Luminous Efficacy Of Radiation Research Articles

Spectral optimization of white light from hybrid metal halide perovskites

Comprehensive studies were carried out to investigate potential applications of hybrid metal halide perovskites in next-generation white light-emitting diodes (LEDs). We investigated the effect of spectral power distributions on the color quality of white light to provide guidelines for designing white LED devices. The white light was obtained by combining appropriate ratios of blue, green, yellow, and red light emitted from hybrid halide perovskites [MAPb(BrxI1-x)3]. The color characteristics of white light were evaluated by calculating CIE 1931 chromaticity coordinates, correlated color temperature (CCT), general color rendering index (Ra), special color rendering indices (R9-R15), Duv, and luminous efficacy of radiation (LER). The high tunability of CCT from 2298 K (warm white) to 8270 K (cool white) with CRI up to 95.4 has been achieved by tuning the ratios of integrated areas of different emissions with extremely small Duv (0.00002-0.0043) indicating the neutral appearance (not greenish or pinkish) of the obtained white light. High LER above 300 lm/W demonstrates that the high vision performance of white LEDs based on perovskite materials. This work provides strong motivation and guidelines for further developing this type of materials for white LEDs with the goal of realizing widespread adoption of white LEDs in general illumination market.

HF-Free Synthesis of Li2SiF6:Mn4+: A Red-Emitting Phosphor.

Li2SiF6:Mn4+ was synthesized via a new HF-free synthesis route by a high-pressure/high-temperature doping experiment at 5.5 GPa and 750 °C. It is proven that the phosphor cannot be synthesized by the common wet-chemical precipitation route in aqueous HF. The sample was characterized by powder X-ray diffraction, EDX, and luminescence spectroscopy. At room temperature, Li2SiF6:Mn4+ exhibits seven emission lines with the strongest line at λmax ≈ 630 nm and a dominant wavelength of λdom ≈ 618 nm. The CIE coordinates are 0.688 and 0.312 for x and y, respectively. The compound shows a luminous efficacy of radiation (LER) of 218 lm Wopt-1, which exceeds the LER of current state-of-the-art red LED phosphor K2SiF6:Mn4+ by 7% due to a blue-shift of the emission. It reveals excellent thermal quenching behavior up to 125 °C.

Investigation on Three-Hump Phosphor-Coated White Light-Emitting Diodes for Healthy Lighting by Genetic Algorithm

In this paper, we carry out the theoretical spectral optimization and perform experiments by varying temperature and electrical current primarily on one-phosphor-coated three-hump white light-emitting diode (LED) for healthy indoor or outdoor lighting by using the genetic algorithm (GA). White light with low circadian action factor (CAF) is propitious for bedroom lighting to help people relax and to promote sleep, whereas high CAF white light is beneficial for increasing working efficiency in the working place. We obtain low CAF or high CAF according to different application occasions (such as bedroom lighting or office lighting), good Commission Internationale de L'Eclairage color rendering index (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) or Illumination Engineering Society of North America color fidelity index (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> ), and color gamut index (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) or other color performances, and possibly high luminous efficacy of radiation (LER, K) at various correlated color temperatures (CCTs) from 2700 to 6500 K. For instance, in the case of one-phosphor-coated three-hump white LEDs at CCT = 3000 K, the low CAF as 0.264 (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> = 80, R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> = 77, R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> = 103, and K = 350.0 lm/W) can be achieved, whereas that of standard light at the same CCT is 0.407. In addition, we also compare four types of three-hump white LEDs in performances of circadian action, luminous efficacy of radiation (LER), and color rendering. This paper can help both industry and academia understand the application of three-hump white LEDs in the healthy indoor or outdoor lighting.

Quantification of Trichromatic Light Sources to Achieve Tunable Photopic and Mesopic Luminous Efficacy of Radiation

ABSTRACT This study attempts to quantify and optimize the luminous efficacy of radiation (LER) of trichromatic light sources in both photopic and mesopic visions. A new chromaticity-guided rather than spectrum-guided optimization approach is provided. LER contour maps formed by the gamut area of the three light source units are drawn in the chromaticity diagram. Based on these, functional models of LER in photopic and mesopic visions as well as scotopic/photopic (S/P) ratios are established as a function of chromaticity coordinates. Increasing trends for LER and S/P ratios in corresponding contour maps are proposed, which allow quick estimation and synthesis for ideal performance. By using the functional model, the ideal LER, S/P ratio of trichromatic light sources, and their combinations can be achieved. Based on this work, tunable combinations of LER and S/P ratios with chromaticity information can be achieved, which could provide more application possibilities for trichromatic light sources.

Establishing Functional Model of Photometric Performance of Trichromatic Light Sources in Chromaticity Diagrams

In this study, we conducted a combined examination of the photometric and colorimetric performances of trichromatic light sources, which are synthesized by three kinds of light-source units, by transferring their photometric performance to chromaticity diagrams. Contour maps of three photometric performances, which are within a triangle gamut-area formed by three chromaticity coordinates relating to three-light-source-units of a trichromatic light source, were drawn to include the luminous efficacy of radiation (LER) in photopic, scotopic, and cirtopic vision. Functional models of the three different photometric performances as a function of chromaticity coordinates x and y based on the contour maps have been proposed. The functional models quantify all the photometric characteristics in the gamut area and can predict the performances for different synthesis of trichromatic light sources. Using the proposed model, it is possible to evaluate the present performance of trichromatic light sources and provide a method for optimization. An example of evaluation and optimization of a commercial RGB-LED light source was conducted using this proposed approach. Results show that the randomly chosen optimized points in the best optimization ways in the chromaticity diagram for LERs reach 16%, 31%, and 72% improvement in photopic, scotopic, and cirtopic vision, respectively.

Control of Luminescence by Tuning of Crystal Symmetry and Local Structure in Mn4+ -Activated Narrow Band Fluoride Phosphors.

Mn4+ -doped fluoride phosphors have been widely used in wide-gamut backlighting devices because of their extremely narrow emission band. Solid solutions of Na2 (Six Ge1-x )F6 :Mn4+ and Na2 (Gey Ti1-y )F6 :Mn4+ were successfully synthesized to elucidate the behavior of the zero-phonon line (ZPL) in different structures. The ratio between ZPL and the highest emission intensity υ6 phonon sideband exhibits a strong relationship with luminescent decay rate. First-principles calculations are conducted to model the variation in the structural and electronic properties of the prepared solid solutions as a function of the composition. To compensate for the limitations of the Rietveld refinement, electron paramagnetic resonance and high-resolution steady-state emission spectra are used to confirm the diverse local environment for Mn4+ in the structure. Finally, the spectral luminous efficacy of radiation (LER) is used to reveal the important role of ZPL in practical applications.

Control of Luminescence by Tuning of Crystal Symmetry and Local Structure in Mn4+‐Activated Narrow Band Fluoride Phosphors

AbstractMn4+‐doped fluoride phosphors have been widely used in wide‐gamut backlighting devices because of their extremely narrow emission band. Solid solutions of Na2(SixGe1−x)F6:Mn4+ and Na2(GeyTi1−y)F6:Mn4+ were successfully synthesized to elucidate the behavior of the zero‐phonon line (ZPL) in different structures. The ratio between ZPL and the highest emission intensity υ6 phonon sideband exhibits a strong relationship with luminescent decay rate. First‐principles calculations are conducted to model the variation in the structural and electronic properties of the prepared solid solutions as a function of the composition. To compensate for the limitations of the Rietveld refinement, electron paramagnetic resonance and high‐resolution steady‐state emission spectra are used to confirm the diverse local environment for Mn4+ in the structure. Finally, the spectral luminous efficacy of radiation (LER) is used to reveal the important role of ZPL in practical applications.

Blue light hazard performance comparison of phosphor-converted LED sources with red quantum dots and red phosphor

In this study, the blue light hazard performances of phosphor converted-light-emitting diodes (pc-LEDs) with red phosphor and red quantum dots (QDs) were compared and analyzed by spectral optimization, which boosts the minimum attainable blue light hazard efficiency of radiation (BLHER) at high values of color rendering index (CRI) and luminous efficacy of radiation (LER) when the correlated color temperature (CCT) value changes from 1800 to 7800 K. It is found that the minimal BLHER value increases with the increase in the CCT value, and the minimal BLHER values of the two spectral models are nearly the same. Note that the QDs' model has advantages at CCT coverage under the same constraints of CRI and LER. Then, the relationships between minimal BLHER, CRI, CCT, and LER of pc-LEDs with QDs' model were analyzed. It is found that the minimal BLHER values are nearly the same when the CRI value changes from 50 to 90. Therefore, the influence of CRI on minimal BLHER is insignificant. Minimal BLHER increases with the increase in the LER value from 240 to 360 lm/W.

Blue light hazard optimization for white light-emitting diode sources with high luminous efficacy of radiation and high color rendering index

Blue light hazard of white light-emitting diodes (LED) is a hidden risk for human’s photobiological safety. Recent spectral optimization methods focus on maximizing luminous efficacy and improving color performances of LEDs, but few of them take blue hazard into account. Therefore, for healthy lighting, it’s urgent to propose a spectral optimization method for white LED source to exhibit low blue light hazard, high luminous efficacy of radiation (LER) and high color performances. In this study, a genetic algorithm with penalty functions was proposed for realizing white spectra with low blue hazard, maximal LER and high color rendering index (CRI) values. By simulations, white spectra from LEDs with low blue hazard, high LER (≥297lm/W) and high CRI (≥90) were achieved at different correlated color temperatures (CCTs) from 2013K to 7845K. Thus, the spectral optimization method can be used for guiding the fabrication of LED sources in line with photobiological safety. It is also found that the maximum permissible exposure duration of the optimized spectra increases by 14.9% than that of bichromatic phosphor-converted LEDs with equal CCT.

Optimizing spectral compositions of multichannel LED light sources by IES color fidelity index and luminous efficacy of radiation: publisher's note.

This note points out additional funding information that was not added to Appl. Opt.56, 1962 (2017).APOPAI1559-128X10.1364/AO.56.001962.

Optimizing spectral compositions of multichannel LED light sources by IES color fidelity index and luminous efficacy of radiation.

The trade-off between the color fidelity index (Rf) released recently by the Illuminating Engineering Society of North America (IES) and luminous efficacy of radiation (LER) was investigated by adjusting the peak wavelengths, spectral widths, and intensities of four-channel LEDs utilizing a multiobjective optimization algorithm based on differential evolution in the correlated color temperature (CCT) ranging from 2800 to 6500 K for general lighting. The results indicate that Rf at a specific LER value decreases with the increasing CCT, and vice versa, and that Rf has significant improvements over the Commission Internationale de l'Eclairage (CIE) color rendering index (CRI) in avoiding spectral gaming and evaluating the light sources even with negative CIE general CRI (Ra). Further, the optimal peak wavelengths with regard to Rf were identified as 629 nm, 568 nm, 504 nm, and 447 nm, yielding high color rendering in terms of Rf(93∼94) and Ra(95∼97) and relatively excellent LER (299 lm/W∼339 lm/W) over a wide range of CCTs from 2800 to 6500 K. This suggests that Rf can be compatible with Ra, making it possible to obtain a common set of optimal peak wavelengths for Rf and Ra. Besides, the IES method could assess saturated red and skin tones more fairly than the CIE CRI. With a practical 17-channel LED array covering the resulted four optimal peak wavelengths, the improvements of the IES method over the CIE CRI were validated further.

Ideal luminous efficacy and color spatial uniformity of package-free LED based on a packaging phosphor-coated geometry.

This paper presents the optical simulation of the luminous efficacy of radiation (LER) and color spatial uniformity (CSU) of a package-free white LED conducted to boost the LER and simultaneously improve the CSU. According to the simulation results, the main effect on the LER and CSU was the change in the geometrical ratio of phosphor coating. Regardless of the packaging size, when the ratio of the top coating to the sidewall coverage of the phosphor layer thickness was in the range from 0.9 to 1.1, the maximum LER and optimal CSU can be simultaneously obtained. Besides, effectively increasing the volume of the overall packaging dimension to be 30 times the size of the chip can enable a package-free LED to achieve 90% saturated maximum LER without affecting the CSU.

Spectral optimization based simultaneously on color-rendering index and color quality scale for white LED illumination

Color performance is an important parameter for high-quality light-emitting diode (LED) lighting. Color-rendering index (CRI) and color quality scale (CQS) are two independent parameters to assess the color performance, but high CRI does not correspond to high CQS, and vice versa. Therefore, it's urgent to find a comprehensive and effective metric for assessing the color performance of LEDs that can simultaneously exhibit high color-rendering index (CRI) and high color quality scale (CQS) values. In this study, a genetic algorithm with a penalty function was proposed for realizing spectral optimization by boosting the maximum attainable luminous efficacy of radiation (LER) of spectra while constraining both high CRI and CQS. By simulations, white spectra from LEDs with CRI≥95 and CQS≥95 were achieved at different correlated color temperatures (CCTs) from 2020K to 7929K. Further, a real spectra-tunable LED module consisting of four LEDs is fabricated, and high LER (344lm/W) and color performance (CRI≥90, CQS=90) was realized by tuning driving currents.

Red-Phosphor-Dot-Doped Array in Mirror-Surface Substrate Light-Emitting Diodes

Micro-patterned red-phosphor-dot-doped array in mirror-surface substrate light-emitting diodes (LEDs), which are primarily characterized with imbedded red phosphor dots in yellow phosphor slurry and an installed mirror on the substrate surface, are investigated. These dots scatter lights in different colors, compensate the long waveband in LED spectra, and allow exiting blue rays to escape in large angles. This mirror surface reflects downward blue lights emitted by chips, blends various-wavelengthed light beams, and enhances the cooling effect on the entire module. With the aid of these two modifications, the quantum efficiency (QE) and the luminous efficacy of radiation (LER) have been typically elevated by 16.6% and 38.8%, respectively. Proposed modules can be used in the LED industry for guiding improvements of QE, LER, and yellow–blue ratio uniformity.

6-4: High Efficacy, High Color Quality Hybrid White OLEDs Incorporating Red Quantum Dots with Narrow Emission Bands

We propose a hybrid WOLED technology that combines colloidal quantum dot narrow red emitters with organic emitters for high efficacy, high color quality solid state lighting. Spectra analysis indicates this approach will lead to WOLEDs that could achieve high color quality (CRI≥91; R9≥32) at high luminous efficacy of radiation (≥359lm/W).

Estimation the Amount of Heat Generated by LEDs under Different Operating Conditions

LEDs are not 100 % efficient at converting input power to light. Some of the energy is converted into heat and must be transferred to the ambient. Two approaches are used to assess the amount of heat generated by the LED - based on the luminous efficacy of radiation and a new one presented in the paper. To provide sufficient accuracy in thermal evaluations closer to the real operating modes for a specific LED module a new technique is developed. The aim is to evaluate the amount of heat generated during design phase of LED lighting. Experiments of LED modules are conducted to evaluate LEDs efficiency at converting input power to heat. For the heat dissipation study and thermal analysis of LED module are used non-contact temperature measurement using an infrared camera combined with computer aided thermal modelling and simulation. DOI: http://dx.doi.org/10.5755/j01.eie.22.2.8522

3-D Microlens Phosphor With Curvatures Manufactured by Imprinting for Chip-on-Board Light-Emitting Diodes

High-power light-emitting diodes (LEDs) assembled in 3-D microlens-phosphor structures are investigated. For the proposed design, the phosphor–air interface with arrayed high-valued curvatures replaces the conventional flat counterpart, enabling abundant randomized lights to exit. Monte Carlo algorithm ray tracing and finite-element method were utilized in this paper to analyze the light propagation and thermal migration, simulatively indicating that the redirecting regulation and heat dispassion way of the proposed architecture are considerably different. Experimentally, in comparison with the conventional flat chip-on-board (COB) LEDs, the proposed single-layered structure is capable of improving the luminous efficacy of radiation by 38.1%. For multilayered structures, within which the light transmits first through red phosphor layer and then yellow phosphor layer molded with microlens, a 7% increase in quantum efficiency and an 8.7 increase in color rendering index have been achieved. The proposed structure can guide LED manufacturing communities to significantly improve the optical performance and designs of COB packaging.

Resonant-enhanced full-color emission of quantum-dot-based micro LED display technology.

Colloidal quantum dots which can emit red, green, and blue colors are incorporated with a micro-LED array to demonstrate a feasible choice for future display technology. The pitch of the micro-LED array is 40 μm, which is sufficient for high-resolution screen applications. The method that was used to spray the quantum dots in such tight space is called Aerosol Jet technology which uses atomizer and gas flow control to obtain uniform and controlled narrow spots. The ultra-violet LEDs are used in the array to excite the red, green and blue quantum dots on the top surface. To increase the utilization of the UV photons, a layer of distributed Bragg reflector was laid down on the device to reflect most of the leaked UV photons back to the quantum dot layers. With this mechanism, the enhanced luminous flux is 194% (blue), 173% (green) and 183% (red) more than that of the samples without the reflector. The luminous efficacy of radiation (LER) was measured under various currents and a value of 165 lm/Watt was recorded.

Why Color Space Uniformity and Sample Set Spectral Uniformity Are Essential for Color Rendering Measures

ABSTRACTRecently, a group including the present authors developed a new color rendering (fidelity) measure, approved by the IES and henceforth referred to as IES Rf, that has two major updates with respect to the general color rendering index Ra of the CIE. First, it proposes an update to the more perceptually uniform CAM02-UCS color space. Secondly, instead of using only a small number (eight) of moderately saturated samples to determine a general color fidelity index, the IES method proposes using a set of 99 samples uniformly distributed in color space. In addition to the latter, the sample set has one other important property: spectral uniformity. This ensures that the sample set is not wavelength biased: all wavelengths contribute equivalently to the general color fidelity score. This article explores the importance of these two updates for color fidelity evaluation. It shows how the color space update results in a substantial spread of the new color fidelity scores relative to the old CIE Ra values and how the sample set update results in an overall reduction of color fidelity scores for most light source spectra with high CIE Ra (≥80) and high luminous efficacy of radiation (LER) values, especially those with narrowband or spiked spectral features. It is shown that calculated color fidelity scores are affected by the degree of sample set spectral uniformity and that, in the absence of sample set spectral uniformity, light source spectra can be tuned to yield anomalously high CIE Ra values without necessarily yielding an increase in actual perceived color fidelity.

Effects of phosphor distribution and step-index remote configuration on the performance of white light-emitting diodes.

Phosphor-converted white light-emitting diodes (pc-WLEDs) are fabricated by combining CaSi2O2N2:Eu2+ and Ca2Si5N8:Eu2+ phosphors with a blue chip. Experimental results demonstrate that placing the red phosphor layer above the yellow one (Y down/R up) yields the highest luminous efficiency, making it the preferable phosphor distribution for pc-WLEDs rather than Y up/R down. This finding suggests that the extent of overlap between the emission spectrum of short-emission-wavelength phosphors and the excitation spectrum of long-emission-wavelength phosphors and their luminous efficacy of radiation should be taken into account simultaneously when studying the optical characteristics of pc-WLEDs. Compared to common pc-WLEDs with silicone gel as the remote layer, the proposed step-index remote configuration exhibits superior luminous efficiency because of reduced total internal reflection and Fresnel loss.