Illumination – Plural light sources – With modifier
Reexamination Certificate
1999-09-24
2001-05-22
O'Shea, Sandra (Department: 2875)
Illumination
Plural light sources
With modifier
C362S227000, C362S800000, C362S084000, C362S293000, C362S231000, C362S545000, C313S499000, C313S512000, C313S312000, C313S313000
Reexamination Certificate
active
06234648
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a lighting system comprising at least two light-emitting diodes, each of said light-emitting diodes emitting, in operation, visible light in a preselected wavelength range.
Lighting systems based on light-emitting diodes (LEDs) are used as a source of white light for general lighting applications.
A lighting system of the type mentioned in the opening paragraph is known. In recent years, apart from red light-emitting diodes based on GaP, also efficient, blue light-emitting diodes and green light-emitting diodes based on GaN have been developed. In order to produce white light, in principle, three LEDs are necessary as the primary light source, namely a blue, a green and a red LED.
It is a drawback of such lighting systems that a combination of three LEDs as the primary light source does not always lead to the desired color rendition, which can be attributed to the fact that LEDs with spectral maxima in the desired spectral regions which at the same time are sufficiently energy-efficient are not available or short.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a lighting system, which exhibits an improved color rendition. The invention further aims at improving the luminous efficacy of the lighting system.
To achieve this, the lighting system includes conversion means for converting a part of the visible light emitted by one of the light-emitting diodes into visible light in a further wavelength range so as to optimize the color rendition of the lighting system.
The conversion means are excited by light originating from one of the at least two LEDs. A part of this light is converted by the conversion means, for example via a process of absorption and emission, into visible light in the further wavelength range. This results in a lighting system which comprises, in fact, three light sources, namely two primary light sources which are formed by the at least two LEDs, which primary light sources each emit visible light in a preselected wavelength range, and one so-called secondary light source which emits visible light in the further wavelength range. By a suitable choice of the wavelength ranges in which these two primary light sources and the secondary light source emit visible light, a lighting system is obtained having an improved color rendition relative to a lighting system based on the two primary light sources. Since the application of a third primary light source (for example a green LED or a red LED) is avoided, an improved color rendition of the lighting system is obtained.
Preferably, the conversion means comprise a luminescent material. Such materials are very suitable because they generally have a high quantum efficiency and a high lumen equivalent (expressed in 1m/W), so that a high luminous efficacy of the lighting system is obtained. In addition, many varieties of (stable) inorganic and organic luminescent materials (phosphors) are known, so that the selection of a material for achieving the aim in accordance with the invention (improving the color rendition) is simplified.
The color rendition of the lighting system can be influenced in two ways. On the one hand, the spatial color rendition is improved by optimally mixing the light originating from the LEDs and the conversion means. On the other hand, the color rendition of the lighting system is improved by taking measures which make sure that the light output of the LEDs is independent of time. Such dependence is obtained, for example, if the light output of a LED changes as a function of the temperature of the LED. In this case, the use of temperature-independent LEDs has advantages.
In accordance with a first aspect of the invention, the luminescent material can be preferably excited by light originating from the wavelength range of 400 to 500 nm. By virtue of this sensitivity, the luminescent material can very suitably be used to absorb, in particular, blue light. This absorbed light is very efficiently converted by the luminescent material into visible light in the further wavelength range, for example green light.
Suitable luminescent materials are (Sr, Ca)
2
SiO
4
:Eu
2+
, Ba
2
SiO
4
:Eu
2+
, SrGa
2
S
4
, ZnS:Cu
+
, ZnS:Au
+
, ZnS:Al
3+
, (Zn,Cd)S:Ag
+
and CaS:Ce
3+
. Said materials have a relatively high quantum efficiency and light absorption at 450 nm. These materials further exhibit a relatively very high lumen equivalent when blue light is converted to the desired green light.
A very attractive embodiment of the lighting system in accordance with a first aspect of the invention is characterized in that the two light-emitting diodes at least comprise a blue light-emitting diode and at least a red light-emitting diode, and that the conversion means comprise a (green light-emitting) luminescent material for converting a portion of the light emitted by the blue light-emitting diode into green light. In this manner, a lighting system in accordance with a first aspect of the invention is obtained which emits white light with a high color rendering index on the basis of three basic colors (red, blue and green), in which only two primary light sources are employed, namely blue and red light, and green light is obtained by converting a portion of the blue light. Preferably, the maximum of the spectral emission of the blue light-emitting diode lies in the wavelength range from 460 to 490 nm, the maximum of the spectral emission of the red light-emitting diode lies in the wavelength range from 610 to 630 nm, and the maximum of the spectral emission of the (green light-emitting) luminescent material lies in the wavelength range from 510 to 530 nm.
In accordance with a second aspect of the invention, the luminescent material can be preferably excited by light originating from the wavelength range of 500 to 560 nm. By virtue of this sensitivity, the luminescent material can very suitably be used to absorb, in particular, green light. This absorbed light is very efficiently converted by the luminescent material into visible light in the further wavelength range, for example red light.
Suitable luminescent materials are CaS:Eu,Mn; CaS:Eu; SrS:Eu; (Zn,Cd)S:Ag; SrO:Eu; Sr
3
B
2
O
6
: Eu; Sr
2
Mg(BO
3
)
2
; CaS:Eu, Mn; CaS:Eu or SrS:Eu. Said materials have a relatively high quantum efficiency and light absorption. These materials further exhibit a relatively very high lumen equivalent when blue light or green light is converted to the desired red light.
A very attractive embodiment of the lighting system in accordance with a second aspect of the invention is characterized in that the two light-emitting diodes at least comprise a blue light-emitting diode and at least a green light-emitting diode, and that the conversion means comprise a luminescent material for converting a portion of the light emitted by the blue and/or green light-emitting diode to red light. An important advantage of the use of blue and green LEDs as the primary light source is that both diode chips can be manufactured by means of the GaN technology known per se. Unlike red GaP diode chips, such blue and green GaN diode chips are not temperature-dependent, so that the use of relatively expensive electronics to compensate for the temperature-dependence of such diode chips can be dispensed with. A further advantage resides in that said blue and green GaN diode chips can be contacted on the same side, so that they can be readily arranged in series. The use of a green-excited luminescent material emitting red light has the additional advantage with respect to a blue-excited luminescent material emitting red light that the quantum deficit is smaller. Preferably, the maximum of the spectral emission of the blue light-emitting diode lies in the wavelength range from 460 to 490 nm, the maximum of the spectral emission of the green light-emitting diode lies in the wavelength range from 510 to 550 nm, and the maximum of the spectral emission of the red light-emitting luminescent material lies in the wavelength range from 610 to 630 nm.
The color rend
Börner Herbert F.
Busselt Wolfgang
Harbers Gerard
Justel Thomas
Nikol Hans
Halajian Dicran
O'Shea Sandra
Ton Anabel M
U.S. Philips Corporation
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