PHOSPHOR CONVERTED LED WITH TEMPERATURE STABLE FLUX AND SATURATED RED COLOR POINT

Abstract

The invention provides a lighting device configured to provide red lighting device light, the lighting device comprising: (i) a first light source configured to provide first light source light having a peak wavelength (λls); (ii) a first red luminescent material configured to absorb at least part of the first light source light and to convert into first red luminescent material light having a first red emission peak wavelength (λm1), the first red luminescent material having an excitation maximum (λx1); (iii) a second red luminescent material configured to absorb at least part of the first light source light and to convert into second red luminescent material light having a second red emission peak wavelength (λm2), the second red luminescent material having a second excitation maximum (λx2); and wherein the first luminescent material and the second luminescent material are Eu2+ based, and wherein λm1<λm2, λx1<λls and λx2>λls.

Claims

1. A lighting device configured to provide red lighting device light, the lighting device comprising: a first light source configured to provide first light source light having a peak wavelength (λls); a first red luminescent material configured to absorb at least part of the first light source light and to convert into first red luminescent material light having a first red emission peak wavelength (λm1), the first red luminescent material having an excitation maximum (λx1); a second red luminescent material configured to absorb at least part of the first light source light and to convert into second red luminescent material light having a second red emission peak wavelength (λm2), the second red luminescent material having a second excitation maximum (λx2); wherein the first luminescent material and the second luminescent material are Eu.sup.2+ based, and wherein λm1<λm2, λx1<λls and λx2>λls, wherein the first red luminescent material comprises a ceramic material comprising a luminescent material of the class of M.sub.2Si.sub.5N.sub.8:Eu, wherein the second red luminescent material comprises a luminescent material of the class of MLiAl.sub.3N.sub.4:Eu dispersed in a light transmissive matrix, wherein the second red luminescent material is configured downstream of the first light source, wherein the first red luminescent material is configured upstream or downstream of the second red luminescent material, and wherein M is independently selected from the group consisting of Ca, Mg, Sr, and Ba.

2. The lighting device of claim 1, wherein λls is selected from within a range of about 430 nm to about 470 nm, wherein λm1 is selected from within a range of about 590 nm to about 630 nm, and wherein λm2 is selected from within a range of about 615 nm to about 660 nm.

3. The lighting device of claim 1, wherein λls is selected from within a range of about 435 nm to about 465 nm, wherein λm1 is selected from within a range of about 600 nm to about 630 nm, and wherein λm2 is selected from within a range of about 625 nm to about 660 nm.

4. (canceled)

5. The lighting device of claim 1, wherein the first red luminescent material and the second red luminescent material are selected from the group consisting of (Ba,Sr,Ca).sub.2Si.sub.5-xAl.sub.xN.sub.8-xO.sub.x:Eu, wherein x is in a range of about 0 to about 4, and (Ba,Sr)LiAl.sub.3N.sub.4:Eu.

6. The lighting device of claim 1, wherein the first red luminescent material comprises (Sr,Ca).sub.2Si.sub.5N.sub.8:Eu, and wherein the second red luminescent material comprises SrLiAl.sub.3N.sub.4:Eu.

7. (canceled)

8. The lighting device of claim 1, configured as a lighting unit to provide lighting unit light.

9. The lighting unit of claim 8, further comprising a second light source configured to generate second light source light, wherein the second light source are is configured to provide one or more of blue light, or green light, or yellow light or UV light.

10. (canceled)

11. The lighting unit of claim 9, further comprising a control unit configured to control the first light source, and the second light source independently.

12. The lighting unit of claim 9, configured to provide white lighting unit light.

13. A converter element comprising a first red luminescent material to absorb at least part of a first light source light and to convert into first red luminescent material light having a first red emission peak wavelength (λm1), the first red luminescent material having an excitation maximum (λx1); a second red luminescent material to absorb at least part of said first light source light and to convert into second red luminescent material light having a second red emission peak wavelength (λm2), the second red luminescent material having a second excitation maximum (λx2); wherein the first luminescent material and the second luminescent material are Eu.sup.2+ based, and wherein λm1<λm2, λx1<λls and λx2>λls, wherein the first red luminescent material comprises a ceramic material comprising a luminescent material of the class of M.sub.2Si.sub.5N.sub.8:Eu, wherein the second red luminescent material comprises a luminescent material of the class of MLiAl.sub.3N.sub.4:Eu dispersed in a light transmissive matrix, wherein the second red luminescent material is configured downstream of the first light source, wherein the first red luminescent material is configured upstream or downstream of said second red luminescent material, and wherein M is independently selected from the group consisting of Ca, Mg, Sr, and Ba.

14. The converter element of claim 13, wherein the light transmissive matrix comprise a polymer.

15. The converter element of claim 14, wherein the light transmissive matrix comprises a silicone.

16. The lighting unit of claim 9, further comprising a third light source configured to generate third light source light, wherein the third light source is configured to provide one or more of blue light, or green light, or yellow light or UV light.

17. The lighting unit of claim 16, further comprising a third luminescent material configured to convert at least part of one or more of the first light source light, the second light source light or the third light source light into third luminescent material light.

18. The lighting unit of claim 16, further comprising a control unit configured to control the first light source, the second light source and the third light source independently.

19. The lighting unit of claim 8, further comprising a backlighting unit of a liquid crystal display (LCD) device.

20. The lighting device of claim 1, further comprising at least a portion of a projection system.

21. The lighting device of claim 1, further comprising at least a portion of a self-lit display system.

22. The lighting device of claim 1, further comprising at least a portion of a projection system.

23. The lighting device of claim 1, further comprising at least a portion of a pixelated display system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] 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:

[0051] FIG. 1a-1c schematically depict some aspects of the invention. These schematic drawings are not necessarily on scale.

[0052] FIG. 2a shows the flux stability with temperature of amongst others a combination of phosphors as defined herein, with on the x-axis the socket temperature in ° C. and on the y-axis the relative flux (F) in lumen (Lm), for a first red luminescent material (a), a second luminescent material (b), the combination of the two luminescent materials (a+b) and a for comparison a red AlInGaP LED (c);

[0053] FIG. 2b shows the centroid wavelength shift with temperature, with on the x-axis the socket temperature in ° C. and on the y-axis the centroid wavelength (nm) for a red LED (c) and the same combination of luminescent materials (a+b);

[0054] FIG. 2c shows the absorption (in fact here substantially identical to the excitation (arbitray units on the y-axis)) of red phosphors (a/b) and blue emission shift behavior of a blue LED at different temperatures, indicated in the drawing. The emission is normalized to 1 (intensity in arbitrary units). The x-axis indicates the wavelength (nm);

[0055] FIG. 2d shows the color point shift of the saturated color points with temperature in the CIE 1976 chromaticity chart, with the diamonds indicating the first luminescent material (a), the triangles indicating the second luminescent material (b), and the squares indicating the combination (a+b) of the luminescent materials.

[0056] FIG. 2e shows excitation and emission spectra of the red luminescent materials of which date are also displayed in FIGS. 2a-2d;

[0057] FIG. 2f shows emission spectra of the same combination of luminescent materials at 30, 60, 85 and 120° C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0058] FIG. 1a schematically depicts a non-limiting number of embodiments of the lighting device 100 of the invention. Each lighting device 100 is configured to provide red lighting device light 101. The lighting device 100 comprises a first light source 1, here by way of example a solid state light source (LED), configured to provide first light source light 111 having a peak wavelength λls. Further, each lighting device 100 comprises a first red luminescent material 21 configured to absorb at least part of the first light source light 111 and to convert into first red luminescent material light 221 having a first red emission peak wavelength λm1, the first red luminescent material 21 having an excitation maximum λx1, as well as a second red luminescent material 22 configured to absorb at least part of the first light source light 111 and to convert into second red luminescent material light 221 having a second red emission peak wavelength λm2, the second red luminescent material 22 having a second excitation maximum λx2. The light escaping from the lighting device 100, which is indicated with reference 101, comprises thus said first red luminescent material light 221 and said second red luminescent material light 221. Optionally, this lighting device light 101 may also include light source light 111. Reference 110 indicates a LED die (i.e. the light emissive surface of a LED).

[0059] Six examples of lighting devices are shown. In example I, a converter, indicated with reference 20, comprises both luminescent materials 21,22. The converter may e.g. comprises a light transmissive matrix including both luminescent materials 21,22. In example II, the first luminescent material 21 and the second luminescent material are embedded in a dome or lens-like transparent matrix, e.g. silicone. In both examples I and II there is substantially no distance between the luminescent materials and the light source 1, especially its LED die 110. Reference 30 indicates a support, such as a PCB (printed circuit board).

[0060] In example III, two layers are provided, such as coatings or ceramic bodies, or polymeric matrices, or a coating and a ceramic body, or a ceramic body and a polymeric matrix, etc. Here, by way of example, the second luminescent material 22 is (more) remote from the light source 1, whereas the first luminescent material 21 may be in direct contact with the light emissive surface, here LED die 110, of the first light source 1. In Example IV substantially the same configuration as in example III is shown. However, now the second luminescent material 22 is embedded in a polymeric matrix, such as silicone, which may be provided in the shape of a lens or dome.

[0061] In example V, a device with a chamber 28 is shown. Both luminescent materials 21 and 22 are configured remote, here by way of example in a single converter layer. Reference 12 indicates a window. Further, downstream of the window 12, further optical elements may optionally be available (not shown). Here, the second red luminescent material 22 is configured as window. Note however that a window (material) may e.g. be also used as support for a coating (of one or more of the luminescent materials 21,22), etc. The distance (of the remote luminescent material to the light source 1) is indicated with reference d, which may e.g. in the range of 0.5-100 mm. Example VI shows a hybrid system of examples III and V, with a chamber 28 with one of the luminescent materials, here the second luminescent material 22, remote and the other luminescent material, here the first luminescent material 21, arranged on the light source 1.

[0062] Note however that much more embodiments are possible, including combinations of some of the above described examples. Further, the arrangement of the first luminescent material 21 and the second luminescent material 22 may also be the other way around. Further, optionally both luminescent materials 21,22 are arranged on both options depicted in examples III-VI.

[0063] FIG. 1b schematically depicts three non-limiting examples of the lighting unit 10 of the invention. Each lighting unit 10 is configured to provide lighting unit light 11. Each lighting unit 10 comprises one or more lighting devices 100 (here by way of example only one is schematically depicted). In these examples I-III, the lighting unit 10 further comprises a second light source 2 configured to generate second light source light 321 (examples I-III) and, and optionally a third light source 3 (example I) configured to generate third light source light 331, wherein the second light source 2 and the optional third light source 3 are configured to provide one or more of blue light, green light, yellow light, orange light, deep red light, and UV light. Reference 12 indicates a window, which may for instance include scattering material. However, such window may also include light direction elements. Further, downstream of the window 12 further optical elements may be available (not shown). In embodiments, the window may be configured as support for a coating.

[0064] In example I, for instance, the lighting device 100, comprising first light source 1, provides red lighting device light 101 (see also above for optional embodiments for this lighting device 100). The second light source 2 may e.g. be configured to provide blue light as second light source light 321 (and is therefore also indicated as light 111, as especially the first light source may provide blue light source light 111; see FIG. 1a) and the third light source 3 may be configured to provide green light as third light source light 331. Hence, the first light source 1 and the second light source 2 might optionally be identical, with the former however the first light source light 111 substantially having converted into red lighting device light 101 by the luminescent materials (not shown in these examples; see for details e.g. FIG. 1a).

[0065] In example II, for instance, a combination of the lighting device and a blue LED (2.sup.nd light source) with yellow converter is schematically depicted. The second light source 2 is provide with a third converter 23, which may be configured to convert part of the (blue) second light source light 321 of the second light source 2 into yellow light. The light emission of the third converter 321 is indicated with reference 231 and indicates the third luminescent material light. The blue light source light 321 and the yellow third luminescent material light 231 may be used to provide white lighting unit light 11; the red lighting device light 101 may be used to tune the lighting unit light 11, e.g. to provide more warm white light. Hence, the lighting unit light 11 may, e.g. assuming white light, include blue second light source light 321 and yellow third luminescent material light 231, optionally also (some) blue first light source light, and optionally, e.g. dependent upon the desired color temperature, also red lighting device light 101.

[0066] In example III, for instance, substantially the same embodiment as in example II is depicted. However, now the third converter 23 is arranged remote.

[0067] FIG. 1c schematically depicts in more detail an embodiment of the lighting device 100 as defined herein. Reference 40 indicates a lens (e.g. silicone, glass, plastic material, etc.). References 21 and 22 indicte the red phosphors, which may be provided as powder in silicone/glass/plastic material, as ceramic, as ceramic or glass with multilayer interference filter, etc. Here, by way of example two luminescent material layers, e.g. ceramic bodies, are provided as converter 20. Reference 110 indicates a blue die. Further, reference 30 indicates a mechanical support/socket. Further, reference 50 indicates a side coat or edge element, such as comprising a silicone, glass, plastic material, or epoxy resin, etc., with for instance one or more of a reflector material (titania, alumina, etc.), a thermal conduction support like crystobalite and/or alumina, optionally also one or more of the red phosphors, a protective layer against humidity, etc. However, such side coat or edge element is not necessarily available. Note that the lens or dome 40 encloses a substantial part of the converter, here the ceramic materials. FIG. 1c especially shows an embodiment wherein the invention provides a sandwich structure with first light source 1, especially solid state light source (die 110), and the ceramic body sandwiching a matrix layer comprising a luminescent material (the latter thus being provided directly on the light source (die 110)), especially a sandwich structure wherein the matrix layer at the edges may further be enclosed with an edge element 50, such as a reflective rim. Hence, the matrix layer may be substantially fully enclosed by the light source (die), the ceramic body and the edge element. Even more especially, the ceramic body comprises the luminescent material of the class of M.sub.2Si.sub.5N.sub.8:Eu and the matrix layer comprises the luminescent material of the class of MLiAl.sub.3N.sub.4:Eu, with the matrix material being e.g. a silicone (glue). Further, especially the first light source comprises a solid state light source, especially a high power solid state light source (configured to provide blue light). Hence, in an embodiment the luminescent material of the class of MLiAl.sub.3N.sub.4:Eu is dispersed in a light transmissive matrix configured downstream of the first light source 1 and the ceramic material comprising a luminescent material of the class of M.sub.2Si.sub.5N.sub.8:Eu is configured downstream of said the luminescent material of the class of MLiAl.sub.3N.sub.4:Eu dispersed in a light transmissive matrix.

[0068] In FIG. 2a the emitted flux normalized to the flux at 30° C. socket temperature as a function of socket temperature is shown. The normalized flux (F) is indicated on the y-axis (lumen). For a direct emitting AlInGaP LED (line c) the flux loss with temperature is evident. At 120° C. it is >40% whereas for the mixture (dashed line: a+b) of two dedicated red converting phosphors (a,b) the emitted flux is almost constant as a function of temperature. In these examples, the first phosphor material comprises (Ba,Sr,Ca).sub.2Si.sub.5-xAl.sub.xO.sub.xN.sub.8-x:Eu (herein also indicated as luminescent material a or phosphor a) in dense sintered ceramic form and the second phosphor material comprises (Sr,Ba)LiAl.sub.3N.sub.4:Eu (herein also indicated as luminescent material b or phosphor b) in powder form suspended in a silicone matrix.

[0069] FIG. 2b shows how the centroid wavelength (CW) for a direct red emitting LED (AlInGaP) and the mixture of red emitting phosphors according to the invention vary with temperature. Typically for a 627 nm direct red emitting AlInGaP LED the CW varies with a temperature coefficient of 0.05 nm/° C. The CW of the luminescent material mixture (a+b) is typically in the range of 630 nm to 640 nm with a temperature coefficient of −0.02 nm/° C., this translates into a more stable color point and into temperature independent flux. In FIG. 2b, the centroid wavelength calculation is made for the emission spectra of FIG. 2F and also includes the remaining blue light in the spectrum. This is an exception; when the centroid wavelength of the two red emissions is to be determined, this only relates to the red emission. In FIG. 2b however, the centroid wavelength is compared between a red LED and a device according to the invention. For the sake of comparison, the small blue contribution is not a problem. It is clear that the device of the invention is much less dependent from the temperature than the red LED.

[0070] The emission spectrum of Eu.sup.2+ activated phosphors shifts to shorter wavelength with increasing temperature. In order to compensate this effect a mixture of two red phosphors is applied on a blue LED, which consists of a first phosphor e.g. emitting at a first peak wavelength λ1 e.g. in the range 600-630 nm with e.g. an absorption maximum<440 nm and a second phosphor emitting at a second wavelength e.g. λ2>630 nm and e.g. an absorption maximum>440 nm. For a blue LED emitting in the range of 430 to 460 nm, the emission spectrum shifts to longer wavelength with increasing socket temperature (herein also indicated as base temperature) (FIG. 2c, Table 1).

TABLE-US-00001 TABLE 1 peak and centroid wavelength (CW) of a blue LED for different temperatures Base temperature Peak wavelength Centroid wavelength [° C.] [nm] [nm] 30 452.3 454.5 60 453.5 455.4 85 454.7 456.3 120 456.5 457.9

[0071] The peak wavelengths of the blue LED in table 1 correspond thus with the maxima in FIG. 2c.

[0072] Thus with increasing temperature, the conversion for the first red phosphor decreases and the conversion with the second red phosphor (with longer wavelength) increases (FIG. 2c), and the change in red flux and color point of the phosphor converted LED is reduced as shown in FIG. 2d. FIG. 2c shows a combination of two red luminescent materials ((Ba,Sr,Ca).sub.2Si.sub.5-xAl.sub.xO.sub.xN.sub.8-x:Eu (a) and (Sr,Ba)LiAl.sub.3N.sub.4:Eu (b), one having a shorter wavelength maximum than the other, and both having an excitation maximum close to the light source emission, but the first luminescent material having an excitation maximum at a wavelength shorter than the excitation maximum, and the second luminescent material having an excitation maximum at a wavelength longer than the excitation maximum. The maxima described here relate to the peak wavelength. The shift of the saturated color points is also positively influenced by mixing the two red phosphors. Referring to FIG. 2c it appears that the red luminescent materials excitations substantially overlap with the emission wavelength distribution of the first light source.

[0073] In FIG. 2d it is obvious, that the mixture is less shifting with temperature compare to the single phosphors. In Table 2 the maximum color point shift from 30° C. to 120° C. is tabulated.

TABLE-US-00002 TABLE 2 color point shift Δu.sup.′ Δu′v′ red phosphor 1 0.014 0.014 red phosphor 2 0.029 0.030 Mixture 0.012 0.012

[0074] FIG. 2e shows in a single graph the excitation spectra and emission spectra of the two red luminescent materials. The respective excitation peak wavelengths are found at 435 and 480 nm and are of the luminescent materials a and b, respectively (see also FIG. 2c); the respective emission peak (centroid) wavelengths are found at 616 (632) nm and 650 (662) nm, also of the luminescent materials a and b, respectively. A combination of the emissions on a blue LED as function of the temperature with on the y-axis intensity in arbitrary units is indicated in FIG. 2f. The centroid wavelength of the combination of these red luminescent materials are found at 640.9 nm, 640.6 nm, 640.2 nm, and 639.6 nm (in the order of increasing temperature), respectively. The from this graph 2f indicated centroid wavelengths are only based on the red emission, and not on the remaining blue emission in the graph. Hence, the centroid wavelength is evaluated in a range of about 510-800 nm.