WO2018108827A1

WO2018108827A1 - Light emitting device

Info

Publication number: WO2018108827A1
Application number: PCT/EP2017/082252
Authority: WO
Inventors: Oscar Fernandez; Andreas Hafner; Kimmo Keränen; Stephan Junger; Rolando Ferrini
Original assignee: CSEM Centre Suisse d'Electronique et de Microtechnique SA - Recherche et Développement
Priority date: 2016-12-12
Filing date: 2017-12-11
Publication date: 2018-06-21
Also published as: EP3551932A1

Abstract

The present invention relates to a light-emitting device comprising a light module comprising a top wall forming a light reflecting component and bottom wall forming a light emitting component, a flexible substrate (1) forming said top wall and having at least one reflective surface (2, 3) facing towards the bottom wall, an array of light emitting components (4) arranged on said flexible substrate such that said components emit, in operation, a divergent light beam towards the bottom wall, and a light modelling arrangement (6) forming the bottom wall in front of the array of light emitting components (4), said bottom wall being transparent to visible light and comprising a light receiving surface (6") and a light emitting surface (6') parallel to each other.

Description

LIGHT EMITTING DEVICE

Technical Field

The invention relates to the field of light sources and illunnination devices. The invention relates in particular to white light sources having a uniform intensity distribution, that include light directing films, for outdoor and indoor lighting applications.

Background of the invention

As of today, indoor and outdoor lighting is commonly based on fluorescent lamps, incandescent lamps and pressure lamps, while there is a general trend to use more and more modern light sources such as semiconductor light sources, in particular light emitting diodes (LED).

To enable interior light designers and architects to provide aesthetic white light sources there is an increasing demand for white light sources of which white light spectrum and intensity distribution can be adapted to the required lighting conditions and which provide also a great uniformity of lighting or illumination characteristics of such light sources. Also, subtle shades of white of these light sources are also required.

Conventional light sources have different drawbacks such as heat generation, the need for high voltages, high power consumption and have a minimum thickness of about 50mm. When arranged in an array such light arrays may also be heavy and are difficult to integrate aesthetically in for example an interior of a building. The possibility to design different shapes of the housings of such array is also quite limited and is mostly expensive.

As the technology developments of LEDs has grown exponentially the last decade there is a general trend to use more and more LEDs in lighting solutions. Colored LEDs, specifically blue and white LEDs are now commonly available and are constantly being improved. The performances of modern LEDs has reached extremely high standards such as very high efficiency and lighting power and a wide range of designs of LED's and LED arrays are available.

More particular, white LED elements have become available by for example combining blue LEDs with fluorescent materials or by combining red, green and blue LED's.

In the field of white light illumination systems comprising an array of light sources, a main challenge is to provide high intensity and high efficiency lighting systems that have a uniform intensity distribution over the entire light emitting surface. While a number of designs may achieve a high intensity, they have a low hiding factor, meaning that the individual light emitting sources will not be completely hidden and will be perceived by the observer, which reduces the aesthetic effect of such devices. To solve this problem, a wide variety of light mixing arrangements have been proposed in the past. However these solutions are based on systems that have a lower light emitting efficiency. This general relation between the hiding factor and the light efficiency is that the higher the hiding factor, the lower is the light emission efficiency. This trend is as well applicable to conventional light sources as for light sources related to LED arrays.

Another requirement that is important is the possible reduction in thickness of white light systems, and therefore LED arrays are particularly well suited as the individual LED elements comprising their plastic housing may be as small as 3 mm, and even less. LEDs are also particularly suited to be arranged in an array.

In order to achieve a small thickness and an elevated hiding factor a number of solutions have been proposed in the past that are based on the combination of LEDs and light guides to achieve a sufficient degree of mixing of light to obtain improved viewing characteristics of the illumination devices, also called luminaires.

WO 2006/034831 discloses an example of a system in which the light guide comprises pyramid-shaped out-coupling faces to improve the emitted light distribution. The drawback of the light guide described in WO 2006/034831 is that the in coupled light beam must be highly collimated in order to achieve a high light intensity. Also, as the intensity stability depends on the in coupling system it is difficult to assure a long term stability of the intensity of the light source. A common approach in the use of light waveguides consists in coupling the light beam provided by a light source by an edge of the waveguide. One example of a light source based on edge in coupling of the light source is disclosed in US 2001/0053075. The main drawback of such a solution is that it imposes a minimum thickness of the waveguide to be compatible with an acceptable amount of collected light and hence with acceptable in-coupling efficiency. Also, the emissive (typically rectangular) area increases quadratically with its length while the available surface of the edge only increases linearly. Thus, for a given required lumen output (i.e. the number of LEDs), the LED pitch decreases with increasing emissive area which increases the complexity of the illumination system as well as the thermal dissipation. Also, in the edge-coupling approach, the design of the needed out-coupling structures, which must be compatible with a homogeneous luminance, depends strongly on the emitting area. To the contrary, the back-coupling approach, as for example described in

2013/0272024 has an in-plane periodicity of the elements of the light emitting array and the optimization of the light output of the emitting light array is limited to the optimization of the light emitting unit cells.

Another common solution to homogenize the light emitted by LEDs over large- areas uses diffusing plates having imbedded scattering particles in for example a backlight configuration. However, the hiding power of such plates increases with increasing distance between the LEDs and the diffuser and with the diffuser thickness, but these two effects are not desired for thin lighting modules.

WO 2014/033576 discloses a combination of edge in coupling and a light guide comprising diffusing particles. This approach has the same limitations as the one mentioned for systems based edge in coupling and the one of systems based on light scattering particles.

Another approach is to use materials with large hiding powers, but these solutions are associated with an efficiency drop. Finally, LED densities may also be increased but this leads to an increase in cost and heating effects become then also an issue. Summary of the invention

According to a main object, the present invention relates to a light emitting device as defined in claim 1. More specifically, the light emitting device of the invention comprises: a light module comprising : o a top wall forming a light reflecting component and bottom wall forming a light emitting component, o a flexible substrate forming said top wall and having at least one reflective surface facing towards the bottom wall, o an array of light emitting components arranged on said flexible substrate such that said components emit, in operation, a divergent light beam towards the bottom wall o a light management arrangement forming the bottom wall in front of the array of light emitting components, said bottom wall being transparent to visible light and comprising a light receiving surface and a light emitting surface parallel to each other, an electrical power management unit configured to provide electrical energy to the light emitting components through electrical circuits arranged in or on the flexible substrate at least to connect the light emitting components and the electrical power management unit at least a light sensing device comprising a light sensor and a light collecting component optically connected to the sensor and arranged outside the module vis a vis the light emitting surface of the module to collect light beams emitted from the light emitting surface to direct at least part of it towards the light sensor a control device configured for receiving input signals from the light sensor and delivering input signals to the electrical power management unit in order to set the total luminous flux and colour coordinates of the light emitted by the module at its light emitting surface according to determined values preset and recorded in a memory of the control device. Preferable structural and functional features of the inventive light ennitting device are further defined in the dependent claims 2 to 17.

In particular, in an embodiment, the flexible substrate may be a foil comprising heat management physical structures in contact with the light emitting components.

Preferably, the flexible substrate comprises heat management physical structures comprising ceramic and/or metallic foil parts embedded in the flexible substrate, preferably inserted in opening arranged in said substrate.

Preferably, the flexible substrate comprises printed electronic circuitry on a surface thereof and connecting the light emitting components to the power management unit.

In a preferred embodiment, at least one surface of the flexible substrate is reflective in the 360-830 nm wavelength range.

Preferably, the flexible substrate comprises a reflective coating or laminate on at least one surface.

In an embodiment, the light modeling arrangement comprises a plurality of flexible foils forming a stack in the module bottom wall.

In another embodiment the plurality of foils comprises: a first foil of transparent material containing optically active microstructures to achieve a highly uniform luminance across the module emitting surface, in the direction normal to the emitting surface of the module; a second foil of transparent material containing optically active microstructures to produce a non-glaring angular radiation pattern; - a third foil having down-converting properties to provide down- converted light.

Still preferably, the first foil comprises microstructured pixels having each a geometrical centre aligned with a geometrical centre of a light emitting component. Still preferably, the second foil comprises a colour changing foil and the third foil is an antiglare foil.

In embodiments, the first foil comprises a stack of three optical foils comprising a surface diffuser and two prismatic bright enhancement foils rotated 90° with respect to one another.

In embodiments, the diffuser and the two prismatic bright enhancement foils

In embodiments, the light collecting component is arranged towards the emitting surface of the module and has a cross-section which is smaller than said emitting surface. In embodiments, the light collecting component is a waveguide component, preferably an optical fiber.

In embodiments, the light emitting components are LEDs.

In embodiments, the LEDs comprises blue and/or cool white LEDs in pumping mode and red/amber and/or green/lime LEDs in tuning mode, said LEDs being distributed regularly onto a top surface of the flexible substrate.

Brief description of the drawings

The present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed descriptions and accompanying drawings, in which:

- Fig. 1 illustrates a schematic cross-section of a light emitting device according to the present invention;

- Fig.2 illustrates a light recycling mechanism within the light emitting module of the light emitting device of figure 1 ;

- Fig. 3 illustrates the manufacturing steps of a flexible substrate comprising heat management structures for using as a flexible substrate in a light emitting module of the device of the invention;

- Fig. 4 illustrates schematically a cross-section of the light emitting module of the device of the invention in an embodiment; Fig. 5 illustrates schematic side-view of periodic microstructures arranged in a foil element of a light modelling arrangement of the light emitting module of the device of the invention and the light refraction of rays incident at different angles

Fig.6 illustrates a similar view as figure 5 but with microstructures being configured as microstructured pixels having a base angle beta;

Fig. 7 illustrates details of the microstructured pixels of figure 6 in perspective view;

Fig. 8 and 9 illustrate in cross-section alternative embodiments of a light modeling arrangement for the light emitting module of the device of the invention;

Fig. 10 illustrates a stack comprising a surface diffuser and the two Bright Enhancement Foils rotated 90° with respect to each other to form a light modeling arrangement as shown in figure 9;

Fig.1 1 illustrates a light sensing arrangement for the device of the invention;

Fig.12A and 12B illustrate views of a control device for the light emitting device of the invention;

Fig. 13 illustrates a preferred layout of light emitting components for the light emitting module of the device on the invention, with LED clusters arranged following a hexagonal spatial distribution with one cluster on the center of the hexagon,

Fig. 14A illustrate predicted irradiance produced by a 5x5 array of lambertian LEDs on a planar target situated at t=12.5mm and 25mm, with a pitch of 25mm;

Fig, 14B illustrates irradiance produced by a 5^5 array of LEDs on a planar target by lambertian and cosine-cubed LISDs;

Fig. 15. illustrates a polar plot of the normalized lambertian (solid line) and inverse cosine-cubed (dotted line) LISDs for the device of the invention; Fig. 16 illustrates a polar plot of the normalized theoretically predicted LISD of the LED module without light modeling arrangement as proposed in the invention (lambertian) and with the proposed light modeling arrangement structure in the C0-C180 and C90-C270 and C45-C225 planes;

- Fig. 17A and 17B illustrate a polar plot of the experimentally measured LISDs of a LED module without and with the light modeling arrangement of the device of the invention and the corresponding normalized luminance variation measured across a row of 7 white LEDs in a module without and with the light emitting module of the device of the invention;

- Fig. 18 illustrates experimentally measured luminance map of an LED module with different combinations of foils for the light modeling arrangement of the device of the invention and the luminance distribution of a LED module measured across a straight line path connecting individual LEDs;

- Fig. 19 illustrates the normalized luminance variation in the highlighted areas. Right) Spectral power distribution measured in an integrating sphere set-up;

- Fig. 20 illustrates a polar plot of the LISDs of an LED module with the two different light modeling arrangement structures foreseen in the light emitting device of the present invention. Detailed description

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which the exemplifying embodiments of the present invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the invention to those skilled in the art.

Furthermore, like numbers refer to the same or similar elements or components throughout the following description.

Fig.1 represents a schematic cross-section of a light module forming a first part of the light emitting device according to the invention. It comprises a LED substrate 1 formed preferably of a thin flexible foil with its bottom 2 or its top 3 surface being highly reflecting. For the sake of comprehension and clarity, references to "top" and "bottom" location of elements in the figures correspond to the position of those elements as shown in the drawings, which may be opposite to their position in use.

On the top surface 3 of the LED substrate 1 a number of LEDs 4 are disposed which emit blue and/or cool white light and/or red and/or orange and/or green and/or lime light towards a bottom surface 6" of a light management arrangement 6. Each LED 4 is separated from its nearest neighbour by a distance d, referred to as the LED pitch. At a distance t above the LEDs 4 is located the light management arrangement 6. The light management arrangement 6 may comprise several optical thin-films in physical contact with each other or separated from each other. Ideally the LED substrate 1 , the LEDs 4 and the light management arrangement 6 are located inside a chamber with highly reflective walls 5. At certain distance from the module edge and closely above the light management arrangement 6, a light sensor system 7 collects the emitted light eL and a microprocessor 8 connected to said light sensor system 7 computes the luminance and colour coordinates and communicates with a LED driver 9.

In order to maximize the recycling of light (see Fig. 2) within the light module, the top surface 3 of the LED substrate 1 is either highly reflective or is coated/laminated with/onto a highly reflective material/foil (3' in Fig.1 ) in the (360- 830nm) wavelength range. Alternatively, the LED substrate 1 and its top surface 3 are highly transparent and the reflective material/foil is coated/laminated onto the bottom surface 2 of the LED substrate 1.

As represented in Fig. 3, an integrated LED foil may be provided combining the LED substrate 1 , the LEDs sources 4, heat management structures 33 and electrically conductive circuitry 34 to deliver the required electrical power to the LEDs 4.

The LEDs 4 are blue and/or cool white (pumping) as well as red/amber and/or green/lime (tuning) LEDs distributed more or less regularly onto the top surface 3 of the LED substrate 1.

The heat management structures 33 are integrated into the LED substrate 1 providing efficient heat management for LEDs 4 so that excess heat created by LEDs is conducted through the LED substrate 1 and dissipated to structures towards the bottom surface 2 of the LED substrate 1.

The manufacturing process flow of heat management structures is shown in Fig.3. The heat management structures 33 are based on a ceramic/metallic foil piece with low thermal resistivity. Ceramic/metallic foil pieces with thickness of a same order of magnitude to that of the LED substrate 1 are assembled and bonded on perforated openings 32 on the LED substrate 1. Processing of the openings to the LED substrate 1 can be implemented by mechanical punching or laser tooling. Both methods have been successfully operated and for both methods both sheet-to-sheet and roll-to-roll operation modes are possible.

In a following step, electrically conductive circuitry 34 is processed on the top surface 3 of the LED substrate 1 by printing or sputtering methods and using for example silver or copper inks, but not limited to these. LEDs 4 are then assembled on that conductive circuitry 34 to create a final functional LED foil.

Excellent performance of the heat management structure 33 is expected if the materials used have the appropriate characteristics and dimensions. In order to optimize the structure performance it is beneficial to utilize materials with high thermal conductivity in all process steps. In practice thermal conductivity of ceramic/metallic foil, LED bonding material and printed ink material thermal conductivity should be as high as possible or thermal resistivity as low as possible. In addition, the surface area of the heat management structure is also important. Large area allows heat spreading and improves heat dissipation to structures on the other side of the LED substrate 1.

Achieved thermal resistivity with implemented heat management structures has varied from 13 K/W to 95 K/W depending on the LED component and the heat management structures. Mentioned values are without LED package intrinsic thermal resistivity, which has varied from 17 K/W to 35 K/W.

Light management arrangement

Due to their very small foot-print and lambertian LISD, lighting modules based on LED arrays do not provide a uniform luminance but produce bright spots (Fig. 14A - left). Besides being aesthetically unappealing, these bright spots often produce multi-shadow effects and glaring. In order to provide a more uniform luminance, the LED array needs to be covered by a diffusing foil or plate located at a distance, t, larger than the LED pitch (Fig. 14A - right), which ultimately imposes a restriction on the minimum thickness of the module.

The light module of the device of the invention incorporates a light management arrangement 6 on the top side of the LED foil that efficiently homogenizes the LED light resulting in a uniform luminance across the top surface 17 of the light management arrangement 6. In addition, the light management arrangement 6 produces a non-glaring LIDS by reducing the light intensity at emission angles over 65° whilst keeping the spatial and angular colour variation within acceptable values.

Two light management arrangements are presented hereinafter and in the drawings. The first one is based on beam-shaping microstructured pixels and the second exploits the recursion mode of Bright Enhancement Foils, BEFs, and the high diffuse reflectance of the top 3 or bottom 2 surface of the LED substrate 1 to mix the LED light inside the thin module cavity.

Beam-shaping microstructured pixels:

This first light management arrangement 6 architecture is represented in Fig. 4. A LED foil with N LEDs 4 illuminates a thin, highly transparent film 10 which contains a collection on N microstructured pixels 1 1 each with its geometrical centre aligned to that of the corresponding LED. The microstructured pixels 1 1 are replicated onto the top surface of the foil 10 opposite to its bottom surface 6". The lambertian beam produced by each LED is shaped into a batwing LISD by the microstructured pixels 1 1. The modified beams produce a more uniform irradiance over the bottom surface 122 of the CCF 12. As a result, the luminance of the down- converted white light is highly uniform over the top surface 123 of the CCF 12.

Due to the nature of the CCF 12, the down-converted light is emitted with a lambertian LISD. A glare-free LISD is then achieved by locating an antiglare foil 13 somewhere above the top surface 123 of the CCF 12. Preferably CFF 12 and antiglare foil 13 are in physical contact but optically de-coupled. Luminance uniformity:

The so-called inverse cosine-cubed LISD, shown in Fig. 15, is known to produce highly uniform irradiance levels on planar targets compared to lambertian sources (see Fig. 14B). A general feature of this LISD is that the maximum light intensity is not emitted normal to the LED but at a certain higher angle. These classes of LISDs are often referred to as batwing. Uniform irradiance can be produced by LED arrays over nearby plane targets if the lambertian LISD of each LED is shaped into a batwing LISD.

Thin flexible transparent substrates with replicated periodic microstructures have beam shaping properties. Well-known examples are the BEFs used in displays to direct the light produced by the backlight unit in the forward direction, i.e. to enhance the luminance towards the viewer.

The opposite effect, the (angular) broadening of a lambertian LISD is far more complex using periodic microstructures as exemplified in Fig. 5 using prismatic structures. Whereas rays emitted close to the normal 16 are refracted towards higher angles 16' by the microprisms 1 1 replicated onto the surface of a transparent foil 10, the opposite effect occurs for rays incident at wide angles 14 and 15 resulting in the formation of bright spots on the on the top surface 17 of the Light management arrangement 6.

This limitation can be overcome by restricting the effect of the microstructures to a limited range of incident angles. As sketched in Fig. 6 this can be achieved by limiting the presence of the microstructures to small areas, i.e. pixels 1 1. As a result, rays incident close to the normal 16 are refracted to higher angles 16' whereas the wide incident angles arrays 14, 15 are transmitted with no change in their direction.

The resulting LISD depends on the particular geometry of the microstructures, the optical properties (n, k) of the foil 10 and the microprisms material as well as on the aperture of the pixels 1 1 seen from the LEDs 4; the latter being determined by the area of the pixels their distance, t, to the corresponding LED.

Optical simulations have shown that batwing LISDs (Fig. 16) can be achieved using 2D microprisms with a base angle beta, between 70 and 75° (Fig. 7) and limited to areas between 10-100 times larger than the emitting area of the LEDs and located at a distance between 0.5-3 times the largest dimension of the emitting LED chip. The refractive index of the substrate and prims materials is between 1.3 and 2.0 and best results are observed with refractive indexes between 1.4 and 1.65.

The predicted luminance uniformity of a 600x600mm² lighting module using the described light management arrangement was determined by calculation, for different module thicknesses and LED pitch. In the optical simulations that led to these results all the LEDs were assumed identical.

Light mixing via Bright Enhancement Foils (BEFs):

Uniform luminance can also be obtained using a stack of three optical foils

(see Fig. 8-10 comprising a surface diffuser 18 and two prismatic BEFs 19, 20 rotated 90° with respect to one another.

Such assembly of prismatic BEFs is widely applied in the display field to squeeze the luminance into a narrow solid angle towards the observer. However, when used in direct-lit LED modules (Fig. 8) it produces, in addition to a highly directional glare-free LISD (Fig. 17A), a highly uniform luminance distribution (Fig.

17B).

Alternatively, the CCF 12 can also be located in between the thin-film diffuser

18 and the BEF 19 as shown in Fig. 9.

Optical simulations predict that the most uniform luminance distribution is achieved by microprisms with a base angle beta between 40 and 50°.

The observed luminance uniformity arises to a large measure from the light recycling via the recursion mode of the prims. For a perfect specular reflecting LED substrate a marked minimum occurs at 45°. In this case, the rays emitted in the forward direction (most in lambertian sources) enter the recursion mode, are

(specularly) reflected back to the recursion mode of the BEF and eventually absorbed by the LED substrate, the diffuser, or the BEFs.

For a diffusing LED substrate, these rays are reflected in all directions and have the chance via the emission and high-angle modes. Accordingly, the optical efficiency increases substantially with respect to the specular case. For a diffusing reflector, the reflectance plays a key role in determining the optical efficiency of the module. The role of the diffuser in the light management arrangement stack:

The particular choice of the diffuser is non-trivial. Although one may expect that the use of a diffuser with a higher hiding power (typically associated with lower optical efficiencies) would produce a more uniform luminance, experimental tests have demonstrated that this does not always hold true.

The inventors have benchmarked the diffuser with another commercial thin- film diffuser, 8B28 from Lexan. The results are given in Fig. 18 and Table 1.

Table 1. Radiant and photometric characteristics of a LED module with different combinations of LM foils.

Although the Lexan 8B28 diffuser (diffuser A) gives, on its own, higher luminance uniformity than our diffuser (diffuser B), when combined with the two BEFs, the opposite is observed. In addition to the higher luminance uniformity, the light management arrangement 6 proposed in the present invention is more efficient.

Benchmarking against a standard light management arrangement:

Standard direct-lit LED modules comprise a rigid board of white LEDs that illuminate a standard diffuser plate and often completed with an anti-glare plate or foil. Standard diffusers are few millimeters thick plates of plastic with scattering particles embedded. An example of such plates is the Makrolon DX-NR diffuser from Bayer (Now Covestro).

We have benchmarked the light management arrangement stack 6 of the present invention to the combination of a Makrolon DX-NR cool diffuser (3mm thick) and a BrightView C-GC90 anti-glare foil. The angular response of and LED module equipped with the two different Light management arrangements was also measured and the experimental results are shown in Fig. 19 and 20 and summarized in Table 2.

Although the benchmark light management arrangement is more efficient, the light management arrangement 6 of the invention gives more directional LISD and more uniform luminance. Also, whereas the benchmark uses a 3mm thick rigid plate, the Light management arrangement stack 6 a thin (less than 0.5mm) flexible structure.

Table 2. Radiant and photometric characteristics of the Light management arrangement in the invention compared to the benchmark Light management arrangement.

Note: The higher LU in Table 2 (86.3%) for the Light management arrangement of the invention compared with that in Table 1 (79.4%) arises from the different module thickness used in both comparisons. In the comparison with Lexan 8B28, the distance between the LEDs and the LM stack is 18mm. However, to compensate for the 3mm thickness of the Makrolon diffuser, the LM stack was located at 21 mm.

Preferred LED layout:

As represented in Fig.13, the LED pitch, i.e. the distance between each LED and its first neighbours is a key aspect when high spatial luminance and colour uniformity is pursued. With respect to the spatial colour uniformity, the distance between LEDs of different colours is also important. Accordingly, in the preferred LED layout the LEDs are spatially distributed forming clusters of closely-packed white, red/orange, green/lime LEDs arranged following a hexagonal pattern.

Colour changing foils (CFFs):

A longer lifetime and a very good energy efficiency are exhibited by most light- emitting diodes (LEDs). LEDs can be produced in different colours. To broaden the emission spectra radiation conversion phosphors are applied, which may consist, for example, of cerium-doped yttrium aluminum garnet, absorb a certain proportion of the blue light and emit longer-wave light with a broad emission band, such that the mixing of the transmitted blue light and of the emitted light gives rise to white light.

However the light is perceived by many humans to be less natural and less pleasant than sunlight or light from incandescent lamps.

Organic fluorescent colorants can in principle produce "good colour" out of any LED by virtue of their broad emission bands. As their stability is limited to withstand the thermal and radiative stresses in the case of direct arrangement on the LED chip the colour converter (also referred to simply as "converter"), which generally comprises a carrier and a polymeric coating, is placed at a certain distance away from the LED chip. Such a structure is referred to as "remote phosphor", as for example described in WO 20121 13884. The spatial distance between the primary light source, the LED, and the colour converter reduces the stress resulting from heat and radiation. The molecular structure can be designed such that the spectral peak position can be tuned. Examples of suited organic luminescent materials based on perylen derivatives are marketed by BASF under the Name Lumogen. However to reach high CRI's the composition of such films needs to be adjusted to the design of the final module.

The use of organic fluorescent dyes in these converters offers various advantages. Firstly, organic fluorescent dyes give a much higher yield due to their substantially higher mass-specific absorption, which means that considerably less material is required for efficient radiation conversion than in the case of inorganic radiation converters. Secondly, they enable good colour reproduction and are capable of producing pleasant light. Furthermore, they do not require any materials comprising rare earths, which have to be mined and provided in a costly and inconvenient manner and are only available to a limited degree. It is therefore desirable to provide colour converters for LEDs which comprise suitable organic fluorescent dyes and have a long lifetime.

Intelligence:

The final aim is to couple the light emitted upwards by the module to the light sensor system 7 using a polymer light guide in order to measure the light output, especially the chromaticity coordinates and the correlated colour temperature (see concept schematics in Fig. 1 1. A low-cost polymer optical fibre 71 is located above the light management arrangement 6 as for example below the light management arrangement. The fibre 71 is connected to the sensor 7 which can then be located almost arbitrarily in the module. Due to the fact that the fibre diameter is 1 mm, all mechanical tolerances are relaxed and low-cost mechanical components can be used.

This concept is shown in more detail in Fig. 12A. The core of the polymer optical fibre 71 is embedded in a thin polymer slab 72 which is mounted under the frame of a module 78. Guided by its fibre jacket 73, the fibre is connected to the light sensor 74 by a fibre receptacle 75 and the light sensor 74 mounted on a board 76. The electronics including the LED drivers 77 can be positioned upright in the frame as sketched or flat under the frame next to the fibre coupling slab.

This sensor system 7 is able to detect the actual colour (chromaticity point) of the emitted light eL, and this data is used as an input for the cost effective microcontroller 8 (fig. 8). The feedback system adjusts the driving currents delivered to LEDs with different wavelengths in a way that the colour of the spectrally tunable luminaire is kept constant over time and despite of temperature changes that cause wavelength and intensity drift of LEDs.

Alternatively, the light sensor system 7 may also be arranged as shown in Fig. 12B, where the light management arrangement 6 sits on to of module 78 accomodating the sensor system components.

Claims

1. A light-emitting device comprising:

- a light module comprising :

o a top wall forming a light reflecting component and bottom wall forming a light emitting component,

o a flexible substrate (1 ) forming said top wall and having at least one reflective surface (2, 3) facing towards the bottom wall, o an array of light emitting components (4) arranged on said flexible substrate such that said components emit, in operation, a divergent light beam towards the bottom wall

o a light management arrangement (6) forming the bottom wall in front of the array of light emitting components (4), said bottom wall being transparent to visible light and comprising a light receiving surface (6") and a light emitting surface (6') parallel to each other,

- a power management unit configured to provide electrical energy to the light emitting components (4) through electrical circuits (34) arranged in or on the flexible substrate (1 ) at least,

- a light sensing device comprising a light sensor (74) and a light collecting component optically connected to the light sensor and arranged outside the module vis a vis the light emitting surface of the module to collect light beams emitted from the light emitting surface to direct at least part of it towards the light sensor,

- a control device configured for receiving input signals from the light sensor and delivering input signals to the power management unit in order to set the total luminous flux and colour coordinates of the light emitted by the module at its light emitting surface according to preset values recorded in a memory of the control device.

2. The light-emitting device according to claim 1 , wherein the flexible substrate is a foil comprising heat management physical structures in contact with the light emitting components.

3. The light-emitting device according to clainn 1 or 2, wherein the flexible substrate comprises heat management physical structures comprising ceramic and/or metallic foil parts embedded in the flexible substrate, preferably inserted in opening arranged in said substrate.

4. The light-emitting device according to any of claims 1 to 3, wherein the flexible substrate comprises printed electronic circuitry on a surface thereof and connecting the light emitting components to the power management unit.

5. The light-emitting device according to any of claims 1 to 4, wherein at least one surface of the flexible substrate is reflective in the 360-830 nm wavelength range.

6. The light-emitting device according to claim 5, wherein the flexible substrate comprises a reflective coating or laminate on at least one surface.

7. The light emitting device according to any of the preceding claims, wherein the light modeling arrangement comprises a plurality of flexible foils (10, 12, 13) forming a stack in the module bottom wall (6).

8. The light emitting device according to claim 7, wherein the plurality of foils comprises:

- a first foil (10) of transparent material containing optically active microstructures to achieve a highly uniform luminance across the module emitting surface, in the direction normal to the emitting surface of the module;

- a second foil (12) of transparent material containing optically active microstructures to produce a non-glaring angular radiation pattern;

- a third foil (13) having down-converting properties to provide down- converted light.

9. The light emitting device according to claim 8, wherein the first foil comprises microstructured pixels (1 1 ) having each a geometrical centre aligned with a geometrical centre of a light emitting component.

10. The light emitting device according to claim 8 and 9, wherein the second foil comprises a colour changing foil (12).

1 1. The light emitting device according to any of claims 8 to 10, wherein the third foil is an antiglare foil (13).

12. The light ennitting device according to claim 8, wherein the first foil (10) comprises a stack of three optical foils comprising a surface diffuser (18) and two prismatic bright enhancement foils (19 and 20) rotated 90° with respect to one another.

13. The light emitting device according to claims 10 and 12, wherein the second foil comprises a colour-changing foil (12) is disposed between the surface diffuser (18) and the two prismatic bright enhancement foils (19, 20)..

14. The light emitting device according to any of claims 1 to 13, wherein the light collecting component is arranged towards the emitting surface (6') of the module and has a cross-section which is smaller than said emitting surface.

15. The light emitting device according to any of claims 1 to 14, wherein the light collecting component is a waveguide component, preferably an optical fiber.

16. The light emitting device according to any of claims 1 to 15, wherein the light emitting components are LEDs (4).

17. The light emitting component according to claim 16, wherein the LEDs comprises blue and/or cool white LEDs in pumping mode and red/amber and/or green/lime LEDs in tuning mode, said LEDs being distributed regularly onto a top surface (3) of the flexible substrate (1 ).