White light illumination system with improved color output

Details White light illumination system with improved color output White light illumination system with improved color output

2000-03-27

2003-03-25

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

With gaseous discharge medium

C313S503000, C252S30140H

Reexamination Certificate

active

06538371

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to a white light illumination system, and specifically to a ceramic YAG:Ce:Gd phosphor for converting blue light emitted by a light emitting diode (“LED”) to white light.
White light emitting LEDs are used as a backlight in liquid crystal displays and as a replacement for small conventional lamps and fluorescent lamps. As discussed in chapter 10.4 of “The Blue Laser Diode” by S. Nakamura et al., pages 216-221 (Springer 1997), incorporated herein by reference, white light LEDs are fabricated by forming a ceramic phosphor layer on the output surface of a blue emitting semiconductor LED. Conventionally, the blue LED is an InGaN single quantum well LED and the phosphor is a cerium doped yttrium aluminum garnet (“YAG”), Y
3
Al
5
O
12
:Ce
3+
. The blue light emitted by the LED excites the phosphor, causing it to emit yellow light. The blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light.
The chromaticity coordinates of the blue LED, the yellow YAG phosphor and the white combined output of the LED and the phosphor may be plotted on the well known CIE chromaticity diagram, as shown in FIG.
1
. The chromaticity coordinates and the CIE chromaticity diagram are explained in detail in several text books, such as pages 98-107 of K. H. Butler, “Fluorescent Lamp Phosphors” (The Pennsylvania State University Press 1980) and pages 109-110 of G. Blasse et al., “Luminescent Materials” (Springer-Verlag 1994), both incorporated herein by reference. As shown in
FIG. 1
, chromaticity coordinates of the prior art blue LEDs used for white emission lie in the circle
1
on the CIE chromaticity diagram in FIG.
1
. In other words, the chromaticity coordinates of the LED will be represented by a single point within circle
1
, the location of the particular point depending on the peak emission wavelength of the LED.
The chromaticity coordinates of the YAG:Ce
3+
phosphor are represented by a point along line
3
in
FIG. 1
, depending on the level of Gd dopant on the Y lattice site and/or the level of Ga dopant on the Al lattice site. For example, the chromaticity coordinates of the YAG phosphor containing a high level of Gd and/or a low level of Ga dopant may be located at point
5
, while the chromaticity coordinates of the YAG phosphor containing a low level of Gd and/or a high level of Ga dopant may be located at point
7
. Chromaticity coordinates of the YAG phosphor containing intermediate levels of Gd and/or Ga dopants may be located at any point along line
3
between points
5
and
7
, such as at points
9
,
11
,
13
or
15
, for example.
The chromaticity coordinates of the combined output of the blue LED and the YAG phosphor may be varied within a fan shaped region on the CIE chromaticity diagram in
FIG. 1
, bordered by lines
17
and
19
. In other words, the combined chromaticity coordinates of the output of the LED and the phosphor may be any point inside the area bordered by circle
1
, line
3
, line
17
and line
19
in
FIG. 1
, as described on page 220 of the Nakamura et al. text book. However, the LED—phosphor system described by Nakamura et al. suffers from several disadvantages.
As shown in
FIG. 1
, the CIE chromaticity diagram contains the well known Black Body Locus (“BBL”), represented by line
21
. The chromaticity coordinates (i.e., color points) that lie along the BBL obey Planck's equation: E(&lgr;)=A&lgr;
−5
/(e
(B/T)
−1), where E is the emission intensity, &lgr; is the emission wavelength, T the color temperature of the black body and A and B are constants. Various values of the color temperature, T, in degrees Kelvin, are shown on the BBL in FIG.
1
. Furthermore, points or color coordinates that lie on or near the BBL yield pleasing white light to a human observer. Typical white light illumination sources are chosen to have chromaticity points on the BBL with color temperatures in the range between 2500K to 7000K. For example, lamps with a point on the BBL with a color temperature of 3900 K are designated “natural white,” a color temperature of 3000 K are designated “standard warm white,” and so on. However, points or color coordinates that lie away from the BBL are less acceptable as a white light to the human observer. Thus, the LED—phosphor system shown in
FIG. 1
contains many points or chromaticity coordinates between lines
17
and
19
that do not yield an acceptable white light for lighting applications.
In order to be useful as a white light source, the chromaticity coordinates LED—phosphor system must lie on or near to the BBL. The color output of the LED—phosphor system varies greatly due to frequent, unavoidable, routine deviations from desired parameters (i.e., manufacturing systematic errors) during the production of the phosphor.
For example, the color output of the LED—phosphor system is very sensitive to the thickness of the phosphor. If the phosphor is too thin, then more than a desired amount of the blue light emitted by the LED will penetrate through the phosphor, and the combined LED—phosphor system light output will appear bluish, because it is dominated by the output of the LED. In this case, the chromaticity coordinates of the output wavelength of the system will lie close to the LED chromaticity coordinates and away from the BBL on the CIE chromaticity diagram. In contrast, if the phosphor is too thick, then less than a desired amount of the blue LED light will penetrate through the thick phosphor layer. The combined LED—phosphor system will then appear yellowish, because it is dominated by the yellow output of the phosphor.
Therefore, the thickness of the phosphor is a critical variable affecting the color output of the system. Unfortunately, the thickness of the phosphor is difficult to control during large scale production of the LED—phosphor system, and variations in phosphor thickness often result in the system output being unsuitable for white light lighting applications or appearing non-white (i.e., bluish or yellowish), which leads to an unacceptably low LED—phosphor system manufacturing yield.
FIG. 2
illustrates a CIE chromaticity diagram containing the chromaticity coordinates at point
11
of a prior art YAG:Ce
3+
phosphor layer that is placed over a blue LED having chromaticity coordinates at point
23
. Thus, the chromaticity coordinates of this system will lie along line
25
connecting points
11
and
23
in FIG.
2
. If the phosphor layer is thinner than required to produce white light, then too much of the blue LED light will penetrate through the phosphor layer and the chromaticity coordinates of the system light output will lie near the LED coordinates, such as at point
27
, below the BBL. The output of this system will appear bluish. If the phosphor layer is thicker than required to produce white light, then too little of the LED light will be absorbed by the phosphor, and the chromaticity coordinates of the system will lie near the phosphor coordinates, such as at point
29
, above the BBL. The output of the system will appear yellowish. The chromaticity coordinates of the system will lie near or on the BBL at point
31
only if the thickness of the phosphor layer is almost exactly equal to the thickness required to produce acceptable white light. Thus,
FIG. 2
illustrates the sensitivity of the system color output to variations in the phosphor layer thickness.
Furthermore, the prior art LED—phosphor system suffers from a further deficiency. In order to obtain a white light illumination system with different color temperatures that have color coordinates on or near the BBL (i.e., a system that yields an acceptable white light for illumination purposes), the composition of the phosphor has to be changed. For example, if a prior art system includes a phosphor having a composition whose color coordinates are located at point
11
in
FIG. 2
, then the LED—phosphor system con

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