WO2008053012A1

WO2008053012A1 - Use of phosphor led's for fine tuning the performance of a lighting assembly

Info

Publication number: WO2008053012A1
Application number: PCT/EP2007/061738
Authority: WO
Inventors: Henk Schutte
Original assignee: Lemnis Lighting Patent Holding B.V.
Priority date: 2006-11-03
Filing date: 2007-10-31
Publication date: 2008-05-08

Abstract

Disclosed is a lighting assembly comprising light emitting diodes (LEDs). A first group of at least one LED emits light having a peak wavelength in the range of from 550 to 620 nm; a second group of at least one LED emits light having a peak wavelength in the range of from 505 to 550 nm; and a third group of at least one phosphor-comprising LED having a peak wavelength in the range of from 450 to 505 nm. The former two groups have a light intensity at least twice that of the third group. In an alternate embodiment a lighting assembly for utility lighting comprises a first group of at least one LED having a peak wavelength in the range of from 480 to 530 nm, and a second group of at least one phosphor comprising LED.

Description

Use of Phosphor LEDs for fine tuning the performance of a lighting assembly

TECHNICAL FIELD

[oooi] The present intention relates to the use of phosphor LEDs for fine tuning performance of a lighting assembly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] It is known to provide light emitting diodes that emit light in the short wavelength range of the visible spectrum with a phosphor. The purpose of the phosphor is to convert the short wavelength to a much broader wavelength distribution at longer wavelengths. The purpose in most cases is to create a light emitting diode that emits white light.

[0003] An important disadvantage of providing LEDs with a phosphor is that the useful life of a phosphor is much shorter than that of the LED itself. For perspective, the performance of a phosphor significantly deteriorates under the influence of temperature and uv after about 20,000 hours of use, and drops to unacceptable levels after about 50,000 hours of use. The useful life of a light emitting diode is on the order of 100,000 hours.

2. Description of the Related Art

[0004] British patents 2 411 010 describes a so-called anomaloscope, which is an optical instrument in which the light lights of a reference source is matched by mixing more or less monochromatic blue and yellow lights. The white light of the reference source in this publication is produced by a combination of a blue LED with a yellow phosphor and a naked green LED. Optionally, a red LED may be included as well. The purpose is to create a white light source with a very flat spectral power distribution. [0005] U.S. Patents 7,061 ,454 discloses a LED device comprising a white LED device and a red LED device. The white LED comprises a blue LED covered with a phosphor.

[0006] The prior art uses a phosphor LED as the main source of light. This results in a flat spectral power distribution. The useful life of such a light source is limited by the useful life of the phosphor.

[0007] It is an object of the present intention to use a phosphor LED to fine tune the performance of a lighting assembly. Because the phosphor LED is merely used for fine tuning purposes, the lighting assembly continues to provide acceptable performance long after the useful life of the phosphor has expired. Accordingly, the lighting assemblies of the present intention have a useful life equal to that of the LEDs, rather than that of the phosphor. Moreover, as will be explained herein below, even after the phosphor stops functioning, it still acts as a filter of the primary light of the LED that is covered with the phosphor. In addition, the lighting assemblies of the present intention emit lights with a wavelength distribution comprising distinct peaks.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention relates to the use of a phosphor coated LED for fine tuning the performance of a lighting assembly. Specifically, the phosphor coated LED is a blue LED with a white or yellow phosphor.

[0009] In one embodiment, the invention relates to an RGB lighting assembly comprising: a) a first group of at least one LED emitting light having a peak wavelength in the range of from 550 to 620 nm, said group emitting light having a relative intensity RI_R; b) a second group of at least one LED emitting light having a peak wavelength in the range of from 505 to 550 nm, said group emitting light having a relative intensity RI_G: c) a third group of at least one phosphor-comprising LED, emitting primary light having a peak wavelength in the range of from 450 to 505 nm, said group emitting light having a relative intensity Rl₆, and secondary light having a peak wavelength in the range of from 515 to 620 nm, said group emitting light having a relative intensity Rl₆; wherein the ratios RI_R/RI_B and RI_G/RI_B are each at least 2.0.

[ooio] In a second embodiment the invention relates to a lighting assembly for utility lighting comprising: a) a first group of at least one LED emitting light having a peak wavelength in the range of from 480 to 530 nm; b) a second group of at least one LED which is a phosphor-coated blue LED; said lighting assembly having an S/P ratio of at least 2.0.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[ooii] The present intention is based on the discovery that a phosphor coated light emitting diode can be used to fine tune the spectral distribution of a lighting assembly, in particular the spectral distribution of a lighting assembly comprising LEDs that are not coated with a phosphor, so-called "naked" LEDs.

[0012] The invention will be illustrated with reference to blue LEDs emitting primary light having a peak wavelength in the range of from 450 to 505 nm, and emitting a secondary light having a peak wavelength in the range of from 515 to 620 nm. It will become clear from the embodiments described herein that the same principle can be used to advantage with other types of phosphor coated LEDs as well.

[0013] Phosphor coated LEDs are well-known in the art. The light emitted by a naked LED generally has a narrow spectral distribution peak. Phosphors are used to convert the primary light emitted by a light emitting diode to light having a broad and flat spectral distribution, which approaches as much as possible the spectral distribution of a conventional light source such as an incandescent light bulb. The -A-

use of phosphors is inherently energy inefficient, because energy is lost in the conversion of the short wavelength primary light to the longer wavelength secondary light. Moreover, the phosphors known today have a much shorter life expectancy than the LEDs themselves. For example, a typical phosphor starts showing a significant deterioration of its performance after about 20,000 hours of use, and after about 50,000 hours of use generally the lumen maintenance is reduced to less than 50%. The LEDs themselves, on the other hand, have a life expectancy on the order of 100,000 hours. The use of phosphors therefore reduces the useful life of a light emitting diode by more than 50 percent.

[0014] It is also known to combine red, green, and blue naked LEDs to form a trichromatic light source, which approximates white light. It has been recognized that these so-called RGB LEDs do not have sufficient emission in the yellow part of the spectrum (the so-called yellow gap). It has been suggested to add an amber LED to an RGB assembly in order to fill the yellow gap.

[0015] Another drawback of RGB LEDs is that the amount of blue light is often too great, resulting in a light that can be considered white, but that has a blue hue which is considered unpleasant for many lighting purposes.

[0016] It has now been discovered that the performance of and RGB LED assembly can be improved by the replacing the naked blue LED of the assembly with a phosphor coated blue LED. The phosphor coating reduces the amount of blue light emitted by the LED. Moreover, the white or yellow light emitted by the phosphor serves to fill the yellow gap in the spectrum of the RGB assembly.

[0017] Importantly, the amount of blue light in the spectrum of the RGB assembly should be relatively small for producing warm white light. When the contribution of blue light to the overall spectrum is kept small, the hue of the light is pure white, or may have the slightly warm, yellow hue. The phosphor will deteriorate during use of the lighting assembly. However, it will continue to function as a filter of the blue light, so that the amount of blue light in the spectrum is reduced as compared to that if a naked blue LED were used. As the phosphor deteriorates, its contribution to filling the yellow gap diminishes. However, this has only a small effect on the overall performance of the lighting assembly. Even when the phosphor has become entirely inactive, the performance of the lighting assembly is still improved over that of a combination of naked LEDs, because of the filtering effect of the phosphor, which reduces the amount of blue light in the spectrum. As a result, the effective life of the lighting assembly is that of the LEDs, that is, about 100,000 hours, as compared to that of the phosphor which is 50,000 hours or less.

[0018] Accordingly, in one aspect of the present invention relates to an RGB lighting assembly comprising: a) a first group of at least one LED emitting light having a peak wavelength in the range of from 550 to 620 nm, said group emitting light having a relative intensity RI_R; b) a second group of at least one LED emitting light having a peak wavelength in the range of from 505 to 550 nm, said group emitting light having a relative intensity RI_G: c) a third group of at least one phosphor-comprising LED, emitting primary light having a peak wavelength in the range of from 450 to 505 nm, said group emitting light having a relative intensity Rl₆, and secondary light having a peak wavelength in the range of from 515 to 620 nm, said group emitting light having a relative intensity Rl₆; wherein the ratios RI_R/RI_B and RI_G/RI_B are each at least 2.0.

[0019] In a preferred embodiment the ratio RI_R/RI_B is at least 2.5. In yet another preferred embodiment the ratio RI_G/RI_B is also at least 2.5. In a further preferred embodiment the ratio RI_R/RI_B is at least 6.0. In yet a further preferred embodiment the ratio RI_G/RIB is also at least 6.0.

[0020] The LEDs in the assembly may be combined in any suitable circuit known to the person skilled in the art. In a preferred embodiment the LEDs are combined in a bridge circuit. In a still further preferred embodiment the first group comprises four LEDs that form a rectifier bridge circuit. This rectifier bridge circuit powers the LEDs of the second group and the LEDs of the third group. This arrangement permits the lighting assembly to be directly connected to a domestic power outlet providing AC voltage of 120 or 130 Volts, as in North America, or 230 Volts as in Europe. However, the same circuitry may be powered by a DC source of 12 Volts or less.

[0021] To obtain the desired ratios in relative intensity, the second group may comprise three green LEDs and the third group may comprise one phosphor coated blue LED. It will be understood that the decisive factor is the relative intensity ratios, which may be obtained by any suitable number of combinations of LEDs.

[0022] The invention will be further illustrated by a second embodiment, which relates to utility lighting. The term utility lighting as used herein refers to lighting situations in which the overriding factor is providing good visibility at an energy cost that is as low as possible. A faithful color rendering is not a primary objective of utility lighting, but a certain level of collar recognition is often desirable as will be explained further herein below. Examples of utility lighting include park and street lighting, lighting of airport aprons, outdoor industrial areas and harbor facilities, parking lots, and the like.

[0023] Commonly used light sources for utility lighting include sodium lamps and high-pressure mercury lamps. Both light sources provide a very poor spectral distribution.

[0024] This particular embodiment of the present invention is based in part on the recognition that the human eye is most sensitive during nighttime in the cyan area of the spectrum, that is in the range of from 480 to 530 nm, more specifically in the range of from 495 to 510 nm.

[0025] An important parameter for determining the efficiency of utility lighting is the so-called scotopic/photopic, or S/P ratio. This ratio reflects the different perception of intensity of light under daylight (photopic) conditions and nighttime (scotopic) conditions. Traditional light sources have an S/P ratio of much less than 2, and even blue LEDs with a white phosphor barely reach an S/P ratio of 2.

[0026] It has now been discovered that cyan LEDs having a peak wavelength in the range of from 480 to 530 nm, preferably in the range of from 495 to 510 nm, provide S/P ratios of 2.0 or higher, even as high as 5. In terms of lighting efficiency, one might consider constructing utility light assemblies comprising only cyan LEDs. However, it is often desirable to provide a minimum level of color recognition even in utility lighting situations. For example, a certain level of color rendition is helpful to customers in finding their cars in a parking lot at night. Also in traffic, it is important for drivers to be able to easily recognize traffic signals and road signs. It has now been discovered that the color recognition of a cyan LEDs light source can be improved in a dramatic way by adding a small amount of blue light and white light. This may be done by including in the lighting assembly a phosphor coated blue LED.

[0027] Specifically, and the present embodiment the invention relates to a lighting assembly for utility lighting comprising: a) a first group of at least one LED emitting light having a peak wavelength in the range of from 480 to 530 nm; b) a second group of at least one LED which is a phosphor-coated blue LED; said lighting assembly having an S/P ratio of at least 2.0.

[0028] In a preferred embodiment, the peak wavelength of the first LED is in the range of from 495 to 510 nm, which is the spectral area where nighttime vision is at its most sensitive.

[0029] It is not necessary to provide a large amount of light in the blue area of the spectrum, or to provide a large amount of white light in order to obtain the desired collar recognition properties. Because the contribution of the phosphor coated blue LED can be kept small, the lighting assembly is not much affected by the inevitable deterioration of the phosphor. First of all, the deterioration of the phosphor does not affect the emission of the primary blue light itself, which continues to contribute to the color recognition during the entire life of the lighting assembly. As a result, the performance of the lighting assembly remains acceptable even after the phosphor has become fully inactive. As result, the useful life of the lighting assembly of this particular embodiment is not determined by the useful life of the phosphor, but by that of the LEDs in the assembly.

[0030] In a preferred embodiment the first group has a relative flux RF₁ and the second group has a relative flux RF₂, such that the ratio RF₁ZRF₂ is at least 0.09. In a more preferred embodiment this ratio is in the range of from 0.2 to 20, preferably from 5 to 15.

[0031] It is desirable to include a third group of at least one LED emitting light having a peak wavelength in the range of from 580 to 620 nm, as this further enhances the color rendition of the lighting assembly. This third group of at least one LED has a relative flux RF₃, preferably such that the ratio RF-1/RF3 is in the range of from 5 to 15.

[0032] The performance of the lighting assembly of the present embodiment is illustrated by the data in the following table, wherein Ra stands for the color rendering index, also known as CRI.

Table 1

[0033] The numbers compare the performances of a white phosphor LED (bottom row) and the combination of a white phosphor LED with one or more naked cyan LEDs. The data show that adding even a small amount of cyan light increases the S/P ratio. Preferred embodiments for utility lighting are those in which the intensity of the cyan light is greater than that of the white light. The actual ratio will be chosen in function of the importance of color rendering in the particular application. It should be recognized that even a slightly negative Ra value corresponds to an appreciable level of color recognition, which may well be acceptable for certain applications.

[0034] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. For example, the lighting assembly may be modified by adding additional monochromatic LEDs. The essence of the invention is the use of a blue LED with a white phosphor in combination with at least one non-phosphor LED.

[0035] Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. An RGB lighting assembly comprising: a) a first group of at least one LED emitting light having a peak wavelength in the range of from 550 to 620 nm, said group emitting light having a relative intensity RI_R; b) a second group of at least one LED emitting light having a peak wavelength in the range of from 505 to 550 nm, said group emitting light having a relative intensity RI_G: c) a third group of at least one phosphor-comprising LED, emitting primary light having a peak wavelength in the range of from 450 to 505 nm, said group emitting light having a relative intensity Rl₆, and secondary light having a peak wavelength in the range of from 515 to 620 nm, said group emitting light having a relative intensity Rl₆; wherein the ratios RI_R/RI_B and RI_G/RI_B are each at least 2.0

2. The RGB lighting assembly of claim 1 wherein the ratio RI_R/RI_B is at least 2.5.

3. The RGB lighting assembly of claim 1 wherein the ratio RI_G/RI_B is at least 2.5.

4. The RGB lighting assembly of claim 1 wherein the ratio RI_R/RI_B is at least 6.0.

5. The RGB lighting assembly of claim 1 wherein the ratio RI_G/RI_B is at least 6.0.

6. The RGB lighting assembly of claim 1 wherein the LEDs form part of a bridge circuit.

7. The RGB lighting assembly of claim 6 wherein the first group comprises four LEDs forming a rectifier bridge circuit, which provides power to the second group and the third group.

8. The RGB lighting assembly of claim 7 wherein the second group comprises three green LEDs.

9. The RGB lighting assembly of claim 7 or 8 wherein the third group comprises one phosphor LED.

10. A lighting assembly for utility lighting comprising: a) a first group of at least one LED emitting light having a peak wavelength in the range of from 480 to 530 nm; b) a second group of at least one LED which is a phosphor-coated blue LED; said lighting assembly having an S/P ratio of at least 2.0.

11. The lighting assembly of claim 10 wherein the peak wavelength of the first LED is in the range of from 495 to 510 nm.

12. The lighting assembly of claim 10 wherein the first group has a relative flux RF₁, and the second group has a relative flux RF₂ such that the ratio RF₁/ RF₂ is at least 0.09.

13. The lighting assembly of claim 12 wherein the RF₁/ RF₂ ratio is in the range of from 0.2 to 20, preferably from 5 to 15.

14. The lighting assembly of claim 13 further comprising a third group of at lease one LED emitting light having a peak wavelength in the range of from 580 to 620 nm.

15. The lighting assembly of claim 14 wherein the third group has a relative flux RF₃ such that the ratio RF-ι/RF₃ is in the range of from 5 to 15.

16. The lighting assembly of any one of claims 10 - 15 having an S/P ratio of at least 2.7.