Special Color Rendering Index Research Articles

A single‐structured LuAG:Ce,Mn phosphor ceramics with high CRI for high‐power white LEDs

AbstractThe realization of high color rendering index (CRI) is still a great challenge for high‐power LEDs (hp‐LEDs), which is hindered by the phosphor converter. In this work, based on the strategy of Ce3+ and Mn2+ multi‐ion substitution, the single‐structured LuAG:Ce,Mn ceramics with high CRI were prepared via regulating the ratio of tri‐color (red, green, and blue) components. The effects of Mn2+‐Si4+ pairs doping content on the crystal structure, morphologies, and luminescence properties were investigated in detail. The red emission centered at 590 and 750 nm were effectively compensated by regulating Mn2+ occupancy sites, resulting in a significant improvement of CRI. Pure white light with general CRI Ra up to 91.0, special CRI R9 reaching 37.9 and LE as high as 85.07 lm/W was achieved, when the hp‐LEDs were constructed from related phosphor ceramic Ce02Mn7. These results suggest that the LuAG:Ce,Mn phosphor ceramics are highly promising color converters for hp‐LEDs application.

Evaluation of changing the components involved in CIE color rendering index

Various improvement methods have been proposed to cope with the deficiencies of the Commission Internationale de l’Eclairage (CIE) color rendering index (CRI). However, standardizing these methods lack quantitative evidences. Accordingly, with numerous light sources having rich spectral properties, the changes in CRI scores before and after improvement were quantitatively evaluated in terms of six critical components, including reference illuminant, standard observer, chromatic adaptation transform, uniform color space, color difference combination, and the rescaling operation, involved in the CIE CRI calculation. The results indicate that the largest change in CRI scores occurs in improving uniform color space, especially with CAM02-UCS, and that the CRI scores of the sources with highly-structured spectra tend to be changed considerably by most improvement methods. Compared with the other CRIs, the change in the special CRI R9 is frequently much larger for almost all the improvement methods. The least change in CRI scores is caused by partly updating reference illuminants as the mixed illuminants of the Planckian radiation and CIE daylight, which should be the most acceptable improvement. Additionally, adopting a rescaling function with the original scaling factor could be a candidate for avoiding negative CRI scores without leading to significantly large change in positive scores.

Phosphor-free, color-mixed, and efficient illuminant: Multi-chip packaged LEDs for optimizing blue light hazard and non-visual biological effects

Currently many evaluation models on the photobiological effects (PBE) of light sources do not consider the influence of age and luminance on the pupil diameter, which affects the light radiation intensity on the human retina. In this study, the pupil diameter is taken into consideration when evaluating the PBE of several light sources. Moreover, the correction factor M is proposed. The blue light hazard (BLH) efficacy and the circadian rhythm (CR) effects of the daylight at dusk, together with three indoor light sources with a correlated color temperature (CCT) of about 3000 K were evaluated by using a corrected evaluation model. The results show that an incandescent lamp is more photobiologically friendly for humans, despite being inefficient. Based on high wall-plug efficiency (WPE) GaN-based yellow (565 nm, 24.3%@20 A/cm2) and green (522 nm, 41.3%@20 A/cm2) LEDs on silicon substrate, incandescent-like spectrum and phosphor-free color-mixed white LEDs (CM-LEDs) with a general color rendering index (CRI) of 94, a CCT of 2866 K, and an efficiency of 131 lm/W were manufactured by mixing blue, cyan, green, yellow and red LEDs. The PBE evaluation results of such CM-LEDs are superior to those of an incandescent lamp. Moreover, blue light free and candlelight-toned LEDs with an efficiency of 120.3 lm/W, a general CRI of 84, a special CRI R9 of 93.3, and a CCT of 1810 K were fabricated by mixing yellow and red LEDs (R&Y-mixed LEDs). The R&Y-mixed LEDs show no blue light weighted quantities and have a weaker influence on the CR shift. They are photobiologically friendly for humans and suitable for nocturnal indoor and outdoor lighting environments.

Improving the color-rendering index of a tandem warm white organic light-emitting device by employing a simple fabrication process.

We realized an efficient spectra-stable tandem white organic light-emitting device (WOLED) with pleasurable warm white emission and an extremely high color-rendering index (CRI) simultaneously by connecting a fluorescent blue unit and a phosphorescent dual-color unit via an easy-fabrication charge generation unit (CGU). Over a wide range of driving current density, the tandem warm white OLED exhibited general CRI (Ra) value exceeding 85 and special CRI (R9) value close to 57. In addition, Duvs are within the tolerance for CRI measurements, ensuring validity and logicality of the calculated results concerning Ra and R9. The obtained tandem OLED showed the maximum efficiencies of 38cd/A and 22.9lm/W, and still maintained high efficiencies of 35.5cd/A and 17.2lm/W at the luminance of 1000 cd/m2. Moreover, we studied the origins of both extreme color stability in dual-color OLED and extremely efficient electron injection within the CGU. To the best of our knowledge, this is the first report regarding quantitative values of both R9 and the Duv for a high-Ra tandem WOLED.

Realizing an efficient warm white organic light-emitting device possessing excellent color-stability and color rendering index

Abstract We were successful in realizing an efficient warm white organic light-emitting device simultaneously exhibiting extreme color-stability and high color rendering index (CRI) just by precisely confining the recombination zone and fully utilizing the energy transfer between excitons. The warm white OLED presents maximum efficiencies of 21.4 cd/A, 25.8 lm/W and still remains at high efficiencies of 17.7 cd/A, 15.6 lm/W at the luminance of 1000 cd/m2. As the operating current density increases from 10 mA/cm2 to 60 mA/cm2, the variations in Commission Internationale de l’Eclairage (CIE) coordinates are only (0.004, 0.007) while the general CRI (Ra) maintains at 80 approximately and the special CRI R9 is higher than 20. What is more, the Duv satisfies the criteria for CRI measurements and the gamut area index (GAI) ensures the vividness of the illuminated objects, as renders the fabricated white OLED a promising candidate for high-quality solid-state lighting application. We also studied the origin of the quite slight color variations for the warm white OLED.

2D photonic crystal layer assisted thiosilicate ceramic plate with red‐emitting film for high quality w‐LEDs

AbstractIn this work, we have succeeded in obtaining high quality warm w‐light‐emitting‐diodes (LEDs) by adopting hybrid two‐dimensional (2D) structure of SiNx photonic crystal layer (PCL) assisted cyan‐emitting ceramic‐plate thiosilicate SrLa2Si2S8:Ce3+ with red‐emitting film SrLiAl3N4:Eu2+ phosphor on a 430 nm blue LED chip at 350 mA. 2D SiNx PCL was capped with thiosilicate is because it can enhance the luminous efficacy and maintain the low correlated color temperature (CCT) and high color‐rendering index (CRI). High luminous efficacy (82.3 lm/W), high special CRI (R9=75) as well as the low CCT (5431 K) of the optimal w‐LED was obtained due to the assistances of 2D SiNx PCL and narrow‐band red‐emitting phosphor with the doping percentage at 10 wt%. The synthesis processes, structural analysis, optical properties and LED device performances were detailed investigated to find out the relationship between the optimum composition and good optical properties. Based on intriguing luminescence properties by the 2D SiNx PCL and red‐emitting film phosphor introducing, we proclaim this method could also have high potential application in other phosphor‐converted w‐LEDs.

Color Temperature Tunable White LED Cluster With Color Rendering Index Above 98

A photometric optimization model for correlated color temperature (CCT) tunable white LED clusters, including down-conversion energy loss, was developed under the constraint of color rendering index (CRI). The CCT tunable R/Y/C white LED cluster, which consists of direct-emission red LEDs, phosphor-coated yellow LEDs (pc-Y LEDs) with green and orange phosphors excited by a blue chip, and pc-C LEDs with green phosphors excited by a blue chip, was obtained under conditions of both CRI and special CRI of R9 for strong red above 98 at CCTs from 2700 to 6500 K. The optimal peak wavelengths of red LED, blue chip, green and orange phosphors, and optimal pc-Y LED as well as pc-C LED are presented. A CCT tunable R/Y/C white LED cluster with CRIs of 97-99, R9s of 98-99, CQSs of 95-97, and LEs of 122-132 lm/W was realized at CCTs from 2731 to 6533 K, which could become an ideal light source for application as Planck’s blackbody radiator or daylight source.

Study of Perovskite QD Down-Converted LEDs and Six-Color White LEDs for Future Displays with Excellent Color Performance.

A narrow-emitting red, green, and blue (RGB) perovskite quantum dot (PeQD)-based tricolored display system can widen the color gamut over the National Television System Committee (NTSC) to 120%, but this value is misleading with regard to the color perception of cyan and yellow reproduced in the narrow RGB spectra. We propose that a PeQD-based six-color display system can reproduce true-to-life spectral distributions with high fidelity, widen the color gamut, and close the cyan and yellow gap in the RGB tricolored display by adding cyan (Cy), yellowish green (Yg), and orange colors (Or). In this study, we demonstrated pure-colored CsPbX3 (X = Cl, Br, I, or their halide mixtures; Cl/Br and Br/I) PeQD-based monochromatic down-converted light-emitting diodes (DC-LED) for the first time, and we incorporated PeQDs with UV-curable binders and long-wavelength-pass-dichroic filters (LPDFs). CsPbX3 PeQD-based pure Cy-, G-, Yg-, Or-, R-emitting monochromatic DC-LED provide luminous efficacy (LE) values of 81, 184, 79, 80, and 35 lm/W, respectively, at 20 mA. We also confirmed the suitability and the possibility of access to future color-by-blue backlights for field-sequential-color liquid crystal displays, using six-color multipackage white LEDs, as well as future six-colored light-emitting devices with high vision and color performance. The fabricated six-color multipackage white LEDs exhibited an appropriate LE (62 lm/W at total 120 mA), excellent color qualities (color rendering index (CRI) = 96, special CRI for red (R9) = 97) at a correlated color temperature (CCT) of 6500 K, and a wide color gamut covering the NTSC up to 145% in the 1931 Commission International de l'Eclairage (CIE) color coordinates space.

Synthesis and characterization of green Zn-Ag-In-S and red Zn-Cu-In-S quantum dots for ultrahigh color quality of down-converted white LEDs.

Eco-friendly green Zn-Ag-In-S (ZAIS) and red Zn-Cu-In-S (ZCIS) core/shell-like alloyed quantum dots (QDs) have been synthesized by a facile hot-injection method with a multiple injection approach. Broad full-width at half-maximum (fwhm) of the photoluminescence (PL) emission and tunability of the green ZAIS and red ZCIS QDs were obtained by adopting a low-temperature core growth and high-temperature multiple alloyed reaction. The alloyed green ZAIS and red ZCIS QDs reached PL quantum yields as high as 0.61 and 0.53; fwhm of the PL peaks were as wide as 81 and 106 nm, respectively. This demonstrates the practical realization of white down-converted light-emitting diodes (DC-LEDs), fully covering the whole visible wavelength range and the cyan gap, using two broad fwhm green ZAIS and red ZCIS QDs. We also characterized the vision and color performance using luminous efficacy (LE), color rendering index (CRI), special CRI for strong red (R9), and color quality scale (CQS) of white DC-LEDs incorporated with green ZAIS and red ZCIS QDs at the correlated color temperature (CCT) range of 2700-10 000 K. The tricolor white DC-LED using broad fwhm green-emitting ZAIS and red-emitting ZCIS core/shell-like alloyed QDs exhibits a moderate LE (31.2 lm/W) and ultrahigh color qualities (CRI = 97, R9 = 97, and CQS = 94) with warm white at a CCT of 3500 K.

Hybrid 2D photonic crystal-assisted Lu3Al5O12:Ce ceramic-plate phosphor and free-standing red film phosphor for white LEDs with high color-rendering index.

This paper reports the combined optical effects of a two-dimensional (2D) SiNx photonic crystal layer (PCL)-assisted Lu3Al5O12:Ce (LuAG:Ce) green ceramic-plate phosphor (CPP) and a free-standing (Sr,Ca)AlSiN3:Eu red film phosphor to enhance luminous efficacy, color rendering index (CRI), and special CRI (R9) of LuAG:Ce CPP-capped white light-emitting diodes (LEDs) for high-power white LEDs at 350 mA. By introducing the 2D SiNx PCL, the luminous efficacy was improved by a factor of 1.25 and 1.15 compared to that of the conventional flat CPP-capped LED and the thickness-increased CPP-capped LED (with a thickness of 0.15 mm), respectively, while maintaining low color-rendering properties. The combining of the free-standing red film phosphor in the flat CPP-capped, the 2D PCL-assisted CPP-capped, and the thickness-increased CPP-capped LEDs led to enhancement of the CRI and the special CRI (R9); it also led to a decrease of the correlated color temperature (CCT) due to broad wavelength coverage via the addition of red emission. High CRI (94), natural white CCT (4450 K), and acceptable luminous efficacy (71.1 lm/W) were attained from the 2D PCL-assisted LuAG:Ce CPP/free-standing red film phosphor-based LED using a red phosphor concentration of 7.5 wt %. It is expected that the combination of the 2D PCL and the free-standing red film phosphor will be a good candidate for achieving a high-power white CPP-capped LED with excellent CRI.

Spectral optimization of color temperature tunable white LEDs with excellent color rendering and luminous efficacy.

The optimization model of limited luminous efficacy (LLE) for correlated color temperature (CCT) tunable phosphor-coated white light-emitting diodes (pc-W LEDs) consisting of blue chips, green and yellow phosphors, and red chips, including downconversion energy loss, has been developed under the constraint of a designated color-rendering index (CRI) and a special CRI of R9 for strong red. The optimal spectra of pc-W LEDs are obtained with a nonlinear program for maximizing LLE under conditions of both CRI and R9 above 90, 95, and 98 at CCTs of 2700-6500 K. The pc-W LEDs with LLE>301 lm/W for both CRI and R9 above 90, LLE>290 lm/W for both CRI and R9 above 95, and LLE>276 lm/W for both CRI and R9 above 98 could be achieved at CCTs of 2700-6500 K. The recommended peak wavelengths and full widths at half-maximum (FWHMs) for the current LEDs and phosphors and their photometric and colorimetric performances are presented.

Photobiologically Safe High Color Rendering Index White Organic Light-Emitting Devices

A high color rendering index (CRI) white organic light-emitting devices with photobiological safety is demonstrated. The effect of the active layer thickness on CRI and blue light hazard is demonstrated. The optimized device shows a high general CRI of 97, a special CRI R 9 of 94, and a power efficiency of 15.2 lm/W. The correlated color temperature is 2161. The device could be classified into low risk group at ~2000 cd/m 2 , which is proved to be safe for our eyes and could be applied to indoor illumination.

색온도 가변 LED 조명 최적화 설계 및 제작

본 논문에서는 현재 양산되고 있는 Red(621 nm), Green(530 nm), Blue(453 nm), Amber(590 nm) 등 4파장의 LED 조합을 이용하여 색온도가 가변되는 동안에도 평균연색지수 및 특별연색지수 R9 값이 높게 유지될 수 있도록 조명의 스펙트럼을 최적화하였다. 최적화 시 설계의 정확도를 높이기 위하여 실제 측정한 LED 스펙트럼을 사용하였고 광선과 기구물 사이의 상호 작용을 고려한 광선 추적 기법을 활용하여 수행하였다 또한 최적화 결과를 검증하기 위하여 색온도 가변 조명 장치를 직접 제작하였으며 광 특성을 평가한 결과 색온도가 3000 K에서 6000 K까지 변하는 동안 CRI 및 R9 값이 각각 87~90, 34~93을 나타내었다. In this paper the spectra of correlated color temperature (CCT) tunable white light-emitting diode (LED) luminaires, consisting of commercial red, green, blue, and amber LED chips, were optimized to increase color rendering index (CRI), and a special CRI of R9 for deep red color was obtained. To improve the design's accuracy, measured LED spectra were used instead of mathematically modeled ones. Real CCT tunable LED luminaires with CRIs of 87-90 and R9s of 34-93 were fabricated and demonstrated at CCTs of 3000-6000 K.

Color Temperature Tunable White-Light LED Cluster with Extrahigh Color Rendering Index

The correlated color temperature (CCT) tunable white-light LED cluster with extrahigh color rendering property has been found by simulation and fabricated, which consists of three WW LEDs (CCT = 3183 K), one red LED (634.1 nm), one green LED (513.9 nm), and one blue LED (456.2 nm). The experimental results show that this cluster can realize the CCT tunable white-lights with a color rendering index (CRI) above 93, special CRI R9 for strong red above 90, average value of the special CRIs of R9 to R12 for the four saturated colors (red, yellow, green, and blue) above 83, and luminous efficacies above 70 lm/W at CCTs of 2719 K to 6497 K.

High general and special color rendering index white organic light-emitting device with bipolar homojunction emitting layers

High general and special color rendering index (CRI) together with gamut area index (GAI) is demanded for lighting source. In a tolerable Duv, an efficient warm white organic light-emitting device (WOLED) with a high CRI and GAI is developed by using a bipolar homojunction emitting layers, i.e., junction between different emitting layers with the same bipolar host material. At a low drive voltage of 4V, the WOLED shows a high luminance of 1594cd/m2 and a power efficiency of 12.95lm/W, and, it shows a high general CRI of 82, a special CRI R9 of 75, an average of R9 to R12 of 69 and a GAI of 95. Besides, the WOLED shows relative stable color coordinates due to the elimination of the charges accumulation at interface between different emitting layers. And the current efficiency roll-off of the WOLED is also reduced due to the balance of carrier injection and transport in emitting layers.

Spectral optimization of the color temperature tunable white light-emitting diode (LED) cluster consisting of direct-emission blue and red LEDs and a diphosphor conversion LED

The correlated color temperature (CCT) tunable white-light LED cluster, which consists of direct-emission blue and red LEDs as well as phosphor-conversion (PC) LEDs packaged by combining green and orange phosphors with a blue LED die, has been obtained by nonlinear program for maximizing luminous efficacy (LE) of radiation (LER) under conditions of both color rendering index (CRI) and special CRI of R9 for strong red above 90 at CCTs of 2700 K to 6500 K. The optimal peak wavelengths of blue LED, red LED, blue LED die, green and orange phosphors are 465 nm, 628 nm, 452 nm, 530 nm and 586 nm, respectively. The real CCT tunable PC/red/blue LED cluster with CRIs of 90~96, R9s of 90~96, CQSs of 89~94, LERs of 303~358 lm/W, and LEs of 105~119 lm/W has been realized at CCTs of 2722 K to 6464 K. The deviation of the peak wavelength should be less than ± 5 nm for blue LED die, ± 1 nm for red LED, and ± 2 nm for blue LED to achieve the PC/R/B LED cluster with high optical performance.

Optimal spectra of white light-emitting diodes using quantum dot nanophosphors

The relationship and trade-offs between the performance parameters including color rendering index (CRI), luminous efficacy of radiation (LER) and correlated color temperature (CCT) of white LEDs using quantum dot nanophosphors (QD-WLEDs) are investigated for CRI ≥ 80 and LER ≥ 300 lm/W at 1500 K ≤ CCT ≤ 6500 K. The optimal spectra of QD-WLEDs with CCTs of 2700-6500 K have been obtained with a nonlinear program for maximizing LER under conditions of both CRI and a special CRI of R9 strong red above 90 or 95. Furthermore, high performance QD-WLEDs with LER = 381 lm/W for CRI = R9 = 90 and LER = 371 lm/W for CRI = R9 = 95 at CCT = 3000 K, with LER = 361 lm/W for CRI = R9 = 90 and LER = 352 lm/W for CRI = R9 = 95 at CCT = 4500 K, and with LER = 346 lm/W for CRI = R9 = 90 and LER = 338 lm/W for CRI = R9 = 95 at CCT = 5700 K could be achieved. The LERs of high performance white LEDs using QD nanophosphors increase by 13% to 32% compared with that of white LEDs using traditional phosphors.

Spectral optimization of warm-white light-emitting diode lamp with both color rendering index (CRI) and special CRI of R9 above 90

The optimal spectra of the warm-white LED (WWLED) lamp consisting of AlGaInP red LED and the p-W LED packaged by combining silicate green and orange phosphors with a InGaN blue LED die was obtained by nonlinear program for maximizing the luminous efficacy of radiation while both color rendering index (CRI) and special CRI of R9 for strong red above 90 at correlated color temeratures (CCTs) of 2700 K. The optimal peak wavelengths of red LED, blue LED die, silicate green and orange phosphors are 626 nm, 454 nm, 535 nm and 584 nm, respectively. Their optimal relative radation fluxs are 9.7%, 33.4 %, 26.1 %, 30.8 %, respectively. The real WWLED lamp with CCT of 2653 K, CRI of 90, R9 of 94 and R(9-12) of 88, as well as luminous efficacy (LE) of 80.2 lm/W have been realized. Furthermore, the WWLED lamp can realized CCT tunable warm-white-light with CRIs of 86 ∼ 93, R9s of 86 ∼ 95 and LEs of 78.2 ∼ 80.3 lm/W at CCTs of 2392 K to 3014 K by adjusting drive current of the red LED.

White Complementary Solid-State Lamp

A complementary source of white light composed of a conventional, white, phosphor-conversion light-emitting diode (LED) complemented with red-orange (625 nm) and cyan (505 nm) colored LEDs was modeled and optimized. In comparison with the conventional white LED, the optimal complementary device features a reduction of color temperature from 6725 K to 5400 K and improvement of the general color rendering index (CRI) from 73 to 90 with the minimal special CRI increased from −18 to 77. We demonstrated that the complementary source can be employed as a versatile white lamp with a trade-off between luminous efficiency and color rendering implemented by variation of contributions from the primary LEDs.