Abstract:
The present invention relates to an LED lighting system producing light having improved Color Rendering Index (CRI). The system includes a plurality of LEDs wherein the LEDs have at least a first and second characteristic wavelength, and a mixed phosphor layer overlying at least one of a portion of the plurality of LEDs, the phosphor layer absorbing at least a portion of the LED-emitted light and producing phosphor-emitted light, wherein the phosphors includes a third characteristic wavelength and a fourth characteristic wavelength, wherein the third characteristic wavelength and fourth characteristic wavelength are not the same.

Description:
RELATED APPLICATION 
       [0001]    The present application claims priority to U.S. Provisional Patent Application No. 60/890,160, filed Feb. 15, 2007, the entire contents of which are hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to LED lighting systems. More specifically, the present invention relates to LED lighting systems using multiple LED light sources overlaid with a phosphor layer. 
       BACKGROUND OF THE INVENTION 
       [0003]    Light sources which produce a high color rendering index (CRI) have many applications in the lighting industry. CRI is an indication of the vibrance of the color of light being produced by a light source, and is an indication of the amount of color shift exhibited by the light source upon reflectance. The higher the CRI value for a particular light source, the more white the light and the better or more true that the light source renders the colors of various objects it is used to illuminate. One method for determining CRI is referred to as “R8” and involves the use of eight color chips or palettes which are illuminated by the light source. A CRI value is determined for each of the eight color chips being illuminated, with a CRI of 100 indicating perfect color rendering for that particular color chip as compared to an ideal planckian radiator or daylight emission spectrum. The eight CRI values are then averaged to produce a single CRI value for the light source being evaluated. Alternatively, other CRI evaluation techniques, such as the use of 14 color chips, may be used. High CRI light, or high color rendering light accents the various colors in objects which are to be illuminated. 
         [0004]    Another factor to be considered in evaluating a light source is the concept of “color temperature,” which is an indication of how much color is perceived to be produced by the light source. Higher color temperature values are used to refer to “cool” color temperatures, where the color of light produced is perceived to have a blue-ish tint. The peak wavelength of the spectral power distribution of the light is inversely proportional to the color temperature of the light. Lower color temperature values are used to refer to “warm” white light which is perceived as having a more yellow color. 
         [0005]    Traditional solid state LED lights used to create white light sources have typically used blue LEDs having an excitation near 440-460 nm in conjunction with 573 nm yellow phosphors overlaid on the LEDs. It should be noted that the designation of the phosphor refers to the color (expressed in wavelengths) of the light emitted by the phosphor based on the light incident to the phosphor. Thus, “yellow” phosphor refers to phosphor which emits light in the yellow range, approximately 573 nm. The phosphor layer converts lower wavelength light, such as that generated by the LEDs, into higher wavelength light. This is commonly referred to as luminescence, or phosphor luminescence. 
         [0006]    However, the resulting light tends to be a “cool” light, having a color temperature in the range of 4000-4700 Kelvin (K). Furthermore, the CRI of such lights tends to be relatively low, often in the range of CRI value of 60. 
         [0007]    Thus, there is a need to produce “warm” white and “cool” white light using LED light sources, which also exhibits a relatively higher CRI value. There is also a need to generate the light efficiently, as measured by the Light Efficiency Ratio (“LER”), also referred to as the Light Efficacy, with higher efficiency being preferable. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is generally directed to a light source made up of multiple LEDS, where the LEDs do not all have the same characteristic wavelength. Also, the phosphor layer which is used may be made as a mixture of different types of phosphor, such that the phosphor layer exhibits two or more characteristic colors. In other words, the incident light into the phosphor layer causes the mixed phosphor layer to emit light of at least two different wavelengths. Such a mixed type of phosphor is commonly referred to as “A/B” phosphor where the designation “A/B” refers to the ratio or the mix of the phosphor and indicates that the light being output by the phosphor is A % light having a first wavelength and B % light having a second wavelength, where A+B=100%. Thus, the designation 90/10 of 600/524 nm phosphor refers to a phosphor which outputs 90% of its output light at 600 nm and 10% of its output light at 524 nm. 
         [0009]    According to the present invention, the phosphor layer may be selectively overlaid on all of the LEDs, some of the LEDs, or on a selected portion of one or more of the LEDs. The phosphor layer may be provided as a separate structure, or layer, above the LEDs, or alternatively may be deposited directly on the surface of each LED die. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components, and wherein: 
           [0011]      FIG. 1  is an illustration of the light characteristics and parameters of a prior art LED light source made up of four blue LEDs and a yellow phosphor (573 nm) overlaid on all four LEDs; 
           [0012]      FIG. 2  is an illustration of the light characteristics and parameters of a prior art LED light source made up of four blue LEDs and an 82/18 orange/green phosphor (600/524 nm) overlaid on all four LEDs; 
           [0013]      FIG. 3  is an illustration of the excitation efficiency for various types of phosphor layers; 
           [0014]      FIG. 4  is an illustration of the light characteristics and parameters of a first embodiment of an LED light source according to the present invention made up of three blue LEDs, one cyan LED and a 90/10 phosphor (600/573 nm) overlaid on all four LEDs; 
           [0015]      FIG. 5  is an illustration of the light characteristics and parameters of a second embodiment of an LED light source according to the present invention made up of three blue LEDs, one cyan LED and a mixed orange/green phosphor (600/524 nm) overlaid on all four LEDs; 
           [0016]      FIG. 6  is a CIE x,y coordinate plot illustrating the color characteristics of LED light sources according to the present invention; 
           [0017]      FIG. 7  is an illustration of the wavelength distribution characteristics of various LED dies used in the different embodiments of the present invention; 
           [0018]      FIG. 8  is an illustration of the layout of a prior art four LED light source using four blue (445-460 nm) LEDs; 
           [0019]      FIG. 9  is an illustration of the layout of a four LED light source using three blue (445-460 nm) LEDs and one cyan (488-495 nm) LED; 
           [0020]      FIG. 10  is an illustration of the layout of a four LED light source using two blue (445-460 nm) LEDs, one cyan (488-495 nm) LED and one red (618-635 nm) LED; 
           [0021]      FIG. 11  is an illustration of Relative Spectral Power as a function of wavelength for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, with a residual transparency flux ratio for the four LED emitters of 0.6/0.5/5.2/4.1 (blue 452, blue 458, cyan, red), and a 18/45/37 mix of 524/573/600 nm phosphor, producing a luminous flux ratio of 16/40/33.5; 
           [0022]      FIG. 12  is an illustration of the Relative Spectral Power as a function of wavelength, as well as the CIE chromaticity chart for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, with a residual transparency flux ratio of 0.5/0.7/4.8/12.2 (blue 452, blue 458, cyan, red), and a 19/48/33 mix of 524/573/600 nm phosphor, producing a luminous flux ratio of 15.7/39.5/26.7; 
           [0023]      FIG. 13  is an illustration of the Relative Spectral Power as a function of wavelength, as well as the CIE chromaticity chart for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, with a residual transparency flux ratio of 1.5/1.3/6.2/6.1 (blue 452, blue 458, cyan, red), and a 41/26/33 mix of 524/573/600 nm phosphor, producing a luminous flux ratio of 34.9/22.1/27.9; 
           [0024]      FIG. 14  is an illustration of the Relative Spectral Power as a function of wavelength, as well as the CIE chromaticity chart for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, with a residual transparency flux ratio of 1.8/1.2/6.0/5.8 (blue 452, blue 458, cyan, red) and a 46/26/28 mix of 524/573/600 nm phosphor, producing a luminous flux ratio of 39.4/22.1/23.7; and 
           [0025]      FIG. 15  is an illustration of the Relative Spectral Power as a function of wavelength, as well as the CIE chromaticity chart for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 450 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, with a residual transparency flux ratio of 2.3/1.6/7.7/4.2 (blue 450, blue 458, cyan, red) and a 43/31/26 mix of 524/573/600 nm phosphor, producing a luminous flux ratio of 36.5/26.0/21.6. 
           [0026]      FIG. 16  is an illustration of the effect of phosphor thickness upon the chromaX and chromaY of the light, using three blue die emitters (wavelength 452 nm dominant) plus one red emitter (623 nm dominant) plus one phosphor (573 nm peak wavelength). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Referring now to  FIG. 1 , therein is illustrated the light characteristics and parameters of a prior art LED light source which utilizes four blue LEDs having a characteristic wavelength of 445-460 nm, along with a yellow phosphor layer (573 nm) overlaid on all four LEDs. This type of conventional light source exhibits a CRI of about 64.3 (as indicated in  FIG. 1 ), which is relatively poor in terms of accurate color rendering. Also, the color temperature of such a light source is found to be rather cold (approximately 4528K). As shown in the spectral waveform at the bottom of  FIG. 1 , this LED light source does not have a sufficient red component in the output light. This is sometimes referred to as having a “hole” in the light spectrum. In addition, as illustrated, there is a minimal amount of light output in the spectrum range between the two wavelength peaks. 
         [0028]    The parameters measured for this type of light source include Total Luminous Flux, which refers to the light emitted by the phosphor, as weighted by the human eye. As is understood, the human eye gives more/less importance to different parts of the light spectrum. Another parameter which is determined includes Radiant Flux, an indication of the light output by the phosphor without taking into account any weighting by the human eye. The parameter Dominant Wavelength refers to the wavelength which approximates the peak achromatic spectral distribution as compared to the monochromatic locus when tracing a directrix from the chromatic E point. The parameter Peak Wavelength refers to the highest power spectral component, regardless of any weighting. 
         [0029]      FIG. 2  illustrates the characteristics and parameters of another prior art LED light source which utilizes four blue LEDs (445-460 nm), along with a mixed orange/green phosphor (600/524 nm) with a ratio of 82/18. As shown in  FIG. 2 , the use of the mixed phosphor increases the CRI of this light source to about 76.55, as compared with 64.3 for the device of  FIG. 1 . Also, the color temperature of the device of  FIG. 2  is 3049K, as compared to a color temperature of 4528K for the device of  FIG. 1 . 
         [0030]    Referring now to  FIG. 3 , therein is illustrated the excitation efficiency for various types of phosphors. The term “excitation efficiency” refers to the proportion of incident light at a first frequency which is converted to output light at a second frequency by the phosphor. Typically, not all of the incident light is converted to output light by the phosphor layer. Some of the incident light is lost due to heat. Also, some of the incident light passes through the phosphor layer (transparently) without any change in the frequency of the light. As shown in  FIG. 3 , the 524 nm phosphor exhibits a relatively high excitation efficiency but over a shorter range of wavelengths. The 573 nm phosphor exhibits a slightly lower excitation efficiency but over a slightly larger range of wavelengths. Finally, the 600 nm phosphor generally exhibits the lowest excitation efficiency, but maintains the excitation efficiency over a wider range of wavelengths. 
         [0031]      FIG. 4  illustrates the characteristics and parameters for a first embodiment of an LED light source according to the present invention. In the embodiment of  FIG. 4 , the LED light source includes three blue LEDs (445-460 nm) and one cyan LED (490-495 nm). The phosphor used is a mixed phosphor, 600/573 nm, with a ratio of 90/10. As shown in  FIG. 4 , the CRI for this embodiment is found to be 80.85, which is significantly higher than the prior art devices of  FIGS. 1 and 2 , which exhibit CRI values of 64.3 and 76.55, respectively. Although the CRI has improved noticeably, the light produced remains a pleasing warm white with a color temperature of 3054K. 
         [0032]      FIG. 5  illustrates the characteristics and parameters for a second embodiment of an LED light source according to the present invention. In the embodiment of  FIG. 5 , the LED light source includes three blue LEDs (445-460 nm) and one cyan LED (490-495 nm). The phosphor used is a mixed phosphor, 600/524 nm, with a ratio of 90/10. As shown in  FIG. 5 , the CRI for this embodiment is found to be 85.56, which is significantly higher than the prior art devices of  FIGS. 1 and 2 , which exhibit CRI values of 64.3 and 76.55, respectively. Although the CRI has improved noticeably, the color temperature has degraded somewhat, and is found to be 3338K. 
         [0033]      FIG. 6  illustrates a CIE x,y coordinate plot showing the color characteristics of various embodiments of LED light sources according to the present invention. As shown in  FIG. 6 , the generally vertical, radiating lines are referred to as “isothermal lines” and represent constant color temperature characteristics. The areas in between the isothermal lines can be divided into relatively small regions, referred to as “color bins”. The color temperature of the various devices can then be plotted and a determination made as to which “color bin” they each fall into. 
         [0034]      FIG. 7  is an illustration of the frequency characteristics of various LED dies used in the different embodiments of the present invention. As shown in  FIG. 7 , each LED die has a spectral characteristic exhibiting a single peak wavelength corresponding to the color of light generated by that particular LED die. 
         [0035]      FIGS. 8-10  are illustrations of the layout of various four-LED light source configurations according to the present invention using a plurality of phosphors in the mixed phosphor layer. The particular layout shown in  FIG. 8  utilizes four blue (445-460 nm) LEDs; the layout of  FIG. 9  utilizes three blue (445-460 nm) LEDs and one cyan (488-495 nm) LED; the layout of  FIG. 10  utilizes two blue (445-460 nm) LEDs, one cyan (488-495 nm) LED, and one red (618-635 nm) LED. The LEDs may be mounted on a single circuit board. For example, the LEDs may be mounted on a ceramic substrate in order to provide better thermal handling properties. 
         [0036]      FIG. 11  is an illustration of Relative Spectral Power as a function of wavelength for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, and a 18/45/37 mix of 524/573/600 nm phosphor. The dashed line in  FIG. 11  represents the theoretical spectral power as a function of wavelength for a light source having a CRI of 100. 
         [0037]    For the device illustrated in  FIG. 11 , the CRI value is approximately 88, and the color temperature is a “warm” color temperature of 3337K. Residual transparency flux ratio refers to the ratio of light output by a particular source which passes through a phosphor unabsorbed (i.e., no conversion of wavelength) versus the total amount of light hitting a detector, such as the human eye. For the device of  FIG. 11 , the residual transparency flux ratio is 0.6/0.5/5.2/4.1 (blue 452, blue 458, cyan, red). This means that 0.6% of the light hitting a detector, such as the eye, is light emitted by the blue 452 nm LED which passes through the phosphor unabsorbed. Similarly, 0.5% of the light hitting the detector is light emitted by the blue 458 nm LED which passes through the phosphor unabsorbed. Further, 5.2% of the light hitting the detector is light emitted by the cyan LED which passes through the phosphor unabsorbed. Finally, 4.1% of the light hitting the detector is light from the red LED which passes through the phosphor unabsorbed. 
         [0038]    Luminous flux ratio (also termed luminescence) refers to the ratio of light emitted by a particular phosphor versus the total amount of light hitting a detector, such as the human eye. For the device of  FIG. 11 , the luminous flux ratio is 16/40/33.5. This means that 16% of the light hitting the detector is light emitted by the 524 nm phosphor; 40% of the light hitting the detector is light emitted by the 573 nm phosphor; and 33.5% of the light hitting the detector is light emitted by the 600 nm phosphor. 
         [0039]      FIG. 12  includes an illustration of Relative Spectral Power as a function of wavelength for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, and a 19/48/33 mix of 524/573/600 nm phosphor. The red emitter is “direct”, i.e., the light emitted by the red LED does not have a significant change of wavelength as it passes through the mixed phosphor layer. The dashed line in the wavelength chart of  FIG. 12  represents the theoretical spectral power as a function of wavelength for a light source having a CRI of 100. For the device illustrated in  FIG. 12 , the CRI value is 97.4 using an R8 approach (eight color palettes), and 96.8 using an R14 approach (14 color palettes). The color temperature is approximately 2850-2851.  FIG. 12  also includes an illustration of the CIE chromaticity chart for the individual LED sources, the phosphor, and the composite spectral power distribution of the light output from the phosphor. For the device of  FIG. 12 , the residual transparency flux ratio is 0.5/0.7/4.8/12.2. For the device of  FIG. 12 , the luminous flux ratio contribution provided by the phosphors is 15.7/39.5/26.7. 
         [0040]      FIG. 13  includes an illustration of Relative Spectral Power as a function of wavelength for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, and a 41/26/33 mix of 524/573/600 nm phosphor. The dashed line in the wavelength chart of  FIG. 13  represents the theoretical spectral power as a function of wavelength for a light source having a CRI of 100. For the device illustrated in  FIG. 13 , the CRI value is 96.4 using an R8 approach (eight color palettes), and 95.5 using an R14 approach (14 color palettes). The color temperature is approximately 4750-4753.  FIG. 13  also includes an illustration of the CIE chromaticity chart for the individual LED sources, the phosphor, and the composite spectral power distribution of the light output from the phosphor. For the device of  FIG. 13 , the residual transparency flux ratio is 1.5/1.3/6.2/6.1. For the device of  FIG. 13 , the luminous flux ratio is 34.9/22.1/27.9. 
         [0041]      FIG. 14  includes an illustration of Relative Spectral Power as a function of wavelength for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, and a 46/26/28 mix of 524/573/600 nm phosphor. The dashed line in the wavelength chart of  FIG. 14  represents the theoretical spectral power as a function of wavelength for a light source having a CRI of 100. For the device illustrated in  FIG. 14 , the CRI value is 97.5 using an R8 approach (eight color palettes), and 96.6 using an R14 approach (14 color palettes). The color temperature is approximately 5248-5250.  FIG. 14  also includes an illustration of the CIE chromaticity chart for the individual LED sources, the phosphor, and the composite spectral power distribution of the light output from the phosphor. For the device of  FIG. 14 , the residual transparency flux ratio is 1.8/1.2/6.0/5.8. For the device of  FIG. 14 , the luminous flux ratio is 39.4/22.1/23.7. 
         [0042]      FIG. 15  includes an illustration of Relative Spectral Power as a function of wavelength for a four LED light source using two blue (445-460 nm) LEDs (one blue LED with a peak wavelength of 452 nm and one blue LED with a peak wavelength of 458 nm), one cyan (488-495 nm) LED, one red (618-635 nm) LED, and a 43/31/26 mix of 524/573/600 nm phosphor. The dashed line in the wavelength chart of  FIG. 15  represents the theoretical spectral power as a function of wavelength for a light source having a CRI of 100. For the device illustrated in  FIG. 15 , the CRI value is 96.5 using an R8 approach (eight color palettes), and 95.7 using an R14 approach (14 color palettes). The color temperature is approximately 6750-6757.  FIG. 15  also includes an illustration of the CIE chromaticity chart for the individual LED sources, the phosphor, and the composite spectral power distribution of the light output from the phosphor. For the device of  FIG. 15 , the residual transparency flux ratio is 2.3/1.6/7.7/4.2. For the device of  FIG. 15 , the luminous flux ratio is 36.5/26.0/21.6. 
         [0043]      FIG. 16  includes an illustration of the effect of phosphor thickness upon the chromaticity of an LED lighting system having three blue die emitters (wavelength 452 nm dominant) plus one red emitter (623 nm dominant) plus one phosphor (573 nm peak wavelength). 
         [0044]    As presented above, by varying the properties of the LEDs used in the light source, and also by varying the properties of the phosphor layer, the CRI, as well as the color temperature, of the resulting device may be modified to achieve desired CRI and color temperature ranges for any particular lighting application. It will also be understood that the peak wavelength of phosphors within the mixed phosphor layer may vary by ±5 nm or more, in order to produce light having a more desirable combination of high CRI and high LER. 
         [0045]    While there have been shown, described, and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions, substitutions, and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. For example, it is expressly intended that all combinations of those elements and/or steps which perform substantially the same function, in substantially the same way, to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale, but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.