Illumination – Plural light sources – Particular wavelength
Reexamination Certificate
1999-09-15
2001-05-22
O'Shea, Sandra (Department: 2875)
Illumination
Plural light sources
Particular wavelength
C362S545000, C362S555000, C362S800000
Reexamination Certificate
active
06234645
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a lighting system for producing white light, which lighting system comprises at least three light-emitting diodes, each one of the light-emitting diodes emitting, in operation, visible light in a preselected wavelength range.
Lighting systems on the basis of light-emitting diodes (LEDs) are used as a source of white light for general lighting applications.
U.S. Pat. No. 5,851,063 discloses a lighting system which employs three LEDs as a light source for; making white light. In this lighting system it is calculated that the maxima of the emission spectra of the LEDs are preferably selected in the wavelength ranges from 455 to 490 mn, 530 to 570 nm and 605 to 630 nm. For such a lighting system it is further calculated that a color rendering index in excess of 80 can be achieved.
Such lighting systems have the drawback that LEDs with spectral maxima in these spectral ranges and, simultaneously, a sufficient energy efficiency are not available. In a lighting system in which such LEDs are employed, particularly the luminous efficacy is adversely influenced.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a lighting system which can be used in practice. The invention further aims at providing a lighting system having a relatively high luminous efficacy.
To achieve this, the lighting system includes at least a fourth light-emitting diode which, in operation, emits visible light in a further wavelength range, the maximum of the spectral emission of the fourth light-emitting diode lying in the further wavelength range from 575 to 605 nm.
By employing four types of LEDs having different spectral ranges, the possibilities of combining LEDs are extended. In order to obtain a lighting system producing white light with a good color rendition, which is based on the three primary colors blue, green and red, it is desirable that the emission maxima of the spectral ranges of the LEDs lie in the ranges from 430 to 490 nm (blue), 530 to 565 nm (green) and 590 to 630 nm (red). In these wavelength ranges, blue and red LEDs with a reasonable luminous efficacy are commercially available, however, green LEDs with the desirable spectral range and comparable luminous efficacy are not or hardly available. Available “green” LEDs having an efficiency which is approximately half that of their blue equivalents emit in the blue-green spectral range between 500 and 525 nm, which does not include the desired spectral range. In addition, as the load at the input of the LED increases, a (further, undesirable) shift towards the blue is observed. Since LEDs with a spectral range in the yellow region (maximum of the spectral emission in the wavelength range from 565 to 605 nm) and a high luminous efficacy are available, a practically usable lighting system producing white light with the desired color rendition is obtained by combining (commercially available) blue, “green” (emission in the blue-green range), yellow and red LEDs. By using commercially available LEDs, a lighting system is obtained which also exhibits a relatively high luminous efficacy.
Highly efficient yellow LEDs on the basis of GaAs have been available for some years and are also increasingly used for signaling purposes, such as rear lights (of vehicles), traffic lights and traffic-signaling systems.
U.S. Pat. No. 5,851,063 also discloses four different types of LEDs as the light source for producing white light, and it is calculated that the maxima of the emission spectra of the LEDs are selected in the wavelength ranges from 440 to 450 nm, 455 to 505 nm, 555 to 565 nm and 610 to 620 nm. These wavelength ranges are based on calculations of the desired light quality on the basis of LEDs with a desired emission spectrum. The known lighting system is apparently not based on commercially available LEDs. The lighting system in accordance with the invention comprises a practically feasible combination of (the spectral characteristics of) known and commercially available LEDs for manufacturing a light source producing white light with a relatively high luminous efficacy.
It is desirable to determine a relatively limited wavelength range within which the maximum of the spectral emission of the fourth light-emitting diode is situated. Preferably, the maximum of the spectral emission of the fourth light-emitting diode lies in the wavelength range from 585 to 600 nm. The use of such yellow LEDs causes the harmony with the other three types of LEDs to be improved. Since the photopic sensitivity of the human eye in the wavelength range is maximal at 555 nm, relatively small variations in the spectral range of the yellow LED have a relatively large effect on the color rendition of the lighting system. A (commercially available) yellow LED having a maximum spectral emission at 595 nm (20 nm FWHM, energy-efficiency 20%) is very suitable.
Preferably, the color rendering index (Ra) of the lighting system is at least equal to or greater than 60(R
a
≧60). By a suitable combination of the spectral emissions of the four light sources, a lighting system is obtained having a relatively high luminous efficacy and a good color rendering index.
Preferably, the luminous efficacy of the lighting system is at least equal to or greater than 30 lm/W. Lighting systems on the basis of LEDs having such an efficiency are commercially attractive. In a particularly preferred embodiment, the luminous efficacy of the lighting system is greater than 40 lm/W. For comparison, a typical 100 W incandescent lamp has a luminous efficacy of 14 lm/W (color temperature 2800 K, color rendering index 100), a 500 W halogen incandescent lamp has a luminous efficacy of approximately 19 lm/W (color temperature 3000 K, color rendering index 100), while a 36 W fluorescent lamp has a luminous efficacy of approximately 89 lm/W (color temperature 4000 K, color rendering index 85). The color rendering index of the lighting system in accordance with the invention is lower than that calculated in the known lighting system, however, the luminous efficacy of the lighting system in accordance with the invention is substantially higher and the lighting system in accordance with the invention is based on a combination of commercially available light-emitting diodes.
Preferably the three light-emitting diodes comprise a blue light-emitting diode, a blue-green light-emitting diode and a red light-emitting diode, and the fourth light-emitting diode includes a yellow light-emitting diode. In this manner, a lighting system is obtained which emits white light with a good color rendition on the basis of four basic colors (blue, blue-green, yellow and red). 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 blue-green light-emitting diode lies in the wavelength range from 510 to 530 nm, and the maximum of the spectral emission of the red light-emitting diode lies in the wavelength range from 610 to 630 nm. LEDs having such spectral ranges and a relatively high energy efficiency are commercially feasible. By using the yellow-type LEDs, the “mismatch” in the color of the green LED, which emits blue-green light, is compensated.
A point of special interest in the lighting system in accordance with the invention is that, in general, LEDs emit light with a high directivity, while it is desirable for the LEDs to emit (diffuse) light in accordance with a Lambertian radiator.
The invention further aims at improving the blending of light of the lighting system. To achieve this, an alternative embodiment of the lighting system in accordance with the invention is characterized in that the lighting system is further provided with reflection means. The LEDs are provided in the lighting system in such a manner that a substantial part of the light originating from the LEDs cannot directly leave the lighting system, but instead is incident on the reflection means. An advantage of the use of reflection means is that light originating from the
Borner Herbert Friedrich
Busselt Wolfgang
Justel Thomas
Nikol Hans
Ronda Cornelis R.
Halajian Dicran
Lee Guiyoung
O'Shea Sandra
U.S. Philips Cororation
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