Light generating system comprising an elongated luminescent body

Abstract

The invention provides an elongated luminescent body (100) comprising an elongated support (170) and a coating layer (180), wherein the elongated luminescent body (100) further comprises a body axis (BA), and a length parameter P of a body dimension perpendicular to the body axis (BA), wherein the length parameter P is selected from height (H), width (W) and diameter (D), wherein: —the elongated support (170) comprises a support material (171), a support material index of refraction n1, wherein the support material index of refraction n1 is at least 1.4, a support surface (172), and a support length (L1); —the coating layer (180) is configured on at least part of the support surface (172) over at least part of the support length (L1), wherein the coating layer (180) comprises a coating layer material (181), a coating layer index of refraction n2, wherein coating layer index of refraction n2 is at least 1.4, and a coating layer thickness (d1), wherein the coating layer material (181) has a composition different from the support material (171), wherein the coating layer material (181) comprises a luminescent material (120) configured to absorb one or more of UV radiation and visible light, and to convert into luminescent material light (8) having one or more wavelengths in one or more of the visible and the infrared; and —the support material (171) is transmissive for the luminescent material light (8), and (i) −0.2≤n1−n2≤0.2 and (ii) d1/P≤0.25 apply.

Claims

1. A light generating system comprising: a plurality of light sources configured to provide light source light; an elongated luminescent body comprising an elongated support and a coating layer, wherein the elongated luminescent body further comprises a body axis (BA), and a length parameter P of a body dimension perpendicular to the body axis (BA), wherein the length parameter P is selected from height (H), width (W) and diameter (D), wherein: the elongated support comprises a support material-, a support material index of refraction n1, wherein the support material index of refraction n1 is at least 1.4, a support surface, and a support length; the coating layer is configured on at least part of the support surface over at least part of the support length, wherein the coating layer comprises a coating layer material, a coating layer index of refraction n2, wherein coating layer index of refraction n2 is at least 1.4, and a coating layer thickness (d1), wherein the coating layer material has a composition different from the support material, wherein the coating layer material comprises a luminescent material configured to absorb one or more of UV radiation and visible light, and to convert into luminescent material light having a wavelength at a spectral maximum of the luminescent material light in one or more of the visible and the infrared; the support material is transmissive for the luminescent material light, and −0.2≤n1−n2≤0.2 and d1/P≤0.25 apply, wherein the coating layer or an outer layer configured over the coating layer, has a root mean square height Sq of at maximum 1/10 of a wavelength at a spectral maximum of the luminescent material light, and wherein the elongated luminescent body comprising one or more side faces, wherein the elongated luminescent body comprises a radiation input face and a radiation exit window, wherein the radiation input face is configured in a light receiving relationship with the plurality of light sources, wherein the luminescent material configured to absorb at least part of the light source light and convert into the luminescent material light, and wherein the radiation input face is configured perpendicular to the light exit window.

2. The light generating system according to claim 1, wherein the coating layer, or where available an outer layer configured over the coating layer, has a root mean square height Sq of at maximum 1/20 of a wavelength at a spectral maximum of the luminescent material light.

3. The light generating system according to claim 1, wherein −0.1≤n1−n2≤0.2, and d1/P≤0.2 applies.

4. The light generating system according to claim 1, wherein the length parameter P is selected from the range of 0.5-100 mm.

5. The light generating system according to claim 1, wherein the support material comprises one or more of a glass material, a single crystal, a ceramic material, and a polymeric material, and wherein the support material has a mean free path for a wavelength at a spectral maximum of the luminescent material light of at least 50 mm.

6. The light generating system according to claim 1, wherein the coating layer material has a mean free path MFP for a wavelength at a spectral maximum of the luminescent material light, wherein MFP≥MFP.sub.min, where
MFP.sub.min=c*L1*d1/P, and where c=0.25.

7. The light generating system according to claim 1, wherein the coating layer material comprises one or more of an organic dye and inorganic luminescent nanoparticles.

8. The light generating system according to claim 1, wherein the coating layer material comprises a matrix material and an inorganic luminescent material comprising particles having weight averaged particle sizes selected from the range of 0.1-20 μm, embedded in a matrix material, wherein the matrix material is selected from the group consisting of a glass material, a ceramic material, and a polymeric material.

9. The light generating system according to claim 8, wherein the matrix material has a matrix material index of refraction n21, and wherein the luminescent material has a luminescent material index of refraction n22, wherein 0.02≤n21−n22≤0.02.

10. The light generating system according to claim 1, wherein the support material comprises one or more of a glass material and a polymeric material, wherein the coating layer material comprises one or more of an organic dye, a glass material, and a polymeric material, and wherein the elongated luminescent body has a round, an oval, or a rectangular cross-section perpendicular to the body axis (BA).

11. The light generating system according to claim 1, wherein the elongated luminescent body is associated to a light extraction body or forms a monolithic body with a light extraction body.

12. The light generating system according to claim 1, wherein the circumferential surface of the elongated luminescent body is partially provided with a coating layer in an amount equal to or less than 60%.

13. The light generating system according to claim 1, wherein the elongated support further comprises an elongated core of a material with a thermal conductivity equal to or larger than 20 W m.sup.−1 K.sup.−1 and wherein the ratio P2/P1 is equal to or smaller than 0.5, with a length parameter P1 of a body dimension perpendicular to the body axis of the elongated support, length parameter P2 of a body dimension perpendicular to the body axis of the elongated core, and the length parameter P1, P2 is selected from height (H), width (W) and diameter (D).

14. A projection system comprising the light generating system according to claim 13.

15. A luminaire or a lamp comprising the system according to claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

(2) FIGS. 1a-1g schematically depict some aspects of the invention; and

(3) FIG. 2a schematically shows an embodiment of a cross section of configuration with single-sided illumination of luminescent rod. The inner sides of the cooling block(s) may be made reflective or covered by a mirror;

(4) FIG. 2b provides a schematic representation of single-sided concept;

(5) FIGS. 3a-3f schematically depict some embodiments of the elongated luminescent body;

(6) FIG. 4 schematically depict some embodiments of the coating layer; and

(7) FIG. 5 schematically depicts yet another embodiment of the light generating system;

(8) FIG. 6 schematically depicts yet another embodiment of the light generating system.

(9) The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(10) A light emitting device according to the invention may be used in applications including but not being limited to a lamp, a light module, a luminaire, a spot light, a flash light, a projector, a (digital) projection device, automotive lighting such as e.g. a headlight or a taillight of a motor vehicle, arena lighting, theater lighting and architectural lighting.

(11) Light sources which are part of the embodiments according to the invention as set forth below, may be adapted for, in operation, emitting light with a first spectral distribution. This light is subsequently coupled into a light guide or waveguide; here the elongated luminescent body. The light guide or waveguide may convert the light of the first spectral distribution to another spectral distribution and guides the light to an exit surface.

(12) An embodiment of the light generating system as defined herein is schematically depicted in FIG. 1a. FIG. 1a schematically depicts a light generating system 1000 comprising a plurality of solid state light sources 10 and a luminescent concentrator 5 comprising an elongated luminescent body 100 having a first face 141 and a second face 142 defining a length L of the elongated luminescent body 100. The elongated luminescent body 100 comprising one or more radiation input faces 111, here by way of example two oppositely arranged faces, indicated with references 143 and 144 (which define e.g. the height H), which are herein also indicated as edge faces or edge sides 147. Further the elongated luminescent body 100 comprises a radiation exit window 112, wherein the second face 142 comprises the radiation exit window 112. The entire second face 142 may be used or configured as radiation exit window. The plurality of solid-state light sources 10 are configured to provide (blue) light source light 11 to the one or more radiation input faces 111. As indicated above, they especially are configured to provide to at least one of the radiation input faces 111 a blue power W.sub.opt of in average at least 0.067 Watt/mm.sup.2. Reference BA indicates a body axis, which will in cuboid embodiments be substantially parallel to the edge sides 147. Reference 140 refers to side faces or edge faces in general.

(13) The elongated luminescent body 100 may comprise a ceramic material 120 configured to wavelength convert at least part of the (blue) light source light 11 into converter light 101, such as at least one or more of green and red converter light 101. As indicated above the ceramic material 120 comprises an A.sub.3B.sub.5O.sub.12:Ce.sup.3+ ceramic material, wherein A comprises e.g. one or more of yttrium (Y), gadolinium (Gd) and lutetium (Lu), and wherein B comprises e.g. aluminum (Al). References 20 and 21 indicate an optical filter and a reflector, respectively. The former may reduce e.g. non-green light when green light is desired or may reduce non-red light when red light is desired. The latter may be used to reflect light back into the elongated luminescent body or waveguide, thereby improving the efficiency. Note that more reflectors than the schematically depicted reflector may be used. Note that the elongated luminescent body may also essentially consist of a single crystal, which may in embodiments also be A.sub.3B.sub.5O.sub.12:Ce.sup.3.

(14) The light sources may in principle be any type of light source, but is in an embodiment a solid state light source such as a Light Emitting Diode (LED), a Laser Diode or Organic Light Emitting Diode (OLED), a plurality of LEDs or Laser Diodes or OLEDs or an array of LEDs or Laser Diodes or OLEDs, or a combination of any of these. The LED may in principle be an LED of any color, or a combination of these, but is in an embodiment a blue light source producing light source light in the UV and/or blue color-range which is defined as a wavelength range of between 380 nm and 490 nm. In another embodiment, the light source is an UV or violet light source, i.e. emitting in a wavelength range of below 420 nm. In case of a plurality or an array of LEDs or Laser Diodes or OLEDs, the LEDs or Laser Diodes or OLEDs may in principle be LEDs or Laser Diodes or OLEDs of two or more different colors, such as, but not limited to, UV, blue, green, yellow or red.

(15) The light sources 10 are configured to provide light source light 11, which is used as pump radiation 7. The luminescent material 120 converts the light source light into luminescent material light 8 (see also FIG. 1e). Light escaping at the light exit window is indicated as converter light 101, and will include luminescent material light 8. Note that due to reabsorption part of the luminescent material light 8 within the luminescent concentrator 5 may be reabsorbed. Hence, the spectral distribution may be redshifted relative e.g. a low doped system and/or a powder of the same material. The light generating system 1000 may be used as luminescent concentrator to pump another luminescent concentrator.

(16) FIGS. 1a-1b schematically depict similar embodiments of the light generating system. Further, the light generating system may include further optical elements, either separate from the waveguide and/or integrated in the waveguide, like e.g. a light concentrating element, such as a compound parabolic light concentrating element (CPC). The light generating systems 1000 in FIG. 1b further comprise a collimator 24, such as a CPC.

(17) As shown in FIGS. 1a-1b and other Figures, the light guide has at least two ends, and extends in an axial direction between a first base surface (also indicated as first face 141) at one of the ends of the light guide and a second base surface (also indicated as second face 142) at another end of the light guide.

(18) Reference 1100 refers to a light generating device comprising the light sources 10 and the elongated luminescent body 100, and optionally the light concentrating element 24. The light generating device has a radiation exit window 112 when there is no light concentrating element 24, and a radiation exit window 212 when there is a light concentrating element 24.

(19) The radiation exit window 112 is in optical contact, such as physical contact, with the light concentrating element 24, such as a CPC like light concentrating element (see also above). The CPC like) light concentrating element 24 has a radiation exit window 212. In embodiments, however, the elongated luminescent body and light concentrating element are essentially a single (monolithic) body. Then, the radiation exit window 212 of the light concentrating element may essentially be the same as the radiation exit window 112 as there is essentially no physical boundary between the elongated luminescent body and the (CPC like) light concentrating element 24.

(20) FIG. 1c schematically depicts some embodiments of possible ceramic bodies or crystals as waveguides or luminescent concentrators. The faces are indicated with references 141-146. The first variant, a plate-like or beam-like elongated luminescent body has the faces 141-146. Light sources, which are not shown, may be arranged at one or more of the faces 143-146 (general indication of the edge faces is reference 147). The second variant is a tubular rod, with first and second faces 141 and 142, and a circumferential face 143. Light sources, not shown, may be arranged at one or more positions around the elongated luminescent body. Such elongated luminescent body will have a (substantially) circular or round cross-section. The third variant is substantially a combination of the two former variants, with two curved and two flat side faces. In the embodiment having a circular cross-section the number of side faces may be considered unlimited (Co).

(21) In the context of the present application, a lateral surface of the light guide should be understood as the outer surface or face of the light guide along the extension thereof. For example in case the light guide would be in form of a cylinder, with the first base surface at one of the ends of the light guide being constituted by the bottom surface of the cylinder and the second base surface at the other end of the light guide being constituted by the top surface of the cylinder, the lateral surface is the side surface of the cylinder. Herein, a lateral surface is also indicated with the term edge faces or side 140.

(22) The variants shown in FIG. 1c are not limitative. More shapes are possible; i.e. for instance referred to WO2006/054203, which is incorporated herein by reference. The ceramic bodies or crystals, which are used as light guides, generally may be rod shaped or bar shaped light guides comprising a height H, a width W, and a length L extending in mutually perpendicular directions and are in embodiments transparent, or transparent and luminescent. The light is guided generally in the length L direction. The height H is in embodiments <10 mm, in other embodiments <5 mm, in yet other embodiments <2 mm. The width W is in embodiments <10 mm, in other embodiments <5 mm, in yet embodiments <2 mm. The length L is in embodiments larger than the width W and the height H, in other embodiments at least 2 times the width W or 2 times the height H, in yet other embodiments at least 3 times the width W or 3 times the height H. Hence, the aspect ratio (of length/width) is especially larger than 1, such as equal to or larger than 2, such as at least 5, like even more especially in the range of 10-300, such as 10-100, like 10-60, like 10-20. Unless indicated otherwise, the term “aspect ratio” refers to the ratio length/width. FIG. 1c schematically depicts an embodiment with four long side faces, of which e.g. two or four may be irradiated with light source light.

(23) The aspect ratio of the height H:width W is typically 1:1 (for e.g. general light source applications) or 1:2, 1:3 or 1:4 (for e.g. special light source applications such as headlamps) or 4:3, 16:10, 16:9 or 256:135 (for e.g. display applications). The light guides generally comprise a light input surface and a light exit surface which are not arranged in parallel planes, and in embodiments the light input surface is perpendicular to the light exit surface. In order to achieve a high brightness, concentrated, light output, the area of light exit surface may be smaller than the area of the light input surface. The light exit surface can have any shape, but is in an embodiment shaped as a square, rectangle, round, oval, triangle, pentagon, or hexagon.

(24) Note that in all embodiments schematically depicted herein, the radiation exit window is especially configured perpendicular to the radiation input face(s). Hence, in embodiments the radiation exit window and radiation input face(s) are configured perpendicular. In yet other embodiments, the radiation exit window may be configured relative to one or more radiation input faces with an angle smaller or larger than 90°.

(25) Note that, in particular for embodiments using a laser light source to provide light source light, the radiation exit window might be configured opposite to the radiation input face(s), while the mirror 21 may consist of a mirror having a hole to allow the laser light to pass the mirror while converted light has a high probability to reflect at mirror 21. Alternatively or additionally, a mirror may comprise a dichroic mirror.

(26) FIG. 1d very schematically depicts a projector or projector device 2000 comprising the light generating system 1000 as defined herein. By way of example, here the projector 2000 comprises at least two light generating systems 1000, wherein a first light generating system 1000a is configured to provide e.g. green light 101 and wherein a second light generating system 1000b is configured to provide e.g. red light 101. Light source 10 is e.g. configured to provide blue light. These light sources may be used to provide the projection (light) 3. Note that the additional light source 10, configured to provide light source light 11, is not necessarily the same light source as used for pumping the luminescent concentrator(s). Further, here the term “light source” may also refer to a plurality of different light sources. The projector device 2000 is an example of a light generating system 1000, which light generating system is especially configured to provide light generating system light 1001, which will especially include light generating system light 101.

(27) High brightness light sources are interesting for various applications including spots, stage-lighting, headlamps and digital light projection.

(28) For this purpose, it is possible to make use of so-called luminescent concentrators where shorter wavelength light is converted to longer wavelengths in a highly transparent luminescent material. A rod of such a transparent luminescent material can be used and then it is illuminated by LEDs to produce longer wavelengths within the rod. Converted light which will stay in the luminescent material such as a doped garnet in the waveguide mode and can then be extracted from one of the surfaces leading to an intensity gain (FIG. 1e).

(29) High-brightness LED-based light source for beamer applications appear to be of relevance. For instance, the high brightness may be achieved by pumping a luminescent concentrator rod by a discrete set of external blue LEDs, whereupon the phosphor that is contained in the luminescent rod subsequently converts the blue photons into green or red photons. Due to the high refractive index of the luminescent rod host material (typically 1.8) the converted green or red photons are almost completely trapped inside the rod due to total internal reflection. At the exit facet of the rod the photons are extracted from the rod by means of some extraction optics, e.g. a compound parabolic concentrator (CPC), or a micro-refractive structure (micro-spheres or pyramidal structures). As a result, the high luminescent power that is generated inside the rod can be extracted at a relatively small exit facet, giving rise to a high source brightness, enabling (1) smaller optical projection architectures and (2) lower cost of the various components because these can be made smaller (in particular the, relatively expensive, projection display panel).

(30) FIG. 1f schematically depicts an embodiment of a luminaire or light generating system 1000 (or other type of lighting device) comprising the light generating system 1000. The luminaire 1200 provide light which may—in a control mode of the luminaire—comprise the lighting generating system light 1001.

(31) The rod and the collimator 24, such as a CPC, may be made from the same material either in one part, as schematically depicted in FIG. 1g or of different parts, as schematically depicted in FIG. 1b. The support and the collimator 24 may e.g. manufactured by pressing glass or molding of silicone. Reference 170 indicates the support (without coating layer). When the support and collimator, such as the CPC 24, are separate parts, they may especially also be of the same material. This may allow an easy gluing or direct-bonding of the connection.

(32) FIGS. 2a-2b schematically depict embodiments of a light generating system 1000 comprising a light source 10 configured to provide light source light 11 and an elongated luminescent body 100 having a length L (see FIG. 2b).

(33) As indicated above, the elongated luminescent body 100 comprises (n) side faces 140, here 4, over at least part of the length. The (n) side faces 140 comprise a first side face 143, comprising a radiation input face 111, and a second side face 144 configured parallel to the first side face 143, wherein the side faces 143, 144 define a height h.

(34) As indicated above, the elongated luminescent body 100 further comprises a radiation exit window bridging at least part of the height h between the first side face 143 and the second side face 144 (see especially FIG. 1a). The luminescent body 100 comprises a garnet type A.sub.3B.sub.5O.sub.12 luminescent material 120 comprising trivalent cerium, wherein the garnet type A.sub.3B.sub.5O.sub.12 luminescent material 120 is configured to convert at least part of the light source light 11 into converter light 101.

(35) Further, the light generating system 1000 comprises one or more heat transfer elements 200 in thermal contact with one or more side faces 140 and a reflector 2100 configured at the second side face 144 and configured to reflect light source light 11 escaping from the elongated luminescent body 100 via second face 144 back into the elongated luminescent body 100.

(36) The one or more heat transfer elements 200 are especially configured parallel to at least part of one or more of the side faces 140 over at least part of the length of the elongated luminescent body 100 at a shortest distance (d1) from the respective one or more side faces 140. The shortest distance d1 is especially 1 μm≤d1≤100 μm.

(37) As shown in FIGS. 2a-2b, the one or more heat transfer elements 200 comprise one or more heat transfer element faces 201 directed to one or more side faces 140. As shown in these schematic drawings, the one or more heat transfer elements 200 are at least in thermal contact with all side faces 140 other than the first side face 143. Further, as also shown in these schematic drawings, the one or more heat transfer elements 200 may be configured as a monolithic heat transfer element 220. In embodiments, this monolithic heat transfer element 220 is configured in thermal contact with a support 240 for the light source 10.

(38) A heat transfer element face 201 of the one or more heat transfer element 200 directed to the second face 144 comprises the reflector 2100. Here, all faces 201 directed to the luminescent body 100 comprise such reflector 2100.

(39) FIG. 2b schematically depict another embodiment of the monolithic heat transfer element 220, including a slit 205 configured to host the luminescent body 100. The light sources 10 may be provided as LED bar. The monolithic heat transfer element 220 is used for cooling of the luminescent body 100.

(40) The optional intermediate plate, indicated with reference 250, may serve as a spacer to keep the luminescent body at the desired distance from the light sources and may also serve as a reflector for the light that escapes from the luminescent body side faces. As an alternative, the spacer could be integrated with the one or more heat transfer element 200, especially a top one or more heat transfer element 200 (such as a top cooling block).

(41) In FIGS. 2a-2b, the one or more heat transfer elements are configured within a circle section of at least 180°, here in fact about 270°.

(42) As shown above, the light generating system 1000 comprises in embodiments a plurality of light sources 10 configured to provide light source light 11 and an elongated luminescent body 100 comprising one or more side faces 140, the elongated luminescent body 100 comprising a radiation input face 111 and a radiation exit window 112, wherein the radiation input face 111 is configured in a light receiving relationship with the plurality of light sources 10, wherein the elongated luminescent body 100 comprises luminescent material 120 configured to convert at least part of light source light 11 (received at the radiation input face 111) into luminescent material light 8.

(43) Amongst others, herein a high lumen density (HLD) source is proposed with potentially a 46% improved efficiency as compared to a reference HLD source and without use of a (cerium comprising) garnet rod.

(44) In embodiments, the HLD comprises a transparent round rod or round tube with a transparent thin layer of phosphor-coating on the cylindrical side. As in existing solutions, a mirror can be placed on one outer rod end and an outcoupling optical part like a CPC on the other. The phosphor coating may be applied on the cylindrical side, either fully around or just at a part of the cylindrical side, e.g. facing the LED positions. The phosphor matrix has a tuned refractive index, matching that of the phosphor particles to prevent scattering in the phosphor layer, or nano-sized phosphor particles are used that do no diffract the converted light. Organic phosphors can be used as well.

(45) As the rod is a light guide, the actual TIR occurs at the coating surface. Especially, this coating is relatively smooth at the outside. As phosphor particles may have the size of several microns, it may be needed to overcoat the phosphor coating with a dedicated overcoat with a matched index. Dip coating seems a good way to achieve such.

(46) The phosphor emits light isotropically, which may be a prerequisite for a highly efficient system. However, as the phosphors are applied in a matrix material it may be very important to prevent scattering in the layer. Scattering in the coating may have two major drawbacks. Firstly the converted light may be readily redirected by scattering in the coating layer. After scattering in the layer, the effective converted light distribution may be tending to a Lambertian distribution, perpendicular on the layer surface. Due to this distribution the side extraction of converted light may be very high. Secondly, a drawback may be that converted light traveling around in the rod in TIR can hit the layer again and after scattering it may be redistributed also towards a Lambertian profile, with the same consequence of efficiency loss. The efficiency of the rod may drop to very low values.

(47) An isotropic radiation can be reached by a matrix material of the phosphor with a refractive index that may be equal to that of the phosphor particles. A silicone matrix refractive index can be tuned from 1.4 to 1.9 with addition of titania (TiO.sub.2) and/or zirconia (ZrO.sub.2) nanoparticles.

(48) Another way to solve the scattering in the phosphor layer may be by nano-sized phosphor particles. These particles may be luminescent material particles comprising a dopant that acts as the activator, or they may be quantum dots, i.e., particles that absorb and re-emit light with optical properties depending on their (nano-scale) size. The effective refractive index of the layer may be that of the mixture of the constituents, which needs to match with the rod.

(49) A third solution may be an organic phosphor, meaning a polymeric phosphor material with no matrix material, or molecularly dispersed in a matrix material.

(50) Some system calculations were performed. The ray efficiency of converted light in the rod-CPC can be calculated using optical principles. Basic assumptions in the modelling were:

(51) A mirror with a typical 95% reflectivity is applied at the back side of the rod

(52) A CPC is placed at the front side with similar index (or in 2 cases with a LuYAG rod with a lower index for the CPC)

(53) Isotropic emission from the skin of the rod.

(54) The following data were used in the modelling or were obtained from the modelling:

(55) TABLE-US-00001 Relative Re- light fractive con- index version (550 effi- Type Material nm) CPC ciencies Round coated rod Fused silica 1.46 Same material 0.72 Round coated rod Schott crown 1.52 Same material 0.75 N-BK7 Round coated rod Schott Duran 1.47 Same material 0.73 boro tube Round coated rod Cdgm crown QK 1.47 Same material 0.73 Round coated rod High n glass 1.84 Same material 0.84 cdgm ZLaF 55D Round rod LuYAG 1.84 Low n CPC 0.73 Round rod LuYAG 1.84 High n CPC 0.84 Block rod LuYAG 1.84 Low n CPC 0.57 Block rod LuYAG 1.84 High n CPC 0.68

(56) The bold entry in the table indicates a reference system. It appears that the reference system has a converted light ray efficiency in a HLD with rectangular rod (a garnet with low n CPC, last 2 rows) is at max 0.57, a high n CPC with a rectangular rod allows up to 0.68. Further, it appears that the round rod in n=1.84 material allows up to ray efficiencies of 0.84 with a high-n CPC; a gain of 46% w.r.t. current HLD with low n CPC and 24% w.r.t. a high-n CPC. Further, it also appears that optical grade glasses like N-BK7 of Schott enables low optical losses of <2%, and fused silica even has zero losses. Extinction in high-n optical glasses can be low as well, as is the case for CDGM ZLaF-55D. For the calculation a typical path length of 100 mm through the rod is assumed. For a LuYAG block-shaped rod of 60 mm length the average path length travelled through the rod by the rays is 80 mm. The term “relative” indicates that the efficiency is determined relative to the total amount of luminescent material light that is generated in the elongated luminescent body. Hence, a value of 0.57 indicates that 57% leaves the rod (at the light exit window).

(57) Next to round solid rods, a tubular structure may be an option. It may allow a bigger outer diameter of the rod, but it creates challenges to narrow down the tube via a tapered section to obtain a small cross section just before the outcoupling part. A thin walled tube would allow a thin walled spherical cap that has the same function as a mirror.

(58) The absorption of pump light may be in a thin coating layer. To obtain high absorption the phosphor content in the coating should be high, and the actual dopant concentration in the phosphor particles should be high as well, in the case of inorganic luminescent materials.

(59) Especially, in embodiments the coating may be fully around the glass cylindrical side, even with 1 sided pumping, as the pump light traverses and can be absorbed at the opposite side. A reflector placed oppositely of the pump LEDs may enable to have another pass of the pump light through both layers, see e.g. FIG. 5.

(60) Assuming a cerium comprising garnet material, preliminary calculations assuming a 2.5% Ce concentration in the phosphor particles and 40% (volume) particle loading in the matrix and application of a mirror result in a preferred coating layer thickness of around 70 μm to come to an absorption level of 90%. Far thinner luminescent coatings can be achieved by organic phosphor materials or by quantum dot materials.

(61) The phosphor coating may comprise an organic polymeric material, a silicone as the matrix for phosphor particles, or glass as the matrix for phosphor particles. Polymer or silicone-based coatings may be applied in various ways such as dip coating or spraying.

(62) When using a ceramic or high-temperature glass material as the light guide, application of a phosphor in glass coating may be very suitable using the co-sintering route.

(63) Otherwise the glass crystallization route may be favorable.

(64) It may be desirable to apply one or more of the following:

(65) a round rod as carrier with a low optical extinction coefficient, and with a refractive index equal to that of the coating or relatively close to that (or even higher).

(66) a coating with phosphor particles that absorbs incident pump light to a sufficient degree (>80%), if needed in a multilayer to reach the absorption level required.

(67) a coating with a minimum of scattering of converted light.

(68) a relatively very smooth outside surface of the light guide, which may be either the carrier rod or the coating, or an outer coating thereon, to sustain TIR. A matched-index overcoat over the phosphor layer with a very smooth surface may be needed if the phosphor layer as such may be rough.

(69) a CPC with similar refractive index as the rod.

(70) a mirror at the back side of the rod, or a thin-walled spherical cap on a thin walled tube.

(71) For a good light guiding performance of the rod, the typical ray travel length up to a scattering event, usually described as mean free path, needs to be sufficient, like in embodiments mfp>100 mm. A mfp of about 100 mm may be a typical length of the mean optical path in the light guide towards the CPC. Assuming the above derived layer thickness of 70 μm on a 1.3 mm high rod, about 10% of the light guide volume may be built up by the coating. Assuming the carrier to have no scattering, the mfp of the coating could e.g. be mfp_coating>10 mm.

(72) In conclusion, in embodiments the coating matrix material should have such a good index match with the phosphor particles that the scatter level results in mfp_coating>10 mm. Scattering of the coating material can be measured independently of the application.

(73) With the present invention, one or more of the following may be provided and/or achieved:

(74) The complicated to grow single crystal or relatively complicated to produce in a good quality polycrystalline aluminum garnet material as the base material for the rod may be not needed anymore. These garnets are difficult to manufacture and require high transparency and optical smoothness of surfaces, which may be very expensive. Nevertheless, e.g. for thermal reasons such materials may still be used as support material.

(75) The HLD efficiency can go up by 46% at max with respect to reference HLD efficiency. Some losses are expected as well, e.g. for coupling in, 5-10%, for lower absorption efficiency 5%-10%, some absorption by the glass (2%) and outcoupling in rough surfaces (5%-10%).

(76) The potential scattering of converted light in the coating may be solved by index matching of the matrix with the phosphor, by using nano-sized phosphor particles or by using a polymeric phosphor.

(77) The rod-CPC can be pressed as a single part in a mold, no additional assembly of a rod with a CPC may be required then.

(78) The solution may be very versatile. Multiple types of phosphors can be applied.

(79) A glass rod as light guide and carrier for a phosphor layer.

(80) A local phosphor layer as a skin layer on the rod. If the phosphor is a composite of phosphor particles in a matrix, the matrix may essentially have a similar index as the phosphor particles, which may be done by index matching of the matrix, typically done with nano-sized zirconia and/or titania particles, or other types of such oxides (see also above). Another solution for composite phosphor layers may be with use of nano-size phosphor particles dispensed in the matrix or the use of an organic phosphor. Especially the phosphor layer has a relative high index of refraction, n>1.5, more especially n>1.7, ideally n>1.8.

(81) An overcoat of the phosphor layer with the same or a comparable index of refraction as the phosphor coating with a very smooth outer surface.

(82) Pump LEDs facing the phosphor layer at a close distance.

(83) A reflector around the pump LED to increase the coupling in of pump light.

(84) A reflecting cavity around the phosphor layer at opposite side of the pump LEDs.

(85) A CPC or other outcoupling device connected to the glass rod.

(86) A cooling structure to lead away the heat generated in the phosphor layer.

(87) Alternatively, a cylindrical tube may be used as light guide and carrier for a phosphor layer.

(88) In further embodiments, multiple concentrically mounted cylindrical tubes are used as an assembly of light guides carrying a phosphor layer.

(89) Alternatively, other transparent polycrystalline or monocrystalline ceramics may be used as light guide and carrier for a phosphor layer, enabling application of a much larger industrial supply base and fully developed materials.

(90) Alternatively, transparent light guides with a rectangular cross section are used as the carrier for a phosphor layer.

(91) FIGS. 3a-3f schematically depict some embodiments of the elongated luminescent body 100. The elongated luminescent body 100 comprises an elongated support 170 and a coating layer 180. The elongated luminescent body 100 comprises a body axis BA. Further, the elongated luminescent body has a length parameter P of a body dimension perpendicular to the body axis BA. The length parameter P is selected from height H and width W (FIGS. 3c and 3d) and diameter D (FIGS. 3a-3b, 3e and 3f).

(92) In the case of an elongated luminescent body having a circular cross-section, the length parameter P is the diameter D of elongated luminescent body. In the case of an elongated luminescent body having a rectangular cross-section, the length parameter P is the height H or width W of elongated luminescent body, especially the height. When referring to e.g. FIG. 2a, it schematically shows an embodiment wherein illumination with the light source light is done in a direction parallel to the height. Hence, especially the height may be chosen as length parameter P. As also shown in these schematically depicted embodiments, the length parameter P=D or P=H may thus also include the layer thickness (see FIGS. 3a-3d).

(93) FIGS. 3a-3f schematically depict embodiments wherein in a cross-sectional view the entire circumference is provided with the coating layer 180. However, alternatively also part may be provided with the coating layers, e.g. a subset of the support surfaces 172 in FIGS. 3c and 3d, or a part of the circular support surface 172 of the support 170 in FIGS. 3a, 3b, 3e and 3f.

(94) The elongated support 170 comprises a support material 171, such as YAG (undoped), PMMA, glass, or quartz glass. The support material 171 has a support material index of refraction n1, wherein the support material index of refraction n1 is at least 1.5. Further, the support 170 has a support surface 172 on which the coating layer 180 may be provided. The support has a support length L1.

(95) The coating layer 180 is configured on at least part of the support surface 172 over at least part of the support length L1. Here, all schematically depicted embodiments 3a-3d have a coating layer 180 over the entire length, with the coating layer circumferentially enclosing the support 170, but with no coating layer at the end faces.

(96) The coating layer 180 comprises a coating layer material 181. The coating layer material has an index of refraction n2. As indicated above, especially the coating layer index of refraction n2 is at least 1.5. The coating layer 180 has a coating layer thickness d1, such as e.g. up to 100 μm, though other layer thicknesses may also be possible.

(97) The coating layer material 181 has a composition different from the support material 171. The coating layer material 181 at least comprises a luminescent material 120, whereas the support material may not include a luminescent material, and especially certainly not a luminescent material that absorbs in the same wavelength range as the luminescence of the luminescent material. The luminescent material 120 is configured to absorb one or more of UV radiation and visible light, such as from a light source (see also FIGS. 1a, 1b, 1d, 1e, 2a, 2b, 5 and 6) and to convert into luminescent material light 8 having one or more wavelengths in one or more of the visible and the infrared.

(98) The luminescent material may e.g. comprise a cerium comprising garnet, as described above. Instead of cerium doped garnets, or in addition to such garnets, also other luminescent materials may be applied, e.g. embedded in organic or inorganic light transmissive matrixes, as luminescent concentrator. For instance quantum dots and/or organic dyes may be applied and may be embedded in transmissive matrices like e.g. polymers, like PMMA, or polysiloxanes, etc. etc. Other light transmissive material as host matrix may be used as well, see also below.

(99) Quantum dots are small crystals of semiconducting material generally having a width or diameter of only a few nanometers. When excited by incident light, a quantum dot emits light of a color determined by the size and material of the crystal. Light of a particular color can therefore be produced by adapting the size of the dots. Most known quantum dots with emission in the visible range are based on cadmium selenide (CdSe) with a shell such as cadmium sulfide (CdS) and zinc sulfide (ZnS). Cadmium free quantum dots such as indium phosphide (InP), and copper indium sulfide (CuInS.sub.2) and/or silver indium sulfide (AgInS.sub.2) can also be used. Quantum dots show very narrow emission band and thus they show saturated colors. Furthermore the emission color can easily be tuned by adapting the size of the quantum dots. Any type of quantum dot known in the art may be used in the present invention. However, it may be preferred for reasons of environmental safety and concern to use cadmium-free quantum dots or at least quantum dots having a very low cadmium content.

(100) Instead of quantum dots or in addition to quantum dots, also other quantum confinement structures may be used. The term “quantum confinement structures” should, in the context of the present application, be understood as e.g. quantum wells, quantum dots, quantum rods, or nano-wires.

(101) Organic phosphors can be used as well. Examples of suitable organic phosphor materials are organic luminescent materials based on perylene derivatives, for example compounds sold under the name Lumogen® by BASF. Examples of suitable compounds include, but are not limited to, Lumogen® Red F305, Lumogen® Orange F240, Lumogen® Yellow F083, and Lumogen® F170.

(102) As indicated above, the support material 171 is transmissive for the luminescent material light and especially also one or more of UV radiation and visible light. Further, the conditions −0.2≤n1−n2≤0.2 (at one or more wavelengths of the luminescent material light) and/or d1/P≤0.25 apply, i.e. the reflective indices of the coating material and the support material are not differing too much from each other and the coating layer is relatively thin.

(103) As the coating layer 180 is relatively thin the height H and width W, or the diameter D, of the elongated luminescent bodies 100 is essentially the same as the height H1 and width W1, or the diameter D1, of the support 170.

(104) FIGS. 3a, 3b, 3e and 3f schematically depict supports 170, and thereby also elongated luminescent bodies 100, which have an essentially circular cross-section. FIGS. 3c and 3d schematically depicts embodiments with an essentially rectangular (here square) cross-section.

(105) Especially, the surface of the coating layer 180 is smooth in order to limit scattering. It may also be possible to provide a coating on the coating layer, with the coating having a relatively flat surface. Hence, as schematically depicted in FIG. 3e, an outer layer 190 configured over the coating layer 180, may be available. Especially, the same conditions in relation to refractive index and mean free path (for scattering) that may apply to the coating layer may also apply to the outer coating layer 190. Reference 191 indicates the surface layer of the outer coating layer.

(106) FIG. 3f schematically depicts an embodiment of an elongated luminescent body 100 wherein the elongated support 170 further comprises an elongated core 175 of a material with a thermal conductivity equal to or larger than 20 W m.sup.−1 K.sup.−1, preferably equal to or larger than 50 W m.sup.−1 K.sup.−1, more preferably equal to or larger than 100 W m.sup.−1 K.sup.−1 and most preferably equal to or larger than 150 W m.sup.−1 K.sup.−1. As a result, the cooling of the elongated luminescent body 100 is further improved. Examples of materials for the elongated core 175 are ceramics like alumina (30 W m.sup.−1 K.sup.−1), SiC (60 W m.sup.−1 K.sup.−1) and AlN (160 W m.sup.−1 K.sup.−1), or metals like iron, aluminium, bronze, brass or copper (40-400 W m.sup.−1 K.sup.−1). The elongated support 170 has a diameter D1 and the elongated core 175 has a diameter d2. The ratio of the diameters d2/D1 is preferably equal to or smaller than 0.5, more preferably equal to or smaller than 0.4, even more preferably equal to or smaller than 0.3, most preferably equal to or smaller than 0.2. In alternative embodiments, with a different shape of the elongated luminescent body 100 or elongated support 170, the ratio P2/P1 is preferably equal to or smaller than 0.5, more preferably equal to or smaller than 0.4, even more preferably equal to or smaller than 0.3, most preferably equal to or smaller than 0.2, with a length parameter P1 of a body dimension perpendicular to the body axis of the elongated support 170, length parameter P2 of a body dimension perpendicular to the body axis of the elongated core 175, and the length parameter P1, P2 is selected from height (H), width (W) and diameter (D). In case of a rectangular cross section the largest dimension (e. height or width) may be selected. Preferably, the coefficient of thermal expension (CTE) of the material used for the elongated core 175 matches with the coefficient of thermal expansion of the material used for the support material 171 such that the difference in the coefficient of thermal expension is equal to or lower than 5×10.sup.−6 K.sup.−1, more preferably equal to or lower than 4×10.sup.−6 K.sup.−1, even more preferably 3×10.sup.−6 K.sup.−1 even more preferably 2×10.sup.−6 K.sup.−1 and most preferably 1×10.sup.−6 K.sup.−1. As an example, YAG or LuAG as a support material 171 has a CTE in the range of 6-8×10.sup.−6 K.sup.−1. In combination with materials like sapphire or AlN (CTE=5×10.sup.−6 K.sup.−1) for the core 175 a good match between the CTE of both materials is obtained. Alternatively metals like iron (CTE=12×10.sup.−6 K.sup.−1), nickel (CTE=13×10.sup.−6 K.sup.−1), platinum (CTE=9×10.sup.−6 K.sup.−1), tungsten (CTE=4.2-4.6×10.sup.−6 K.sup.−1), the nickel-cobalt alloy Kovar (CTE=5.3×10.sup.−6 K.sup.−1), the nickel-molybdenum alloys (CTE=5.2-7.2×10.sup.−6 K.sup.−1), and titanium (CTE=8.4-8.6×10.sup.−6 K.sup.−1) or some of its alloys with e.g. tin (for which the CTE goes up to 9.4×10.sup.−6 K.sup.−1) are suitable materials for the elongated core 175.

(107) The rod can be fully round with a layer on top of the cylindrical surface. A dip coating process using a vertical orientation of the rod seems a viable route. This may thus apply to a rod-shaped support, but this may also apply to a cuboid-shaped support.

(108) Another solution is a partial covering of the cylindrical surface. In that case the phosphor layer may cause un-roundness of the rod, which may lead to TIR losses. It may especially be intended to embed the phosphor layer in the rod to have a final round shape, like in e.g. FIGS. 3a and 3b. With an embedded phosphor layer, the profiled rod may be made by a pressing process. A profiled rod may in embodiments refer to a shape that has a shallow cavity to be filled with the (local) phosphor containing coating, in order to provide an essentially circular cross-section.

(109) The drawings of FIG. 4 schematically depict a number of embodiments. For instance, embodiment I may schematically depict an embodiment of the elongated luminescent body wherein the coating layer material 181 comprises an organic dye 121. The organic dye may be provided as such. However, the organic dye may also be provided as e.g. molecular dispersion in a matrix material 182, see embodiment II. Embodiment III may be an embodiment of the elongated luminescent body wherein the coating layer material 181 comprises inorganic luminescent nanoparticles 122, such as quantum dots. Embodiment IV may be an embodiment of the elongated luminescent body 100, wherein the coating layer material 181 comprises a matrix material 182 and an inorganic luminescent material 123 comprising particles (having e.g. weight averaged particle sizes selected from the range of 0.1-20 μm), embedded in a matrix material 182. The matrix material 182 may e.g. be selected from the group consisting of a glass material, a ceramic material, and a polymeric material.

(110) Reflectors can be used to increase the coupling-in efficiency of the blue pump light, and a reflector can be put at the opposite side of the pump LEDs to send back the light that transmits the layers, see e.g. FIGS. 5 and 6. References 2100 indicate a reflector or reflective surface. Reference 200 indicates a thermal conductive element, such as a heat sink. Part of the rod is in contact with a heat sink that encloses the rod tightly to cool the rod. The enclosure should be done in such a way that there is hardly any optical contact, but the average distance should be kept around 10 micrometer or less. The expansion coefficient of the heat sink should not deviate too much from the rod material. Heat sink materials can be ceramics (like Alumina), copper or aluminum.

(111) More optical elements than depicted herein may be comprised by the system(s).

(112) A further alternative embodiment of a light generating system 1000 is shown in FIG. 6. Only part of the circumferential surface of the elongated luminescent body 100 is provided with a coating layer 180, which is the part that is relatively close to the heat transfer element 200, and this part may be equal to or less than 60%, preferably equal to or less than 50%, more preferably equal to or less than 40%, even more preferably equal to or less than 30%. The reflector or reflective surface 2100 is optional and may be omitted. The elongated luminescent body 100 has a first surface 105 that comprises the light input face. The elongated luminescent body 100 has a second surface 107 on which the coating layer 180 is configured. The first surface 105 is opposed to the second surface 107. As the coating layer 180 is relatively close to the heat transfer element 200, the cooling of the elongated luminescent body 100 is improved, because the transfer of the heat generated in coating layer 180 to the heat transfer element 200 is improved. Hence, the average temperature of the elongated luminescent body 100 is lower compared to a light generating system with a luminescent body 100 that has a coating layer 180 around its full circumferential surface. The elongated luminescent body 100 is partly enclosed by the heat transfer element 200. Alternatively, the elongated luminescent body 100 is enclosed by the heat transfer element 200 for at least 60% of the circumferential surface of the elongated luminescent body 100, preferably for at least 70%, more preferably for at least 80%. As a result, the cooling of the elongated luminescent body 100 is further improved. The distance between the elongated luminescent body 100 and the heat transfer element 200 is in the range of 2-20 μm. The term “plurality” refers to two or more.

(113) The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%.

(114) The term “comprise” includes also embodiments wherein the term “comprises” means “consists of”.

(115) The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term “comprising” may in an embodiment refer to “consisting of” but may in another embodiment also refer to “containing at least the defined species and optionally one or more other species”.

(116) Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. 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.

(117) The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

(118) It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

(119) In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

(120) Use of the verb “to comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

(121) The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.

(122) The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

(123) The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

(124) The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

(125) The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.