LED based device with wide color gamut

Abstract

The invention provides a lighting unit comprising a source of blue light, a source of green light, a first source of red light comprising a first red luminescent material, configured to provide red light with a broad band spectral light distribution, and a second source of red light comprising a second red luminescent material, configured to provide red light with a spectral light distribution comprising one or more red emission lines. Especially, the first red luminescent material comprises (Mg,Ca,Sr)AlSiN.sub.3:Eu and/or (Ba,Sr,Ca).sub.2Si.sub.5-xAl.sub.xO.sub.xN.sub.8-x:Eu, and the second red luminescent material comprises K.sub.2SiF.sub.6:Mn.

Claims

1. A device comprising: a light emitting diode; a green light source; a first red luminescent material to provide red light with a broad band spectral light distribution, the first red luminescent material disposed in a path of light emitted by the light emitting diode; and a second red luminescent material to provide red light with a spectral light distribution comprising one or more red emission lines, the second red luminescent material disposed in a path of light emitted by the light emitting diode, wherein the second red luminescent material provides red light with a spectral light distribution comprising one or more red emission lines having a centroid emission wavelength ?610 nm and with one or more red emission lines having a full width half maximum (FWHM) of ?6 nm.

2. The device of claim 1, wherein the first and the second red luminescent materials are mixed with resin and disposed on top of the light emitting diode.

3. The device of claim 1, wherein the light emitting diode is a first light emitting diode emitting blue light, and the green light source is a second light emitting diode emitting green light.

4. The device of claim 3, further comprising a light transmissive window disposed in a path of light emitted by the first and the second light emitting diodes.

5. The device of claim 4, wherein the first and the second light emitting diodes are disposed in a chamber.

6. The device of claim 3, further comprising RGB filters disposed in a path of light emitted by the first and the second light emitting diodes.

7. The device of claim 1, wherein the first and the second red luminescent materials are arranged at a non-zero distance from the light emitting diode.

8. The device of claim 1, wherein the non-zero distance is between 0.1 and 100 mm.

9. The device of claim 1, wherein the first and the second red luminescent materials are embedded in silicone on the light emitting diode.

10. The device of claim 1, wherein the green light source comprises a green luminescent material disposed in a path of light emitted by the light emitting diode.

11. The device of claim 1, wherein the light emitting diode emits UV light, the device further comprising a blue luminescent material.

12. The device of claim 1, wherein the first red luminescent material provides red light with a broad band spectral light distribution having a centroid emission wavelength ?590 nm and with a full width half maximum (FWHM) of ?70 nm.

13. The device of claim 1, wherein the first red luminescent material is selected from (Mg,Ca,Sr,Ba)AlSiN.sub.3:Eu and (Ba,Sr,Ca).sub.2Si.sub.5-xAl.sub.xO.sub.xN.sub.8-xEu, wherein 0?x?4.

14. The device of claim 1, wherein the second red luminescent material is selected from M.sub.2AX.sub.6 doped with tetravalent manganese, wherein M comprises monovalent cations, selected from Li, Na, K, Rb, Cs, and NH.sub.4, wherein A comprises a tetravalent cation selected from Si, Ti, Ge, Sn, and Zr, and wherein X comprises a monovalent anion selected from F, Cl, Br and I, but at least comprising F.

15. The device of claim 1, wherein the second red luminescent material comprises core-shell quantum dots.

16. The device of claim 1, wherein the light emitting diode is a first light emitting diode that emits blue light, wherein the green light source comprises a second light emitting diode with centroid emission wavelength in the range of 510-540 nm, wherein the first red luminescent material is selected from (Mg,Ca,Sr,Ba)AlSiN.sub.3:Eu and (Ba,Sr,Ca).sub.2Si.sub.5-xAl.sub.xO.sub.xN.sub.8-x:Eu, wherein x=0?x?4, and wherein the second red luminescent material comprises core-shell quantum dots.

17. The device of claim 1, wherein: the green light source comprises a green luminescent material selected from a divalent europium containing oxynitride, a divalent europium containing thiogallate, a trivalent cerium containing nitride, a trivalent cerium containing oxynitride, and a trivalent cerium containing garnet; the first red luminescent material is selected from (Mg,Ca,Sr,Ba)AlSiN.sub.3:Eu and (Ba,Sr,Ca).sub.2Si.sub.5-xAl.sub.xO.sub.xN.sub.8-x:Eu, wherein 0?x?4; and the second red luminescent material is selected from M.sub.2AX.sub.6 doped with tetravalent manganese, wherein M comprises monovalent cations, selected from Li, Na, K, Rb, Cs, and NH.sub.4, wherein A comprises a tetravalent cation selected from Si, Ti, Ge, Sn, and Zr, and wherein X comprises a monovalent anion selected from F, Cl, Br and I, but at least comprising F.

18. The device of claim 17, wherein the green luminescent material and the first and the second red luminescent materials are disposed in a light converter arranged on the light emitting diode.

19. The device of claim 1, wherein the first and the second red luminescent materials are different materials.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIGS. 1a-1e schematically depict some aspects of the invention; these drawings are not necessarily on scale;

(3) FIG. 2a: Reflection and emission spectra of a K.sub.2SiF.sub.6:Mn powder (R1,E1), a ECAS powder (R2,E2) and a silicone-based layer including 18 vol. % K.sub.2SiF.sub.6:Mn and 2 vol. % ECAS (R3). FIG. 2b shows the emission spectra (E4) of a blue LED (B) and a silicone-based layer including 18 vol. % K.sub.2SiF.sub.6:Mn and 2 vol. % ECAS, The narrow band luminescent material comprises at least one emission line that is beyond 610 nm, and which has the indicated centroid wavelength CW and full width half maximum FWHM.

(4) FIG. 3: Transmission functions selected for RGB pixels of a LCD display;

(5) FIGS. 4a-4b: show with a 510 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit;

(6) FIGS. 5a-5b: show with a 520 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit;

(7) FIGS. 6a-6b: show with a 530 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit; and

(8) FIGS. 7a-7b: show with a 540 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(9) FIG. 1a schematically depicts an embodiment of a lighting unit 100 as described herein. The lighting unit 100 comprising a light source 107, here a light source of blue light 110 (i.e. a source 110 of blue light; the reference 110 refers to the source; the further indication of blue light indicates the nature of the source), a source (of green light) 120, a first source (of red light) 1310 comprising a first red luminescent material 1311, configured to provide red light 31 with a broad band spectral light distribution, and a second source (of red light) 1320 comprising a second red luminescent material 1321, configured to provide red light 32 with a spectral light distribution comprising one or more red emission lines.

(10) Here, in this embodiment, the source of blue light 110 comprises a blue LED. Over this source of blue light 110, such as in a resin on top of the LED die, indicated with reference 111, a light converter 1300 may be arranged. This light converter 1300 may comprise one or more luminescent materials. Here, the light converter 1300 comprises both the first red luminescent material 1311 and the second red luminescent material 1321. These two luminescent material are sources of red light, as they are able to absorb the light source light of the source of blue light 110, and to convert into the broad band red light 31 and the narrow band red light 32. The blue light is indicated with reference 11; the source thereof is indicated with reference 110.

(11) By way of example, in this embodiment the source of green light 120 is shown as LED that is configured to generate green light, indicated with reference 21. This may thus be a LED only, without luminescent material.

(12) Here, the lighting unit comprises a light chamber 105, with a light transmissive window 102. Light from the light sources escape from this window 102, i.e. broad band red light 31, narrow band red light 32, green light 21 and blue light 11. All light escaping from the light exit window or transmissive window 102 is indicated as lighting unit light 101. As indicated above, this light is composed of components, which in combination with a set of RGB filters generate a front of screen (FOS) broad color gamut. The color gamut is defined with the color points resulting for selective transmission of the light from the light source for all color filters, separately.

(13) The blue light source 110 in combination with the two red luminescent materials 1311,1321 is herein also indicated as the pc-pink LED (which may thus provide pink light, which is in this embodiment a combination of blue light 11, broad band red light 31, and narrow band red light 32).

(14) FIG. 1b schematically depicts an embodiment wherein the source of blue light 110 is a LED (light source 107 comprises a source 110 configured to provide blue light). This LED is used to provide blue light to the luminescent materials that are provided as upstream layer or coating, to the light transmissive window 102. Hence, in this embodiment the converter 1300 is arranged at a non-zero distance from the LED die 111. This distance is indicated with reference d1 (see further below). The converter 1300 may especially be arranged at a non-zero distance d1 from the light source 110 (or other light source(s), which may for instance be a light emitting diode, although the distance d may also be zero, for instance when the light converter 1300 is applied on a LED die or embedded in a (silicone) cone on the LED die (see also FIG. 1c). The converter 1300 may optionally allow at least part of the light source light 11 penetrate through the converter. In this way, downstream of the converter, a combination of converter light, based on the luminescence(s) of the luminescent material(s) comprised by the light converter 1300, and light source light 11 may be found. The light downstream of the light converter is indicated a lighting device light 101. The distance d1 may especially be in the range of 0.1-100 mm, especially 0.5-100 mm, such as 1-20 mm, like especially 1-50 mm, like about 1-3 for applications close by the light source and 5-50 mm for more remote applications. Note however that the invention is not limited to applications wherein d1>0. The invention, and the herein described specific embodiments, may be also applied in other embodiments with d1=0. In such instances, the light converter may especially be configured in physical contact with the LED die.

(15) Note that the light source 107 might alternatively be a light source configured to provide UV light. In such instance, the lighting unit 100 may be configured to (substantially) prevent light source light downstream of the light exit window/downstream of the convert. For instance, the light converter may be configured to convert substantially all UV light source light into luminescence light of the one or more luminescent materials comprised by the light converter. In this embodiment, the source (of green light) 120 comprises a green luminescent material, indicated with reference 1200. This green luminescent material 1200 provides when being excited the green light 21. The light converter 1300 may further comprise a blue luminescent material, configured to generate blue light upon excitation with the light source light 11.

(16) Alternatively or additionally, the source of blue light 110 comprises a UV LED with a blue luminescent material that at least partly converts the UV light source light into blue light.

(17) FIG. 1c schematically depicts an embodiment wherein the light converter 1300 is arranged on the LED die 111 of the light source 107. This light source 107 may be a UV LED, or, as depicted, a source of blue light 110, i.e. a blue LED. The converter 1300 in such embodiment comprises a broad band red luminescent material 1311; thereby the first source of red light 1310 (note that this reference refers to the source, which is a source of red light) is provided. Further, the converter 1300 comprises the narrow band red luminescent material 1321. In this way, the second source of red light 1320 is provided; this second source (of red light) 1320 (i.e. the second source of red light 1320) comprises the narrow band luminescent material 1321, which provides when being excited the narrow band red light 32. Here, the converter 1300 also optionally comprises green luminescent material 1200, which provides when being excited the green light 21 (and is thereby also a source 120 of green light).

(18) Note that the above described embodiments may be combined. Further, the invention also relates to alternative arrangements, as will be clear to a person skilled in the art.

(19) FIG. 1d schematically depicts one of the applications of the lighting unit 100, here in a liquid crystal display device 2, which comprises a back lighting unit 200 which comprises one or more lighting units 100 (here, one lighting unit is schematically depicted), as well as a LCD panel 300, which can be backlighted with the lighting device light 101 of the lighting unit(s) 100 of the back lighting unit 200. Again, by way of example, the light source(s) 107 are sources of blue light 110. In this embodiment, a plurality of such sources 110 is depicted, which are used to excite the luminescent material(s) comprised by the converter 1300. Such LCD display device may further include one or more color filters, especially arranged downstream of the backlighting unit (but upstream of a display of the LCD display device). These filters may filter the (back) lighting device light 101. For the sake of clarity, these filters are not depicted.

(20) FIG. 1e schematically depicts an emission spectrum of a broad band luminescent material. The FWHM line indicates the middle between the top of the band and the background signal; the reference CW, indicates the wavelength of which left and right of the dotted line at this wavelength, equal intensities are found. This is known as the centroid wavelength.

EXAMPLES

Example 1

(21) The red-emitting phosphor layers necessary for amongst others the pc-pink LED described herein may be obtained by suspending commercially available ECAS (Sr.sub.0.8Ca.sub.0.2SiAlN.sub.3:Eu(0.8%).) and K.sub.2SiF.sub.6:Mn (prepared as reported by Adachi et al., Journal of Applied Physics 104, 023512, 2008, Direct synthesis and properties of K.sub.2SiF.sub.6:Mn.sup.4+ phosphor by wet chemical etching of Si wafer) in a silicone-based polymer at room temperatures. The well-mixed slurry is tape-casted at a glass substrate and cured at 150? C. for 4 hours in air. The cured layers are about 120 ?m in thickness, the filling grade of the phosphor is 20 vol. % in total (18 vol. % K.sub.2SiF.sub.6:Mn, 2 vol % ECAS). The measured reflection spectra of the K.sub.2SiF.sub.6:Mn and ECAS powder and the silicone-based layer including 18 vol % K.sub.2SiF.sub.6:Mn and 2 vol. % ECAS is visible in FIG. 2a. The emission spectra of the layer including K.sub.2SiF.sub.6:Mn and ECAS measured is visible in FIG. 2b.

(22) FIG. 2a shows the reflection and emission spectra of a K.sub.2SiF.sub.6:Mn powder (R1 (i.e. reflection); E1 (i.e. emission)), a ECAS powder (R2;E2) and a silicone-based layer including 18 vol. % K.sub.2SiF.sub.6:Mn and 2 vol. % ECAS (R3). FIG. 2b shows the emission spectra (E4) of a blue LED (B) and a silicone-based layer including 18 vol. % K.sub.2SiF.sub.6:Mn and 2 vol. % ECAS. The narrow band luminescent material comprises at least one emission line that is beyond 610 nm, and which has the indicated centroid wavelength CW and narrow full width half maximum FWHM, which is well below 50 nm (only a few nanometers).

(23) With an additional green LED centered at 530 nm a large color gamut located within NTSC can be obtained.

(24) Hence, the invention provides in an embodiment a phosphor converted LED with one narrow deep red emitting component as MnIV doped K.sub.2SiF.sub.6 in combination with a narrow green emitting component with a FWHM?50 nm, with another phosphor added to the MnIV component that maximizes the gamut color space coverage for NTSC and sRGB definitions. The light source can therefore consist of:

(25) A: a direct emitting green LED with a peak emission wavelength >510 nm and <540 nm and a pc-LED with two phosphors, one being MnIV doped and the second phosphor having a peak emission between 590 and 630 nm and a FWHM?70 nm.

(26) B: a three phosphor pcLED with a green phosphor being ?-SiAlON or (Sr,Ca).sub.(1-x)Ga.sub.2S.sub.4:Eu.sub.x doped with Eu (0.01<x<0.1), a MnIV doped phosphor and a third phosphor having a peak emission between 590 and 630 nm and a FWHM?70 nm.

(27) Further Examples for the Application of a Phosphor-Converted Red Phosphor in a LCD Backlight.

(28) The following example show the Front Of Screen (FOS) performance for a LCD display with one set of RGB color filters shown in FIG. 3, in relation to the spectral composition of a LED backlight unit, with R indicating the red filter, G indicating the green filter and B indicating the blue filter.

(29) First, some reference examples are given: typical pcLED backlight units combine a blue emitting LED with a green emitting and a red emitting phosphor.

Reference Example 1

(30) Green phosphor: LuAG (Lu.sub.2.94Al.sub.5O.sub.12:Ce.sub.0.06)

(31) Red phosphor: Eu doped (Sr,Ca)AlSiN.sub.3

(32) The FOS color gamut in comparison to the sRGB and NTSC definition gamuts together with the color points of the backlight unit and the white FOS color point and the emission spectrum of the backlight unit, for blue LEDs with peak emission at 440, 450, 460 and 470 nm, respectively, were determined:

(33) TABLE-US-00001 TABLE 1 Area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent. Blue peak LE [nm] sRGB NTSC [lm/W] pr_redbroad pr_green pr_blue 440 98.29% 69.62% 273 23.97% 36.65% 39.38% 450 96.86% 68.60% 275 27.89% 36.41% 35.71% 460 91.23% 64.62% 264 32.34% 31.27% 36.39% 470 79.69% 56.44% 241 38.27% 21.29% 40.43%
Herein, pr_redbroad, pr_green, pr_blue, pr_redKSiF (see below) and pr_redQD (see below) are the power fractions for the different emitters. These values add up to 100%.

Reference Example 2

(34) In order to increase the NTSC gamut coverage, a pink emitting LED (blue LED+red phosphor) is combined with a green emitting LED in the range of 520-530 nm peak emission.

(35) Green: direct emitting LED

(36) Red phosphor Eu doped (Sr,Ca)AlSiN.sub.3

(37) Blue LED: 450 nm peak emission

(38) The following data were obtained:

(39) TABLE-US-00002 TABLE 2 Area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent for 450 nm blue LED and different green LEDs. Green peak LE [nm] sRGB NTSC [lm/W] pr_redbroad pr_green pr_blue 510 103.73% 73.47% 243 47.09% 23.57% 29.34% 520 109.20% 77.34% 257 45.29% 22.07% 32.64% 530 109.68% 77.68% 270 42.38% 22.50% 35.12% 540 106.30% 75.29% 283 37.82% 24.87% 37.32%
Compared to the pcLED green backlight unit,

(40) TABLE-US-00003 Blue peak [nm] sRGB NTSC LE [lm/W] 450 96.86% 68.60% 275
nor the color gamut nor the LE increase significantly.

Reference Example 3

(41) Due to the limited cut-off of the red color filter, the NTSC gamut coverage, can only be increased significantly, if a narrow red emitting phosphor is used.

(42) Known materials with the desired properties are MnIV doped fluoride phosphors:

(43) Green: direct emitting LED

(44) Red phosphor: MnIV doped K.sub.2SiF.sub.6

(45) Blue LED: 450 nm peak emission

(46) TABLE-US-00004 TABLE 3 Area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent for 450 nm blue LED and different green LEDs: Green peak LE [nm] sRGB NTSC [lm/W] pr_green pr_redKSiF pr_blue 510 132.14% 93.59% 233 28.79% 41.40% 29.80% 520 138.77% 98.29% 250 26.79% 39.53% 33.68% 530 136.61% 96.76% 265 26.89% 36.54% 36.57% 540 128.58% 91.07% 280 28.99% 32.00% 39.00%

(47) Now the gamut approaches the area of NTSC definition, but it covers not the same color space. Deficit is a too red color point, generating issues in image color reproduction.

(48) Further, the FOS color gamut with backlight units using different direct green emitting LEDs of 510, 520, 530 and 540 nm peak, respectively, were determined.

Example 2

(49) The aim of this invention isamongst othersto build an LCD backlighting unit with maximum overlap of the FOS color gamut with the NTSC definition.

(50) This is done by combining a direct green emitting LED with a pcLED consisting of a blue LED and two phosphors, one being a Mn(IV) doped phosphor, the other one being Eu doped (Sr,Ca)AlSiN3

(51) FIGS. 4a to 7b show the FOS color gamut in comparison to the sRGB and NTSC definition gamuts together with the backlight emissions.

(52) FIGS. 4a-4b: show with a 510 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit.

(53) FIGS. 5a-5b: show with a 520 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit.

(54) FIGS. 6a-6b: show with a 530 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit.

(55) FIGS. 7a-7b: show with a 540 nm green LED, on the left (a) a chromaticity chart (CIE chromaticity diagram) and on the right 9b) an emission spectrum of backlight unit.

(56) It appears that the lighting unit according to the invention gives a high NTSC value but also has a very good overlap with the NTSC color space, more than 5 percent better than all the reference examples.

(57) Summarized, the following results were obtained:

(58) TABLE-US-00005 TABLE 4 Area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent for 450 nm blue LED and different green LEDs: Green peak LE [nm] sRGB NTSC [lm/W] pr_redbroad pr_green pr_redKSiF pr_blue 510 108.19% 76.63% 241 40.69% 24.19% 5.64% 29.47% 520 113.99% 80.74% 255 39.11% 22.62% 5.43% 32.84% 530 114.18% 80.87% 269 36.64% 22.99% 4.99% 35.38% 540 110.12% 77.99% 282 32.81% 25.30% 4.30% 37.60%

Reference Example 4

(59) The following combination was evaluated:

(60) Green: direct emitting LED

(61) Red phosphor: quantum dot phosphor with peak emission at 630 nm

(62) Blue LED: 460 nm peak emission

(63) The FOS color gamut with backlight units using different direct green emitting LEDs of 510, 520, 530 and 540 nm peak, respectively, were evaluated. In below table the area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent for 450 nm blue LED and different green LEDs is displayed.

(64) TABLE-US-00006 TABLE 5 area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent for 450 nm blue LED and different green LEDs Green peak LE [nm] sRGB NTSC [lm/W] pr_redbroad pr_green pr_redKSiF pr_blue 510 137.73% 97.55% 224 10.27% 32.08% 20.80% 36.85% 520 142.29% 100.78% 238 10.07% 28.68% 19.82% 41.44% 530 138.20% 97.88% 252 9.94% 27.64% 17.80% 44.62% 540 128.57% 91.06% 266 9.83% 28.58% 14.58% 47.01%

Example 3

(65) Further, a combination according to the invention was evaluated:

(66) Green: direct emitting LED

(67) Red phosphor: Qdot with peak emission at 630 nm+red CASN phosphor, peak emission 620 nm

(68) Blue LED: 460 nm peak emission

(69) The FOS color gamut with backlight units using different direct green emitting LEDs of 510, 520, 530 and 540 nm peak, respectively, were evaluated. In below table the area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent for 450 nm blue LED and different green LEDs is displayed.

(70) TABLE-US-00007 TABLE 6 area of the FOS color gamut relative to sRGB and NTSC definitions and FOS lumen equivalent for 450 nm blue LED and different green LEDs Green peak LE [nm] sRGB NTSC [lm/W] pr_redbroad pr_green pr_redQD pr_blue 510 118.52% 83.94% 237 9.86% 27.38% 23.71% 39.05% 520 122.76% 86.95% 249 9.68% 24.62% 22.79% 42.91% 530 121.73% 86.22% 260 9.54% 24.08% 20.88% 45.49% 540 117.08% 82.93% 270 9.42% 25.53% 17.71% 47.34%

(71) Also here, it appears that the lighting unit according to the invention gives a high NTSC value but also has a very good overlap with the NTSC color space, more than 5 percent better than all the reference examples.