Abstract:
A white light emitting device includes a device having at least one light emitting diode (LED) serving as a light source and being capable of emitting a light between blue and green in color; and a phosphor consisted of first and second phosphors each made of a compound from different materials. The phosphor is excited by the light from the light source to emit a first-color light and a second-color light, which are both blended with a portion of the light and altogether release to obtain a white light having enhanced color rendering effects, thereby offering the white light with illuminant contrast.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (a) Field of the Invention  
         [0002]     The invention relates to a white light emitting device, and more particularly, to a white light emitting device having enhanced light rendering effects and a phosphor that is resistant to quantitative change, thereby ensuring stability and reliability of a wavelength range of the white light.  
         [0003]     (b) Description of the Prior Art  
         [0004]     Referring to the U.S. Pat. No. 6,351,069B1 disclosing Red-Efficiency-Compensating Phosphor LED, a phosphor thereof is consisted of two materials namely SrS:E u  and YAG:Pr 3+ .  
         [0005]     Wherein, a light emitting diode (LED) thereof is for emitting a blue light serving as a light source, which excites the phosphor in the Srs: E u  material to further excite a light (first-color light) having a specific wavelength. The light source also excites the phosphor of the YAG:Pr 3+  to further excite another light (second-color light) having a different wavelength from that of the first-color light. The two lights (the first-color and second-color lights) having different wavelengths are blended with a portion of the light source and then released, with the blended light being defined as “white light” according to naked eye.  
         [0006]     However, the phosphor of the Srs: E u  material according to the prior invention, due to chemical properties of sulfur, has unsatisfactory heat resistance, and often incurs oxidation and quantitative change caused by ambient temperature rise. The light wavelength (first-color light) released from exciting the Srs: E u  material having undergone quantitative change can hardly be controlled within an expected range, and therefore it also becomes more difficult to control quality of white light having better color rendering effects in a long term.  
         [0007]     Referring to the U.S. Pat. No. 6,504,179B1 disclosing LED-Based White-Emitting Illumination Unit, a phosphor thereof is formed by mixing green emitting phosphor that emits green lights when excited by a light source, and a yellow emitting phosphor that emits yellow lights when excited by a light source.  
         [0008]     The LED thereof emits a blue light serving as a light source. When a portion of the light blue from of the light source is blended with the aforesaid yellow light and green light having different wavelengths, a white light is obtained. Yet, according to this prior invention, using the two distinct light having different wavelengths and excited by the blue light source of the phosphor thereof, due to lack of wavelengths ranging within the red spectrum, the white light produced has comparatively inadequate color rendering effects; that is, the white light appears rather dull.  
       SUMMARY OF THE INVENTION  
       [0009]     The primary object of the invention is to provide a white light emitting device having enhanced light rendering effects, thereby allowing the white produced with illuminant contrast.  
         [0010]     The secondary object of the invention is to provide a white light emitting device having a phosphor thereof being resistant from quantitative change by being a heat-resistant and stable material, thereby ensuring high quality and enhanced coloring rendering effects of the white light produced in a long term. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a flow chart of the method according to the invention.  
         [0012]      FIG. 2  shows a curve diagram illustrating a spectrum of a first-color light emitted from a second phosphor excited by a light source.  
         [0013]      FIG. 3  shows a curve diagram illustrating a spectrum of a second-color light emitted from a first phosphor excited by a light source.  
         [0014]      FIG. 4  shows a curve diagram illustrating a spectrum of a white light according to the invention.  
         [0015]      FIG. 5  shows a sectional view of the device in an embodiment according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     To accomplish the aforesaid objects, descriptions of the invention shall be given with the accompanying drawings below.  
         [0017]     Referring to  FIGS. 1 and 5 , a white light emitting device according to the invention comprises characteristics of: 
        a device  100  having at least one light emitting diode (LED)  10  for serving as a light source and being capable of emitting a light  20  located between blue and green spectra;     a phosphor  30  capable of effectively absorbing the light  20  and being further excited to emit a second-color light  40  and a first-color light  50 , wherein the first-color and second-color lights  50  and  40  are blended with a light  20 ′ from a portion of the light source to obtain a white light  60  having good light rendering effects; and consisted of a second phosphor  32  and a first phosphor  34 , wherein the second phosphor  32  is selected from one or several of the following compounds:     (a) YAG:C e  with C e  as an activator thereof;     (b) TbAG: C e  with C e  as an activator thereof; 
 
 and the first phosphor  34  is selected from one or several of the following compounds: 
    (a) Gd 3 Al 5 O 12  with C e  as an activator thereof;     (b) YAG: E u  with E u  as an activator thereof;     (c) Y 2 O 3 : E u , Bi with a compound of E u  and Bi as an activator thereof;     (d) Y(Gd)BO 3 : E u  with E u  as an activator thereof;     (e) 6MgO.A S2 O 5 :M n  with M n  as an activator thereof;     (f) 4MgOF 2 GeO 2 : M n  with M n  as an activator thereof; and     (g) GdMgB 5 O 10  with a compound at least consisting C e , T b , M n  or E u  as an activator thereof.        
 
         [0029]     According to the aforesaid primary characteristics, a wavelength of the second-color light  40  is preferably selected from a range between 580 to 700 nm, and a wavelength of the first-color light is preferably selected from a range between 520 to 565 nm and has a color tone defined within a green spectrum. The second-color light is produced from the first phosphor  34  excited by the light  20  of the light source, and has a color tone defined within the red spectrum. The first-color light  50  is produced from the second phosphor  32  excited by the light  20  of the light source as shown in  FIG. 3 .  
         [0030]     According to the aforesaid primary characteristics, wavelengths emitted by the LED  20  are selected from a range between 360 and 560 nm.  
         [0031]     According to the aforesaid primary characteristics, the phosphor  30  is mounted on or located approaching the LED  10 , thereby effectively absorbing the light  20  of the light source.  
         [0032]     According to the aforesaid primary characteristics, the LED  10  has a conductive connecting end  12  thereof connected to a circuit board  80 .  
         [0033]     According to the aforesaid primary characteristics, when the second phosphor  32  is made of YAG, an activator thereof may be either be an individual element or a compound from Pr and Dy. When the second phosphor  32  is made of TbAG, an activator thereof may either be an individual element or a compound from Pr and Dy.  
         [0034]     Referring to  FIGS. 1 and 5  again, the device  100  according to the invention comprises an LED  10  serving as a light source. The LED  10  has two conductive connecting ends  12  thereof connected to a circuit board  80  to form an electric loop. When the circuit is conducted, the LED  10  emits a light  20  having a wavelength ranging between 360 and 560 nm, wherein a color of the light  20  is defined between blue and green spectra. In this embodiment according to the invention, the light  20  having a peak wavelength preferably between 400 and 450 nm is selected as shown in  FIGS. 2 and 3 . The phosphor  30  is consisted of the first and second phosphors  34  and  32 , and is mounted on, covered by or located approaching the LED  10 . The second phosphor  32  may be selected from one or more of the following compounds: 
        (a) YAG: C e  with C e  as an activator thereof; and     (b) TbAG: C e  with C e  or T b  as an activator thereof.        
 
         [0037]     Therefore, when the phosphor  30  containing a material from the second phosphor  32  is illuminated and excited by the light  20  of the light source, a first-color light  50  having a wavelength ranging between 520 and 565 nm is emitted by the phosphor  30 , wherein the first-color light  50  is defined as a wavelength range within the green spectrum. When the second phosphor  32  is made of YAG, an activator thereof can either be an individual element from Pr and Dy, or a compound of the two activators. When the second phosphor  32  is made of TbAG, an activator thereof may either be individual element selected from Dy or Pr, or a compound of the two activators.  
         [0038]     The first phosphor  34  may be an individual element or a compound of at least two elements from the following: 
        (a) Gd 3 Al 5 O 12 : C e  with C e  as an activator thereof;     (b) YAG: E u  with E u  as an activator thereof;     (c) Y 2 O 3 : E u , Bi with a compound of E u  and Bi as an activator thereof;     (d) Y(Gd)BO 3 : E u  with E u  as an activator thereof;     (e) 6MgO. A S2 O 5 : M n  with M n  as an activator thereof;     (f) 4MgOF 2 GeO 2 : M n  with M n  as an activator thereof; and     (g) GdMgB 5 O 10  with a compound at least consisting C e , T b , M n , or E u  as an activator thereof.        
 
         [0046]     When illuminated and excited by the light  20  of the light source, the second phosphor  32  emits a first-color light  50  having a wavelength ranging between 520 and 565 nm as shown in  FIG. 2 . The first-color light  50  is defined as having a wavelength range within a green spectrum.  
         [0047]     A portion light  20 ′ from the light  20  of the light source having not excited the first and second phosphors  34  and  32 , is blended with the first-color and second-color lights  50  and  40 , and altogether released out of the device  100  to obtain a white light  60  having good color rendering effects. Referring to  FIG. 4  showing a curve diagram illustrating a white light spectrum obtained by experiments according to the invention, the spectrum is a curve diagram of the three wavelengths from the first-color and the second-color lights  50  and  40 , and the portion light  20 ′. In the diagram, a curve of the second-color light  40  in a marked area appears rather distinct, so as to enhance rendering effects of the white light  60  blended by the three different wavelengths.  
         [0048]     Using the invention, a white light obtained approaches natural sunlight (white light) and is therefore regarded as having an excellent white light tone.  
         [0049]     In addition, the first and second phosphors  34  and  32  according to the invention are free from sulfur, and are thus provided with higher heat-resistance for preventing the first and second phosphors from quantitative change caused by ambient temperature change.  
         [0050]     It is of course to be understood that the embodiment described herein is merely illustrative of the principles of the invention and that a wide variety of modifications thereto may be effected by persons skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.