FIELD OF THE INVENTION

The present invention generally relates to the field of wavelength converting screens for lighting devices. BACKGROUND OF THE INVENTION

Wavelength converting materials, such as phosphors, are generally used in lighting devices to adjust the wavelength of light emitted from light sources. Light having a wavelength within a blue spectrum may for instance be converted into light having a longer wavelength within a yellow spectrum by passing through a phosphor screen. The desired emission colors and color renditions are usually provided by phosphor screens comprising mixtures of green and red emitting materials. Such phosphor screens are normally realized with solid or liquid transparent matrices in which phosphor particles are arranged. A problem with such solid matrices is that the difference between the refractive index of the matrix and the refractive index of the phosphor material is substantial. This refractive index difference results in light scattering, which reduces the efficiency of the lighting device. The availability of suitable solid transparent materials having refractive indices matched to the phosphor particles in the matrix is limited. Liquid matrices, on the other hand, provide a wide variety of refractive indices but may give rise to irregular wavelength converting characteristics across the screen.

SUMMARY OF THE INVENTION

Thus, there is a need for providing alternatives and/or new devices that would overcome, or at least alleviate or mitigate, at least some of the above mentioned drawbacks. It is with respect to the above considerations that the present invention has been made. An object of the present invention is to provide an improved alternative to the above mentioned technique and prior art.

More specifically, it is an object of the present invention to provide a screen for a lighting device, the screen having improved wavelength converting characteristics. It is also an object of the present invention to provide a method of manufacturing such a screen. These and other objects of the present invention are achieved by means of a screen and a method of manufacturing such a screen having the features defined in the independent claims. Preferable embodiments of the invention are characterized by the dependent claims.

Hence, according to a first aspect of the present invention, a screen for a lighting device comprising a light source arranged to emit light towards the screen is provided. The screen comprises two at least partly transparent sheets opposing each other, and a structure arranged between the two sheets. The structure provides a plurality of compartments having sidewalls extending from one of the sheets to the other sheet. Further, at least one of the compartments encapsulates fluid (such as liquid or gas) and wavelength converting particles.

According to a second aspect of the invention, a method of manufacturing a screen for a lighting device comprising a light source arranged to emit light towards the screen is provided. The method comprises providing an at least partly transparent first sheet and providing a structure at the first sheet. The structure provides a plurality of compartments having sidewalls extending from the first sheet. Further, at least one of the compartments is filled with fluid and wavelength converting particles and an at least partly transparent second sheet is provided at the structure such that the fluid and the wavelength converting particles are encapsulated in the compartment, the second sheet opposing the first sheet.

According to a third aspect of the present invention, a lighting device is provided. The lighting device comprises a light source and a screen according to the first aspect of the invention, wherein the screen is arranged to receive and transmit light emitted from the light source.

The present invention is based on the idea of dividing a fluid matrix of a screen into compartments (or containers), whereby the wavelength converting particles are prevented from being settled in one portion of the screen due to gravity. Thus, the present invention provides a compartmented (transparent) carrier sheet which reduces problems related to gravity induced lateral separation of wavelength converting particles and fluid over large areas.

The present invention is advantageous in that wavelength converting characteristics can be made more regular across the screen while the screen can be arbitrary mounted in any orientation without the wavelength converting characteristics being affected by gravity. Further, the compartments provide a structure that allows accurate control of the conversion strength over a large area, thereby obtaining uniform color distribution over the screen. Further, the sidewalls of the compartments allow light manipulation for increasing light extraction efficiency and light mixing efficiency over the screen.

Further, the present invention is advantageous in that the structure, which is compartmented and sandwiched between the two transparent sheets, makes the screen rigid (torsion resistant) and robust, whereby the risk of damage (e.g. breaking) is reduced as compared to prior art techniques wherein a single container is used as a fluid matrix. Further, the robust screen can withstand large pressure increases which may arise due to changes in temperature of the fluid (e.g. incompressible liquid) in the screen. Further, the fluid efficiently facilitates heat transport to the sidewalls by convection in the compartment.

Further, the present invention is advantageous in that inexpensive fluids suitable to be used in the screen (i.e. encapsulated in the compartments) are available, whereby material costs for the screen can be reduced compared to if a solid matrix is used. Further, the present invention is advantageous in that suitable fluids are available in a wide variety of refractive indices, whereby the possibility to match the refractive index of the fluid with the refractive index of the wavelength converting particles is facilitated.

Moreover, the present invention is advantageous in that the compartmented screen can be provided by simply assembling a few parts. Hence, the manufacturing of the screen of the lighting device according to the present invention is relatively simple, whereby manufacturing costs are reduced. Moreover, the manufacturing of the screen requires less accuracy as compared to prior art techniques wherein wavelength converting material has to be accurately deposited in a regular pattern on/in a solid matrix (or substrate). With the present invention, the fluid and wavelength converting particles may be mixed in a desired concentration and then distributed into the compartments.

Because of the manufacturing simplicity and the robustness of the screen, the present invention may be advantageous for large-scale wavelength converting screens which, hence, may be provided in a relatively simple and cheap way.

According to an embodiment of the present invention, the compartments may form a tessellation pattern (in a cross-sectional view) and/or a honeycomb structure, which is advantageous in that such a pattern/structure provides the compartments to seamlessly cover (without gaps) a planar surface. Hence, a great share of the screen can be covered by wavelength converting portions. In embodiments of the present invention, the tessellation pattern may comprise a continuous pattern of squares, equilateral triangles, hexagons, or any other pattern reducing gaps between the compartments. The present embodiments are advantageous in that they provide a continuous coverage of wavelength converting portions over the screen. Further, the compartments may be arranged in abutment to each other, which likewise reduces gaps between the compartments and contributes to a full (or at least almost full) coverage of wavelength converting portions across the screen.

According to an embodiment of the present invention, several of the plurality of compartments may contain substantially equal amounts of wavelength converting particles, which is advantageous in that wavelength converting characteristics are

substantially constant (uniform) across the screen. Hence, tinting of light in certain areas of the screen is reduced and color uniformity becomes independent of viewing angles.

In an embodiment of the present invention, the structure (i.e. the sidewalls of the compartments) may be transparent and/or reflective. A transparent structure is advantageous in that it facilitates light extraction from the screen and increases the efficiency of the lighting device. In case of scattering of light in the compartments, the sidewalls provide an efficient extraction channel. In case of (at least almost) fully transparent wavelength converting particles in the compartments, the reflectance of the sidewalls can be used to prevent light trapping in the screen, which is advantageous in that it enhances light extraction and color uniformity across the screen.

According to an embodiment of the invention, the refractive index of the fluid (which preferably is at least partly transparent) and the refractive index of the wavelength converting particles may be substantially equal (or matched) and preferably differing by less than 0.2, more preferably by less than 0.1, and even more preferably by less than 0.01. In other words, the refractive index of fluid and the refractive index of the wavelength converting particles are preferably matched. The present embodiment is advantageous in that a sufficiently low amount of scattering for extracting converted light from the screen is provided. In particular, in case of large amount of particles in the light conversion layer due to low absorption strength of light from the light source, the refractive index of the fluid and the refractive index of the wavelength converting particles may preferably be substantially equal.

In an embodiment of the invention, the wavelength converting particles may comprise phosphor particles. A wide variety of phosphors having different wavelength converting characteristics is available and, hence, it is possible to select a desired specific wavelength range into which the light is to be converted. The selection may e.g. be based on the wavelength of the light emitted by the light source and the desired wavelength/color of the light passed through the screen. In other words, a phosphor having absorption

characteristics suitable for absorbing the light from the light source and emission

characteristics suitable for emitting the desired color may be selected. In particular, the present embodiment is advantageous in that particles of weakly light absorbing phosphor with high quantum efficiency and the desired spectral characteristics may be used. The photometric efficiency of a lighting device with a given light source (e.g. a blue light source) depends on the spectral characteristic of the phosphor converting particles. In case of a red line emitter, the lumen equivalent can be more than 380 lumen per Watt, with a color rendering index (CIE publication 13.3) close to 90. Examples are materials activated with Eu ³⁺ and Mn ⁴⁺ ions, which are weakly absorbing materials (for 400 - 490 nm excitation) with narrow red emission.

Weakly absorbing phosphors result in low light output due to a limited absorption of the exciting light by the phosphor. To increase the light output, an increase in the layer thickness of the phosphor has to be made (i.e. the concentration of phosphor and/or the thickness of the screen has to be increased). According to an embodiment of the present invention, such an increase may be made while an increased scattering is avoided since the refractive indices of the fluid and the wavelength converting particles are allowed to be substantially equal. Hence, the reduction of the output from the screen while using weakly absorbing phosphors is limited.

In an embodiment of the present invention, an inner side of at least one of the compartments may be porous, which is advantageous in that a pressure increase caused by a rising temperature in the compartment (e.g. under effect of the light source being turned on) is reduced since the fluid may be absorbed by the pores present in the sidewalls as it expands.

Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following. In particular, it will be appreciated that the various embodiments described for the lighting device are all combinable with the method as defined in accordance with the second aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, in which:

Figure 1 shows a lighting device according to an embodiment of the present invention;

Figure 2 shows an exploded view of a screen according to an embodiment of the present invention;

Figure 3 shows compartment patterns/structures according to embodiments of the present invention;

Figure 4 shows a honeycomb structure of the compartments and an enlarged view of a compartment according to an embodiment of the present invention;

Figure 5 shows a method of manufacturing a lighting device according to an embodiment of the present invention; and

Figure 6 shows an embodiment of the lighting device according to each step in the method shown in Figure 5.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to Figure 1, there is shown a lighting device 1 according to an embodiment of the present invention. The lighting device 1 comprises a light source 10 adapted to emit light of a certain wavelength range. For example, the lighting device may comprise one or more light emitting diodes, LEDs, or fluorescent lamps. Advantageously, the light source includes LEDs with peak emission in the range of 250-490 nm, preferably in the range of 400-470 nm. The lighting device 1 further comprises a screen 100 arranged to receive and transmit light from the (one or a plurality of) light source 10 (i.e. to let light from the light source 10 pass through the screen 100) in order to convert the wavelength of the light emitted from the light source 10 into a different wavelength range. Such an arrangement may for instance be provided for converting wavelengths within a blue spectrum (roughly 400-500 nm) into wavelengths within a warm yellow spectrum (roughly 550-600 nm).

Figure 2 shows an exploded view of a screen 200 according to an embodiment of the present invention. The screen 200 comprises two sheets 210, 240 being at least partly transparent for allowing light transmission. Between the two sheets, which oppose (face) each other, a structure 220 is provided. In other words, the two sheets 210, 240 are arranged opposite to each other relative to the structure 220 or a separate sheet comprising the structure 220. The structure is provided by wall portions forming a grid (or net), whereby the structure 220 provides a plurality of compartments 221 having sidewalls 222 extending from one of the sheets 210 to the other 240. Hence, the two sheets 210, 240 and the structure 220 together provide a transparent compartmented carrier matrix. It will be appreciated that the sheets 210, 240 and the structure 220 may be separate parts or provided in the same piece, e.g. molded in the same piece or adhered to each other. The compartments may form a tessellation pattern, for example comprising squares 32 or equilateral triangles 31, as shown in Figure 3. Preferably, the compartments are arranged in abutment to (i.e. adjacent) each other for providing wavelength converting portions (at least almost) all over the screen.

If weakly absorbing conversion particles are used, the depth of the

compartments (or the thickness of the screen) may be in the range of 1-10 mm. The projected area of the sidewalls on the sheets may be smaller than 20 percent of the total surface of the screen. The concentration of wavelength converting particles (e.g. phosphor particles) in the fluid may range from 1 percent to 30 percent by volume.

With reference to Figure 4, another embodiment of the present invention is described, wherein the compartments 400 forms a honeycomb structure 40 and the tessellation pattern comprises hexagons. The present embodiment is advantageous in that the honeycomb structure provides stability and robustness of the screen. A compartment 400 of the structure with its sidewalls 430, a portion 410 of one of the sheets and a portion 420 of the other sheet form an enclosure in which a fluid, preferably a liquid 440, and wavelength converting particles 450 are arranged. The wavelength converting particles 450 are dispersed in the liquid 440. Preferably, a substantially equal amount of wavelength converting particles are distributed in each one the plurality of compartments in the screen and the concentration of wavelength converting particles in the liquid may be selected to provide a desired light absorption strength. Further, the number of individual compartments 400 may be sufficiently high and the size of the compartments 400 may be sufficiently small to avoid tinting of the emitted light in certain areas due to gravity induced separation of the wavelength converting particles 450 as obtained with too big/few compartments, thereby ensuring color uniformity independent of viewing angles. However, for ease of fabrication, the number of

compartments 400 may be kept as low as possible and the size of the compartments 400 may be kept as big as possible without lowering the uniformity of the wavelength converting characteristics across the screen. For example, the surface area of the screen may be in the range of 20-1000 cm ² , the depth of the compartments from 1-10 mm and the aspect ratio of the side walls (width/height) between 1 and 100. Further, the average diameter of the compartments may be greater than one time the compartment depth and less than five times the compartment depth.

The liquid 440 is transparent (or at least semitransparent) for allowing transmission of light through the screen and is preferably selected to have a matched refractive index in relation to the wavelength converting particles 450. The refractive indices of the liquid 440 and the wavelength converting particles 450 are preferably substantially equal (or preferably differing < 0.2) in a part of the visible spectrum where the phosphor does not absorb, such as the part where the phosphor emits its (secondary) light. For example, a liquid having the refractive index 1.36 and phosphor having the refractive index 1.34 may be selected. The refractive index of the liquid 440 is preferably selected to provide a required amount of scattering for extracting the converted light from the screen. Further, the liquid 440 may be non-toxic, non-flammable and have relatively high boiling point and low vapor pressure. The wavelength converting particles 450 may be selected with respect to their wavelength converting characteristics. The wavelength converting particles may for instance be phosphor particles and in particular, weakly light absorbing phosphor particles having high quantum efficiency. Examples of phosphors which may be used are manganese-doped potassium hexafluorsiliciate (refractive index: 1.34), manganese-doped sodium

hexafluorosilicate (refractive index: 1.31) and manganese-doped caesium hexafluorosilicate. Examples of liquids which may be used are silicone (refractive index: 1.4-1.6), ethanol (C ₂ H ₅ OH, refractive index: 1.36), ethylene glycol (refractive index: 1.44) and acetonitrile (refractive index: 1.35). In the above examples, the values of the refractive indices are given for a wavelength of 589 nm

When primary light is emitted from the light source, it passes through the screen, wherein light is scattered in the liquid 440 by the wavelength converting particles 450 in the compartments 400. The wavelength converting particles 450 at least partially absorb the light and convert it into another wavelength range, whereupon the secondary light (or at least a part of the light) of the converted wavelength range is emitted from the screen.

Alternatively, a mixture of primary and secondary light is emitted from the screen.

The structure and the sheets (i.e. the closed compartments) may be made e.g. in transparent (or at least semitransparent) plastics, which is advantageous in that such plastics are relatively inexpensive. Alternatively, the structure and the sheets may be made of glass or any other solid, at least partly transparent material. Further, the sheets may be rugged for scattering light out of the screen. With reference to Figures 5 and 6, a method of manufacturing a lighting device according to an embodiment of the invention will be described. Figure 5 shows a method 5000 comprising the steps of providing 501 an at least partly transparent first sheet 610 and providing 502 a structure 620 on the first sheet 610. The structure 620 provides a plurality of compartments 621 having sidewalls 622 extending from the first sheet 610. The first sheet 610 and the structure 620 may be separate parts joined together, or molded in a single piece. The method 5000 further comprises the step of filling 503 the

compartments 621 with liquid 631 and wavelength converting particles 632. The liquid 631 and the wavelength converting particles 632 may be mixed before they are distributed into the compartments 621. Alternatively, the liquid 631 and the wavelength converting particles 632 may be distributed separately into the compartments 621. The method 5000 further comprises the step of providing (or attaching) 504 an at least partly transparent second sheet 640 at the structure 620 such that the second sheet 640 opposes the first sheet 610. In other words, the second sheet 640 is arranged on top of the compartmented structure 620 such that the liquid 631 and the wavelength converting particles 632 are enclosed in the compartments 621. Preferably, the second sheet 640 is attached to the structure 620 such that no air (or gas) bubbles are enclosed in the compartments 621, and in a sealing and gas tight manner for avoiding liquid leakage and evaporation.

Further, a light source 650 (or a plurality of light sources) may be provided 505 and the screen 660 comprising the compartments 621 may be arranged (or mounted) 506 such that it is able to receive and transmit light from the light source 650.

The wavelength converting screen according to the present invention may be used in new generation lighting devices but may also be used as an upgrade in already existing lighting devices or systems. Further, the screen may be used as a remote phosphor component in backlights, downlighters, TL (tube lighting) and GLS (general lighting service) retrofit lamps. Further, the lighting device according to the present invention may be used in office lighting, home lighting, spot lighting, theater lighting, fiber optics, displays, medical lighting, automotive applications etc.

While specific embodiments have been described, the skilled person will understand that various modifications and alterations are conceivable within the scope as defined in the appended claims. For example, the screen may comprise any suitable number of compartments of which some or several may be filled with fluid and wavelength converting particles. Moreover, different compartments may comprise different kinds of wavelength converting particles whereby light of different colors may be emitted from the screen. The light source or light sources assembled behind these different compartments or group of compartments may be arranged to emit primary light in a wavelength range commensurate with the absorption spectrum of the wavelength converting particles applied in the respective compartments.