GB2480758A - Light guide with first and second light output surfaces - Google Patents

Abstract

A lighting apparatus 1 that provides an energy efficient illumination source that is suitable for use as an interior, ambient lighting source is described. The lighting apparatus 1 comprising a light guide 2 suitable for guiding light 5 coupled to a first surface 3 to a second surface 6a, 6b of the light guide 2 thus providing a first light output 7a, 7b emitted from the second surface 6a, 6b. The light guide 1 further comprises a plurality of extraction features 10 located on a further surface 8 thereof, the extraction features 10 redirect a portion of the guided light 5 to provide a second light output 11 that is more diffuse than the first light output 7a, 7b. Extraction features 10 may comprise an interleaved pattern of first and second extraction features having first and second luminance profiles respectively.

Description

1 LiQhtinQ Aiaratus 3 The present invention relates to lighting apparatus and particularly to lighting apparatus 4 suitable for use as a diffuse light source.

6 Diffuse light sources are the preferred option for interior, ambient lighting as the glare from 7 point light sources, such as incandescent bulbs or LEDs, is often found to be distracting 8 and generally unattractive.

It is known in the art to employ lamp shades located over an incandescent bulb in order to 11 convert this point light source into a more attractive lighting solution. However, lamp 12 shades exhibit very low optical efficiency and so a significant proportion of the light 13 generated by these bulbs is simply absorbed and thus not available for the desired lighting 14 purpose. To compensate for the absorbed light bulbs of higher wattage are employed such that even in the presence of the lamp shade sufficient diffuse light is emitted. This is 16 obviously a rather energy inefficient solution.

18 With regards LED point light sources the standard way to achieve diffuse lighting is to 19 locate the LEDs within a deep box or casing and have a highly diffusing sheet located at 1 the exit of the box or casing in order to convert the generated light into a more diffuse 2 form. Diffusing the light generated by the LEDs also reduces the lighting efficiency due to 3 losses through absorption and reflection within the diffuser.

In addition to the above, there exist industry standards regarding the control of the angle of 6 light distribution for luminary requirements e.g. European Standard EN 12464-1: "The 7 Lighting of Workplaces". There are various reasons for the existence of these standards, 8 for example, lighting can cause interfering reflections on modern computer screens. For 9 this reason the standard specifies requirements for controlling the average luminances.

For normal workstations, a limit applies of 1000 cd/rn2 or 200 cd/rn2 depending on the 11 computer screen. This limit applies for angles starting from 65° all-around. For critical 12 computer screen activities this starts from 55° all-around.

14 It is therefore an object of an aspect of the present invention to obviate or at least mitigate the foregoing disadvantages of the lighting apparatus known in the art.

17 It is a further object of an aspect of the present invention to provide a lighting apparatus 18 that provides an efficient source of light that is suitable for use as interior ambient lighting.

Summary of Invention

22 According to a first aspect of the present invention there is provided a lighting apparatus 23 the lighting apparatus comprising a light guide suitable for guiding light coupled to a first 24 surface thereof to a second surface of the light guide so as to provide the lighting apparatus with a first light output emitted from the second surface wherein the light guide 26 comprises a plurality of extraction features located on a third surface thereof the extraction 27 features being arranged so as to redirect a portion of the guided light so as to provide the 28 lighting apparatus with a second light output, the second light output being more diffuse 29 than the first light output.

31 The arrangement of the light guide provides the lighting apparatus with a low-glare 32 functional source of illumination from the first light output and a diffuse source of 33 illumination from the second light output. The lighting apparatus provides an energy 34 efficient illumination source that is suitable for interior, ambient lighting.

1 The second surface may be located opposite to the first surface.

3 The light guide may further comprise a plurality of extraction features located on a fourth 4 surface of the light guide the extraction features again being arranged so as to redirect a portion of the guided light so as to enhance the second light output.

7 The second light output may be emitted from the third surface. Alternatively, the second 8 light output may be emitted from the fourth surface. In a yet further alternative the second 9 light output may be emitted from both the third and fourth surfaces.

11 The light-guide may be planar. Alternatively the light guide may be non-planar e.g. bent or 12 curved. Preferably the light guide comprises a hollow cylinder. Such an embodiment 13 allows the light apparatus to replicate the light provided be the combined effects of an 14 incandescent bulb and a lamp shade.

16 Preferably the lighting apparatus further comprises a light source arranged to couple light 17 into the first surface of the light guide.

19 The plurality of extraction features may comprise an interleaved pattern of first and second extraction features. By providing an interleaved pattern of first and second extraction 21 features predetermine luminance profiles for the second light output can be produced.

23 The first extraction features preferably produce a first luminance profile component for the 24 second light output. The second extraction features preferably produce a second luminance profile component for the second light output.

27 Most preferably the interleaved pattern of the first and second extraction features and 28 hence the combination of the first and second luminance profiles provide the second light 29 output with a uniform luminance profile. Advantageously, this uniform light output of the second light output is achieved without the need to employ a diffuser. This provides a 31 lighting apparatus suitable for providing a second light output of uniform luminance that is 32 significantly more energy efficient when compared to those known in the art.

34 Optionally the interleaved pattern comprises alternative rows or columns of the first and second extraction features. Alternatively the interleaved pattern comprises a chequered 1 pattern of first and second extraction features. The interleaved pattern may alternatively 2 comprise an irregular arrangement of the first and second extraction features. In a yet 3 further alternative embodiment the interleaved pattern may comprise more than two types 4 of extraction features.

6 The extraction features may comprise reflective extraction features or refractive extraction 7 features.

9 According to a second aspect of the present invention there is provided a lighting apparatus the lighting apparatus comprising a non-planar light guide suitable for guiding 11 light coupled to a first surface thereof to a second surface of the light guide so as to 12 provide the lighting apparatus with a first light output emitted from the second surface 13 wherein the light guide comprises a plurality of extraction features located on a third 14 surface thereof the extraction features being arranged so as to redirect a portion of the guided light so as to provide the lighting apparatus with a second light output, the second 16 light output being more diffuse than the first light output.

18 Embodiments of the second aspect of the invention may comprise features to implement 19 the preferred or optional features of the first aspect of the invention or vice versa.

21 According to a third aspect of the present invention there is provided a method of 22 producing a lighting apparatus the method comprising: 23 -providing a light guide suitable for guiding light coupled to a first surface thereof to a 24 second surface of the light guide so as to provide the lighting apparatus with a first light output emitted from the second surface; and 26 locating a plurality of extraction features on a third surface of the light guide, the 27 extraction features being arranged so as to redirect a portion of the guided light so 28 as to provide the lighting apparatus with a second light output, the second light 29 output being more diffuse than the first light output.

31 The step of locating a plurality of extraction features on the third surface of the light guide 32 may comprise locating an interleaved pattern of first and second extraction features on the 33 third surface.

1 The step of locating the interleaved pattern may provide the second light output with a 2 uniform angular luminance profile.

4 The step of locating the interleaved pattern may comprise locating the first and second extraction features on a common surface of the light guide. The extraction features may 6 however be located on opposite surfaces of the light guide.

8 The method may further comprise the step of coupling a light source to the light guide.

Embodiments of the third aspect of the invention may comprise features to implement the 11 preferred or optional features of the first and second aspects of the invention or vice versa.

13 According to a fourth aspect of the present invention there is provided a method of 14 producing a lighting apparatus the method comprising the steps of: -providing a non-planar light guide suitable for guiding light coupled to a first surface 16 thereof to a second surface of the light guide so as to provide the lighting apparatus 17 with a first light output emitted from the second surface; and 18 locating a plurality of extraction features on a third surface of the light guide, the 19 extraction features being arranged so as to redirect a portion of the guided light so as to provide the lighting apparatus with a second light output, the second light 21 output being more diffuse than the first light output.

23 Embodiments of the fourth aspect of the invention may comprise features to implement the 24 preferred or optional features of the first, second and third aspects of the invention or vice versa.

27 Brief Descriition of DrawinQs 29 Aspects arid advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings in which: 32 Figure 1 presents a schematic representation of a flat panel lighting apparatus in 33 accordance with an aspect of the present invention; 1 Figure 2 presents a schematic representation of an alternative embodiment of the lighting 2 apparatus of Figure 1; 4 Figure 3 presents a schematic representation of a yet further alternative embodiment of the lighting apparatus of Figure 1; 7 Figure 4 presents a side view of a yet further alternative embodiment of the lighting 8 apparatus of Figure 1; and Figure 5 presents a schematic representation of the interaction of a mirrored micro lens 11 extraction feature with the light coupled within the lighting apparatus of Figure 4; 13 Figure 6 presents luminance versus angle of observation graphs for: 14 (a) the first extraction feature of the lighting device of Figure 4; (b) the second extraction feature of the lighting apparatus of Figure 4; and 16 (c) the combined output of the first and second extraction features of Figure 4; 18 Figure 7 presents an example of structures suitable for use as refractive extraction 19 features, namely cylinders having asymmetric triangular shaped cross sections; 21 Figure 8 presents a further alternative example of structures suitable for use as: 22 (a) the first feature of the light-guide device of Figure 4, namely a micro lens having a 23 reflective coating; and 24 (b) the second feature of the light-guide device of Figure 4, namely a micro lens; 26 Figure 9 side profiles of alternative surface features that may be employed as the 27 extraction features within the light-guide device.

29 Detailed Description

31 A schematic representation of a lighting apparatus 1 in accordance with an aspect of the 32 present invention is presented in Figure 1. The lighting apparatus 1 can be seen to 33 comprise a planar light guide 2 having a substantially cuboid shape. Located along a first 34 surface 3 of the planar light-guide 2 is an array of light sources 4, in the form of LEDs that are edge coupled to the planar light-guide 2. The light 5 generated by the light source 4 1 then propagates within the planar light-guide 2 due to the effects of total internal reflection.

2 Surface 6a of the planar light-guide 2, located opposite to the first surface 3, and surfaces 3 6b allow for first light outputs 7a and 7b, respectively, to exit the lighting apparatus 1, as 4 described in further detail below.

6 Located across surface 8 and surface 9 of the planar light-guide 2 are a plurality of 7 extraction features 10. The extraction features 10 enable the light 5 to escape the total 8 internal reflection condition so as to allow for a second light output 11 to exit the lighting 9 apparatus 1, as described in further detail below.

11 There are two main physical principles employed by the extraction features 10 to disturb 12 the total internal reflection, namely reflection and refraction. Reflection involves the light 5 13 interacting with a reflecting material on the surface of the light-guide 2 which is absorbed 14 and then re-emitted and so breaks down the effect of total internal reflection. A white material, for example, a white ink is commonly employed as the reflecting medium. The 16 white ink emits the light in random directions (commonly called scattering) and as a result 17 a proportion of the scattered light is incident on the opposing surface at an angle lower 18 than the critical angle for the total internal reflection condition to apply and so the light 5 19 can then exit the light guide 2 from this surface.

21 An alternative solution is to use refractive effects, where the shape of the exit surface is 22 locally modified on a small scale such that the effective angle of incidence of the guided 23 light 5 is changed below the critical angle and light can therefore exit in accordance with 24 Snell's Law. Mechanical machining or laser machining of the surfaces of the light guide 2 may be deployed to achieve this effect. Alternatively, injection moulding, micro-moulding 26 or micro-embossing of transparent hemispherical micro lens features, or uneven surfaced 27 features may be employed to produce the extraction features 10 on the light guide 2. In 28 practice refraction techniques are found to give more efficient and controllable interference 29 with the guided light 5 than those employing reflection extraction features.

31 In the presently described lighting apparatus 1 the majority of the light 5 is guided towards 32 the surface 6a and 6b of the light guide 2 and thus exits the light guide 2 so as to form the 33 first light outputs 7a and 7b. The beam angles, Th and 8, of the first light outputs 7a and 34 7b are defined by the light source 4 beam angle and refractive index of the light guide 2 and so form low-glare functional sources of illumination.

2 A minority portion of the light 5 is however extracted from the surfaces 8 and 9 of the light 3 guide 2 in the form of the second light output 11, having a beam angle 3,due to the 4 presence and formation of the extraction features 10. Thus the second light output 11 provides a diffuse source of illumination that provides the lighting apparatus 1 with an 6 attractive decorative effect to an observer.

8 The overall result is that the lighting apparatus 1 provides a thin plate device which to an 9 observer has two diffusely illuminated surfaces, provided by the second light output 11, when viewed from a substantially normal direction to the surfaces 8 and 9, but the light 11 energy is primarily delivered within the first output lights 7a and 7b, that exits the lighting 12 apparatus via the surface 6a and 6b, respectively.

14 The employment of the light guide 2 provides the lighting apparatus with increased functionality. For example, Figures 2 and 3 presents embodiments where the light guide 2 16 is non-planar. In particular, Figure 2 presents an S-shaped light guide 2b while Figure 3 17 presents a cylindrical light guides 2c and 2d having a circular cross section. The 18 difference between the embodiments of Figure 3(a) and Figure 3(b) is the location of the 19 array of light sources 4and 4b, respectively. Such embodiments allow the light apparatus to replicate the light provided by the combined effects of an incandescent bulb and a lamp 21 shade. It will be appreciated that the light guide may take on a variety of regular or 22 irregular shapes over and above those shown in Figures 1 to 3.

24 The extraction features 10 generally exhibit a low angular luminance uniformity which translates onto the second light output 11 from the apparatusi. In order to provide an 26 observer with the appearance of uniform angular luminance it is known in the art to locate 27 a diffuser between the light-guide 2 and the observer. A reflector, located on the opposite 28 side of the planar light-guide 2 to the observer, is also often employed to increase the light 29 levels reaching the observer. However, the incorporation of a diffuser and/or a reflector within the lighting apparatus 1, lb or lc would not only be impractical since it would detract 31 from attractive and decorative nature of the apparatus 1 but would also add to the cost of 32 the manufacture and overall thickness of the apparatus 1.

34 As will now be described in further detail the profile of the second light output 11 can be predetermined through the careful choice and location of the extraction features 10.

1 Referring to Figure 4, a side view of an alternative embodiment of the lighting apparatus is 2 presented, as depicted generally by reference numeral 1 d. In this embodiment the 3 extraction features comprise two different types, referred to hereinafter as the first and 4 second light extraction features 12 and 13. The first 12 and second light extraction features 13 are deployed across the surface 8 of the light-guide 2 within an inter-leaved 6 pattern. As can be seen the interleaved pattern comprises alternative rows of first 12 and 7 second light extraction features 13. The surface area of the extraction features 12 and 13 8 can be seen to increases the further the extraction features are from the light source 4.

In the embodiment presented in Figure 4 the first 12 and second light extraction features 11 13 are reflective extraction features and are located on a common surface of the planar 12 light-guide 2. In particular the first extraction features 10 comprise micro lenses coated 13 with a reflective coating. Further detail of the interaction of these mirrored micro lenses 14 with the light 5 coupled within the light guide 2 is presented schematically within Figure 5.

The second light extraction features 13 may comprises asymmetrical or symmetrical 3-d 16 shapes forming, triangle or pyramid shapes, random micro lenses, v-grooves, cylinders as 17 presented below within Figure 9(a).

19 Figure 6 presents three, luminance versus angle of observation graphs. The first of these graphs, presented in Figure 6(a), shows the luminance versus angle of observation for the 21 first extraction feature 12 of the lighting apparatus id. The luminance profile for the first 22 extraction feature 12 is non-uniform. In particular, the luminance curve of Figure 6(a) is 23 seen to be asymmetric about the zero angle of observation with more light being extracted 24 within the -180° to 0° range.

26 The second of these graphs, presented in Figure 6(b), shows the luminance versus angle 27 of observation for the second extraction feature 13 of the lighting apparatus id. Again the 28 luminance profile is non-uniform but this time the luminance curve is seen to be 29 asymmetric about the zero angle of observation with more light being extracted within the 0° to 180°range.

32 Since the first 12 and second light extraction features 13 are deployed across the surface 33 of the planar light-guide 2 within an inter-leaved pattern they tend to compensate for each 34 other such that the luminance versus angle of observation graphs for the second light output 11 of the lighting apparatus id as a whole is substantially uniform, as presented in 1 Figure 6(c). As a result the second light output 1 lexhibits a light output of substantially 2 uniform angular luminance.

4 By arranging for the surface area of the extraction features 12 and 13 to increase the further the extraction features 12 and 13 are from the light source 4 the extraction features 6 12 and 13 also provide a means for also improving the spatial luminance uniformity of the 7 second light output 11. Alternatively, improved uniformity of the spatial luminance of the 8 second light output 11 can be achieved by arranging for the spacing between the 9 extraction features 12 and 13 to decrease the further the extraction features 12 and 13 are located from the light source 4.

12 In an alternative embodiment of the lighting apparatus the first 12b and second light 13 extraction features 13b are both refractive extraction features and are again located on a 14 common surface of the planar light-guide 2. Figure 7 presents an example of such extraction features 12b and 13b in the form of cylinders having an asymmetric triangular 16 shaped cross sections. It will be appreciated by those skilled in the art that in order to 17 provide a uniform angular luminance profile with the extraction features 12b and 13b of 18 Figure 7 it would be necessary to employ a light source 4 at either end of the light guide 2, 19 however this is not desirable since this would detract from the functionality of the first light T 20 output 7. When light is coupled from the right hand side of Figure 7 then the luminance C\J 21 versus angle of observation curve is as shown in Figure 6(a) while light coupled from the 22 left provides the luminance versus angle of observation curve of Figure 6(b). In this way 23 the second light output 11 can be profiled to project in a particular direction. Such a 24 feature may be desirable for room lighting or for interior roof lights within in cars where it is desirable to direct the output light towards only the two front passengers 27 In a similar manner to that described above appropriate choice of surface area or spacing 28 of the extraction features 12b and 13b can be employed to also provide the light-guide 29 device lb with a predetermined spatial luminance profile.

31 It will be appreciated by those skilled in the art that the first extraction features 12 may 32 comprise reflective extraction features while the second light extraction features 13 may 33 comprise refractive extraction feature, or vice versa. Figure 8(a) presents one such 34 suitable reflective extraction feature 12c which comprises a change in shape of the surface of the planar light-guide 2, in the form of a first transparent hemispherical micro lens 1 having a reflecting coating. Such components provide a luminance versus angle of 2 observation graphs of the type shown in Figure 6(a). In a similar manner a second 3 transparent hemispherical micro lens 13c, with no mirrored coating, is also employed to 4 provide the second refractive extraction features, see Figure 8(b). Such components provide a luminance versus angle of observation graph of the type shown in Figure 6(b).

6 In this embodiment it is necessary to incorporate a reflector on the side of the planar light- 7 guide 2 opposite to where the second light output 11 is to be viewed in order to provide 8 this output with a substantially uniform luminance.

It will be appreciated that alternative interleaved patterns may be adopted. For example 11 the interleaved pattern may comprise: 12 1) alternative columns of first 12 and second light extraction features 13; 13 2) the first 12 and second light extraction features 13 arranged as chequered pattern; 14 3) less regular arrangements of the first 12 and second light extraction features 13 e.g. the extraction features may be grouped in two or more rows or columns, or 16 randomly located within an array across the surface of the planar light-guide 2. In 17 such embodiments the uniformity of the second light output may be reduced.

18 4) more than two types of extraction features arranged in any of the patterns 19 described in 1)to3).

21 In all of the above described embodiments, except for those comprising solely reflective 22 extraction features, the extraction features may be located on opposite surfaces of the 23 planar light-guide 2. This is a less preferable arrangement however since it is likely to 24 make the device thicker and increase the manufacturing costs 26 With the above arrangements it will be appreciated by those skilled in the art that the 27 uniform nature of the output light from the light-guide device 1 is not dependent upon the 28 light guide being planar. Thus the light guide may now be curved or bent so as to form the 29 lighting apparatus of Figures 2 and 3 without significant deterioration of the uniform nature of the second light output 11. Thus an observer will not see bright and dark areas as they 31 change their viewing angle of the second light output 11, even when it is non-planar. In 32 practice high uniformity levels (>70%) for an observer have been achieved with the above 33 described apparatus for light-guide devices exhibiting large curvatures (bend radius> 34 10cm).

1 By way of example Figure 9 presents alternative surface features that may be employed 2 as the extraction features 12 and 13. The extraction features shown in Figure 9(a) 3 comprise a reflective coating and so are suitable for use as reflective extraction features 4 while the extraction features shown in Figure 9(b) do not comprise a reflective coating and so are suitable for use as refractive extraction features. As can be seen the surface 6 features of Figure 9 include asymmetrical or symmetrical 3-d shapes forming micro lenses, 7 triangle or pyramid shapes, random micro lenses, v-grooves, cylinders.

9 The reflective coatings employed with the extraction features may be specular (mirror) or non-specular (white).

12 The light source 4 may comprise a single LED or multiple LEDs of the same or differing 13 colours. The light-sources can be located at one or more edges of the light-guide. The 14 light sources 4 can be embedded within the light guide 2 or located in apertures or ports formed in the light guide 2. The light sources 4 can be edge-emitting, side emitting, or top 16 emitting LED packages. The light sources 4 may be LED, super luminescent LED, micro 17 cavity or laser diode chips. The light sources 4 may be fluorescent lamps.

19 The light guide 2 may be square, rectangular, circular or any other regular shape.

Alternatively, the light guide 2 may be an irregular shape. The size of the light guide 2 21 may vary from 1cm to 2m, with a thickness range from 0.1mm to 10mm.

23 The size (x,y,z) of the individual light extraction features 10, 12 and 13 may be in the range 24 of 1 micron to 1cm. The light extraction features 10, 12 and 13 may be a nanostructure with photonic bandgap, diffractive or non-linear optical prosperities.

27 Although the above described interleaved patterns of the first 12 and second extraction 28 features 13 have generally been designed to produce a second light output 11 for the 29 apparatus that is uniform it will be appreciated that alternative predetermine luminance profiles for the second light output 11 can be produced through the careful selection and 31 deployment of the extraction features 12 and 13. Being able to control the angular 32 distribution of "task" lighting and/or "decorative" lighting is important for example, to reduce 33 glare, for interior roof lights within cars where it is desirable to direct the output light 34 towards only the two front passengers, or within an office environment where it is desirable 1 for the majority of the light to be directed towards desks in work area while still retaining a 2 level of attractive decorative diffuse light from the surface of the lighting apparatus..

4 The present invention is inherent with significant advantages over the lighting apparatus known in the art. In the first instance the described lighting apparatus provides the 6 required functional light output but combines this with a secondary light output that 7 provides the lighting apparatus with an attractive decorative effect that is known to be 8 desirable to observers. This effect is achieved in an energy efficient manner when 9 compared with those systems known in the art.

11 A uniform secondary light output can be achieved for a planar lighting apparatus without 12 the need to employ a diffuser or a reflector. This provides a lighting apparatus having a 13 reduced thickness and manufacturing costs when compared to those known in the art.

In addition, the uniform nature of the secondary light output from the lighting apparatus is 16 maintained even when the apparatus is curved or bent i.e. non-planar. This significantly 17 increases the design of lights that can be produced.

19 A lighting apparatus that provides an energy efficient illumination source that is suitable for use as an interior, ambient lighting source is described. The lighting apparatus comprising 21 a light guide suitable for guiding light coupled to a first surface to a second surface of the 22 light guide thus providing a first light output emitted from the second surface. The light 23 guide further comprises a plurality of extraction features located on a further surface 24 thereof the extraction features being arranged so as to redirect a portion of the guided light so as to provide a second light output that is more diffuse than the first light output.

27 The foregoing description of the invention has been presented for purposes of illustration 28 and description and is not intended to be exhaustive or to limit the invention to the precise 29 form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others 31 skilled in the art to best utilise the invention in various embodiments and with various 32 modifications as are suited to the particular use contemplated. Therefore, further 33 modifications or improvements may be incorporated without departing from the scope of 34 the invention as defined by the appended claims.