Communications: electrical – Visual indication
Reexamination Certificate
2000-05-15
2002-10-15
Crosland, Donnie L. (Department: 2632)
Communications: electrical
Visual indication
C340S907000, C340S815450, C362S230000, C362S293000, C362S800000, C257S089000, C345S044000, C345S083000, C428S690000, C313S502000, C313S486000
Reexamination Certificate
active
06466135
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to a blue-green light illumination system, and specifically to a ceramic phosphor blend for converting UV radiation emitted by a light emitting diode (“LED”) to blue-green light.
Semiconductor light emitting diodes are semiconductor chips that are mounted in a package and which emit radiation in response to an applied voltage or current. These LEDs are used in a number of commercial applications such as automotive, display, safety/emergency and directed area lighting.
One important application of semiconductor LEDs is as a light source in a traffic light. Presently, a plurality of blue-green emitting LEDs containing III-V semiconductor layers, such as GaN, etc., are used as the green light of a traffic signal (also known as a traffic light).
Industry regulations often require traffic light colors to have very specific CIE color coordinates. For example, according to the Institute of Transportation Engineers (ITE), a green traffic light in the United States is typically required to have emission CIE color coordinates located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following color coordinates:
a) x=0.000 and y=0.506;
b) x=0.224 and y=0.389;
c) x=0.280 and y=0.450; and
d) x=0.000 and y=0.730. The following CIE color coordinates are most preferred for green traffic light applications: x=0.1 and y=0.55.
Likewise, industry regulations require automotive display colors to have specific CIE color coordinates. According to the Society of Automotive Engineers (SAE), a green automotive display, such as a vehicle dashboard display, is typically required to have emission CIE color coordinates located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following color coordinates:
e) x=0.0137 and y=0.4831;
f) x=0.2094 and y=0.3953;
g) x=0.2879 and y=0.5196; and
h) x=0.0108 and y=0.7220.
The color coordinates (also known as the chromaticity coordinates) and the CIE chromaticity diagram are explained in detail in several text books, such as on pages 98-107 of K. H. Butler, “Fluorescent Lamp Phosphors” (The Pennsylvania State University Press 1980) and on pages 109-110 of G. Blasse et al., “Luminescent Materials” (Springer-Verlag 1994), both incorporated herein by reference.
Presently, GaN based LEDs are designed to emit blue-green light with a peak wavelength of 505 nm, which has the desired CIE color coordinates of x=0.1 and y=0.55. Table I illustrates the optical properties of an LED having a In
1-x
Ga
x
N active layer that was manufactured according to desired parameters.
TABLE I
Peak
Emission
External
Color Coordinates
Emission
Peak Half
Quantum
Efficacy
x
y
Wavelength
Width
Efficiency
(lm/W)
0.1
0.55
505 nm
35 nm
10%
25 lm/W
In Table I, external quantum efficiency refers to a ratio of a number of photons emitted per number of electrons injected into the LED.
However, these LEDs with the In
1-x
Ga
x
N active layer suffer from the following disadvantage. Due to frequent deviations from desired parameters (i.e., manufacturing systematic variations), the LED peak emission wavelength deviates from 505 nm, and thus, its CIE color coordinates deviate from the desired x=0.1 and y=0.55 values. For example, the LED color output (e.g., spectral power distribution and peak emission wavelength) varies with the band gap width of the LED active layer. One source of deviation from the desired color coordinates is the variation in the In to Ga ratio during the deposition of the In
1-x
Ga
x
N active layer, which results in an active layer whose band gap width deviates from the desired value. This ratio is difficult to control precisely during mass production of the LEDs, which leads to inconsistent color coordinates in a given batch of LEDs. Thus, the In
1-x
Ga
x
N LEDs which are suitable for use in traffic lights have a lower production yield because a large number of such LEDs with unsuitable emission color coordinates have to be discarded. The present invention is directed to overcoming or at least reducing the problem set forth above.
BRIEF SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a blue-green illumination system, comprising a light emitting diode, and at least one luminescent material having at least two peak emission wavelengths, wherein the emission CIE color coordinates of the at least two peak emission wavelengths are located within an area of a pentagon on a CIE chromaticity diagram, whose corners have the following CIE color coordinates:
e) x=0.0137 and y=0.4831;
b) x=0.2240 and y=0.3890;
c) x=0.2800 and y=0.4500;
g) x=0.2879 and y=0.5196; and
h) x=0.0108 and y=0.7220.
In accordance with another aspect of the present invention, there is provided a traffic signal, comprising a housing, at least one lens, a radiation source having a peak emission wavelength of 420 nm and below, and at least one luminescent material having at least two peak emission wavelengths, wherein the emission CIE color coordinates of the at least two peak emission wavelengths are located within an area of a quadrilateral on a CIE chromaticity diagram, whose corners have the following CIE color coordinates:
a) x=0.000 and y=0.506;
b) x=0.224 and y=0.389;
c) x=0.280 and y=0.450; and
d) x=0.000 and y=0.730.
In accordance with another aspect of the present invention, there is provided a method of making a blue-green light illumination system, comprising, blending a first phosphor powder having a first peak emission wavelength and a second phosphor powder having a second peak emission to form a phosphor powder mixture having emission CIE color coordinates located within an area of a pentagon on a CIE chromaticity diagram, whose corners have the following CIE color coordinates:
e) x=0.0137 and y=0.4831;
b) x=0.2240 and y=0.3890;
c) x=0.2800 and y =0.4500;
g) x=0.2879 and y=0.5196; and
h) x=0.0108 and y=0.7220. and placing the phosphor powder mixture into the illumination system adjacent a radiation source.
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Keith Butler: Fluorescent Lamp Phosphors, pp. 98-107 (The Pennsylvania State University Press 1980).
S. Nakamura et al.: The Blue Laser Diode, pp. 216-221, 328-329 (Springer 1997).
G. Blasse et al.: Luminescent Materials, pp. 109-110 (Springer-Verlag 1994).
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Comanzo Holly Ann
McNulty Thomas Francis
Srivastava Alok Mani
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