WO2012059866A1

WO2012059866A1 - Light emitting sheet

Info

Publication number: WO2012059866A1
Application number: PCT/IB2011/054857
Authority: WO
Inventors: Maarten Marinus Johannes Wilhelmus Van Herpen; Hugo Johan Cornelissen
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2010-11-03
Filing date: 2011-11-02
Publication date: 2012-05-10
Anticipated expiration: 2013-05-03
Also published as: TW201232070A

Abstract

A lighting system comprises a light guide comprising a sheet of light guiding material and an array of light out-coupling structures formed as a regular pattern at a surface of the sheet of light guiding material. At least one LED module is mounted on said surface of the sheet of light guiding material for directing light into the sheet.

Description

Light emitting sheet

FIELD OF THE INVENTION

The present invention relates to a light emitting sheet, for example using LED light sources. BACKGROUND 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 (Poly(methyl methacrylate) - a transparent thermoplastic) waveguide, 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.

Figure 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.

Figure 2 shows the structure of Figure 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.

One application of light emitting sheet is a so-called "transmissive emissive window", which is transparent and therefore can form part of (or be provided over) a window, but is also emissive and can therefore be used for lighting. The light skin is thus used as a glazing material (generally over a substrate such as glass).

SUMMARY OF THE INVENTION

The invention is directed to the problem of making a low-cost general lighting system in the form of a sheet, with a cut-to-measure architecture. A light guide can provide a low-cost way to distribute the light from LEDs, but the challenge then becomes how to make the light guide cut-to-measure.

Known designs have out-coupling structures that depend on the position of the LEDs as explained above, and this prevents a larger sheet being cut to size, with the remainder of the sheet being usable for another application.

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 regular pattern at a surface of the sheet of light guiding material; and at least one LED module mounted on said surface of the sheet of light guiding material for directing light into the sheet.

This design enables a general lighting system to be formed including LEDs and a light-guide. The light guide comprises a regular, i.e. periodic, out-coupling structure that does not vary globally across the light guide (although it may comprise a repeating local pattern). The LEDs are provided as separate components to be mounted on the light guide, for example placed anywhere on the light guide. The regular pattern means that the light guide can be cut to size without reference to the number or positioning of the LED light sources.

The lighting system gives light output near the location of the LEDs, where the light from the LEDs is distributed in the light guide. The exact shape of the light emitting area can be selected by designing the angular emission distribution of the LED module.

The sheet of light guiding material can comprise PMMA, glass or polycarbonate. Thus, known light guide materials can be used, and the invention does not place any unusual requirement on the material to be used. Similarly, known light out- coupling structures can be used, such as such as scattering paint dots, micro-grooves, micro- prisms, microlenses, domains with surface roughness, phosphor dots.

The LED module can comprise means for directing the light output within a range of angles away from a direction normal to the sheet. This helps to couple the light into the light guide. The LED module can comprise means for directing the light output within a range of lateral angles. This can be used to control how the light output is formed, and thereby dictate the desired positioning of the LED module with respect to the patter of light out-coupling structures.

A plurality of LED modules is preferably provided, which are interconnected electrically by a transparent conductive sheet, or by very thin unobtrusive wires.

The pattern can be regular in both dimensions of the surface of the sheet and repeats at each light out-coupling structure. This means the LED modules can be placed anywhere on the sheet and in any number, since the pattern has no features linked to the LED module.

Instead, the pattern can again be regular in both dimensions of the surface of the sheet but it may repeat at the level of sub-sets of light out-coupling structures. Thus, a sub-set of light out-coupling structures can be designed with a particular mounting position for the LED for the best coupling of light into the light guide. An adhesive is preferably provided which makes optical contact between the LED module and the light guide.

The invention also provides a method of configuring a lighting system, comprising:

cutting a light guide to a desired size, the light guide comprising a sheet of light guiding material and an array of light out-coupling structures formed as a regular pattern at a surface of the sheet of light guiding material; and

mounting at least one LED module on said surface of the sheet of light guiding material for directing light into the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a know light emitting sheet in perspective view;

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

Figure 3 shows a first example of lighting system of the invention;

Figure 4 shows a second example of lighting system of the invention; and

Figure 5 shows a third example of lighting system of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a lighting system having a light guide comprising a sheet of light guiding material and an array of light out-coupling structures formed as a regular pattern at a surface of the sheet of light guiding material. At least one LED module is mounted on said surface of the sheet of light guiding material for directing light into the sheet. This arrangement couples light into the sheet from above, and this means a special pattern of light out-coupling structures is not needed as for edge lit arrangements. The regular pattern means the sheet can be cut to size and the remaining part of the sheet can be used for other lighting systems.

Figure 3 shows a first example of lighting system of the invention.

The general lighting system 30 includes LEDs 32 and a light-guide 31. The light guide 31 comprises a periodic out-coupling structure 35 that does not vary across the light guide 31 and the LEDs 32 are provided as part of an LED module 33.

The LED module 33 may be placed anywhere on the light guide 31, using an adhesive 38 to make optical contact between the LED module 33 and the light guide 31. When the LED module 33 has been applied, LED light 34 will couple into the light guide 31 and will be coupled out using the out-coupling structure 35, for example as indicated at location 36.

The resulting light effect is that the bright LED emission of a single LED is distributed over a larger area, reducing glare to a level where it becomes suitable to use as general lighting.

The pattern of light spreading from the LED is symmetric around the location of the LED in this example. As shown, the pitch of the light out-coupling structures can be less than the size of the LED module, so that the positioning of the LED module is not critical - no matter where the LED is positioned, the pattern of light out-coupling structures around it will be substantially the same.

The LED module is designed such that light is not reflected at the interface between the LED module and the surface of the sheet. Thus, light is coupled into the sheet.

The light out-coupling structures may cause some reflection of light from the LED module back into the LED module, where it will be partly absorbed (approximately 40%) and partly reflected back again. The absorbed light will however be partly re-emitted (approximately 60%) by the light recycling mechanism. Thus, the fact that the light out- coupling structures prevent perfect coupling from the LED module to the light guide does not significantly reduce the efficiency, the light out-coupling structures also occupy a relatively small area.

Because the pattern of light out-coupling structures is entirely regular, the sheet can be used for any number and position of LED modules.

The regular pattern has no spatial non-uniformities or spatial dependency of its pattern. This opens up the possibility to use new light out-coupling mechanisms not able to provide a non-uniform pattern as previously required. For example, the pattern can comprise a uniformly applied light paint which provides scattering particles in a very low density. By applying this paint to the light guide, the paint layer acts as a uniform out-coupling layer.

Another example is a paint comprising phosphor particles, such as organic phosphors. By applying a thin layer of such a paint, a uniform out-coupling layer is again provided.

In these arrangements, the intensity of the light pattern around the LED module will be greater nearer the LED module, due to the closer proximity to the light source. The pattern can instead have a local design for the individual LED module, for example with closer out-coupling structures further away. Thus, the pattern can repeat at the level of a sub-array of light out-coupling structures, rather at the level of the individual light out-coupling structures. This requires a regular pattern, but not one which is perfectly uniform - in other words the regularity is not at the level of an individual light out-coupling structure.

The LED module then has a preferred attachment point with respect to that local design. This can also mean that LED modules should ideally be placed at each designed attachment point of the cut sheet, but this is not essential.

In order to couple light into the lighting guide efficiently, the angular distribution of the LED light emitted by LED module 33 should be optimized. Preferably, the LED module only emits light at larger angles (i.e. away from the normal), because these angles are captured in the light guide. LED light that would normally be emitted under smaller angles should be redirected towards these larger angles.

The redirection of the LED light to optimize the angular distribution may be done in several ways:

mirrors, lenses and/or obstructions;

interference coatings;

a retroreflector layer (which efficiently reflects light at small angles, but transmits light at larger angles).

The purpose of these measures is to direct the light output of the LED module within a range of angles away from a direction normal to the sheet.

The optical measures for implementing this will be routine to those skilled in the art. For instance, an interference coating can be designed that reflects light admitted at small angles and passes light emitted at large angles. This is particularly suitable for blue or near-UV LEDs, where part of the incoupled light is converted to another part of the spectrum by a phosphor. The same approach can be applied to all red, green and blue LEDs, where each LED has its own specifically designed interference filter.

The LED module can also be designed to direct light output within a range of lateral angles. In this way, a predefined LED angular distribution is used to make the light output spatially uniform.

In particular, the LED modules can have an angular emission distribution such that a certain predetermined area of the periodic out-coupling structure is illuminated spatially uniformly. This angular distribution can for instance resemble a so-called "bat- wing" distribution. The predetermined illuminated area can have a regular geometrical shape such that full tiling of the surface is possible. Some examples of such an illuminated area are given in Figure 4.

The plan view in Figure 4 shows an LED module 40 at the centre of a triangular area, with multiple triangles forming a lattice. Module 42 is at one corner of a square area, with the squares forming a lattice. Module 44 is at the centre of a square area, with the squares forming a lattice and module 46 is at the centre of a hexagonal area, with the hexagons forming a lattice.

The cross section in Figure 4 shows that this angular light pattern is generated by controlling the intensity (represented by the length of the arrows 48) in different directions. Thus, in cross section through the sheet, there is a 180 degree range of angles. By controlling the intensity in different directions, the range over which the light out-coupling structures are illuminated, as well as the relative intensity, is controlled. By making the intensity pattern different angularly around the LED module, the outer shape of the illuminated area is controlled.

The end user can chose to illuminate the full surface by placing an LED module at each area, or a customized lighting pattern can be obtained by placing the LEDs in a specific grid.

Placement of the LEDs on the light guide is still free because the light extraction pattern can be kept uniform. The control of the angular distribution is part of the LED module design.

The lateral angular pattern can be controlled by a specifically designed interference coating as described above. Red, green, blue and near-UV LEDs require their own design, tuned to the specific wavelength. Another example is shutters, for example lamella or apertures, that block light travelling in unwanted directions.

The power supply to the LED module may be achieved using very thin conductive wires, but preferably using a transparent conductive material such as ITO provided on a transparent carrier that can be adhered to the light guide.

For example, Figure 5 shows an LED module comprising two LEDs 32 and a patterned transparent conductive coating 39.

By using a light extraction pattern that is uniform, the LED modules can be placed at will, and the light guide can be cut-to-measure.

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 (30) comprising:

a light guide (31,35) comprising a sheet of light guiding material (31) and an array of light out-coupling structures (35) formed as a regular pattern at a surface of the sheet of light guiding material (31); and

at least one LED module (32,33) mounted on said surface of the sheet of light guiding material (31) for directing light into the sheet.

2. A system as claimed in claim 1, wherein the sheet of light guiding material (31) comprises PMMA, glass or polycarbonate.

3. A system as claimed in claim 1, wherein the LED module (32,33) comprises means for directing the light output within a range of angles away from a direction normal to the sheet.

4. A system as claimed in claim 1, wherein the LED module (32,33) comprises means for directing the light output within a range of lateral angles.

5. A system as claimed in claim 1, wherein the at least one LED module comprises a plurality of LEDs (32) which are interconnected electrically by a transparent conductive sheet (39).

6. A system as claimed in claim 1, wherein the pattern is regular in both dimensions of the surface of the sheet (31) and repeats at each light out-coupling structure (35).

7. A system as claimed in claim 1, wherein the pattern is regular in both dimensions of the surface of the sheet (31) and repeats at sub-sets of light out-coupling structures (35).

8. A system as claimed in claim 1, wherein an adhesive (38) is provided which makes optical contact between the LED module (32,33) and the sheet (31).

9. A method of configuring a lighting system, comprising:

cutting a light guide (31,35) to a desired size, the light guide comprising a sheet (31) of light guiding material and an array of light out-coupling structures (35) formed as a regular pattern at a surface of the sheet of light guiding material; and

mounting at least one LED module (32,33) on said surface of the sheet of light guiding material for directing light into the sheet (31).

10. A method as claimed in claim 9, wherein the sheet (31) of light guiding material comprises PMMA, polycarbonate or glass.

11. A method as claimed in claim 9, comprising directing the LED module (32,33) light output within a range of angles away from a direction normal to the sheet.

12. A method as claimed in claim 9, comprising directing the LED light module (32,33) light output within a range of lateral angles.

13. A method as claimed in claim 9, comprising providing a plurality of LEDs (32) which are interconnected electrically by a transparent conductive sheet (39).

14. A method as claimed in claim 9, wherein the pattern is regular in both dimensions of the surface of the sheet and repeats at each light out-coupling structure (35) or at sub-sets of light out-coupling structures.

15. A method as claimed in claim 9, comprising using an adhesive (39) to make optical contact between the LED module and the light guide.