Phosphor blends for generating white light from near-UV/blue light-emitting devices

#This invention relates to blends of phosphors that convert electromagnetic radiation in the near-ultraviolet (“near-UV”)-to-blue wavelength range to white light. More particularly, this invention relates to phosphor blends that convert electromagnetic radiation emitted by near-UV/blue light-emitting devices to white light. This invention also relates to light sources using such phosphor blends.

#A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and host inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. Well-known phosphors have been used in mercury vapor discharge lamps to convert the ultraviolet (“UV”) radiation emitted by the excited mercury vapor to visible light. Other phosphors are capable of emitting visible light upon being excited by electrons (used in cathode ray tubes) or x rays (for example, scintillators in x-ray detection systems).

#The efficiency of a lighting device that uses a phosphor increases as the difference between the wavelength of the exciting radiation and that of the emitted radiation narrows. Therefore, in the quest for improving efficiency of white light sources, effort has been dedicated to finding a source of stimulating radiation that has wavelengths longer than that of UV radiation and phosphors that respond to those wavelengths. Recent advances, in light-emitting diode (“LED”) technology have brought efficient LEDs emitting in the near UV-to-blue range. The term “LEDs” as used herein also includes laser diodes. The term “near UV” as used herein means UV radiation having wavelengths in the range from about 315 nm to about 410 nm. These LEDs emitting radiation in the near UV-to-blue range will be hereinafter called “UV/blue LEDs.” As used herein, a UV/blue LED may emit radiation having wavelengths in the near UV range, in the blue light range, or in a broad range from near UV to blue. It would be an advance to the technology of lighting to provide a range of phosphors that can be stimulated by the radiation emitted from these UV/blue LEDs radiation sources to allow for a flexibility in the use of phosphors for generating various color LEDs. Such phosphors when combined with the emission from the UV/blue LEDs can provide efficient and long lasting lighting devices that consume little power.

#FIG. 1A shows the room-temperature excitation spectrum of the Sr pyrophosphate phosphor.

#FIG. 1B shows the room-temperature emission spectrum of the Sr pyrophosphate phosphor.

#FIG. 2A shows the room-temperature excitation spectra of two calcium halophosphate phosphors.

#The present invention provides convenient phosphor blends that are excitable by electromagnetic radiation having wavelengths in the near UV-to-blue range (from about 315 nm to about 480 nm) to emit efficiently visible light in the wavelength range from about 490 nm to about 770 nm. The wavelength of the exciting radiation is preferably in the range from about 315 to about 420 nm, more preferably from about 350 nm to about 410 nm. A suitable near UV/blue LED for use with a phosphor blend of the present invention is one having an InGaN active layer as disclosed in U.S. Pat. No. 5,777,350. Particularly useful are those LEDs having a GaN layer or having only a very small amount of In dopant in the GaN layer as these LEDs would emit radiation predominantly in the wavelength range less than about 400 nm. The phosphor blends of the present invention comprise at least two phosphors selected from the group consisting of (a) Sr2P2O7:Eu2+,Mn2+; (b) (Ca,Sr, Ba)5(PO4)3(F,Cl,OH):Eu2+,Mn2+; (c) 3.5MgO.0.5MgF2.GeO2:Mn4+; (d) Sr4Al14O25:Eu2+; (e) (Sr,Ba,Ca)5(PO4)3(Cl,OH):Eu2+; (f) an europium-activated aluminate phosphor selected from the group consisting of (Ba,Ca,Sr)2MgAl16O27:Eu2+, (Ba,Ca,Sr)MgAl10O17:Eu2+, and (Ba,Ca,Sr)Mg3Al14O25:Eu2+; and (g) an europium and manganese co-activated aluminate phosphor selected from the group consisting of (Ba,Ca,Sr)2MgAl16O27:Eu2+,Mn2+, (Ba,Ca,Sr)MgAl10O17:Eu2+,Mn2+, and (Ba,Ca,Sr)Mg3Al14O25:Eu2+,Mn2+ (hereinafter called “BaMn”). In the formulas of the phosphors, the elements following the colons represent the activators or sensitizers and are present at low atomic proportions compared to the metals, such as less than about 20 percent. A group of elements separated by commas in a set of parentheses represent those elements that are interchangeable at the same lattice site.

#By mixing appropriate proportions of these phosphors, a composite emission spectrum of the blend may be created that provides a desired CT and CRI with maximum luminosity (as defined by the lumens per watt of radiation input energy or “L/Wrad”). CT of a light source is the temperature of a blackbody radiator that has the same color. An incandescent light bulb has a color temperature of about 2000 K. Between 3000 K and 5000 K, a light source appears intensely white. Sunlight has a color temperature of about 5000 K. As the color temperature increases to between 8000 K and 10000 K, the light source appears blue. The CRI of a test light source is a rating that represents the degree of resultant color shift of a test object under that test light source in comparison with its color under a standard lamp of the same temperature. When the CRI value is 100, the test object appears to have the same color as when it is illuminated by the standard light source. Compositions of phosphor blends may be chosen to provide emission of white light having coordinates close to the black body locus of the CIE chromaticity diagram. In general lighting applications, it is desirable to provide light sources having a color temperature in the range of 4000 K to 6000 K and a CRI of greater than 80. Table 1, which is shown in its entirety in FIG. 9, presents the result of a simulation of different blends of the phosphors. In this Table, the numerical value for each individual phosphor represents the proportion of the composite spectrum that the emission from the individual phosphor must contribute to result in the specified CT, CRI, luminosity, and distance from the black body locus; and it is not the weight fraction of the individual phosphor in the blend. The phosphor blend of this invention also is advantageously used with near-UV LEDs to produce less variation among the finished lamps.

#In addition to white light, light of other colors may be generated from other selected blends of the phosphors. FIG. 7 shows the coordinates of the phosphors in the CIE chromaticity diagram. A tie line between any two points representing two phosphors gives the set of color coordinates of the blends of these two phosphors. Similarly, an area inside a polygon, the vertices of which represent the coordinates of the individual phosphors, gives the set of possible colors given by blends of these phosphors.