LIGHTING DEVICE

Abstract

A lighting device includes: a light conversion unit including a light conversion element including a material containing an optically active element from lanthanoids, wherein: the light conversion element includes a front side, a rear side, and a thickness (t), the light conversion element is set up for an irradiation on the front side with a primary light (I.sub.0), for a diffuse reflectance of the primary light (I.sub.REM), for a specular reflection of the primary light (I.sub.FRE), and for a diffuse emission of a secondary light (I.sub.EM) with an altered wavelength compared to the primary light, and the light conversion unit has a specific diffuse reflectance SDR=t.sup.?1.Math.I.sub.REM/(I.sub.0?I.sub.FRE), such that a luminous flux emitted by the light conversion unit at an irradiance limit of the light conversion unit with regard to a variation in the proportion of the optically active element is at most 4 mm.sup.?1 away from a maximum.

Claims

1. A lighting device, comprising: a light source configured for emitting a primary light; and a light conversion unit formed by or comprising a light conversion element comprising a material containing a proportion of at least one optically active element from a group of lanthanoids, wherein the light conversion element includes a front side, a rear side, and a thickness (t) that extends from the front side to the rear side, wherein the light conversion element is set up for an irradiation on the front side with the primary light (I.sub.0), for a diffuse reflectance of the primary light (I.sub.REM), for a specular reflection of the primary light (I.sub.FRE), and for a diffuse emission of a secondary light (I.sub.EM) with an altered wavelength compared to the primary light, and wherein the light conversion unit has a specific diffuse reflectance SDR=t.sup.?1.Math.I.sub.REM/(I.sub.0?I.sub.FRE), which is selected such that a luminous flux emitted by the light conversion unit at an irradiance limit of the light conversion unit with regard to a variation in the proportion of the at least one optically active element is at most 4 mm.sup.?1 away from a maximum.

2. The lighting device according to claim 1, wherein the specific diffuse reflectance SDR of the light conversion unit is selected such that the luminous flux emitted by the light conversion unit at the irradiance limit of the light conversion unit with regard to the variation in the proportion of the at least one optically active element is at least 0.25 mm.sup.?1 away from a maximum.

3. The lighting device according to claim 1, wherein the specific diffuse reflectance SDR of the light conversion unit is greater than 0.1 mm.sup.?1.

4. The lighting device according to claim 1, wherein the specific diffuse reflectance SDR of the light conversion unit is less than 7 mm.sup.?1.

5. The lighting device according to claim 1, wherein the light conversion unit has at least one highly reflective layer or coating.

6. The lighting device according to claim 5, wherein the light conversion unit includes at least one optical separation layer.

7. A lighting device according to claim 5, wherein the lighting device further includes an adhesion promoter layer beneath the at least one highly reflective layer.

8. The lighting device according to claim 1, wherein the light conversion unit further includes a substrate and a binder, the substrate being bonded directly or indirectly to the rear side of the light conversion element, the binder being between the light conversion element and the substrate, and wherein the binder is formed as at least one of an organic adhesive, at least one glass, at least one ceramic adhesive, at least one inorganic adhesive, at least one sintered sinter paste, and at least one metallic solder compound.

9. The lighting device according to claim 8, wherein a bond strength of the light conversion element on the substrate is greater than 1 MPa.

10. The lighting device according to claim 8, wherein the thickness (t) of the light conversion element is not more than 250 ?m.

11. The lighting device according to claim 8, wherein the substrate at least one of (a) includes at least one ceramic, at least one metal, or at least one ceramic-metal composite, and (b) has a thermal conductivity of greater than 30 W/mK.

12. The lighting device according to claim 1, wherein the light conversion element consists wholly of or predominantly includes at least one material of a composition (A.sub.1-y C.sub.y).sub.3B.sub.5O.sub.12, with A being selected from at least one of Y, Lu, and Gd, with B being selected from at least one of Al and Ga, and with C being selected from the at least one optically active element from the lanthanoids.

13. The lighting device according to claim 1, wherein the material of the light conversion element is wholly or partly a ceramic.

14. The lighting device according to claim 1, wherein the light conversion element comprises a first component composed of at least one material of a composition (A.sub.1-y C.sub.y).sub.3 B.sub.5O.sub.12, with A being selected from at least one of Y, Lu, and Gd, with B being selected from at least one of Al and Ga, and with C being selected from at least one of the lanthanoids, and wherein the light conversion element comprises a second component composed of a material having higher thermal conductivity.

15. The lighting device according to claim 1, wherein the material of the light conversion element includes a plurality of pores or a plurality of other light-scattering inclusions or particles.

16. The lighting device according to claim 1, wherein the lighting device includes: the thickness (t) of the light conversion element being not more than 90 ?m; a coefficient of scatter (s) of the light conversion element, applicable to a wavelength of 600 nm, of 150 cm.sup.?1<s<550 cm.sup.?1; a thermal conductivity (1) of the light conversion element, applicable to a room temperature, of 5 Wm.sup.?1K.sup.?1<1<15 Wm.sup.?1K.sup.?1; and a Ce content y of the light conversion element of y.sub.eff>0.0125 at %.

17. The lighting device according to claim 1, wherein the lighting device includes: the thickness (t) of the light conversion element being not more than 170 ?m; a coefficient of scatter (s) of the light conversion element, applicable to a wavelength of 600 nm, of 150 cm.sup.?1<s<550 cm.sup.?1; a color temperature (CCT)>5500 K; and a Ce content y of the light conversion element of y.sub.eff>0.025 at %.

18. The lighting device according to claim 1, wherein the lighting device includes: the thickness (t) of the light conversion element being not more than 170 ?m; a coefficient of scatter (s) of the light conversion element, applicable to a wavelength of 600 nm, of 150 cm.sup.?1<s<550 cm.sup.?1; a color temperature (CCT) of 4000<CCT<5500 K; and a Ce content y of the light conversion element of y.sub.eff>0.025 at %.

19. A light conversion unit, comprising: a light conversion element comprising a material containing a proportion of at least one optically active element from a group of lanthanoids, wherein the light conversion element includes a front side, a rear side, and a thickness (t) that extends from the front side to the rear side, wherein the light conversion element is set up for an irradiation on the front side with a primary light (I.sub.0), for a diffuse reflectance of the primary light (I.sub.REM), for a specular reflection of the primary light (I.sub.FRE), and for a diffuse emission of a secondary light (I.sub.EM) with an altered wavelength compared to the primary light, and wherein the light conversion unit has a specific diffuse reflectance SDR=t.sup.?1.Math.I.sub.REM/(I.sub.0?I.sub.FRE), which is selected such that a luminous flux emitted by the light conversion unit at an irradiance limit of the light conversion unit with regard to a variation in the proportion of the at least one optically active element is at most 4 mm.sup.?1 away from a maximum.

20. The light conversion unit according to claim 19, wherein the light conversion unit is configured for being used, and wherein at least one of: (a) the light conversion unit is configured for operating at a margin from the irradiance limit of the light conversion unit of less than 50 percent; and (b) the light conversion unit is configured for operating at a margin from the irradiance limit of the light conversion unit of greater than 5 percent.

21. A method of determining a specific diffuse reflectance SDR of a light conversion unit, the method comprising the steps of: providing that the light conversion unit includes a light conversion element; irradiating a front side of the light conversion element with a primary light (I.sub.0); measuring a diffuse reflectance of the primary light (I.sub.REM) and a specular reflection of the primary light (I.sub.FRE); measuring or determining a thickness (t) of the light conversion element; and calculating the specific diffuse reflectance by a formula, wherein the specific diffuse reflectance SDR=t.sup.?1.Math.I.sub.REM/(I.sub.0?I.sub.FRE).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0441] The invention is described in more detail hereinafter with reference to the figures that follow. The figures show: The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:

[0442] FIGS. 1A, 1B, and 1C show schematic section diagrams of light conversion units;

[0443] FIG. 2 shows a schematic section diagram of a measurement setup including an illumination device and a detector;

[0444] FIG. 3 shows graphs of emitted luminous flux against irradiance for 14 samples according to Table 1, the graphs being based on experimental measurements;

[0445] FIG. 4 shows graphs of emitted luminous flux against irradiance for 4 samples according to Table 2, the graphs being based on numerical simulations;

[0446] FIG. 5 shows graphs of emitted luminous flux against irradiance for 4 samples according to Table 2, the graphs being based on experimental measurements;

[0447] FIGS. 6A, 6B, 6C, 6D, and 6E show graphs of emitted luminous flux at the irradiance limit for various Ce contents of the light conversion element, plotted against irradiance at the irradiance limit;

[0448] FIG. 7 shows a schematic section diagram of a test setup including an illumination device and detectors;

[0449] FIGS. 8A, 8B, 8C, and 8D show schematic section diagrams of measurement devices for ascertaining the thickness t of the light conversion element;

[0450] FIGS. 9A, 9B, 9C, 9D, and 9E show graphs of emitted luminous flux at the irradiance limit for various Ce contents of the light conversion element, plotted against specific diffuse reflectance;

[0451] FIGS. 10A, 10B, 10C, 10C, 10D, and 10E show graphs of efficacy at the irradiance limit for the same variations as in FIGS. 9A, 9B, 9C, 9D, and 9E, plotted against specific diffuse reflectance; and

[0452] FIGS. 11A, 11B, 11C, 11D, and 11E shows graphs of efficient luminous flux (ELF) at the irradiance limit for the same variations as in FIGS. 9a-e, plotted against specific diffuse reflectance.

[0453] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one embodiment of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

[0454] The characteristics of the converter material optimized for operation close to the irradiance limit were found by varying absorption, scatter, thermal conductivity and thickness for the case of YAG:Ce, GYAG:Ce and LuAG:Ce-based converter materials, firstly by way of controlled experiments (production and analysis of corresponding samples) and secondly by numerical simulations of the materials and their properties.

[0455] For the experimental determination of the irradiance limit, 4?4 mm.sup.2 dies of a particular thickness, composed of various materials, polished on both sides, AR-coated on the front side, provided with a silver coating on the rear side, were bonded on the rear side to copper heat sinks by way of a silver sintering paste. Excitation was effected by way of a laser beam (450 nm) outcoupled from a fiber, of about 490 ?m in diameter and having a virtually uniform beam profile (top hat). The radiation emitted was ascertained with increasing excitation power until the light output dropped.

[0456] FIGS. 1A, 1B, and 1C show light conversion units 200. The light conversion unit 200 shown in FIG. 1A and FIG. 1B includes a light conversion element 1, a substrate 3 and optionally a binder 2. The light conversion element 1 may especially take the form of a ceramic converter. The binder may take the form, for example, of a solder, adhesive, or sintered sinter paste. The substrate may take the form, for example, of a heat sink, for example including or composed of copper, or of a wheel including or composed of aluminium.

[0457] The light conversion unit 200 shown in FIG. 1C likewise includes a light conversion element 1, a substrate 3 and optionally a binder 2. The diagram is especially intended to illustrate that the light conversion element 1 on its front side may optionally have a coating, e.g. an AR coating 11. The light conversion element 1 may optionally also have, on its rear side, a coating 12, e.g. an HR coating and/or an optical separation layer and/or an adhesion promoter. Alternatively or additionally, a coating 13 may be provided, which constitutes a reflection layer of the light conversion element 1 and which has been applied, for example, on an adhesion promoter layer 12.

[0458] The substrate 3 may optionally also have, on its side facing the light conversion element 1 or the binder 2, a coating 31, for example including or composed of Au, NiP/Au and/or Ag.

[0459] FIG. 2 shows a test setup including a lighting device 100 and a detector 10. Unless stated otherwise, the elucidations given in connection with FIGS. 1A, 1B, 1C are applicable to the individual constituents and reference numerals. The lighting device 100 includes a light conversion unit 200, in this case with converter 1, binder 2 and substrate 3, wherein the substrate 3 has been applied to a top layer 4 that may have, for example, an adjustable temperature, a primary light source 5, for example a blue laser beam source, especially with beamforming, set up so as to emit a laser beam 6 that hits the front side of the converter 1 so as to give rise to emitted radiation 7 composed of diffusely reflected primary light and diffusely emitted secondary light.

[0460] The basic construction of the converter component in FIG. 1 is not limited to the above-described variant used for the measurements. Instead, this is one of many possible embodiments. This is correspondingly applicable to the basic measurement setup in FIG. 2.

[0461] A fundamental factor that should be mentioned is that the light conversion properties (the level of light output emitted or of the luminous flux emitted, the irradiance limit) depend essentially on the following properties of the converter material and further boundary conditions: [0462] Characteristics of the incident blue laser beam: wavelength, power, power density, beam profile. [0463] Characteristics of the converter material: coefficient of absorption and coefficient of scatter for the incident blue laser radiation and for the converted radiation of greater wavelength, quantum efficiency, Stokes shift, refractive index, thermal conductivity and thickness. These properties (except for thickness t) are more or less temperature-dependent. [0464] Characteristics of the converter surfaces or interfaces: reflectivity of the incidence and emission side (front side), reflectivity of the rear side, heat transfer on the rear side to an actively or passively cooled wheel (dynamic case) or to an actively or passively cooled heat sink (static case), and the surface quality of the converter ceramic (generally polished).

[0465] Numerical simulations were fundamentally conducted as described in V. Hagemann, A. Seidl, G. Weidmann: Static ceramic phosphor assemblies for high power high luminance SSL-light sources for digital projection and specialty lighting. Proc. of SPIE Vol. 11302 113021N-11, SPIE OPTO, San Francisco 2020, [1] hereinafter. Some simulations were firstly used to simulate the experimental measurements, and hence the simulation was intrinsically verified, and secondly, in order to extend the parameter space, further material properties that are not covered by the samples available were simulated.

[0466] The materials listed in Table 1 were available for the experimental study. These are materials from the range of YAG:Ce, GYAG:Ce and LuAG:Ce. x and y denote the respective proportions of Gd and of Ce at the Y and Lu positions in the crystal lattice. t is the thickness of the converter material. In the case of a mixed ceramic (composite), z denotes the proportion by volume of the added component (e.g. aluminium oxide). In this case, it is important in respect of the absorber properties that are proportional to the Ce content to replace the Ce content y with the effective Ce content y.sub.eff, which is calculated as follows: y.sub.eff=(1?z).Math.y. In the case of single-phase converter materials (z=0), y.sub.eff is thus equal to y. If the scatter properties of the components of a mixed ceramic are distinctly different, an effective value is ascertained here too. The same may also apply to thermal conductivity and refractive index.

[0467] The laser beam power density attained at maximum measured light output is the irradiance limit. The accuracy of the determination depends on the degree to which the increase in laser power is stepped in the region of the irradiance limit. This applies both to experiment and to simulation.

[0468] FIG. 3 shows, for the 14 samples from Table 1 below, the progression of emitted luminous flux with increasing laser power.

TABLE-US-00001 TABLE 1 Laser power density attained at maximum light output (irradiance limit) for different converter materials (experimental determination). Irradiance s t limit No. Material x y z y.sub.eff [cm.sup.?1] [?m] #1 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0036 0.0036 ~350 80 107 #2 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0036 0.0036 ~350 90 93 #3 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0063 0.0063 ~350 80 73 #4 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0063 0.0063 ~350 100 66 #5 [(Lu.sub.1?yCe.sub.y).sub.3 Al.sub.5O.sub.12].sub.1?z[Al.sub.2O.sub.3].sub.z 0.0050 0.05 0.0048 ~350 80 93 #6 [(Lu.sub.1?yCe.sub.y).sub.3 Al.sub.5O.sub.12].sub.1?z[Al.sub.2O.sub.3].sub.z 0.0050 0.05 0.0048 ~350 100 73 #7 [(Lu.sub.1?yCe.sub.y).sub.3 Al.sub.5O.sub.12].sub.1?z[Al.sub.2O.sub.3].sub.z 0.0063 0.5 0.0032 ~350 100 121 #8 (Y.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0030 0.0030 ~500 150 52 #9 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.055 0.0027 0.0027 ~550 150 38 #10 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.1 0.0024 0.0024 ~600 150 38 #11 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.05 0.0012 0.0012 ~450 150 59 #12 (Y.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0012 0.0012 ~400 150 87 #13 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.1 0.0012 0.0012 ~500 150 52 #14 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0050 0.0050 ~350 100 73 indicates data missing or illegible when filed

[0469] In order to verify the numerical simulation, the experimental data of samples #2, #4, #7 and #14 were used. For this purpose, the material parameters reported in [1] (for YAG:Ce) were replaced by those for LuAG:Ce. In the case of #7, in addition, elevated thermal conductivity owing to the presence of a mixed LuAG-Al2O3 ceramic was taken into account, and the absorption is a result of the effective Ce content y.sub.eff.

[0470] FIG. 4 shows, as a result, the simulation data for the emitted luminous flux with increasing irradiance for the four samples mentioned.

[0471] By way of comparison, FIG. 5 shows the experimentally determined progressions, i.e. in each case the emitted luminous flux measured with increasing irradiance for the four different samples. The progressions and especially the limit are in good agreement with the experimental results, as also shown in Table 2 below.

TABLE-US-00002 TABLE 2 Irradiance limit for different converter materials (comparison of experimental determination and numerical simulation). Thick- Irradiance Irradiance ness limit limit No. Material y z y.sub.eff #2 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0036 0.0036 90 93 88 #4 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0063 0.0063 100 66 69 #7 [(Lu.sub.1?yCe.sub.y).sub.3 Al.sub.5O.sub.12].sub.1?z[Al.sub.2O.sub.3].sub.z 0.0063 0.5 0.0032 100 121 122 #14 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0050 0.0050 100 73 74 indicates data missing or illegible when filed

[0472] FIG. 6A shows the emitted luminous flux calculated in each case at the irradiance limit, for LuAG:Ce with coefficient of scatter s=350 cm.sup.?1, at different thickness t, for various Ce contents y, here in at %. What is thus shown is the maximum luminous flux ascertained by simulation (luminous flux of the converted emitted light at the irradiance limit, i.e. the maximum of calculated progressions of the type as shown in FIG. 4) depending on the Ce content y, for 3 different thicknesses of the converter, by way of example for LuAG:Ce with an illustrative coefficient of scatter of 350 cm.sup.?1. It can be seen that, within the range examined, thinner converters lead to higher possible luminous fluxes, and also that there is an optimal range in each case for the Ce content (range of highest possible luminous flux).

[0473] FIG. 6B shows the corresponding comparison for an illustrative variation in the coefficient of scatter, i.e. the emitted luminous flux calculated in each case at the irradiance limit, for LuAG:Ce with thickness t=80 ?m at a different coefficient of scatter s for various Ce contents y, here in at %.

[0474] A similar effect to a reduction in converter thickness is possessed by the illustrative addition of aluminium oxide to YAG:Ce or to LuAG:Ce. Since aluminium oxide has about 3 times the thermal conductivity of YAG or LuAG, this increases the thermal conductivity of the converter material overall (in accordance with the proportion by volume of Al.sub.2O.sub.3). Alternatively, it would also be possible to add a different component having comparatively high thermal conductivity with low absorption in the region of the converted light, e.g. aluminium nitride.

[0475] FIG. 6C shows the emitted luminous flux calculated in each case at the irradiance limit, for LuAG:Ce by comparison with a mixed LuAG-Al.sub.2O.sub.3 ceramic, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various (effective) Ce contents y.sub.eff, here in at %. This shows the effect using the example of LuAG:CeAl.sub.2O.sub.3 at an illustrative converter thickness of 80 ?m in the case of illustrative addition of aluminium oxide in such a way that thermal conductivity is increased in comparison by a factor of 2. An illustrative coefficient of scatter here too (for both components) is 350 cm.sup.?1.

[0476] In FIG. 6D, LuAG:Ce is compared with YAG:Ce; what is shown in each case is the emitted luminous flux calculated at the irradiance limit, for LuAG:Ce by comparison with YAG:Ce, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various Ce contents y, here in at %.

[0477] The reflectivity R of a rear-side reflective coating (for the wavelength range of the converted light) also affects the possible luminous flux, since more or less is absorbed at the rear side.

[0478] In FIG. 6E, by way of example, this reflectivity is varied, and hence shows the emitted luminous flux calculated at the irradiance limit, for LuAG:Ce with thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, at different rear-side reflectivity R, for various Ce contents y, here in at %.

[0479] However, experimental determination of the irradiance limit is complex and harbors the risk of destruction as a result of overheating at the irradiation point.

[0480] By contrast, specific diffuse blue reflectance at low power (low power specific diffuse blue remission, low power SDBR, unit: mm.sup.?1) can be ascertained via a non-destructive measurement on a finished component.

[0481] For the determination of low power SDBR, the converter material is irradiated with a (blue) laser beam of such low power or power density (irradiance) that it is not associated with any significant heating of the component, for example at about 1 mW/mm.sup.2 (10.sup.?3 W/mm.sup.2). In the case of such low irradiance, there is a strictly linear increase in the emitted luminous flux of the converted radiation with increasing irradiance, meaning that efficiency/efficacy (luminous flux based on laser power) is independent of irradiance. Low power in the meaning intended here thus means a linear increase in luminous flux emitted with incident irradiance. But it is never the case that all the incident blue light is absorbed. A portion of the (blue) laser light is subject to direct specular reflection at the surface (Fresnel reflection). A portion of the proportion that penetrates is absorbed (and a portion of that in turn is converted), but another portion is diffusely backscattered again (reflected) without being absorbed. This diffusely backscattered, unabsorbed component of the penetrating (non-Fresnel-reflected) (blue) laser radiation is the diffuse (blue) reflectance DBR, and the SDBR is that value based on the thickness of the converter t:

SDBR=DBR/t

[0482] FIG. 7 shows the measurement principle for the determination of diffuse (blue) reflectance DBR. Unless stated otherwise, the elucidations given in association with FIGS. 1A, 1B, 1C and FIG. 2 are applicable to the individual constituents and reference numerals. Primary light with irradiance I.sub.0 hits the front side of the light conversion element 1. This results in diffuse reflectance of primary light (I.sub.REM), specular reflection of primary light (I.sub.FRE) and diffuse emission of secondary light (I.sub.EM) with an altered wavelength compared to the primary light. These components are detected by detectors 9, 10, with detector 9 being designed for mirror-reflected radiation and detector 10 for diffusely reflected (blue) and diffusely emitted converted radiation.

[0483] FIGS. 8A, 8B, 8C, and 8D show measurement principles for ascertaining the thickness t of the converter, where 51 represents a mechanical measurement of thickness, for example with a slide rule, 52 a measurement of thickness using ultrasound sensors, 53 a measurement of thickness using NIR laser Doppler interferometry, and 54 a mechanical measurement of thickness with callipers (the approximate thickness of the binder is measured as well). Unless stated otherwise, the elucidations given in connection with FIGS. 1A, 1B, 1C, FIG. 2 are applicable to the individual constituents and reference numerals.

[0484] The thickness t of the converter bonded to a substrate is thus either determined before the coating and bonding of the converter and is thus known, or it can be determined on the finished component on one side in a non-destructive manner, for example by acoustic (ultrasound sensors) or optical (NIR laser Doppler interferometry) ways, if the diameter of the component is large enough. Rapid and simple measurement of the finished component is also possible with a calliper as a way of measuring the relative distance of the surface from the heat sink surface, but it is still necessary in this case to subtract a known, approximate value for the typical thickness of the binder (generally in the order of magnitude of 10 to 30 ?m).

[0485] FIGS. 9A, 9B, 9C, 9D, and 9E correspond to FIGS. 6A, 6B, 6C, 6D, and 6E, except that the irradiance limit is plotted against the value of low power SDBR. FIG. 9A: emitted luminous flux calculated at the irradiance limit, for LuAG:Ce with coefficient of scatter s=350 cm.sup.1, at different thickness t, for various Ce contents y, here in at %. FIG. 9B: emitted luminous flux calculated at the irradiance limit, for LuAG:Ce with thickness t=80 ?m, with different coefficient of scatter s, for various Ce contents y, here in at %. FIG. 9C: emitted luminous flux calculated at the irradiance limit, for LuAG:Ce by comparison with a mixed LuAG-Al2O3 ceramic, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various (effective) Ce contents y.sub.eff, here in at %. FIG. 9D: emitted luminous flux calculated at the irradiance limit, for LuAG:Ce by comparison with YAG:Ce, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various Ce contents y, here in at %. FIG. 9E: emitted luminous flux calculated at the irradiance limit, for LuAG:Ce with thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, at different rear-side reflectivity R, for various Ce contents y, here in at %.

[0486] For maximization of possible luminous flux, an option is given especially to a low power SDBR between 3 and 6.

[0487] However, it should be remembered that efficiency or, better, efficacy (lm/W) is likewise significant in respect of performance. Efficacy at the irradiance limit is emitted luminous flux based on incident laser power.

[0488] FIGS. 10A, 10B, 10C, 10D, and 10E show the dependence of efficacy on the irradiance limit for the same variations as in FIGS. 9A, 9B, 9C, 9D, and 9E. FIG. 10A: calculated efficacy at the irradiance limit, for LuAG:Ce with coefficient of scatter s=350 cm.sup.?1, at different thickness t, for various Ce contents y. FIG. 10B: calculated efficacy at the irradiance limit, for LuAG:Ce with thickness t=80 ?m, at different coefficient of scatter s, for various Ce contents y. FIG. 10C: calculated efficacy at the irradiance limit, for LuAG:Ce by comparison with a mixed LuAG-Al2O3 ceramic, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various (effective) Ce contents y.sub.eff. FIG. 10D: calculated efficacy at the irradiance limit, for LuAG:Ce by comparison with YAG:Ce, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various Ce contents y. FIG. 10E: calculated efficacy at the irradiance limit, for LuAG:Ce with thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, at different rear-side reflectivity R, for various Ce contents y.

[0489] Both may be significant for the performance of a converter component: maximum luminous flux F and maximal efficacy h. This is expressed by the product of the two parameters, efficient luminous flux:

ELF=F*h

[0490] FIGS. 11A, 11B, 11C, 11D, and 11E show the dependence of efficient luminous flux at the irradiance limit for the same variations as in the previous figures. FIG. 11A: calculated ELF at the irradiance limit, for LuAG:Ce with coefficient of scatter s=350 cm.sup.?1, at different thickness t, for various Ce contents y. FIG. 11B: calculated ELF at the irradiance limit, for LuAG:Ce with thickness t=80 ?m, at different coefficient of scatter s, for various Ce contents y. FIG. 11C: calculated ELF at the irradiance limit, for LuAG:Ce by comparison with a mixed LuAG-Al2O3 ceramic, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various (effective) Ce contents y.sub.eff. FIG. 11D: calculated ELF at the irradiance limit, for LuAG:Ce by comparison with YAG:Ce, for thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, for various Ce contents y. FIG. 11E: calculated ELF at the irradiance limit, for LuAG:Ce with thickness t=80 ?m, coefficient of scatter s=350 cm.sup.?1, at different rear-side reflectivity R, for various Ce contents y.

[0491] Specific diffuse (blue) reflectance SDR (SDBR) can be determined in a simple and non-destructive manner using a light conversion unit. This is largely independent of composition, thickness, scatter properties etc. SDR is an adjustable parameter of a light conversion unit.

[0492] SDR is optionally in the range of 0<SDR<3 mm.sup.?1, optionally 0.5<SDR<2.5 mm.sup.?1, optionally 0.8<SDBR<2 mm.sup.?1.

[0493] As an example, Table 3 shows the measured samples #1 to #14, with the diffuse blue reflectance DBR determined in each case according to FIG. 8, the converter thickness t, and the specific diffuse blue reflectance SDBR (low power SDBR) formed therefrom.

TABLE-US-00003 TABLE 3 Measured diffuse blue reflectance DBR, converter thickness t, and specific diffuse blue reflectance SDBR calculated therefrom for different converter materials. t SDBR No. Material x y z y.sub.eff DBR [?m] [mm.sup.?1] #1 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0036 0.0036 0.074 80 0.93 #2 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0036 0.0036 0.085 90 0.95 #3 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0063 0.0063 0.043 80 0.54 #4 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0063 0.0063 0.040 100 0.40 #5 [(Lu.sub.1?yCe.sub.y).sub.3 Al.sub.5O.sub.12].sub.1?z[Al.sub.2O.sub.3].sub.z 0.0050 0.05 0.0048 0.066 80 0.82 #6 [(Lu.sub.1?yCe.sub.y).sub.3 Al.sub.5O.sub.12].sub.1?z[Al.sub.2O.sub.3].sub.z 0.0050 0.05 0.0048 0.064 100 0.64 #7 [(Lu.sub.1?yCe.sub.y).sub.3 Al.sub.5O.sub.12].sub.1?z[Al.sub.2O.sub.3].sub.z 0.0063 0.5 0.0032 0.055 100 0.55 #8 (Y.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 1 0.0030 0.0030 0.120 150 0.80 #9 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.055 0.0027 0.0027 0.137 150 0.92 #10 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.1 0.0024 0.0024 0.143 150 0.95 #11 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.05 0.0012 0.0012 0.198 150 1.32 #12 (Y.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0012 0.0012 0.201 150 1.34 #13 (Y.sub.1?x?y Gd.sub.x Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.1 0.0012 0.0012 0.190 150 1.27 #14 (Lu.sub.1?y Ce.sub.y).sub.3 Al.sub.5 O.sub.12 0.0050 0.0050 0.054 100 0.54

[0494] For the production of these respective samples, powders of the pure oxides yttrium oxide, lutetium oxide, aluminium oxide, gadolinium oxide and cerium oxide were mixed according to the composition of the desired compound #1 to #14 and, after addition of ethanol, dispersing aids and compressing aids, and of grinding balls, they were finely ground in a drum by way of a roller bench. The slip was then dried by way of a rotary evaporator and then compressed uniaxially to give cylindrical green bodies. The green bodies were debindered at about 600? C., followed by reactive sintering under air at about 1600? C. (for several hours). The sintered bodies were sawn into wafers by way of a wire saw and then ground and polished to the desired thickness. Subsequently, the wafers were printed on the rear side by screenprinting with a solder glass-containing Ag thick-layer paste. The paste was fired at about 900? C. The wafers were subjected to vapor deposition on the front side of an about 97 nm thin AR layer of SiO2. The wafers that had thus been coated on both sides were individualized by dicing into dies of the 4?4 mm.sup.2 format. For bonding to an Au-plated Cu heat sink (20?20?3 format), an Ag sinter paste was applied by way of a dispenser in the center of the heat sink and a die was pressed on in each case, then this compound was sintered under air at about 200? C. for about 2 h.

[0495] While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.