EP3695163A1

EP3695163A1 - Lightguide plate

Info

Publication number: EP3695163A1
Application number: EP18783022.9A
Authority: EP
Inventors: Youri Meuret; Jana RYCKAERT; Zeger HENS; Igor NAKONECHNYI
Original assignee: Katholieke Universiteit Leuven; Universiteit Gent
Current assignee: Katholieke Universiteit Leuven; Universiteit Gent
Priority date: 2017-10-10
Filing date: 2018-10-10
Publication date: 2020-08-19
Also published as: WO2019072938A1

Abstract

A composite light guide is disclosed. The composite light guide comprises a light guide layer having a first face and a second opposite face, and an edge portion configured to receive input light, wherein at least one of the first face and the second face is an output face. The composite light guide 5 comprises a plurality of low-scattering photo-luminescent elements in optical contact with the light guide layer and a spatially inhomogeneous out-coupling layer comprising a plurality of out-coupling structures disposed at least one of the first face and the second face. The plurality of low-scattering photo-luminescent elements comprises at least one element capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range. Each of the 10 plurality of out-coupling structures has a dimension which is substantially larger than wavelengths in the first wavelength range and wavelengths in the second wavelength range.

Description

LIGHTGUIDE PLATE

Field of the Invention

The present invention relates to a light guide plate, a lighting unit including a light guide plate, and methods for the manufacture of a light guide plate.

Background

The need for light sources with a tailored output spectrum is ubiquitous. Displays and projectors require spectra containing narrow red, green, and blue lines; light bulbs for domestic applications require spectra close to that of a black body radiator; horticultural applications require spectra containing mainly blue and red light.

Many current lighting devices rely on a combination of one or more blue LEDs or laser diodes and one or more luminescent materials that absorb part of the blue light and convert it by photo- luminescence to a longer wavelength range. In some cases, powder phosphors are used as the luminescent material, which typically consists of luminescent ions embedded in a crystalline host or matrix. Typical characteristics of powder phosphors are (1) broad emission, often exceeding 100 nm full width at half maximum (FWHM) and (2) pronounced light scattering.

These characteristics can lead to disadvantages for the use of powder phosphors in light sources. The broad emission spectrum precludes provision of a light source emitting a predesigned light spectrum. Powder phosphors exist which have a relatively narrow emission line, for example K2SiF6:Mn, which has a plurality of narrow emission lines in the red region of the spectrum and can be excited at 450 nm. However, such emitters tend to have a low absorption coefficient. Consequently, thick phosphor layers tend to be required in order to absorb a proportion of excitation light sufficient to provide the required intensity of output light. In addition, each desired emission colour requires a further phosphor material to be incorporated.

Colloidal quantum dots (QDs) are an alternative to powder phosphors. Colloidal QDs offer narrower emission lines which can be 30 nm FWHM or less, and can be tuned in color by changing the dot size. However, the photo-thermal stability of QDs is inferior to that of the most suitable powder phosphors, for example YAG:Ce, which can limit their use as a luminescent downconverter.

QDs typically have dimensions that are 10 to 100 times smaller than the wavelength of visible light, and can therefore be substantially non-scattering, or low-scattering, elements. The absence of scattering by QDs can result in light passing without scattering through QD-based composites, which can limit the interaction length between the light and the QDs. US9199842 B2 ("US'842") describes a quantum dot light conversion film and scattering particles. The scattering particles have a dimension which is similar to the wavelength of primary light which excites the quantum dots. US'842 introduces scattering particles with the aim to increase the optical path length of primary light in a QD light conversion film. Document WO2009017794A1 discloses an illumination assembly with a lightguide and a light homogenization region including wavelength converting material distributed in- homogeneously. Document WO0102772A1 discloses a display assembly with front light guide, including a surface facing a display panel and an array of light extractors. Document CN105700242A discloses a backlight module with two light guide plates including a respective bottom surface and a quantum dot fluorescence film being held between the bottom surfaces of the plates.

Summary

According to a first aspect of the present invention, there is provided a composite light guide comprising a light guide layer having a first face and a second opposite face, and an edge portion configured to receive input light, wherein at least one of the first face and the second face is an output face. The composite light guide comprises a plurality of low-scattering photo-luminescent elements in optical contact with the light guide layer and a plurality of out-coupling structures disposed on at least one of the first face and the second face. The plurality of low-scattering photo- luminescent elements comprises at least one element capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range. Each of the plurality of out-coupling structures has a dimension which is substantially larger than wavelengths in the first wavelength range and wavelengths in the second wavelength range. The plurality out- coupling structures are comprised in a spatially inhomogeneous out-coupling layer. For example, the distribution of the out-coupling structures may be inhomogeneous. For example, the out-coupling structures may present different sizes. For example, the out-coupling structures may present different shapes. For example, both the sizes and/or the shapes of the out-coupling structures and their distribution may be inhomogeneous.

It is an advantage of embodiments of the present invention that a light guide can be provided which is capable of emitting light with a tailored output spectral irradiance distribution (e.g. intensity and spectral distribution) across the output face, for example white light having a spatially homogeneous distribution.

It is an advantage of embodiments of the present invention that the number of low-scattering photo- luminescent elements required to provide a particular output intensity and spectral distribution, can be significantly reduced.

The plurality of low-scattering photo-luminescent elements may be dispersed within the light guide layer.

The low-scattering photo-luminescent elements have a substantially homogeneous spatial distribution, for example they may be homogeneously distributed, across a photoluminescent layer where the low-scattering photo-luminescent elements are provided, or across the light guide layer. It is an advantage of embodiments of the present invention that the low-scattering photo- luminescent elements are located along the optical path of the excitation light which is trapped by total-internal reflection inside the light guide plate, which decreases the number of required photo- luminescent elements in order to have a certain chance of absorption of the excitation light.

The plurality of low-scattering photo-luminescent elements may be disposed on at least one of the first face and the second face.

It is an advantage of embodiments of the present invention that a composite light guide including photo-luminescent elements may be easily fabricated, for example using a printing process.

The light guide layer may comprise a first light guide layer portion having first and second opposite faces and a second light guide layer portion having first and second opposite faces, wherein the second face of the first light guide layer portion is next to the first face of the second light guide layer portion, and wherein the plurality of low-scattering photo-luminescent elements is disposed between the first light guide layer portion and the second light guide layer portion.

It is an advantage of embodiments of the present invention that a light guide layer may be easily fabricated, for example using a lamination process and that the QD layer is automatically protected/sealed from air, which can provide enhanced photothermal stability.

The plurality of out-coupling structures may be arranged in a predetermined configuration, wherein the configuration of the plurality of out-coupling structures is chosen so as to obtain a desired output light distribution.

It is an advantage of embodiments of the present invention that a desired output light distribution can be provided by simply changing the configuration of out-coupling structures.

The at least one low-scattering photo-luminescent element capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range may be a low- scattering photo-luminescent element of a first type. The plurality of low-scattering photo- luminescent elements may further comprise at least one low-scattering photo-luminescent element of a second type. The at least one low-scattering photo-luminescent element of the second type is capable of absorbing light within a first wavelength range and emitting light within a third wavelength range which is different to the first wavelength range and the second wavelength range, and each of the plurality of out-coupling structures has a dimension which is substantially larger than wavelengths in the third wavelength range.

It is an advantage of embodiments of the present invention that a wide range of output wavelengths is available. It is an advantage of embodiments of the present invention that an output spectrum having localized regions with different wavelength ranges can be provided.

The plurality of out-coupling structures may comprise at least one geometrical feature. It is an advantage of embodiments of the present invention that the plurality of out-coupling structures can be easily manufactured using e.g. a roll-to-roll manufacturing method and that the out-coupling structures induce no optical losses.

The plurality of out-coupling structures may comprise at least one printed element.

It is an advantage of embodiments of the present invention that the plurality of out-coupling structures can be easily manufactured using a printing process.

The plurality of out-coupling structures may comprise at least one out-coupling structure having a first spectral property and at least one out-coupling structure having a second spectral property which is different to the first spectral property.

The at least low-scattering photo-luminescent element may be a colloidal quantum dot.

The composite light guide may further comprise a reflective layer spaced apart from the first face or the second face by an air gap.

It is an advantage of embodiments of the present invention that emission towards the output face can be enhanced.

According to a second aspect of the present invention there is provided a lighting unit comprising a composite light guide according to the first aspect and at least one excitation light source configured to emit excitation light at the first wavelength, wherein the at least one excitation light source is optically coupled to the light guide at the edge portion.

It is an advantage of embodiments of the present invention that a lighting unit having a configurable output spectrum can be provided with a single input light source.

According to a third aspect of the present invention there is provided use of a composite light guide according to the first aspect or a lighting unit according to the second aspect to generate a desired output light distribution.

According to a fourth aspect of the present invention there is provided a method of manufacturing a composite light guide according to the first aspect, the method comprising providing the light guide layer or light guide layer portion and forming the plurality of out-coupling structures on a face of the light guide layer or light guide layer portion.

The out-coupling structure may be formed using a printing process. It is an advantage of embodiments of the present invention that a light guide can be manufactured easily and cheaply. The out-coupling structure may be formed by removing material or reshaping the material from the light guide layer or light guide layer portion. It is an advantage of embodiments of the present invention that a composite light guide can be manufactured using a high accuracy process for the out-coupling structures The method may further comprise printing the plurality of low-scattering photo-luminescent elements on a face of the light guide layer or light guide layer portion.

It is an advantage of the present invention that out-coupling structures and photo-luminescent elements can be provided using a printing process, which can provide a simple and cheap manufacturing method.

According to a fourth aspect of the present invention there is provided a method of providing a composite light guide having a predetermined output spectral irradiance distribution, preferably a uniform output spectral irradiance distribution. The method comprises the steps of:

a) providing a light guide comprising a light guide layer having a first and a second opposite face and an edge portion configured to receive input light, wherein at least one of the first face and the second face is an output face. It also includes a plurality of low-scattering photo-luminescent elements in optical contact with the light guide layer, and an out-coupling layer disposed on at least one of the first face and the second face. The out-coupling layer comprises a plurality of out-coupling structures in an initial configuration. The plurality of low-scattering photo- luminescent elements comprises at least one element capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range. The low- scattering photo-luminescent elements have a substantially homogeneous spatial distribution, for example they may be homogeneously distributed, for example along a luminescent layer in optical contact with the light guide layer, or across the light guide layer itself. The initial configuration of the out-coupling layer comprises the out-coupling structures in a homogeneous spatial distribution.

b) Calculating or determining the output spectral irradiance distribution of the light guide with the initial configuration of the out-coupling layer to obtain the initial output spectral irradiance distribution of the light guide and comparing the initial output spectral irradiance distribution with the desired output spectral irradiance distribution (preferably uniform output spectral irradiance distribution).

c) Modifying the configuration of the out-coupling layer in a further, improved configuration by varying the size (or shape) of the out-coupling structures, by varying the distance between different out-coupling structures and/or by varying the size (or shapes) of the out-coupling structures and the distance between the different out-coupling structures;

d) Calculating or determining the output spectral irradiance distribution of the light guide with the further, improved configuration of out-coupling layer to obtain the further, improved output spectral irradiance distribution of the light guide and comparing the further, improved output spectral irradiance distribution with the desired output spectral irradiance distribution (preferably uniform output spectral irradiance distribution) and

e) repeating step c) and step d) in dependence upon the difference between the further, improved output spectral irradiance distribution and the desired output spectral irradiance distribution.

Brief Description of the Drawings

Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of a composite light guide according to embodiments of the present invention, the cross-section view being taken in a plane substantially perpendicular to the plane of the light guide layer;

Figure 2 is a schematic perspective view of a light guide layer;

Figure 3 illustrates total internal reflection at an interface between a luminescent layer and air; Figure 4 is a cross-sectional view of a composite light guide according to embodiments of the present invention which includes a reflector layer, the cross-section view being taken in a plane substantially perpendicular to the plane of the light guide layer;

Figure 5 is a schematic representation of an output light distribution including a plurality of differently shaped areas, each area having an associated wavelength of light;

Figure 6 is a flow chart of an optimization method which can be used to determine a configuration of the out-coupling structures;

Figure 7 is a cross-sectional view of a lighting device including a composite light guide according to embodiments of the present invention;

Figure 8 is a cross-sectional view of a composite light guide according to embodiments of the present invention;

Figure 9 is a flow chart of a method of manufacturing a composite light guide according to embodiments of the present invention.

Figure 10 is a graph showing the low scattering requirements of a photo-luminescent element depending on its volume.

Figure 11 is a flow chart of a method of providing a composite light guide having a predetermined output spectral irradiance distribution according to embodiments of the present invention.

Figure 12 is a flow chart of a method of optimization of the luminance uniformity taking into account colour uniformity. Figure 13 to 16 show the mean illuminance and colour point in CIExy as a function of three different (inhomogeneous) distributions of out-coupling structures obtained by methods of embodiments of the present invention.

Figure 17 shows the mean illuminance and colour point in CIExy for a homogeneous distribution of out-coupling structures.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting the scope.

In the different drawings, the same reference signs refer to the same or analogous elements.

Detailed Description of Certain Embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. The term "comprising" therefore covers the situation where only the stated features are present and the situation where these features and one or more other features are present. Thus, the scope of the expression "a device comprising means A and B" should not be interpreted as being limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

In certain embodiments, the present invention is related to display devices. As used herein, a display device refers to any system with a lighting display. Such devices include, but are not limited to, devices encompassing a liquid crystal display (LCD), televisions, computers, mobile phones, smart phones, personal digital assistants (PDAs), gaming devices, electronic reading devices, digital cameras, and the like.

The light guides of the present invention can be used in any suitable application, including but not limited to down lighting, indoor or outdoor lighting, stage lighting, decorative lighting, accent lighting, museum lighting, and highly-specific wavelength lighting for horticultural, biological, or other applications, as well as additional lighting applications which will be apparent to those of ordinary skill in the art upon investigating the invention described herein.

As used herein, where two elements are described as being in "optical contact", it is meant that the elements are either 1) in physical contact with each other or 2) spaced apart by one or more intermediate elements, each of the one or more intermediate elements having a respective refractive index which differs from the refractive index of each of the two elements by no more than 0.2, preferably by no more than 0.1

For example, where a low-scattering photo-luminescent element is described as being in optical contact with the light guide layer, it is meant that the low-scattering photo-luminescent element is either 1) in physical contact with the light guide layer or 2) spaced apart from the light guide layer by one or more intermediate layers, each of the one or more intermediate layers having a respective refractive index which differs from the refractive index of the light guide by no more than 0.2, preferably by no more than 0.1.

Referring to Figure 1, a composite light guide 1 according to embodiments of the present invention is shown in schematic cross-section. The composite light guide 1 includes a light guide layer 2, a plurality of low-scattering photo-luminescent elements 3 provided in a luminescent layer 4, and a plurality of out-coupling structures 5.

The light guide layer 2 has a first face 6 and a second, opposite face 7.

In preferred embodiments the light guide layer 2 is substantially planar, that is, referring to Figure 2, the light guide layer 2 has a thickness t, a length /, and a width w, wherein the thickness t is substantially smaller than the length and the width. The light guide layer may be flexible, that is, capable of being bent so as to have a radius of curvature which is substantially greater than the wavelength of light propagating within, coupled into, or coupled out of the light guide layer.

The light guide layer 2 comprises a substantially optically transparent material, for example glass, poly(methyl methacrylate), also called PMMA, polycarbonate, zeonex, polymethacrylmethylimid, also called PMMI, Pleximid, optical silicone resin. Herein, substantially optically transparent means a transmittance at 20 mm of at least 0.95, preferably at least 0.98, more preferably at least 0.99. The light guide layer 2 may comprise a flexible material, for example PMMA. The light guide layer 2 may comprise a material having an impact strength in the range of 10 to 40 kJ/m². The light guide layer 2 may comprise a material having a tensile strength at 23°C of between 50 and 100 MPa. The light guide layer 2 may comprise a material having a flexural strength of between 80 and 120 MPa.

The light guide layer 2 has a light guide layer refractive index n_LcM- One or more surfaces of the light guide layer 2 may be polished to reduce scattering of incident light. The light guide layer 2 comprises an edge portion 18 which extends between the first face 6 and the second face 7.

The plurality of low-scattering photo-luminescent elements 3 are provided in luminescent layer 4 which is disposed on at least a portion of the first face of the light guide layer 2. The luminescent layer has a luminescent layer refractive index of n_Li_. The plurality of low-scattering photo- luminescent elements 3 are in optical contact with the light guide layer 2, that is, they are either in direct contact with the light guide layer or are spaced apart from the light guide layer by a portion of the luminescent layer which has a refractive index which differs from the refractive index of the light guide by no more than 0.2, preferably by no more than 0.1.

In some embodiments, luminescent layer 4 may be in optical contact with an intermediate layer and the intermediate layer may be in optical contact with the light guide layer 2, and this can provide a plurality of low-scattering photo-luminescent elements in optical contact with the light guide layer. That is, one or more intermediate layers may be present between the luminescent layer 4 and the light guide layer 2 provided that adjacent layers are in optical contact.

The luminescent layer refractive index n_Li_ is substantially equal to the light guide layer refractive index DLGM, that is, the luminescent layer refractive index n_Li_ and light guide layer refractive index riLGM differ by no more than 0.2 and preferably no more than 0.1. For example, the light guide layer may comprise PMMA, which has a refractive index of 1.49, and the luminescent layer may also comprise PMMA . The luminescent layer may comprise a kraton polymer.

The luminescent layer refractive index n_Li_ and the light guide layer refractive index DLGM are both greater than a refractive index of a medium which surrounds the composite light guide 1, that is, the luminescent layer refractive index n_Li_ and the light guide layer refractive index DLGM are chosen so as to allow trapping of light within the composite light guide 1 by total internal reflection. The luminescent layer refractive index n_Li_ and the light guide layer refractive index DLGM are chosen such that total internal reflection at an interface between the light guide layer 2 and the luminescent layer 4 is minimized, for example to less than 25%. In preferred embodiments the luminescent layer refractive index n_Li_ and light guide layer refractive index DLGM differ by no more than 0.2 and preferably no more than 0.1.

The luminescent layer 4 comprises the plurality of low-scattering photo-luminescent elements 3. As will be described in more detail hereinafter, primary light which is input into the light guide plate can be converted to secondary light by the low-scattering photo-luminescent elements and coupled out of the light guide by the out-coupling structures. As used herein, for a photo-luminescent element to be classified as low-scattering, no more than 25% of primary light input into the light guide plate is coupled out of the light guide plate through elastic scattering by the low-scattering photo- luminescent elements, preferably no more than 10%, more preferably no more than 5%, still more preferably no more than 1%. By elastic scattering it is meant an interaction between primary light and the low-scattering photo-luminescent element wherein the wavelength of primary light is not changed.

Figure 10 is a graph showing the absorbance and scattering as the cross-sections vs particle volume. The actual values will differ depending on the material used, but the trend will be similar. For a 4 nm particle 1001, the ratio is roughly 10^s. This ratio decreases with increasing the volume, as scattering increases faster than absorption. In any case, for 20 nm particles the scattering is still below 1% of the absorption. For a volume near 3 10^"23 cubic meters (30000 nm³) or higher, corresponding to a particle with a radius of 40 nm of more, the ratio between scattering and absorbance cross section will exceed 1% (see dark region 1002). Note that clustering of the particles can occur during processing, which increases scattering (a cluster behaves in a simple way as a larger particle). In embodiments of the present invention, parameters such as the materials, the size and/or distribution of the photo-luminescent elements are such that low-scattering can be provided. A photo- luminescent element is an element which is capable of absorbing primary light having an input wavelength within a first wavelength range and subsequently emitting secondary light having a wavelength within a second wavelength range which may be different to the first wavelength range. A portion of the first wavelength range may overlap, or coincide, with a portion of the second wavelength range. In preferred embodiments, the first wavelength range does not substantially overlap the second wavelength range.

In preferred embodiments the second wavelength range is a relatively narrow wavelength range, which can help to avoid colour leakage. For example, the secondary light may have a secondary light spectrum having a full width at half maximum (FWHM) of no more than lOOnm, preferably no more than 50nm, more preferably no more than 35 nm or no more than 25 nm.

For example, in some embodiments, the plurality of low-scattering photo-luminescent elements comprises at least one quantum dot, preferably at least one colloidal quantum dot.

For example, in some embodiments, the low-scattering photo-luminescent elements are quantum dots in optical contact with a light guide layer, increasing the amount of interactions of the blue LED light with the luminescent layer 4. This increased amount of interactions allows reducing the amount of quantum dots needed to obtain a desired white point, resulting in a reduction of the total cost of a display (without sacrificing other properties, e.g. it may allow retaining the typical good stability of remote phosphor systems).

In other embodiments, the low-scattering photo-luminescent elements may comprise an organic dye. The low-scattering photo-luminescent elements may comprise a semiconductor nanocrystal having dimensions too large for the element to show size quantization but small enough to be non- scattering, for example a perovskite nanocrystal. A perovskite nanocrystal in the form of a particle having a typical dimension of, for example, 10 nm may not show size quantization but may be sufficiently small to be substantially non-scattering and have a narrow emission determined by the bandgap of the semiconductor. Emission tuning is possible by changing the composition of the semiconductor nanocrystal instead of changing the size of the semiconductor nanocrystal. The low- scattering photo-luminescent element may be a carbon dot. The low-scattering photo-luminescent element may be a nanoparticle containing luminescent dopants such as transition metal ions or lanthanide ions.

In some embodiments of the present invention, the luminescent layer 4 may provide homogeneous distribution of wavelength conversion. Factors that influence the amount of wavelength conversion of light include for example the thickness of the layer and the concentration of photo-luminescent material (e.g. particles, elements) in the layer. Thus, changing the thickness or concentration will have the same effect on the overall performance.

Homogeneity of the layer may depend on different aspects of the total design such as properties of the photo-luminescent elements (e.g. QD properties), size of the light guide 1, color and/or luminance requirements, etc. In some embodiments of the present invention, the deviation of layer thickness (e.g. of thickness of the luminescent layer), or the concentration of photo-luminescent material across the light guide plate, or both together, may be below 1%. For instance, the concentration of photo-luminescent elements may be homogeneous across a layer with homogeneous thickness, the average deviation of them being under 1%. For example, the average deviation of the thickness over the whole light guide layer, plus the average deviation of the concentration over the whole light guide layer, should be less than 5%, more preferably less than 1%, still more preferably less than 0.2 %. For example, the deviations can be calculated over an averaging area of lxlcm² on the output face of the light guide plate. Providing such homogenous layer may be done in an easy way.

Referring to Figure 3, a light ray i of primary light within the light guide layer is shown. The light ray Ri is incident at the interface between the light guide layer 2 and the luminescent layer 4 at an angle greater than or equal to the critical angle for a light guide layer-air interface or a luminescent layer-air interface.

If an air gap (not shown) were present between the luminescent layer 4 and the light guide layer 2, then light ray Ri would undergo total internal reflection at a light guide layer-air interface and would not continue to propagate into the luminescent layer 4 and interact with photo-luminescent elements 3. By providing the luminescent layer in optical contact with the first face 6 of the light guide layer, light ray i can propagate across interface into the luminescent layer 4 and can interact with photo-luminescent elements 3. If the light ray i does not interact with a photo- luminescent element 3, it can be totally internally reflected at a luminescent layer-air interface h, thus being directed back towards the light guide layer 2.

Therefore by providing the luminescent layer 4 and low-scattering photo-luminescent elements 3 in optical contact with the light guide layer 2, primary light can be confined within the light guide layer 2 and can pass through the luminescent layer 4 multiple times. This enhancement of primary light- luminescent element interactions can allow a much lower loading of luminescent elements to be provided as compared to a luminescent layer not in optical contact with a light guide layer. For example, the required weight of low-scattering photo-luminescent elements per surface area of the light guide can be reduced.

Referring again to Figure 1, the plurality of out-coupling structures 5 are disposed on the first face 6 of the light guide layer 2. The plurality of out-coupling structures 5 each has a first dimension in a plane substantially parallel to the first face 6 and the second face 7, wherein the dimension is substantially larger than wavelengths in the first wavelength range and wavelengths in the second wavelength range. For example, an out-coupling structure may have a circular shape in the plane substantially parallel to the first face 6 and the second face 7, and the first dimension may be a radius of the dot. An out-coupling structure may have a square shape in the plane substantially parallel to the first face 6 and the second face 7, and the first dimension may be a width of the dot. Herein, in relation to the out-coupling structures, "substantially larger" in relation to a wavelength or wavelength range means that the light scattering by the out-coupling structure is substantially wavelength-independent for the wavelength or wavelength range. For example, for a primary light wavelength of approximately 400 nm, the out-coupling structures may have a dimension that is at least 30 times, for example at least 50 times, the primary light wavelength and the secondary light wavelength, in some embodiments the dimension may be 100 times or 150 times the primary light wavelength and the secondary light wavelength).

The plurality of out-coupling structures 5 each has a second dimension in a direction substantially perpendicular to the first face 6 and the second face 7. The second dimension is such as to substantially prevent transmission of light through the out-coupling structure in the second dimension. For example, in some embodiments the second dimension may be no less than 10 micrometers and no more than 100 micrometers. However, it will be understood that the second dimension may be chosen in dependence upon the wavelengths of primary and secondary light and the colour of the out-coupling structure and an optimal value may be determined through experimentation. For example, by providing a plurality of out-coupling structures each having a different second dimension and measuring the transmission in each case and selecting a second dimension which provides a desired transmission. A desired transmission is preferably less than 0.05, more preferably less than 0.02, still more preferably less than 0.01.

An out-coupling structure 5 may comprise a printed element, that is, an element formed by a printing process. For example, an out-coupling structure may comprise a printed dot or other printed shape. An out-coupling structure 5 may comprise a geometrical feature. For example, an out- coupling structure 5 may comprise a recess in the first face 5 or the second face 6 of the light guide layer. An out-coupling structure 5 may comprise an embossed shape in the first face 5 or the second face 6 of the light guide layer. An out-coupling structure 5 may comprise an etched shape in the first face 5 or the second face 6. A geometrical feature may be formed, for example, using one or more of an etching process, a laser ablation process, an embossing process.

Referring to Figure 4, schematic cross-section of part of a lighting device 10 including a composite light guide 1 according to embodiments of the present invention is shown. The lighting device 10 includes a reflector layer 11.

The reflector layer 11 is disposed adjacent to the first face 6 of the light guide layer 2. The reflector layer 11 is spaced apart from the first face 6 by an air gap 16. This can help to provide high efficiency of light trapping within the light guide layer 2 due to a reduced number of interactions with a reflective layer. The air gap 16 has a width (measured in a direction perpendicular to the first face 6) which is preferably greater than 2 μιτι and less than 5 mm. The air gap width is preferably greater than twice the wavelength of light propagating within the light guide layer, more preferably greater than three times or four times the wavelength of light propagating within the light guide layer. The reflective layer 11 preferably has a reflectivity no less than 95%; however, in some embodiments, the reflectivity can be less than 95% and by providing the air gap 16, total internal reflection can occur at a first interface between the first face and the air gap, thus reducing the proportion of light incident on the first face from the light guide layer which interacts with the reflector layer 11. Some light may be incident at first interface at an angle insufficient for total internal reflection at the first interface and for this light, the reflector layer 11 allows to redirect the light back towards the light guide layer. However, in some embodiments, the reflector layer 11 may be in direct contact with at least a portion of the first face 6 of the light guide layer 2. For example, the plurality of out-coupling structures 5 may be deposited on the first face 6 and the reflector layer 11 may be subsequently deposited over the first face and the plurality of out-coupling structures 5. As will be described in further detail herein, in some embodiments, the plurality of out-coupling structures are provided only on the second face 7 of the light guide layer 2 and the reflector layer 11 may be disposed on a portion or on the entirety of the first face 6. In embodiments wherein output light comprises light output from the composite light guide 1 through the first face 6, the reflector layer 11 may be disposed adjacent to the second face 7 of the light guide layer 2. The reflector layer 11 may be in direct contact with at least a portion of the second face 7 of the light guide layer 2. For example, the plurality of out-coupling structures 5 may be deposited on the second face 7 and the reflector layer 11 may be subsequently deposited over the second face 7 and the plurality of out-coupling structures 5. In some embodiments, the plurality of out-coupling structures are provided only on the first face 6 of the light guide layer 2 and the reflector layer 11 may be disposed on a portion or on the entirety of the second face 7. In preferred embodiments, the reflector layer may be spaced apart from the second face 7 of the light guide layer 2 by an air gap.

The lighting device 10 includes a first light source 12 configured to emit primary light 13 towards the light guide layer 2. The first light source 12 is positioned so as to allow efficient coupling of primary light 13 into light guide layer 2. Efficient coupling as used herein means that primary light 13 is coupled into light guide layer 2 and is trapped within the composite light guide 1 by multiple total internal reflections at the first interface and at a second interface h between the first face 6 and the air gap 16 between the light guide layer 2 and the reflector 11. Light which is incident at second interface h at an angle less than a critical angle for total internal reflection at second interface h passes through first face 6 and is reflected at the reflector layer 11 towards the first face 6.

The reflector layer 11 preferably has a high reflectance, for example greater than 95%. The primary light 13 (and secondary light as will be described herein) can have multiple interactions with the reflectors, thus providing a reflector layer 11 with a high reflectance can allow efficient trapping of primary light, that is, reduced loss of primary light 13 from the light guide plate.

The lighting device 10 may comprise an edge reflective layer 19 disposed adjacent to one or more edge faces of the light guide layer 2. The edge reflective layer 19 is preferably in physical contact with the one or more edge faces. For example, the edge reflective layer 19 may comprise a reflective coating. The edge reflective layer 19 may comprise a reflective foil layer attached to one or more edge faces, for example attached using an adhesive such as glue. The edge reflective layer 19 may also be partially disposed on one or more edge faces which are adjacent to one or more first and/or second light sources (for example, edge face 15 is adjacent to the first light source 12), in areas such that coupling of primary light 13 into the light guide layer 2 is not substantially prevented.

The first light source 12 is configured to emit primary light 13 having wavelengths within a primary light wavelength range. In some embodiments, the primary light may have a primary light spectrum with a full width at half maximum (FWHM) which is no more than 60 nm. For example, the first light source 12 may comprise a light-emitting diode (LED), for example a blue light LED, a green light LED, a red light LED, or any LED or other light source configured to emit visible light. The first light source 12 may comprise a blue LED configured to emit primary light 13 having wavelengths in the range 420 to 480 nm. The first light source 12 may comprise a blue LED configured to emit primary light 13 having wavelengths in the range 435 to 465 nm. The first light source 12 may comprise a blue LED configured to emit primary light 13 within a wavelength range having a central wavelength of 465 nm. In some embodiments the first light source may have a FWHM which is less than 1 nm, for example the first light source 12 may comprise a laser diode.

In some embodiments, the first light source 12 may have a primary light spectrum with a relatively large FWHM, for example the first light source may be configured to emit white light. The first light source 12 may be a white LED. In some embodiments, the first light source 12 comprises a white LED and the low-scattering photo-luminescent elements are capable of emitting red light. The first light source 12 may be configured to emit light which is not visible, for example the first light source 12 may be a UV LED. In some embodiments, a first light source comprising a UV LED may be used in combination with a first low-scattering photo-luminescent element capable of emitting red light upon absorption of UV light, a second low-scattering photo-luminescent element capable of emitting green light upon absorption of UV light, and a third low-scattering photo-luminescent element capable of emitting blue light upon absorption of UV light.

The first light source 12 is disposed adjacent to an edge face 15 of the light guide layer 2. The edge face 15 is located in an edge portion 18 of the light guide layer 2. For example, the first light source 12 may be spaced apart from an edge face 15 by an air gap. The first light source 12 may be provided with or without a lens. For example, providing a first light source 12 without a lens can allow the first light source 12 to be disposed closer to the edge face 15 than if the first light source were provided with a lens. This can allow more efficient coupling of primary light into the light guide layer. Providing a first light source 12 with a lens can allow collimation of primary light before coupling into the light guide layer 2.

In some embodiments the first light source 12 may be attached to an edge face 15 of the light guide layer 2 using glue (not shown) which has optical properties allowing efficient coupling of light into the light guide layer.

Although one first light source 12 is illustrated in Figure 4, more than one first light source 12 can be provided. For example, a plurality of first light sources 12 may be provided in a close packed array at one or more edge faces of the light guide layer 2. One or more first light sources may be provided at each edge face of the light guide layer 2. One or more first light sources may be provided at opposing edge faces of the light guide layer 2. A light source configured to emit primary light may be referred to as a primary light source. Although first light sources have been described, embodiments of the present invention may also include one or more second light sources, wherein a second light source is configured to emit second primary light within a second primary light wavelength range which is different to the primary light wavelength range. A second light source may have any of the properties specified herein in relation to the primary light source. Further light sources may be provided configured to emit light within wavelength ranges which are different to those of the second and first light sources.

Primary light 13 coupled into light guide layer 2 can interact with low-scattering photo-luminescent elements 3 in the luminescent layer 4. Primary light 13 having a wavelength within the first wavelength range, that is, within the wavelength range for which the low-scattering photo- luminescent elements 3 can absorb light, can be absorbed by low-scattering photo-luminescent elements 3. After absorption of primary light 13, a low-scattering photo-luminescent element 3 can emit secondary light 17 which is light having a wavelength within the second wavelength range. Primary light 13 may pass through luminescent layer 4 without interacting with a low-scattering photo-luminescent element 3 and be totally internally reflected at interface . As primary light 13 is trapped within the composite light guide 1, multiple passes through the luminescent layer 4 are possible without loss of the primary light 13. This can allow a lower loading of photo-luminescent elements to be provided in luminescent layer 4 compared to implementations wherein primary light can only pass through a photo-luminescent element interaction area once.

Secondary light 17 is emitted substantially isotropically, that is, with equal intensity in all directions. Thus secondary light 17 may escape the composite light guide 1 if it is emitted outside the range of emission angles for which secondary light 17 can be trapped within composite light guide 1 by total internal reflection. Secondary light 17 may also be trapped within composite light guide 1 in the same manner as primary light 13, that is, by total internal reflections at interface and reflections at reflector layer 11. Reflector layer 11 can help to retain secondary light 17, which is emitted in a direction towards the first face 6, within the composite light guide 1.

Secondary light 17 which is trapped within composite light guide 2 by multiple reflections and total internal reflections can be coupled out of the composite light guide 2 by interaction with an out- coupling structure 5.

The plurality of out-coupling structures 5 are disposed on the first face 6 of the light guide layer 2. However, as will be discussed in more detail in the following, the plurality of out-coupling structures 5 may be disposed on the second face 7. The plurality of out-coupling structures may be disposed on the first face 6 and the second face 7.

Interaction of light, which may be primary and/or secondary light, with an out-coupling structure 5 can result in a change in the direction of propagation of the light which can allow the light to no longer be trapped within the composite light guide by total internal reflections. The light can therefore escape from the composite light guide, that is, be coupled out of the composite light guide. Light, which may be primary and/or secondary light, may still be trapped within the composite light guide by total internal reflection after interaction with an out-coupling structure. Such light may then need to interact with a further out-coupling structure in order to escape the composite light guide. The plurality of out-coupling structures 5 are arranged in a predetermined configuration, wherein the configuration of the plurality of out-coupling structures is chosen so as to obtain a desired output light distribution.

In embodiments of the present invention, output light comprises light 20 which is output from the composite light guide 1 through the second face 7 of the light guide layer. However, in some embodiments, output light comprises light which is output from the composite light guide through the first face 6 of the light guide layer. In some embodiments, output light comprises first output light which is output from the composite light guide 1 through the first face 6 of the light guide layer and second output light which is output from the composite light guide 1 through the second face 7 of the light guide layer. Although in the following output light is referred to which is light output from the composite light guide 1 through the second face 7 of the light guide layer, it will be understood that output light may additionally or alternatively comprise light output from the composite light guide through the second face 6 of the light guide layer. In some embodiments, a reflector layer 11 may not be present.

Output light comprises a mixture of primary light and secondary light.

The desired proportion of primary and secondary light in the output light can depend on the particular application of the composite light guide and thus on one or more properties of the desired output light, for example, on the range of colours required in the output light, the intensity of the output light, the correlated colour temperature of the output light. The proportion of primary and secondary light in the output light can be controlled using methods as described herein, for example, using out-coupling structures of two or more different colours.

In embodiments where the low-scattering photo-luminescent elements are provided in luminescent layer 4 on the second face of the light guide layer 2, the output light spectrum comprises light which is output through the second face 7 and through the first interface and thus is output from the composite light guide 1. However, in some embodiments, the low-scattering photo-luminescent elements are not provided in a luminescent layer on the second face 7 of the light guide layer 2 and the output light spectrum comprises light which is output through the second face 7 of the light guide layer 2 and thus is output from the composite light guide 1. In some embodiments, the output light spectrum comprises light which is output through the first face 6. A desired output light distribution may be a desired spectral irradiance distribution of output light 20. For example, in some embodiments a desired output distribution may be a substantially homogeneous spectral irradiance distribution of output light 20, that is, the spectral irradiance of output light 20 does not substantially vary with position in a plane parallel to the plane of the second face 7. In some embodiments, the composite light guide 1 may be flexible and a substantially homogeneous spectral irradiance distribution of output light 20 means that the spectral irradiance distribution of the output light 20 along a surface normal of the composite light guide 1 does not vary substantially over the exit surface (e.g. the output surface).

A desired output light distribution may be a non-homogeneous spatial distribution of output light 20. For example, a composite light guide according to embodiments of the present invention may be used as a backlight for a static touch screen user display and it may be desired to provide a first area on the display which is capable of outputting white light, a second area on the display which is capable of outputting blue light, and a third area on the display which is capable of outputting red light. One or more areas may be capable of outputting light which comprises secondary light and no substantial component of primary light. One or more areas may be capable of outputting light which comprises primary light and no substantial component of secondary light.

The out-coupling structures may define a layer in the light guide. This layer may be inhomogeneous, meaning that one or more characteristics thereof may be not homogeneous, such as for example the size of the out-coupling structures, their shape, the distribution of the out-coupling elements, a combination of both, etc. Where inhomogeneous spatial distribution is required, the variation of the spatial density of the out-coupling structures can be moderate, even becoming zero in the limit of very low average spatial density, although the efficiency also drops. A good compromise can be found within the range of a 10% spatial density difference as lower limit, and a factor of 10-fold the spatial density difference as upper limit.

A desired output light distribution may be a desired spectral distribution of output light 20. For example, a composite light guide according to embodiments of the present invention can be used as a user interface backlight, for example as a backlight in a touch screen, a tablet device, a smartphone, Referring to Figure 5, the present invention allows to provide an output light distribution 20 having more than one output light region Ai, A2, A3, A₄, each output light region providing corresponding output light having a different wavelength λι, λ2, λ3, λ₄, or colour, for example one or more of blue, green, red, and/or a combination of colours such as white or purple. The configuration of the out-coupling structures is determined in dependence upon the desired output light spectrum. This configuration can be determined using an optimization process as follows. Referring to Figure 6, a flow chart of an optimization process is shown. An initial uniform, pseudo-random configuration of out-coupling structures is chosen and is set as the trial configuration of out-coupling structures (step S601). A trial output light distribution is calculated based upon the trial configuration of out-coupling structures and relevant parameters of the composite light guide (step S602). The trial output light distribution is compared with the desired output light distribution and a figure of merit is calculated (step S603). The figure of merit is compared with the threshold value (step S604). If the figure of merit is below a predetermined threshold value, the trial configuration of out-coupling structures is output (step S605a). If the figure of merit is not below a predetermined threshold value, the trial configuration of out-coupling structures is modified (step S605b) and the process returns to step S602.

The optimization process may be a computer-implemented method. For example, the LightTools software package includes a Backlight Pattern Optimiser module which can be used to implement the optimization process.

Relevant parameters of the composite light guide 2 may include dimensions of the light guide layer and luminescent layer, optical properties of the layers, distribution and optical properties of the low- scattering photo-luminescent elements 3. Other relevant parameters for the optimization process may include the type, spectrum, intensity, and linewidth of a light source 12. Relevant parameters may be fixed or variable during the optimization process.

The modification step comprises varying one or more properties of the configuration of out-coupling structures. For example, one or more of the size, shape, colour, spacing and position of the out- coupling structures can be varied.

The configurations of the out-coupling structures may be determined using an optimisation area divided into a mesh, or an array of sub-areas, and a local density for each sub-area can be determined.

One of the parameters for the optimization of the out-coupling structures is the distribution of the average spatial density of out-coupling structures. In some embodiments, the distribution is inhomogeneous, with the values of spatial density mentioned earlier.

The distribution can be optimized over the full light guide surface on which the out-coupling structures are positioned. The full light guide surface, on which the out-coupling structures are positioned (and which can be considered an out-coupling layer), is divided in small surface areas or bins. The average spatial density of out-coupling structures equals the surface area of non- overlapping out-coupling structures (e.g. non-overlapping dots, such as white dots) in a certain bin divided by the surface area of that bin. Again, the range of used average spatial densities in the light guide design varies largely depending on quantum dot properties, light guide size, required color and/or luminance requirements. Typical values fall between 50% and 1%.

The averaging area would be the same as for the determination of homogeneity of the photo- luminescent layer: lxl cm² can be considered a good surface bin size, given the fact that typical out- coupling structure sizes are in the range of 0.1 to 1 mm and spatial density values typically do not drop below 1-2 %.

These values of mesh are only indicative. For example, in some embodiments, each sub-area or bin can have a size of approximately 2x2 mm². However, the size of the bin is not limited to this size and can be chosen as larger or smaller than 2x2 mm².

The merit function for a given configuration of out-coupling structures can be determined in dependence upon the desired output light distribution and the achieved output light distribution for the given configuration of out-coupling structures. The output light distributions can be calculated with reference to the distribution of output light received at a first receiver area close to the composite light guide 1, adjacent to the second face 7 of the light guide layer 2. However, in embodiments wherein output light comprises light output from the composite light guide 1 through the first face 6, the output light distribution may additionally or alternatively be calculated with reference to the distribution of output light received at a second receiver area close to the composite light guide 1, adjacent to the first face 6 of the light guide layer 2. The receiver area preferably has the same dimensions as the optimisation area and is divided into sub-areas in the same way as for the optimisation area. However, in some embodiments the receiver area may be divided into sub- areas of a different size to the sub-areas of the otpimisation area.

For example, in embodiments where the target output light distribution includes a target x colour coordinate CIE target(i,j) for a bin labelled by (i ) and a target y colour coordinate C/f_y, target(i ) for a bin labelled by (i ), and an x colour coordinate achieved by the optimisation process is CIE_{X/ a}cwai(i,j) and a y colour coordinate achieved by the optimisation process is C/f_y, acwai(i ) the merit function may be

(CIE_X, actual target (i ))² + (CIEy, actual (U) - IEy, target (i ))²

The modification step may comprise changing a local density of out-coupling structures in one or more bins of the optimisation area. A local density of out-coupling structures in a bin is the fraction of the area of the bin which is covered by out-coupling structures, that is, the area covered by out- coupling structures divided by the area of the bin. Changing a local density may comprise changing one or more of a number of out-coupling structures in the bin, a size of one or more out-coupling structures in the bin, a shape of one or more out-coupling structures in the bin. A maximum local density value may be imposed. In preferred embodiments the maximum local density value is approximately 10% but this value may be greater than or less than 10%. The maximum local density value may be chosen so as to allow light to be confined within the composite light guide 1 for a sufficient number of total internal reflections to ensure that primary light and secondary light are sufficiently mixed. The maximum local density value may be chosen to avoid a configuration in which primary light is coupled out of the composite light guide before sufficient mixing with the secondary light has occurred as this can be detrimental to obtaining color uniformity and illuminance uniformity simultaneously. A low total density of out-coupling structures can cause the light to be trapped within the composite light guide for a large number of total internal reflections of the light. This can lead to better color homogeneity, but can lead to a decrease in efficiency by absorption in the light guide layer and/or by the reflector. Thus, the optimsation process aims to maximize the average density of out-coupling structures with the aim of achieving a high efficiency (less loss of light through absorption/reflection) while, in some embodiments, being a density that is low enough to ensure good color uniformity of output light. For example, in some embodiments a maximum local density value of 80% or more can be achieved. In some embodiments a maximum local density value of 10% or less is required.

The modification step may comprise changing a spectral property of one or more out-coupling structures in one or more bins of the optimisation area. Changing a spectral property may comprise for example changing the reflectance spectrum of printed out-coupling structures in a bin. An out- coupling structure may comprise a photonic structure and changing a spectral property of an out- coupling structure may comprise changing a parameter of the photonic structure. An out-coupling structure may comprise a diffractive feature such as a grating and changing a spectral property of an out-coupling structure may comprise changing a parameter of the diffractive feature such as the spacing or pitch of the grating.

For example, it may be desired that output light comprises a particular ratio of primary light to secondary light. It may be desired that output light comprises a first ratio of primary light to secondary light in a first region and a second, different ratio of primary light to secondary light in a second region. Varying the spectral properties of one or more out-coupling structures in an optimisation process can allow to achieve a desired spectral distribution of output light.

Providing out-coupling structures having different spectral properties can allow to reduce the average mixing path length of light in the composite light guide (that is, can allow to reduce a number of reflections required for primary and secondary light to be mixed), and therefore increase the efficiency (as a shorter path length results in less absorption of light by the light guide layer). This can allow to determine an optimized pattern for each of the primary and secondary light and reduce the need for extensive primary and secondary light mixing.

An out-coupling structure may comprise a dot, for example an out-coupling structure may have a shape which is substantially circular. A dimension of the out-coupling structure in the plane of the light guide layer 2 is substantially larger than the first wavelength and the second wavelength. For example, an initial distribution of out-coupling structures may comprise a plurality of white dots having a shape which is substantially circular in the plane of the light guide layer 2, each having a radius of 175 μιτι. However, the present invention is not limited to this particular initial configuration and includes any suitable initial configuration of out-coupling structures. An out-coupling structure may have a shape in the plane of the light guide layer 2 which is square, rectangular, oval, any regular or non-regular polygon shape. A dimension of the out-coupling structure in a direction perpendicular to the plane of the light guide layer 2 is preferably sufficient to allow high reflectivity, for example preferably allowing more than 95% reflectivity.

An out-coupling structure may comprise a coloured paint, for example white, red, green, blue, or any other colour of paint.

In embodiments of the present invention the plurality of low-scattering photo-luminescent elements 3 includes at least one low-scattering photo-luminescent element of a first type and at least one low- scattering photo-luminescent element of a second type. The low-scattering photo-luminescent element of the first type is capable of absorbing light within a first wavelength range (primary light) and emitting light within a second, different wavelength range (first secondary light). The low- scattering photo-luminescent element of the second type is capable of absorbing light within a first wavelength range (primary light) and emitting light within a third wavelength range (second secondary light) which is different to the first wavelength range and the second wavelength range, and each of the plurality of out-coupling structures has a dimension which is substantially larger than wavelengths in the third wavelength range.

By providing low-scattering photo-luminescent elements having different emission wavelengths, the spectral distribution of output light can be further controlled.

In preferred embodiments, the first wavelength range does not substantially overlap the third wavelength range. In preferred embodiments, the third wavelength range is a relatively narrow wavelength range, this can help to avoid colour leakage.

The distribution and density of the photo-luminescent elements of the first type and the second type may be determined in dependence upon a desired output light distribution. For example, the desired output light distribution comprises a first area having first output light with a first ratio of primary light to secondary light and a second area having second output light with a second, different ratio of primary light to secondary light. A first type of photo-luminescent elements may then be disposed in or on a first region of the light guide layer and a second type of photo-luminescent elements may be diposed in or on a second region of the light guide layer. The configuration of out-coupling structures may then be optimised to provide substantially uniform emission in the first area and the second area.

If only the spatial properties (size, shape, spacing, density) of the out-coupling structures are optimized to obtain a uniform irradiance distribution of output light, it can be the case that the colour distribution (spectral irradiance distribution) of output light is not uniform, that is, output light may comprise regions where the first secondary light is more intense than the second secondary light and vice versa. In some applications this can be desirable. However in some applications this may not be desirable and the present invention allows to solve this problem. A low density is set for the out-coupling structures so that the primary light, first secondary light, and second secondary light have a longer path length within the composite light guide and are thus better mixed. This can affect the efficiency, but the density of the out-coupling structures can be tuned so as to provide an acceptable efficiency. By varying a spectral property of out-coupling structures, such as the reflection spectrum (related to what commonly in known as "colour"), a substantially spectrally uniform output light distribution can be provided, wherein output light comprises regions where the first secondary light is not substantially more intense than the second secondary light and vice versa. In some embodiments, varying a spectral property of out-coupling structures can allow to provide a output light distribution having regions of different colours and intensities of output light.

Referring to Figure 7, a second lighting device 30 is shown which includes the composite light guide 1. The second lighting device 30 includes an extended reflector element 31 which surrounds the composite light guide 1 adjacent to the edge faces and the first face of the light guide layer 2. The extended reflector element 31 has an opening adjacent to the second face 7 of the light guide layer 2 which allows output light 20 to be output from the second lighting device 30. The extended reflector element 31 can help to contain light which may escape the composite light guide 1 from the edge faces and the first face 6 of the light guide layer 2 and reflect escaped light back into the composite light guide 1. The extended reflector element 31 may support the primary light source 12.

The second lighting device 30 includes an optical stack 35 adjacent to and substantially parallel to the second face 7 of the light guide layer 2. The optical stack 35 may comprise one or more of a diffuser, a brightness enhancement film (BEF), one or more polarizers, a color filter. The optical stack 35 may comprise a colour filter having a particular filter wavelength range for which light is transmitted and the low-scattering photo-luminescent elements 3 may be chosen so as to have a spectrum of secondary light which closely matches the filter wavelength range. One or more intermediate layers may be provided between the optical stack 35 and the light guide layer 2. For example, a bezel may be provided between the optical stack 35 and the light guide layer 2 which can help to prevent primary light from propagating into the optical stack 35 if it is desired that output light 20 does not comprise primary light.

Although some embodiments described herein relate to a composite light guide having out-coupling structures disposed on the second face of the light guide plate and low-scattering photo-luminescent elements on the first face of the light guide plate, other positionings of the out-coupling structures and the low-scattering photo-luminescent elements are possible. For example, in some embodiments the out-coupling structures are disposed on the first face of the light guide layer and the low-scattering photo-luminescent elements are disposed on the first face of the light guide layer. In some embodiments the out-coupling structures are disposed on the first face of the light guide layer and the low-scattering photo-luminescent elements are disposed on the second face of the light guide layer. In some embodiments the out-coupling structures are disposed on the second face of the light guide layer and the low-scattering photo-luminescent elements are disposed on the second face of the light guide layer.

The low-scattering photo-luminescent elements 3 need not be disposed on a face of the light guide layer 2. For example, the low-scattering photo-luminescent elements 3 may be dispersed within the light guide layer 2. In embodiments wherein the low-scattering photo-luminescent elements 3 are dispersed within the light guide layer 2, the light guide layer can comprise PMMA. The light guide layer may comprise S1O2, Polydimethylsiloxane (PDMS), or any suitable polymer. One possible way providing low-scattering photo-luminescent elements dispersed within a transparent medium is described in: Meinardi, F. et al., Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots, Nature Nanotechnology 2015, 10, 878-885.

Referring to Figure 8, in a composite light guide 1, the low-scattering photo-luminescent elements 3 may be provided in a luminescent layer which is 'sandwiched' between two portions of the light guide layer. The light guide layer comprises a first portion 40 and a second portion 41. The first portion 40 has a first face 42 and a second, opposite face 43. The second portion 41 has a first face 44 and a second, opposite face 45. The second face 43 of the first portion 40 is next to the first face 44 of the second portion 41. The plurality of low-scattering photo-luminescent elements 3 is disposed in a luminescent layer 4 between the first portion 40 and the second portion 41. The first face 6 of the light guide layer 2 is provided by the first face 42 of the first portion 40 and the second face 7 of the light guide layer 2 is provided by the second face 45 of the second portion 41. The first and second portions 40, 41 can act as barriers preventing oxygen and/or water from reaching the luminescent layer 4. This can significantly enhance the photo-thermal stability of the plurality of low-scattering photo-luminescent elements 3.

In the cases where the photo-luminescent elements 3 are dispersed within a transparent medium or sandwiched, homogeneity of the distribution of wavelength conversion can also be provided, where the deviation of the thickness of the layer including the photo-luminescent elements, or their concentration across the light guide plate, or both together, may be below 1%, or their average deviation of the thickness plus the concentration may be under 5% or under 1% or under 0.2%, e.g. over an averaging area of lxlcm², as explained earlier.

In embodiments of the present invention wherein a luminescent layer 4 is disposed on the first face 6 or the second face 7 of the light guide layer 2, a capping layer may be disposed over the luminescent layer 4 which can act as barriers preventing oxygen and/or water from reaching the luminescent layer 4. This can significantly enhance the photo-thermal stability of the plurality of low-scattering photo-luminescent elements 3. A capping layer may comprise one or more of AI2O3, S1O2, any inorganic transparent material such as T1O2, ZrC>2, HfC>2, ZnS. The capping layer may be deposited on the luminescent layer for example by an atomic layer deposition process.

The primary light source can include any light source capable of creating secondary light emission from the low-scattering photo-luminescent elements. An appropriate primary light source will be capable of emitting light having a wavelength capable of exciting the photo-luminescent elements, thereby initiating secondary light emission. An ideal primary light source will also exhibit high efficiency, low operating temperatures, high flux, and high brightness. Additional considerations for choosing the primary light source can include availability, cost, size, tolerance, emission color and purity, spectral width, direction of emitted light, lifetime, quality, consistency of features. The primary light source can be any suitable light source, such as a LED, a blue or ultraviolet sources such as blue or UV LEDs, a laser, an arc lamp, a black-body light source, and other solid state light sources. Preferred embodiments will include a LED primary light source. Preferably, the primary light source is a blue or UV light source, most preferably a blue LED which emits in the range of 440-470 nm, for example at 445 nm. In some embodiments the primary light source is a blue LED which emits in the range of 450-460nm. For example, the primary light source can be a GaN LED such as a GaN LED which emits blue light at a wavelength of 450 nm. The light source may comprise, for example, one or more LXZ1-P 01 light emitting diodes, available from Lumileds (Netherlands).

In preferred embodiments, a portion of the blue light emitted by the blue primary light source will be apportioned to absorbance and reemission by the photo-luminescent elements, and a portion of the blue primary light will function as a blue light component of the light out-coupled from the light guide. In these embodiments, light emitted from the composite light guide will include a mixture of primary excitation light from the primary source and secondary light emitted from the photo- luminescent elements following excitation by the primary light.

In some embodiments of the invention the low-scattering photo-luminescent elements are luminescent semiconductor nanocrystals, or quantum dots (QDs). The various properties of the photo-luminescent elements, including their absorption properties, emission properties, quantum efficiencies and refractive indices, can be tailored and adjusted for various applications, for example, by the size, the shape or the composition of the photo-luminescent elements. A nanocrystal has at least one region or characteristic dimension with a dimension of less than about 500 nm, and down to on the order of less than about 1 nm. As used herein, when referring to any numerical value, "about" means a value of ±10% of the stated value (e.g. about 100 nm encompasses a range of sizes from 90 nm to 110 nm, inclusive). The terms "nanocrystal," "quantum dot," and "nanodot," are readily understood by the ordinarily skilled artisan to represent like structures and are used herein interchangeably. The present invention also encompasses the use of polycrystalline or amorphous nanocrystals.

Typically, the region of characteristic dimension will be along the smallest axis of the structure. The QDs can be substantially homogenous in material properties, or in certain embodiments, can be heterogeneous. The optical properties of QDs can be set by their particle size, particle shape, chemical or surface composition; and/or by suitable optical testing available in the art. The ability to tailor the nanocrystal size in the range between about 1 nm and about 15 nm enables photoemission coverage in the entire optical spectrum to offer great versatility in color rendering. Additionally or alternatively, such variations can be provided by changing the composition of the photo-luminescent element. Particle encapsulation offers robustness against chemical and UV deteriorating agents. Additional exemplary nanostructures include, but are not limited to, nanowires, nanorods, nanotubes, branched nanostructures, nanotetrapods, tripods, bipods, nanoparticles, and similar structures having at least one region or characteristic dimension (optionally each of the three dimensions) with a dimension of less than about 500 nm, e.g., less than about 200 nm, less than about 100 nm, less than about 50 nm, or even less than about 20 nm or less than about 10 nm. Typically, the region or characteristic dimension will be along the smallest axis of the structure. Nanostructures can be, e.g., substantially crystalline, substantially monocrystalline, polycrystalline, amorphous, or a combination thereof.

The low-scattering photo-luminescent elements for use in the present invention can be produced using any method known to those skilled in the art. For example, suitable QDs and methods for forming suitable QDs include but are not limited to those disclosed in: U.S. Pat. No. 6,225,198, US Patent Application Publication No. 2002/0066401, filed Oct. 4, 2001, U.S. Pat. No. 6,207,229, U.S. Pat. No. 6,322,901, U.S. Pat. No. 6,949,206, U.S. Pat. No. 7,572,393, U.S. Pat. No. 7,267,865, U.S. Pat. No. 7,374,807, U.S. patent application Ser. No. 11/299,299, filed Dec. 9, 2005, and U.S. Pat. No. 6,861,155, each of which is incorporated by reference herein in its entirety.

The QDs (or other nanostructures) for use in the present invention can be produced from any suitable material, suitably an inorganic material, and more suitably an inorganic conductive or semiconductive material. Suitable semiconductor materials include any type of semiconductor, including group ll-VI, group lll-V, group IV-VI and group IV semiconductors. Suitable semiconductor materials include, but are not limited to, Si, Ge, Sn, Se, Te, B, C (including diamond), P, BN, BP, BAs, AIN, AIP, AIAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, AIN, AIP, AIAs, AlSb, GaN, GaP, GaAs, GaSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdSeZn, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCI, CuBr, Cul, Si3N4, Ge3N4, AI203, (Al, Ga, ln)2 (S, Se, Te)3, AI2CO, and appropriate combinations of two or more such semiconductors. Suitable materials include chalcopyrites (I-III-VI2) such as CulnS2, CulnSe2, CuGaS2, CuGaSe2. Suitable materials include perovskites (Cs Pb halides, methylammonium Pb halides, formamidimium Pb halides). Suitable materials include carbon dots (c-dots). Suitable materials include nanocrystals doped with transition metal ions or lanthanide ions.

In certain aspects, the semiconductor nanocrystals or other nanostructures may comprise a dopant from the group consisting of: a p-type dopant or an n-type dopant. The nanocrystals (or other nanostructures) useful in the present invention can also comprise ll-VI or lll-V semiconductors. Examples of ll-VI or lll-V semiconductor nanocrystals and nanostructures include any combination of an element from Group II, such as Zn, Cd and Hg, with any element from Group VI, such as S, Se, Te, Po, of the Periodic Table; and any combination of an element from Group III, such as B, Al, Ga, In, and Tl, with any element from Group V, such as N, P, As, Sb and Bi, of the Periodic Table. Other suitable inorganic nanostructures include metal nanostructures. Suitable metals include, but are not limited to, u, Pd, Pt, Ni, W, Ta, Co, Mo, Ir, Re, Rh, Hf, Nb, Au, Ag, Ti, Sn, Zn, Fe, FePt, and the like.

The QDs used in embodiments of the present invention may be chosen based on the desired emission properties of the application for which the light guide is used. Preferred QD characteristics include high quantum efficiency (e.g., about 90% or greater), continuous and tunable emission spectrum, and narrow and sharp spectral emission (e.g., less than 50 nm, 35 nm or less, or 25 nm or less full width at half max (FWHM)).

In preferred embodiments, the QDs will include at least one population of QDs capable of emitting red light and at least one population of QDs capable of emitting green light upon excitation by a blue light source. The QD wavelengths and concentrations can be adjusted to meet the optical performance required. In still other embodiments, the QD phosphor material can comprise a population of QDs which absorb wavelengths of light having undesirable emission wavelengths, and reemit secondary light having a desirable emission wavelength. In this manner, the QD film comprises at least one population of color-filtering QDs to further tune the lighting device emission and reduce or eliminate the need for color filtering.

The QDs are preferably coated with one or more ligand coatings, embedded in one or more matrix materials, and/or sealed by one or more barrier layers. Such ligands, matrix materials, and barriers can provide photo-stability of the QDs and protect the QDs from environmental conditions including elevated temperatures, high intensity light, external gasses, moisture, and other harmful environmental conditions. Additional effects can be achieved with these materials, including a desired index of refraction in the host matrix material, a desired viscosity or QD dispersion/miscibility in the host matrix material, and other desired effects. In preferred embodiments, the ligand and matrix materials will be chosen to have a sufficiently low thermal expansion coefficient, such that thermal curing does not substantially affect the QD phosphor material.

The light guide according to the present invention can comprise one or more layer materials between adjacent elements of the light guide. The light guide can include one or more layer material disposed between any of the adjacent elements in the light guide, including between the plurality of low-scattering photo-luminescent elements and the light guide layer, between the plurality of out- coupling structures and the light guide layer, between the plurality of low-scattering photo- luminescent elements and the plurality of out-coupling structures. The one or more layers can include any suitable materials, including, but not limited to, optical materials, adhesives, optical adhesives, glass, polymers, solids, liquids, gels, cured materials, optical coupling materials, index- matching materials, cladding or anti-cladding materials, spacers, epoxy, silica gel, silicones, any matrix materials described herein, reflective or anti-reflective materials, wavelength-selective materials, wavelength-selective anti-reflective materials, color filters, or other suitable media known in the art. Suitable layer materials include optically transparent, non-yellowing, pressure-sensitive optical adhesives. Suitable materials include silicones, silicone gels, silica gel, epoxies (e.g., Loctite™ Epoxy E-30CL), acrylates (e.g., 3M™ Adhesive 2175). The one or more layer materials can be applied as a curable gel or liquid and cured during or after deposition, or pre-formed and pre-cured prior to deposition. Suitable curing methods include UV curing, thermal curing, chemical curing, or other suitable curing methods known in the art. Suitably, index-matching media materials can be chosen to minimize reflection between elements of the lighting device.

In certain embodiments, the light guide can include a plurality of spatial regions having multiple different light emission characteristics. In one embodiment, the light guide comprises a first plurality of spatial regions comprising a first population of low-scattering photo-luminescent elements capable of emitting light having a first secondary light wavelength or within a first secondary light wavelength range (e.g., green light-emitting low-scattering photo-luminescent elements), and at least a second plurality of spatial regions comprising a second population of low-scattering photo- luminescent elements capable of emitting light having a second secondary light wavelength or within a second secondary light wavelength range which is different than the first secondary light wavelength or first secondary light wavelength range (e.g., red light-emitting low-scattering photo- luminescent elements). The light guide can further comprise a third plurality of spatial regions comprising a third population of low-scattering photo-luminescent elements capable of emitting third secondary light having a third secondary light wavelength or within a third secondary light wavelength range which is different from at least one of the first and second secondary light wavelengths or first and second secondary light wavelength ranges (e.g., blue light-emitting low- scattering photo-luminescent elements). The light guide can comprise additional pluralities of spatial regions comprising additional populations of low-scattering photo-luminescent elements capable of emitting light having additional wavelengths or wavelength ranges different from at least one of the first, second, and third secondary light wavelengths or secondary light wavelength ranges. For example, the light guide can include a plurality of distinct spatial regions or pixels, wherein each pixel comprises a plurality of smaller spatial regions or subpixels which emit different colors of light. For example, the light guide can include a plurality of pixels, wherein each pixel includes a first subpixel comprising one or more red light-emitting low-scattering photo-luminescent elements, a second subpixel comprising one or more green light-emitting low-scattering photo-luminescent elements, and a third subpixel comprising one or more blue light-emitting low-scattering photo-luminescent elements.

Referring to Figure 9, a flow chart of a first method of manufacturing a composite light guide according to embodiments of the present invention is shown. A light guide layer 2 or light guide layer portion 40, 41 is provided (step S901). At least one of the plurality of out-coupling structures is formed on at least one of the first face and the second face of the light guide layer or light guide layer portion (step S902). The out-coupling structure may be formed using a printing process. This can allow a composite light guide to be manufactured easily and cheaply. The out-coupling structure may be formed by removing material from or reshaping the material of the light guide layer or light guide layer portion. This can allow a composite light guide to be manufactured using a high accuracy process for the out-coupling structures. Removal of material may comprise an etching process, a laser ablation process, a diamond machining process. Reshaping of material may comprise a hot embossing process, a mechanical imprinting process. The method may optionally further comprise a depositing the plurality of low-scattering photo-luminescent elements on at least one of the first face and the second face of the light guide layer or light guide layer portion (step S903). Depositing the plurality of low-scattering photo-luminescent elements on at least one of the first face and the second of the light guide layer or light guide layer portion may comprise printing the plurality of low- scattering photo-luminescent elements on at least one of the first face and the second of the light guide layer or light guide layer portion.

In some embodiments, the provided light guide layer comprises a plurality of low-scattering photo- luminescent elements dispersed within the light guide layer and step S903 may not be required. In some embodiments, providing the light guide layer (step S901) may further comprise a step of dispersing a plurality of low-scattering photo-luminescent elements within a transparent material, which then comprises the light guide layer.

In a further aspect, the present invention provides a method of determining the configuration of the out-coupling structures, by providing a starting configuration (homogeneous configuration) of the out-coupling structures, and providing optimization and variation of the configuration of the out- coupling structures, which may result in an inhomogeneous final configuration. This optimization method allows providing, not only good luminance uniformity (as in the case of Backlight Pattern Optimiser of LightTools), but also good colour uniformity.

The method allows to provide a composite light guide having a predetermined output spectral irradiance distribution, preferably a uniform output spectral irradiance distribution. The flowchart of Figure 11 shows the steps of the method, together with optional steps (in dashed boxes). The method can be implemented at least partially in a simulation software, and when the result of the simulation is satisfactory (for example, when the difference between the output spectral irradiance distribution and the predetermined spectral irradiance distribution are below a threshold), the actual physical light guide can be manufactured. The method comprises the step of:

(a) Providing S1101 a light guide comprising a light guide layer with two opposite faces and an edge portion configured to receive input light. At least one of the faces is an output face. The light guide includes a plurality of low-scattering photo-luminescent elements. They may have a substantially homogeneous spatial distribution, in particular they may be homogeneously distributed (e.g. along the luminescent layer). The plurality of low-scattering photo-luminescent elements are in optical contact with the light guide layer. The light guide comprises an out-coupling layer disposed on at least one of its faces, which comprises a plurality of out-coupling structures in an initial configuration. The method provides Sllll an initial configuration is a homogeneous spatial distribution, where the out-coupling structures may have the same shape and size and may be homogeneously distributed (uniform distances between them, etc). As explained with reference to the first aspect, the plurality of low-scattering photo-luminescent elements comprises at least one element capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range.

The method further comprises:

(b) Determining S1102 the output spectral irradiance, for example by calculations. The output spectral irradiance distribution is typically quantified in W/m^A2.nm, and its quantity represents both the luminance and colour distribution of the emitted light. The method could be applied using the output light distribution. However, since it is especially difficult to realize both the desired (uniform) luminance and colour distribution due to the different origins and propagation directions of the excitation light and emitted secondary light in the light guide layer, the output spectral irradiance distribution is a much more relevant quantity than the output light distribution (or luminance) alone. The output spectral irradiance distribution of the light guide, calculated with the initial configuration of the out-coupling layer, gives the "initial output spectral irradiance distribution" of the light guide. It is then compared with a predetermined output spectral irradiance distribution. Preferably, the predetermined output spectral irradiance distribution is uniform.

The method further comprises:

(c) Modifying S1103 the configuration of the out-coupling layer in a further, improved configuration. In some embodiments, this is performed by:

Modifying S1113 the size or shape of the out-coupling structures,

- Modifying S1123 the distance between different out-coupling structures

Modifying S1133 both distance between different out-coupling structures and size or shape of the out-coupling structures

In some embodiments, for example, only the distance of virtually identical out-coupling structures is varied.

The method further comprises:

(d) Determining S1104 the output spectral irradiance distribution of the light guide with the further configuration of out-coupling layer to obtain the further output spectral irradiance distribution of the light guide. This is compared with a predetermined (e.g. uniform) output spectral irradiance distribution.

In some embodiments of the present invention, the method may comprise obtaining S1105 the difference between the further output spectral irradiance distribution and the predetermined output spectral irradiance distribution, and repeating step (c) and step (d) in dependence upon that difference. In some embodiments of the present invention, the method provides termination S1106 of the process when the output spectral irradiance distribution difference between the improved output spectral irradiance distribution and the predetermined output spectral irradiance distribution is below of a predetermined threshold. However, the present invention is not limited to this, and other considerations such as illumination uniformity, colour uniformity, etc. may be considered or given more weight in order to set the termination condition.

This method can be implemented as a simulation method, for example executed in a computer or the like as a program.

Figure 12 shows steps of a general routine (e.g. implementable via software such as, but not exclusively, MATLAB) to configure the out-coupling structures, for example to provide an optical contact backlight design with an appropriate white point and good colour and luminance homogeneity, including reducing the density of out-coupling structures per bin. It is also assumed a homogeneous distribution of photo-luminescent elements. In the start step, the maximum density of out-coupling structures is set S1201, and a concentration of photo-luminescent elements (in this case, green and red QDs) is chosen S1202. With the BPO utility of LightTools, the pattern of out- coupling structures is altered S1603. Because the maximum density of out-coupling structures per bin should be respected, the total amount of out-coupling structures will be lowered, so the concentration of the QDs may need to be changed S1204 to obtain again the desired average white point. This process of altering the pattern of out-coupling structures and then altering the QD concentration may be repeated until the system converges and a design with uniform luminance and the desired average white point is obtained. If the color coordinates of the light incident on each bin of the receiver are not be sufficiently uniform, the local density of the out-coupling structures can be altered S1205 in accordance with some embodiments of the present invention, for example using the equation

This can lead to the three different situations in which CIEx and CIEy are not uniform, or show different trends, or they remain constant over the light guide layer. When CIE x and CIE y follow a different trend, or when it is not possible to simultaneously obtain a uniform color point and an acceptable luminance uniformity, the design is reconstructed sl206 for a lower maximal density of out-coupling structures which allows more color mixing. Because the light is trapped inside the light guide for a longer period, the total efficiency goes down, but also the amount of QDs needed to reach the desired white point can be further reduced, as the amount of interactions with the QD- layer increases. Overall, the uniformity in luminance output is better, while the color point remains uniform.

In some embodiments of the present invention, by including an inhomogeneous scattering layer of essentially identical elements and a homogeneous luminescent element layer, particularly advantageous output light distributions can be achieved. Figure 13 to Figure 15 show the results of illuminance and colour point CIExy for QD coated lightguides (assuming a perfectly homogeneous QD layer) as a function of the distance from the LED in mm, with two different optimization strategies of the pattern of out-coupling structures. These calculations can be done following the routine shown in Figure 12, in accordance with embodiments of the present invention. Figure 13 shows the plots of the total amount of out-coupling structures per bin, as a function of the distance from the source (LED) in mm, with the two plots A, B having different distribution. Figure 14 shows the illuminance incident on each bin of the receiver, for the two distributions of the plots A, B of Figure 13. Figure 15 shows the CIEx and CIEy coordinate of the light incident on each bin.

For the distribution corresponding to the left plot A, the pattern of out-coupling structures can be varied to ensure a good (e.g. optimal) output illuminance distribution uniformity. For the distribution corresponding to the right plot B, the pattern of out-coupling structures can be varied to ensure good (e.g. optimal) colour uniformity, and acceptable output illuminance distribution uniformity.

Reducing the spatial density of the out-coupling structures shows an improvement of colour uniformity. The right plot B of Figures 13 to 15 shows a maximum density of out-coupling structures of 50%. The plots of Figure 16 shows a third distribution of the out-coupling structure (top), the illuminance incident on each bin of the receiver (middle) and the CIEx and CIEy coordinate of the light incident on each bin of the receiver, but in this case with a maximum density of out-coupling structures of 25%. By comparing the right plot B of Figures 13 to 15 with the results shown in Figure 16 for a reduced amount of out-coupling structures in the design, it is apparent that a better illuminance uniformity is provided while maintaining a uniform colour output. In addition, for the 25% maximal density, only

78% of the amount of QDs are needed in comparison to the 50% case.

It is shown that providing and optimizing an inhomogeneous layer of out-coupling elements give good results and improved uniformity. For example, Figure 17 shows, for comparison, the illuminance and colour uniformity of a light guide comprising a uniform layer of out-coupling structures.

The requirements of output spectral irradiance distribution vary wildly with the particular application of the light guide. For example, in displays the requirements are usually very stringent, while in horticulture lightning, the requirements may be more relaxed. It is proposed to quantify the uniformity of the output spectral irradiance via the normalised root-mean-square-deviation, between the desired and obtained output spectral irradiance distribution. This deviation makes no difference between spectral (color) variations and irradiance variations. The deviation may lay within a range between 0.25 (maximally) and 0.01 (minimally).

Claims

1. - A composite light guide (1) comprising:

a light guide layer (2) having a first face (6) and a second opposite face (7), and an edge portion (15, 18) configured to receive input light, wherein at least one of the first face (6) and the second face (7) is an output face;

a plurality of low-scattering photo-luminescent elements (3) in optical contact with the light guide layer (2);

an out-coupling layer disposed on at least one of the first face (6) and the second face (7) and comprising a plurality of out-coupling structures (5); and

wherein the plurality of low-scattering photo-luminescent elements (3) comprises at least one element capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range;

characterized in that the out-coupling layer is a spatially inhomogeneous out-coupling layer, and wherein each of the plurality of out-coupling structures (5) has a dimension which is substantially larger than wavelengths in the first wavelength range and wavelengths in the second wavelength range.

2. - A composite light guide according to claim 1, wherein the plurality of low-scattering photo- luminescent elements (3) are dispersed within the light guide layer (2).

3.- A composite light guide according to claim 1 or 2, wherein the plurality of low-scattering photo- luminescent elements (3) is disposed on at least one of the first face (6) and the second face (7).

4. - A composite light guide according to any one of claims 1 to 3, wherein the plurality of low- scattering photo-luminescent elements (3) is homogeneously distributed.

5. - A composite light guide according to any one of claims 1 to 4, wherein the light guide layer (2) comprises a first light guide layer portion (40) having first and second opposite faces (42, 43) and a second light guide layer portion (41) having first and second opposite faces (44, 45), wherein the second face (43) of the first light guide layer portion (40) is next to the first face (44) of the second light guide layer portion (41), and wherein the plurality of low-scattering photo-luminescent elements (3) is disposed between the first light guide layer portion (40) and the second light guide layer portion (41).

6. - A composite light guide according to any preceding claims, wherein the plurality of out-coupling structures (5) are arranged in a predetermined configuration, wherein the configuration of the plurality of out-coupling structures is chosen so as to obtain a desired output light distribution.

7. - A composite light guide according to any preceding claim, wherein the at least one low-scattering photo-luminescent element (3) capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range is a low-scattering photo-luminescent element of a first type, and the plurality of low-scattering photo-luminescent elements (3) further comprises at least one low-scattering photo-luminescent element of a second type, wherein the at least one low-scattering photo-luminescent element of the second type is capable of absorbing light within a first wavelength range and emitting light within a third wavelength range which is different to the first wavelength range and the second wavelength range, and wherein each of the plurality of out-coupling structures (5) has a dimension which is substantially larger than wavelengths in the third wavelength range.

8. - A composite light guide according to any preceding claim, wherein the plurality of out-coupling structures (5) comprises at least one geometrical feature.

9. - A composite light guide according to any preceding claim, wherein the plurality of out-coupling structures (5) comprises at least one printed element.

10.- A composite light guide according to any preceding claim, wherein the plurality of out-coupling structures (5) comprises at least one out-coupling structure having a first spectral property and at least one out-coupling structure having a second spectral property which is different from the first spectral property .

11. - A composite light guide according to any preceding claim, wherein the at least low-scattering photo-luminescent element (3) is a colloidal quantum dot.

12. - A composite light guide according to any preceding claim, further comprising a reflective layer (11) spaced apart from the first face (6) or the second face (7) by an air gap.

13. - A lighting unit (10, 30) comprising:

a composite light guide (1) according to any preceding claim; and

at least one excitation light source (12) configured to emit excitation light at the first wavelength, wherein the at least one excitation light source (12) is optically coupled to the light guide layer at the edge portion (15, 18).

14. - Use of a composite light guide (1) according to any one of claims 1 to 12 or a lighting unit (10, 30) according to claim 13 to generate a desired output light distribution.

15.- A method of manufacturing a composite light guide according to any one of claims 1 to 13, the method comprising:

providing (S901) the light guide layer or light guide layer portion; and

forming (S902, Sllll) the plurality of out-coupling structures on at least one of the first face and the second face of the light guide layer or light guide layer portion .

16.- A method according to claim 15, the method further comprising:

printing (S903) the plurality of low-scattering photo-luminescent elements on a face of the light guide layer or light guide layer portion.

17.- The method of claim 15 or 16 , providing the composite light guide having a desired output spectral irradiance distribution, wherein providing a light guide or comprising a light guide layer portion comprises providing (S1101) a light guide layer or light guide portion having a first face and a second opposite face and an edge portion configured to receive input light, wherein at least one of the first face and the second face is an output face; providing a plurality of low-scattering photo- luminescent elements in optical contact with the light guide layer, wherein the plurality of low- scattering photo-luminescent elements comprises at least one element capable of absorbing light within a first wavelength range and emitting light within a second, different wavelength range, and wherein the low-scattering photo-luminescent elements have a substantially homogeneous spatial distribution, the method further comprising

(a) wherein forming (S902) the plurality of out coupling structures comprises providing (Sllll) an out-coupling layer disposed on at least one of the first face and the second face and comprising a plurality of out-coupling structures in an initial configuration; wherein the initial configuration of the out-coupling layer comprises the outcoupling structures in a homogeneous spatial distribution;

(b) calculating (S1102) the output spectral irradiance distribution of the light guide with the initial configuration of the out-coupling layer to obtain the initial output spectral irradiance distribution of the light guide and comparing the initial output spectral irradiance distribution with the predetermined output spectral irradiance distribution;

(c) modifying (S1103) the configuration of the out-coupling layer in a further configuration by varying the size or shape of the out-coupling structures, by varying the distance between different out- coupling structures and/or by varying the size or shapes of the out-coupling structures and the distance between the different out-coupling structures;

(d) calculating (S1104) the output spectral irradiance distribution of the light guide with the further configuration of out-coupling layer to obtain the further output spectral irradiance distribution of the light guide and

(e) comparing the further output spectral irradiance distribution with the predetermined output spectral irradiance distribution and repeating step c) and step d) in dependence upon the difference between the further output spectral irradiance distribution and the predetermined output spectral irradiance distribution.