The disclosure relates to the field of LED-based illumination products and more particularly to light emitting diodes devices configured to change color along blackbody curves during dimming.
Users desire an LED source that mimics the color changing effect caused by dimming of a filament (such as tungsten or halogen). The effect can be described as a lowering of Correlated Color Temperature (CCT) along the blackbody curve from a nominal 2700-3000K at full power to 2000-2200K at 10% dimming. There have been different attempts at lowering of CCT along the blackbody curve in LED sources. Some attempts use multiple color points to achieve a target color mix at a given power level. For example, using a high CCT LED and low CCT LED driven at differing currents to change intensity balance and final mixed color point have been implemented. Other attempts involve three or more monocolor LEDs (e.g., red, orange, green, blue, etc.) with multiple drivers to model an emanated spectra.
Such attempts are generally costly and require active electrical components in order to obtain desired effects. Therefore, there is a need for a dimming LED source that lowers CCT along the blackbody curve without excessive cost or complexity. The present invention fulfills this need among others.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the invention is a lighting system having a phosphor having a varying conversion efficiency. In one embodiment, the lighting system comprises: (a) at least one light emitting diode (LED) light source, the light source being configured to receive variable power; (b) a first phosphor for converting light; and (c) a second phosphor for converting light, the second phosphor having a conversion efficiency which varies as the power varies; (d) wherein light from the LED, the first phosphor and the second phosphor combine to form emitted light.
Another aspect of the invention is method of varying the conversion efficiency of a phosphor to change the color of light emitted from the lighting system described above. In one embodiment, the method comprises: (a) varying the electrical power to the LED light source causing at least one of flux of the LED light source to vary or the temperature of the lighting system to vary; and (b) whereon the emitted light varies in color as the conversion efficiency varies based on at least one of the flux varying or the temperature varying.
This approach eliminates the additional complication of multiple driver sources. Tuning of the behavior to attain specific shifts/colors can be done by adjusting phosphor temperature- and/or photosensitivities. The foregoing approach can be used to calibrate a specific color in a fixed lamp application by targeting the desired color (e.g., using a one-time driver power calibration adjustment).
In one embodiment, the lighting system is ‘passive’ in that it does not require multiple channel drivers to modulate the spectrum. Therefore, such an embodiment may be integrated to a retrofit lamp or more generally a lighting system in absence of any advanced control circuits. In some such cases, standard-dimming switches provides the needed control.
Those skilled in the art will understand that the drawings, described herein, are for illustration purposes only. The drawings are not intended to limit the scope of the present disclosure.
Reference is now made in detail to certain embodiments. The disclosed embodiments are not intended to be limiting of the claims.
In one embodiment of a lighting system of the present invention comprises: (a) at least one light emitting diode (LED) light source, the light source being configured to receive variable power; (b) a first phosphor for converting light; and (c) a second phosphor for converting light, the second phosphor having a conversion efficiency which varies as the power varies; (d) wherein light from the LED, the first phosphor and the second phosphor combine to form emitted light. Each of these elements is considered below in greater detail.
The LED light source functions to provide the primary source of light or flux to excite the first and/or second phosphors, and to contribute to the emitted light. The LED light source is configured to receive variable power such that its optical output varies. Suitable LED and their associated drive circuitry are well known, and, thus, the details of which are not described herein. Suffice to say that the LEDs can be used in any conventional configuration, including, for example, individual LED and LED array configurations. Additionally, suitable LEDs include LEDs, OLEDs, laser diodes, and LED/laser diode combinations.
Although any LED may be used, typically LEDs having a shorter wavelength such as violet or blue LEDs are preferred (although are not necessary) as they tend to have higher excitation energy for exciting the first and second phosphors as described below. In one particular embodiment, the LED is a violet LED, which Applicants have discovered have various advantages. They may exhibit unusually-high wall-plug efficiency at high current density and high temperature, which makes them desirable as pump LEDs for some lighting systems. In addition, violet LEDs enable a white-balanced spectrum at various CCTs having little or no blue light. This can be desirable in some applications where varying CCT is sought. In one embodiment, the LED is a bulk GaN LED configured to emit light having a wavelength of about 400 nm to 460 nm. Such LEDs are commercially available from Soraa Inc (Freemont, Calif.). Other embodiments will be known or obvious to those of skill in the art in light of this disclosure.
In one embodiment, the LED light source receives variable power. As mentioned above, drivers for providing variable power to the LED devices are well known. The variable power may range from a relatively low power to a relatively high power. Generally, although not necessarily, the ratio of low to high power will be at least 1:2, and, in one particular embodiment, 1:10, or 1:100 , or more. Such large ratios are typical in deep-dimming applications where the user may dim light to a very low level. In the case of conventional (filament) lights, such dimming ratios are associated with a shift of the light's color temperature, as will be illustrated below. These color temperature shifts are familiar to users, and therefore it can be desirable to emulate them with LED systems. Other embodiments will be known or obvious to those of skill in the art in light of this disclosure.
As is known, applying variable power to the LED light source will result in certain effects. First, as the power increases/decreases, the light output or flux of the LED device increases/decreases, respectively. Second, as the power increases/decreases, overall heat generated by the light system increases/decreases. It should be understood that the heat may be emitted not only from the LED, but also from the drive circuitry, phosphors, and other components of the light system. The heat naturally affects the temperature of the phosphors and other components of the light system. These effects are referred to herein as the “driving conditions” of the phosphors. For example, in one embodiment, the driving condition may be the optical flux exciting the phosphor, and in another embodiment, the driving condition may the temperature of the phosphor. In an alternative embodiment, the light system comprises a temperature-control heating element that is dependent on the LED power level. Thus, heat from this element may also be a driving condition for a temperature dependent phosphor.
The phosphors or other wavelength-converting materials (collectively referred to as “phosphors”) function to convert light from one wavelength to one or more different wavelengths. This is a well-known function of phosphors. In various embodiments, the phosphors may be optically excited by light from LED emitters, from other phosphors, or by a combination thereof
In the present invention, at least one of the phosphors reacts to the effect(s) of varying power (i.e., the driving conditions as described above) to alter the color of the emitted light. Specifically, as the power decreases, the longer wavelength emissions increase relative to the shorter wavelength emissions from the phosphor(s) in the emitted light. Likewise, as the power increases, the longer wavelength emissions decrease relative to the shorter wavelength emissions of the phosphor(s) in the emitted light. In one embodiment, the result of this is emitted light that mimics the color changing effect caused by dimming of a tungsten filament.
As mentioned above, the effect can be described as a lowering of CCT along the blackbody curve from a nominal 2800K at full power to 2000-2200K at 10% dimming. Other CCT ranges can be considered, in order to emulate a variety of systems having variable color temperature. This includes emulation of halogen lamps (typically having a CCT of 3000K at full power), or of natural daylight (whose CCT may vary in a wide range from 3000K to 10.000K or more, depending on time of the day)
Various embodiments of phosphors may be used that have a conversion efficiency that varies with varying power to the LED. As used herein, the term “conversion efficiency” of the phosphor is the product of two terms: absorption efficiency and quantum yield for conversion. In some embodiments, the absorption efficiency (i.e. the absorption coefficient) depends on the driving conditions (e.g. in the case of a saturable phosphor, in which the absorption decreases as exciting flux increases). In some embodiments, the quantum yield depends on the driving conditions (e.g., in the case of a phosphor which has a quantum yield that is temperature-dependent).
In the case of phosphors having saturable absorption, the saturation may be caused by a long lifetime of carriers (electrons) optically excited in the phosphor—also termed the decay time of the phosphor. For instance, the lifetime may be longer than 1 us (e.g., it may be 1 us, 10 us, 100 us, 1 ms, etc . . . ). For a given lifetime, the saturation will occur at an inversely proportional optical excitation flux. A phosphor having a specific lifetime may be selected. An example of a flux-saturable phosphor is magnesium fluoro-germanate (MFG) family.
In the case of the phosphors having a temperature-dependent quantum yield, the output tends to decrease as its temperature increases. For example, the quantum yield may vary from 95% at room temperature to 50% at a high temperature (such as 100C, 150C, 200C); such variations mean that the amount of light emitted by the phosphor is roughly halved between room temperature and high temperature. This may correspond to a significant shift of CCT for the light emitted by the lighting system—for instance, a shift of 500K, 1000K or 2000K or more.
Other effects may induce a dependence of conversion efficiency versus driving conditions. For instance, the absorption spectrum of a phosphor may undergo a spectral shift with temperature; likewise, LED wavelength usually shifts with temperature (typically by a few nm over a temperature range of 100C). These shifts may be sufficient to significantly alter the net absorption of a phosphor at a given wavelength, for instance by offsetting the emission wavelength of pump LEDs and the peak absorption of the phosphor. Thus, the net excitation of the phosphor by the pump LED may vary with temperature. Phosphor materials having a such a temperature-dependent behavior may include semiconductor quantum dots.
Another possible effect is a temperature-induced shift in the emission spectrum of a phosphor. For instance, a red phosphor may have a spectrum that shifts to longer-wavelength with higher temperature. Such shift may be caused by the rigid shift of the shape of the spectrum towards longer wavelength. Such a shift may be caused by a change in shape (for instance, if several optical transitions make up the phosphor spectrum and the relative intensities of these transitions varies with temperature).
Various configurations of the phosphors may be used to alter the color of the emitted light as power to the LED device varies. For example, one or more of the following phosphor configurations may be used: temperature-sensitive orange or red phosphor, for which emission intensity decreases with increasing LED power (and temperature); photo-sensitive orange or red phosphor, for which emission decreases with increasing photo-flux; temperature- and photo-sensitive orange or red phosphors to increase the amount of red intensity attenuation; wavelength-sensitive blue or green/yellow phosphors, for which conversion efficiency increases with increasing or decreasing driving conditions, thereby increasing the blue/yellow intensity relative to the orange or red intensity; color-shifting red, orange, green/yellow or blue phosphors, for which emission shifts to decreasing u′ or v′ values with increasing temperature or photo-flux, such that the combined color shifts in the desired direction; any number of other non-saturated phosphor combinations that result in a balance between orange or red and the rest of the spectrum decreasing with dimming; temperature- or photo-sensitive filter materials that subtract red with increasing temperature/photo-flux; infrared-emitting phosphors or quantum dots that have temperature or photo-sensitive absorption in the red spectral region; up-conversion materials with temperature- or photo-sensitive red absorption; temperature-sensitive dyes with red absorption; temperature quenching red down-converting material, such as an organic dye or quantum dot such as CdSe, InP, and other materials including various Cd-free materials; and combinations of any of the foregoing that results in the desired effect.
In one embodiment, the first and second phosphors emit light of different wavelengths. For example, in one embodiment, one phosphor emits light in the blue/green/yellow spectrum, while the other emits light in the orange/red spectrum. The desired color change for varying output can be achieved by the different phosphors emitting their light at different intensities: at lower power, light in the blue/green/yellow spectrum can decease relative to light in the orange/red spectrum or light in the orange/red spectrum can increase relative to the light in blue/green/yellow spectrum, and at higher power, light in the blue/green/yellow spectrum can increase relative to light in the orange/red spectrum or light in the orange/red spectrum can decrease relative to the light in blue/green/yellow spectrum. Thus, either phosphor (relatively short or long wavelength) can affect the color of the emitted light at different power levels. Thus, the second phosphor, which has varying conversion efficiency (i.e. its output changes as the power varies), may be either the phosphor emitting a relatively short wavelength (e.g., in the blue/green/yellow spectrum), or the phosphor emitting a relatively long wavelength (e.g. in the orange/red spectrum). In one embodiment, the second phosphor emits a relatively long wavelength compared to the first wavelength. In one embodiment, the second phosphor is a red or an orange phosphor.
Although only second phosphor needs to have varying conversion efficiency, it should be understood that the first phosphor may have varying conversion efficiency too. In this embodiment, the effects of the varying conversion efficiency of the first phosphor preferably (although not necessarily) do not counteract the effects of the varying conversion efficiency of the second phosphor.
The compositions of phosphors or other wavelength-converting materials referred to in the present disclosure (collectively referred to as “phosphors”) comprise any combinations of known wavelength-converting materials. Suitable phosphors are well know. Wavelength conversion materials can be crystalline (single or poly), ceramic or semiconductor particle phosphors, ceramic or semiconductor plate phosphors, organic or inorganic down converters, up converters (anti-stokes), nano-particles and other materials which provide wavelength conversion. Examples of major classes of down converter phosphors used in solid-state lighting include garnets doped at least with Ce3+; nitridosilicates or oxynitridosilicates doped at least with Ce3+; chalcogenides doped at least with Ce3+; silicates or fluorosilicates doped at least with Eu2+; nitridosilicates, oxynitridosilicates or sialons doped at least with Eu2+; carbidonitridosilicates or carbidooxynitridosilicates doped at least with Eu2+; aluminates doped at least with Eu2+; phosphates or apatites doped at least with Eu2+; chalcogenides doped at least with Eu2+; and oxides, oxyfluorides or complex fluorides doped at least with Mn4+. Specific examples of phosphors include, for example, one or more of the following: (Ba,Sr,Ca,Mg)5(PO4)3(Cl,F,Br,OH):Eu2+, Mn2+; (Ca,Sr,Ba)3MgSi208:Eu2+, Mn2+; (Ba,Sr,Ca)MgAl10017:Eu2+, Mn2+; (Na,K,Rb,Cs)2[(Si,Ge,Ti,Zr,Hf,Sn)F6]:Mn4+; (Mg,Ca,Zr,Ba,Zn) [(Si,Ge,Ti,Zr,Hf,Sn)F6]:Mn4+; (Mg,Ca,Sr,Ba,Zn)2SiO4:Eu2+; (Sr,Ca,Ba)(Al,Ga)2S4:Eu2+; (Ca,Sr)S:Eu2+,Ce3+; (Y,Gd,Tb,La,Sm,Pau)3(Sc,Al,Ga)5O12:Ce3+; and combinations of two or more thereof
Other examples of phosphors include, for example, one or more of the following: Ca1−xAlx−xySi1−x+xyN2−x−xyCxy:A; Ca1−x−zNazM(III)x−xy−zSi1−x+xy+zN2−x−xyCxy:A; M(II)1−x−zM(I)zM(III)x−xy−zSi1−x+xy+zN2−x−xyCxy:A; M(II)1−x−zM(I)zM(III)x−xy−zSi1−x+xy+zN2−x−xy−2w/3CxyOw−v/2Hv:A; M(II)1−x−zM(I)zM(III)x−xy−zSi1−x+xy+zN2−x−xy−2w/3−v/3CxyOwHv:A; and combinations or two or more thereof; wherein 0<x<1, 0<y<1, 0≦z<1, 0≦v<1, 0<w<1, x+z<1, x>xy+z, and 0<x−xy−z<1, M(II) is at least one divalent cation, M(I) is at least one monovalent cation, M(III) is at least one trivalent cation, H is at least one monovalent anion, and A is a luminescence activator doped in the crystal structure.
Still other examples of phosphors include, for example, one or more of the following: LaAl(Si6−z Al z)(N10−z Oz):Ce3+(wherein z=1); (Mg,Ca,Sr,Ma)(Y,Sc,Gd,Tb,La,Lu)2S4:Ce3+; (Ba,Sr,Ca)xxSiyNz:Eu2+ (where 2x+4y=3z); (Y,Sc,Lu,Gd)2−nCanSi4N6+nCl-n:Ce3+, (wherein 0≦n≦0.5); (Lu,Ca,Li,Mg,Y) a-SiAlON doped with Eu2+ and/or Ce3+; (Ca,Sr,Ba)SiO2N2:Eu2+,Ce3+; (Sr,Ca)AlSiN3:Eu2+; CaAlSi(ON)3:Eu2+; (Y,La,Lu)Si3N5:Ce3+; (La,Y,Lu)3Si6N11:Ce3+; and combinations of two or more thereof.
For purposes of the application, it is understood that when a phosphor has two or more dopant ions (i.e. those ions following the colon in the above phosphors), this is to mean that the phosphor has at least one (but not necessarily all) of those dopant ions within the material. That is, as understood by those skilled in the art, this type of notation means that the phosphor can include any or all of those specified ions as dopants in the formulation.
Further, it is to be understood that nanoparticles, quantum dots, semiconductor particles, and other types of materials can be used as wavelength converting materials. The list above is representative and should not be taken to include all the materials that may be utilized with embodiments described herein.
Those of skill in the art will be able to select suitable first phosphors having the above-described properties from the phosphor lists above in light of this disclosure. Likewise, those of skill in the art will be able to select suitable second phosphors having the desired conversion efficiencies in light of this disclosure. For example, suitable temperature-sensitive quantum yield phosphors include europium-doped alkaline earth ortho-silicate orange phosphor with peak wavelength of 605 nm; suitable temperature-sensitive emission spectrum phosphors include manganese-doped alkaline earth germanium oxy-fluoride red phosphor with peak wavelength of 659 nm; and suitable flux-sensitive phosphors include Mn-doped phosphors such as K2[TiF6]:Mn4+. Still other embodiments will be known or obvious to those of skill in the art in light of this disclosure.
Upon dimming from full-power to <10% power, the contribution of the emission from phosphor(s) with color points that fall within Region 2 must decrease with respect to the intensity of phosphor(s) within Region 1 in order to emulate CCT-change along the blackbody. Such a change may be carried out by either increasing Region 1 phosphor(s) intensity with respect to stable phosphor(s) within Region 2 or by decreasing Region 2 phosphor(s) intensity with respect to stable phosphor(s) within Region 1. In order to reproduce a continuous CCT- change, the trade-off between phosphor intensities within these two regions must also be continuous.
This approach may be implemented using either single or multiple phosphors within each region or multiple phosphors that may fall outside the regions in which the composite color falls within the regions. As an example, as shown in
The case of dimming at lower CCT values (1700K to 2000K) is illustrated in
The implementations illustrated in
The color-shifting behavior was implemented by taking advantage of two properties of the wavelength-converting materials: (1) the larger thermal quenching of emission intensity of the orange silicate compared to the other emissive materials and, (2) an increase in the probability of higher energy manganese transitions relative to the transition at 659 nm in the fluoride phosphor, which shifts the fluoride color point by (0.007, 0.001) in CIE 1976 uniform chromaticity space. As the electrical power to the light module was decreased, both the temperature of the phosphor and the violet flux density decreased, leading to a relative increase in the orange and red contents of the spectra. The CCT decreased, accordingly. The junction temperature of the LED in the light module used in this example varied between 25° C. and 125° C. from ˜1% to 100% drive power.
A similar embodiment consists of the same light source and phosphors as the example shown in
CRI values were tuned by changing the relative phosphor loadings. These examples included a violet light source with the same peak wavelength as the example shown in
For samples a and c, a 650 nm peak CASN was used, and for sample b, a 655 nm peak CASN was used. The blue-to-green-to-orange-to-red phosphor loading ratios by weight were as follows: a: 100:508:183:43, b: 100:290:45:23, c: 100:498:175:25. The total weight of phosphors in each of the examples was set so that the fractional content of violet in their spectra (i.e., violet leakage) were the same for all three. The color rendering indices (i.e., CRI) for these examples were: a: Ra=87/R9=34, b: Ra=97/R9=74, c: Ra=83/R9=23. Thus, with the same light source and down-converting materials, including a temperature-sensitive orange phosphor and three temperature-stable phosphors, color rendering indices may be tuned by varying the phosphor loading ratio and the peak wavelength of the red phosphor.
For samples a and c, a 650 nm peak CASN was used, and for sample b, a 655 nm peak CASN was used. The blue-to-green-to-orange-to-red phosphor loading ratios by weight were as follows: a: 100:508:183:43, b: 100:290:45:23, c: 100:498:175:25. The total weight of phosphors in each of the examples was set so that the fractional content of violet in their spectra (i.e., violet leakage) were the same for all three. The CRI for these examples were: a: Ra=87/R9=34, b: Ra=97/R9=74, c: Ra=83/ R9=23. Thus, with the same light source and down-converting materials, including a temperature-sensitive orange phosphor and three temperature-stable phosphors, color rendering indices may be tuned by varying the phosphor loading ratio and the peak wavelength of the red phosphor.
It will be apparent to those skilled in the art that many modifications, both to materials and methods, may be practiced without departing from the scope of the disclosure. For example, the aforementioned dimming capability can be incorporated into an MR-16 lamp, a PAR-30 lamp, an AR-111 lamp, etc., and/or any other known lamp, fixture, application, or form factor. Such lamps and fixtures include replacement and/or retro-fit directional lighting fixtures.
As already discussed, in some embodiments, it is desirable to closely match the behavior of an incandescent or halogen lamp—thus emulating the behavior of
In some embodiments, the chromaticity is below the Planckian locus. For example,
The chromaticity distance from the white locus can be characterized by the distance Dwhite=sqrt((x−xw)2+(y−yw)2) where (x, y) is the chromaticity of the embodiment and (xw, yw) is the chromaticity of the white locus at the same CCT as the embodiment. In some embodiments, this distance is smaller than a limit value such as 1 point, 2 points, 3 points. This expresses that such embodiments have a chromaticity similar to that of the white locus.
In addition to uses of the aforementioned temperature-sensitive phosphors flux-sensitive saturable phosphors can be employed to achieve CCT variations. Such an approach is illustrated in
The embodiments of
Various aspects of this embodiment can be advantageously controlled. For example, the optical properties of the pump LED can be varied, and the selection of phosphors can be varied, and the relative loading of phosphors can be varied to accomplish an optimization objective. The optimization criteria may include the CRI of the source at various dimming levels, its chromaticity at various dimming levels. The loading of the saturable phosphor can be chosen so that its saturation occurs at a desired drive such as, for example, 10% dimming. In other embodiments, more than one saturable phosphor is used.
Embodiments may be integrated to various systems. This includes lighting systems (e.g., lamps, troffers and others) and non-lighting systems (e.g., display applications). Some of such applications are discussed hereunder. Various embodiments use different packages. In some cases the package is a COB package with multiple LEDs. In some cases it is a single-LED high-power package. In some cases it is a mid-power package, such as a lead frame package including one LED, two LEDs or more.
While this description is made with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings hereof without departing from the essential scope. Also, in the drawings and the description, there have been disclosed exemplary embodiments and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the claims therefore not being so limited. Moreover, one skilled in the art will appreciate that certain steps of the methods discussed herein may be sequenced in alternative order or steps may be combined. Therefore, it is intended that the appended claims not be limited to the particular embodiment disclosed herein.
This application claims the benefit of U.S. Provisional Application No. 62/069,528, filed Oct. 28, 2014, incorporated herein by reference in its entirety.
Number | Date | Country | |
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62069528 | Oct 2014 | US |