Optical converter for high luminances

Abstract

An optical converter for producing colored or white light from blue excitation light is provided. The converter has good scattering properties to be able to produce nearly white light from the scattered blue light components and the scattered, converted yellow light components. The optical converter includes material including one or more of a YAG ceramic, a LuAG ceramic, and a magnesium-aluminum ceramic exhibiting strong scattering.

Claims

1. A converter for producing colored or white light from blue excitation light, comprising: a converter body having an excitation face and an opposite face, the spacing therebetween defining the thickness of the converter body, the converter body comprising a cerium doped optoceramic converter material selected from the group consisting of a YAG ceramic, a LuAG ceramic, and a magnesium-aluminum ceramic, wherein the optoceramic converter material has a degree of cerium doping and is sintered at a temperature that are sufficient to provide an embedded grain structure in the doped optoceramic material which exhibits scattering to obtain an emission spot of a common size as a cross section of a light beam of the excitation light, to obtain a remission factor of longer wavelength light of 600 nm wavelength of R.sub.emission>0.6, and to obtain a quantum efficiency QE of greater than 0.80.

2. The converter as claimed in claim 1, wherein the thickness of the converter body is used for fine-tuning fractions of remitted excitation light and remitted longer wavelength light.

3. The converter as claimed in claim 1, wherein the optoceramic converter material is a doped YAG ceramic comprising cerium doped Y.sub.3Al.sub.5O.sub.12.

4. The converter as claimed in claim 1, wherein the optoceramic converter material is a LuAG ceramic comprising cerium doped Lu.sub.3(Ga,Al).sub.5O.sub.12.

5. The converter as claimed in claim 1, wherein the optoceramic converter material is a magnesium-aluminum ceramic comprising cerium doped Mg.sub.3Al.sub.8[SiO].sub.3.

6. The converter as claimed in claim 1, wherein the optoceramic converter material is doped with an amount of Ce.sub.2O.sub.3 in a range from 0.01 to 2%.

7. The converter as claimed in claim 1, wherein the optoceramic converter material is doped with an amount of Ce.sub.2O.sub.3 in a range from 0.3 to 1%.

8. The converter as claimed in claim 1, wherein the optoceramic converter material is doped with an amount of Ce.sub.2O.sub.3 in a range from 0.1 to 0.5%.

9. The converter as claimed in claim 1, wherein the thickness of the converter body is 1 mm thickness.

10. The converter as claimed in claim 1, further comprising an optical figure of merit of QE*R.sub.emission>0.6.

11. The converter as claimed in claim 1, wherein the optoceramic converter material has a thermal figure of merit (FOM.sub.therm,stat)>1000 (W/m), and wherein said thermal figure of merit is calculated as FOM.sub.therm=thermal conductivity*min(T.sub.fail,T.sup.0.8.sub.quench).

12. The converter as claimed in claim 1, wherein the optoceramic converter material has a thermal conductivity of at least 5 W/m*K.

13. The converter as claimed in claim 1, wherein the optoceramic converter material has a thermal conductivity of at least 10 W/m*K.

14. The converter as claimed in claim 1, wherein the optoceramic converter material has a thermal conductivity of at least 12 W/m*K.

15. The converter as claimed in claim 1, further comprising a luminance of >>100 cd/mm.sup.2 in a static operation mode.

16. The converter as claimed in claim 1, further comprising a mirror secured to the opposite face.

17. The converter as claimed in claim 1, wherein the excitation face is coated with a dichroic filter.

18. The converter as claimed in claim 1, comprising scattering properties that remit between 10% and 30% of the excitation light that, in combination with the excitation light, converted light leads to a white color impression for the viewer.

19. The converter as claimed in claim 1, further comprising a scattering layer at the excitation face to provide scattering properties that remit between 10% and 30% of the excitation light that achieves a white light-near region.

20. The converter as claimed in claim 1, wherein the excitation face is roughened to provide scattering properties that remit between 10% and 30% of the excitation light that achieves a white light-near region.

21. A converter for producing colored or white light from blue excitation light, comprising: a converter body having an excitation face and an opposite face, the spacing therebetween defining the thickness of the converter body, wherein the converter material comprises a cerium doped optoceramic having a degree of cerium doping and being sintered at a temperature that are sufficient to provide an embedded grain structure in the doped optoceramic which exhibits scattering to obtain an emission spot of a common size as the cross section of the light beam of the excitation light, to obtain a remission factor of longer wavelength light of 600 nm wavelength of R.sub.emission>0.6, and to obtain a quantum efficiency QE of greater than 0.80, wherein the converter material is a doped YAG ceramic or LuAG ceramic.

22. A converter comprising a ceramic converter material having a cerium doped optoceramic that has a degree of cerium doping and is sintered at a temperature that are sufficient to provide an embedded grain structure in the doped optoceramic so that the ceramic converter material exhibits an optical figure of merit of FOM.sub.opt=QE*R.sub.emission that is greater than 0.5, wherein the QE is a quantum efficiency and the R.sub.emission is a remission factor of longer wavelength light of 600 nm wavelength.

23. The converter as claimed in claim 22, wherein the optical figure of merit of FOM.sub.opt=QE*R.sub.emission that is greater than 0.7.

24. The converter as claimed in claim 22, wherein the optical figure of merit of FOM.sub.opt=QE*R.sub.emission that is greater than 0.8.

Description

DESCRIPTION OF THE FIGURES

(1) The invention will now be described in more detail by way of exemplary embodiments and with reference to the accompanying drawings.

(2) In the drawings:

(3) FIG. 1 schematically shows the configuration of a converter operated in remission, as a first exemplary embodiment;

(4) FIG. 2 schematically shows the configuration of a modification of the first exemplary embodiment including a dichroic filter;

(5) FIG. 3 schematically illustrates the use of the optoceramic in a converter operated in transmission, as a third exemplary embodiment;

(6) FIG. 4 schematically illustrates a converter operated in transmission and with an improved frame, as a fourth exemplary embodiment;

(7) FIG. 5 schematically illustrates the configuration of a dynamic, i.e. rotating, converter operated in remission, in which the optoceramic is formed as a disc;

(8) FIG. 6 schematically illustrates the configuration of a dynamic, i.e. rotating, converter operated in remission, in which the optoceramic 1 is formed as a ring;

(9) FIG. 7 schematically illustrates the configuration of a dynamic converter operated in transmission;

(10) FIG. 8 is a graph illustrating the dependence of quantum efficiency and remission;

(11) FIG. 9 is an overlay graph illustrating the dependence of remission and quantum efficiency on the sintering temperature of an optoceramic of a composition Lu.sub.3(Ga,Al).sub.5O.sub.12—Ce;

(12) FIG. 10 is an overlay graph illustrating the dependence of remission and quantum efficiency on the sintering temperature of an optoceramic of a composition Y.sub.3Al.sub.5O.sub.12—Ce;

(13) FIG. 11 is a graph illustrating the dependence of remission on the cerium content; and

(14) FIG. 12 illustrates a converter and the adjusting of a defined fraction of excitation light for generating a white field-near point in the chromaticity diagram.

(15) The exemplary embodiments of the invention will now be described with reference to the figures.

(16) FIG. 1 schematically shows a side view of the configuration of a converter operated in remission, as a first exemplary embodiment.

(17) The converter comprises an optoceramic converter body 1 and a mirror 3 which are joined to each other by an adhesive layer 2. The converter material, i.e. the optoceramic, is a doped YAG ceramic or a LuAG ceramic and defines a grain structure with strong scattering.

(18) From above, i.e. from the excitation side, blue excitation light 4 of high power density, i.e. with a small light beam cross section is incident upon the excitation face of converter body 1, and upon penetration into the optoceramic longer wavelength light 5 is generated by conversion and is emitted to the outside by scattering, as indicated by arrows.

(19) Adhesive layer 2 has a thickness d.sub.adh of 10 μm, for example. Mirror 3 reflects the excitation light 4 into optoceramic 1 and thus enhances luminous efficacy of the converted, longer wavelength light 5.

(20) Converter area A is 4 mm.sup.2, for example, and the converter has a thickness d.sub.conv of 200 μm. The maximum pump power can be approximated as:
P.sup.max.sub.opt˜4(T.sup.max.sub.conv−T.sub.RT)/R.sup.th
with:
T.sup.max.sub.conv=T.sub.RT+Q/A*d/λ

(21) with R.sup.th=d/(Aλ) being the thermal resistance and λ the thermal conductivity of the converter.

(22) Thermal conductivity of optoceramic λ.sub.OC=10 W/mK.

(23) Thermal conductivity of adhesive λ.sub.adh=0.3 W/mK.

(24) The thermal resistance of the converter results from the absolute thermal resistances R.sup.th of the converter material and of the adhesive used.

(25) Thus, with thermal resistances of the optoceramic of R.sup.th.sub.OC=5 K/W and of the adhesive layer of R.sup.th.sub.adh=8.3 K/W, a thermal resistance of the converter R.sup.th.sub.conv of 13.3 K/W is resulting. Adhesive layer 2 has a higher thermal resistance than optoceramic 1 and therefore largely determines the thermal resistance, i.e. the lower limit thereof, in this case.

(26) With a failing threshold of the optoceramic of T.sup.max.sub.OC=250° C., a maximum pump power P.sup.max.sub.opt of 68 W is resulting. Only the adhesive layer has a limiting effect.

(27) With a failing threshold of the adhesive of 100° C., a maximum optical pump power of P.sup.max.sub.opt=36 W is yet resulting.

(28) With an irradiation with 10 W upon 4 mm.sup.2, for example, this surprisingly high pump power allows for luminances of 200 cd/mm.sup.2 and for a luminous flux of 600 lm.

(29) By comparison, converters known from prior art based on a phosphor in silicone (PIS) can achieve maximum optical pump powers P.sup.max.sub.opt of about 3 W. This is in particular due to the high thermal resistance R.sup.th.sub.PIS of about 100 K/W and the low failing threshold of the PIS converter material.

(30) FIG. 2 schematically shows the configuration of the second exemplary embodiment which is a variation of the first exemplary embodiment, in which additionally an anti-reflective coating 6 was applied on optoceramic 1. This reduces reflection of the excitation beam 4 at the surface of optoceramic 1 and improves emission of scattered converted light 5.

(31) FIG. 3 shows the configuration of the third exemplary embodiment, a converter excited from the rear side, i.e. with transmissive excitation. The excitation light 4 is incident on optoceramic 1 at excitation side 11. The converted longer wavelength light 5 exits to emission side 12. Optoceramic 1 has a thickness of 200 micrometers. This ensures sufficiently high transmission with good quantum efficiency. In this example, optoceramic 1 is fixed only laterally, i.e. left and right of the light spot of excitation light, by a metallic frame 70. Thus, the major part of optoceramic 1 hangs freely between the frame. This design is only made possible by the fact that the optoceramic 1 is self-supporting. By contrast, in systems known from prior art, such as PIS substrates, a support material is needed, so that rear excitation is not possible. Frame 70 is designed as a heat sink and has a diameter of 2 mm. In order to reduce the size of the emission spot, optoceramic 1 is coated with an edge filter 91 on emission side 12. On excitation side 11, in turn, optoceramic 1 is coated with a broadband AR coating.

(32) The configuration schematically shown in FIG. 4 is a modification of the third exemplary embodiment. Here optoceramic 1 is framed in a manner so that frame 71 covers a major part of the bottom surface of optoceramic 1, apart from the area on which the light spot of excitation light 4 is incident. Frame 71 is metallic and reflects light scattered in optoceramic 1, including excitation light 4 and converted light 5. In this way, luminous efficacy in the optoceramic is increased. Therefore, an additional coating of optoceramic 1 on the excitation side 11 can be dispensed with in this case.

(33) Assuming the same material parameters as in the first exemplary embodiment, the effective thermal resistance R.sup.th.sub.eff resulting is 55 K/W. This permits optical pump powers P.sup.max.sub.opt of up to 16 W.

(34) The difference to the optical pump powers of the first exemplary embodiment is caused by the configuration.

(35) Depending on the operating configuration, the performance here additionally depends on geometry factors, for example. A result thereof is that the maximum pump power in transmission is lower than in remission.

(36) The table below gives an overview of the data relevant for the thermal FOM of a static converter, for converter material according to the invention, OC1 and OC2, and for converter materials based on silicone or glass/glass-ceramic.

(37) The thermal FOM for a statically operated converter has been defined as the product of the maximum allowable temperature and thermal conductivity.

(38) Converters according to the invention exhibit an FOM.sub.stat of up to 7322 W/m, prior art phosphor-in-silicone converters with a similar configuration, by contrast, only exhibit an FOM of 95 W/m.

(39) In the last column, the FOM was supplemented by a heat capacity figure. The heat capacity figure is the product of heat capacity and density. The FOM extended in this manner, FOM.sub.dyn, describes the behavior of a converter on a rotating wheel, and thus a dynamic converter.

(40) Optoceramics OC1 and OC2 of the invention exhibit extremely high FOMs.

(41) The two optoceramics OC1 and OC2 differ in their doping. Optoceramic OC1 comprises YAG (yttrium aluminum garnet), optoceramic OC2 comprises Mg.sub.3Al.sub.8[SiO].sub.3.

(42) However, the differences in FOM of the two optoceramics are in particular attributable to different densities.

(43) In terms of the FOM, the materials listed in the table can be divided into three groups. The low FOMs of the silicone matrix based converters are in particular attributable to the low thermal conductivity.

(44) Glass or glass ceramic based materials exhibit FOMs which are significantly higher than those based on silicone matrices, i.e. on organic systems. However, the FOMs of the optoceramics according to the invention are more than 5 times as high as those of glass ceramic based converters.

(45) In particular, the optoceramics advantageously exhibit exceptionally high thermal conductivities λ.

(46) TABLE-US-00001 TABLE 1 Overview of FOM relevant parameters Cp Cp * p λ TQE/80 T.sub.abs Min FOM.sub.stat FOM.sub.dyn Material [J/K * g] [J/cm.sup.3K] [W/mK] [° C.] [° C.] (TQE80/T.sub.abs) [W/m] [J.sup.2/cm.sup.4sK] OC1 0.6 2.76 14 250 1700 523 7322 202 OC2 0.6 2.28 10 250 1700 523 5230 119 Silicone 1.4 1.7 0.2 220 200 473 95 1.6 Glass 1 2.5 0.9 300 500 573 443 11.1 Glass 1 2.5 1.5 250 900 523 859 22 ceramic

(47) The fifth exemplary embodiment shown in FIG. 5 schematically illustrates the cross section of a dynamic converter which is operated in remission. Optoceramic 1 is formed as a circular disc and is applied to support 3 by adhesive layer 2. The surface of optoceramic 1 is coated with an edge filter 9. In its center, support 3 is coupled to a hub 8. Excitation is localized at one point of the converter. By means of hub 8, support 3 with optoceramic 1 is rotated. Thus, a predetermined localized area of optoceramic 1 is irradiated by the excitation light 4 for only a short duration. The duration of local irradiation may be set via the rotary frequency of hub 8. Because of the short duration of local irradiation, high excitation powers of far more than 25 W can be obtained.

(48) In the exemplary embodiment schematically illustrated in FIG. 6, the optoceramic 1 has an annular shape and is applied on circular support 3 by means of adhesive layer 2. By using an annular optoceramic 1, material costs can be reduced without affecting the optical properties of the converter. This is possible because in this embodiment only those portions have been eliminated which are not covered by the excitation beam 4 anyway.

(49) FIG. 7 schematically illustrates a dynamic, i.e. rotating converter operated in transmission. The disc-shaped support 3 fixes the annular optoceramic 1 which is configured as a reflective frame. Here, support 3 only covers a portion of the bottom surface of optoceramic 1 that is necessary for fixing it. The major part of the bottom surface of the optoceramic is not covered by support 3 and can thus be used for conversion.

(50) Here, again, optoceramic 1 is provided with an AR coating 10 on the excitation side, i.e. on its bottom surface, and with an edge filter 91 on its upper surface.

(51) FIG. 8 shows the relationship between quantum efficiency and remission. However, the relationship between scattering and remission can advantageously be used to quantify the scattering properties of a converter. For this purpose, the remission is measured at a wavelength above the excitation wavelength. In this case, the remission of a sample of the converter material of a thickness of 1 mm was measured at a wavelength of 600 nm. The spectrometer used was a spectrometer with an integration sphere. Below, the remission R.sub.emission measured according to these measurement rules is representative of an optical measurement of scattering properties. The remission was measured on a sample of a thickness of 1 mm at a wavelength of 600 nm. Here, a strongly scattering material exhibits high remission.

(52) In strongly scattering samples the quantum efficiency is expected to drop. The expected behavior is indicated in the chart by a straight line.

(53) Most surprisingly, however, a regime 14 was found for which the samples exhibit high remission without causing a significant decrease in quantum efficiency.

(54) Thus, optoceramics have been created which exhibit a remission at 600 nm from 0.7 to 0.95, and which have a quantum efficiency of more than 0.85.

(55) These extraordinary property of the inventive optoceramics is a result of a specific composition of the initial mixture and of production conditions. Surprisingly, it has been found that the scattering of the optoceramics can be adjusted by the choice of the sintering temperature. This will be explained hereinafter with reference to the examples shown in FIG. 9 and FIG. 10. FIG. 9 and FIG. 10 illustrate the dependence of remission, quantum efficiency, and of the product of remission and quantum efficiency of two cerium-doped optoceramics of different composition.

(56) FIG. 9 shows an overlay chart of an optoceramic of a composition Lu.sub.3(Ga,Al).sub.5O.sub.12—Ce, the optoceramic in FIG. 10 has a composition of Y.sub.3Al.sub.5O.sub.12—Ce.

(57) In both cases, remission is a function of sintering temperature, while for quantum efficiency there is only a weak dependence. This is particularly evident from FIG. 9.

(58) For example, an increase in sintering temperature by 100° C. results in a halving of remission, while quantum efficiency remains almost constant.

(59) As shown in FIG. 10, this effect is much less pronounced with the second optoceramic.

(60) The extent of the effect may especially depend on the composition of the starting material. For example, the relationship of the two parameters depends on the density of conversion centers, i.e. on the degree of doping.

(61) FIG. 11 shows the relationship between absorption efficiency and remission or scattering of optoceramics according to the invention having different degrees of doping of Ce.sub.2O.sub.3.

(62) Absorption efficiency refers to the fraction of absorbed excitation light of the incident excitation light. It is calculated according to 1−R.sub.excitation from the remission of blue excitation light. The remission of blue excitation light was measured on samples of 1 mm thickness in a quantum efficiency measurement station.

(63) Optoceramics with high doping levels (0.2 wt % Ce.sub.2O.sub.3) show a strong dependence of absorption efficiency and remission. Especially at high remission values of 0.8 and more, absorption efficiency decreases rapidly.

(64) It has been shown that the absorption efficiency for blue light, or blue remission, R.sub.excitation, can be adjusted within wide limits by adjusting scattering or yellow remission R.sub.emission.

(65) By adjusting scattering largely independently from quantum efficiency, a good light confinement and thus high luminances can be achieved on the one hand.

(66) On the other, it is possible to adjust scattering in a manner so that a combination of remitted excitation radiation R.sub.excitation, and converted light emission results in a certain color impression which falls on the conversion line in the chromaticity diagram. The conversion line is the line connecting the color coordinates of the excitation light and the color coordinates of the emission spectrum.

(67) Referring to FIG. 12, another exemplary embodiment will be described which allows for adjusting a white field-near point in the color spectrum diagram. For operation in remission the converter 1 is provided with a mirror 31. In order to arrive near the white field on the conversion line in the color spectrum diagram, the converter 1 is selected with respect to its scattering properties such that 20% of the blue excitation light is remitted and 80% is absorbed. The absorbed fraction of excitation light is predominantly converted into yellow light and is emitted either directly or after having been reflected at mirror 31, as a scattering lobe 51.

(68) The blue remitted light is composed of a Fresnel reflection 40 and a scattering lobe 41. This scattering lobe 41 has a Lambertian radiation pattern.

(69) Shifting of the white field-near point is now effected by adjusting the converter thickness. If the latter is so small that excitation light is reflected at mirror 31 to re-exit at the surface, a second scattering lobe 42 is produced. This second scattering lobe 42 will have a more or less directional radiation pattern, depending on the converter thickness and the scattering. A result of the superposition of scattering lobes 40, 41, 42 of the excitation light and scattering lobe 51 of the converted light is an angular dependence of the color impression.

(70) The angular dependence can be reduced by roughening the surface of the converter.

(71) An alternative of fine-tuning to obtain the white field-near point is to apply a thin scattering layer on the excitation face of converter 1. The scattering layer may for example consist of TiO.sub.2 or of non-doped optoceramic material. In both cases, the blue fraction of the emitted light is increased relative to the yellow fraction thereof.

(72) In order to arrive in the white field of the chromaticity diagram, the yellow light may be depleted from green fractions by filtering. This may be achieved, for example, by a dichroic mirror which reflects the blue light but transmits the incident green light, so that the latter will not exit on the utilization side of the converter.

(73) The converter materials according to the invention exhibit tailored thermal, thermo-optical, and optical properties.

(74) Synergetic interaction of these properties provides for very high luminances of more than 100 cd/mm.sup.2.