Patent Application
20050275333
1. Field of the Invention
The present invention relates to a white light illumination device and a method for producing the same. It particularly relates to a white light illumination device and a method for producing the same that uses two fluorescent materials and an ultraviolet excitation light source to achieve white light with high-luminosity according to the principle of complementary color.
2. Description of Prior Art
White light is a mixture of multiple colors of light, including at least two kinds of light with different wavelengths. For instance, the viewer can perceive the mixture of red, blue and green light or the mixture of blue and yellow as white light. Therefore, a white light illumination device is attainable according to the principle as above described.
Conventionally, there are many methods of obtaining a white light illumination device. The first conventional method is to use three light emitting diodes (LEDs) to provide red, green and blue light by controlling a corresponding driving current respective to each LED. One of the three LEDs is made of InGaAlP, and the other two are made of GaN. The three LEDs are packaged in a lamp, and a lens is used to mix light from the three LEDs to generate white light.
The second conventional methods is similar to the first conventional methods, the difference therebetween is that the second methods only uses two LEDs to provide blue and yellow-green light for producing white light. The two LEDs include one made of GaN and another made of GaP. At present, the light emission efficiency of the described conventional methods has achieved 20 lm/w. However, the conventional methods suffer from the following disadvantages. White light can no longer be produced if one of the LEDs fails. Moreover, a different controlling circuit responding to each LED is necessary, which is expensive.
The third conventional method was developed by Japan Nichia Chemical Industries, Ltd. in 1996, and provides a white light illumination device using a blue light-emitting diode made of an InGaN semiconductor, and a yttrium aluminum garnet (“YAG:Ce”) fluorescent material, which emits yellow light. The mixture of these blue and yellow emission lights also can be perceived as white light by an observer. Although the light emission efficiency of this kind of white light illumination device is slightly lower than that of the first and second conventional methods, it can greatly reduce fabrication cost because only one LED chip is needed. Furthermore, fluorescent material technology improves daily. Thus, a white light illumination device of this method has been available on the market.
However, the second and the third conventional methods utilize the principle of complementary colors to generate white light, of which the spectrum distribution is not as continuous as that of natural sunlight in the visible portion of the spectrum. In result, the white light generated by these methods has a lower color saturation, and is only suitable for simple illumination purposes. Recently, an ultraviolet light-emitting LED has been developed, which can be used to excite conventional fluorescent material to generate white light. Furthermore, the ultraviolet light emitting LED has a valuable merit of low power consumption. Thus, a white light illumination device formed by this method can very likely be a substitute for conventional lamp or fluorescent lamp in the future. Conventional white light illumination device commonly uses three or more kinds of fluorescent materials in order to improve color rendering characteristics. However, to obtain a perfect white light with many kinds of fluorescent materials, the following conditions must be met. First, a chosen excitation light should be completely absorbed by the fluorescent materials, and the difference between the light absorption coefficient of each fluorescent material should be as small as possible, Likewise, the light energy transferring efficiency of the fluorescent materials preferably should be as close as possible. Therefore, the above conditions greatly limit number of fluorescent materials that can be utilized, causing difficulties in choosing suitable fluorescent materials.
Typically, a conventional light color modulation method is implementing by adding or mixing two kinds of impurity ions as active centers at the same time. A fluorescent material capable of radiating two kinds of wavelengths light is thus obtained. Moreover, different colors of light can be obtained by adjusting the proportions of these active centers. However, most impurity ions need different excitation wavelengths, so the feasibility of this method is not as high as expected.
Therefore, the present invention provides an improved white light illumination device and a manufacturing method for the same that can overcome or at least reduce the disadvantages set forth above.
In accordance with one aspect of the present invention, there is provided a white light illumination device that comprises a light-emitting diode capable of emitting ultraviolet light, a first fluorescent powder excited by the ultraviolet light from the light-emitting diode and emitting blue-green fluorescent light having a peak emission wavelength of about 470 to about 500 nm, and a second fluorescent powder being excited by the ultraviolet light from the light-emitting diode and emitting orange fluorescent light having a peak emission wavelength of about 570 to about 600 nm. A chemical formula of the first fluorescent powder is (Ba1-x-yEuxSry)MgAl10O17, where 0<x≦1, 0≦y≦1; and a chemical formula of the second fluorescent powder is (Ca,Eu,Mn)(PO4)3Cl. The blue-green and the orange fluorescent lights combine to produce white light.
In accordance with another aspect of the present invention, a method of making a white light illumination device is provided and comprises steps described as follows. An ultraviolet light source is provided. A first fluorescent powder is synthesized and emits a blue-green light having a peak wavelength of about 470 to about 500 nm by excitation with ultraviolet light emitted from the ultraviolet light source. A second fluorescent powder is synthesized and emits an orange light having a peak wavelength of about 570 to about 600 nm by excitation with ultraviolet light emitted from the ultraviolet light source. The first and second fluorescent powders are blended in a predetermined proportion.
Other objects, advantages, and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:
A white light illumination device of the present invention comprises a light source capable of emitting ultraviolet light, a first fluorescent powder and a second fluorescent powder. The ultraviolet light emitted by the light source excites the first fluorescent powder, causing it to radiate blue-green fluorescent light having a peak emission wavelength of about 470 to about 500 nm. The ultraviolet light emitted by the light source excites the second fluorescent powder, causing it to radiate orange fluorescent light having a peak emission wavelength of about 570 to about 600 nm. When the blue-green fluorescent light and the orange fluorescent light blend together, the viewer perceives the mixture of blue-green and orange light as white light.
The first fluorescent powder can be made of (Ba1-x-yEuxSry)MgAl10O17, where 0<x≦1, 0≦y≦1, and the second fluorescent powder can be made of (Ca,Eu,Mn)(PO4)3Cl.
In order to make the peak emission wavelength of the first fluorescent powder (Ba1-x-yEuxSry)MgAl10O17 shift toward the blue, the present invention implements color modulation of the fluorescent light by the principal that a radius of a metal ion in a host crystal lattice can change the force of a crystal lattice field. The 4f orbital of a Eu2+ ion separates into 2F5/2and 2F7/2 states due to the effect of spin-orbital coupling, and the 5d orbital is also split by the effect of the crystal lattice field. Referring to
Furthermore, the present invention replaces Ba2+ ion in the conventional yellow fluorescent powder (Ba1-x-yEuxSry)MgAl10O17 with an Sr2+ ion. To a substitutional solid solution, the adulterate concentration of its heterogeneous ions mainly depends on the structural difference between its resultant and primary reactants. Both BaMgAl10O17 and SrMgAl10O17 have a same spatial group symmetry of P63/mmc, and both the Sr2+ ion and Ba2+ ion thereof are in D3h symmetry. Thus, it can be deduced that the Sr2+ ion has excellent solubility in the BaMgAl10O17. The Sr2+ ion has a relatively bigger electron cloud expansion effect compared with Ba2+ ion, and makes the wavelength of light generate a red shift. Thus, in general, when the Ba2+ ion is replaced with the Sr2+ ion, the peak wavelength of light emitted from the Eu2+ ion when transitioning from the 4f orbital to the 5d orbital should shift from 450 nm (y=0.00) to 480 nm (y=0.85).
The first and the second fluorescent powders of the present invention can be excited by the same excitation light, and radiate blue-green light (from 470 nm to 500 nm) and orange light (from 570 nm to 600 nm), respectively. Accordingly, the white light illumination device of the present invention can be made by blending the first and the second fluorescent powders in suitable proportions.
A preferred fabrication method of the white light illumination device of the present invention includes the following process. An ultraviolet light source is provided, and the first fluorescent powder (Ba1-x-yEuxSry)MgAl10O17, for example, (Ba0.425Eu0.15Sr0.425)MgAl10O17, is synthesized. The second fluorescent powder (Ca,Eu,Mn)(PO4)3Cl is synthesized, and then the first and second fluorescent powders are blended in predetermined proportions. The first fluorescent powder and the second fluorescent powder are produced by a solid reaction method or a chemical synthesizing method, such as a citrate gel method or a co-precipitation method, and the like.
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It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
