LIGHTING DEVICE WITH CERAMIC GARNET

Abstract

The invention provides a lighting device comprising a plurality of solid state light sources and an elongated ceramic body having a first face and a second face defining a length (L) of the elongated ceramic body, the elongated ceramic body comprising one or more radiation input faces and a radiation exit window, wherein the second face comprises the radiation exit window, wherein the plurality of solid state light sources are configured to provide blue light source light to the one or more radiation input faces and are configured to provide to at least one of the radiation input faces a photon flux of at least 1.0*10.sup.17 photons/(s.Math.mm.sup.2), wherein the elongated ceramic body comprises a ceramic material configured to wavelength convert at least part of the blue light source light into at least converter light, wherein the ceramic material comprises an A.sub.3B.sub.5O.sub.12:Ce.sup.3+ ceramic material, wherein A comprises one or more of yttrium (Y), gadolinium (Gd) and lutetium (Lu), and wherein B comprises aluminum (Al).

Claims

1. A lighting device comprising a plurality of solid state light sources and an elongated ceramic body having a first face and a second face and a length (L) of the elongated ceramic body, the elongated ceramic body comprising one or more radiation input faces and a radiation exit window, wherein the second face comprises the radiation exit window, wherein the plurality of solid state light sources are configured to provide blue light source light to the one or more radiation input faces and are configured to provide to at least one of the radiation input faces a photon flux of at least 1.0*10.sup.17 photons/(s.Math.mm.sup.2), wherein the elongated ceramic body comprises a ceramic material configured to wavelength convert at least part of the blue light source light into converter light, wherein the ceramic material comprises an A.sub.3B.sub.5O.sub.12:Ce.sup.3+ ceramic material, wherein A comprises one or more of yttrium (Y), or gadolinium (Gd) or lutetium (Lu), wherein B comprises aluminum (Al), and wherein the elongated ceramic body is obtained by performing a vacuum sintering process and an isostatic pressing process at elevated temperatures of starting material, to provide the elongated ceramic body, followed by a method comprising an annealing process in an oxidizing atmosphere at a temperature of at least 1000° C.

2. (canceled)

3. The lighting device of claim 1, wherein the elongated ceramic body is obtained by performing a vacuum sintering process and an isostatic pressing process at elevated temperatures of starting material in a neutral or reducing atmosphere, followed by the annealing process.

4. The lighting device of claim 1, further comprising an optical reflector configured upstream of the radiation exit window, wherein the optical reflector is configured to reflect light back into the elongated ceramic body, wherein the radiation exit window is configured perpendicular to the one or more radiation input faces, and wherein the lighting device further comprises an optical filter configured downstream of the radiation exit window and configured to reduce a relative contribution of one or more of non-green and non-red light in the converter light.

5. The lighting device of claim 1, wherein the length (L) is at least 20 mm, wherein a concentration of cerium is in a range of about 0.1% to about 3.0% of A, and wherein the photon flux is at least 4.5*10.sup.17 photons/(s.Math.mm.sup.2).

6. The lighting device of claim 5, having a lumen output of the converter light downstream from the radiation exit window, wherein at a fixed photon flux per mm.sup.2 the device is configured so that the lumen output is scalable with the length (L) of the elongated ceramic body at least within a length (L) range of about 20 mm to about 100 mm.

7. The lighting device of claim 1, wherein the ceramic material is configured so that a thermo luminescence spectrum of the ceramic material shows a maximum in a range of about 50° C. to about 100° C.

8. The lighting device of claim 1, wherein, in a first option, A in the ceramic material comprises at least 90% Lu, or wherein, in a second option, A in the ceramic material comprises about 50% to about 95% Y and comprises about 5% to about 50% Gd, and wherein in both the first option and in the second option, B in the ceramic material comprises at least 95% Al and Ga.

9. The lighting device of claim 1, wherein the elongated ceramic body comprises a geometrical concentration factor, defined as a ratio of the radiation input faces area to the radiation exit window area, of at least 2.

10. The lighting device of claim 1, further comprising at least one light concentrator of a projector.

11. The lighting device of claim 10, further comprising at least one second lighting device, wherein the lighting device is configured to provide green light, and wherein the second lighting device is configured to provide red light.

12. A method for production of an elongated ceramic body, the elongated ceramic body comprising one or more radiation input faces and a radiation exit window, wherein the elongated ceramic body is configured to receive, at at least one of the radiation input faces, a photon flux of at least 1.0*10.sup.17 photons/(s.Math.mm.sup.2) and wherein the elongated ceramic body comprises a ceramic material configured to wavelength convert at least part of a blue light source light into at least converter light, wherein the ceramic material comprises an A.sub.3B.sub.5O.sub.12:Ce.sup.3+ ceramic material, wherein A comprises one or more of yttrium, gadolinium and lutetium, wherein B comprises aluminum, and wherein the method comprises processing starting material at elevated temperatures to provide the elongated ceramic body, and annealing the elongated ceramic body in an annealing process in an oxidizing atmosphere at a temperature of at least 1000° C.

13. The method of claim 12, wherein the method comprises processing starting material at elevated temperatures, a vacuum sintering process, and an isostatic pressing process, and wherein the oxidizing atmosphere comprises O.sub.2.

14. The method of claim 12, wherein processing the starting material at elevated temperatures is performed in a neutral or reducing atmosphere.

15. The method of claim 12, wherein the starting material is chosen such that, in a first option, A in the ceramic material comprises at least 90% Lu, or, in a second option, such that A in the ceramic material comprises in a range of about 50% Y to about 95% Y and in a range of about 5% Gd to about 50% Gd, and wherein in both the first option, and in the second option, B in the ceramic material comprises at least 95% Al and Ga.

16. The lighting device of claim 1, further comprising a collimator configured downstream of the radiation exit window and configured to collimate the converter light.

17. The lighting device of claim 7, wherein the maximum is in a range of about 2 to about 10 times higher than the maximum as when the ceramic material is provided as a single crystal.

18. The lighting device of claim 10, wherein the projector is a DLP projector.

19. The lighting device of claim 10, wherein the projector is an LCD projector.

20. The lighting device of claim 10, further comprising a heat sink configured to facilitate cooling of the light concentrator.

21. The lighting device of claim 10, further comprising a heat sink configured to facilitate cooling of the solid state light sources.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] 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:

[0053] FIGS. 1a-1e schematically depict some aspects of the invention;

[0054] FIGS. 2a-2d show some results of some ceramic bodies and lighting devices made.

[0055] The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] 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.

[0057] 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 ceramic 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.

[0058] An embodiment of the lighting device as defined herein is schematically depicted in FIG. 1a. FIG. 1a schematically depicts a lighting device 1 comprising a plurality of solid state light sources 10 and an elongated ceramic body 100 having a first face 141 and a second face 142 defining a length L of the elongated ceramic body 100. The elongated ceramic 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 width W). Further the ceramic body 100 comprises a radiation exit window 112, wherein the second face 142 comprises said 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.

[0059] The elongated ceramic body 100 comprises 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 ceramic body or waveguide, thereby improving the efficiency. Note that more reflectors than the schematically depicted reflector may be used.

[0060] The light sources may in principle be any type of point 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.

[0061] FIGS. 1a-1b schematically depict similar embodiments of the lighting device. Further, the lighting device 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 lighting devices 1 in FIG. 1b further comprise a collimator 24, such as a CPC.

[0062] FIG. 1c schematically depicts the (non-)scalability of the ceramic waveguides. When the ceramic body with the light sources are scalable, the intensity of the lighting device light 101 scales with the length of the ceramic body (see FIGS. 1b-1c). The length is indicated with rl (relative rod length; x-axis) and the intensity is indicated with I (relative lumens, y-axis). A perfect scalability would be curve a (ideal scalability) in FIG. 1c. A single crystal comes in general very close to such scalable waveguide. Curve b (scalable, but non-ideal) shows a substantially scalable waveguide. The ceramic body as described herein (especially due to the post-annealing in an oxygen atmosphere), is close to ideally scalable. Over a substantial length variation, such as in the range of 20-100 mm, the ceramic bodies or waveguides as described herein appear to be scalable. Non-scalability is shown with curve c (non-scalable). For instance, ceramic bodies not having been subjected to the post-annealing are substantially less or even no scalable.

[0063] FIG. 1d schematically depicts some embodiments of possible ceramic bodies as waveguides or luminescent concentrators. The faces are indicated with references 141-146. The first variant, a plate-like or beam-like ceramic body has the faces 141-146. Light sources, which are not shown, may be arranged at one or more of the faces 143-146. 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 ceramic body. Such ceramic 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. The variants shown in FIG. 1d are not limitative. More shapes are possible; i.e. for instance referred to WO2006/054203, which is incorporated herein by reference. The ceramic bodies, 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 ration (of length/width) is especially larger than 1, such as equal to or larger than 2. Unless indicated otherwise, the term “aspect ratio” refers to the ratio length/width.

[0064] 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.

[0065] FIG. 1e very schematically depicts a projector or projector device 2 comprising the lighting device 1 as defined herein. By way of example, here the projector 2 comprises at least two lighting devices 1, wherein a first lighting device (1a) is configured to provide e.g. green light 101 and wherein a second lighting device (1b) is configured to provide e.g. red light 101. Light source 5 is configured to provide blue light. These light sources may be used to provide the projection 3.

[0066] Several lighting devices were built and/or evaluated. Below, the blue flux density for pumping of 48 mm long rods; rod size: 1.2 (cool side)×1.9 (pump side)×48 mm:

TABLE-US-00001 Flux density on rod averaged Total over rod area electric Total optical Flux density on (91.2 mm.sup.2) @ Blue flux/ power in blue power LED 2 mm.sup.2 45% DC I peak LED # of (W) (Wopt) (Wopt/mm.sup.2) Wopt/mm.sup.2 per (Amp) (Wopt) LEDs @ 45% DC @ 45% DC @ 45% DC side 0.26 0.165 52 16 8.6 0.165 0.047 2.24 1.11 52 165 57.8 1.11 0.317 4 1.54 52 300 80 1.54 0.439

[0067] The value of 2.24 A (ampere; Amp) is the maximum current in test module used. The 4 A values are extrapolated. The coupling efficiency to the rod is assumed to be 100%, in reality 85-90% is more likely. Further, over the rod surface an incident uniform blue flux distribution is assumed. In reality some hot spots may occur, but always less severe than the flux density on the LEDs itself. The rod is pumped from 2 sides; the incident blue flux density on the rod is calculated per side, assuming that one side is excited by the LEDs positioned on the corresponding side. This will hold when the dopant concentration, typically Ce, is sufficiently high to absorb the blue light over a thin layer. The data in the table holds for a pulsed operation with at duty cycle (DC) of 45%. More power can be generated at the same peak current by increasing the DC, e.g. up to 60%, but not much higher as there is also time needed for the sequential switching of the other 2 color channels in a DLP projector. For a LCD projector direct current drive is used (100% DC).

[0068] In a further example, green and red luminescent ceramics were compared. Flux output density is taken as output from a CPC extraction optic with the rod placed inside a blue LED pump module. Rod length=48 mm; LED count: 52×2 mm.sup.2 high power LEDs. The rod cross-section area is 1.2×1.9 mm=2.28 mm.sup.2:

TABLE-US-00002 Green flux LuAG green density YGdAG red Red flux Wopt, green (Wopt/mm.sup.2) Wopt, red (Wopt/mm.sup.2) 2.2 A, 45% DC 14 6.1 7.2 3.2 4 A, 45% DC 20 8.8 10 4.4 estimate

[0069] LuAG (Lu.sub.3Al.sub.5O.sub.12:Ce.sup.3+) garnet is used for green emission; YGdAG ((Y+Gd).sub.3Al.sub.5O.sub.12:Ce.sup.3+) garnet for red. YGdAG is actually yellow emitting, also containing green and red. The green part is filtered out in order to use the red part in the beamer. Part of the light may bounce several times up and down inside the rod. There is still further improvement potential in green and especially in red. More optimized red may be only slightly lower performing than green. Instead of 52 LEDs, also 56 LEDs of 1 mm.sup.2 could be used to pump a 52 mm rod, increasing flux with 8%. Improvements to the blue LED output (previous slide) directly improves the green/red output with the same factor. Doubling the rod length (and LED count) may ideally double the output, hence double the density. Therefore a 10 cm rod could have a green density of 18-20 Wopt/mm.sup.2 on the nose exit.

[0070] FIG. 2a shows the excitation spectrum of a ceramic garnet rod with a composition according to the invention, here LuAG. The excitation (at maximum emission) is shown for a ceramic body having been treated in a H.sub.2/N.sub.2 mixture with H.sub.2O (a) and having been treated under oxidative conditions (b) as defined in the present application. The ratio of the intensity of the lowest excitation band relative to the intensity of the one but lowest excitation band increases (from 1.17 to 1.44) due to the oxidative annealing.

[0071] The impact of oxidative annealing is shown in FIGS. 2b and 2c-2d. FIG. 2b shows thermo luminescence data of different LuAG (pure Lu samples), with the lowest, a, being the thermo luminescence spectrum of a single crystal and with the highest, c, being the thermo luminescence spectrum of a non-annealed ceramic body. The middle curve, b, which is much closer to the single crystal curve, is the annealed ceramic body. The x-axis indicates the temperature in ° C., and the y-axis indicates the thermo luminescence intensity in arbitrary units. For the thermo luminescence data, the samples are shorty submitted to UV illumination (360 nm) and then heated in the dark with a linear heating rate. The emitted light intensity is measured as a function of the sample temperature.

[0072] The polycrystalline YGdAG rod shows after HIP and wet forming gas anneal (reducing atmosphere) a low performance. Afterglow of trap states is visible when the rod is put at elevated temperature (due to release of trapped charges, followed by radiative recombination). By an oxidative post-anneal step, such as the extra anneal in an O.sub.2 containing atmosphere, for 4 hrs. at 1250° C., a strong increase in light output is observed, related to reduction of trap states. No afterglow is seen. The relative difference between the oxidized rod and the native rod is clearly increasing with increased LED power/flux density.

[0073] Ceramic bodies with 0.25% Ce and 25% Gd (FIG. 2c), and with 1% Ce and 25% Gd (FIG. 2d) were tested on the impact of oxidative annealing. Rods of 1.2 mm*1.9 mm*52 mm, with 52 LED modules, with 10% duty cycle were used as devices for the tests. In FIG. 2c, the order of the curves, from the lowest curve to the highest curve is: No anneal; 1% O2; 5% O2; 21% O2; CO anneal. When no anneal is performed, the results are substantially lower than all oxidative anneals (at 1250° C.). In FIG. 2d, the order of the curves, from the lowest curve to the highest curve is: 1st reference (no oxidative anneal); 1% O2 anneal; 0.5% O2 anneal; 0.1% O2 anneal; N2 Anneal (with a few ppm O2).

[0074] Now, an example of a synthesis of a ceramic body is described. The desired combination of raw materials, Lu2O3, Y2O3, Gd2O3, Al2O3 and CeO2 are weighed out in a jar. The raw materials are wet-mixed in water using alumina milling balls. Binder is added and the suspension is dried (e.g.: spray drying). The granulate is dry-pressed into the desired from using a steel die, with pressures in the range of about 1 ton/cm.sup.2. The pressed samples are first fired in air at 1000° C. in order to remove all organic substances (binder). Then the samples are sintered in a vacuum oven typically at 1700° C., pressure <10.sup.−5 bar. The vacuum sintered samples are subsequently Hot Isostatic Pressed (HIP), typically at 1700° C./1000 bar Ar or N.sub.2 to achieve near zero porosity. After HIP the samples are to be annealed under varying oxidizing conditions typically at 1250° C. (see above experiments). Finally the samples are polished to obtain final product. For the here described stick samples the annealing was standard done in wet Forming gas (WFG) before polishing. Subsequent described annealing was done afterwards on the polished samples.

[0075] Hence, herein ceramic garnet compositions, doped with Ce.sup.3+, in combination with suitable annealing steps, are described. The resulting ceramic sticks show light conversion efficiencies that are on par with single crystals of the same composition, also at high excitation densities using optimized processing conditions. Without annealing, the light conversion efficiencies at high excitation densities are significantly lower (30% or more). Thermally stimulated luminescence can be used to check whether the annealing step(s) that are essential part of the invention have been applied. Ceramic sticks that have not been annealed properly show a clear afterglow, especially at elevated temperatures.