Patent Publication Number: US-2009231832-A1

Title: Solid-state lamps with complete conversion in phosphors for rendering an enhanced number of colors

Description:
REFERENCE TO PRIOR APPLICATION  
     The current application claims the benefit of co-pending U.S. Provisional Application No. 61/069,354, entitled “Solid-State Lamp with Complete Conversion in Phosphors for Rendering an Enhanced Number of Rendered Colors,” which was filed on Mar. 15, 2008, and which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD  
     Aspects of the invention relate to phosphor-conversion (PC) sources of white light, which are composed of at least two groups of emitters, such as ultraviolet (UV) light-emitting diodes (LEDs) and wide-band (WB) or narrow-band (NB) phosphors that completely absorb and convert the flux generated by the LEDs to other wavelengths, and to improving the color quality of the white light emitted by such light sources. In particular, embodiments of the present invention describe new 2-4 component combinations of peak wavelengths and bandwidths for white PC LEDs with complete conversion. These combinations are used to provide spectral power distributions that enable lighting with a considerable portion of a high number of spectrophotometrically calibrated colors rendered almost indistinguishably from a blackbody radiator or daylight illuminant, and which differ from distributions optimized using standard color-rendering assessment procedures based on a small number of test color samples. 
     BACKGROUND ART  
     Composing white light from colored components in an optimum way has been a key problem of the lighting industry since the introduction of fluorescence lamps in the 1930s. Presently, the ability of white light to properly render the colors of illuminated objects is optimized by maximizing the general color rendering index, R a , a figure of merit introduced by the International Commission of Illumination (Commission Internationale de I&#39;Éclairage, CIE) in 1974 and updated in 1995 (CIE Publication No. 13.3,1995). A trichromatic system with a maximized R a  composed of red (610 nm), green (540 nm) and blue (450 nm) components (W. A. Thornton, U.S. Pat. No 4,176,294, 1979) is widely accepted in lighting technology as the white light standard. 
     The development of efficient LEDs radiating in the short-wavelength range of the visible spectrum has resulted in the emergence of solid-state lighting. Since LEDs employ injection electroluminescence and potentially offer radiant efficiency that exceeds the physical limits of other sources of light, solid-state lighting is a tremendous lighting technology with the promise of the highest electric power conservation and vast environmental benefits. 
     Composite white light from LEDs can be obtained by means of partial or complete conversion of short-wavelength radiation in phosphors, using a set of primary LED chips with narrow-band emission spectra or a complementary use of both phosphor-conversion and colored LEDs. The phosphor-conversion approach based on UV and blue LEDs with complete or partial conversion in phosphors offers an unsurpassed versatility in color control, since the peak wavelengths of the LEDs can be tailored by varying the chemical contents and thickness of the active layers in the electroluminescent structures, and the peak wavelengths and the bandwidths of the phosphors can be tailored by varying the chemical content of the phosphor converters. 
     Using phosphors with different wavelengths and bandwidths allows for tailoring continuous illumination spectra similar to those of blackbody radiators or daylight illuminants, which are widely accepted as the ultimate-quality sources of white light. This requires the determination of phosphor wavelengths and phosphor bandwidths providing the best possible quality of light for a given number of phosphors contained in a white light source, and the minimal number of phosphors with particular bandwidths required for attaining the ultimate quality of white light emitted by LEDs with partial or complete conversion. 
     The existing approach of assessing the color rendering properties of PC LEDs is based on the CIE 1995 procedure (CIE Publication No. 13.3, 1995), which traces back to halophosphate fluorescent lamp technology, and which employs the general color rendering index R a  based on eight test color samples selected from the Munsell system of colors (and possible additional six test color samples). This number of colors (eight to fourteen) is much smaller than that resolved by human vision and is not suitable for tailoring phosphor blends in white PC LEDs that are designed to emit light with ultimate color quality. 
     SUMMARY OF THE INVENTION  
     Aspects of the invention relate to phosphor-conversion (PC) sources of white light, which are composed of at least two groups of emitters, such as ultraviolet (UV) electroluminescent light-emitting diodes (LEDs) and wide-band (WB) or narrow-band (NB) phosphors that completely absorb and convert the flux generated by the LEDs to other wavelengths, and to improving the quality of the white light emitted by such light sources. In particular, embodiments of the present invention describe new 2-4 component combinations of peak wavelengths and bandwidths for white PC LEDs with complete conversion. These combinations are used to provide spectral power distributions that enable lighting with a considerable portion of a high number of spectrophotometrically calibrated colors rendered almost indistinguishably from a blackbody radiator or daylight illuminant, and which differ from distributions optimized using standard color-rendering assessment procedures. 
     A first aspect of the invention provides a lighting source, having a predetermined correlated color temperature, comprising: a light emitter comprising an ultraviolet light-emitting diode generating a flux that is completely absorbed and converted to other wavelengths by a set of phosphors, each phosphor having a primary color, peak (or average) wavelength, and bandwidth, and with the peak wavelengths and relative fluxes generated by the set of phosphors being selected such that in comparison with a reference lighting source, when each of more than fourteen test color samples resolved by an average human eye as different is illuminated: (a) chromaticity shifts with a chromatic adaptation of human vision taken into account are preserved within corresponding regions of a chromaticity diagram, each containing all colors that are indistinguishable, to the average human eye, from a color at a center of the region; and (b) lightness shifts are preserved within predetermined values. 
     Another aspect of the invention provides lighting method, comprising: generating white light, having a predetermined correlated color temperature, using a light emitter, the light emitter comprising an ultraviolet light-emitting diode generating a flux that is completely absorbed and converted to other wavelengths by a set of phosphors, each phosphor having a primary color, peak (or average) wavelength, and bandwidth, and with the peak wavelengths and relative fluxes generated by the set of phosphors being selected such that in comparison with a reference lighting source, when each of more than fourteen test color samples resolved by an average human eye as different is illuminated: (a) chromaticity shifts with a chromatic adaptation of human vision taken into account are preserved within corresponding regions of a chromaticity diagram, each containing all colors that are indistinguishable, to the average human eye, from a color at a center of the region; and (b) lightness shifts are preserved within predetermined values. 
     Another aspect of the invention provides a method for generating white light having a predetermined correlated color temperature, comprising: selecting a light emitter including an ultraviolet light-emitting diode generating a flux that is completely absorbed and converted to other wavelengths by a set of phosphors, each phosphor having a primary color, peak (or average) wavelength, and bandwidth, and with the peak wavelengths and relative fluxes generated by the set of phosphors being selected such that in comparison with a reference lighting source, when each of more than fourteen test color samples resolved by an average human eye as different is illuminated: (a) chromaticity shifts with a chromatic adaptation of human vision taken into account are preserved within corresponding regions of a chromaticity diagram, each containing all colors that are indistinguishable, to the average human eye, from a color at a center of the region; and (b) lightness shifts are preserved within predetermined values. 
     Other aspects of the invention may include and/or implement some or all of the features described herein. The illustrative aspects of the invention are designed to solve one or more of the problems herein described and/or one or more other problems not discussed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  depicts a schematic diagram of an illustrative complete conversion white phosphor-conversion (PC) light emitting diode (LED) according to an embodiment. 
         FIG. 2  depicts optimization results for the emission spectra of white PC LEDs with complete conversion provided in accordance with aspects of the present invention. 
         FIG. 3  depicts the peak positions (a), relative radiant fluxes (b), and minimal bandwidth (c) of the wide-band (WB) phosphors as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in two WB phosphors, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered, according to an embodiment. 
         FIG. 4  depicts the peak positions (a) and relative radiant fluxes (b) of the phosphors, and the minimal bandwidth (c) of the WB phosphors, as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in two WB phosphors and one red narrow-band (NB) phosphor, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered, according to an embodiment. 
         FIG. 5  depicts the peak positions (a), relative radiant fluxes (b), and minimal bandwidth (c) of the WB phosphors as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in three WB phosphors, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered, according to an embodiment. 
         FIG. 6  depicts the peak positions (a) and relative radiant fluxes (b) of the phosphors, and the minimal bandwidth (c) of the WB phosphors, as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in three WB phosphors and one red NB phosphor, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered, according to an embodiment. 
         FIG. 7  depicts the peak positions (a), relative radiant fluxes (b), and minimal bandwidth (c) of the WB phosphors as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in four WB phosphors, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     In accordance with embodiments of the present invention, a lighting source having a predetermined correlated color temperature is provided. The lighting source comprises phosphor-conversion (PC) sources of white light, which are composed of at least two groups of emitters, such as ultraviolet (UV) electroluminescent light-emitting diodes (LEDs) and wide-band (WB) or narrow-band (NB) phosphors that completely absorb and convert the flux generated by the LEDs to other wavelengths. Embodiments of the present invention describe new 2-4 component combinations of peak (or average) wavelengths and bandwidths for white PC LEDs with complete conversion. These combinations are used to provide spectral power distributions that enable lighting with a considerable portion of a high number of spectrophotometrically calibrated colors rendered almost indistinguishably from a blackbody radiator or daylight illuminant, and which differ from distributions optimized using standard color-rendering assessment procedures. As used herein, unless otherwise noted, the term “set” means one or more (i.e., at least one) and the phrase “any solution” means any now known or later developed solution. 
     DEFINITIONS  
     Electroluminescent LED—light emitting diode, which converts electric power to light due to electroluminescence.
         Phosphor—a substance that converts light of particular wavelengths (usually shorter ones) to light with other wavelengths (usually longer ones) due to photoluminescence.       

     White phosphor-conversion (PC) LED—a solid-state lamp in which radiation emitted from an electroluminescent LED is completely or partially absorbed and converted in one or a plurality of phosphors in order to generate white light by means of color mixing. 
     Complete-conversion PC LED—a PC LED that contains an electroluminescent LED (e.g., an ultraviolet (UV) LED) emitting light and a plurality of phosphors that completely absorb and convert the flux generated by the electroluminescent LED to visible light in such a way that a mixture of the light generated by different phosphors is perceived as white light. 
     Color space—a model for mathematical representation of a set of colors. 
     Munsell samples—a set of color samples introduced by Munsell and then updated, such that each sample is characterized by the hue, value (lightness scale), and chroma (color purity scale). 
     MacAdams ellipses—the regions on the chromaticity plane of a color space that contain all colors which are almost indistinguishable, to the average human eye, from the color at the center of the region. 
     Standard illuminant—a standardized spectral power distribution of visible light, which allows colors recorded under different lighting to be compared, such as of blackbody radiator or reconstituted daylight-phase illuminant. 
     Embodiments of the present invention provide sources of white light nearly identical to a blackbody radiator or daylight-phase illuminant in terms of its perception by the human eye. In order to characterize and compare different sources of white light, aspects of the invention introduce a characteristic of the light source related to the rendering of colors of illuminated objects, which is used to evaluate the white light source quality. 
     To characterize the quality of white light, embodiments of the present invention provide an advanced color rendering assessment procedure. A common approach for the assessment of the color-rendering properties of a light source is based on the estimation of color differences (e.g., shifts of the color coordinates in an appropriate color space) for test samples when the source under consideration is replaced by a reference source (e.g., blackbody radiator or reconstituted daylight illuminant). The standard CIE 1995 procedure, which initially was developed for the rating of halophosphate fluorescent lamps with relatively wide spectral bands, and which was later refined and extended, employs only eight to fourteen test samples from the vast palette of colors originated by the artist A. H. Munsell in 1905. When applied to sources composed of narrow-band emitters, such as LEDs, the CIE 1995 procedure receives criticism that is mainly due to the small number of test samples (eight to fourteen) employed. Another drawback is the use of equally treated shifts for all samples in a color space, which lacks uniformity in terms of perceived color differences. In fact, the CIE 1960 Uniform Chromaticity Scale (UCS) space, which is employed in the standard color rendering assessment procedure, is completely symmetrized only around the very central point. 
     Aspects of the present invention are based on using a much larger number of test samples and on the color differences distinguished by human vision for each of these samples. To this end, the entire Munsell palette is employed, which specifies the perceived colors in three dimensions: hue; chroma (saturation); and value (lightness). A spectrophotometrically calibrated set of 1269 Munsell samples is used, which (with some exceptions for highly saturated colors) can be referred to as all colors of the real world. The Joensuu Spectral Database, available from the University of Joensuu Color Group, is an example of a spectrophotometrically calibrated set of 1269 Munsell samples that can be used in the practice of an embodiment of the present invention. 
     The perceived color differences are evaluated using MacAdam ellipses, which are the experimentally determined regions in the chromaticity diagram (hue-saturation plane), containing colors that are almost indistinguishable by human vision. A nonlinear interpolation of the ellipses determined by MacAdam for 25 colors is employed to obtain the ellipses for the entire 1269-element Munsell palette. For instance, using the inverse distance weighted (geodesic) method, an ellipse centered at the chromaticity coordinates (x, y) has an interpolated parameter (a minor or major semiaxis or an inclination angle) given by the formula 
     
       
         
           
             
               
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     where P 0 (x 0i , y 0i ) is a corresponding experimental parameter, and hi is the distance from the center of the interpolated ellipse to an original MacAdam ellipse 
         h   i =√{square root over (( x -x 0i )+(y-y 0i ))}{square root over (( x -x 0i )+(y-y 0i ))} 2 . 
     In an embodiment, a rendered chromaticity of a sample is defined as that which shifts only within the 3-step MacAdam ellipse (i.e., by less than three radii of the ellipse) with the chromatic adaptation taken into account (e.g., in the way used in CIE Publication No. 13.3, 1995). Further, in an embodiment, the allowed difference in lightness (the third coordinate) is set to 2% for all the samples. If the color point moves out of such an elliptical cylinder when switching from the reference illuminant to that under test, the distortion of the sample color will be noticed by over 99% of individuals with normal vision. As a figure of merit for the overall assessment of color rendering properties of a lamp, embodiments of the present invention utilize a new methodology involving a Number of Rendered Colors (also named as Color Fidelity Index), N r , measured in percents in respect of the total number of the test Munsell samples (1269), which is the proposed alternative to the general color rendering index R a  based on eight test samples. 
     Aspects of the present invention perform optimization of white phosphor-conversion LEDs with complete conversion for different numbers n of spectral components (e.g., n equal to two, three, or four) to attain the highest number of rendered colors N r  for a set of colors, such as the aforementioned spectrophotometrically calibrated set of 1269 Munsell samples. Correlated color temperatures in the entire relevant range of 2500 K to 10000 K are used. In particular, the color temperature of 6500 K is of importance, since it almost fits the chromaticity of daylight. 
     The employed spectral components comprise Gaussian shapes, which are very similar to spectral shapes of the emission bands of most real phosphors. The spectra of complete-conversion LEDs are simulated using phosphor bands (e.g. from two to four). One set of solutions is obtained for phosphors with emission bands of equal full width at half magnitude (FWHM, Δ), which are designated here as wide-band (WB) phosphors. In another set, the longest-wavelength phosphor is preset to a 10-nm bandwidth (narrow-band, NB, phosphor) in order to mimic a rear-earth activator with screened (4f-4f) transitions, such as in Eu 3+ , Sm 3+ , or Pr 3+ . 
     A method of optimization in the 2n-dimensional parametric space of peak wavelengths and relative fluxes is applied in order to maximize N r . For example, with a gradual increase of the bandwidth of the WB phosphors, N r  is continually maximized until the peak value (100%) is attained. At that point, the optimization routine is terminated and the peak wavelengths of the primary emitters and the width of the WB phosphor bands are recorded. To rate the energy-conversion usefulness of each ultimate-quality solution, luminous efficacy of radiation, which is the ratio of luminous and radiant fluxes, is determined as well. 
     The optimized spectral power distributions that provide or attain the ultimate quality of white light are discussed below. These spectra with N r =100% have the general color rendering indexes (CRIs) of 96-98 points, whereas common PC LEDs with partial conversion in YAG:Ce 3+ (“cool-white”) and in diphosphor blend (“warm-white”) render about 20% (R a ≈70) and 70% (R a ≈90) of the palette, respectively. The indicated bandwidths of the WB phosphors are the smallest required (larger widths do not decrease the quality of light but they can result in shifting of the peaks and reduced luminous efficacy of radiation). For non-Gaussian shapes of phosphor bands, the indicated peak wavelengths might somewhat differ but are still close to the average wavelengths. If the emission band is noticeably asymmetric, the average wavelength might be more meaningful than the peak wavelength. To this extent, in the following discussion, one can use an average wavelength in place of a peak wavelength. 
       FIG. 1  shows a schematic diagram of a complete conversion white PC LED  10  in accordance with embodiments of the present invention. The white PC LED  10  includes a semiconductor chip  12  containing an electroluminescent light-emitting diode  14  that is configured to emit violet or UV light  16  with a peak wavelength, for example, shorter than 430 nm, and a phosphor converter layer  18  including a plurality of phosphors that completely absorb and convert the flux generated by the diode  14  to visible light. The visible light emitted by the phosphors, when mixed, is perceived as white light  20 . In this example, the phosphor converter layer  18  includes three different phosphors, namely a “red” phosphor P r , a “green” phosphor P g , and a “blue” phosphor P b . The red phosphor P r  converts the flux generated by the diode  14  to visible red light L r , while the green and blue phosphors P g , P b , convert the flux generated by the diode  14  to visible green and blue light L g , L b , respectively. The semiconductor chip  12  is coupled (not shown) to electrical leads  22 , and the semiconductor chip  12  and phosphor converter layer  18  are disposed within an enclosure  24 . 
     In accordance with this and other embodiments of the present invention, the peak wavelengths and relative fluxes of the plurality of phosphors in the white PC LED  10  are selected to maximize the number of rendered colors N r  such that, when compared to a reference lighting source, when each of more than fourteen test color samples resolved by an average human eye as different is illuminated: (a) chromaticity shifts with a chromatic adaptation of human vision taken into account are preserved within corresponding regions of a chromaticity diagram, each containing all colors that are indistinguishable, to the average human eye, from a color at a center of the region; and (b) lightness shifts are preserved within predetermined values. For example, in an embodiment, the peak wavelengths and relative fluxes can be selected such that in comparison with a reference source for the illuminated test color samples the chromaticity shifts are preserved within 3-step MacAdam ellipses and the lightness shifts are preserved within 2%. The relative fluxes generated by each of the phosphors can be controlled via at least one of: the concentration of the phosphor particles in the phosphor converter layer  18 ; the thickness of the phosphor converter layer  18 ; the refraction index of the materials forming the phosphor converter layer  18 ; the distance of the phosphor converter layer  18  from the LED  14 ; the location of the phosphor converter layer  18  within the enclosure  24 , and/or the like. 
       FIG. 2  and Table 1 show the optimization results provided in accordance with aspects of the present invention for white PC LEDs with complete conversion. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Wide-band (WB) Phosphors 
                 Narrow-band (NB) 
                 Luminous 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Color 
                   
                 Blue 
                 Green 
                 Yellow 
                 Red 
                 (Δ = 10 nm) 
                 Efficacy of 
               
               
                 Converter 
                 Temperature 
                 Bandwidth 
                 Peak 
                 Peak 
                 Peak 
                 Peak 
                 Red Phosphor 
                 Radiation 
               
               
                 Phosphors 
                 (K) 
                 (nm) 
                 (nm) 
                 (nm) 
                 (nm) 
                 (nm) 
                 Peak (nm) 
                 (lm/W) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 2 WB 
                 3000 
                 157 
                 — 
                 520 
                 — 
                 664 
                 — 
                 205 
               
               
                   
                 6500 
                 164 
                 464 
                 — 
                 627 
                 — 
                 — 
                 210 
               
               
                 2 WB + 1 NB 
                 3000 
                 110 
                 496 
                 — 
                 608 
                 — 
                 659 
                 269 
               
               
                   
                 6500 
                 117 
                 462 
                 — 
                 586 
                 — 
                 657 
                 244 
               
               
                 3 WB 
                 3000 
                 83 
                 482 
                 572 
                 — 
                 661 
                 — 
                 244 
               
               
                   
                 6500 
                 88 
                 456 
                 550 
                 — 
                 664 
                 — 
                 248 
               
               
                 3 WB + 1 NB 
                 3000 
                 55 
                 468 
                 536 
                 604 
                 — 
                 657 
                 285 
               
               
                   
                 6500 
                 57 
                 455 
                 527 
                 598 
                 — 
                 655 
                 271 
               
               
                 4 WB 
                 3000 
                 46 
                 462 
                 529 
                 592 
                 657 
                 — 
                 270 
               
               
                   
                 6500 
                 51 
                 453 
                 521 
                 586 
                 652 
                 — 
                 268 
               
               
                   
               
            
           
         
       
     
     The solid lines in  FIG. 2  show the spectra with a color temperature of 6500 (e.g., daylight), whereas the dashed lines show the spectra with a color temperature of 3000 K (e.g., warm white/halogen). A dichromatic solution with two WB phosphors (a) includes phosphors with bandwidths that are not readily available in common phosphors (Δ&gt;150 nm). A trichromatic solution with two extra-WB blue and yellow phosphors (Δ&gt;110 nm) supplemented with a NB red phosphor (b) is more attainable and deserves additional attention. However, more feasible solutions are based on three (c) or four (e) WB phosphors with a minimal FWHM of about 90 nm and 50 nm, respectively, or three WB phosphors (Δ&gt;55 nm) supplemented with a NB red phosphor (d). Again, all these spectra contain a deep-red component peaked at about 650-660 nm. In particular, the trichromatic (3 WB) solution (c) includes primary emitters with peak wavelengths of about 470 nm, 560 nm, and 660 nm that considerably differ from those of Thornton (450 nm, 540 nm, and 610 nm), especially in the long-wavelength region. In terms of luminous efficacy of radiation, a more favorable solution is a tetrachromatic lamp with blue, green, and yellow WB phosphors and a red NB phosphor (d). Unfortunately, the required peak wavelength of the NB phosphor (around 655 nm) is somewhat longer in comparison with those of common 4f -4f phosphors, which have narrow red lines in a range of 610-630 nm. Therefore, at this time, trichromatic and tetrachromatic spectra containing wide bands are technologically more attractive for complete-conversion PC LEDs with ultimate quality of white light. Many blue, green, and yellow phosphors with an excitation spectra in the near-UV or violet region and a bandwidth in excess of 50 nm are available, whereas the phosphors of choice for the red WB component can comprise, for example, Eu 2+ -activated nitrides or oxyn itrides. 
       FIGS. 3 to 7  show the peak positions (a) and relative radiant fluxes (b) of the phosphors, and the minimal bandwidth (c) of the WB phosphors, as functions of correlated color temperature for optimized white PC LEDs with complete conversion with different numbers and types of phosphors, determined in accordance with aspects of the present invention. Connecting lines in  FIGS. 3 to 7  are provided merely as guides to the eye. 
       FIG. 3  shows the peak positions (a), relative radiant fluxes (b), and minimal bandwidth (c) of the WB phosphors as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in two WB phosphors, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered. 
     Based on data such as that provided in Table 1 and  FIGS. 2 and 3 , an embodiment of an optimized white PC LED provided in accordance with the present invention comprises: two WB phosphors with bandwidths of at least about 120 nm and peak wavelengths in a range of about 430-555 nm and 610-690 nm, respectively, with a correlated color temperature in a range of about 2500 to 10000 K set by adjusting the relative fluxes generated by each of the WB phosphors, wherein the chromaticity and lightness shifts are preserved for more than about 1000 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. Another embodiment provides an optimized white PC LED with a correlated color temperature of about 6500 K, comprising two WB phosphors with bandwidths of at least about 140 nm, peak wavelengths of about 464 nm and 627 nm, respectively, and relative radiant fluxes of about 0.57 and 0.43, respectively, wherein the chromaticity and lightness shifts are preserved for more than about 1200 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. 
       FIG. 4  shows the peak positions (a) and relative radiant fluxes (b) of the phosphors, and the minimal bandwidth (c) of the WB phosphors, as functions of correlated color temperature for white PC LEDs with complete conversion of UV in two WB phosphors and one red NB phosphor, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered. 
     Based on data such as that provided in Table 1 and  FIGS. 2 and 4 , an embodiment of an optimized white PC LED provided in accordance with the present invention comprises: a NB phosphor with a peak wavelength in a range of about 640-675 nm and two WB phosphors with bandwidths of at least about 80 nm and with peak wavelengths in a range of about 440-520 nm and 565-630 nm, respectively, with a correlated color temperature in a range of about 2500 to 10000 K set by adjusting the relative fluxes generated by each of NB and WB phosphors, wherein the chromaticity and lightness shifts are preserved for more than about 1000 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. Another embodiment provides an optimized white PC LED with a correlated color temperature of about 6500 K, comprising a NB phosphor with a bandwidth of about 10 nm, peak wavelength of about 657 nm, and relative radiant flux of about 0.09, and two WB phosphors with bandwidths of at least about 100 nm, peak wavelengths of about 462 nm and 586 nm, respectively, and relative radiant fluxes of about 0.50 and 0.41, respectively, wherein the chromaticity and lightness shifts are preserved for more than about 1200 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. 
       FIG. 5  shows the peak positions (a), relative radiant fluxes (b), and minimal bandwidth (c) of the WB phosphors as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in three WB phosphors, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered. 
     Based on data such as that provided in Table 1 and  FIGS. 2 and 5 , an embodiment of an optimized white PC LED provided in accordance with the present invention comprises: three WB phosphors with bandwidths of at least about 65 nm and peak wavelengths in a range of about 430-505 nm, 525-595 nm, and 625-670 nm, respectively, with a correlated color temperature in a range of about 2500 to 10000 K set by adjusting the relative fluxes generated by each of the WB phosphors, wherein the chromaticity and lightness shifts are preserved for more than about 1000 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. Another embodiment provides an optimized white PC LED with a correlated color temperature of about 6500 K, comprising three phosphors with bandwidths of at least about 70 nm, peak wavelengths of about 456 nm, 550 nm, and 644 nm, respectively, and relative radiant fluxes of about 0.39, 0.32, and 0.29, respectively, wherein the chromaticity and lightness shifts are preserved for more than about 1200 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. 
       FIG. 6  shows the peak positions (a) and relative radiant fluxes (b) of the phosphors, and the minimal bandwidth (c) of the WB phosphors, as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in three WB phosphors and one red NB phosphor, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered. 
     Based on data such as that provided in Table 1 and  FIGS. 2 and 6 , an embodiment of an optimized white PC LED provided in accordance with the present invention comprises: a NB phosphor with a peak wavelength in a range of about 640-675 nm and three WB phosphors with bandwidths of at least about 40 nm and peak wavelengths in a range of about 435-490 nm, 505-560 nm, and 580-625 nm, respectively, with a correlated color temperature in a range of about 2500 to 10000 K set by adjusting the relative fluxes generated by each of NB and WB phosphors, wherein the chromaticity and lightness shifts are preserved for more than about 1000 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. Another embodiment provides an optimized white PC LED with a correlated color temperature of about 6500 K, comprising a NB phosphor with a bandwidth of about 10 nm, peak wavelength of about 655 nm, and relative radiant flux of about 0.15, and three WB phosphors with bandwidths of at least about 50 nm, peak wavelengths of about 455 nm, 527 nm, and 598 nm, respectively, and relative radiant fluxes of about 0.32, 0.27, and 0.26, respectively, wherein the chromaticity and lightness shifts are preserved for more than about 1200 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. 
       FIG. 7  shows the peak positions (a), relative radiant fluxes (b), and minimal bandwidth (c) of the WB phosphors as functions of correlated color temperature for white PC LEDs with complete conversion of UV light in four WB phosphors, with all 1269 colors of the spectrophotometrically calibrated Munsell palette rendered. 
     Based on data such as that provided in Table 1 and  FIGS. 2 and 7 , an embodiment of an optimized white PC LED provided in accordance with the present invention comprises: four WB phosphors with bandwidths of at least about 30 nm and peak wavelengths in a range of about 430-485 nm, 500-550 nm, 565-610 nm, and 635-675 nm, respectively, with a correlated color temperature in a range of about 2500 to 10000 K set by adjusting the relative fluxes generated by each of WB phosphors, wherein the chromaticity and lightness shifts are preserved for more than about 1000 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. Another embodiment provides an optimized white PC LED with a correlated color temperature of about 6500 K, comprising four WB phosphors with bandwidths of at least about 40 nm, peak wavelengths of about 453 nm, 521 nm, 586 nm, and 652 nm, respectively, and relative radiant fluxes of about 0.29, 0.26, 0.23, and 0.22, respectively, wherein the chromaticity and lightness shifts are preserved for more than about 1200 different test color samples of the 1269 spectrophotometrically calibrated samples of the Munsell palette. 
     Further objects and advantages are to provide a design for the high quality solid state white light source that can be used to replicate sunlight in any color-sensitive applications, such as filming, photographing, and designing, in medicine for the seasonal disease treatment and prophylactics, in psychology for depression treatment and prophylactics, etc. The same method based on the evaluation of the number of rendered colors N r , from a given set of samples can be used for color compensation calibrations in digital cameras, color printing, and other applications. 
     The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to an individual in the art are included within the scope of the invention as defined by the accompanying claims.