PHOSPHOR CONVERTED WHITE LIGHT EMITTING DEVICES AND PHOTOLUMINESCENCE COMPOUNDS FOR GENERAL LIGHTING AND DISPLAY BACKLIGHTING

Abstract

A phosphor converted white light emitting device comprises a solid-state light emitter (LED) operable to generate blue light with a dominant wavelength in range 440 nm to 470 nm; yellow to green-emitting phosphor operable to generate light with a peak emission wavelength in a range 500 nm to 550 nm; and a red-emitting manganese-activated fluoride phosphor such a manganese-activated potassium hexafluorosilicate phosphor (K.sub.2SiF.sub.6:Mn.sup.4+). The yellow to green and red-emitting phosphors are incorporated as a mixture and dispersed throughout a light transmissive material with an index or refraction of 1.40 to 1.43. In some embodiments the light transmissive comprises a dimethyl-based silicone. The device can further comprise an orange to red-emitting phosphor operable to generate light with a peak emission wavelength of 580 nm to 620 nm.

Claims

1. A white light emitting device comprising: a solid-state light emitter operable to generate blue light with a dominant wavelength in a range 440 nm to 470 nm; a yellow to green-emitting phosphor excitable by blue light and operable to generate light with a peak emission wavelength in a range 500 nm to 575 nm; a red-emitting manganese-activated complex fluoride phosphor; and a light transmissive material with an index of refraction of 1.40 to 1.43 comprising a mixture of the yellow to green-emitting phosphor and red-emitting manganese-activated fluoride phosphor.

2. The white light emitting device of claim 1, wherein the red-emitting manganese-activated fluoride phosphor comprises a manganese-activated potassium hexafluorosilicate phosphor.

3. The white light emitting device of claim 1, wherein the light transmissive material comprises a methyl-based silicone.

4. The white light emitting device of claim 1, wherein the yellow to green-emitting phosphor comprises a cerium-activated garnet phosphor.

5. The white light emitting device of claim 4, wherein the cerium-activated garnet phosphor is represented by the chemical formula Y.sub.3x(Al.sub.1yGa.sub.y).sub.5O.sub.12:Ce.sub.x where 0.01<x<0.2 and 0<y<2.5.

6. The white light emitting device of claim 5, wherein the cerium-activated garnet phosphor further comprises at least one of F, Cl and Br.

7. The white light emitting device of claim 4, wherein the cerium-activated garnet phosphor is represented by the chemical formula Lu.sub.3x(Al.sub.1yM.sub.y).sub.5O.sub.12:Ce.sub.x where M is at least one of Mg, Ca, Sr, Ba, Ga and combinations thereof, 0.01<x<0.2 and 0<y<1.5.

8. The white light emitting device of claim 7, wherein the cerium-activated garnet phosphor further comprises at least one of F, Cl and Br.

9. The white light emitting device of claim 1, wherein the yellow to green-emitting phosphor comprises a europium activated -SiAlON phosphor.

10. The white light emitting device of claim 1, wherein the yellow to green-emitting phosphor comprises a europium-activated sulfide phosphor represented by the general formula SrGa.sub.2S.sub.4:Eu.

11. The white light emitting device of claim 1, further comprising an orange to red-emitting phosphor excitable by blue light and operable to emit light with a peak emission wavelength in a range 580 nm to 620 nm, and wherein the light transmissive material comprises a mixture of the yellow to green-emitting phosphor, red-emitting manganese-activated fluoride phosphor and orange to red-emitting phosphor.

12. The white light emitting device of claim 11, wherein the orange to red-emitting phosphor comprises a europium activated silicon nitride-based phosphor.

13. The white light emitting device of claim 12, wherein the europium activated silicon-nitride phosphor is represented by the chemical formula (Ca.sub.1xSr.sub.x)AlSiN.sub.3:Eu where 0.5<x1.

14. The white light emitting device of claim 12, wherein the europium activated silicon-nitride phosphor is represented by the chemical formula Ba.sub.2xSr.sub.xSi.sub.5N.sub.8:Eu where 0x2.

15. The white light emitting device of claim 1, wherein the device is operable to generate white light with a Correlated Color Temperature of between 2700K and 3000K, a general Color Rendering Index (Ra) of 90 or higher and a Color Rendering Index (R9) of 90 or higher.

16. The white light emitting device of claim 1, wherein the weight proportion of red-emitting manganese activated fluoride phosphor to green-emitting phosphor is greater than 50%.

17. The white light emitting device of claim 1, wherein the weight percent proportion of the red-emitting manganese activated fluoride phosphor to yellow to green-emitting phosphor is between about 70 wt % and about 90 wt %.

18. A white light emitting device comprising: a solid-state light emitter operable to generate blue light with a dominant wavelength in range 440 nm to 470 nm; a yellow to green-emitting phosphor excitable by blue light and operable to emit light with a peak emission wavelength in a range 500 nm to 575 nm; a red-emitting manganese-activated potassium hexafluorosilicate phosphor excitable by blue light and operable to emit light with a peak emission wavelength between 631 nm and 632 nm; and an orange to red-emitting phosphor excitable by blue light and operable to generate light with a peak emission wavelength in a range 575 nm to 600 nm, wherein the device is operable to generate white light with a Correlated Color Temperature of between about 2700K and about 3000K and wherein over a wavelength range 460 nm to 600 nm a maximum deviation between the intensity of the light emitted by the device normalized to a CIE 1931 XYZ relative luminance Y=100 compared with the intensity of light of a black-body curve of the same Correlated Color Temperature that is normalized to a CIE 1931 XYZ relative luminance Y=100 is less than 0.3.

19. The white light emitting device of claim 18, wherein the device is operable to generate white light with a general Color Rendering Index (Ra) of 90 or higher and a Color Rendering Index (R9) of 90 or higher.

20. The white light emitting device of claim 18, further comprising a light transmissive material with an index of refraction of 1.40 to 1.43 comprising a mixture of the yellow to green-emitting phosphor, red-emitting manganese-activated potassium hexafluorosilicate phosphor and the orange to red-emitting phosphor.

21. The white light emitting device of claim 18, wherein the yellow to green-emitting phosphor comprises a cerium-activated yttrium garnet phosphor.

22. The white light emitting device of claim 18, wherein the yellow to green-emitting phosphor comprises a cerium-activated lutetium garnet phosphor.

23. The white light emitting device of claim 18, wherein the orange to red-emitting phosphor comprises a europium activated silicon nitride-based phosphor.

24. A photoluminescence compound comprising: a light transmissive material with an index of refraction of 1.40 to 1.43 comprising a mixture of a yellow to green-emitting phosphor with a peak emission wavelength in a range 500 nm to 575 nm and a red-emitting manganese-activated fluoride phosphor.

25. The photoluminescence compound of claim 24, wherein the red-emitting manganese-activated fluoride phosphor comprise a potassium hexafluorosilicate phosphor.

26. The photoluminescence material of claim 24, wherein the light transmissive material comprises a methyl-based silicone.

27. The photoluminescence material of claim 24, wherein the yellow to green-emitting phosphor comprises a cerium-activated garnet phosphor.

28. The photoluminescence material of claim 24, further comprising an orange to red-emitting phosphor with a peak emission wavelength in a range 580 nm to 620 nm, and wherein the light transmissive material comprises a mixture of the yellow to green-emitting phosphor, red-emitting manganese-activated potassium hexafluorosilicate phosphor and orange to red-emitting phosphor.

29. The photoluminescence material of claim 28, wherein the orange to red-emitting phosphor comprises a europium activated silicon nitride-based phosphor.

30. A display backlight comprising: a solid-state light emitter operable to generate blue light; a narrow-band green-emitting phosphor excitable by blue light and operable to generate light with a peak emission wavelength of about 535 nm; a red-emitting manganese-activated potassium hexafluorosilicate phosphor; and a light transmissive material with an index of refraction of 1.40 to 1.43 comprising a mixture of the narrow-band green-emitting phosphor and the red-emitting manganese-activated potassium hexafluorosilicate phosphor.

Description

DESCRIPTION OF THE DRAWINGS

[0040] In order that the present invention is better understood, a LED-based white light emitting devices and photoluminescence compounds in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which like reference numerals are used to denote like parts, and in which:

[0041] FIG. 1 is schematic representation of an LED-based white light emitting device in accordance with an embodiment of the invention;

[0042] FIG. 2 is a plot of luminous flux versus CIE x for LED-based white light emitting devices for which a) the phosphors are incorporated as a mixture in a phenyl-based silicone and b) the phosphors are incorporated as a mixture in a dimethyl-based silicone;

[0043] FIG. 3 is an emission spectrum of an LED-based white light emitting device in accordance with an embodiment of the invention;

[0044] FIG. 4 is an emission spectrum of an LED-based white light emitting device (Device 11) in accordance with an embodiment of the invention;

[0045] FIG. 5 is an emission spectrum of an LED-based white light emitting device (Device 12) in accordance with an embodiment of the invention;

[0046] FIG. 6 is an emission spectrum of an LED-based white light emitting device (Device 13) in accordance with an embodiment of the invention;

[0047] FIG. 7 normalized intensity (normalized to a CIE 1931 XYZ relative luminance Y=100) versus wavelength for (i) Device 14 (solid line), (i) Device 15 (dotted line) and (i) Plankian locus (dashed line) for a CCT of 2700 K;

[0048] FIG. 8 normalized intensity deviation (normalized for a CIE 1931 XYZ relative Luminance Y=100) from the black-body curve (2700 K) versus wavelength for Devices 14 and 15;

[0049] FIG. 9 is an emission spectrum of an LED-based white light emitting device, display backlight (Device 16), in accordance with an embodiment of the invention; and

[0050] FIG. 10 is an emission spectrum of an LED-based white light emitting device, display backlight (Device 17) in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0051] FIG. 1 is a schematic representation of a white light emitting device 10, according to an embodiment of the invention. The device 10 is configured to generate warm white light with a CCT (Correlated Color Temperature) of approximately 2700 K and a General CRI (Color Rendering Index) CRI (Ra) of 90 and higher.

[0052] The device 10 can comprise one or more blue-emitting GaN (gallium nitride)-based LED chips 12 that are housed within a package 14. The one or more LED chips are operable to generate blue light with a dominant wavelength within a range of 440 nm to 470 nm, typically 450 nm to 455 nm. The package, which can for example comprise Surface Mountable Device (SMD) such as an SMD 5630 LED package, comprises upper and lower body parts 16, 18. The upper body part 16 defines a recess 20 which is configured to receive the one or more LED chips 12. The package further comprises electrical connectors 22 and 24 on the base that are electrically connected to corresponding electrode contact pads 26 and 28 on the floor of the recess 20. Using adhesive or solder, the LED chip(s) 12 can be mounted to a thermally conductive pad 30 located on the floor of the recess 20. The thermally conductive pad 30 is thermally connected to a thermally conductive pad 32 on the base of the package. The LED chip's electrode pads are electrically connected to corresponding electrode contact pads 26 and 28 on the floor of the package using bond wires 34 and 36 and the recess 20 is completely filled with a transparent silicone 38 which is loaded with a mixture of a yellow to green-emitting phosphor, orange to red-emitting phosphor and a red-emitting manganese-activated fluoride phosphor such that the exposed surfaces of the LED chip 12 are covered by the phosphor/silicone material mixture. To enhance the emission brightness of the device the walls of the recess 20 are inclined and have a light reflective surface.

[0053] The red-emitting manganese-activated fluoride phosphor can comprise a potassium hexafluorosilicate phosphor which can be represented by the chemical formula K.sub.2SiF.sub.6:Mn.sup.4+ excitable by blue excitation light and operable to generate red light with a peak emission wavelength .sub.p of about 631 nm to about 632 nm. An example of such a phosphor is NR6931 KSF phosphor from Intematix Corporation, Fremont, Calif., USA which has a peak emission wavelength of 632 nm. For the sake of brevity manganese-activated potassium hexafluorosilicate phosphor and K.sub.2SiF.sub.6:Mn.sup.4+ phosphor will be referred to as KSF.

[0054] The yellow to green-emitting phosphor can comprise any phosphor excitable by blue light and operable to generate light with a peak emission wavelength .sub.p in a range 500 nm to 575 nm and can include for example silicate-based phosphors, a garnet-based phosphor such as YAG or LuAG phosphors. Examples of such phosphors are given in TABLE 1.

TABLE-US-00001 TABLE 1 Example yellow to green-emitting phosphors Wavelength Phosphor General Composition .sub.p (nm) YAG Y.sub.3x(Al.sub.1yGa.sub.y).sub.5O.sub.12:Ce.sub.x 0.01 < x < 0.2 & 0 < y < 2.5 520-550 LuAG Lu.sub.3x(Al.sub.1yM.sub.y).sub.5O.sub.12:Ce.sub.x 0.01 < x < 0.2 & 0 < y < 1.5 500-550 M = Mg, Ca, Sr, Ba, Ga, LuAG Lu.sub.3x(Al.sub.1yGa.sub.y).sub.5O.sub.12:Ce.sub.x 0.01 < x < 0.2 & 0 < y < 1.5 500-550 Silicate A.sub.2SiO.sub.4:Eu A = Mg, Ca, Sr, Ba 500-550 Silicate (Sr.sub.1xBa.sub.x).sub.2SiO.sub.4:Eu 0.3 < x < 0.9 500-550

[0055] In one embodiment the yellow to green-emitting phosphor comprises a green-emitting LuAG-based phosphor as taught in U.S. Pat. No. 8,529,791 entitled Green-Emitting, Garnet-Based Phosphors in General and Backlighting Applications which is hereby incorporated in its entirety. Such a green-emitting phosphor comprises a cerium-activated, green-emitting lutetium aluminate phosphor consisting of lutetium, cerium, at least one alkaline earth metal, aluminum, oxygen, and at least one halogen, wherein the phosphor is configured to absorb excitation radiation having a wavelength ranging from about 380 nm to about 480 nm, and to emit light having a peak emission wavelength .sub.p ranging from about 500 nm to about 550 nm. An example of such a phosphor is GAL540 phosphor from Intematix Corporation, Fremont, Calif., USA which has a peak emission wavelength of 540 nm.

[0056] The orange to red-emitting phosphor can comprise any phosphor excitable by blue light and operable to emit light with a peak emission wavelength .sub.p in a range 580 nm to 620 nm and can include for example a silicate, europium activated silicon nitride-based phosphor or -SiAlON phosphor. Examples of such orange to red-emitting phosphors are given in TABLE 2. In one embodiment the orange-emitting phosphor comprises a red-emitting phosphor as taught in U.S. Pat. No. 8,597,545 entitled Red-Emitting Nitride-Based Calcium-Stabilized Phosphors which is hereby incorporated in its entirety. Such a red emitting phosphor comprises a nitride-based composition represented by the chemical formula M.sub.aSr.sub.bSi.sub.cAl.sub.dN.sub.eEu.sub.f, wherein: M is Ca, and 0.1a0.4; 1.5<b<2.5; 4.0c5.0; 0.1d0.15; 7.5<e<8.5; and 0<f<0.1; wherein a+b+f>2+d/v and v is the valence of M. Alternatively, the red-emitting phosphor comprises a red light emitting nitride-based phosphor as taught in U.S. Pat. No. 8,663,502 entitled Red-Emitting Nitride-Based Phosphors which is hereby incorporated in its entirety. Such a red emitting phosphor comprising a nitride-based composition represented by the chemical formula M.sub.x/v)M.sub.2Si.sub.5xAl.sub.xN.sub.8:RE, wherein: M is at least one monovalent, divalent or trivalent metal with valence v; M is at least one of Mg, Ca, Sr, Ba, and Zn; and RE is at least one of Eu, Ce, Tb, Pr, and Mn; wherein x satisfies 0.1x<0.4, and wherein said red-emitting phosphor has the general crystalline structure of M.sub.2Si.sub.5N.sub.8:RE, Al substitutes for Si within said general crystalline structure, and M is located within said general crystalline structure substantially at the interstitial sites. An example of one such a phosphor is XR600 red nitride phosphor from Intematix Corporation, Fremont, Calif., USA which has a peak emission wavelength of 600 nm.

TABLE-US-00002 TABLE 2 Example orange to red-emitting phosphors Wavelength Phosphor General Composition .sub.p (nm) -SiAlON Ca.sub.(x/2)Si.sub.12xAl.sub.xN.sub.16:Eu 0 < x < 6 580-610 -SiAlON M.sub.xSi.sub.12(m+n)Al.sub.m+nO.sub.nN.sub.16n:Eu M = Ca, Sr, Y..; x < 2 580-600 CASN (Ca.sub.1xSr.sub.x)AlSiN.sub.3:Eu 0.5 < x 1 600-620 258 nitride Ba.sub.2xSr.sub.xSi.sub.5N.sub.8:Eu 0 x 2 580-620 Silicate (Ba.sub.xSr.sub.1x).sub.3SiO.sub.5:Eu 0 x 0.2 586-600 Silicate (Ba.sub.xY.sub.ySr.sub.1xy).sub.3(Al.sub.ySi)O.sub.5:Eu 0 x 0.2, 0 y 0.4 600-615

[0057] In accordance with an embodiment of the invention, the material into which the mixture of phosphor materials is incorporated can comprise a light transmissive material with an index of refraction n=1.40 to 1.43. For example the light transmissive material can comprise a dimethyl-based silicone such as a polydimethylsiloxane (PDMS). An example of such a suitable silicone material is OE-6370 HF optical encapsulant from Dow Corning.

[0058] FIG. 2 is a plot of luminous flux versus CIE x for an LED-based white light emitting device in accordance with the invention (.square-solid. designated dimethyl silicone OE-6370HF). The variation in CIE x results from different loadings of the phosphor mixture within the silicone. For comparison, data is shown for an identical device in which the same phosphor mixture is incorporated within a phenyl-based silicone (.diamond-solid. designated phenyl silicone). The phenyl-based silicone used in these devices is OE-6650 optical encapsulant from Dow Corning. Phenyl-based silicone encapsulants are typically used to encapsulate phosphor within LED devices.

[0059] FIG. 2 shows that by using a dimethyl-based silicone as the phosphor encapsulant results in about a 10% increase in luminous flux from the device as compared with the same device that uses a phenyl-based silicone as the phosphor encapsulant. It is believed that the increase in luminous flux results from the lower index of refraction (n1.4) of the dimethyl-based silicone compared with the index of refraction (n1.54) for a phenyl-based silicone. This lower index of refraction is believed to increase red light extraction from the red-emitting manganese-activated potassium hexafluorosilicate (KSF) phosphor which typically have an index of refraction (K.sub.2SiF.sub.6:Mn.sup.4+n=1.3991) by reducing total internal reflection at the interface of the phosphor particle and surrounding optical medium (silicone). For comparison, the index of refraction for other phosphors (including the yellow to green-emitting LuAGs and orange-emitting nitrides) is typically around 1.8 which may account for the widespread use of phenyl-based silicone encapsulants in LED-based light-emitting devices. It might be expected that the use of a dimethyl-based silicone would have a detrimental effect on light emission from phosphors other than the red-emitting manganese-activated potassium hexafluorosilicate phosphor (KSF) and degrade the overall performance of the device. However, as shown FIG. 2 when using a red-emitting manganese-activated fluoride phosphor in combination with other phosphor the net result is an increase in luminous flux. The increase in luminous flux is found to be greater when a majority (i.e. more than 50% by weight) of the total phosphor comprises a red-emitting manganese-activated fluoride phosphor. In other embodiments, the yellow to green-emitting, orange to red-emitting (when present) and KSF phosphors can be incorporated as a mixture in other light transmissive materials with an index of refraction of about 1.40 to about 1.43 such as for example a light transmissive epoxy resin.

Device 1: CCT 2700 K and CRI (Ra)90 White Light Emitting Device

[0060] TABLES 3A and 3B tabulate details of a white light emitting device designated Device 1 which is configured to generate white light with a nominal CCT of 2700 K and a General CRI (Ra) of 90 and higher.

TABLE-US-00003 TABLE 3A Yellow to green-emitting KSF phosphor Device phosphor (Wavelength .sub.p) (Wavelength .sub.p) Encapsulant 1 NYAG4454 NR6931 KSF OE-6370 HF (558 nm) (632 nm) dimethyl

[0061] Device 1 comprises a SMD 5630 LED package that contains a single 451 nm GaN LED chip with a mixture of two phosphors: (i) a yellow to green-emitting phosphor (Intematix's NYAG4454 cerium-activated green-emitting YAG phosphor) and (ii) a red-emitting manganese-activated fluoride phosphor (Intematix NR6931 KSF). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a dimethyl-based silicone (Dow Corning OE-6370 HF optical encapsulant). The proportion of KSF phosphor of the total phosphor weight is 86.5 wt % with the remaining 13.5wt % comprising NYAG4454 (TABLE 3B).

TABLE-US-00004 TABLE 3B Yellow KSF to green-emitting phosphor Total phosphor Device phosphor (wt %) (wt %) content per 100 g silicone (g) 1 13.5 86.5 100

[0062] TABLE 3C tabulates the optical characteristics of the white light emitting device, Device 1. As can be seen from the table the device generate white light with a CCT2700 K, a General CRI (Ra) of 90 and greater, and a CRI (R9) of greater than 90. Further, as can be seen from TABLE 3C, Device 1 has a Luminous Efficacy (LE) of 335 lm/W.

TABLE-US-00005 TABLE 3C Luminous Flux CCT CRI CRI LE Device (lm) CIE x CIE y (K) (Ra) (R9) (lm/W) 1 54.0 0.4587 0.4134 2738 90.4 93.2 335

[0063] Devices 2 to 10: CCT 2700 K and CRI (Ra)95 White Light Emitting Devices

[0064] TABLE 4A tabulates details of various white light emitting devices designated Devices 2 to 10. Devices 2 to 10 are nominally the same device and each is configured to generate white light with a nominal CCT of 2700 K and a General CRI (Ra) of 95 and higher.

TABLE-US-00006 TABLE 4A Yellow to green-emitting KSF Orange to red-emitting phosphor phosphor phosphor Device (wt %) (wt %) (wt %) 2 to 10 15 82 3

[0065] Each Device comprises a SMD 5630 LED package that contains a single 451 nm GaN LED chip with a mixture of three phosphors: (i) a yellow to green-emitting phosphor (Intematix's GAL540 cerium-activated green-emitting LuAG phosphor), (ii) a red-emitting manganese-activated fluoride phosphor (Intematix NR6931 KSF), and (iii) an orange to red-emitting phosphor (Intematix XR600 nitride-based phosphor). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a dimethyl-based silicone (Dow Corning OE-6370 HF optical encapsulant). The proportion of red phosphor (XR600+KSF) of the total phosphor weight is 85 wt % with the remaining 15 wt % comprising GAL540 (TABLE 4A). The proportion of KSF is 82 wt % and the proportion of XR600 is 3 wt %.

[0066] TABLE 4B tabulates the optical characteristics of the white light emitting devices, Devices 2 to 10. As can be seen from the table each device generates white light with a CCT2700 K, a General CRI (Ra) of 95 and greater, and a CRI (R9) of greater than 92. Further as can be seen from TABLE 4B the devices have a Luminous Efficacy (LE) ranging between 334 lm/W and 339 lm/W and an average LE of 335 lm/W.

[0067] FIG. 3 is an emission spectrum for one of the devices of TABLE 4B.

TABLE-US-00007 TABLE 4B Luminous Flux LE CCT CRI CRI Devices (lm) CIE x CIE y (lm/W) (K) (Ra) (R9) 2 49.46 0.4576 0.4048 338.9 2686 96.1 92.7 3 48.72 0.4560 0.4032 338.1 2696 95.9 92.4 4 49.07 0.4581 0.4103 335.9 2722 95.6 95.7 5 49.36 0.4565 0.4067 334.4 2717 94.8 96.7 6 49.59 0.4564 0.4106 336.4 2748 95.6 96.0 7 49.17 0.4567 0.4064 333.7 2712 95.3 95.7 8 48.99 0.4566 0.4069 333.9 2717 95.3 96.3 9 49.17 0.4599 0.4081 333.7 2680 95.0 96.3 10 49.15 0.4590 0.4090 335.1 2699 95.4 95.3 Average 49.19 0.4574 0.4073 335.6 2709 95.4 95.2
Devices 11, 12 and 13: CCT 3000 K, CRI (Ra)95 and CRI (R9)90 White light emitting devices

[0068] TABLES 5A and 5B tabulate details of various white light emitting devices designated Devices 11 (ref), 12 and 13. Each Device is configured to generate warm white light with a CCT of approximately 3000 K and comprises a SMD 2835 LED package that contains a single 451 nm GaN LED chip.

TABLE-US-00008 TABLE 5A Yellow to Orange to red- green-emitting phosphor KSF phosphor emitting phosphor Device (Wavelength .sub.p) (Wavelength .sub.p) (Wavelength .sub.p) Encapsulant 11 (ref) GAL535 NR6931 KSF OE-6370 HF (535 nm) (632 nm) dimethyl 12 GAL540 NR6931 KSF XR600 OE-6370 HF (540 nm) (632 nm) (600 nm) dimethyl 13 GAL540 NR6931 KSF XR600 OE-6636 (540 nm) (632 nm) (600 nm) phenyl

[0069] Device 11 (ref) comprises a mixture of two phosphors: (i) a yellow to green-emitting phosphor (Intematix's GAL535 cerium-activated green-emitting LuAG phosphor) and (ii) a red-emitting manganese-activated fluoride phosphor (Intematix's NR6931 KSF). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a dimethyl-based silicone (Dow Corning OE-6370 HF optical encapsulant). The proportion of KSF of the total phosphor weight is 82% wt % with the remaining 18 wt % comprising GAL 535 (TABLE 5B).

TABLE-US-00009 TABLE 5B Orange to red- Yellow to green- emitting Total phosphor Total emitting phosphor KSF phosphor phosphor content per 100 g phosphor in Device (wt %) (wt %) (wt %) silicone (g) device (mg) 11 (ref) 18 82 0 150 3.69 12 23.5 73.5 3 110 3.44 13 22 75 3 83 3.51

[0070] Device 12 comprises a mixture of three phosphors: (i) a yellow to green-emitting phosphor (Intematix's GAL540 cerium-activated green-emitting LuAG phosphor), (ii) a red-emitting manganese-activated fluoride phosphor (Intematix's NR6931 KSF), and (iii) an orange to red-emitting phosphor (Intematix's XR600 nitride-based phosphor). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a dimethyl-based silicone (Dow Corning OE-6370 HF optical encapsulant). The proportion of red phosphor (XR600+KSF) of the total phosphor weight is 76.5 wt % with the remaining 23.5 wt % comprising GAL540 (TABLE 5B). The proportion of KSF is 73.5 wt % and the proportion of XR600 is 3 wt %.

[0071] Device 13 comprises a mixture of three phosphors: (i) a yellow to green-emitting phosphor (Intematix's GAL540 cerium-activated green-emitting LuAG phosphor), (ii) a red-emitting manganese-activated fluoride phosphor (Intematix's NR6931 KSF), and (iii) an orange to red-emitting phosphor (Intematix's XR600 nitride-based phosphor). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a phenyl-based silicone (Dow Corning OE-6636 optical encapsulant). The proportion of red phosphor (XR600+KSF) of the total phosphor weight is 78 wt % with the remaining 22 wt % comprising GAL540 (TABLE 5B). The proportion of KSF is 75 wt % and the proportion of XR600 is 3 wt %.

[0072] TABLE 5C tabulates the optical characteristics of the Devices 11 (ref), 12, 13 and FIGS. 4 , 5 and 6 respectively show the emission spectra for the Devices 11 (ref), 12 and 13.

[0073] The benefits of including a third phosphor (i.e. orange to red-emitting phosphor) in addition to the yellow to green-emitting and KSF phosphors is evidenced by comparing the optical characteristics of Devices 11 (ref) and 12 (TABLE 5C). It can be seen that the inclusion of a third orange to red-emitting phosphor gives a brightness increase of 9%, increases the General CRI (Ra) from 69 to 95 and increases the CRI (R9) from 7 to 93. In summary the benefits of including a third phosphor (i.e. orange to red-emitting phosphor) can be an increase in brightness, an increase in General CRI (Ra) and an increase in CRI (R9).

TABLE-US-00010 TABLE 5C Lumi- nous Bright- Flux ness CCT CRI CRI LE Device (lm) (%) CIE x CIE y (K) (Ra) (R9) (lm/W) 11 (ref) 40.21 100.0% 0.4162 0.3668 3057 69.3 6.8 298 12 43.71 108.7% 0.4382 0.3999 2947 94.8 92.8 334 13 42.30 105.2% 0.4414 0.3925 2833 94.0 95.5 328

[0074] The benefits of encapsulating the three phosphor mixture in a dimethyl silicone (more particularly a light transmissive material with an index of refraction n 1.40 to 1.43) compared with encapsulating the three phosphor mixture in a phenyl silicone can be determined by comparing the optical characteristics of Device 12 with Device 13 (TABLE 5C). It can be seen that use of a dimethyl silicone increases the brightness by 2.5% (i.e. 105.2% to 108.7%). The use of a dimethyl silicone, which as described above is believed increases light extraction of light generated by KSF phosphor, which may account for the increase in luminous efficacy (LE). Whilst, when using a dimethyl silicone, overall phosphor usage may increase (110 g per 100 g siliconeDevice 12 versus 83 g per 100 g of siliconeDevice 13) the substantial increases in brightness, General CRI (Ra) and CRI (R9) potential far outweighs any additional cost increase.

Devices 14 (ref) and 15: CCT 2700 K White Light Emitting Devices

[0075] To further illustrate and explain the benefits of using a three phosphor solution and a phosphor encapsulant having an index of refraction n=1.40 to 1.43, two further devices, Device 14 (ref) and Device 15 are now discussed. TABLES 6A and 6B tabulate details of the white light emitting devices, Device 14 (ref) and Device 15. Each Device is configured to generate warm white light with a CCT of 2700 K and comprises a SMD 5630 LED package that contains a single 451 nm GaN LED chip.

TABLE-US-00011 TABLE 6A Yellow to Orange to red- green-emitting phosphor KSF phosphor emitting phosphor Device (Wavelength .sub.p) (Wavelength .sub.p) (Wavelength .sub.p) Encapsulant 14 (ref) GAL535 NR6931 KSF OE-6370 HF (535 nm) (632 nm) Dimethyl 15 GAL540 NR6931 KSF XR600 OE-6336 (540 nm) (632 nm) (600 nm) Phenyl

[0076] Device 14 (ref) comprises a mixture of two phosphors: (i) a yellow to green-emitting phosphor (Intematix's GAL535 cerium-activated green-emitting LuAG phosphor) and (ii) a red-emitting manganese-activated fluoride phosphor (Intematix's NR6931 KSF). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a phenyl-based silicone (Dow Corning OE-6336 optical encapsulant). The proportion of KSF of the total phosphor weight is 82% wt % with the remaining 18 wt % comprising GAL 535 (TABLE 6B).

TABLE-US-00012 TABLE 6B Orange to Yellow to green- KSF red-emitting Total phosphor emitting phosphor phosphor phosphor content per 100 g Device (wt %) (wt %) (wt %) silicone (g) 14 (ref) 18 82 0 140 15 23.5 73.5 3 110

[0077] Device 15 comprises a mixture of three phosphors: (i) a yellow to green-emitting phosphor (Intematix's GAL540 cerium-activated green-emitting LuAG phosphor), (ii) a red-emitting manganese-activated fluoride phosphor (Intematix's NR6931 KSF), and (iii) an orange to red-emitting phosphor (Intematix's XR600 nitride-based phosphor). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a dimethyl-based silicone (Dow Corning OE-6370 HF optical encapsulant). The proportion of red phosphor (XR600+KSF) of the total phosphor weight is 76.5 wt % with the remaining 23.5 wt % comprising GAL540 (TABLE 6B). The proportion of KSF is 73.5 wt % and the proportion of XR600 is 3 wt %.

[0078] TABLE 6C tabulates the optical characteristics of the Devices 14 (ref) and 15. It can be seen that the combined effect of including a third orange to red-emitting phosphor and the use of a dimethyl silicone encapsulant gives a brightness increase of 12%, an increase in the general CRI (Ra) from 70 to 95, an increase in CRI (R9) from 17 to 90 and an increase in the luminous efficacy (LE) from 311 to 333.

TABLE-US-00013 TABLE 6C Lumi- nous Bright- Flux ness CCT CRI CRI LE Device (lm) (%) CIE x CIE y (K) (Ra) (R9) (lm/W) 14 (ref) 77 100.0% 0.4579 0.4070 2699 70.3 17.3 311 15 87 112.0% 0.4598 0.4107 2701 95.8 89.5 333

[0079] FIG. 7 shows the normalized intensity versus wavelength for (i) the Device 14 (ref) (dotted line), (ii) the Device 15 (solid line) and (iii) black-body curve (dashed line) for a CCT of 2700 K. To make a meaningful comparison of the spectra, each spectra has been normalized such each has a CIE 1931 XYZ relative luminance Y=100. The data are normalized using the CIE 1931 luminosity function y() of a standard observer which takes account of the photopic response of an observer.

[0080] The Plankian curve or black-body curve (dashed lineFIG. 7) represents the spectrum for a General CRI (Ra) equal to 100. Accordingly, for a white light emitting device to have the highest color rendering possible, its emission spectrum should match the black-body spectrum as closely as possible.

[0081] Referring to FIG. 7 it can be seen that the addition of an orange to red-emitting phosphor and use of encapsulant having an index of refraction that closely matches the index of refraction of the KSF phosphor results in an emission spectrum (solid line) that more closely matches the black-body spectrum (dotted line) in three respects.

[0082] First, as indicated by cross hatched area 50, the green peak between about 500 nm and about 540 nm is reduced such that the emission spectrum (solid line) in this region more closely follows the black-body curve (dotted line). It is postulated that the reduction of the green peak results from the dimethyl silicone which increases light scattering and reduces light extraction from the yellow to green-emitting phosphor.

[0083] Second, as indicated by cross hatched area 52, the valley between about 550 nm and about 610 nm has been filled by the inclusion of the orange to red-emitting phosphor such that the emission spectrum (solid line) in this region more closely follows the black-body curve (dotted line).

[0084] Third, it is postulated that the combined effects of including an orange to red-emitting phosphor and use of a dimethyl silicone, reduces the total amount of KSF phosphor which reduces the KSF emission spikes 54, 56, 58, 60 (Device 14 (ref)115 g KSF per 100 g of silicone, Device 1581 g KSF per 100 g silicone) such that the emission spectrum (solid line) in this region more closely follows the black-body curve (dotted line).

[0085] FIG. 8 is a plot of normalized intensity deviation (normalized for a CIE 1931 XYZ relative luminance Y=100) from the black-body curve (2700 K) versus wavelength for the Devices 14 (ref) and 15. As can been from FIG. 8, over a wavelength range 460 nm to 500 nm a maximum deviation between the intensity of the light emitted by the device normalized to a CIE 1931 XYZ relative luminance Y=100 compared with the intensity of light of a black-body curve is less than 0.2. Further, it can be seen that over a wavelength range 500 nm to 570 nm a maximum deviation between the intensity of the light emitted by the device normalized to a CIE 1931 XYZ relative luminance Y=100 compared with the intensity of light of a black-body curve is less than 0.1. Hence, it can be seen that the best effects are achieved over the wavelength range of 500 nm to 570 nm where the emission spectrum for Device 15 follows the black-body curve very closely to almost ideal conditions. Across the overall wavelength range of 460 nm to 600 nm, it can be seen that a maximum deviation between the intensity of the light emitted by the device normalized to a CIE 1931 XYZ relative luminance Y=100 compared with the intensity of light of a black-body curve is less than 0.3.

Display Backlight

[0086] While the foregoing embodiments have been described in relation to high CRI white light emitting device for general, embodiments of the invention also find utility in white light emitting devices for use as display backlights. More particularly, although not exclusively, embodiments of the invention concern display backlights for use in high color gamut liquid crystal displays such as for example televisions, computer monitors, laptops, tablet devices and smart phones. In a display backlight the yellow to green-emitting phosphor comprises a narrow-band green-phosphor having a peak emission wavelength that corresponds to the green filter element of the display. Typically, in most liquid crystal displays the peak emission wavelength is about 535 nm (2 nm). In this specification a narrow-band green-emitting phosphor refers to a phosphor whose emission peak has a FWHM (Full Width Half Maximum) of about 50 nm of less. Examples of suitable narrow-band phosphors are given in TABLE 7. In a backlight the emission spectrum is matched to the color filter plate of the display with red, green and blue components matched to the red, green and blue filters. As a consequence there is no benefit in including an orange to red-emitting phosphor.

TABLE-US-00014 TABLE 7 Example narrow-band green-emitting phosphors for white light emitting devices for display backlighting FWHM Wavelength Phosphor General Composition (nm) .sub.p (nm) Sulfide SrGa.sub.2S.sub.4:Eu 46-48 536 -SiAlON:Eu M.sub.xSi.sub.12(m+n)Al.sub.m+nO.sub.nN.sub.16n:Eu 0.01 < x < 0.1; 50-52 525-545 0.01 < m < 0.12 & 0.1 < n < 0.5 M = Sr, Ca Mg

[0087] Devices 16 and 17: Display Backlights

[0088] TABLES 8A and 8B tabulate details of various white light emitting devices designated Devices 16 and 17. Each Device is configured for use as a display backlight and comprises a SMD 5630 LED package that contains a single 452 nm GaN LED chip.

TABLE-US-00015 TABLE 8A Yellow to green-emitting phosphor KSF phosphor Device (Wavelength .sub.p) (Wavelength .sub.p) Encapsulant 16 -SiAlON:Eu NR6931 KSF OE-6370 HF (535 nm) (632 nm) dimethyl 17 SrGa.sub.2S.sub.4:Eu NR6931 KSF OE-6370 HF (536 nm) (632 nm) dimethyl

[0089] Device 16 comprises a mixture of two phosphors: (i) a yellow to green-emitting phosphor (narrow-band green-emitting -SiAlON:Eu phosphor with a peak emission wavelength .sub.p=535 nm) and (ii) a red-emitting manganese-activated fluoride phosphor (Intematix's NR6931 KSF). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a dimethyl-based silicone (Dow Corning OE-6370 HF optical encapsulant). The proportion of KSF of the total phosphor weight is 82% wt % with the remaining 18 wt % comprising -SiAlON:Eu phosphor (TABLE 8B).

TABLE-US-00016 TABLE 8B Yellow to green-emitting KSF phosphor Total phosphor content per Device phosphor (wt %) (wt %) 100 g silicone (g) 16 24 76 55 17 14 86 55

[0090] Device 17 comprises a mixture of two phosphors: (i) a yellow to green-emitting phosphor (narrow-band green-emitting Sulfide phosphor: SrGa.sub.2S.sub.4:Eu with a peak emission wavelength .sub.p=536 nm) and (ii) a red-emitting manganese-activated fluoride phosphor (Intematix's NR6931 KSF). The mixture of phosphors is incorporated in, and homogeneously distributed throughout, a dimethyl-based silicone (Dow Corning OE-6370 HF optical encapsulant). The proportion of KSF of the total phosphor weight is 82% wt % with the remaining 18 wt % comprising SrGa.sub.2S.sub.4:Eu (TABLE 8B).

[0091] TABLE 8C tabulates the optical characteristics of the Devices 16 and 17 and FIGS. 4 9 and 10 respectively show the emission spectra for the Devices 16 and 17. As can be seen from TABLE 4C Devices 16 and 17 respectively produce light with color gamut of % of the NTSC (National Television System Committee) colorimetry 1953 (CIE 1931).

TABLE-US-00017 TABLE 8C Luminous Color Gamut Device Flux (lm) Brightness (%) CIE x CIE y % NTSC 16 44.01 100.0% 0.2805 0.2607 88.8 17 44.93 102.1% 0.2806 0.2608 91.9

[0092] Although the present invention has been described with particular reference to certain embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. For example while embodiments of the invention have been described in relation to manganese-activated potassium hexafluorosilicate phosphor (KSF) it is expected that the present invention finds utility to other manganese-activated fluoride phosphors with an index of refraction of about 1.4 (typically n1.39 to n1.43). It is believed that such manganese-activated fluoride phosphors having these properties may include K.sub.2TiF.sub.6:Mn.sup.4+, K.sub.2SnF.sub.6:Mn.sup.4+, Na.sub.2TiF.sub.6:Mn.sup.4+, Na.sub.2ZrF.sub.6:Mn.sup.4+, Cs.sub.2SiF.sub.6:Mn.sup.4+, Cs.sub.2TiF.sub.6:Mn.sup.4+, Rb.sub.2SiF.sub.6:Mn.sup.4+, Rb.sub.2TiF.sub.6:Mn.sup.4+, K.sub.3ZrF.sub.7:Mn.sup.4+, K.sub.3NbF.sub.7:Mn.sup.4+, K.sub.3TaF.sub.7:Mn.sup.4+, K.sub.3GdF.sub.6:Mn.sup.4+, K.sub.3LaF.sub.6:Mn.sup.4+ and K.sub.3YF.sub.6:Mn.sup.4+.