GB2589110A

GB2589110A - Uniform lighting device

Info

Publication number: GB2589110A
Application number: GB1916878.0A
Authority: GB
Inventors: Gourlay James; Jankauskas Marius
Original assignee: Design LED Products Ltd
Current assignee: Design LED Products Ltd
Priority date: 2019-11-20
Filing date: 2019-11-20
Publication date: 2021-05-26
Anticipated expiration: 2039-11-20
Also published as: GB2589110B; WO2021099789A1; GB201916878D0

Abstract

A uniform lighting device 13 has one or more light sources 14 embedded within a planar light-guide 3. The planar light-guide has a light output surface 6, wherein the one or more light sources and the planar light-guide combine to produce a non coupled light region (7, figure 4) of the planar light-guide around the one or more light sources, wherein light rays emitted from the associated one or more light sources exit the planar light-guide via the light output surface without being coupled into the planar light-guide and a coupled light region (8, figure 4) wherein light rays are coupled into the planar light-guide. A light reduction medium 16 is located within the non-coupled light region to reduce the intensity of the non-coupled light rays exiting from the non-coupled light region. A light extracting medium 17 is located within the coupled light region to extract coupled light rays.

Description

Intellectual Property Office Application No. GII1916878.0 RTM Date 3 May 2020 The following terms are registered trade marks and should be read as such wherever they occur in this document: Osram Everlight Makrofol Melinex, Lexan Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo 1 Uniform Lighting Device 3 The present invention relates to the field of lighting, and in particular, to a uniform lighting 4 device that can be used for illumination, backlighting, signage or display purpose. The described uniform lighting device finds particular application within the field of 6 transportation e.g. the automotive, train and aerospace industries.

8 Background to the Invention

Lighting is a key means of making interior vehicle spaces, where passengers stand or sit 11 during transportation, more attractive and pleasant environments. One of the most 12 effective ways to deliver light into these environments, while saving space, is to backlight 13 the interior surfaces of the vehicles. As a result, there is a requirement for a uniform, low 14 intensity light level to be provided over a large surface area. This uniform low intensity light level is required in order to keep the glare experienced by passengers being 16 transported within the vehicles to a minimum, whilst also providing a means to attractively 17 decorate and illuminate the interior surfaces.

1 Due to space and weight constraints within vehicles, any light source solution must be very 2 thin, of the order of --I mm. In addition, due to vibration and integration constraints, the 3 lighting device must also be capable of being mechanically attached, bonded, joined or 4 moulded onto the internal surface of the vehicle.

6 A number of light source technologies exist which can be employed within the field of 7 transportation. Two such examples are electroluminescent film and organic light emitting 8 diodes (OLED). Both solutions involve an active light emitting material that covers the 9 entire surface to be backlit. However, both technologies are expensive, have a low reliability and lifetime and so neither are ideally suited as an integrated solution for 11 transportation interiors.

13 Inorganic light emitting diodes (LEDs) are the most common lighting technology employed 14 for transportation lighting. LEDs are small solid state, semiconductor chip-based devices, that can be designed to emit different colours of light, or when used in combination with 16 colour converting materials, to provide white light. However, LEDs are small points of 17 light, and therefore require an external optical system to turn them into large area, 18 homogeneous lighting surfaces.

The simplest configuration of optical system to achieve the desired large area, 21 homogeneous lighting surface is to deploy the LED devices in a 2D matrix across a printed 22 circuit board (PCB) and then place a diffuser layer on top of the 2D matrix. This is 23 conventionally known as a direct-lit LED backlight. An advantage of the direct-lit LED 24 backlight approach is that each LED is independently addressable, and so a pixelated area light source can be produced. However, such systems require either the LEDs to be 26 very closely packed, which results in high power density and cost per area, or the 27 employment of a very thick optical system (e.g. an air gap and/or diffuser thickness), which 28 then makes them unsuitable for deployment within the limited interior transport spaces.

29 For example, if the LEDs are spaced 20 mm apart, the optical system depth is required to be >20 mm.

32 It is known in the art to employ light guides to distribute light from a light source to an area 33 that requires illumination. One known type of light-guide is an optical fibre, which is 34 typically made up of a transparent material (glass or plastic) with thin filaments that are 1 capable of transmitting light. An alternative known type of light-guide is a planar light- 2 guide. These are plates or panel light-guides, which are typically formed as thin cuboids.

4 Both light-guide designs exploit the effects of refraction caused by two materials having different refractive index. In particular, a light-guide transports light from one location to 6 another, by exploiting the effects of total internal reflection experienced by the light 7 propagating within the material when it encounters a boundary surrounding the material. A 8 further useful property of the aforementioned light-guides is their ability to take the light 9 output from an LED and spread it evenly and or change its shape or distribution to achieve a desired result.

12 One controllable parameter of a light-guide is its numerical aperture (NA). Numerical 13 aperture is defined as the maximum acceptance angle for a light-guide that allows light be 14 coupled into the light-guide. This parameter is dependent upon the refractive index of the light-guide and surrounding media. Light incident upon the light-guide above the 16 maximum angle is not coupled into the light-guide.

18 The above described, light-guiding approaches have been developed to try and meet the 19 space limitations of backlighting within the field of transportation. One approach has been the employment of woven, optical fibre mats. These systems typically have the light 21 source located separately from the backlighting surface. However, optical fibre mats are 22 expensive and do not easily support a pixelated, uniform area light source.

24 A second approach is that commonly known as the edge-lit LED backlight approach, see for example US patent publication number US 2004/0136173. Here a machined, printed 26 or moulded, light-guide plate is employed, and the LEDs are mounted along one or more 27 edges. Light is thereafter coupled from the LEDs, into the light-guide plate, before 28 propagating though the light-guide plate. Light extraction features on the surface of the 29 light-guide plate provide a means for the light to exist from the light-guide plate. Correct design of the light extraction features (variation in size, density etc.), gives a homogeneous 31 or uniform backlighting of a surface material or a diffuser layer located across the light- 32 guide plate. A limitation of the edge-lit LED approach is that the LEDs are only located at 33 the edge of the light-guide plate, thus although the light source appears as a uniform area, 34 individual areas or pixels are not addressable.

1 Another approach is conventionally known as a composite light-guide device, see for 2 example international patent publication number WO 2007/138294. Here, LEDs are 3 distributed in a 2D matrix that is embedded within a light-guide structure. The light-guide 4 structure acts to guide the light from the LEDs in the plane of the light-guide structure.

Light extraction features inside or on surface of the composite light-guide device are then 6 employed to provide a means for the light to exit the light-guide structure. The design of 7 the light extraction features (variation in size, density etc.) again provides a means for 8 homogeneously or uniformly backlighting a surface material across the light-guide 9 structure.

11 One of the challenges of employing a composite light-guide device to produce 12 homogeneous or uniform backlighting structure is the issue of the integrated LEDs being 13 seen as visible "hot spot" artefacts. This issue will now be described in further detail with 14 reference to Figure 1 to 4.

16 Figure 1 presents a two-dimensional, cross sectional side view of a composite light-guide 17 device, depicted generally be reference numeral 1 while Figure 2 presents a plan view of 18 the composite light-guide device 1 of Figure 1(a).

As can be seen from Figure 1, a light source 2 is presented embedded within a planar 21 light-guide 3. Light emitted from the light source 2 has range of angles. In Figure 1, two 22 different classes of light ray are presented namely, light rays 4 that do not get coupled into 23 the planar light-guide 3 and light rays 5 that do get coupled into the planar light-guide 3.

24 The reason that two classes of light rays 4 and 5 exist is a direct result of the effects of total internal reflection i.e. the combined effects of the angle of propagation of the light 26 emitted from the light source 2 when taken in conjunction the wavelength of the light and 27 the refractive index of planar light-guide 3 and the surrounding media. A light ray 5 28 emitted from the light source 2 at an angle greater than the angle of total internal reflection 29 will be coupled into the planar light-guide 3 while light rays 5 emitted at an angle less than the angle of total internal reflection will not be coupled into the planar light-guide 3 and 31 thus exit the device via a light output surface 6.

33 The region around the light source 2 where light rays 4 emitted from the light source 2 are 34 not coupled into the planar light-guide 3 is depicted generally be reference numeral 7 while, the region around the light source 2 where light rays 5 emitted from the light source 1 2 are coupled into the planar light-guide 3 is depicted generally be reference numeral 8.

2 As a result of the above, regions 7 appear brighter to an observer 9 viewing the light 3 output surface 6 of the composite light-guide device 1 relative to regions 8 and thus are 4 seen as a visible "hot spot" artefact, producing an overall non-uniform appearance to the observer 9.

7 In order to reduce the effects of the visible "hot spot" artefact being seen by the observer 9 8 it is known to locate a diffuser 10 between the observer 9 and the light output surface 6 so 9 producing a more uniform light output 11,. Figures 3 and 4 present cross sectional side views of the composite light-guide device 1 of Figure 1 with two different arrangements for 11 incorporating the diffuser 10, namely Figure 3 shows an arrangement that incorporates an 12 air gap 12 between the diffuser 10 and the light output surface 6; while Figure 4 shows an 13 arrangement where the diffuser 10 is optically bonded to the light output surface 6.

As previously discussed with reference to direct-lit LED backlights, the effectiveness of a 16 diffuser 10 in producing a uniform light output 11 is limited by how thick this component, 17 and any associated air gap 12, can be made. The applicants have been unable to design 18 a composite light-guide device 1 that satisfactorily removes the effects visible "hot spot" 19 artefact while meeting the space limitations of backlighting within the field of transportation.

Significantly, when the diffuser 10 is optically bonded to the composite light-guide1 (see 21 Figure 4) in an attempt to reduce the overall thickness of the device, the applicants found 22 that the area of the "hot spot" regions 7 actually increases. This was a direct result of the 23 difference in the refractive index of the air gap 12 and the material from which the diffuser 24 10 was formed.

26 Summary of the Invention

28 It is therefore an object of an embodiment of the present invention to provide an alternative 29 lighting device that provides a uniform, low intensity light output over a large surface area.

31 It is a further object of an embodiment of the present invention to provide a uniform lighting 32 device that is thinner than those uniform lighting devices known in the art.

1 According to a first aspect of the present invention there is provided a uniform lighting 2 device comprising one or more light sources embedded within a planar light-guide, the 3 planar light-guide having a light output surface, 4 wherein the one or more light sources and the planar light-guide combine to produce a non-coupled light region of the planar light-guide around the one or more light sources, 6 wherein light rays emitted from the associated one or more light sources exit the planar 7 light-guide via the light output surface without being coupled into the planar light-guide, 8 and 9 a coupled light region of the planar light-guide around the one or more light sources, wherein light rays emitted from the associated one or more light sources are coupled into 11 the planar light-guide, 12 the uniform lighting device further comprising 13 a light reduction medium located within the non-coupled light region to reduce the intensity 14 of the non-coupled light rays exiting from the non-coupled light region via the light output surface, 16 and 17 a light extracting medium located within the coupled light region to extract coupled light 18 rays from the coupled light region via the light output surface.

The combined effects of the light reduction medium and the light extracting medium is 21 such that the spatial intensity distribution of the reduced light intensity within the non- 22 coupled regions can be balanced with the spatial intensity distribution of the light emitted 23 from the light output surface within the coupled light regions. The overall result is a 24 uniform light output that does not exhibit any visible "hot spot", or indeed "dark spot", artefacts being seen by the observer.

27 The light reduction medium preferably extends over the whole area, or at least a 28 substantial part of the area, of the non-coupled light regions. It may be located on the light 29 output surface, embedded within the planar light-guide, or comprise a combination of both locations.

32 The light reduction medium preferably comprises an ink layer, a dye layer, a thin film or 33 other absorbing or reflecting medium or material. The light reduction medium may 34 comprise a specular or non-specular reflector, such as a metal film or white ink. If an absorption ink is employed this may be printed to be semi-transparent or with small holes, 1 to allow a small amount of light to be emitted and so avoid a "dark spot" artefact being 2 seen by an observer.

4 Optionally, the light reduction medium is arranged to vary the amount of light reflected or absorbed depending on the spatial distance from the associated one or more light 6 sources. This has the advantage of increasing the uniformity of light emitted from the light 7 output surface within the non-coupled light regions. Variation of the amount of light 8 reflected or absorbed may be achieved by incorporating a patterned formed from dots or 9 holes within the light reduction medium.

11 The light extracting medium preferably extends over the whole area, or at least a 12 substantial part of the area, of the coupled light regions. It may be located upon a surface 13 of the planar light-guide opposite to the light output surface. Alternatively, the light 14 extracting medium may be located on the light output surface, embedded within the planar light-guide or comprise a combination of two or more of these locations.

17 The light extracting medium preferably comprises a light scattering medium. It may be 18 patterned or provided as a thin layer.

It is preferable for the light extracting medium to be patterned into a dot matrix. The dot 21 matrix may comprise a regular or irregular array of dots. The irregular array may comprise 22 dots of varying radius and or varying separation.

24 Most preferably the one or more light sources are independently addressable.

26 The one or more light sources may comprise a light emitting diode electrically and 27 mechanically mounted onto a printed circuit board (PCB).

29 The PCB may be a transparent or non-transparent PCB.

31 Preferably, the LEDs are of a type designed to emit light from all five surfaces that are not 32 in contact with the PCB. Alternatively, the LEDs comprise top emitting or side emitting 33 LEDs. If the LED package consists of more than one LED chip, then preferably, the 34 transparent material of the LED is of a diffusing material, so as to improve the colour mixing of the light from the more than one LED chips.

2 The planar light-guide may be made from a layer of transparent material such glass or a 3 polymer resin.

The uniform lighting device may further comprise a diffuser arranged to diffuse light exiting 6 the light output surface. An air gap may be incorporated between the diffuser and the light 7 output surface. Alternatively, the diffuser may be optically bonded to the light output 8 surface.

The uniform lighting device may further comprise a transparent substrate upon which the 11 one or more light sources are mounted. Preferably the refractive index of the transparent 12 substrate is less than or equal to the refractive index of the planar light guide.

14 Optionally, the one or more light sources are embedded within a planar light-guide such that the PCB is located between the LED and the light output surface. In this embodiment 16 the PCB can be employed as the light reduction medium.

18 Optionally, a section of the planar light-guide located around the one or more light sources 19 is formed as a separate mechanical component that is removably mounted within an aperture formed in the planar light-guide.

22 The uniform lighting device may further comprise an electrical connector may be located 23 on the opposite side of the PCB to which the LED is mounted.

The uniform lighting device may further comprise a reflector arranged such that the planar 26 light-guide is located between the reflector and the light output surface. The reflector may 27 be located within the non-coupled light region to reflect uncoupled light rays towards the 28 light output surface. The reflector recycles light and improves efficiency. The reflector can 29 be separated from the light-guide by an air gap or attached onto the surface. To improve the uniformity control, the reflector can be attached with a low refractive index layer, which 31 increases the Numerical Aperture of the light-guide attached to the reflector.

33 According to a second aspect of the present invention there is provided a method of 34 forming a uniform lighting device, the method comprising 1 -embedding one or more light sources within a planar light-guide, the planar light-guide 2 having a light output surface, 3 -combining the one or more light sources and the planar light-guide to produce 4 a non-coupled light region of the planar light-guide around the one or more light sources, wherein light rays emitted from the associated one or more light sources exit the planar 6 light-guide via the light output surface without being coupled into the planar light-guide, 7 and 8 a coupled light region of the planar light-guide around the one or more light sources, 9 wherein light rays emitted from the associated one or more light sources are coupled into -the planar light-guide, 11 -providing a light reduction medium located within the non-coupled light region to reduce 12 the intensity of the non-coupled light rays exiting from the non-coupled light region via the 13 light output surface, and 14 -providing a light extracting medium located within the coupled light region to extract coupled light rays from the coupled light region via the light output surface.

17 The light reduction medium is preferably provided over the whole area, or at least a 18 substantial part of the area, of the non-coupled light regions. It may be provided on the 19 light output surface, embedded within the planar light-guide, or comprise a combination of both locations.

22 Optionally, the light reduction medium is arranged to vary the amount of light reflected or 23 absorbed depending on the spatial distance from the associated one or more light 24 sources. Variation of the amount of light reflected or absorbed may be achieved by providing a patterned formed from dots or holes within the light reduction medium.

27 The light extracting medium is preferably provided over the whole area, or at least a 28 substantial part of the area, of the coupled light regions. It may be provided upon a 29 surface of the planar light-guide opposite to the light output surface. Alternatively, the light extracting medium may be provided on the light output surface, embedded within the 31 planar light-guide or comprise a combination of two or more of these locations.

33 Optionally, the light extraction medium is arranged to vary the amount of light extracted 34 depending on the spatial distance from the associated one or more light sources.

1 Variation of the amount of light extracted may be achieved by providing a patterned 2 formed from dots or holes within the light extraction medium.

4 Embedding one or more light sources within a planar light-guide may comprise embedding a light emitting diode electrically and mechanically mounted onto a printed circuit board 6 (PCB).

8 Optionally, the one or more light sources are embedded within a planar light-guide such 9 that the PCB is located between the LED and the light output surface. In this embodiment the PCB can be employed as the light reduction medium.

12 The method of forming a uniform lighting device may further comprise providing a diffuser 13 arranged to diffuse light exiting the light output surface.

The method of forming a uniform lighting device may further comprise providing a 16 transparent substrate upon which the one or more light sources are mounted. Preferably 17 the refractive index of the transparent substrate is less than or equal to the refractive index 18 of the planar light guide.

The method of forming a uniform lighting device may further comprise providing a reflector 21 arranged such that the planar light-guide is located between the reflector and the light 22 output surface. The reflector may be located within the non-coupled light region to reflect 23 uncoupled light rays towards the light output surface.

Embodiments of the second aspect of the invention may include one or more features of 26 the first aspect of the invention or its embodiments, or vice versa.

28 Brief Description of the Drawings

There will now be described, by way of example only, various embodiments of the 31 invention with reference to the drawings, of which: 33 Figure 1 presents a two-dimensional, cross sectional side view of a composite light-guide 34 device known in the art; 1 Figure 2 presents a plan view of the composite light-guide device of Figure 1; 3 Figure 3 presents a two-dimensional, cross sectional side view of an alternative composite 4 light-guide device known in the art; 6 Figure 4 presents a two-dimensional, cross sectional side view of a further alternative 7 composite light-guide device known in the art; 9 Figure 5 presents a two-dimensional, cross sectional side view of uniform lighting device in accordance with an embodiment of the present invention; 12 Figure 6 presents a two-dimensional, cross sectional side view of uniform lighting device in 13 accordance with an alternative embodiment of the present invention; Figure 7 presents a two-dimensional, cross sectional side view of uniform lighting device in 16 accordance with a further alternative embodiment of the present invention; and 18 Figure 8 presents a two-dimensional, cross sectional side view of uniform lighting device in 19 accordance with a yet further alternative embodiment of the present invention.

21 In the description which follows, like parts are marked throughout the specification and 22 drawings with the same reference numerals. The drawings are not necessarily to scale 23 and the proportions of certain parts have been exaggerated to better illustrate details and 24 features of embodiments of the invention.

26 Detailed Description of Preferred Embodiments

28 Figure 5 presents a two-dimensional, cross sectional side view of a uniform lighting device 29 13 in accordance with an embodiment of the present invention. The uniform lighting device 13 comprises one or more light sources 2 embedded within a planar light-guide 3 31 (although, for ease of understanding, only one is shown in Figure 5).

33 In the presently described embodiment, the light sources 2 comprise a light emitting diode 34 (LED) 14 electrically and mechanically mounted onto a printed circuit board (PCB) 15.

Preferably, the LEDs 14 are of a type designed to emit light from all five surfaces that are 1 not in contact with the PCB 15. A Chip Scale Package (CSP) LED (e.g. an OSRAM 2 CHIPLED® 0402, LW OH8G that emits white light) or an ROB LED such as Everlight 3 EAST1616RGBAO are two example LEDs 14 that may be incorporated within the uniform 4 lighting device 13. Both these LEDs 14 are low power and have dimension of -1 mm.

6 The sixth surface of the LEDs 14 is where the electrical contacts are located. The LED 7 electrical contacts are used to electrically and mechanically mount the LEDs 14, with 8 conventional solder, onto metal pads located on the PCB 15.

The PCB 15 may comprise a thin, non-transparent PCB, made from a 0.4 mm thick FR4 or 11 a 0.15 mm thick polyimide substrate. The metal pads connect with copper wires/traces 12 which have been etched at a size of typically < 1 mm. It is preferable for the PCB 15 to 13 have a small surface area relative to the planar light-guide 3 and be cut into stripes, small 14 circular modules or into a grid structure.

16 The planar light-guide 3 that encapsulates the light sources 2 is made from a layer of 17 transparent material such as glass or a polymer resin, such as acrylic, polymethyl 18 methacrylate (PMMA), polycarbonate, silicone or polyurethane. The transparent planar 19 light-guide 3 is arranged to cover all the LEDs 14 on the PCB 15 and define the light output surface 6 for the uniform lighting device 13. The planar light-guide 3 may have a 21 thickness up to 3 mm depending on the particular LEDs 14 employed within the uniform 22 lighting device 13.

24 As can be seen from Figure 5, light is emitted from the LEDs 14 over a range of angles.

The emitted light can therefore again be classed into those light rays 4 that do not get 26 coupled into the planar light-guide 3 and those light rays 5 that do get coupled into the 27 planar light-guide 3. As a result, non-coupled light regions 7 and coupled light regions 8 28 are formed within the uniform lighting device 13.

A light reduction medium 16 is applied within non-coupled light regions 7 to provide a 31 means for reducing the intensity of the light emitted from the light output surface 6 within 32 these regions 7. The light reduction medium 16 preferably extends over the whole area, or 33 at least a substantial part of the area, of the non-coupled light regions 7. It may be located 34 on the light output surface 6 as shown in Figure 5, embedded within the planar light-guide 3, or comprise a combination of both locations. The light reduction medium 16 is therefore 1 employed to absorb or reflect a portion of the light rays 4 in order to avoid any visible "hot 2 spot" artefacts being seen by an observer 9 while still allowing a portion of the light rays 4 3 to be emitted from the light output surface 6 within these regions 7.

The light reduction medium 16 preferably comprises an ink layer, a dye layer, a thin film or 6 other absorbing or reflecting medium or material. The light reduction medium 16 may 7 comprise a specular or non-specular reflector, such as a metal film or white ink. If a black 8 absorption ink is employed this may be printed to be semi-transparent or with small holes, 9 to allow a small amount of light to be emitted and so avoid a "dark spot" artefact being seen by an observer 9. ii

12 Optionally, the light reduction medium 16 is arranged to vary the amount of light reflected 13 or absorbed depending on the spatial distance from the light sources 2. This has the 14 advantage of increasing the uniformity of light emitted from the light output surface 6 within regions 7. This may be achieved by incorporating a patterned formed from dots or holes 16 within the light reduction medium 16.

18 By contrast, in the coupled light regions 8, located at a distance from the light sources 2, a 19 light extracting medium 17 is provided which provides a means for extracting light rays 5 from the planar light-guide 3, via the light output surface 6. In the presently described 21 embodiment, the light extracting medium 17 is located upon a surface of the planar light- 22 guide 3 opposite to the light output surface 6. It will however be appreciated that in 23 alternative embodiments the light extracting medium 17 could be located on the light 24 output surface 6, embedded within the planar light-guide 3 or comprise a combination of two or more of these locations.

27 The light extracting medium 17 preferably comprises a light scattering medium. It may be 28 patterned or provided as a thin layer.

In order to increase uniformity of the light emitted from the light output surface 6 within the 31 coupled light regions 8 it is preferable for the light extracting medium 17 to be patterned 32 into a dot matrix. The dot matrix may comprise a regular or irregular array of dots. The 33 irregular array may comprise dots of varying radius and or varying separation.

1 It will be appreciated by the skilled reader that the combined effects of the light reduction 2 medium 16 and the light extracting medium 17 is such that the spatial intensity distribution 3 of the reduction in light intensity within the non-coupled regions 7 can be balanced with the 4 spatial intensity distribution of the light emitted from the light output surface 6 within the coupled light regions 8. The overall result is a uniform light output that does not exhibit 6 any visible "hot spot', or indeed "dark spot", artefacts being seen by the observer 9.

8 It should be noted that each the light source 2 of the uniform lighting device 13 can be 9 made independently addressable. Thus, the uniform lighting device 13 provides for independent control of the intensity of each the light source 2 which provides the uniform 11 lighting device 13 with the ability to be employed to produce a pixelated area light source.

13 As can be seen from Figure 5, the uniform lighting device 13 may further comprise a 14 diffuser 10 and air gap 12 based optical system located between the light output surface 6 and the observer 9 in order to further improve the uniformity of the light output 11 from the 16 uniform lighting device 13. Because of above described combined effects of the light 17 reduction medium 16 and the light extracting medium 17, the diffuser can be significantly 18 thinner than those employed in the prior art. For example, the diffuser may comprise a 19 polycarbonate film such as Makrofol DX cool, that is 3mm thick, and is 70% light transmitting.

22 The uniform lighting device 13 presented in Figure 5 may be formed by placing the PCB 23 15 in a Reactive Injection Mould (RIM) or regular injection moulding machine, to have the 24 planar light guide 3 moulded around it. The diffuser 10 can then be joined together with the planar light guide 3 in the moulding machine. The light reduction medium 16 and the 26 light extracting medium 17 can then be printed onto the planar light guide 3. Alternatively, 27 the light extracting medium 17 comprises a refractive surface feature that is patterned into 28 the mould tool and transferred onto the planar light guide 3 during the moulding process.

It will be appreciated that the surfaces of the PCB 15 around the LEDs 14 may also be 31 modified to reflect or absorb light and so improve the balance of light rays 4 emitted from 32 the light output surface 6 in the non-coupled light regions 7, with the light rays 5 extracted 33 from the coupled light regions 8. In a similar manner, the shape and size of the PCB 15 34 can be selected to further improve the uniformity of output light 11 from the uniform lighting 1 device 13. The surfaces of the PCB can be coated with low refractive index material, to 2 decouple the light-guiding from the non-transparent PCB surface.

4 In alternative embodiments, the light reduction medium 16 or the light extracting medium 17 may be located on a surface of the PCB 15.

7 In further alternative embodiments, the PCB 15 may comprise a thin, transparent PCB. As 8 such, the refractive index of the transparent PCB 15 can be selected to adjust the surface 9 area of the non-coupled light regions 7. In this embodiment, the light reduction medium 16, or the light extracting medium 17, can be located on a surface of, or within, the 11 transparent PCB 15.

13 The uniform lighting device 13 can be made with an LED pitch of > 50 mm while 14 maintaining an overall system thickness of less than c lOmm while maintain a highly uniform light output 11. As a result, the uniform lighting device 13 is ideally suited to be

16 employed within the field of transportation.

18 Figure 6 presents a two-dimensional, cross sectional side view of an alternative 19 embodiment of the uniform lighting device 18. This embodiment shares a number of features in common with the uniform lighting device 13 presented in Figure 5 and so these 21 common elements are marked with the same reference numerals.

23 The first difference between the uniform lighting device 13 presented in Figure 5 and the 24 uniform lighting device 18 presented in Figure 6 is the inclusion of a transparent substrate 19 employed to act as a carrier for the light sources 2 and the light extracting medium 17.

26 In order to further increase the effectiveness of the device in generating a uniform light 27 output 11 it is preferable for the refractive index of the transparent substrate 19 to be less 28 than or equal to the refractive index of the planar light guide 3. With this arrangement the 29 light rays 5 coupled into the planar light guide 3 are guided within a composite structure formed by the planar light guide 3 and the transparent substrate 19.

32 In the presently described embodiment the transparent substrate 19 comprises a thin 33 (0.1 mm to 0.2 mm) polymer film made from PET Melinex 506. In alternative 34 embodiments, other polymer films known to those skilled in the art may be employed for the transparent substrate 19 e.g. polycarbonate Lexan 8040.

2 The second difference between the uniform lighting device 13 presented in Figure 5 and 3 the uniform lighting device 18 presented in Figure 6 is the fact that the diffuser 10 is 4 bonded directly onto the light output surface 6 of the planar light guide 3 by means of an adhesive layer 20 e.g. a pressure sensitive lamination adhesive. Such an arrangement 6 results in the uniform lighting device 18 being thinner than the uniform lighting device 13 7 presented in Figure 5. As discussed above this arrangement also results in an increase in 8 the surface area of the non-coupled light regions 7. However, this increase can be 9 compensated for by introducing a corresponding increasing the surface area covered by the light reduction medium 16. In addition, the surface area of the non-coupled light 11 regions 7 can be reduced by ensuring that the refractive index of the adhesive layer 20 is 12 lower than the refractive index of the planar light guide 3.

14 It will be appreciated that further alternatives may be made to the uniform lighting device 18 presented in Figure 6. For example, the transparent substrate 19 may be employed to 16 function as the PCB for the LEDs 14. Electrical tracks would then printed or etched 17 directly onto the transparent substrate 19 and the LEDs 14 would then be mounted onto 18 these electrical tracks with silver epoxy, other conducting adhesives or a low temperature 19 solder.

21 Figure 7 presents a two-dimensional, cross sectional side view of a yet further alternative 22 embodiment of the uniform lighting device 21. This embodiment again shares a number of 23 features in common with the uniform lighting device 13 presented in Figure 5 and so these 24 common elements are again marked with the same reference numerals.

26 The main difference between the uniform lighting device 21 and the one presented in 27 Figure 5 is the orientation of light sources 2 embedded within a planar light-guide 3. In this 28 embodiment, the light sources 2 are arranged such that the PCB 15 lies in the plane of, or 29 adjacent to, the light output surface 6 i.e. the light sources 2 can be considered to be upside down, or rotated through 180°, relative to the light sources 2 of the uniform lighting 31 device 13 presented in Figure 5. With this arrangement the PCB 15 provides the function 32 of the light reduction medium 16.

34 A second difference is the presence of a reflector 22 located within the non-coupled light region 7. The reflector 22 is employed to reflect the light rays 4 that are not coupled into 1 the planar light-guide 3, and which would otherwise exit the planar light guide 3 via the 2 surface opposite to the light output surface 6, back towards the light output surface 6. The 3 reflector recycles light and improves efficiency. The reflector can be separated from the 4 light-guide by an air gap or attached onto the surface. The reflector can be bonded onto the light-guide surface with a low refractive index material to increase the numerical 6 aperture of the light-guide around the light source and improve coupling and uniformity 7 controllability.

9 Figure 8 presents a two-dimensional, cross sectional side view of a yet further alternative embodiment of the uniform lighting device 23. This embodiment again shares a number of 11 features in common with the uniform lighting device 13 presented in Figure 5 and so these 12 common elements are again marked with the same reference numerals.

14 In this embodiment, a sections 24 of the planar light-guide 3 located around the light sources 2 are formed as a separate mechanical component. As such, an LED 14 and its 16 associated PCB 15 and light reduction medium 17 can be inserted into a complementary 17 aperture 25 located in the planar light-guide 3 during manufacture. As shown in Figure 8, 18 an electrical connector 26 may be located on the underside of the PCB 15.

This embodiment provides a means for replacing a light source 2 if there is an LED 14 21 failure or other mechanical failure within the device. The LED mounted on the PCB or with 22 another electrical connection solution, can be bonded permanently or temporarily into the 23 aperture, by means of a transparent adhesive or other polymer.

Although the above described embodiments have been described in conjunction with 26 LEDs 14 designed to emit light from all five surfaces that are not in contact with the PCB 27 15 it will be appreciated by the skilled reader that any alternative LED known to those 28 skilled in the art may alternatively be employed e.g. top emitting or side emitting LEDs. If 29 the LED package consists of more than one LED chip, then preferably, the transparent material of the LED is of a diffusing material, so as to improve the colour mixing of the light 31 from the more than one LED chips.

33 In addition, a reflector (not shown) may be incorporated such that the planar light-guide 3 34 is located between the reflector and the light output surface 6. Incorporation of the 1 reflector ensures that all light from the uniform lighting devices is effectively directed 2 towards the observer 9.

4 The present invention provides a number of alternative uniform lighting devices, capable of providing low intensity light level over a large surface area, to those known in the art.

7 A significant advantage of the present invention is that the uniform lighting devices can be 8 made much thinner than those devices known in the art without introducing the 9 problematic features of "hot spot" or "dark spot" artefacts i.e. a thin device can be produced that exhibits a highly uniform light output over a large surface area.

12 The disclosed uniform lighting devices are also cheaper to manufacture, and have a higher 13 reliability and lifetime, than alternative solutions known in the art.

Since the uniform lighting devices comprise a plurality of individual light sources, they 16 exhibit the further advantage that each light source can be made independently 17 addressable, and so a pixelated area light source can be produced.

19 As a result of the above described advantages, the uniform lighting devices of the present invention find particular application within the field of transportation e.g. the automotive, 21 train and aerospace industries where there is a requirement for a thin, robust device that is 22 capable of being mechanically attached, bonded, joined or moulded onto the internal 23 surface of the vehicle.

Throughout the specification, unless the context demands otherwise, the terms "comprise" 26 or "include", or variations such as "comprises" or "comprising", "includes" or "including' will 27 be understood to imply the inclusion of a stated integer or group of integers, but not the 28 exclusion of any other integer or group of integers. Furthermore, unless the context 29 demands otherwise, the term "or" will be interpreted as being inclusive not exclusive.

31 The foregoing description of the invention has been presented for the purposes of illustration 32 and description and is not intended to be exhaustive or to limit the invention to the precise 33 form disclosed. The described embodiments were chosen and described in order to best 34 explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various 1 modifications as are suited to the particular use contemplated. Therefore, further 2 modifications or improvements may be incorporated without departing from the scope of the 3 invention as defined by the appended claims.

Claims

1 Claims: 3 1. A uniform lighting device comprising one or more light sources embedded within a 4 planar light-guide, the planar light-guide having a light output surface, wherein the one or more light sources and the planar light-guide combine to produce a non- 6 coupled light region of the planar light-guide around the one or more light sources, 7 wherein light rays emitted from the associated one or more light sources exit the 8 planar light-guide via the light output surface without being coupled into the planar 9 light-guide, and a coupled light region of the planar light-guide around the one or more light sources, wherein light rays emitted from the associated one or more light 11 sources are coupled into the planar light-guide, 12 the uniform lighting device further comprising 13 a light reduction medium located within the non-coupled light region to reduce the 14 intensity of the non-coupled light rays exiting from the non-coupled light region via the light output surface, and a light extracting medium located within the coupled 16 light region to extract coupled light rays from the coupled light region via the light 17 output surface.19

2. A uniform lighting device as claimed in claim 1 wherein the light reduction medium extends over the whole area, or at least a substantial part of the area, of the non- 21 coupled light regions.23

3. A uniform lighting device as claimed in either of claims 1 or 2 wherein the light 24 reduction medium is located on the light output surface, embedded within the planar light-guide, or comprises a combination of both locations.27

4. A uniform lighting device as claimed in any of the preceding of claims wherein the 28 light extracting medium extends over the whole area, or at least a substantial part of 29 the area, of the coupled light regions.31

5. A uniform lighting device as claimed in any of the preceding of claims wherein the 32 light extracting medium are located upon a surface of the planar light-guide opposite 33 to the light output surface or on the light output surface or embedded within the 34 planar light-guide or comprise a combination of two or more of these locations.1

6. A uniform lighting device as claimed in any of the preceding of claims wherein the 2 light reduction medium and or the light extracting medium are arranged to vary the 3 amount of light reflected or absorbed depending on the spatial distance from the 4 associated one or more light sources.6

7. A uniform lighting device as claimed in any of the preceding of claims wherein the 7 one or more light sources comprise a light emitting diode electrically and 8 mechanically mounted onto a printed circuit board (PCB).

8. A uniform lighting device as claimed in claim 7 wherein the PCB comprises a 11 transparent or non-transparent PCB.13

9. A uniform lighting device as claimed in any of the preceding claims wherein the 14 uniform lighting device further comprise a diffuser arranged to diffuse light exiting the light output surface.17

10. A uniform lighting device as claimed in claim 9 wherein an air gap is incorporated 18 between the diffuser and the light output surface.

11. A uniform lighting device as claimed in claim 9 wherein the diffuser is optically 21 bonded to the light output surface.23

12. A uniform lighting device as claimed in any of the preceding claims wherein the 24 uniform lighting device further comprises a transparent substrate upon which the one or more light sources are mounted wherein the refractive index of the transparent 26 substrate is less than or equal to the refractive index of the planar light guide.28

13. A uniform lighting device as claimed in any of the claims 7 to 12 wherein the one or 29 more light sources are embedded within a planar light-guide such that the PCB is located between the LED and the light output surface.32

14. A uniform lighting device as claimed in any of the preceding claims wherein a section 33 of the planar light-guide located around the one or more light sources is formed as a 34 separate mechanical component that is removably mounted within an aperture formed in the planar light-guide.2

15. A uniform lighting device as claimed in any of the preceding claims wherein the 3 uniform lighting device further comprises a reflector arranged such that the planar 4 light-guide is located between the reflector and the light output surface.6

16. A method of forming a uniform lighting device, the method comprising: 7 -embedding one or more light sources within a planar light-guide, the planar light- 8 guide having a light output surface, 9 -combining the one or more light sources and the planar light-guide to produce a non-coupled light region of the planar light-guide around the one or more light 11 sources, wherein light rays emitted from the associated one or more light sources 12 exit the planar light-guide via the light output surface without being coupled into the 13 planar light-guide, and 14 a coupled light region of the planar light-guide around the one or more light sources, wherein light rays emitted from the associated one or more light sources are coupled 16 into -the planar light-guide, 17 -providing a light reduction medium located within the non-coupled light region to 18 reduce the intensity of the non-coupled light rays exiting from the non-coupled light 19 region via the light output surface, and -providing a light extracting medium located within the coupled light region to extract 21 coupled light rays from the coupled light region via the light output surface.23

17. A method of forming a uniform lighting device as claimed in claim 16 wherein the 24 light reduction medium is provided over the whole area, or at least a substantial part of the area, of the non-coupled light regions.27

18. A method of forming a uniform lighting device as claimed in either of claims 16 or 17 28 wherein the light reduction medium is provided on the light output surface, 29 embedded within the planar light-guide, or comprise a combination of both locations.31

19. A method of forming a uniform lighting device as claimed in any of claims 16 to 18 32 wherein the light extracting medium is provided over the whole area, or at least a 33 substantial part of the area, of the coupled light regions.1

20. A method of forming a uniform lighting device as claimed in any of claims 16 to 19 2 wherein the light extracting medium is provided upon a surface of the planar light- 3 guide opposite to the light output surface, on the light output surface, embedded 4 within the planar light-guide or comprise a combination of two or more of these locations.7

21. A method of forming a uniform lighting device as claimed in any of claims 16 to 20 8 wherein the light reduction medium and or the light extracting medium is arranged to 9 vary the amount of light reflected or absorbed depending on the spatial distance from the associated one or more light sources.12

22. A method of forming a uniform lighting device as claimed in any of claims 16 to 21 13 wherein embedding the one or more light sources within the planar light-guide 14 comprises embedding a light emitting diode electrically and mechanically mounted onto a printed circuit board (PCB).17

23. A method of forming a uniform lighting device as claimed in claim 22 wherein the one 18 or more light sources are embedded within a planar light-guide such that the PCB is 19 located between the LED and the light output surface.21

24. A method of forming a uniform lighting device as claimed in any of claims 16 to 23 22 wherein the method further comprises providing a diffuser arranged to diffuse light 23 exiting the light output surface.

25. A method of forming a uniform lighting device as claimed in any of claims 16 to 24 26 wherein the method further comprises providing a transparent substrate upon which 27 the one or more light sources are mounted wherein the refractive index of the 28 transparent substrate is less than or equal to the refractive index of the planar light 29 guide.