WO2012085730A1

WO2012085730A1 - Light emitting sheet and its fabrication

Info

Publication number: WO2012085730A1
Application number: PCT/IB2011/055547
Authority: WO
Inventors: Giovanni Cennini; Hugo Johan Cornelissen; Hendrikus Hubertus Petrus Gommans
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-12-21
Filing date: 2011-12-08
Publication date: 2012-06-28

Abstract

A lighting system and manufacturing method in which a light guide comprises a laminate of at least two layers, comprising an outer layer of optically transparent cladding and a core layer which comprises a layer of first refractive index and regions of different second refractive index which define light out-coupling structures, or comprising a multi- layer core.

Description

LIGHT EMITTING SHEET AND ITS FABRICATION

FIELD OF THE INVENTION

The present invention relates to a light emitting sheet, for example using LED light sources, and to its fabrication.

SUMMARY OF THE INVENTION

Light output devices utilizing light emitting diodes (LEDs) as their light sources have become increasingly popular. Such light output devices can be used for illumination of objects, for display of an image, or simply for decorative purposes.

LEDs are made by connecting the n-type semiconductor region and the p-type semiconductor region of an LED chip to respective terminal pins for drawing electric current. The LED chip is embedded in a package, for example of a resin. The package may be arranged so that light from the LED chip is emitted in one or more designated directions.

LEDs have a small form factor, which enables thin and versatile designs to be formed. One example is a light emitting sheet which can be placed over, or integrated with a surface. A light emitting sheet is for example provided with an embedded LED or an array of embedded LEDs. The LEDs emit light at their locations within the sheet.

The small form factor of the LEDs translates to very high brightness, for example exceeding 10⁶ cd/m².

Thus, a problem with light emitting sheets with integrated arrays of LEDs is that the sheet has local high intensity regions at the LEDs. The individual LEDs can create glare as well as unwanted shadowing effects. In many applications, it is desired to obtain a more uniform light output intensity across the area of the sheet, for example spreading the LED light output over a larger area of for example 1 to 10cm². Secondary optics, such as light diffusing layers or scattering surfaces can be used for this purpose.

Another example of a light emitting sheet uses a light cavity to spread light and thereby generate a more uniform output. Light cavities are for example used in backlight units for LCDs, where the uniformity of the output is of particular importance. Such a cavity can be in the form of a foil, which is illuminated by edge-mounted LEDs.

One example of this type of foil is a PMMA waveguide (Poly(methyl methacrylate) - a transparent thermoplastic), sometimes known as a light skin. Light is captured within the waveguide by total internal reflection, and light out-coupling structures are used to generate the uniform illumination at the light output surface. These light out- coupling structures provide a change in refractive index or a change in the angle of the light, such as to interrupt the total internal reflection. For example they can comprise light scattering regions. The out-coupling areas are arranged with reference to the LED positions - for example closer together further away from the LEDs, because the intensity is lower, so that more light output areas are needed for a uniform intensity over the area of the light output surface.

Fig. 1 shows a PMMA waveguide light emitting sheet, and shows edge coupled LEDs 10 (for example red, green and blue), the waveguide 12 and the irregular pattern of light out-coupling structures 14. The pattern is calculated precisely to ensure a good uniformity. The waveguide is for example 1mm thick.

Fig. 2 shows the structure of Fig. 1 in cross section.

The light out-coupling structures can take various forms, such as scattering paint dots, micro-grooves, micro-prisms, microlenses, domains with surface roughness, phosphor dots.

The waveguide is usually produced by injection moulding.

The PMMA (or other plastic material transparent to light, such as polycarbonate) is injected into metal moulds with carefully polished or micro-structured walls and inserts. After injection at high pressure and temperature, the plastic cools down and solidifies. It is clear that this is a slow batch process. Moreover, there are limitations inherent in this technological process. On the one hand, large sizes objects require large moulds that cannot be easily realized. On the other hand, each time a different shape has to be changed (even slightly modified) a new mould or insert has to be created.

Another significant limitation is the length-to-thickness-ratio. It is well known that for a light guide of a certain thickness, the length is limited because of limited pressures that can be applied to the molten plastics. For instance, in practice an injection moulded light guide of thickness 800 μιη cannot be made longer than approximately 200mm.

The invention relates to light guide designs that aim to provide uniform light distribution, and to the method of fabrication, in order to address the problems identified above.

SUMMARY OF THE INVENTION

According to the invention, there is provided a lighting system comprising: a light guide comprising a sheet of light guiding material and an array of light out-coupling structures formed as a pattern; and at least one LED module for coupling light into the light guide, wherein the light guide comprises an outer layer of optically transparent cladding and a core layer which comprises a layer of first refractive index and regions of different second refractive index which define the light out-coupling structures, and wherein the light guide comprises a laminate of at least two layers.

The system preferably comprises at least three layers, comprising the outer layer, the core layer and a further outer layer, with the two outer layers on opposite sides of the core layer.

The regions can comprise air spaces within a plastics layer or they can comprise first plastics portions within a plastics layer of different plastic material.

In one example, the core layer comprises a first region of uniform first refractive index over a first depth range of the layer, a second region over a second depth range of the layer and which includes the regions of second refractive index surrounded by the first refractive index material. The term "over a depth range" is intended to mean that within the slab, there is a region corresponding to a range of depths, and the depth is the dimension perpendicular to the surface of the slab. For example from the top surface down in depth to 25% of the slab will constitute depth range of 0 to 25%.

This provides a core layer with at least two characteristics at different depths - a solid part and the part with the light out-coupling regions. The core layer itself may therefore be a multi-layer laminated structure.

The core layer can comprise a third region over a third depth range of the layer which comprises spacers which support the outer layer.

The core layer can comprise a fourth region over a fourth depth range of the layer which comprises spacers which support a further outer layer, with the two outer layers on opposite sides of the core layer.

The spacers can ensure that light that can leave the core layer by the out- coupling structures does not become trapped in the cladding layer or layers.

The first region can cover a first depth range on one side of the second region and a second depth range on the opposite side of the second region, so that the second region is sandwiched between layers of uniform refractive index. Thus, the core layer may itself define the light out-coupling regions as embedded structures between solid layers, in addition to the core layer being protected by one or more cladding layers. The second region can then comprise a foil having openings which define the regions of second refractive index, wherein at least two opposing side walls of the openings are slanted with respect to the direction perpendicular to the layer. This slanting means that only the slanted surfaces of the openings need to be of optical quality.

The cladding and the layer of first refractive index can comprise PMMA. The invention also provides a method of manufacturing a light guide for a lighting system, comprising:

combining at least two layers together, comprising an outer layer of optically transparent cladding and a single layer or multi-layer core which comprises at least a layer of first refractive index and regions of different second refractive index, the regions defining light out-coupling structures for coupling light out of the light guide, wherein a lamination process is used to combine the core later and the cladding layer and/or to combine multiple layers of the core.

The lamination process means that the light out-coupling structures are embedded within the light guide. For example, they can comprise micrometer or nanometer scale regions of second refractive index. The design of the core layer is tailored to the particular application. The individual layers can be formed by a roll-to-roll process.

The cladding layer (or layers) can function as a protection layer or layers, or further protection layers can be provided. The presence of these layers guarantees that the light propagates inside the core without being extracted by scratches, stains, and other surface imperfections.

The invention avoids the need for an injection moulding process. The method is thus fast and suitable for large size objects.

The core can comprise an array of regions, and the positioning of the regions determines the light output characteristics. For example, the positioning can be to create uniform illumination from the output surface from an edge-mounted light source.

The regions can comprise air spaces within a plastics layer. These air spaces create refractive index boundaries which interrupt the total internal reflection within the light guide.

The method preferably comprises laminating at least three layers, comprising the outer layer, the core layer and a further outer layer, with the two outer layers on opposite sides of the core layer.

The invention also provides a method of manufacturing a lighting system, comprising manufacturing a light guide using the method of the invention, and mounting a light source along at least one edge of the light guide. This provides an edge-lit lighting system. BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 shows a know light emitting sheet in perspective view;

Fig. 2 shows the light emitting sheet of Fig. 1 in cross section;

Fig. 3 is used to explain the manufacturing method of the invention;

Fig. 4 shows a first example of lighting system of the invention;

Fig. 5 shows a second example of lighting system of the invention; and

Fig. 6 shows two variations of a third example of lighting system of the invention.

Fig. 7 shows two variations of a fourth example of lighting system of the invention.

Fig. 8 shows a fifth example of lighting system of the invention.

Fig. 9 shows a sixth example of lighting system of the invention.

Fig. 10 shows in more detail the components of the light guide of the system of Fig. 8; and

Fig. 11 shows a seventh example of lighting system of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a lighting system and manufacturing method in which a light guide comprises a laminate of at least two layers, comprising an outer layer of optically transparent cladding and a core layer which comprises a layer of first refractive index and regions of different second refractive index which define light out-coupling structures.

Fig. 3 shows the lamination process for the large area light guides with light out-coupling structures (typically micrometer or nanometer scale) embedded in them.

In this example, the light guide has three layers - a cladding layer 30 in the form of a foil provided from a first drum 31 , a core layer 32 in the form of a foil provided from a second drum 33 and a cladding layer 34 in the form of a foil provided from a third drum 35.

The two cladding layers 30,34 surround the core layer 32. There may be only one cladding layer, for example if the opposite side of the core layer is to be attached to a substrate which will itself function as a protection layer and give the require reflection surface for total internal reflection within the core. The three foils 30,32,34 are used to build the core and cladding of a 2D slab (which can be flexible) of transparent material, in which the light can propagate and spread.

The central foil core 32 has micrometer scale and/or nanometer scale structures embedded within the foil. The core 32 thus has a refractive index n that is a function of the position.

The structures inside the foil 32 can for example be realized by laser cutting, water cutting and hot embossing techniques.

In Fig. 3, the central core is shown as a single layer, but it will become apparent from the description below that the central core layer 32 may itself comprise a laminate of multiple layers.

In Fig. 3, the layers 30 and 34 are cladding layers on each side of the central core layer 32.

The three foils 30,32,34 are brought together by roller arrangements to implement a lamination process. Protective coatings 36,38 (for mechanical protection) can also be applied on the opposite surfaces of the light guide as part of the lamination process.

Optionally, the layers 30 and 34 can be part of a three-layer core 32 and the coatings 36 and 38 then act as cladding and as mechanical protection. In this case, the lamination process is applied only to a multi-layer core.

The lamination process is a standard industrial process. It can be implemented with hot rolls or cold rolls, for example as used to apply a protective coating layer to a photograph or poster or book. Lamination processes are also known for providing protective carriers for an optical layer such as a polarizer film for a liquid crystal display. The lamination processes that are suitable will be well known to those skilled in the art.

Fig. 4 shows a first example of lighting system of the invention, comprising an edge mounted light source 10 and a light guide, having the core as a two-layer structure and a single cladding layer on top..

The light guide comprises a 2D (by which is meant the depth is small compared to the linear dimensions of the surface) slab with internal indentations which define void areas 40, as part of the core layer 32.

The core layer 32 comprises a first region 32a of uniform first refractive index over a first depth range of the layer (the bottom part of the core layer in Fig. 4), and a second region 32b over a second depth range of the core layer (the top part of the core layer in Fig. 4) and which includes the regions 40 of second refractive index surrounded by the first refractive index material 42. No lower cladding layer is shown in Fig. 4. The sequence of voids 40 and material 42 of the core layer creates a merlon- shaped pattern (castellation). The height and the width of the voids can be designed in such a way that a suitable quantity of light can be redirected upwards towards an additional 2D layer in the form of one of the cladding layers 34. The refractive index of the upper cladding layer 34 can be the same as the refractive index of the core layer material 42 and the material of the solid part 32a.

Preferably, air fills the gaps between the cladding 34 and the adjacent part 32b of the core. This can be ensured by inserting spacers between the cladding 34 and the core 32 as shown in Fig. 5.

The structure provides redirection of the light at the steep edges of the void structures 40. This provides an interruption to the light guiding defined by the interface between the core layer and the cladding layer (or layers).

The light propagating inside the core can be redirected upwards in a well defined cone angle by a sharp edge. Fig. 5 shows the light paths.

Considering the corner A of the void cavity 40 defined the four points ABCD, the light rays that can be out-coupled at this point are those enclosed in the angular range 9i- 9c, where 9c is the critical angle defined as by the Snell law; for PMMA (n = 1.49) this angle is approximately 43 degrees. This means that light rays inside a PMMA light guide propagate in the cone ±43°.

The rays that propagate at 9c will be out coupled perpendicular with respect to the guide. From the vertex A, the steepest angle is 9i defined as:

6_j = arcsin j— sin [θ₀ ] .

θ„ = arctan

(0.1) where h is the depth of the void cavity (height of the merlon) and b is the length of the void cavity.

It is clear that rays that hit the surface AD near D will have larger angles. The parameters h and b can be designed to have an opportune angular distribution of rays.

The rays that are out-coupled do not become trapped in the cladding layer as a result of the arrangement of air-cladding-air, which is created by the spacers. In Fig. 5, the spacers 38 can be particles placed on top of the core 32. The particles can be reflective or absorptive. Alternatively, the spacers can be defined by portions of an additional 2D foil placed between the core and the cladding.

The main differences between Fig. 6 and Fig. 5 are represented by the spacers between the cladding and the core.

In Fig. 6, the spacers 38 are parts of the core itself.

Thus, the spacers are defined by a third region 36 over a third depth range (the top thickness part in the example of Fig. 6) of the core layer. The spacers 38 support the cladding layer 34.

Fig. 6 also shows cladding layers 30,34 below and above the core layer 32.

In Fig. 6a, the spacer arrangement is provided against one cladding layer only, whereas in Fig. 6b it is provided against both cladding layers. Thus, in Fig. 6b, the core has a fourth region 60 over a fourth depth range of the layer which comprises spacers 38 which support the lower cladding layer 30.

The spacers can be defined by extruding the core to define wedge forms. The cladding can be of a different refractive index (but not necessarily).

In the example of Fig. 7, the voids 40 are placed more inside the core.

In particular, the solid layer 32a of the core is now provided above and below the void structures 40 (i.e. it defines two different depth ranges of the core) so that the second region of the core, with the voids 40, is sandwiched between layers of uniform refractive index.

The cladding layers are of different refractive index in Fig. 7a, which avoids the need for the air spacing, whereas in Fig. 7b, the spacers 38 are provided to enable the cladding layer to have the same refractive index as the core. In Fig. 7b, the core has wedge extrusions to define projections which function as the spacers 38.

Fig. 8 shows an example where the core 32 is itself built up from 3 foils

80,82,84.

Fig. 8a shows the core in expanded form. The central foil 82 defines the void structures (the second region of the core). This foil 82 has through holes or vias defining slits 86 at a slanted angle. The three foils are joined together by a lamination process, in the manner shown in Fig. 3, either at the same time as the cladding layers (so that four or five layers are laminated) or before laminating the cladding layers, so that the completed core can be considered as represented by the central layer 32 in Fig. 3.

These slanted-angle slits act as total internal reflection mirrors and very efficiently extract the light form the core, through the cladding, into the air. An advantage of this structure is that the slits only need to have two side walls that have to be optical quality, not three as in Fig. 5, where the edges DA and BC as well as the bottom AB have to be optically smooth in order to control the nature of the light output.

If rough surfaces are used (DA, AB and BC in Fig. 5), the effect is that of a scattering element like a dot of white paint, or a patch of rough surface on a smooth lightguide. The use of smooth surfaces enables the direction of light extraction to be engineered. The perpendicular side walls DA as in Fig. 5 result in a certain beam cone, and the tilted side walls of Fig. 8 result in a different beam cone.

Thus, the slits do not have to be slanted. In this case, the multiple foils can be used to create the structure of Fig. 5, with the central foil having the thickness h in Fig. 5. The sidewalls of the slits than constitute the edges DA and BC in Fig. 5.

Fig. 8b shows the assembled light guide, with the core 32 defined by the three layers of Fig. 8a, together with the cladding layers 30,34.

The cladding layers and the core can have the same refractive index.

Fig. 9 shows an alternative design for the foils 80,82,84 making up the core. The central foil 82 has an array of pyramid structures instead of an array of openings. These define oppositely slanted faces at opposite sides.

Fig. 10 shows the three foil layers 80,82,84 of Fig. 8 in perspective 3D view. The array of slits can be manufactured with laser ablation in a PMMA foil, for example of thickness 80μιη.

Fig. 11 shows a further embodiment in which the air spacing defined by the spacers is replaced with a material having a different refractive index.

Fig. 11a shows a similar core pattern with regions of different refractive index, but implemented by a solid of lower refractive index than the main core material. The same solid is used for the continuous layer 110, which functions in the same way as the air spacing described above.

In Fig. 1 lb, embedded out-coupling structures are shown, but instead of voids, these can be solid regions 112 of different refractive index, for example forming a diffractive grating. The air spacing is again replaced with a solid layer 110 of lower refractive index than the main core layer material.

Examples of transparent materials that can be used, and their refractive indices

(n), are:

- main refractive index material for the core 32: polymethylmethacrylate

(PMMA) (n=1.49), polyStyrene (n=1.59), polycarbonate (n=1.59), polyvinylchloride (PVC) (n=1.54), polyethylene (n=1.51), polysulfone (n=1.63), cellulose acetate (n=1.45),

solid material of lower refractive index for the spacer or for solid out-coupling structures: silicone (n= 1.42- 1.45), poly(tetrafluoroethylene) (PTFE) (n=1.35), silica

(n=l .43), fluorinated polyacrylates (n=l .34),

cladding layer materials: preferably transparent polymers,

material of additional protective layer 36,36 of Fig. 3: fiber reinforced polymers.

The invention can be used in illumination applications generally, including backlighting.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does 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 measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A lighting system comprising:

a light guide comprising a sheet of light guiding material and an array of light out-coupling structures formed as a pattern; and

at least one LED module for coupling light into the light guide, wherein the light guide comprises an outer layer of optically transparent cladding and a core layer which comprises a layer of first refractive index and regions of different second refractive index which define the light out-coupling structures, and wherein the light guide comprises a laminate of at least two layers.

2. A system as claimed in claim 1, comprising at least three layers, comprising the outer layer, the core layer and a further outer layer, with the two outer layers on opposite sides of the core layer.

3. A system as claimed in claim 1, wherein the regions comprise air spaces within a plastics layer.

4. A system as claimed in claim 1, wherein the regions comprise first plastics portions within a plastics layer of different plastic material.

5. A system as claimed in claim 1, wherein the core layer comprises a first region of uniform first refractive index over a first depth range of the layer, a second region over a second depth range of the layer and which includes the regions of second refractive index surrounded by the first refractive index material.

6. A system as claimed in claim 5, wherein the core layer comprises a third region over a third depth range of the layer which comprises spacers which support the outer layer.

7. A system as claimed in claim 6, wherein the core layer comprises a fourth region over a fourth depth range of the layer which comprises spacers which support a further outer layer, with the two outer layers on opposite sides of the core layer.

8. A system as claimed in claim 5, wherein the first region covers a first depth range on one side of the second region and a second depth range on the opposite side of the second region, so that the second region is sandwiched between layers of uniform refractive index.

9. A system as claimed in claim 8, wherein the second region comprises a foil having openings which define the regions of second refractive index, wherein at least two opposing sides walls of the openings are slanted with respect to the direction perpendicular to the layer.

10. A system as claimed in claim 1, wherein the cladding and the layer of first refractive index comprise PMMA.

11. A method of manufacturing a light guide for a lighting system, comprising:

12. A method as claimed in claim 11 , wherein the outer layer and the core layer are formed by a roll-to-roll process.

13. A method as claimed in claim 11, wherein the regions comprise air spaces within a plastics layer.

14. A method as claimed in claim 11, comprising laminating at least three layers, comprising the outer layer, the core layer and a further outer layer, with the two outer layers on opposite sides of the core layer. A method of manufacturing a lighting system, comprising manufacturing a light guide using the method of claim 11 ; and mounting a light source along at least one edge of the light guide.