U.S. patent application number 11/572586 was filed with the patent office on 2008-04-24 for illumination system comprising a radiation source and a luminescent material.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS, N.V.. Invention is credited to Helmut Bechtel, Thomas Justel, Hans Nikol, Cornelis Reinder Ronda.
Application Number | 20080093979 11/572586 |
Document ID | / |
Family ID | 34982339 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080093979 |
Kind Code |
A1 |
Bechtel; Helmut ; et
al. |
April 24, 2008 |
Illumination System Comprising a Radiation Source and a Luminescent
Material
Abstract
Illumination system comprising a radiation source and a
luminescent material comprising a first phosphor capable of
absorbing part of the light emitted by the radiation source and
emitting light of a wavelength different from that of the absorbed
light; wherein said first phosphor comprises europium(III) as an
activator in a host lattice selected from the compounds of an
anionic oxygen-containing species with a cationic metal species,
comprising yttrium(III) and gadolinium(III). A light-emitting diode
as a radiation source is especially contemplated. The invention
also relates to a red to amber yellow emitting
europium(III)-activated phosphor comprises europium(III) as an
activator in a host lattice selected from the compounds of an
anionic oxygen-containing species with a cationic metal species,
comprising yttrium(III) and gadolinium(III). Furthermore the
invention relates to the use of the phosphor for general
illumination, traffic and signage lighting, automotive and for
backlighting of liquid crystal displays.
Inventors: |
Bechtel; Helmut; (Roetgen,
DE) ; Justel; Thomas; (Witten, DE) ; Ronda;
Cornelis Reinder; (Aachen, DE) ; Nikol; Hans;
(Aachen, DE) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS,
N.V.
EINDHOVEN
NL
|
Family ID: |
34982339 |
Appl. No.: |
11/572586 |
Filed: |
July 21, 2005 |
PCT Filed: |
July 21, 2005 |
PCT NO: |
PCT/IB05/52442 |
371 Date: |
January 24, 2007 |
Current U.S.
Class: |
313/503 ;
252/301.4H; 252/301.4R |
Current CPC
Class: |
H01L 2224/8592 20130101;
H01L 2224/48091 20130101; H01L 2224/48247 20130101; C09K 11/7734
20130101; H01L 33/502 20130101; H01L 2924/00012 20130101; H01L
2924/00014 20130101; H01L 2924/181 20130101; C09K 11/7774 20130101;
C09K 11/7787 20130101; H01L 2924/181 20130101; H01L 2224/48091
20130101; C09K 11/7731 20130101 |
Class at
Publication: |
313/503 ;
252/301.4H; 252/301.4R |
International
Class: |
H01J 1/62 20060101
H01J001/62; C09K 11/78 20060101 C09K011/78 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 2004 |
EP |
04103645.0 |
Claims
1. Illumination system, comprising a radiation source and a
luminescent material comprising a first phosphor capable of
absorbing part of the light emitted by the radiation source and
emitting light of a wavelength different from that of the absorbed
light; wherein said first phosphor comprises europium(III) as an
activator in a host lattice selected from the compounds of an
anionic oxygen-containing species with a cationic metal species,
comprising yttrium(III) and gadolinium(III).
2. Illumination system according to claim 1, wherein the radiation
source is a light-emitting diode having an emission with a peak
emission wavelength in the range of 325 to 495 nm.
3. Illumination system according to claim 1, wherein the
luminescent material in addition comprises a second phosphor.
4. Illumination system according to claim 3, wherein the second
phosphor is a green phosphor selected from the group of
terbium(III)-activated compounds.
5. Illumination system according to claim 4, wherein the second
phosphor is selected from the group of
Y.sub.xGd.sub.1-x)BO.sub.3:Tb (0<x<1),LaPO.sub.4:Tb;
LaPO.sub.4:Ce,Th; (Y.sub.xGd.sub.1-x).sub.3Al.sub.5O.sub.12:Tb
(0<x<1); CeMgAl.sub.11O.sub.19:Tb; GdMgB.sub.5O.sub.10:Ce,Th;
(Y.sub.xGd.sub.1-x)BO.sub.3:Tb(0<x<1);
(Y.sub.xGd1-x).sub.2SiO.sub.5:Tb (0<x<1),
Gd.sub.2O.sub.2S:Tb; LaOBr:Tb, and LaOCl:Tb.
6. Illumination system according to claim 1, wherein the
luminescent material comprises a first phosphor combined with a
photonic bandgap material.
7. Illumination system according to claim 1, wherein the
luminescent material comprises a first phosphor having a medium
(median?) grain size d.sub.m1>500 nanometers.
8. Illumination system according to claim 1, wherein the
luminescent material comprises the first phosphor exhibiting a
transparent monolithic ceramic microstructure.
9. Illumination system according to claim 1, wherein the
luminescent material comprises a first phosphor having a grain size
d.sub.m1 and a second phosphor having a grain size
d.sub.m2<d.sub.m1.
10. Phosphor capable of absorbing part of the light emitted by the
radiation source and emitting light of a wavelength different from
that of the absorbed light; wherein said phosphor comprises
europium(III) as an activator in a host lattice selected from the
compounds of an anionic oxygen-containing species with a cationic
metal species, comprising yttrium(III) and gadolinium(III).
11. Phosphor according to claim 10, wherein the molar proportion of
the amount of gadolinium in the host lattice is less than 50 mole
percent.
12. Phosphor according to claim 12, comprising in addition a
co-activator selected from bismuth(III) and praseodymium(III).
13. Phosphor according to claim 12, wherein the anionic
oxygen-containing species is selected from the group of oxide,
oxysulfide, oxyhalides, borates, aluminates, gallates, silicates,
germanates, phosphates, arsenate, vanadate, niobate, tantalate, and
mixtures thereof.
14. Phosphor as claimed in claim 12, which phosphor comprises the
activator in a molar proportion of 0.001 to 20 mole % relative to
the cation in the host lattice.
15. Phosphor as claimed in claim 12, which phosphor comprises the
co-activator in a molar proportion of 0.001 to 2 mole % relative to
the cation in the host lattice.
16. Phosphor as claimed in claim 12, selected from the group of:
(Y.sub.1-x-yGd.sub.x).sub.2O.sub.2S:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)VO.sub.4:Eu.sub.y,
(Y.sub.1-x-y-zGd.sub.x)OCl:Eu.sub.yBi.sub.z,
(Y.sub.1-x-yGd.sub.x)(V,P,B)O.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)NbO.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)TaO.sub.4:Eu.sub.y, and
(Y.sub.1-x-y-zGd.sub.x).sub.2O.sub.3:Eu.sub.yBi.sub.z, wherein
0<x<1; 0<y<0.2 and 0<z<0.02.
17. Phosphor according to claim 12, wherein the phosphor is
provided with a coating selected from the group of fluorides and
orthophosphates of the elements aluminum, scandium, yttrium,
lanthanum, gadolinium, and lutetium, the oxides of aluminum,
yttrium, and lanthanum, and the nitride of aluminum.
18. Use of a phosphor according to claim 10 for general
illumination, traffic and signage lighting, automotive and for
backlighting of liquid crystal displays.
Description
[0001] The present invention generally relates to an illumination
system comprising a radiation source and a luminescent material
comprising a phosphor. The invention also relates to a phosphor for
use in such an illumination system.
[0002] More particularly, the invention relates to an illumination
system for the generation of specific, colored light, including
white light, by luminescent down conversion and additive color
mixing based on a blue to violet radiation-emitting diode in
combination with a luminescent material comprising a phosphor.
Today light emitting illumination systems comprising visible
colored light emitting diodes as radiation sources are used single
or in clusters for all kind of applications where rugged, compact,
lightweight, high efficiency, long-life, low voltage sources of
white or coloured illumination are needed.
[0003] Such applications comprise inter alia illumination of small
LCD displays in consumer products such as cellular phones, digital
cameras and hand held computers. Pertinent uses include also status
indicators on such products as computer monitors, stereo receivers,
CD players, VCRs, and the like. Such indicators are also found in
systems such as instrument panels in aircraft, trains, ships, cars,
etc.
[0004] Recently, various attempts have been made to make white
light emitting illumination systems by using light-emitting diodes
as radiation sources. When generating white light with an
arrangement of red, green and blue light emitting diodes, the
problem has been that white light of the desired tone cannot be
generated owing to variations in the tone, luminance, and other
properties of the individual light-emitting diodes.
[0005] In order to solve this problem, various illumination system
concepts have been developed for converting the color of
light-emitted by light-emitting diodes by means of a luminescent
material comprising a phosphor so as to provide a visible white
light illumination.
[0006] Such white light illumination systems have been based in
particular either on the trichromatic (RGB) approach, i.e. mixing
of three colors: namely red, green and blue, in which case the
latter component of the output light may be provided by a phosphor
or by the primary emission of the LED; or in a second, simplified
solution, on the dichromatic (BY) approach, i.e. mixing of yellow
and blue, in which case the yellow secondary component of the
output light may be provided by a yellow phosphor and the blue
component may be provided by a phosphor or by the primary emission
of blue LEDs.
[0007] One dichromatic (BY) approach, as disclosed, for example, in
U.S. Pat. No. 5,998,925, uses a blue light emitting diode of InGaN
semiconductor material combined with
Y.sub.3Al.sub.5O.sub.12:Ce(YAG-Ce) as a phosphor. The YAG-Ce
phosphor is coated on the InGaN LED, and a portion of the blue
light emitted from the LED is converted into yellow light by the
phosphor. Another portion of the blue light from the LED is
transmitted through the phosphor. This system thus emits both blue
light emitted from the LED and yellow light emitted from the
phosphor. The mixture of blue and yellow emission bands is
perceived as white light by an observer with a typical CRI in the
middle 70 s and a color temperature Tc, that ranges from about 6000
K to about 8000 K.
[0008] Although the method is advantageously simple and readily
implemented, it is disadvantageously poor in color rendering at low
color temperatures, resulting from a lack of red color content, and
it suffers from a color-shifting problem as the operational current
increases. Therefore it is not an ideal light source for
illumination.
[0009] One trichromatic RGB approach for manufacturing a white LED
may be implemented by exploiting an ultraviolet-emitting UV LED for
excitation of a set of phosphors. In this approach the visible part
of the emitting spectrum is completely generated by phosphors. The
UV radiation emitted by the LED excites the phosphors to emit red,
green and blue light, and these tri-color lights are further mixed
into white light. However, moving the pump source into the UV
spectral range results in a reduced radiant efficacy because of
increased energy losses in the conversion process. Besides, the
packaging materials have an ageing problem due to the UV light
damages. Therefore this is not a proper way to produce a white
illumination source.
[0010] Another trichromatic lamp approach for generating white
light is disclosed in U.S. Pat. No. 6,686,691. The invention
according to U.S. Pat. No. 6,686,691 relates to a tri-color lamp
with specific red and green phosphors excitable by a common blue
light emitting diode (LED). This arrangement provides a mixing of
three light sources: light emitted from the two phosphors and
unabsorbed light emitted from the LED. Power fractions of each of
the light sources can be varied to achieve good color
rendering.
[0011] Yet, it is a general concern with phosphor-converted LED
lamps using blue to violet emitting LEDs for excitation of
phosphors that currently known phosphors have not been developed
and optimized for such excitation.
[0012] Currently known phosphors were developed and optimized for
two major applications: (1) fluorescent lamps that utilize 254 nm
UV radiation from a Hg discharge for excitation and (2) CRTs, where
the RGB-phosphors are excited by an electron beam.
[0013] This leads to a new challenge to be met by phosphors in
phosphor- converted LEDs.
[0014] It has been found in particular that the electro-optical
efficiency of conventional red phosphors, for example
Y.sub.2O.sub.3:Eu(III), as mentioned in US2004/0000862, is
unsatisfactory in illumination systems, using a LED die? as a
radiation source, as these red phosphors only slightly absorb
radiation having a wavelength above 300 nm.
[0015] Therefore, there is a need to provide an illumination system
comprising a novel luminescent material that is excitable by a
radiation source with an emission in the blue-violet range and
emits in the visible yellow-amber-red range of the electromagnetic
spectrum.
[0016] Desirable characteristics for illumination systems for
general purposes are also a high brightness at economical cost.
[0017] Thus the present invention provides an illumination system,
comprising a radiation source and a luminescent material comprising
a first phosphor capable of absorbing part of the light emitted by
the radiation source and emitting light of a wavelength different
from that of the absorbed light; wherein said first phosphor
comprises europium(III) as an activator in a host lattice selected
from the compounds of an anionic oxygen-containing species with a
cationic metal species, comprising yttrium(III) and
gadolinium(III).
[0018] According to a first aspect of the invention, a white-light
illumination system comprises a blue light emitting diode having a
peak emission wavelength in the range of 325 to 495 nm as a
radiation source.
[0019] An illumination system comprising this phosphor has an
improved quantum yield for the blue to violet excitation radiation
having a wavelength .lamda. in the range between 325 and 495
nm.
[0020] As the phosphor comprising europium(III) as an activator in
a host lattice selected from the compounds of an anionic
oxygen-containing species with a cationic metal species, comprising
yttrium(III) and gadolinium(III), emits in the red-amber-yellow
range of the electromagnetic spectrum, the illumination system
comprising such a phosphor is capable of providing red to amber to
yellow or white light.
[0021] An essential factor is that the europium(III)-activated
phosphors are narrow-band emitters in the red-amber-yellow
wavelength range emitting between 580 and 700 nm range of the
electromagnetic spectrum, so that little or no light is generated
at wavelengths that are positioned in the visible spectrum away
from the desired red, amber, or yellow wavelengths.
[0022] Such a narrow-band emission helps to increase the efficacy
of the illumination system.
[0023] Applications of the invention include inter alia indicators,
traffic lighting, street lighting, security lighting and lighting
of automated factory, and signal lighting for cars and traffic as
well as general illumination. Applications of the invention include
colored also security lighting as well as signage lighting for cars
and traffic. Another field of applications includes backlighting of
liquid crystal displays.
[0024] One embodiment of the invention provides a white light
illumination system comprising a blue light emitting diode having a
peak emission wavelength in the blue-violet range of 400 to 495 nm
as a radiation source and a luminescent material comprising a first
phosphor capable of absorbing part of the light emitted by the
radiation source and emitting light of a wavelength different from
that of the absorbed light; wherein said first phosphor comprises
europium(III) as an activator in a host lattice selected from the
compounds of an anionic oxygen-containing species with a cationic
metal species, comprising yttrium(III) and gadolinium(III), and at
least one second phosphor.
[0025] In particular, the luminescent material may be a phosphor
blend comprising a first phosphor comprising europium(III) as an
activator in a host lattice selected from the compounds of an
oxo-anion species with a cationic metal species, comprising
yttrium(III) and gadolinium(III), and a green phosphor.
[0026] The emission spectrum of such a luminescent material
comprising additional green phosphors has the appropriate
wavelengths for obtaining, together with the blue to violet light
of the LED and the yellow to red light of the europium-activated
oxygen -dominated type phosphor according to the invention, a
high-quality white light with good color rendering at the required
color temperature.
[0027] Such a green phosphor may be selected from the group of
terbium(III)-activated phosphor compounds.
[0028] It is furthermore preferred that the green Tb(III)
-activated phosphor is selected from the group of:
(Y.sub.xGd.sub.1-x)BO.sub.3:Tb (0<.times.<1),LaPO.sub.4:Tb;
LaPO.sub.4:Ce,Th; (Y.sub.xGd.sub.1-x).sub.3Al.sub.5O.sub.12:Tb
(0<.times.<1); CeMgAl.sub.110.sub.19:Tb;
GdMgB.sub.5O.sub.10:Ce,Th;
(Y.sub.xGd.sub.1-x)BO.sub.3:Tb(0<.times.<1);
(Y.sub.xGd.sub.1-x).sub.2SiO.sub.5:Tb (0<.times.<1),
Gd.sub.2O.sub.2S:Tb; LaOBr:Tb, and LaOCl:Tb.
[0029] An illumination system comprising terbium(III)-activated
phosphors as a second phosphor can provide a composite white output
light that is well-balanced with respect to color. In particular,
the composite white output light has a narrow-band emission in the
red color range, in contrast to the broad-band emission of the
conventional lamp. This characteristic makes the device ideal for
applications in which a high lumen equivalence is required.
[0030] According to one embodiment of the invention, the
luminescent material comprises the first phosphor combined with a
photonic bandgap material to concentrate the primary radiation
emitted by the LED in the position of the phosphor in the
luminescent material for enhanced absorption.
[0031] The luminescent material may comprise the first phosphor,
having a grain size d.sub.m1>500 nanometers--also to enhance
absorption of the primary radiation.
[0032] According to one embodiment of the invention, the
luminescent material comprises the first phosphor as a transparent
monolithic ceramic microstructure material.
[0033] The application of the phosphor as a transparent monolithic
ceramic material renders it possible to adjust a much stronger
optical absorption, while the material remains transparent to the
pump radiation.
[0034] Said application of transparent ceramic material as a
conversion layer in blue pumped LED lamps has even more
advantages:
[0035] In high-power LEDs, a significant heating up of the phosphor
material, possibly leading to thermal quenching cannot be
prevented. Thermal conductivity is better when a transparent
ceramic material is used. In addition, a larger volume is
heated.
[0036] Lower concentrations of luminescent ions can be chosen,
preventing or reducing concentration quenching.
[0037] Optical functionality can be easily integrated, e.g. in the
form of lenses.
[0038] According to one embodiment of the invention, the
luminescent material may comprise the first phosphor having a grain
size d.sub.m1 and the second phosphor having a grain size
d.sub.m2<d.sub.m1.
[0039] Another aspect of the present invention provides a phosphor
capable of absorbing part of the light emitted by the radiation
source and emitting light of a wavelength different from that of
the absorbed light; wherein said phosphor comprises europium(III)
as an activator in a host lattice selected from the compounds of an
anionic oxygen-containing species with a cationic metal species,
comprising yttrium(III) and gadolinium(III).
[0040] The luminescent material is excitable by UV-radiation with
wavelengths from 200 nm to 400 nm, but is excited with higher
efficiency by blue to violet light emitted by a blue light emitting
diode having a wavelength around 400 to 495 nm. Thus the
luminescent material has ideal characteristics for conversion of
the blue light of a nitride semiconductor light-emitting component
into white light.
[0041] These phosphors are narrow-band emitters whose visible
emission is so narrow that the wavelength range of their emission
is less than 20 nm, where the visible emission is predominantly
located.
[0042] Additional important characteristics of the phosphors
include 1) resistance to thermal quenching of luminescence at
typical device operating temperatures (e.g. 80.degree. C.); 2) lack
of interfering reactivity with the encapsulating resins used in the
device fabrication; 3) suitable absorptive profiles to minimize
dead absorption within the visible spectrum; 4) a temporally stable
lumen output over the operating lifetime of the device and; 5)
compositionally controlled tuning of the phosphors' excitation and
emission properties.
[0043] Preferably the molar proportion of the amount of gadolinium
in the host lattice is less than 50 mole percent.
[0044] The phosphor may comprise in addition a co-activator
selected from bismuth(III) and praseodymium(III), which are able to
absorb blue light and to transfer the absorbed energy to the
europium(III)-activator cations.
[0045] Preferably the anionic oxygen-containing species is selected
from the group of oxide, oxysulfide, oxyhalides, borates,
aluminates, gallates, silicates, germanates, phosphates, arsenate,
vanadate, niobate, tantalate, and mixtures thereof
[0046] It is also preferred that the phosphor comprises the
activator in a molar proportion of 0.001 to 20 mole % relative to
the cation in the host lattice and the co-activator in a molar
proportion of 0.001 to 2 mole % relative to the cation in the host
lattice.
[0047] Especially preferred are phosphors selected from the group
of: (Y.sub.l-x-yGd.sub.x).sub.2O.sub.2S:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)VO.sub.4:Eu.sub.y,
(Y.sub.1-x-y-zGdx)OCl:Eu.sub.yBi.sub.z,
(Y.sub.1-x-yGd.sub.x)(V,P,B)O.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)NbO.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)TaO.sub.4:Eu.sub.y, and
(Y.sub.1-X-Y-zGd.sub.x).sub.2O.sub.3:Eu.sub.yBi.sub.z, wherein
0<x <1,0<y<0.2 and 0<z<0.02.
[0048] These phosphors may have a coating selected from the group
of fluorides and orthophosphates of the elemeunts aluminum,
scandium, yttrium, lanthanum, gadolinium and lutetium, the oxides
of aluminum, yttrium, and lanthanum, and the nitride of
aluminum.
[0049] The illumination system according to the invention comprises
a luminescent material comprising a phosphor capable of absorbing
part of the radiation emitted by the radiation source and emitting
light of a wavelength different from that of the absorbed light;
wherein said phosphor comprises europium(III) as an activator in a
host lattice selected from the compounds of an anionic
oxygen-containing species with a cationic metal species, comprising
yttrium(III) and gadolinium(III).
[0050] While the use of the present phosphors and luminescent
materials is contemplated for a wide array of illumination
applications, the present invention is described with particular
reference to and finds particular application in illumination
systems comprising light-emitting diodes, especially ultraviolet to
blue light-emitting diodes as their radiation sources.
[0051] The type and amount of the cationic metal species yttrium
and gadolinium present in the phosphor compound dictate the
physical and/or chemical properties of the compound, while the
local bonding environments of europium(III) in the oxygen-dominant
host lattice determine the characteristics of its emission and
absorption spectra.
[0052] Anionic oxygen-containing species (or oxygen-dominant
anions) are generally defined as oxygen-containing species having a
net negative ionic charge. Host lattices comprising oxygen-dominant
anions have the following properties: a) they have a large bandgap
so as not to absorb the emitted radiation from the activator and b)
they are relatively stiff, so that lattice vibrations, which lead
to non-radiative relaxation and decrease efficiency, are not easily
excited.
[0053] Of particular interest are the oxides, oxysulfides,
oxyhalides, and oxo-anions of boron, aluminum, gallium, silicon,
germanium, phosphorus, arsenic, vanadium, niobium, and tantalum,
and combinations or mixtures thereof.
[0054] Typically, said oxo-anions are comprised of individual
monomer subunits [A.sup.a+O.sub.xO.sub.y/2].sup.a-2x-y, wherein A
is selected from boron, aluminum, gallium, silicon germanium,
phosphorus, arsenic, vanadium, niobium, and tantalum, a is the
respective oxidation number thereof, O is oxygen, and x+y is an
integer equal to 3 or 4. The subunits may be bound together by
conventional covalent oxygen-bridge bonds (i.e., shared
electrons).
[0055] Oxo-anions are either isolated (finite), or oligomeric, i.e.
connected to a limited number of adjacent oxo-anions by oxygen
bridges, or alternatively they are directly interconnected through
oxygen bonding into infinite chains, sheets, or 3-dimensional
framework structures.
[0056] The host lattice may contain a single oxo-anion species, a
mixture of different oxo-anions, or combination of more than one
element selected from boron, aluminum, gallium, silicon germanium,
phosphor, arsenic, vanadium in one oxo-anionic species.
[0057] By way of example, such combinations may comprise
borosilicate, phosphosilicate, phosphovanadate, phosphotantalate,
aluminosilicate, and aluminoborate.
[0058] In the host lattice of the phosphors according to the
invention, the anionic oxygen-containing species are used with
specific counter-ions, i.e. cationic metal species comprising
yttrium and gadolinium.
[0059] The incorporation of gadolinium(III) in the host lattice
increases the proportion of covalent bonding and ligand-field
splitting. This leads to a shift of excitation (and emission) bands
to longer wavelengths in comparison with the basic host lattice
comprising only yttrium(III).
[0060] A gadolinium-containing host lattice is excellent in acting
as a sensitizer host lattice , because both the ground state and
excited states lie within the bandgap of about 6 eV of the host
lattice. Gadolinium absorbs and emits radiation via 4f-5df
transitions, i.e. electronic transitions involving f-orbital energy
levels. While f-f transitions are quantum-mechanically forbidden,
resulting in weak emission intensities, it is known that Gd(III)
strongly absorbs radiation through allowed 4f-5df transitions (via
d- orbital/f-orbital mixing) and consequently produces high
emission intensities in the UV-B range of the electromagnetic
spectrum.
[0061] This host lattice typically preferably comprises a major
proportion of yttrium, up to about 50 mole percents of gadolinium,
and a minor activating proportion, typically about 0.03 to 2 mole
percent of the rare earth activator Eu(III), plus possibly a
co-activator, selected from bismuth and praseodymium.
[0062] Especially useful materials of the phosphor according to the
invention are: (Y.sub.1-x-yGd.sub.x).sub.2O.sub.2S:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)VO.sub.4:Eu.sub.y,
(Y.sub.1-x-y-zGd.sub.x)OCl:Eu.sub.yBi.sub.z,
(Y.sub.1-x-yGd.sub.x)(V,P,B)O.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)NbO.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)TaO.sub.4:Eu.sub.y,
(Y.sub.1-x-y-zGd.sub.x).sub.2O.sub.3:Eu.sub.yBi.sub.z, wherein
0<x<1; 0<y<0.2 and 0<z<0.02.
[0063] Table 1 discloses color points and lumen equivalencies of a
selection of phosphors according to the invention in comparison
with two conventional phosphors (in italics).
TABLE-US-00001 TABLE 1 lumen equivalency Chem. Composition .lamda.
[max] color point x, y [lm/W] CaS:Eu 655 0.69, 0.30 90 Sr2Si5N8:Eu
625 0.62, 0.38 180 (Y.sub.1-x-yGd.sub.x).sub.2O.sub.2S:Eu.sub.y 620
0.66, 0.33 210 (Y.sub.1-x-yGd.sub.x)VO.sub.4:Eu.sub.y 615 0.65,
0.33 225 (Y.sub.1-x-y-zGd.sub.x)OCl:Eu.sub.yBi.sub.z 612 0.64, 0.35
230 (Y.sub.1-x-yGd.sub.x)(V,P,B)O.sub.4:Eu.sub.y 615 0.65, 0.33 225
(Y.sub.1-x-yGd.sub.x)NbO.sub.4:Eu.sub.y 615 0.64, 0.33 255
(Y.sub.1-x-yGd.sub.x)TaO.sub.4:Eu.sub.y 615 0.64, 0.35 250
(Y.sub.1-x-y-zGd.sub.x).sub.2O.sub.3:Eu.sub.yBi.sub.z 611 0.65,
0.34 280
[0064] Especially oxo-anionic compounds of yttrium and gadolinium
are useful hosts for europium (III), because the oxygen ligation of
europium(III) substantially impacts its emission and absorption
spectra. The limited electronegativity of the oxo-anions of yttrium
and gadolinium decreases the degeneration of the electronic states
of europium, producing emission and absorption bands which differ
substantially from those produced in e.g. halide hosts: they are
narrower and have different relative intensities and different
positions. In general, the absolute position and width of an
emission or absorption band shifts to lower energy as the
electronegativity of the surrounding anions decreases.
[0065] The emission spectra of the phosphor comprising gadolinium
and yttrium in the host lattice resemble that of the phosphor
comprising only yttrium. It exhibits a narrow emission band at 580
to 700 nm due to the 4f-4f transitions of Eu(III).
[0066] The molar proportion z of Eu(III) is preferably in a range
of 0.003<z<0.2.
[0067] When the proportion z of Eu(III) is 0.003 or lower,
luminance decreases because the number of excited emission centers
of photoluminescence due to Eu(III) decreases and, when z is
greater than 0.2, density quenching occurs. Concentration quenching
refers to the decrease in emission intensity which occurs when the
concentration of an activation agent added to increase the
luminance of the luminescent material is increased beyond an
optimum level.
[0068] Replacing some of the europium in a europium-activated
phosphor with bismuth as a sensitizer has the effect that the
incident energy, photon or excited electron, is absorbed from the
discharge of the discharge-maintaining composition by the bismuth,
and the activator forms the site where the electron radiatively
relaxes.
[0069] These europium(III)-activated oxygen-dominant phosphors
according to the invention are responsive to more energetic
portions of the electromagnetic spectrum than just the visible
portion of the spectrum.
[0070] In particular, the phosphors according to the invention are
excitable by UV-emission lines with wavelengths from 200 to 400 nm,
but are also excited with high efficiency by LED light emitted by a
blue-violet light emitting component having a wavelength from 400
to 495 nm. Thus the luminescent material has ideal characteristics
for converting blue light of nitride semiconductor light-emitting
components into white light.
[0071] Preferably the europium(III)-activated type phosphors
according to the invention may be coated with a thin, uniform
protective layer of one or more compounds selected from the group
formed by the fluorides and orthophosphates of the elements
aluminum, scandium, yttrium, lanthanum, gadolinium, and lutetium,
the oxides of aluminum, yttrium, and lanthanum, and the nitride of
aluminum.
[0072] The protective layer thickness customarily ranges from 0.001
to 0.2 micrometers and is thus so thin that it can be penetrated by
the radiation of the radiation source without substantial loss of
energy. The coatings of these materials on the phosphor particles
may be applied, for example, by deposition from the gas phase or a
wet coating process.
[0073] To improve their absorption characteristics, these phosphors
are preferably used in a grain size distribution in a range of
grain size d.sub.m1>500 nm. The grain size is determined by the
capabilities of the phosphor to absorb radiation and absorb as well
as scatter visible radiation, but also by the necessity to form a
phosphor coating that bonds well to the substrate. The latter
requirement is met only by very small grains, but the light output
is improved in slightly larger grains in a range of grain size
d.sub.m1>500 nm.
[0074] The invention also relates to an illumination system
comprising a radiation source and a luminescent material comprising
a first phosphor capable of absorbing part of the light emitted by
the radiation source and emitting light of a wavelength different
from that of the absorbed light; wherein said first phosphor
comprises europium(III) as an activator in a host lattice selected
from the compounds of an anionic oxygen-containing species with a
cationic metal species, comprising yttrium(III) and
gadolinium(III).
[0075] Any configuration of an illumination system which includes a
radiation source and phosphor composition is contemplated in the
present invention, preferably with the addition of other well-known
phosphors, which may be combined to achieve a specific color or
white light when irradiated by a primary UV or blue-violet light as
specified above.
[0076] Radiation sources such as those found in discharge lamps and
luminescent lamps, such as mercury low- and high-pressure discharge
lamps, sulfur discharge lamps, and discharge lamps based molecular
radiators are contemplated for use as radiation sources with the
present inventive phosphor compositions.
[0077] Preferred radiation sources include any semiconductor
optical radiation emitters and other devices that emit optical
radiation in response to electrical excitation. Semiconductor
optical radiation emitters include inter alia light-emitting diode
LED chips, light-emitting polymers (LEPs), organic light-emitting
devices (OLEDs), polymer light-emitting devices (PLEDs), etc.
[0078] In a preferred embodiment of the invention, the radiation
source is a blue to violet light-emitting diode having an emission
with a peak emission wavelength in the range of 325 to 495 nm.
[0079] A detailed construction of one embodiment of such an
illumination system comprising a radiation source and a luminescent
material as shown in FIG. 1 will now be described.
[0080] FIG. 1 is a schematic view of a chip type light-emitting
diode with a coating comprising the luminescent material. The
device comprises a chip type light-emitting diode (LED) 1 as a
radiation source. The light-emitting diode chip is positioned in a
reflector cup lead frame 2. The chip 1 is connected to a first
terminal 6 via a bond wire 7, and directly to a second electric
terminal 6. The recess of the reflector cup is filled with a
coating material, which contains a luminescent material according
to the invention so as to form a coating layer which is embedded in
the reflector cup. The phosphors are applied either separately or
in a mixture.
[0081] The coating material typically comprises a polymer for
encapsulating the phosphor or phosphor blend. In this embodiment,
the phosphor or phosphor blend should exhibit high stability
properties against the encapsulant. Preferably, the polymer is
optically clear to prevent any significant light scattering. A
variety of polymers are known in the LED industry for making LED
lamps.
[0082] In one embodiment, the polymer is selected from the group
consisting of epoxy and silicone resins. Adding the phosphor
mixture to a polymer precursor liquid can achieve the
encapsulation. For example, the phosphor mixture may be a granular
powder. Introducing phosphor particles into a polymer precursor
liquid results in formation of a slurry (i.e. a suspension of
particles). Upon polymerization, the phosphor mixture is fixed
rigidly in place by the encapsulation. In one embodiment, both the
luminescent material and the LED dice? are encapsulated in the
polymer.
[0083] The transparent coating material may comprise
light-diffusing particles, advantageously so-called diffusers.
Examples of such diffusers are mineral fillers, in particular
CaF.sub.2, TiO.sub.2, SiO.sub.2, CaCO.sub.3, and BaSO.sub.4, or
else organic pigments. These materials can be added to the
above-mentioned resins in a simple manner.
[0084] According to one embodiment of the invention, the coating
material is selected from coating materials with a refractive index
in the range of the refractive index of the phosphor material.
[0085] According to a further preferred embodiment of the
invention, the coating layer is a double layer, wherein the first
phosphor is provided as a thin-film layer or a layer comprising the
first phosphor as nanoparticles. This first layer is provided in
contact with the LED chip. Layer thickness should be sufficient to
provide efficient absorption. A second layer comprising a
green-to-yellow emitting phosphor is provided on top of the first
layer.
[0086] Another embodiment uses a monolithic transparent ceramic
material comprising the first phosphor as a conversion layer in LED
lamps. If the first phosphor is provided as a monolithic
transparent ceramic material, absorption as well as scattering by
the first phosphor material is low.
[0087] Optical transmission characteristics from translucent to
transparent provide a high light yield in the conversion of the
high-energy radiation and assure a high transmissivity of the
luminescent emission within the luminescent material
[0088] The use of a transparent ceramic material renders it
possible to adjust a much stronger optical absorption, while the
material remains transparent to the pump radiation.
[0089] Such optical transmission characteristics of the phosphor
material can be achieved with a high-density ceramic material that
has an optimized low residual porosity. In addition to a crystal
anisotropy of the optical refractive index due to a non-uniform
crystal structure, foreign-phase inclusions as well as grain
boundaries and, in particular, pores are disruptive for an optimum
transmission of the luminescent emission.
[0090] A phosphor material with a transparent monolithic ceramic
microstructure may be obtained in a conventional flux process or by
a wet chemical method where precursor co-precipitates are formed,
followed by a conversion of the precursor precipitates into an
oxygen-containing monolithic ceramic material by sintering,
preferably under isostatic pressure. The possibility to machine the
ceramic monolithic material may improve light extraction and render
it possible to create lenses and light-guiding effects.
[0091] The absorption of primary radiation by the phosphors
according to the invention can be further improved if they are
combined with a photonic band gap material.
[0092] Photonic band-gap (PBG) materials represent a new class of
dielectric materials capable of guiding and manipulating the flow
of light on the scale of the wavelength of light. Photonic band-gap
materials consist of a periodic arrangement of dielectric elements,
e.g. hollow spheres or hollow cylinders, in a dielectric host
material with high refractive index with a lattice constant
comparable to the wavelength of light.
[0093] In analogy to the forbidden energy range--the band-gap--for
electrons in a semiconductor, such materials can show a band-gap
for the energy spectrum of photons. Thus the surface of such PBG
materials can act like a perfect dielectric mirror for impinging
light. This renders it possible to capture, i.e. localize light in
two and three dimensions with radii of curvature which were
previously inaccessible.
[0094] According to one embodiment of the invention, photonic
band-gap materials consisting of a one-dimensional periodic
arrangement of dielectric elements may be used as a PBG layer in a
layer stack alternating with phosphor layers. The PBG layer may be
a double layer, comprising alternating layers of materials of high
and low refractive index, each having a layer thickness
q=.lamda./4, the phosphor layer having a layer thickness of
.lamda., wherein .lamda. is the wavelength of the incident blue to
violet primary radiation.
[0095] According to another embodiment, the phosphor may be
incorporated in a known three-dimensional photonic band-gap
material such as opals or inverse opals.
[0096] According to a further embodiment, a three-dimensional PBG
material comprised by the first phosphor material of the invention
is contemplated.
[0097] In operation, electrical power is supplied to the LED chip
to activate the chip. When activated, the chip emits the primary
light, e.g. blue to violet light. A portion of the emitted primary
light is completely or partly absorbed by the luminescent material
in the coating layer. The luminescent material then emits secondary
light, i.e. the converted light having a longer peak wavelength,
primarily yellow to amber to red in a sufficiently broad band,
specifically with a significant proportion of narrow-band red in
response to absorption of the primary light. The remaining
unabsorbed portion of the emitted primary light is transmitted
through the luminescent laye along with the secondary light. The
encapsulation directs the unabsorbed primary light and the
secondary light in a general direction as output light. The output
light is thus a composite light that is composed of the primary
light emitted from the die? and the secondary light emitted from
the luminescent layer.
[0098] The color temperature or color point (color location in the
CIE chromaticity diagram) of the output light of an illumination
system according to the invention will vary in dependence on the
spectral distributions and intensities of the secondary light in
comparison with the primary light.
[0099] Firstly, the color temperature or color point of the primary
light can be varied by a suitable choice of the light-emitting
diode.
[0100] Secondly, the color temperature or color point of the
secondary light can be varied by a suitable choice of the phosphor
in the luminescent material, its particle size and its
concentration. Furthermore, these arrangements also advantageously
afford the possibility of using phosphor blends in the luminescent
material, as a result of which, advantageously, the desired hue can
be set even more accurately.
[0101] According to one embodiment of the invention, the output
light may have a spectral distribution such that it appears to be
"white" light.
[0102] A white light emitting illumination system according to the
invention can advantageously be produced by choosing the
luminescent material such that a blue radiation emitted by a blue
light emitting diode is converted into complementary wavelength
ranges so as to form trichromatic white light according to the RGB
concept. In this case, the luminescent material may be a blend of
two phosphors, a yellow to red europium(III)-activated compound of
an anionic oxygen-containing species with a cationic metal species,
comprising yttrium(III) and gadolinium(III), and a green
phosphor.
[0103] The red light emitting phosphor may be especially selected
from the group of: (Y.sub.1-x-yGd.sub.x).sub.2O.sub.2S:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)VO.sub.4:Eu.sub.y,
(Y.sub.1-x-y-zGd.sub.x)OCl:Eu.sub.yBi.sub.z,
(Y.sub.1-x-yGd.sub.x)(V,P,B)O.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)NbO.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)TaO.sub.4:Eu.sub.y, and
(Y.sub.1-x-y-zGd.sub.x).sub.2O.sub.3:Eu.sub.yBi.sub.z, wherein
0<x<1, 0<y<0.2 and 0<z<0.02.
[0104] The green-emitting phosphor is selected from the group of
the terbium(III)-activated green phosphors, especially from the
group of (Y.sub.xGd.sub.1-x)BO.sub.3:Tb
(0<x<1),LaPO.sub.4:Tb; LaPO.sub.4:Ce,Tb;
(Y.sub.xGd.sub.1-x).sub.3Al.sub.5O.sub.12:Tb (0<x<1);
CeMgAl.sub.11O.sub.19:Tb; GdMgB.sub.5O.sub.10:Ce,Tb;
(Y.sub.xGd.sub.1-x)BO.sub.3:Tb(0<x<1);
(Y.sub.xGd1-x).sub.2SiO.sub.5:Tb (0<x<1),
Gd.sub.2O.sub.2S:Tb; LaOBr:Tb, and LaOCl:Tb.
[0105] The luminescent material may comprise the first phosphor
having a grain size dm1 second phosphor having a grain size
dm2<dm1.
[0106] A second red luminescent material may also be used, in
addition, in order to improve the color rendering of this
illumination system. The luminescent material may be a blend of two
phosphors, a yellow to amber to red europium(III)-activated
compounds of an anionic oxygen-containing species with a cationic
metal species, comprising yttrium(III) and gadolinium(III), and a
red phosphor selected from the group of (Ca.sub.1-xSr.sub.x)S:Eu,
wherein 0<x<1, and
(Sr.sub.1-x-yBa.sub.xCa.sub.y).sub.2-zSi.sub.5-aAl.sub.aN.sub.8-aO.sub.a:-
Eu.sub.z, wherein ps
0.ltoreq.a<5,0<x.ltoreq.1,0.ltoreq.y.ltoreq.1, and
0<z.ltoreq.0.2.
[0107] Particularly good results are achieved with a blue LED whose
emission maximum lies at 400 to 495 nm. An optimum has been found
to lie at 445 to 465 nm, taking particular account of the
excitation spectrum of the europium(III)-activated compounds of an
anionic oxygen-containing species with a cationic metal species,
comprising yttrium(III) and gadolinium(III).
[0108] A white light emission with an even higher luminous efficacy
is possible when red and green phosphors are used together with a
blue-emitting LED and a yellow to red emitting
europium(III)-activated compound of an anionic oxygen-containing
species with a cationic metal species, comprising yttrium(III) and
gadolinium(III).
[0109] In the example given here, the white-light emitting
illumination system according to the invention can particularly
preferably be realized by admixing the inorganic luminescent
material comprising a mixture of two phosphors with a silicone
resin used to produce the luminescence conversion encapsulation or
layer. A first phosphor (1) is the red-emitting
(Y,Gd).sub.2O.sub.3:Eu(III), the second phosphor (2) is the
red-emitting CaS:Eu, and the third phosphor (3) is a green-emitting
phosphor of type (Ce,Tb)MgAl.sub.11.sub.19.
[0110] Part of a blue radiation emitted by a 462 nm InGaN light
emitting diode is shifted by the inorganic luminescent material
(Y,Gd).sub.2O.sub.3:Eu(III) into the red spectral region and,
consequently, into a wavelength range which is complementarily
colored with respect to the color blue. Another part of the blue
radiation emitted by the 462 nm InGaN light emitting diode is
shifted by the inorganic luminescent material
(Ce,Tb)MgAl.sub.11O.sub.19 into the green spectral region. A human
observer perceives the combination of blue primary light and the
polychromatic secondary light of the phosphor blend as white
light.
[0111] The hue (color point in the CIE chromaticity diagram) of the
white light thus produced can be varied in this case by a suitable
choice of the phosphor mixture and concentration.
[0112] FIG. 2 shows the emission spectrum of a white light emitting
LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED
chip, (Y,Gd).sub.2O.sub.3:Eu(III) as a red-emitting phosphor, and a
broad-band green phosphor.
[0113] FIG. 3 shows the luminous efficacy and CRI of a white light
emitting LED with Tc=2500 K (CRI=76). comprising a blue (460 nm)
emitting LED chip, (Y,Gd).sub.2O.sub.3 :Eu(III) as a red-emitting
phosphor, and a broad-band green phosphor.
[0114] FIG. 4 shows the emission spectrum of a white light emitting
LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED
chip, (Y,Gd).sub.2O.sub.3:Eu(III) as a red-emitting phosphor, and
(Ce,Tb)MgAl.sub.11O.sub.19 as a green phosphor.
[0115] FIG. 5 shows the luminous efficacy and CRI of a white light
emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm)
emitting LED chip, (Y,Gd).sub.2O.sub.3:Eu(III) as a red-emitting
phosphor, and (Ce,Tb)MgAl.sub.11O.sub.19 as a green phosphor.
[0116] The luminescent material may be a blend of three phosphors,
a yellow to red europium(III)-activated compound of an anionic
oxygen-containing species with a cationic metal species, comprising
yttrium(III) and gadolinium(III), such as
(Y.sub.1-x-yGd.sub.x).sub.2O.sub.2S:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)VO.sub.4:Eu.sub.y,
(Y.sub.1-x-y-zGd.sub.x)OCl:Eu.sub.yBi.sub.z,
(Y.sub.1-x-yGd.sub.x)(V,P,B)O.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)NbO.sub.4:Eu.sub.y,
(Y.sub.1-x-yGd.sub.x)TaO.sub.4:Euy, and
(Y.sub.1-x-y-zGd.sub.x).sub.20.sub.3:Eu.sub.yBi.sub.z, wherein
0<x<1; 0<y<0.2 and 0<z<0.02, a second red
phosphor selected from the group (Ca.sub.1-xSr.sub.x) S:Eu, wherein
0.ltoreq.x.ltoreq.1, and
(Sr.sub.1-x-yBa.sub.xCa.sub.y).sub.2-zSi.sub.5-aAl.sub.aN.sub.8-aO.sub.a:-
Eu.sub.z, wherein 0.ltoreq.a<5, 0<x.ltoreq.1;
0.ltoreq.y.ltoreq.1 and 0<z.ltoreq.0.2, and a green Tb(III)
-activated phosphor selected from the group of
(Y.sub.xGd.sub.1-x)BO.sub.3:Tb (0<x<1),LaPO.sub.4:Tb;
LaPO.sub.4:Ce,Tb; (Y.sub.xGd.sub.1-x).sub.3Al.sub.5O.sub.12:Tb
(0<x<1); CeMgAl.sub.11O.sub.19:Tb; GdMgB.sub.5O.sub.10:Ce,Tb;
(Y.sub.xGd.sub.1-x)BO.sub.3:Tb(0<x<1);
(Y.sub.xGd.sub.1-x).sub.2SiO.sub.5:Tb (0<x<1),
Gd.sub.2O.sub.2S:Tb; LaOBr:Tb and LaOCl:Tb.
[0117] Useful second green and red phosphors and their optical
properties are summarized in the following Table 2.
TABLE-US-00002 TABLE 2 Composition .lamda..sub.max [nm] Color point
x, y (Ba.sub.1-xSr.sub.x).sub.2SiO.sub.4:Eu 523 0.272, 0.640
SrGa.sub.2S.sub.4:Eu 535 0.270, 0.686 SrSi.sub.2N.sub.2O.sub.2:Eu
541 0.356, 0.606 SrS:Eu 610 0.627, 0.372
(Sr.sub.1-x-yCa.sub.xBa.sub.y).sub.2Si.sub.5N.sub.8:Eu 615 0.615,
0.384
(Sr.sub.1-x-yCa.sub.xBa.sub.y).sub.2Si.sub.5-xAl.sub.xN.sub.8-xO.sub.x:Eu
615-650 * CaS:Eu 655 0.700, 0.303 (Sr.sub.1-xCa.sub.x)S:Eu 610-655
*
[0118] In the example given here, the white-light emitting
illumination system according to the invention can particularly
preferably be realized by admixing the inorganic luminescent
material comprising a mixture of two phosphors with a silicone
resin used to produce the luminescence conversion encapsulation or
layer. A first phosphor (1) is the red-emitting
(Y,Gd).sub.2O.sub.3:Eu(III), the second phosphor (2) is the
red-emitting CaS:Eu, and the third phosphor (3) is a green-emitting
phosphor of the (Ce,Tb)MgAl.sub.11O.sub.19 type.
[0119] According to a further aspect of the invention, an
illumination system that emits output light having a spectral
distribution such that it appears to be "yellow to red" light is
contemplated.
[0120] In one embodiment, a yellow to red light emitting
illumination system according to the invention can advantageously
be produced by choosing the luminescent material such that a blue
radiation emitted by the blue light emitting diode is converted
into complementary wavelength ranges so as to form dichromatic
yellow to red light. In this case, yellow light is produced by the
luminescent material that comprises phosphors including
europium(III)-activated compounds of an anionic oxygen-containing
species with a cationic metal species, comprising yttrium(III) and
gadolinium(III).
[0121] A luminescent material comprising a europium(III)-activated
compounds of an anionic oxygen-containing species with a cationic
metal species, comprising yttrium(III) and gadolinium(III), as a
phosphor is particularly well suited as a yellow component for
stimulation by a primary UVA or blue radiation source such as, for
example, an UVA-emitting LED or blue-emitting LED.
[0122] It is possible thereby to implement an illumination system
emitting in the yellow to amber to red regions of the
electromagnetic spectrum.
[0123] In a further embodiment, a yellow-light emitting
illumination system according to the invention can advantageously
be produced by choosing the luminescent material such that a blue
radiation emitted by the blue light emitting diode is converted
into complementary wavelength ranges so as to form dichromatic
yellow to red light.
[0124] In this embodiment, yellow to red light is produced by the
luminescent materials that comprise a europium(III)-activated
compounds of an anionic oxygen-containing species with a cationic
metal species, comprising yttrium(III) and gadolinium(III).
[0125] Particularly good results are achieved with a blue LED whose
emission maximum lies at 400 to 480 nm. An optimum was found to lie
at 445 to 465 nm, taking particular account of the excitation
spectrum of the europium(III)-activated compounds of an anionic
oxygen-containing species with a cationic metal species, comprising
yttrium(III) and gadolinium(III).
[0126] Part of a blue radiation emitted by a 462 nm InGaN light
emitting diode is shifted by the inorganic luminescent material
into the yellow to red spectral region and, consequently, into a
wavelength range which is complementarily colored with respect to
the color blue. A human observer perceives the combination of blue
primary light and the excess secondary light of the yellow to red
emitting phosphor as yellow to red light.
[0127] The color output of the LED-phosphor system is very
sensitive to the thickness of the phosphor layer: if the phosphor
layer is thick and comprises an excess of a yellow to red
europium(III)-activated compounds of an anionic oxygen-containing
species with a cationic metal species, comprising yttrium(III) and
gadolinium(III), then a lesser amount of the blue LED light will
penetrate through the thick phosphor layer. The combined
LED-phosphor system will then appear yellow to red, because it is
dominated by the yellow to red secondary light of the phosphor.
Therefore, the thickness of the phosphor layer is a critical
variable affecting the color output of the system.
[0128] The hue (color point in the CIE chromaticity diagram) of the
yellow to red light thereby produced can be varied in this case by
a suitable choice of the phosphor mixture and concentration.
[0129] A red light emitting illumination system according to the
invention can particularly preferably be realized by admixing an
excess of the inorganic luminescent material
(Y,Gd).sub.2O.sub.3:Eu(III) with a silicone resin used to produce
the luminescence conversion encapsulation or layer. Part of a blue
radiation emitted by a 462 nm InGaN light-emitting diode is shifted
by the inorganic luminescent material (Y,Gd).sub.2O.sub.3:Eu(III)
into the red spectral region, i.e. into a wavelength range which is
complementarily colored with respect to the color blue. A human
observer perceives the combination of blue primary light and the
excess secondary light of the red-emitting phosphor as red to cyan
light.
[0130] FIG. 1 is a schematic view of a dichromatic white LED lamp
comprising a phosphor of the present invention positioned in a
pathway of light emitted by an LED structure.
[0131] FIG. 2 shows the emission spectrum of a white light emitting
LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED
chip, (Y,Gd).sub.2O.sub.3:Eu(III) as a red-emitting phosphor, and a
broad-band green phosphor.
[0132] FIG. 3 shows luminous efficacy and CRI of a white light
emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm)
emitting LED die, (Y,Gd).sub.2O.sub.3:Eu(III) as a red-emitting
phosphor and a broad-band green phosphor.
[0133] FIG. 4 shows the emission spectrum of a white light emitting
LED with Tc=2500 K (CRI=76) comprising a blue (460 nm) emitting LED
die, (Y,Gd).sub.2O.sub.3:Eu(III) as a red-emitting phosphor, and
(Ce,Tb)MgAl.sub.11O.sub.19 as a green phosphor.
[0134] FIG. 5 shows the luminous efficacy and CRI of a white light
emitting LED with Tc=2500 K (CRI=76) comprising a blue (460 nm)
emitting LED die, (Y,Gd).sub.2O.sub.3:Eu(III) as a red-emitting
phosphor, and (Ce,Tb)MgAl.sub.11O.sub.19 as a green phosphor.
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