WO2011007305A1

WO2011007305A1 - Lighting device

Info

Publication number: WO2011007305A1
Application number: PCT/IB2010/053169
Authority: WO
Inventors: Michel Cornelis Josephus Marie Vissenberg; Willem Lubertus Ijzerman; Fetze Pijlman; Bastiaan Uitbeijerse
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2009-07-15
Filing date: 2010-07-12
Publication date: 2011-01-20

Abstract

It is presented a lighting device (10,100) in which the light from a light source (11,101) is entered into a hollow light guide (13, 130). The hollow light guide is formed by a transmissive optical element (12, 120), which comprises a light output surface (17,121) for outputting light from the lighting device, a reflector (14, 135), and a diffuser (15, 137). Further, the reflector is arranged inclined relative to the transmissive optical element. The diffuser is arranged to form a light input surface (18, 141) into the hollow light guide. Light received in the hollow light guide and being by the reflector is directed towards the transmissive optical element. The transmissive optical element is a redirection element.

Description

Lighting device

FIELD OF THE INVENTION

The present invention relates generally to lighting devices and more particularly to a lighting device comprising a light source, and a hollow light guide formed by a transmissive optical element comprising a light output surface for outputting light from the lighting device, and a reflector, which is arranged inclined relative to the transmissive optical element.

BACKGROUND OF THE INVENTION

A known lighting device having the above-mentioned basic structure is disclosed in US. Patent No. 5,046,805, and more particularly as an embodiment as described in reference to Fig. IB therein. A reflective surface is constructed and distributed at an inclined angle with respect to an output modifier, such that a collimated light beam injected in the wedge shaped hollow light guide formed by the reflective surface and the output modifier reaches the reflective surface at an inclined angle. The light is reflected by the reflective surface towards the output modifier surface, which may be e.g. a diffuser. Part of the light is transmitted without any further internal reflections in the hollow light guide, and part of the light is further reflected within the hollow light guide. Due to the reflective surface being inclined, the inputted collimated light is in this way homogenously distributed along the modifier surface before exiting the lighting device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved and alternative lighting device.

The invention is defined by the appended independent claim. Preferred embodiments are set forth in the dependent claims and in the following description and drawings.

According to a first aspect of the present invention there has been provided a lighting device comprising a light source, and a hollow light guide. The hollow light guide is formed by a transmissive optical element comprising a light output surface for outputting light from the lighting device, a reflector, which is arranged inclined relative to the transmissive optical element, and a diffuser. The diffuser is arranged to form a light input surface into the hollow light guide. The transmissive optical element is a redirection element.

Thus, a lighting device which in an efficient way provides a widened and directionally controlled beam of light is achieved. The diffuser provides efficient incoupling of the emitted light from the light source into the hollow light guide. Further, the diffuser provides a relatively large light input area into the hollow light guide, and thereby allows for utilizing discrete light sources in the lighting device. In the hollow light guide, diffused light reaches the reflector and is reflected towards the redirection element in a spatially broadened distribution to finally be redirected and outputted via the light output surface of the redirection element. The redirection element is preferably arranged to redirect light before exiting the lighting device into one single direction. The redirection element may be arranged to change the direction of light by means of prisms.

According to an embodiment of the lighting device, a partially reflective element is arranged inside the hollow light guide and in front of the redirection element. The partially reflective element enhances the amount of light being reflected back into the hollow light guide, such that the light is more efficiently spread before exiting the device which is advantageous for providing increased quality of the output beam. Further, the partially reflective element may be arranged to at least partly collimate the light before entering the redirection element.

According to an embodiment of the lighting device, the device further comprises a collimating element arranged between the diffuser and the hollow light guide, which is advantageous for improving the beam properties and the light intensity distribution at the light output surface. Collimation in a first direction helps for beam spreading, since the light will travel farther towards the outer rim of the hollow light guide before encountering the redirection element. Further, collimation in any direction increases the control of the direction of light as it enters the redirection element. This way narrow beams, and beams with less glare light can be made which is advantageous.

According to an embodiment of the lighting device, the collimating element is a prismatic collimating foil or plate. The prismatic collimating foil allows for providing pre- collimated light in an arbitrary direction (depending on the prism orientation) or in both directions. These foils are advantageous since they only take a very limited space.

According to an embodiment of the lighting device, the collimating element is tapered at least in a first direction. This advantageously increases the uniformity of the light beam in this direction. In case of a rotationally symmetric lighting device, the collimation in a second direction comes automatically because of the symmetry of the system. In case of a rectangular lighting device, also the second direction is preferably tapered for collimation. Thereby, pre-collimation of light in both directions upon entering into the hollow light guide is achievable.

According to an embodiment of the lighting device, the collimating element is asymmetrically arranged such that the light beam entering into the hollow light guide is mainly directed towards the reflector. By this, the center of the distribution of light from the light input surface is aimed in the direction of the reflector instead of the horizontal direction or at the redirection element. This provides a longer propagation of the light towards the outer rim of the hollow light guide before encountering the redirection element and exiting the lighting device. This advantageously allows the light to travel farther to the outer edge of the system, thereby improving the uniformity of the luminance profile across the exit of the optical system.

According to an embodiment of the lighting device, the hollow light guide and the redirection element are rotationally symmetric, thereby providing a large light output surface and a symmetric light beam which is desirable for lighting applications, such as e.g. downlighting etc. Rotational symmetry gives automatic collimation of the tangential (or azimuth) direction in the beam. Non-rotational symmetric system typically requires additional arrangements to control the orthogonal direction of the beam, e.g. a tapered guide to collimate the light that enters the wedge.

According to an embodiment of the lighting device, the device further comprises a center cavity at least partly enclosed by a wall surface including the light input surface. The light source is arranged in the center cavity. By arranging the light source inside a cavity with diffuse exit surfaces, the beam formation by the optical system becomes independent of the exact properties of the source (size or position of the source, and direction, color and intensity of the light). Thus the number, type and positions of the sources may be changed without altering the beam distribution (both luminance and illuminance distributions). This is particularly advantageous for LED systems, because of the natural spread in light output of LEDs due to the manufacturing process or due to differences in ageing or temperature. This further allows for the spread to be averaged out by combining multiple sources inside a cavity. Also, the rapid development of LED efficiency and flux requires optical systems that are robust against the number of applied LEDs. (I.e. this year, the downlight may contain 10 LEDs, but next year the same illumination may be achieved having 8 LEDs, etc.)

According to an embodiment of the lighting device, the center cavity comprises a light guide. For a rotationally symmetric system, the light guide is preferably cylinder shaped to fit the center cavity.

According to an embodiment of the lighting device, the light source comprises light emitting diodes, which are efficient light sources. Further, LEDs are small, which allows for compact lighting systems. Also, combinations of LEDs with different colors inside a cavity, allows for a light source with variable color.

According to an embodiment of the lighting device, the partially reflecting element is or comprises one of a transparent plate, a multilayered stack, a multilayered dielectric coating, and a semitransparent mirror.

The degree of reflectivity and transmissivity may be set to match a required beam spreading. By selecting a high reflectance, a larger amount of the light is distributed closer to the outer rim of the hollow light guide before exiting the lighting device.

According to an embodiment of the lighting device, the partially reflecting element is angle selective. The angle selectivity may be such that it is more reflective for light having a large angle of incidence than for light having a small angle of incidence, with respect to the normal to the plane of the partially reflecting element, thus broadening the light output beam.

An angle-selective filter between the hollow light guide and the redirection plate also improves the control over the beam directions (beam width and glare). Preferably the angle-selective partially reflecting element is arranged to reflect parallel light and transmitting perpendicular light.

According to an embodiment of the lighting device, the partially reflecting element is provided with a pattern with transparent portions, and reflective portions. This provides an advantageous way to control uniformity of the outputted light. The partly reflecting element may be a perforated metallic reflector, or a plastic plate with reflective paint dots.

According to an embodiment of the lighting device, the partially reflecting element is specular. This conserves the outward direction of the light, thus improving the uniformity, and also the beam control.

According to an embodiment of the lighting device, the partially reflecting element is spatially varying in reflectivity and transmissivity. In this embodiment the transmission of the partially reflecting element is position dependent which is advantageous for improving the uniformity of the illuminance profile of the outputted light beam.

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

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention. In all figures, the dimensions as sketched are for illustration only and do not reflect the true dimensions or ratios. All figures are schematic and not to scale. In particular the thicknesses are exaggerated in relation to the other dimensions. In addition, details such as LED chip, wires, substrate, housing, etc. have been omitted from the drawings for clarity.

Figs, la-b are cross-sectional side views of embodiments of a lighting device according to the present inventive concept;

Figs. 2a-b are cross-sectional side views of embodiments of a lighting device according to the present inventive concept;

Figs. 3a-b are cross-sectional side views of embodiments of a lighting device according to the present inventive concept;

Fig. 4 is a cross-sectional side view of a prior art lighting device; Fig. 5 is a graph illustrating the concept of beam width and glare for a light beam;

Fig. 6a is a graph illustrating beam profiles for a prior art lighting device and an embodiment of a lighting device according to the present inventive concept, and 6b-c are illustrations of the illuminance distribution at the output surface of the prior art lighting device and the embodiment of a lighting device according to the present inventive concept;

Figs. 7a-d are illustrations of the illuminance distribution at the output surface of embodiments of the lighting device according to the present inventive concept;

Figs. 8a-b show beam distributions from embodiments of a lighting device in accordance with the present inventive concept;

Figs. 9a-b are illustration of the illuminance distribution at the output surface of embodiments of the lighting device according to the present inventive concept; and

Figs. 10a-b show beam distributions from embodiments of a lighting device in accordance with the present inventive concept. In the drawings the same reference numerals are used for similar or corresponding elements, also when the numerals refer to elements in different embodiments.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain 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, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

Referring to Fig. Ia, a lighting device 10 comprises a light source 11, a diffuser 15, and a light transmissive optical element, embodied by a redirection element 12, for forming of an output light beam, and a reflector 14. The reflector 14 is arranged inclined relative to the redirection element 12. Further, the redirection element 12, the reflector 14 and the diffuser 15 confine a hollow light guide 13 in which light from the light source 11 is entered via a light input surface 18 formed by the diffuser 15. In this embodiment, the diffuser 15 has been realized as a diffusive layer that emits light upon illumination, preferably a phosphor layer. When the light emitting layer 15 emits light as a response from illumination by the light source 11 , it emits light having virtually all possible angles of incident, i.e. from about +90° to -90° in relation to the normal of the light input surface 18 into the hollow light guide 13. The light entering the hollow light guide 13 via the diffuser 15 propagates in all directions, and some of it is reflected by the reflector 14 that is inclined and facing in the direction of the diffuser 18. The reflecting surface of the reflector 14 is arranged with an angle β in relation to the redirection element 12 (and in relation to the normal of the diffuser 18).

The reflecting surface 14 reflects light towards the redirection element 12, which is in a perpendicular relationship to the diffuser 18. The angle β is selected so that a uniform light beam with a desirable beam width (at full-width-at-half-maximum, FWHM) can be achieved. In most practical applications the angle β will be relatively small, such as in the range of l°- 15°.

The reflector 14 can be realized in several ways, e.g. via refraction, a mirror layer (metal or dielectric stack) etc.

As explained above, part of the diffused light in the hollow light guide 13 is reflected by the reflector 14 towards the redirection element 12. Part of the light reaching the redirection element 12 is reflected back into the hollow light guide 13 and towards the reflector 14. Thus, some of the reflected light is reflected several times within the hollow light guide 13. A major part of the light that reaches the redirection element 12 is transmitted out through the light output surface 17. The redirection element 12 is arranged to shape and direct the light beam that is output by the lighting device 10.

Other parts, such as a housing in which the just mentioned parts are mounted, etc. may be comprised in the lighting device. However, 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 inventive concept.

In Fig. Ib, an embodiment of the lighting device 20 with basically the same structure as previously described is presented. In this embodiment a partially reflective element 16 is arranged in front of the redirection element 17, and inside the hollow light guide 13. The partially reflective element 16 increases the number of internal reflections of light within the hollow light guide 13 before exiting the lighting device 20 via the redirection element 17. This provides an increased illuminance uniformity of the beam shape of the outputted without changing the beam distribution. The partially reflective element 16 is in an embodiment of the lighting device a simple transparent plate, partially reflective due to Fresnel reflection.

In alternative embodiments the partially reflective element can be a

multilayered stack or have a multilayer dielectric coating provided by means of polymer processing techniques, evaporation techniques, sputtering, ion beam etch, chemical etching etc. on a glass substrate or other suitable plate to enhance the reflectivity. The partially reflective element may be angle-selective, i.e. be more strongly reflective for light with a large angle of incidence and less reflective for light with a small angle of incidence (with respect to the normal to the plane of the partially reflective element). In fact, this is already the case for the Fresnel reflector mentioned above, but the effect may be enhanced by using the appropriate multilayer stack.

Further, in yet another embodiment the partially reflective element is a semitransparent mirror having a predetermined degree of reflectivity (and transmissivity). A higher reflectance moves the light more to the outer rim of the lighting device. Metallization and dielectric films of varying density and spatial variation may be provided by means of e.g. sputtering, thermal evaporation techniques etc.

In yet another embodiment the partially reflective element comprises a dithered or dotted surface, i.e. comprises patches that are highly transmissive and patches that are highly reflective, in order to create a semi-transparency effect. Typically a glass substrate is patterned and provided with a reflective coating, such that uncoated patches are formed.

The partially reflective element is preferably (nearly) specular, but diffuse scattering reflectors may also be of advantage. In particular if the forward scattering component is dominant (i.e. the propagation direction towards the outer rim of the lighting device is not altered too much).

According to an embodiment, the partially reflective element is spatially varying in reflectivity/transmissivity. This improves the uniformity of the light at the exit, i.e. the light output surface 17 of the lighting device 20. In particular, the reflectivity should be higher at the light input surface 18 than at the opposite end of the lighting device 20, i.e. at the outer rim of the lighting device 20. The spatial variations may be achieved by varying the thickness of the (metallic) reflecting layer, or by varying the density (pitch and/or size) of highly transparent patches with respect to the density of highly reflective patches.

According to an embodiment of the lighting device, herein under described with reference to Fig. 2a, which shows a cross-sectional side view a lighting device according to the present inventive concept, the lighting device 100 is circle symmetric in a plane z-x. The lighting device 100 has a cylindrical center cavity 140 in which, at a lower part of the center cavity, light emitting diode (LED) light sources 101 are arranged on a base substrate 150, such as a PCB. The LEDs 101 may be omnidirectional. Opposite to the light sources 101, at the upper part of the cylindrical center cavity 140, a mirror 160 is arranged to cover the opening of the center cavity 140. The mirror 160 reflects light from the light source 101, and hinders light which else would escape via the cylinder opening from exiting the lighting device. The mirror 160 is optional, although it increases efficiency for providing a reduced brightness at the center of the lighting device 100. Alternatively the mirror may be inclined, cone shaped, and/or may have diffusely reflective properties for light spreading. The center cavity 140 is radially confined by a circumventing diffuser 137 comprising a light emitting layer, which here is a layer that emits light upon illumination, preferably a phosphor layer. The diffuser 137 further forms a light input surface 141 at the opening of a hollow light guide 130. The hollow light guide 130 is confined by the diffuser 137, a partially reflective element 138 arranged perpendicular to and at the upper part of the diffuser 137 having its first end at the diffuser 137, and a reflector 135 arranged inclined with respect to the partially reflective element 138 and at a second end of the partially reflective element 138, and a second reflector 136 which is perpendicular to the diffuser 137 and arranged at the lower part of the diffuser 137. The second reflector 136 is optional in which case the reflector 135 is arranged to abut on the lower part of the diffuser.

The light entering the hollow light guide 130 via the light input surface 141 first passes a cavity portion of constant thickness, confined by the second reflector 136 and the partially reflective element 138. The diffused light from the diffuser 137 is reflected by the second reflector 136, the partially reflective element 138, and the tapering reflector 135, which is inclined and facing in the direction of the light input surface 141. The reflector 135 is arranged inclining an angle β in relation to the normal of the partially reflective element 138 and the plane z-x of the light guide. Owing to the enclosing light input surface 141, light entering via the light input surface 141 and traveling in the plane z-x of the hollow light guide 130 is being redirected by the reflector 135 and thus is reflected towards the partially reflective element 138, through which part of the light escapes to a redirection element 120. The redirection element 120 is arranged parallel to the partially reflective element 138. The redirection element comprises an upper light output surface 121 through which the light exits the lighting device 100 in the y-direction in Fig. 2a.

The redirection element 120 is arranged to achieve the final adjusting and tuning of the light distribution. Here, the redirection element 120 comprises triangular elements 123 formed in the surface of the layer facing the partially reflective layer 138. The triangular elements 123 are in the form of protrusions, or ridges, encircling the center of the light guide in the z-x plane. Each triangular element 123 presents a first surface 125 facing in the direction of the center of the lighting device 100, and a second surface 127 facing away from center of the lighting device 100. The first surface 125 is arranged at a first angle in relation to the normal to the plane of the layer and the second surface 127 at a second angle. The surfaces 125, 127 meet and form the tip of the triangular element 123, which tip may be in contact, but preferably not in optical contact, with the partially reflective element 138. It should be noted that mechanical contact not necessary results in optical contact, as will be recognized be the skilled person. A light ray leaving the upper surface of the partially reflective element 138 will thus first be refracted in the first surface 125 of a triangular element 123 at an air to redirection element interface, and then be reflected by total internal reflection, TIR, in the second surface 127 of the triangular element 123 at a redirection element to air interface. The last reflection directs the light ray towards the opposite surface of the redirection layer 121, which it passes by refraction at a re-direction layer to air interface. The redirection element 121 may thus have a collimating and/or focusing effect on the light from the light guide. The redirection element 120 is in this exemplifying

embodiment made of Polycarbonate (PC) and has a refractive index of about 1.6.

The material of the redirection element 120 may generally and advantageously have an optical absorption of less than 4/m, provide low haze and scattering, contain particles smaller than 200 nm, be able to sustain an operational temperature higher than 75° C.

According to an embodiment, the redirection element 120 may be similar to a so-called re-direction foil, such as the transmissive right angle film (TRAF) as currently is available under the name Vikuti™ from 3M.

According to embodiments of the lighting device, the device may further comprise a collimating element arranged at the opening of the hollow light guide 130. Fig. 3a illustrates an embodiment of the lighting device 200 in which collimator 237 is arranged between the diffuser 137 and the hollow light guide 130. The collimator 237 has a constant thickness, that in this embodiment forms a cylindrical light guide circumventing the center cavity (in an embodiment which is not rotationally symmetric, the corresponding collimator will typically be cubic). The collimator 237 collimates the diffusive light from the diffuser 137 such that more light travels further in the hollow light guide 130 before being reflected, thus the beam properties of the outputted light beam is altered such that the illumination distribution is shifted towards the outer rim of the lighting device. The light exiting the collimator 237 has the same beam width in the plane of the cross-section (the plane of the paper), but a narrower beam width perpendicular to this plane, i.e. the azimuth direction in cylindrical coordinates. Thus the total beam width may be narrowed. As the collimator 237 is introduced in the light path, and keeping the length of the hollow light guide 130 the same as in the embodiment presented with reference to Fig. 2, the center cavity 140 radius will decrease such that the diffuser 137 is thus positioned closer to the center of the lighting device 200.

According to an embodiment of the lighting device 100, as illustrated in Fig. 2b, the partially reflective element 138 is omitted.

According to another embodiment of the lighting device 300, as presented in Fig. 3b, a similar pre-collimating light guide, the collimating element 337, is tapered in a first direction. Thus, the light beam exiting the collimator 337 is collimated in both directions upon entering the hollow light guide 130. Further, collimation in the first direction increase the beam spreading, since the light will travel farther towards the outer rim before encountering the redirection element 120. This effect on beam spreading is illustrated by results from simulations, as will be discussed further down. Also, in this embodiment the size of the center cavity is decreased, not only in radius but also in height.

According to an embodiment of the lighting device, pre-collimation as discussed above, is arranged by utilizing a prismatic collimating foil or plate, such as the prismatic BEF (Brightness enhancement foil for displays) from 3M that shapes the beam in one direction, or the micro-optic MLO plate used in the Savio range of Philips Luminaires, which collimates in both directions. These collimating optical foils can be formed as a cylinder (for the rotationally symmetric embodiments), and arranged in between the diffuser 137 and the hollow light guide 130, and produce a pre-collimated beam in an arbitrary direction (depending on prism orientation) or in both directions. An advantage of this foil method is that it takes only a very limited space. Hence, the center cavity size can remain almost the same as without the collimator.

According to an embodiment of the lighting device, the collimator is asymmetrically arranged (not shown) such that the pre-collimated light is asymmetrically distributed. Preferably the collimator is arranged such that the center of the light distribution is aimed in the direction of the reflector 135 instead of the horizontal direction or at the redirection element 120. This gives rise to a further propagation of light towards the outer rim, before encountering the redirection element 120 and exiting the lighting device.

A cross-sectional side view of a prior art lighting device is presented in Fig. 4. The lighting device 400 is rotationally symmetric and has a similar construction as the embodiments previously described. More particularly the lighting device 400 has a light source 101 in a center cavity 440, a diffuser 437 encompassing the center cavity 440, which is a light input surface for a wedge shaped light guide 470 arranged between a reflector 435 and a redirection element 420, through which light is outputted. Simulation have been performed utilizing LightTools simulation software from Optical Research Associates on the embodiments 100, 200, 300 of the present inventive concept and the prior art lighting device 400 as a comparative reference.

For the purposes of this inventive concept, the light beam properties of interest are beam width and glare, which are illustrated in Fig. 5. 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 center 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. Further, 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. 5, where the graph illustrates light intensity versus polar angle. In this graph FWHM=2xα. 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.

Fig. 6a illustrates beam profiles of a prior art lighting device as described with reference to Fig. 4, and a lighting device according to the present inventive concept as described with reference to Fig. 2b. Both lighting devices have an outer diameter of 165mm, 6 mm height and no substantially flat part (i.e. the second reflector 136 is not present). The beam profile of the prior art lighting device, graph A, has a FWHM: 2x15 degrees, and the lighting device according to Fig. 2b, graph B, has a FWHM: 2x25 degrees, i.e. the beam width of the lighting device 100 is broadened and the glare intensity is higher than the reference, but still reasonably low, implying that the redirection element is essential for controlling the glare.

In Fig. 6b and 6c the respective illuminance distribution at the output surface 421, 121 of the prior art lighting device and the lighting device 100 is illustrated. The picture illustrates a top view of the lighting devices, such that the simulated intensity of light outputted from the light output surfaces 421, 121 is plotted for the respective device. It can be seen that for the lighting device 100 according to the present inventive concept, most of the light is concentrated around the exit of the center of the light output surface 121. Further the illuminance levels are 3-4 times higher at this center as compared to the prior art device 400. In Fig. 6c, it may be seen that the light is evenly distributed over the light output surface 421. The respective counts registered by the detector for each illumination value within the detected range are presented to the right.

Fig. 7a-d are illustrations of the spatial illuminance distribution at the output surface 121 of embodiments 100, 200 and 300 of the lighting device according to the present inventive concept without the partly refractive element 138. The light source is a so called cavity source, i.e. the presented simulations have been done with the diffuser surface acting as a diffuse light source, i.e. the details inside the source cavity are ignored. Further, the cavity source is collimated in the plane of the cross-section: Fig. 7a for a diffuse light source, Fig. 7b for a light beam confined within 2x45 degrees, Fig. 7c for a light beam confined within 2x10 degrees, and Fig. 7d for a light beam confined within 2x5 degrees. Note that the distribution is most uniform at about 2x10 degrees width. The respective counts registered by the detector for each illumination value within the detected range are presented to the right.

Beam distributions for these lighting devices with different degrees of pre- collimation of the light as it enters the hollow light guide are shown in Fig. 8a-b. The collimation is arranged in the direction perpendicular to the light output surface only. There is no azimuth collimation. Graph A corresponds to the prior art lighting device, graphs B - F corresponds to the lighting device according to Fig. 2b with B: diffuse light source, C: light collimated 2x45 degrees in plane, D: light collimated 2x10 degrees in plane, E: collimated 2x5 degrees in plane, and F: collimated 2x15 degrees in plane. The graphs in Fig. 8b are the same as in Fig. 8a, but with a logarithmic scale.

Figs. 9a-c illustrate the spatial illumination distribution at the light output surface 121 of embodiments of the present invention which have a partially reflective element 138, and a Lambertian diffuse cavity source (again, the diffuser is modeled as a diffuse source, ignoring all details inside the source cavity). Fig. 9a illustrates the effect of applying a Fresnel plate (thin, flat PMMA slab) as partially reflective element 138. Slightly increased beam spreading is achieved as compared to the results in Fig. 7a. Fig. 9b illustrates the effect of applying a 50% reflective semitransparent mirror as partially reflective element 138. Fig. 9c illustrates the effect of applying a PMMA plate with a pattern of circular specular reflective dots (100% coverage at the center and low density at the outer edge). The respective counts registered by the detector for each illumination value within the detected range are presented to the right.

The effect on the beam distributions for these lighting devices with different degrees of partially reflective elements 138, and for a Lambertian diffusive cavity source, is shown in Fig. 10a-b. Note, that the partially reflective plate has almost no effect on beam distribution, but mainly on the efficiency of the lighting device (since improved uniformity requires more reflection and hence gives more absorption losses). The graph in Fig. 10b is the same as in Fig. 10a, but with a logarithmic scale.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

CLAIMS:

1. A lighting device (10, 100) comprising:

a light source (11, 101); and

a hollow light guide (13, 130) formed by:

a transmissive optical element (12, 120) comprising a light output surface (17,121) for outputting light from the lighting device;

a reflector (14, 135), which is arranged inclined relative to said transmissive optical element, and

a diffuser (15, 137);

wherein said diffuser is arranged to form a light input surface (18, 141) into said hollow light guide, and wherein said transmissive optical element is a redirection element.

2. A lighting device (20, 100) according to claim 1, further comprising a partially reflective element (16, 138) arranged inside said hollow light guide and in front of said redirection element (12, 120).

3. A lighting device ( 200) according to claim 1 or 2, further comprising a collimating element (237) arranged between said diffuser (137) and said hollow light guide (130).

4. A lighting device (200) according to claim 3, wherein said collimating element

(237) is a prismatic collimating foil or plate.

5. A lighting device (300) according to claim 3, wherein said collimating element (337) is tapered at least in a first direction.

6. A lighting device (300) according to claim 3, wherein said collimating element (337) is asymmetrically arranged such that the light beam entering into said hollow light guide (130) is mainly directed towards said reflector (135).

7. A lighting device according to anyone of the preceding claims, wherein the hollow light guide (130) and the redirection element (120) are rotationally symmetric.

8. A lighting device according to any one of the preceding claims, further comprising a center cavity (140) at least partly enclosed by a wall surface including said light input surface (141), and wherein said light source (101) is arranged in the center cavity.

9. A lighting device according to claim 8, wherein said center cavity (140) comprises a light guide.

10. A lighting device according to any one of the preceding claims, wherein said light source (101) comprises light emitting diodes.

11. A lighting device according to anyone of claims 2 - 10, wherein said partially reflecting element (138) is or comprises one of a transparent plate, a multilayered stack, a multilayered dielectric coating, and a semitransparent mirror.

12. A lighting device according to anyone of claims 2 - 11, wherein said partially reflecting element (138) is angle selective.

13. A lighting device according to anyone of claims 2 - 12, wherein said partially reflecting element (138) is provided with a pattern with transparent portions, and reflective portions.

14. A lighting device according to anyone of claims 2 - 13, wherein said partially reflecting element (138) is specular.

15. A lighting device according to anyone of claims 2-14, wherein said partially reflecting element (138) is spatially varying in reflectivity and transmissivity.