Lighting Device

Abstract

The present invention relates to an illumination device (10) comprising at least one light source (12) emitting light (28a, 28b), a uniform diffuser (20) forming a light exit window of the illumination device, and a non-uniform volumetric diffuser (16) arranged between the at least one light source and the uniform diffuser.

Description

[Technical field]

[0001] The present invention relates to a lighting device, for example for office lighting. [Background technology]

[0002] The traditional surface appearance of the light exit window of an office lighting device is designed to be smooth and uniform.

[0003] US2010031544A1 discloses a cover plate for a lighting fixture that "realizes excellent light diffusion and uniformity, as well as excellent light transmission." The cover plate is disposed outside the light source of the light fixture so as to emit light emitted from the light source to the outside. The cover plate is made of a light-transmitting resin material having a diameter in the range of 60 μm to 700 μm and 5 to 35% air bubbles for light scattering, and emits light emitted from the light source while diffusing the light over the entire surface of the cover plate. Summary of the Invention [Problem to be solved by the invention]

[0004] Because the surface appearance of the light exit windows of office lighting devices is traditionally designed to be smooth and uniform, traditional lighting devices can be difficult to integrate with human centric and biophilic lighting concepts that benefit from the familiar feeling patterns and textures of nature.

[0005] It is an object of the present invention to overcome this problem and to provide an improved lighting device. [Means for solving the problem]

[0006] According to a first aspect of the invention, this and other objects are achieved by an illumination device comprising at least one light source emitting light, a uniform diffuser forming a light exit window of the illumination device, and a non-uniform volumetric diffuser arranged between the at least one light source and the uniform diffuser such that the light exit window is non-uniformly lit when the illumination device is on. The non-uniform volumetric diffuser may have a first surface facing the at least one light source and an opposite second surface facing the light exit window.

[0007] The invention is based on the realization that by providing a non-uniform volumetric diffuser between the light source(s) and a uniform diffuser, texture and depth beyond the light exit window can be provided when the lighting device is on, thus allowing the lighting device to be well integrated with human-centric and biophilic lighting concepts and natural lighting systems, such as Signify's NatureConnect. The texture may be a natural pattern, for example a marble like pattern. On the other hand, the uniform diffuser allows the non-uniform volumetric diffuser / texture to be concealed when the lighting device is off.

[0008] The non-uniform diffuser may be "volumetric" in that (most, if not all) of the optical gaps between at least one light source and the light exit window are filled with diffuser material (=non-uniform diffuser).

[0009] The non-uniform volumetric diffuser may have open and / or closed cells (bubbles), preferably open and closed cells. The non-uniform volumetric diffuser may be, for example, a foam, preferably a rigid foam. Furthermore, the volumetric diffuser may be non-uniform in three dimensions (i.e., X, Y, and Z directions). In particular, the non-uniform volumetric diffuser may have non-uniform cell sizes, resulting in a texture through the light exit window.

[0010] The non-uniform volumetric diffuser may be configured such that a portion of the emitted light is diffused (scattered) by the non-uniform volumetric diffuser before hitting the light-exit window, resulting in a texture through the light-exit window, while at least one other portion of the emitted light passes through the non-uniform volumetric diffuser without (substantially) being diffused. In other words, at least one other portion of the emitted light hits the light-exit window directly or nearly directly. The at least one other portion of the emitted light that passes through the non-uniform volumetric diffuser without being substantially diffused can result in local sparkle and highlights when the lighting device is on. The local sparkle and / or highlights, in combination with the uniform diffuser, make the lighting device of the present invention perfectly matched with, for example, painted and acoustic ceiling tiles.

[0011] (At least) some of the open and / or closed cells may have a cell diameter of at least 0.6 cm or at least up to 1.0 cm. If the non-uniform volumetric diffuser is thin, e.g., 2.5-3 cm, such large cells / bubbles will diffuse much less light than cells / bubbles that are, e.g., 0.1-0.3 cm. A non-uniform volumetric diffuser having a thickness, which is the distance between a first surface and a second opposing surface, of 2.5-3 cm may have 3-5 cells / bubbles from the first surface to the second opposing surface, viewed perpendicular to the surfaces. Thus, the lighting device may be characterized in that the non-uniform volumetric diffuser may have relatively large cells / bubbles with a relatively large diameter, the maximum diameter of the cells / bubbles may be in the range of 25-40% of the thickness of the non-uniform volumetric diffuser, and optionally the non-uniform volumetric diffuser may not include relatively small cells / bubbles with a diameter in the range of less than 5% of the thickness, such as, for example, a diameter in the range of 0.05-3% or 0.1-2% of the thickness. Furthermore, a certain number (e.g., 3-5) of large cells (e.g., in the range of 0.6-1.0 cm) may be aligned from the first surface to the second surface, which allows the light to pass essentially undiffused, i.e., the diffusion is, for example, less than 10%. In addition to the large cells / bubbles, other open and / or closed cells of the non-uniform volumetric diffuser may be as small as 0.01 cm. Thus, the open and / or closed cells of the non-uniform volumetric diffuser may range from 0.01 to 1.0 cm, more preferably from 0.01 to 0.6 cm.

[0012] A non-uniform volumetric diffuser can have 80-95% cells, more preferably 90-95% cells, i.e. the air fraction of the volume of the non-uniform volumetric diffuser can be 80-95% (or 90-95%), while the perimeter / wall material of the cells can be 5-20% (or 5-10%) of the volume of the non-uniform volumetric diffuser, and thus the non-uniform volumetric diffuser is essentially cellular / bubbly and can be very lightweight.

[0013] The non-uniform volumetric diffuser may have a thickness, for example, in the range of 2-4 cm, preferably in the range of 2.5-3 cm, e.g., 2.5 cm or 3 cm. However, if the LED pitch is small, the non-uniform volumetric diffuser can be thinner (the cells / bubbles are smaller). The thickness may be uniform across the non-uniform volumetric diffuser. Another option is to have pockets, dents, and / or embossed pattern(s) on the first back surface of the volumetric diffuser, resulting in different textures and light intensities on the second front surface of the volumetric diffuser. Thus, when looking at the lighting device, these different patterns can be observed, but these different patterns cannot be observed in the light field emitted into the space that the lighting device lights up, e.g., an office space. The light is distributed uniformly.

[0014] A non-uniform volumetric diffuser may have a gradient of scattering properties through the volume of the non-uniform volumetric diffuser. For example, the average cell size of the open and / or closed cells of the non-uniform volumetric diffuser may increase from the first surface towards the second surface (smaller cells / bubbles scatter more than larger cells / bubbles). In this way, smaller cells at the first surface can help blurring the larger cells at the second surface facing the light exit window. More specifically, a bubble or cell has material only around it and is a (hollow) transparent ball. When light passes through the cell wall, some light is absorbed and some light is scattered in different directions. Hence, a "shadow" of the cell can be cast on the back side of the uniform diffuser. This happens for every cell, bubble. When the shadows overlap, textures are generated. When many shadows of small size cells / bubbles overlap, the information of the individual bubbles, cells is "lost", resulting in blur. The aforementioned gradient may depend on the foaming direction and poling technology.

[0015] A non-uniform volumetric diffuser may include at least two substantially mirror-symmetrical portions on the first and / or second surfaces formed by cleaving (or cleavage). That is, the first surface of both portions, the second surface of both portions, or the first surface of one portion and the second surface of the other portion may be substantially or seemingly the same or appear the same. This may be achieved by cutting a (precursory) non-uniform volumetric diffuser in half and opening it up like a "book". This may be done more than once to provide symmetry in multiple directions. For example, a non-uniform volumetric diffuser including two mirror-symmetrical portions may be aesthetically pleasing. This may also improve the perception of the non-uniformity of the volumetric diffuser.

[0016] The uniform diffuser should be selected such that the texture of the non-uniform volumetric diffuser is visible through the uniform diffuser when the lighting device is on, but not through the uniform diffuser when the lighting device is off. To achieve the former, the uniform diffuser may be sufficiently transparent / highly light-transmitting (e.g., 92%) and / or thin (e.g., thickness in the range of 200-475 μm). On the other hand, the light-transmitting should be low enough to completely obscure the foam / non-uniform volumetric diffuser in the off state of the lighting device.

[0017] The uniform diffuser may be, for example, a uniform plate or sheet diffuser, especially an amorphous (sheet) diffuser having an amorphous surface layer, coating, or film. Such uniform diffusers perform very well in the off state, i.e., in the off state and with ambient (day) light, they have a very uniform appearance and no texture, making them easy to blend with, for example, acoustic surface tiles.

[0018] Note that a non-uniform volumetric diffuser may also scatter ambient internal light on its way to the back side of the volumetric diffuser where it is reflected and diffused again on its way out. Thus, the volumetric diffuser and the amorphous diffuser can work together to achieve the appearance of an off state.

[0019] The uniform diffuser and the non-uniform volumetric diffuser may form a (two-stage) diffuser stack arranged to be backlit by at least one light source. The non-uniform volumetric diffuser may be arranged upstream of the uniform diffuser, and the uniform diffuser may be arranged downstream of the non-uniform volumetric diffuser.

[0020] The diffuser stack is typically flat. Alternatively, the diffuser stack may be curved in multiple dimensions, either uniformly or non-uniformly. For example, the diffuser stack may be curved cylindrically to create a marbled pendulum lamp that has a uniform appearance in the off state, but changes appearance and glows dynamically as the viewing angle changes (as one moves around the lamp) in the on state.

[0021] The at least one light source may include a plurality of discrete light sources arranged on a (light source) carrier. The discrete light sources may beneficially contribute to a sparkle effect. The discrete light sources may be, for example, LEDs (light emitting diodes). The discrete light sources may be arranged in an array. The discrete light sources / LEDs may be white. At least some of the discrete light sources / LEDs may be of different colors to further enhance the texture / marble effect. Furthermore, the lighting device may be configured such that only a portion of the light exit window is illuminated. This may be achieved by turning on only a portion of the light sources or by not distributing the light sources over the entire lighting device. Instead of discrete light sources, uniform light sources such as conventional LED tiles may be used. This also allows to achieve a texture over the light exit window, but without the sparkle effect. The light source carrier may be, for example, a printed circuit board (PCB). The light source carrier may have or be coupled to side walls to form an optical mixing box. The light source carrier may be reflective and may optionally include a decorative pattern to absorb some of the diffuse light and create a light intensity gradient at the light exit window.

[0022] The lighting device may further include a perforated reflector sheet that is overlaid on the carrier and acts as a mechanical spacer between the discrete light sources and the non-uniform volumetric diffuser such that there is no mechanical contact between the discrete light sources and the non-uniform volumetric diffuser. The perforated reflector sheet may include multiple holes that match the multiple discrete light sources. In case of contact, the refractive index between the light sources / LEDs and the touching material will change, thus changing the photon recycling in the LED package, resulting in a different color of the LED. This is avoided due to the perforated reflector sheet. Furthermore, the perforated reflector sheet may provide a higher reflectivity than the carrier alone, thereby improving the light output of the lighting device. The perforated reflector sheet may additionally include a pattern.

[0023] The lighting device of the present invention may for example be applied to a ceiling, in particular as a lighting tile in an office ceiling mixed with acoustic tiles. A plurality of lighting devices / lighting tiles may be provided on such a ceiling. Alternatively, the lighting devices may for example be wall-mounted / vertically arranged.

[0024] Furthermore, the lighting device may be integrated into an acoustic tile. That is, according to a second aspect of the present invention, an acoustic tile is provided that includes a lighting device according to the first aspect. The lighting device may for example be placed in a cut hole in the acoustic tile. The hole may have any shape or size, such as square, rectangular, irregular, etc. Optionally, the entire acoustic tile is covered with a uniform diffuser, but the parts that cover the acoustic tile parts where no lighting device is present are provided with micro-perforations. Thus, in the off state, the lighting device acts as an acoustic tile part, and the ceiling appears completely uniform, but in the on state, one or more parts of the acoustic tile light up, and the illuminated part(s) shine through the ceiling, showing texture and depth, giving the impression of a space behind the ceiling.

[0025] The lighting device may also be incorporated into other devices or objects, such as a (free hanging) 3-D body, one or more sides, or parts of a side, are (independently) illuminated.

[0026] The invention relates to all possible combinations of the features recited in the claims. [Brief description of the drawings]

[0027] This and other aspects of the invention will now be described in more detail with reference to the accompanying drawings, which illustrate embodiments of the invention. As shown in the figures, the sizes of layers and regions may be exaggerated for illustrative purposes and are thus provided to illustrate the general structure of embodiments of the invention. Like reference numerals refer to like elements throughout. [Figure 1] FIG. 1 is a schematic exploded perspective view of a lighting device according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic side view of the lighting device of FIG. [Diagram 3] FIG. 3 is a photograph of a foam that can be used as a non-uniform volumetric diffuser in the lighting device of the present invention. [Figure 4] FIG. 4 is a photograph of a lighting device of the present invention when turned on and viewed from the exit window side. [Diagram 5] FIG. 5 is a schematic side view of an illumination device according to one embodiment having a scattering gradient. [Figure 6] 6a-c show the fabrication of a non-uniform volumetric diffuser with a mirror foam structure. [Figure 7] Figures 7a-b show an illumination device according to one embodiment having a perforated reflector sheet. [Figure 8] FIG. 8 shows a lighting device of the present invention as a ceiling lighting tile. [Figure 9] FIG. 9 shows a lighting device of the present invention incorporated into an acoustic tile. [Figure 10] FIG. 10 is a schematic perspective view of a lighting device having a curved diffuser stack according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention is described more fully below with reference to the accompanying drawings, in which presently preferred embodiments of the invention are shown. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided for thoroughness and completeness, so as to fully convey the scope of the present disclosure to those skilled in the art.

[0029] FIG. 1 shows a lighting device 10 according to one embodiment of the present invention.

[0030] The lighting device 10 comprises at least one light source, here a plurality of discrete light sources 12, adapted to emit light. The light source 12 may be, for example, a white LED. The light source 12 is arranged or disposed on a (light source) carrier 14, for example a PCB. The carrier 14 may be, for example, square or rectangular. The carrier 14 may have side walls (not shown) or may be coupled to side walls to build a light mixing box. The light sources 12 may be arranged in an array, as in FIG. 1, with a plurality of rows and columns.

[0031] The lighting device 10 further includes a first diffuser, i.e., a non-uniform volumetric diffuser 16. The non-uniform volumetric diffuser 16 may also be referred to as a non-uniform bulk diffuser or a non-homogenous bulk / volumetric diffuser. The non-uniform volumetric diffuser 16 is disposed on the carrier 14 so as to cover the light source 12. The non-uniform volumetric diffuser 16 may have, for example, a square or rectangular base area and a thickness t in the range of (for example) 2.5-3 cm.

[0032] 2, the non-uniform volumetric diffuser 16 may have a wide distribution of open and closed cells (bubbles) 18, 18' with cell diameter d ranging from 0.01 to 1.0 cm, more preferably 0.01 to 0.6 cm. That is, the non-uniform volumetric diffuser 16 is characterized by non-uniform cell sizes. The non-uniform volumetric diffuser 16 may be a foam, preferably a rigid foam, for example, as shown in FIG. 3. The non-uniform volumetric diffuser 16 may have 80 to 90% of the cells 18, 18'.

[0033] The lighting device 10 further includes a second diffuser, namely, a uniform diffuser 20. The uniform diffuser 20 may also be referred to as a homogeneous diffuser or a uniform / homogeneous light exit window diffuser. The uniform diffuser 20 covers the non-uniform volumetric diffuser 16. Accordingly, the non-uniform volumetric diffuser 16 is disposed between the light source 12 and the uniform diffuser 20. The first non-uniform volumetric diffuser 16 and the second uniform diffuser 20 preferably form a two-stage diffuser stack 22, and the uniform diffuser 20 may be, for example, square or rectangular so as to match the non-uniform volumetric diffuser 16. As shown in FIGS. 1-2, the two-stage diffuser stack 22 is arranged to be illuminated from behind by the light source 12, and the downstream uniform diffuser 20 also forms the light exit window of the lighting device 10. The uniform diffuser 20 is preferably a thin and uniform plate or sheet diffuser. The uniform diffuser 20 may especially have an amorphous surface layer, coating or film 24 on the observer side. For example, the uniform diffuser 20 may have high light transmittance (e.g., 88% < T < 92%) and a thickness in the range of 200-475 μm.

[0034] The non-uniform volumetric diffuser 16 may be disposed between the light source 12 and the uniform diffuser 20 such that when the light source 12 of the lighting device 10 is on (in the on state), the uniform diffuser / light exit window 20 is unevenly illuminated. Especially, the non-uniform volumetric diffuser 16 including a texture may be imaged on the back side 26 of the uniform diffuser / light exit window 20 when the lighting device 10 is on.

[0035] Furthermore, the non-uniform volumetric diffuser 16 may be configured such that a portion 28a of the emitted light is diffused by the non-uniform volumetric diffuser 16 before hitting the light exit window 20, while at least one other portion 28b of the emitted light passes through the non-uniform volumetric diffuser 16 substantially without being diffused. In other words, some light rays 28a are scattered and other light rays 28b pass almost undisturbed to the back side 26 of the light exit window 20. The former gives rise to a texture through the light exit window 20, which is visible through / behind the uniform diffuser / light exit window 20 when the lighting device 10 is on (see FIG. 4 ) thanks to the thin and sufficiently transparent diffuser 20. The latter may be achieved by the non-uniform volumetric diffuser 16 having a number (e.g., 3-5) of large, preferably open cells 18 (e.g., in the range of 0.6-1.0 cm) aligned across a thickness t, as shown at 30 in Fig. 2. Light 28b can provide localized glow and highlights when the lighting device 10 is on. At 30, three cells 18, 18' with diameters in the range of 25-40% of said thickness t span the thickness t of the non-uniform volumetric diffuser 16.

[0036] When the light source 12 of the lighting device 10 is off (off state) and the light exit window 20 is illuminated with ambient light, the uniform diffuser 20 can help to prevent the texture of the non-uniform volumetric diffuser 16 from being visible through the uniform diffuser 20. In other words, the uniform diffuser 20 completely conceals the foam texture when in the off state, at least in part due to the limited light transmission of the uniform diffuser 20. As indicated above, a sheet diffuser with an amorphous surface film may be used to achieve this. Optionally, a small air gap (not shown) may be present between the first diffuser 16 and the second diffuser 20 to enhance the diffusing effect.

[0037] Note that the non-uniform volumetric diffuser 16 also scatters ambient internal light on its way to the backside 32a of the volumetric diffuser where it may be (diffusely) reflected (e.g. by the perforated reflector 36 (see Figs. 7a-b)) and diffused again on its way out. Thus, there are at least four diffusions (two (in and out) by the non-uniform volumetric diffuser 16 and two (in and out) by the uniform diffuser 20) and at least one diffusion by the diffused perforated reflector sheet 36, which together serve to make the light exit window appear uniform in the off state.

[0038] FIG. 5 illustrates an embodiment of a lighting device 10 similar to FIGS. 1-2, but in which the non-uniform volumetric diffuser 16 has a gradient of scattering properties throughout the volume of the non-uniform volumetric diffuser 16. Specifically, the average cell size of the open and / or closed cells 18a-b increases from a first surface 32a to a second surface 32b of the non-uniform volumetric diffuser 16, with the first surface 32a facing the light source 12 and the second opposite surface 32b facing the uniform diffuser / light exit window 20. Here, the smaller cells / bubbles 18a scatter light from the light source 12 more than the larger cells / bubbles 18b. The smaller cells / bubbles 18a may be as small as 0.01 cm in diameter and the larger cells / bubbles 18b may be as large as 0.6-1.0 cm in diameter.

[0039] It should be noted that because the non-uniform volumetric diffuser 16 may have open cells at the surfaces 32a-b, these surfaces may, and preferably should, be considered as surface planes 32a-b.

[0040] Figures 6a-c show the fabrication of an alternative non-uniform volumetric diffuser 16 that may be used in the lighting device 10. The non-uniform volumetric diffuser 16 in Figure 6c, a top view, includes two (substantially / apparently) mirror-symmetric portions 34a-b on the second surface 32b. These may be formed by cleaving, i.e., cutting a prior non-uniform volumetric diffuser 16' (e.g., a block of foam) in half (Figure 6a) and opening it like a "book" (Figure 6b). The first surface 32a may also have two such mirror-symmetric portions (not shown).

[0041] 7a-b show an embodiment of the lighting device 10 similar to that of FIGS. 1-2, but further including a perforated reflector sheet 36. The perforated reflector sheet 36 is superimposed on the carrier 14 and acts as a mechanical spacer between the discrete light sources 12 and the non-uniform volumetric diffuser 16. That is, the perforated reflector sheet 36 ensures that there is no mechanical contact between the discrete light sources 12 and the non-uniform volumetric diffuser 16. The perforated reflector sheet 36 may include a number of holes 38 that match the multiple discrete light sources 12. The perforated reflector sheet 36 may be made of, for example, MCPET, white polyethylene, or a coating of white paint including TiO2 and / or BaSO4, etc.

[0042] The lighting device 10 of the present invention may for example be applied in a ceiling, in particular as a lighting tile 10 of an acoustic ceiling 100 mixed with acoustic tiles 102 arranged in a grid, as shown in Fig. 8. The lighting device / tile 10 may for example be a square (e.g. 60x60cm). A number of such lighting devices / tiles 10 may be provided on the ceiling 100. When the lighting device 10 is off (as in Fig. 8), it blends perfectly with the normal acoustic tiles 102.

[0043] Furthermore, the lighting device 10 may be integrated into an acoustic tile 102', as shown in FIG. 9. The lighting device 10 may be placed in a (cut) hole 104 in the acoustic tile 102'. The hole 104 may have any shape or size. The hole 104 may be, for example, square, as in FIG. 9. The periphery of the cut hole 104 may be provided with a light reflecting layer 106 to hide the transition edge between the lighting device 10 and the rest of the acoustic tile. In the off state, the lighting device 10 acts as an acoustic tile part and the ceiling 100 appears completely uniform. On the other hand, in the on state, the part of the acoustic tile 102' corresponding to the lighting device 10 lights up (FIG. 9) and the illuminated part shows the sparkle, texture and depth through the ceiling 100.

[0044] Figure 10 shows an embodiment of a lighting device 10 similar to Figures 1-2, but where the two-stage diffuser stack 22 is now curved into a (right circular) cylindrical shape, instead of being flat or planar as in Figures 1-2. The carrier 14 with the light sources 12 can also be curved into a matching cylindrical shape, for example by using a flexible PCB. The lighting device 10 of Figure 10 may be used, for example, to build a marble pendulum lamp that has a uniform appearance in the off state, but changes appearance and glows dynamically in the on state as the viewing angle changes as one moves around the lamp.

[0045] Not all of the curved lighting device 10 need to be illuminated (at the same time). The pattern of the light sources 12 and the perforated reflector sheet can be adapted to achieve a desired / customized effect, such as, for example, a spiral LED pattern. Optionally, the carrier 14 is a partitioned carrier comprising different segments that can be individually controlled, or the carrier comprises an active or passive matrix LED array.

[0046] Those skilled in the art will recognize that the present invention is in no way limited to the above-described preferred embodiments: on the contrary, many modifications and variations are possible within the scope of the appended claims.

[0047] Furthermore, variations to the disclosed embodiments can be understood by those skilled in the art, by studying the drawings, the disclosure, and the appended claims, and can be implemented in practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. 1. A lighting device comprising: at least one light source that emits light; a uniform diffuser forming a light exit window of the lighting device; a non-uniform volumetric diffuser arranged between the at least one light source and the uniform diffuser such that the light exit window is non-uniformly illuminated when the lighting device is on, the non-uniform volumetric diffuser having a first surface facing the at least one light source and an opposite second surface facing the light exit window; Including, the non-uniform volumetric diffuser has open and / or closed cells; An illumination device wherein at least some of the open and / or closed cells have a cell diameter in the range of 0.6 to 1.0 cm, and at least some of the open and / or closed cells have a cell diameter in the range of 0.01 to 0.1 cm.

2. The lighting device of claim 1 , wherein the first back surface of the volumetric diffuser comprises pockets, indentations, and / or an embossed pattern.

3. A lighting device as described in claim 1 or 2, wherein the non-uniform volumetric diffuser has a thickness t perpendicular to the first surface and the second surface, and the non-uniform volumetric diffuser includes cells having diameters in the range of 25 to 40% of the thickness t.

4. 3. The lighting device of claim 1, wherein the non-uniform volumetric diffuser is configured such that a portion of the emitted light is diffused by the non-uniform volumetric diffuser before hitting the light exit window, while at least one other portion of the emitted light passes through the non-uniform volumetric diffuser substantially without being diffused.

5. 3. The lighting device of claim 1, wherein the non-uniform volumetric diffuser has 80-95% cells.

6. 3. A lighting device according to claim 1 or 2, wherein an average cell size of the open and / or closed cells of the non-uniform volumetric diffuser increases from the first surface towards the second surface.

7. 3. A lighting device according to claim 1 or 2, wherein the non-uniform volumetric diffuser comprises at least two substantially mirror-symmetric portions on the first surface and / or the second surface formed by cleaving.

8. 3. The lighting device of claim 1, wherein the uniform diffuser is selected such that the texture of the non-uniform volumetric diffuser is visible through the uniform diffuser when the lighting device is on, but is not visible through the uniform diffuser when the lighting device is off.

9. 3. The lighting device according to claim 1 or 2, wherein the uniform diffuser is a uniform plate or sheet diffuser.

10. 3. A lighting device according to claim 1 or 2, wherein the uniform diffuser is an amorphous diffuser, preferably having an amorphous surface layer, coating or film.

11. 3. The lighting device of claim 1, wherein the uniform diffuser and the non-uniform volumetric diffuser form a diffuser stack arranged to be backlit by the at least one light source.

12. The lighting device of claim 11 , wherein the diffuser stack is curved in multiple dimensions.

13. 3. The lighting device according to claim 1 or 2, wherein the at least one light source comprises a plurality of discrete light sources arranged on a carrier.

14. 14. The lighting device of claim 13, further comprising a perforated reflector sheet overlaid on the carrier and acting as a mechanical spacer between the discrete light sources and the non-uniform volumetric diffuser so that there is no mechanical contact between the discrete light sources and the non-uniform volumetric diffuser, the perforated reflector sheet comprising a plurality of holes matching the plurality of discrete light sources.

15. An acoustic tile comprising a lighting device as described in claim 1 or 2.

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