Phosphor-coated White Light-emitting Diodes Research Articles

More light components and less light damage on rats' eyes: evidence for the photobiomodulation and spectral opponency.

The blue-light hazard (BLH) has raised concerns with the increasing applications of white light-emitting diodes (LEDs). Many researchers believed that the shorter wavelength or more light components generally resulted in more severe retinal damage. In this study, based on the conventional phosphor-coated white LED, we added azure (484nm), cyan (511nm), and red (664nm) light to fabricate the low-hazard light source. The low-hazard light sources and conventional white LED illuminated 68 Sprague-Dawley (SD) rats for 7 days. Before and after light exposure, we measured the retinal function, thickness of retinal layers, and fundus photographs. The expression levels of autophagy-related proteins and the activities of oxidation-related biochemical indicators were also measured to investigate the mechanisms of damaging or protecting the retina. With the same correlated color temperature (CCT), the low-hazard light source results in significantly less damage on the retinal function and photoreceptors, even if it has two times illuminance and blue-light hazard-weighted irradiance ([Formula: see text]) than conventional white LED. The results illustrated that [Formula: see text] proposed by IEC 62471 could not exactly evaluate the light damage on rats' retinas. We also figured out that more light components could result in less light damage, which provided evidence for the photobiomodulation (PBM) and spectral opponency on light damage.

Multi-User Precoder Designs for RGB Visible Light Communication Systems.

In this paper, we design linear precoders for the downlink of a visible light communication (VLC) system that simultaneously serves multiple users. Instead of using phosphor-coated white light-emitting diodes (PWLEDs), we focus on Red-Green-Blue light-emitting diodes (RGB-LEDs) that allow modulating three separate data streams on the three primary colors of the RGB-LEDs. For this system, we design a zero-forcing (ZF) precoder that maximizes the weighted sum rate for a multilevel pulse amplitude modulation (M-PAM). The precoding design in RGB-based systems presents some challenges due to the system constraints, such as the limited power, the non-negative amplitude constraints per light-emitting diode (LED), and the need to guarantee white light emission while transmitting with RGB-LEDs. For comparison purposes, we also consider the ZF design for a PWLED-based system and evaluate the performance of both a PWLED- and an RGB-based system.

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.

Impact of blue filtering on effective modulation bandwidth and wide-angle operation in white LED-based VLC systems

This work studies the impact of employing blue filters on the resulting effective modulation bandwidth of visible light communication (VLC) systems using phosphor-coated white light emitting diodes (LEDs) under wide-angle operation. Effective modulation bandwidths are assessed and compared for VLC systems with ideal and non-ideal transmission blue filters at the receiver. Angle tolerance capabilities of the employed filters have also been investigated in the study. Commercially available phosphor-coated white LED and photodetector (PD) models are utilized to obtain realistic results. We show that the choice of the blue filter structure, combined with the white LED and PD, affects the system’s effective modulation bandwidth. We also show that there exists a trade-off between obtaining a relatively large modulation bandwidth and preserving the wide-angle operation of the system. A figure of merit (FOM) is also introduced to represent such a trade-off. Moreover, a simple estimation of the expected effective modulation bandwidth of a VLC system using phosphor-coated white LED is developed as a result of the weighted average of the blue and yellow light responsivity of the filter. A numerical example is also included in which the optical signal to noise ratio (OSNR) is computed in VLC systems with and without the addition of a blue filter. The OSNR shows enhancement when a blue filter with a wide rejection region is inserted in the studied VLC system.

Multi-Azimuth Failure Mechanisms in Phosphor-Coated White LEDs by Current Aging Stresses

We have experimentally analyzed multi-azimuth degradation mechanisms that govern failures of commercially-available high-power (1 Watt) phosphor-coated white (hppc-W) light-emitting diodes (LEDs) covered with peanut-shaped lenses under three current-stress aging (CSA) conditions. Comprehensive analyses focus on photometric, chromatic, electrical, thermal and packaging characteristics. At the packaging level, (a) the decrease of the phosphor-conversion efficiency; (b) the yellow-browning of the optical lens; and (c) the darkening of the silver-coated reflective layer deposited with extraneous chemical elements (e.g., C, O, Si, Mg, and Cu, respectively) contribute collectively to the integral degradation of the optical power. By contrast, Ohmic contacts, thermal properties, and angles of maximum intensity remain unchanged after 3840 h aging in three cases. Particularly at the chip level, the formation of point defects increases the number of non-radiative recombination centers, and thus decreases the optical power during aging stages. Nevertheless, in view of the change of the ideality factor, the Mg dopant activation and the annealing effect facilitate the increase of the optical power in two specific aging stages (192 h~384 h and 768 h~1536 h). This work offers a systematic guidance for the development of reliable LED-based light sources in general-lighting areas.

Study on color-tunable phosphor-coated white light-emitting diodes with high S/P ratios

In this study, we have investigated the trade-off between the color rendering index (CRI, Ra) and the scotopic/photopic ratio (S/P) for color-tunable phosphor-coated white light-emitting diodes (LEDs) at two CRI limitations (Ra ≥ 70 and Ra ≥ 96). First, luminescent spectra measurements have been conducted to determine experimental results of Ra and S/P under various correlated color temperatures (CCTs). Then, a nonlinear programming method has been adopted for the optimization of Ra and S/P by varying spectral shapes through adjusting spectral parameters, such as peak wavelengths, full-width at half-maxima, and relative intensities. Therefore, polynomial curves of optimal S/P versus CCT at two Ra limitations have been discovered, enabling users to obtain optimal S/P under arbitrary CCTs within [2700 K, 6500 K]. In addition, a comparison study between the present work and our previous work has also been conducted at Ra = 70, and a fair agreement of optimal S/P has been observed.

Analysis and Modeling of High-Power Phosphor-Coated White Light-Emitting Diodes With a Large Surface Area

Modern high-power white light-emitting diodes (LEDs) composed of multiple blue LED chips and yellow phosphor coatings have been successfully commercialized because of their high-luminous efficacy. The multiple-chip LED packages usually come with flat structures that have large surface areas for considerable heat loss. Starting from the analysis and modeling of the blue LED chip, this paper introduces the thermal path through the phosphor layer to form the white phosphor-coated (PC) white LED device model for photometric, electric, and thermal performance analysis. The power distribution of the blue LED chip and that of the PC white LED device are compared. Based on this new analysis, the increase in the heat dissipation coefficient, equivalent thermal resistance, and power loss caused by the phosphor coating can be quantified. New equations suitable for device manufacturers to qualify their devices and design engineers to optimize LED system designs are derived. The analytical results are in good agreement with the practical mesurements.

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.

Dynamic Prediction of Correlated Color Temperature and Color Rendering Index of Phosphor-Coated White Light-Emitting Diodes

Light-emitting diode (LED) technology is a multidisciplinary subject that involves photometry, electric power, heat, and chromaticity which are interdependent on one another. So far, the photoelectrothermal (PET) theory has linked up the first three aspects. This research includes chromaticity into the dynamic PET theory so that even the correlated color temperature (CCT) and color rendering index (CRI) of phosphor-coated white LEDs can be dynamically predicted, thus overcoming the low bandwidth problem of some light measurement equipment. This dynamic modeling of CCT and CRI has been verified with favorable agreements between theoretical predictions and measurements of several LED samples. The outcome of this project offers a new research and development tool for practicing LED system designers to predict the instantaneous variations of CCT and CRI when the power varies in a LED system.

A model for LED spectra at different drive currents

A mathematical model for the spectra of monocolor light-emitting diodes (LEDs) and phosphor-coated white LEDs at different drive currents is established. The simulation program of the color rendering of a white light LED cluster is developed based on this model. The program can predict not only the spectral power distribution and color rendering index (CRI), but also the number of LEDs, drive currents, input power, and luminous efficacy of a white light LED cluster at a given color temperature according to the requirement of the luminous flux. The experimental results show that the relative spectral power distributions (SPDs) and chromaticity coordinates of the model LED are very close to that of the real LED at different drive currents. Moreover, the correlated color temperature (CCT), CRI, special color rendering index (R9) luminous flux, input power, and luminous efficacy of the white light LED cluster predicted by simulation are also very close to the measured values. Furthermore, a white/red cluster with high rendering (CCT = 2903 K, CRI = 91.3, R9 = 85) and a color temperature tunable warm-white/red/green/blule LED cluster with high rendering (CCT = 2700-6500 K, CRI > 90, R9 > 96) are created.

Theory of Illuminance Properties of Multi-point Light Sources with Individual Phosphor-coated White Light-emitting Diodes under Direct Illumination Using Radiation Intensity Distribution Function

We described a basic theory of multi-point light sources that comprise white light-emitting diode (LED) arrays for lighting applications. Evaluating the luminous intensity distribution is very important to understand the illuminance properties in the multi-point LED sources. We thoroughly studied each property of conventional white LEDs using blue LED excitation and a novel type of white LED using near ultraviolet (UV) LED excitation, making use of our theory. Individual quaternary multi-point light sources composed of orange, yellow, green and blue (OYGB) phosphors, combined with near UV LEDs were fabricated. A theoretical method using the radiation intensity distribution function was carried out to determine chromaticity and luminous properties of the light from these OYGB white LEDs under direct illumination. This shows that OYGB white LEDs have some characteristics that are superior to those of the conventional white LEDs.