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A model for spectra of the phosphor-coated white LED (p-W LED) with a blue chip, a red chip, and green and yellow phosphors is presented. The optimal spectra of p-W LEDs with correlated color temperatures (CCTs) of 2700–6500 K have been obtained with a nonlinear program for maximizing luminous efficacy of radiation (LER) under conditions of both color-rendering indices (CRIs) and special CRIs of R9 strong red above 98. The simulation results show that p-W LEDs with one InGaN blue (450 nm) chip, one AlGaInP red (634 nm) chip, and green (507 nm) and yellow (580 nm) silicate phosphors can realize white lights with CRIs of about 98 and special CRIs of R9 for strong red above 98. The average of the special CRIs R9 to R12 for the four saturated colors (red, yellow, green, and blue) is above 95. R13 for the skin of women’s faces at about 100, as well as LERs above 296 lm/W at CCTs of 2700–6500 K. LERs of excellent CRI p-W LEDs with one InGaN blue chip, one AlGaInP red chip, and green and yellow silicate phosphors increased by 19–49% when compared with that of excellent CRI p-W LEDs with one InGaN blue chip and green and yellow silicate phosphors, as well as red nitride phosphor.
©2011 Optical Society of America
1. Introduction
It has been projected that semiconductor white LEDs will broadly replace conventional incandescent and fluorescent lamps for general lighting in the future due to their potential for substantial energy savings, high efficiency, small size, and long lifetime [1]. An important factor is how well a light source renders the true colors of objects. This factor is known as the color rendering index (CRI) [2]. Another important aspect to consider is the luminous efficacy (lumens per watt). The term luminous efficacy is normally used as the conversion efficiency from the input electrical power (watt) to the output luminous flux (lumin), and is hereinafter denoted as LE. The LE of a source is determined by two factors: the conversion efficiency from electrical power to optical power (called radiant efficiency) and the conversion factor from optical power (watt) to luminous flux (lumen). The latter is called luminous efficacy of radiation (lumen/watt) and is hereinafter denoted as LER. There are two different approaches for producing white light using LEDs. In the first approach, emission from multiple single color LEDs is additively mixed to generate white light [3,4]. Since there is no loss due to the down-conversion, this approach offers white light sources with potentially very high LE. Theoretically dichromatic white light sources are most efficient, offering an LER of > 440 lm/W [5]. However, the CRI of dichromatic sources is low. The CRI can be improved dramatically by increasing the number of primary-color LEDs for a white source [6,7]. However, sources with a greater number of primary-color LEDs have a lower LE. In the second approach, which employs phosphors, partial down-conversion of high-energy photons to lower-energy photons takes place in the phosphor. This type of approach has the advantage of a compact package, a single power supply, and a high CRI due to the broad emission spectrum of a phosphor [8–10]. The approach has great color stability, particularly if a UV source is used to excite the phosphor. However, the phosphor-coated white LEDs (p-W LEDs) suffer from limited LE due to down-conversion and a relatively broad emission spectra. It was reported that p-W LEDs by use of dual-blue emitting active regions can be tuned to achieve higher CRIs and luminous efficacies of radiation, as opposed to single blue white sources [11]. One problem with the CRI is that it can give fairly high scores to sources that render some saturated object colors very poorly. An improved indicator, a color quality scale (CQS), has recently been proposed by the National Institute of Standards and Technology (NIST). However, the CQS provides scores consistent with the CRI for most recent phosphor-type LED products [12]. So the CRI as a metric for evaluating the color rendering abilities of white light sources is suitable for p-W LEDs. In this investigation, a solution for producing white light using the p-W LED with a blue chip, a red chip, and green and yellow phosphors is proposed in order to overcome the low conversion efficiency and LE of red phosphor. To determine optimized peak wavelengths of a blue chip, a red chip, and green and yellow phosphors for maximizing the LER while the CRI and the special CRI of R9 for strong red is above 98, a model for the spectra of the p-W LED with blue and red chips is presented. The special CRI R9 is considered because the red–green contrast is very important for color rendering [13,14], and red tends to be problematic. Lack of a red component shrinks the reproducible color gamut and makes the illuminated scene look dull. The p-W LEDs with an InGaN blue chip, a AlGaInP red chip, and green and yellow silicate phosphors are simulated at CCTs of 2700–6500 K, and the simulation results are presented. As compared with the p-W LEDs using red phosphor, the p-W LED consisting of an InGaN blue chip, green and yellow silicate phosphors, as well as a red nitride phosphor at CCTs of 2700–6500 K are also simulated.
2. Model for spectra of a p-W LED with blue and red chips
The relative spectral power distribution (SPD) of a single color LED, , was given by [15]
where ;; (i = 1,2); λ0 refers to the peak wavelength; Δλ1 refers to the left half spectral width (HSW), which is ; Δλ2 refers to the right HSW, which is ; Δλ0 refers to the HSW, which is (Δλ1 + Δλ2)/2; and ki (i = 1,2) are characteristic parameters of the spectral shape.In order to overcome the low conversion efficiency and low LE of red phosphor, we adopt a red chip instead of a red phosphor. The mixture of green and yellow phosphors is coated on the blue and red chips. Owing to the emission spectra of yellow and green phosphors, blue and red spectra transmitted through phosphors are mixed diffusely into white light. The relative SPD of a p-W LED consisting of a blue chip, a red chip, and green and yellow phosphors, Sp-W(λ), is given by
where Sp-W(λ) refers to the spectrum of the p-W LED; SB(λ) and SR(λ) refer to the blue and red spectra transmitted through the phosphors and transparent encapsulated material, respectively; Sg(λ) and Sy(λ) refer to the emission spectra of green and yellow phosphors, respectively; and qB, qg, qy, and qR are proportions of the relative emission spectra of the blue chip, green phosphor, yellow phosphor, and the red chip, respectively. SB(λ), Sg(λ), Sy(λ), and SR(λ) can be expressed by Eq. (1). The model and real SPDs of typical green, yellow, and red phosphors are shown in Fig. 1 .3. Optimization of p-W LEDs with blue and red chips
To analyze the possible performance of the p-W LED with blue and red chips, a simulation program is developed according to the principle of additive color mixture and the CIE method of measuring and specifying color-rendering properties of light sources [2]. Consider an SPD that contains emission spectrum from a blue chip, a red chip, and green and yellow phosphors in the p-W LED. We employ the model SPD of Eq. (1) for blue and red chips as well as for the phosphors. Subjecting the 2 × 4-dimensional parameter space to three-color mixing constrains results in the location of feasible vectors on the hyper-surface with 5 dimensionality [16]. In order to optimize the spectra of the p-W LED with excellent color rendering, we introduce an objective function
where λB, λg, λy, and λR are wavelengths of a blue chip, green phosphor, yellow phosphor, and a red chip, respectively. Hence, the optimization problem reduces to finding the maxima of the objective function under conditions of CRI ≥ 98 and R9≥ 98. Since photon energy linewidths of InGaN blue and AlGaInP red chips are about 5.5 kT and 2.0 kT, respectively, their half-spectral widths are assumed to be 30 nm and 20 nm, respectively. Eleven green (507–546 nm) and nine yellow/orange (554–606 nm) silicate phosphors are used in optimization. The real emission spectra of these silicate phosphors are shown in Fig. 2 . The simulation program can determine the optimized peak wavelengths of InGaN blue and AlGaInP red chips, green and yellow silicate phosphors, and the relative radiation fluxes (Φe %) of spectra from InGaN blue and AlGaInP red chips. It also can determine the emission spectra from green and yellow silicate phosphors according to the requirements of CCT and the distance from the Planckian locus on the CIE 1960 UV chromaticity diagram (dC) with polarity, plus (above the Planckian locus) or minus (below the Planckian locus) [2], by a well-known nonlinear program for maximizing the LER under conditions of both CRI and R9 above 98.
Fig. 2 Relative emission spectra of eleven green (507–546 nm), nine yellow/orange (554 –606 nm) silicate phosphors and six red (631–667 nm) nitride phosphors.
To validate the simulation program for the p-W LED with InGaN blue and AlGaInP red chips, the WW LED (CCT = 2730K, dC = - 0.0013, CRI = 93.6) consisting of one InGaN blue chip (λB = 452.8 nm and Δλ0B = 29.8 nm), one AlGaInP red chip (λR = 623.9 nm and Δλ0R = 19.9 nm), and green (λg = 528 nm and Δλ0g = 81.7 nm) and yellow (λg = 580 nm andΔλ0y = 81.1 nm) silicate phosphors is tested. The simulation results show that qB, qR, qg, and qy are 0.3060, 0.8536, 0.2017, and 0.2724, and that the relative radiation fluxes of the InGaN blue chip, the red AlGaInP chip, and the green and yellow silicate phosphors are 13.5%, 26.7%, 18.6%, and 41.2%, respectively. The predicted and measured SPDs of the WW LED are shown in Fig. 3 and the predicted and measured special CRIs of R1 to R14 of the WW LED are shown in Fig. 4 . The SPD of the WW LED is measured by using a spectro-radiometer with an integrating sphere. The results show that the predicted spectra of the WW LED is very close to the measured one and that the predicted special CRIs of R1 to R14 are almost equal to the measured values.
The excellent CRI as well as high LER p-W LEDs with InGaN blue and AlGaInP red chips at CCTs of 2700–6500 K have been found by simulation analysis. The optimized peak wavelengths of the InGaN blue chip, the AlGaInP red chip, and the green and yellow silicate phosphors are 450 nm, 634 nm, 507 nm, and 580 nm, respectively. The relative SPDs of the InGaN blue (λB = 450.0 nm and Δλ0B = 30.0 nm) chip, the AlGaInP red (λR = 634.0 nm and Δλ0R = 20.0 nm) chip, and the green (λg = 507.0 nm and Δλ0g = 66.1 nm) and yellow (λy = 580.0 nm and Δλ0y = 81.1 nm) silicate phosphors are shown in Fig. 5 . The simulation results of p-W with InGaN blue and AlGaInP red chips are shown in Table 1 . The optimal SPDs of p-WLEDs with InGaN blue and AlGaInP red chips at CCTs of 2700–6500 K are shown in Fig. 6 . Table 1 indicates that p-W LEDs with InGaN blue and AlGaInP red chips could realize white lights with CRIs about 98 and special CRIs of R9 for strong red above 98. The average of the special CRIs R9 to R12 for the four saturated colors (red, yellow, green, and blue) are above 95, R13 for women’s faces about 100, and LERs above 296 lm/W, as well as dCs below 0.0001 at CCTs of 2700–6500 K. Simulation results show that the p-W LEDs with InGaN blue and AlGaInP red chips could achieve excellent color-rendering properties as well as high luminous efficacy of radiation.
Fig. 5 Relative SPDs of a InGaN blue (λB = 450.0 nm and Δλ0B = 30.0 nm) chip, a AlGaInP red (λR = 634.0 nm and Δλ0R = 20.0 nm) chip, green (λg = 507.0 nm and Δλ0g = 66.1 nm) and yellow (λy = 580.0 nm and Δλ0y = 81.1 nm) silicate phosphors.
Table 1. Simulation results of p-W LEDs with the InGaN blue (λB = 450.0 nm and Δλ0B = 30.0 nm) chip, the AlGaInP red (λR = 634.0 nm and Δλ0R = 20.0 nm) chip, and Green (λg = 507.0 nm and Δλ0g = 66.1 nm) and Yellow (λy = 580.0 nm and Δλ0y = 81.1 nm) Silicate Phosphors
Fig. 6 Optimal SPDs of p-W LEDs with the InGaN blue (λB = 450.0 nm and Δλ0B = 30.0 nm) chip, the AlGaInP red (λR = 634.0 nm and Δλ0R = 20.0 nm) chip, green (λg = 507.0 nm and Δλ0g = 66.1 nm) and yellow (λy = 580.0 nm and Δλ0y = 81.1 nm) silicate phosphors at different CCTs. (a) CCT = 2700 K. (b) CCT = 3000 K. (c) CCT = 3500 K. (d) CCT = 4000 K. (e) CCT = 4500 K. (f) CCT = 5000 K. (g) CCT = 5700 K. (h) CCT = 6500 K.
As compared with the p-W LEDs using red phosphor, the p-W LED consisting of a blue chip and green, yellow, and red phosphors at CCTs of 2700–6500 K are simulated. Six red nitride phosphors (631–667 nm) are used in optimization. The real emission spectra of six red nitride phosphors are shown in Fig. 2. The optimized peak wavelengths of the InGaN blue chip, green and yellow silicate phosphors, as well as red nitride phosphor are 450 nm, 507 nm, 580 nm, and 655 nm, respectively. The simulation results of p-W LEDs with the InGaN blue chip (λB = 450.0 nm and Δλ0B = 32.0 nm), green (λg = 507.0 nm and Δλ0g = 66.1 nm) and yellow (λy = 580.0 nm and Δλ0y = 81.1 nm) silicate phosphors, as well as red (λr = 655.0 nm and Δλ0r = 95.3 nm) nitride phosphor are shown in Table 2 . The optimal SPDs of p-W LEDs with the red nitride phosphor at CCTs of 2700–6500 K are shown in Fig. 7 . Table 2 indicates that p-W LEDs with the red nitride phosphor could realize white lights with CRIs about 98, special CRIs of R9 about 99, the average of the special CRIs R9 to R12 for the four saturated colors (red, yellow, green, and blue) above 95, R13 for women’ faces above 96, but LERs below 250 lm/W. The color rendering properties of p-W LEDs with the red nitride phosphor are as excellent as that of p-W LEDs with the AlGaInP red chip. However, LERs of excellent CRI p-W LEDs with the AlGaInP red chip increase by 19–49% compared with that of excellent CRI p-W LEDs with red nitride phosphor because the half-spectral width of the spectra from the AlGaInP red chip is narrower than that of the emission spectra from red nitride phosphor.
Table 2. Simulation Results of p-W LEDs with the InGaN Blue Chip (λB = 450.0 nm and Δλ0B = 30.0 nm), Green (λg = 507.0 nm and Δλ0g = 66.1 nm) and Yellow (λy = 580.0 nm and Δλ0y = 81.1 nm) Silicate Phosphors, and Red (λr = 655.0 nm and Δλ0r = 95.3 nm) Nitride Phosphor
Fig. 7 Relative SPDs of p-W LEDs with the InGaN blue chip (λB = 450.0 nm and Δλ0B = 30.0 nm), green (λg = 507.0 nm and Δλ0g = 66.1 nm) and yellow (λy = 580.0 nm and Δλ0y = 81.1 nm) silicate phosphors, as well as red (λr = 655.0 nm and Δλ0r = 95.3 nm) nitride phosphor at different CCTs. (a) CCT = 2700 K. (b) CCT = 3000 K. (c) CCT = 3500 K. (d) CCT = 4000 K. (e) CCT = 4500 K. (f) CCT = 5000 K. (g) CCT = 5700 K. (h) CCT = 6500 K.
Also, excellent CRI and high LER white lights could be realized by white/red (W/R) LED clusters consisting of red LEDs and p-W LEDs (with an InGaN blue chip and green and yellow silicate phosphors). The CCTs of p-W LEDs in W/R LED clusters, relative radiation fluxes, and relative luminous fluxes of red LEDs and p-W LEDs in W/R LED clusters are shown in Table 3 . The optimal SPDs of p-W LEDs in W/R LED clusters at different CCTs are shown in Fig. 8 .
Table 3. CCTs of p-W LEDs (with a InGaN Blue Chip, Green and Yellow Silicate Phosphors), Relative Radiation Fluxes, and Relative Luminous Fluxes of the Red LEDs and p-W LEDs in the W/R LED Clusters
Fig. 8 Relative SPDs of p-W LEDs in W/R clusters at different CCTs. (a) CCT = 4167 K. (b) CCT = 4474 K. (c) CCT = 5001 K. (d) CCT = 5543 K. (e) CCT = 6123 K. (f) CCT = 6652 K. (g) CCT = 7447 K. (h) CCT = 8396 K.
We think that the simulated results are very important for solid-state lighting. The high-LE LED light source with CRI at about 98, R9 for strong red above 98, the average of the special CRIs R9 to R12 for the four saturated colors (red, yellow, green, and blue) above 95, and R13 for skin of women’s faces at about 100 will become ideal illuminations such as Planck’s blackbody radiator and daylight.
4. Conclusion
The optimal spectra of p-W LEDs with one blue InGaN chip, one red AlGaInP chip, and green and yellow silicate phosphors at CCTs of 2700–6500 K have been obtained by a nonlinear program for maximizing LER under conditions of both CRI and R9 above 98. The optimized peak wavelengths of the blue InGaN chip, the red AlGaInP chip, and green and yellow silicate phosphors are 450 nm, 634 nm, 507 nm, and 580 nm, respectively. The simulation results show that the p-W LEDs with the blue InGaN chip, the red AlGaInP chip, and green and yellow silicate phosphors could realize white lights with CRIs about 98, special CRIs of R9 above 98. The average of the special CRIs R9 to R12 for the four saturated colors (red, yellow, green, and blue) was above 95, R13 for women’s faces at about 100, and for LERs it was above 296 lm/W at CCTs of 2700–6500 K. LERs of excellent CRI p-W LEDs with one blue InGaN chip, one red AlGaInP chip, and green and yellow silicate phosphors increased by 19– 49% when compared with that of excellent CRI p-W LEDs with one blue InGaN chip, green and yellow silicate phosphors, as well as red nitride phosphors.
Acknowledgments
This work was supported by Shanghai Science and Technology Committee (No. 09DZ1141100) and the National “ITER” Project of Ministry of Science and Technology of China (No. 2009GB107006 and No. 2010GB107003).
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