of the grooves is comparatively simple.

In accordance with an embodiment of the lighting device, the angle is an obtuse angle.

In accordance with an embodiment of the lighting device, the grooves are V- shaped in cross-section. The V-shape provides for an efficient manufacture of the lighting device, and causes a significant impact on the beam width.

In accordance with an embodiment of the lighting device, the grooves are arc- shaped in cross-section. For example the grooves can be concave, convex or a mix thereof. In comparison with the V-shape, the arc-shape causes a more smooth intensity distribution of the output light beam. In accordance with an embodiment of the lighting device, the grooves are arranged adjacent to each other. This embodiment is advantageous in that is achieves a larger influence on the beam properties than one having a distance between the grooves.

In accordance with an embodiment of the lighting device, the light guide comprises a centre optical cavity at least partly enclosed by a wall surface including said light entrance surface. The light source is arranged in the centre optical cavity. Thereby different desirable shapes of the light guide are easily obtainable.

In accordance with an embodiment of the lighting device, the reflective element comprises a reflective layer, and said at least a portion of the rear surface is provided with the reflective layer. This embodiment provides for a simple manufacture of the reflective element.

In accordance with an embodiment of the lighting device, the output light beam forming element is a light transmissive redirection plate which is arranged in front of the front surface. In accordance with an embodiment of the lighting device, the redirection plate comprises elongate redirection prisms at a rear side thereof, which rear side faces the front surface of the light guide, wherein each prism has a plurality of facets wherein a first facet thereof is a light entrance facet receiving light from the light guide, and a second facet thereof is a total internal reflection facet which reflects internal light received from said first facet out of the redirection plate through a front side thereof, wherein at least said second facet is curved. In this embodiment two beam width tuning features are combined, thereby causing a combined effect.

In accordance with an embodiment of the lighting device, the light guide and the redirection plate are rotationally symmetric and the prisms extend in a circumferential direction of the redirection plate, and the curvature of each curved facet is provided along a radial cross-section of the prism.

In accordance with an embodiment of the lighting device, the light source comprises light emitting diodes, which is an efficient source.

In accordance with an embodiment the lighting device further comprises a diffuser arranged between the light source and the light entrance surface. The diffuser adds to the diffusing function provided by the grooves of the entrance surface. In accordance with another aspect of the present invention there is provided a luminaire comprising at least one lighting device. In accordance with another aspect of the present invention there is provided a method of manufacturing a lighting device comprising: providing a light source and a light guide, having a rear surface, a front surface, wherein at least a portion of the rear surface is inclined relative to the front surface, and a light entrance surface, arranged to receive light from the light source ; arranging a reflective element at said inclined portion of the rear surface, such that light generated by the light source, which light has entered the light guide through the light entrance surface, is directed out through the front surface; providing an output light beam forming element, arranged to form light exiting from the light guide into an output light beam; and performing at least one beam width tuning measure chosen from a group of beam width tuning measures including: providing the wall surface with output light beam broadening grooves extending substantially perpendicular to said front surface.

These and other aspects, features, and advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail and with reference to the appended drawings in which: Fig. 1 is a schematic cross-sectional view of a part of a an embodiment of a lighting device according to the present invention;

Fig. 2 is a graph illustrating beam features;

Fig. 3 is a schematic cross-sectional view of a part of an embodiment of the lighting device according to the invention; Fig. 4 is a schematic cross-sectional view of a portion of the part shown in

Fig. 3;

Fig. 5 is a diagram showing beam shapes of different embodiments of the lighting device;

Fig. 6 is a diagram illustrating the influence on some beam characteristics that are obtained with a prior art modification of the lighting device;

Fig. 7 is a diagram similar to that in Fig. 6 but for embodiments of the lighting device;

Fig. 8 is a cross-sectional view of a detail of the lighting device in Fig. 1; Fig. 9 is a diagram similar to that in Fig. 6 but for an embodiment of the lighting device;

Fig. 10 is a diagram showing beam widths for different embodiments of the lighting device; Fig. 11 is a diagram similar to that in Fig. 6 but for embodiments of the lighting device;

Figs. 12 to 14 are schematic cross-sectional views of a portion of different embodiments of the lighting device; and

Fig. 15 is a schematic front view of another embodiment of the lighting device according to this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Fig. 1, a lighting device 1 comprises a light source 3, a light guide 5, and an output light beam forming element, embodied by a redirection plate 7, which is arranged in front of the light guide 5. Of course there may be other parts as well, such as a housing in which the just mentioned parts are mounted, etc., but these are well known to the person skilled in the art and will not be described here since they are not necessary for the understanding of the present invention.

The light guide 5 guides the light that is generated by the light source 3, and the redirection plate 7 redirects the light that is output by the light guide 5 and shapes the light beam that is output by the lighting device 1. The light guide 5 has an overall shape of a rotationally symmetric plate, and is typically circular. It has a rear surface 9, shown as a lowest surface in Fig. 1, and a front surface 11. Further, the light guide 5 has a centre optical cavity 17, which is enclosed by a wall surface 19, which is cylindrical in this embodiment, a rear end surface 21, and a front end surface 23. The rear and front end surfaces 21, 23 are attached to the respective opposite open ends of the cylindrical wall surface 19. Thereby the wall surface 19 and the end surfaces 21, 23 together form a full enclosure. The wall surface 19 is light transmissive and comprises a light entrance surface of the light guide 5, which is also called an incoupling facet. In this embodiment the light entrance surface constitutes the whole wall surface 19, but embodiments where the light entrance surface constitutes a part of the wall surface are also feasible. The end surfaces 21, 23 are reflective. Alternatives are possible, for example where one end surface is at least partly transmissive or is excluded. Basically the light guide 5 has initially been provided with a centre through-hole, which has then been covered by rear and front lids, aligned with the rear and front surfaces, respectively, and having reflective inner surfaces 21, 23. A diffuser 25 is arranged between the light source 3 and the light entrance surface 19. In this embodiment, the diffuser 25 has been realized as a diffusive layer 25 covering the wall surface 19. Here the diffusive layer 25 is a layer that emits light upon illumination, preferably a phosphor layer. The diffusive layer 25 is typically not in direct contact with the light entrance surface 19, instead there is a small, equidistant air gap between the light entrance surface 19 and the diffusive layer 25. The gap is preferably as small as possible without there being any optical contact between the wall surface 19 and the layer 25. The layer 25 may even be in mechanical contact with the wall surface 19, as long as it does not undesirably affect the optical properties of the lighting device 1. The light source 3 has a plurality of light emitting diodes (LEDs) and is mounted on the rear inner surface 21. It should be noted that the optical cavity, although advantageous, is not necessary, since other structures for arranging the light entrance surface and the light source are possible, while keeping the basic function, i.e. the generated light is received by the light entrance surface, which couples the light into the light guide. A light output portion 13 of the rear surface 9 causes the light to exit the light guide through the front surface 11. The light output portion 13 is at least a portion of the rear surface 9, and in this embodiment it is an outer ring of the rear surface 9, which is inclined to the front surface 11 and to the rest of the rear surface 9. Thus, a portion of the light guide 5 cooperating with the light output portion 13 is wedge shaped in radial cross-section. The light output portion 13 comprises a reflective element, which here is a reflective layer 15, which can be constituted by for instance a reflective coating, e.g. deposited on the basic light guide material, or a reflective film, or a reflective plate, e.g. adhered to or arranged adjacent to the basic light guide material. As illustrated in fig. 1, the inclination of the light output portion, i.e. the angle to the front surface has been chosen according to common optic rules such that reflected light rays will exit the light guide 5 through the front surface 11. In Fig. 1 three radii have been indicated, rl, r2, and r3, where rl is the radius of the centre optical cavity 17, the inner radius of the light guide 5, and the radius of an inner ring shaped portion 27 thereof. Further, r2 is the outer radius of the inner ring shaped portion 27, and the inner radius of the light output portion 13, and r3 is the outer radius of the light output portion 13, and of the light guide 5 as a whole. Finally, H is the height of the inner ring shaped portion 27. An example of dimensions is: rl=3.5mm; r2=13.5mm; r3=55mm; H=4mm.

The inner ring shaped portion 27 of the light guide 5 is arranged for providing total internal reflection (TIR) of the LED light that enters the light guide 5, while the wedge shaped light output portion 13 is arranged to alter the angle of incidence towards the front surface 11 in order to admit light to be coupled out through the front surface 11. The light which is output of the light guide 5 then passes the redirection plate 7, while being redirected, and more particularly despread into a distinct light beam, thereby.

Referring now to Fig. 3, a simplified illustration of the light guide 50 of an embodiment of the lighting device is shown in cross-section along a plane parallel with the front surface of the light guide 50. The view is a front view, wherein the LEDs 3 are arranged at the bottom of the cavity 53. In order to tune the beam width of the output light beam, the light entrance surface, here the wall surface 51 surrounding the optical cavity 53, is provided with grooves 55. The grooves are formed in the longitudinal direction of the optical cavity 53, i.e. in parallel with the longitudinal centre axis A-A thereof and perpendicular to the front surface of the light guide 50, and are positioned adjacent to each other. In this embodiment each groove 55 is constituted by two flat surfaces 57, 59, see Fig. 4, forming a V-shape, i.e. the grooves 55 are V-shaped in cross-section. It should be noted that in preferred embodiments the groove width is substantially smaller in relation to the circumference of the cavity 53, and thus the number of grooves 55 is substantially greater, than illustrated in Fig.3. By arranging the grooves 55 in comparison to a smooth wall surface 51 a broadening, or widening, of the output beam is obtained. When the wall surface 51, or incoupling facet, is smooth the beam width in azimuth direction is narrowed according to Snell's law, since the light is refracted towards the normal to the interface, i.e. the incoupling facet 51. The normal corresponds with the radial direction of the cavity 53. This beam narrowing is counteracted by the grooves 55. The amount of the broadening, inter alia, is dependent on the top angle A between the two surfaces, or subfacets 57, 59 of each groove 55. Each subfacet 57, 59 causes a narrowed beam, but due to the grooves 55 the orientation of the beam is rotated away from said radial direction, and as a total result the output beam is broadened, or widened, in comparison to that of the same lighting device with a smooth incoupling facet. By varying the top angle A, the beam width can be tuned. Some examples are given in Fig. 5, where the beam shape is compared for the cases of no grooves, i.e. a smooth incoupling facet, and top angles 105, 98, and 90 degrees. It can be seen that in this very example, 90 degrees causes the largest broadening but also causes a beam with two lobes, while 98 degrees causes a considerable broadening but the beam is substantially a one-lobe beam. Thus, in order to produce lighting devices with different characteristics, as regards the beam width, the only part that needs to be modified is the light guide, and only to a minor extent, i.e. the dimensions of the grooves in the incoupling facet. Another way to tune the beam width by means of the grooves 55 is to vary their density, i.e. instead of positioning them adjacent to each other, they are positioned at a distance from each other, which is varied depending on the desired beam width. It should be noted that the above-defined direction of the grooves is preferred. However, a minor deviation from the strictly perpendicular direction is acceptable, in the order of a few degrees, for instance up to three degrees, without excessively deceasing the beam properties of the lighting device. Thus, for the purposes of this application the direction of the grooves is defined as substantially perpendicular to the front surface 11 of the light guide 5, which is for the presented embodiments the same as substantially parallel to the longitudinal centre axis A-A of the optical cavity, where the term "substantially" is to be interpreted as covering said minor deviation. For purposes of comparison with prior art, in EP 0 760 962, some kind of grooves in a surface arranged in the optical cavity are disclosed, see figs. 120 and 28C. Those grooves may provide a broadening effect, however in a plane perpendicular to that of the broadening according to this invention, since the grooves extend parallel to the corresponding front surface rather than substantially perpendicular to the front surface. In addition to the broadening effect, the grooves 55 cause a diffusing effect.

As a result, in many applications the separate diffuser can be omitted, while providing a desirable additional diffusing property in others.

As mentioned above in the "background of the invention", It is desired that the widening does not cause excessive glare. It should be noted that for the purposes of this application, and also as a commonly used definition, beam width is defined in terms of

FWHM, i.e. Full Width at Half Maximum, as illustrated in Fig. 2, where the graph illustrates light intensity versus polar angle. In this graph FWHM=2xα. In practice the appropriate scale for the intensity axis in the graph is logarithmic, as in Figs. 6, 7, and 9.

For reasons of comparison, as an example of a less desirable way of broadening the light beam, it is now referred to Fig. 6. The curve drawn between triangular value points shows the intensity distribution for a lighting device, which has a relatively concentrated and narrow beam. The radius of the optical cavity, which also influences the beam width as will be explained below, is rl=3.5 mm. The beam width for the narrow beam lighting device is 2x5.0 deg FWHM. The curve drawn between the circular value points shows the intensity distribution when the same lighting device has been provided with a

Gaussian diffusor at the exit of the lighting device. The beam has indeed been broadened to 2x25 deg FWHM, but at the same time the intensity at large polar angles has increased significantly. As a contrast, when introducing the grooves of the incoupling facet, the widening becomes much more favorable, as illustrated in Fig. 7. Here a lighting device having r 1=2 Omm, r2=20mm, and the other dimensions equal to those given in the above example, is considered. When given a smooth incoupling facet the lighting device has the beam characteristic of the square value points, i.e. the beam width is 2x15 deg FWHM. The glare is small. In comparison, a lighting device having a grooved incoupling facet, where the facet top angle is 98 degrees has the beam characteristic of the rhomb value points. The beam width has broadened to 2x31 deg FWHM, while the tail intensity at angles above α=60 deg remains substantially unaltered. As a connection to Fig. 6 the same graph for rl=3.5mm, r2=13.5mm, and a smooth incoupling facet is shown in Fig. 7 as well.

Referring again to Fig. 1, an additional variable that can be used is the structure of the rear surface of the redirection plate. The redirection plate 7 is provided with elongate redirection prisms 29 at a rear side thereof, which rear side faces the front surface of the light guide 5, wherein each prism 29 has a plurality of facets 31, 32, and more particularly two facets 31, 32 in this embodiment. Each prism 29 extends circularly around the redirection plate 7, which is rotationally symmetric. One of the facets 32 is curved, as illustrated in an enlarged view of a radial, i.e. relative to the lighting device as a whole, cross-section of the prism 29 in Fig. 8. The arc shaped line 41 represents the curved facet 32, and the straight line 43 extending at an angle to the arc shaped line 41, represents the flat facet 31. A prism with two flat facets 45, 47 is additionally indicated in Fig. 8 with broken lines for reasons of comparison and for enhancing the viewer's perception of the curvature, which is relatively moderate. By designing different redirection plates with more or less curved facets it is possible to adjust the width of the output light beam without changing the outer dimensions of the redirection plate 7. Consequently the other parts of the lighting device can be kept the same and still lighting devices with different illumination properties can be obtained. More particularly, the beam width of the lighting device can be broadened by means of providing the redirection plate 7 with the curved facets 32. By changing the radius of the curvature the beam width will be tuned.

However, as discussed initially, when broadening the beam it is desirable that the glare is not increased, or at least increased to a minor extent. By modifying the facets of the redirection plate this is achieved, as will be exemplified below.

As a contrast to the diffusor example, by applying the curved facets, in a lighting device having rl=20mm, the light beam broadens from 2x15 deg FWHM to 2x24 deg FWHM, while the intensity is substantially the same at polar angles in excess of about 53 degrees, see Fig. 9. In this example, the radius of the curvature of the curved facets 41 is r4=0.96mm, while the height h of the prisms is h=0.354mm, as shown in fig. 8.

As mentioned above, also the radius of the optical cavity influences the width of the output light beam. Thus, according to one aspect of the invention the manufacturing of the lighting device includes an active choice of this cavity radius. In order to illustrate this radius influence on the beam width, for a certain design of the lighting device the beam width varied from 2x5 deg FWHM at r 1=3.5 mm to 2x15 deg FWHM at rl=20 mm, as illustrated by Fig. 10.

Above, three different ways of tuning the beam width by only minor modifications of a single part of the lighting device have been shown. The forming of the grooves in the light entrance surface is the primary light beam tuning measure, but it can be combined with one or more of the others. For example, by combining V-grooves and curved facets and a large cavity radius a beam width of 2x37deg FWHM is obtained, see Fig. 11. The dimensions are rl=20mm, r2=20mm, r3=55mm, H=4mm, top angle of V-grooves=103 deg, and r4=0.96mm.

Above, embodiments of the lighting device according to the present invention as defined in the appended claims have been described. These should be seen as merely non- limiting examples. As understood by a skilled person, many modifications and alternative embodiments are possible within the scope of the invention. For example, referring to figs. 12-14, the grooves of the wall surface 121, 131,

141, and more particularly the light entrance surface thereof, which can constitute the whole or a part of the wall surface, can be given a different shape than the V- shape, such as an arc- shape in cross-section. Thus, in an alternative embodiment the grooves 121 are convex, in a further embodiment the grooves 131 are concave, and in yet another embodiment the grooves 141 are alternatively convex and concave. In the latter embodiment, when the grooves 141 are adjacent to each other the wall surface is approximately sine-shaped.

Referring now to Fig. 15, in alternative embodiments the lighting device is not fully circular, but embodies a segment of a circle. In the particular embodiment illustrated in Fig. 15, the lighting device 151 is a quarter segment. Thereby the lighting device 151 is adapted to be mounted in a corner of a room. While being placed at the point of the segment 151, still the optical cavity 153 is regarded as a "centre optical cavity", where the "centre" is the centre of the full circle of which the lighting device is a segment. In this embodiment the light entrance surface constitutes an arc-shaped portion of the wall surface of the optical cavity 153. The wall surface additionally includes plane portions 155, 157 attached to the arc- shaped portion, arranged at a right angle, and forming the pointed end of the lighting device 151. While being included in the radially extending sides 159, 161 of the lighting device 151, the plan wall surface portions 155, 157 are not part of the light entrance surface. Typically the plan wall surface portions 155, 157 are made reflective, while other alternatives are possible, and also they may have alternative shapes, while still closing the cavity 153.

It should be noted that, for instance, the lighting device according to the present invention is applicable in a luminaire. The luminaire can contain a single lighting device or plural lighting devices depending on the aimed at use of the luminaire.

It is to be noted, that for the purposes of this application, and in particular with regard to the appended claims, the word "comprising" does not exclude other elements or steps, that the word "a" or "an", does not exclude a plurality, which per se will be apparent to a person skilled in the art.

Claims

CLAIMS:

1. A lighting device (1) comprising a light source, a light guide (5), an output light beam forming element (7), arranged to form light exiting from the light guide into an output light beam, and a reflective element (15), wherein the light guide has a rear surface (9), a front surface (11), and a light entrance surface (19), arranged to receive light from the light source (3), wherein at least a portion of the rear surface is inclined relative to the front surface, and wherein the reflective element is arranged at said inclined portion of the rear surface, such that light which has entered the light guide through the light entrance surface is directed out through the front surface, characterized in that the light entrance surface is provided with output light beam broadening grooves (55) extending substantially perpendicular to said front surface.

2. A lighting device according to claim 1, wherein said grooves (55) are defined by at least two surface portions (57, 59) arranged at an angle to each other.

3. A lighting device according to claim 2, wherein said angle is an obtuse angle.

4. A lighting device according to claim 1, 2 or 3, wherein the grooves (55) are V- shaped in cross-section.

5. A lighting device according to claim 1, 2, or 3, wherein said grooves (121,

131, 141) are arc-shaped in cross-section.

6. A lighting device according to any one of the preceding claims, wherein the grooves (55) are arranged adjacent to each other.

7. A lighting device according to any one of the preceding claims, wherein the light guide (5) comprises a centre optical cavity (17) at least partly enclosed by a wall surface including said light entrance surface, and wherein said light source is arranged in the centre optical cavity.

8. A lighting device according to any one of the preceding claims, wherein the reflective element (15) comprises a reflective layer, and wherein said portion of the rear surface (9) is provided with the reflective layer.

9. A lighting device according to any one of the preceding claims, wherein the output light beam forming element (7) is a light transmissive redirection plate which is arranged in front of the front surface (11).

10. A lighting device according to claim 9, wherein the redirection plate (7) comprises elongate redirection prisms (29) at a rear side thereof, which rear side faces the front surface (11) of the light guide (5), wherein each prism has a plurality of facets (31, 32) wherein a first facet thereof is a light entrance facet receiving light from the light guide, and a second facet thereof is a total internal reflection facet which reflects internal light received from said first facet out of the redirection plate through a front side thereof, wherein at least said second facet is curved.

11. A lighting device according to claim 9 or 10, wherein the light guide (5) and the redirection plate (7) are rotationally symmetric and the prisms (29) extend in a circumferential direction of the redirection plate, and the curvature of each curved facet is provided along a radial cross-section of the prism.

12. A lighting device according to any one of the preceding claims, wherein the light source (3) comprises light emitting diodes.

13. A lighting device according to any one of the preceding claims, wherein the lighting device further comprises a diffuser (25) arranged between the light source (3) and the light entrance surface (19).

14. A luminaire comprising at least one lighting device according to any one of the preceding claims.

15. A method of manufacturing a lighting device comprising: providing a light source (3) and a light guide (5), having a rear surface (9), a front surface (11), wherein at least a portion of the rear surface is inclined relative to the front surface, and a light entrance surface, arranged to receive light from the light source ; arranging a reflective element (15) at said inclined portion of the rear surface, such that light generated by the light source, which light has entered the light guide through the light entrance surface, is directed out through the front surface; providing an output light beam forming element (7), arranged to form light exiting from the light guide into an output light beam, characterized by performing at least one beam width tuning measure chosen from a group of beam width tuning measures including providing the wall surface with output light beam broadening grooves (55) extending substantially perpendicular to said front surface.