Decrease In Light Output Research Articles

An analytical and machine learning model for SPD estimation and its prediction for lumen and chromaticity shift based LED lifetime performance analysis

The LED lifetime is commonly estimated by manufacturers using an exponential model to evaluate L70 criteria. However, it ignores colour characteristic variation and does not explain the root cause of LED failure. In this paper, a spectral power distribution (SPD) based approach is proposed to estimate lifetime performance of cool white LED considering both colour characteristics and lumen maintenance, as all the lighting performance parameters are extracted from SPD. The exponential model does provide the lifetime using only lumen data and does not explain the colour characteristics. As an alternate to the exponential model, a quadratic polynomial, and machine learning (ML) models with hours and temperature as input factors, is proposed to determine SPD for experimental conditions as well as to predict for other operating conditions. Further, the lifetime performance analysis is performed for reliability assessment conditions through both lumen and colorimetric performances. The outcomes of all the models are analysed and it is found that the results are comparable. As ML models are simpler than analytical models for more than two inputs, further it is used to predict SPD at different temperatures and the LED performance is validated. Further analysis shows that a decrease in blue light is the primary cause of the overall decrease in light output and decrease in yellow emission due to phosphor degradation is the reason for chromaticity shift.

Reduction of light output of plastic scintillator tiles during irradiation at cold temperatures and in low-oxygen environments

The advent of the silicon photomultiplier has allowed the development of highly segmented calorimeters using plastic scintillator as the active media, with photodetectors embedded in the calorimeter, in dimples in the plastic. To reduce the photodetector’s dark current and radiation damage, the high granularity calorimeter designed for the high luminosity upgrade of the CMS detector at CERN’s Large Hadron Collider will be operated at a temperature of about −30 °C. Due to flammability considerations, a low oxygen environment is being considered. However, the radiation damage to the plastic scintillator during irradiation in this operating environment needs to be considered. In this paper, we present measurements of the relative decrease of light output during irradiation of small plastic scintillator tiles read out by silicon photomultipliers. The irradiations were performed using a 60Co source both to produce the tiles’ light and as a source of ionizing irradiation at dose rates of 0.3, 1.3, and 1.6 Gy/h, temperatures of −30, −15, −5, and 0 °C, and with several different oxygen concentrations in the surrounding atmosphere. The effect of the material used to wrap the tile was also studied. Substantial temporary damage, which annealed when the sample was warmed, was seen during the low-temperature irradiations, regardless of the oxygen concentration and wrapping material. The relative light loss was largest with 3MTM Enhanced Specular Reflector Film wrapping and smallest with no wrapping, although due to the substantially higher light yield with wrapping, the final light output is largest with wrapping. The light loss was less at warmer temperatures. Damage with 3% oxygen was similar to that in standard atmosphere. Evidence of a plateau in the radical density was seen for the 0 °C data.

Performance evaluation of pulse shape discrimination capable organic scintillators for space applications

Scintillators with pulse-shape discrimination (PSD) capability are of great interest to many fields in the scientific community. The ability to discern a gamma ray from a neutron using PSD varies between different types of scintillator materials and dopants. A new generation of organic scintillator materials with PSD capability were studied to determine their radiation hardness to neutron and gamma-ray radiation. The PSD capability, average pulse shapes, and light output of four types of organic scintillator were characterized before and after neutron and gamma-ray irradiation. The main goal of this investigation is to study the effects of long-term irradiation that may be experienced in space applications on the light output and particle discriminating capabilities of each material. EJ-270, EJ-276, organic glass, and Stilbene were tested. Damage due to neutron irradiation (displacement damage) was not observed in any of the scintillators up to 2.56×1011 n/cm2, except for Stilbene which showed a small (12%) decrease in light output. All scintillators presented some light output reduction after gamma-ray irradiation (total ionizing dose), with reductions of 17% (EJ-276 and OGS), 32% (EJ-270), and 42% (Stilbene) observed immediately after 100 kRad.

Optimization of scintillator-reflector optical interfaces for the LUT Davis model.

Designing and optimizing scintillator-based gamma detector using Monte Carlo simulation is of great importance in nuclear medicine and high energy physics. In scintillation detectors, understanding the light transport in the scintillator and the light collection by the photodetector plays a crucial role in achieving high performance. Thus, accurately modeling them is critical. In previous works, we developed a model to compute crystal reflectance from the crystal 3D surface measurement and store it in look-up tables to be used in the Monte Carlo simulation software GATE. The relative light output comparison showed excellent agreement between simulations and experiments for both polished and rough surfaces in several configurations, that is, without and with reflector. However, when comparing them at the irradiation depth closest to the photodetector face, rough crystals with a reflector overestimated the predicted light output. Investigating the cause of this overestimation, we optimized the LUT algorithm to improve the reflectance computation accuracy, especially for rough surfaces. However, optical Monte Carlo simulations carried out with these newly generated LUTs still overestimate the light output. Based on previous observations, one probable cause is the erroneous assumption of perfect couplings between the reflector and crystal and between the crystal and photodetector, which likely results in an important overestimation of the light output compared to experimental values. In practice, several factors could degrade it. Here, we investigated possible suboptimal optical experimental configurations that could lead to a degraded light collection when using Teflon or ESR reflectors coupled to the crystal with air or grease. We generated look-up tables with a mixture of air and grease and showed the effect of three possible sources of light loss: the presence of a small gap between the crystal and the reflector edges close to the photodetector face, the infiltration of grease in the crystal-reflector coupling, and the presence of inhomogeneities in the photodetector-crystal interface. The strongest effect is linked to the presence of a small gap of grease between the edges of the reflector material and the crystal (light loss of 10%-12% for 0.2mm gap). The optical grease infiltrating the crystal-reflector air coupling decreases the light output, depending on the infiltration's extent and the amount of grease infiltrated. Five percent of air in the crystal-photodetector coupling can cause a light output decrease of 2% to 4%. The individual and combined effect of these advanced models can explain the discrepancy of the relative light output obtained with ESR in simulations and experiments. With Teflon, the study indicates that the light output loss strongly depends on the reflectance deterioration caused by grease absorption. Our results indicate that when studying scintillation detector performance with different finishes, performing simulations in ideal coupling conditions can lead to light output overestimation. To perform an accurate light output comparison and ultimately have a reliable detector performance estimation, all potential sources of practical limitations must be carefully considered. To broadly enable high-fidelity modeling, we developed an interface for users to compute their own LUTs, using their surface, scintillator, and reflector characteristics.

Enhanced Heat Dissipation in Gallium Nitride-Based Light-Emitting Diodes by Piezo-phototronic Effect.

As a new generation of light sources, GaN-based light-emitting diodes (LEDs) have wide applications in lighting and display. Heat dissipation in LEDs is a fundamental issue that leads to a decrease in light output, a shortened lifespan, and the risk of catastrophic failure. Here, the temperature spatial distribution of the LEDs is revealed by using high-resolution infrared thermography, and the piezo-phototronic effect is proved to restrain efficaciously the temperature increment for the first time. We observe the temperature field and current density distribution of the LED array under external strain compensation. Specifically, the temperature rise caused by the self-heating effect is reduced by 47.62% under 0.1% external strain, which is attributed to the enhanced competitiveness of radiative recombination against nonradiative recombination due to the piezo-phototronic effect. This work not only deepens the understanding of the piezo-phototronic effect in LEDs but also provides a novel, easy-to-implement, and economical method to effectively enhance thermal management.

Improvement of a PET Detector Performance by Setting Reflectors in Parallel With PMT Face

A whole-body positron emission tomography scanner must be equipped with many detectors so that they are often composed of low-cost photomultiplier tubes (PMTs), that is, large anode-type PMTs. The general detector structure has a scintillation crystal element array coupled on an array of the large anode PMTs, and for crystal element identification, the method is needed to get sufficient spread of scintillation light for distribution among distant PMT anodes. Besides the common method of using a light guide, some methods have been proposed for better element identification performance. In this paper, we introduce a new method, in which the scintillation light spread is promoted not only by removing reflectors between crystal elements but also by restricting light exit to the PMTs by placing additional reflectors at the bottom of the crystal element array. Because the additional reflectors are parallel to the PMT surface, we call them parallel reflectors. We verified our method with a detector consisting of the 2.45 mm $\times \,\, 5$ mm $\times \,\,15$ mm Lu2 x Gd2(1– x )SiO5:Ce crystal elements and two dual-photocathode PMTs. Each photocathode was 8 mm $\times \,\,18$ mm in size. We set a $9 \times 10$ crystal element array on the two PMTs and tried to identify the elements by the $2\times 2$ PMT signals. Detector performance was evaluated with 137Cs point sources (662-keV gamma rays). The results showed that despite a significant decrease in light output at the boundary of the two PMTs, the method made crystal element identification possible. We measured energy resolutions of 13.1% and 17.8% for the elements on the PMT photocathode area and the PMT boundary, respectively. Our method is applicable to the depth-of-interaction detector consisting of multilayer crystal element arrays. By inserting parallel reflectors between the layers, we would be able to control the path of scintillation light originating in the upper layer. The effect was first examined in a basic study with two layers of a $2 \times 5$ crystal element array. Then, the array of each layer was increased to $9\times 9$ for performance evaluation. It was difficult to identify all 162 crystal elements with four PMT signals; however, results suggest the possibility of scintillation light path control in each layer separately by the parallel reflectors and the potential for better performance by more precise adjustment of parallel reflectors.

Temperature Quenching of Radio- and Photoluminescence of Y3(Ga,Al)5O12:Ce3+ and Gd3(Ga,Al)5O12:Ce3+ Garnet Ceramics

Cerium-doped mixed oxide garnets are promising luminescent materials to use as LED phosphors or scintillators for radiation detectors. In this paper, temperature-dependent characteristics of Ce-doped multicomponent garnets ceramics have been studied under photo and X-ray excitation. Ceramic samples of Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> :Ce <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.006</sub> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> = 1 , 2, 3, and 4) and of Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> :Ce <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.006</sub> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> = 1 , 2, and 3) have been used. At high temperature, all samples showed decrease in the Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> luminescence intensity, luminescence decay, and luminescence lifetime due to thermal ionization of Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> excited 5d states. Values of activation energies of the thermal ionization process have been determined from photoluminescence lifetime measurements and shown in comparison with literature data. At low temperature, Gd-containing samples showed significant decrease in light output. In the same temperature range, large thermoluminescence peaks have been observed, indicating that the reduction of Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> emission at low temperatures can be attributed to the localization of charge carriers on traps.

Lock-in Thermography for the Development of New Materials

Thermal management is one of the crucial concerns for mid- and high-power white light-emitting diodes (LEDs). Most of the currently available white LEDs on the market consist of a blue LED chip coated with a yellow phosphor-polymer composite. Besides the heat-induced degradation of the phosphor-polymer composite, the so-called thermal quenching, i.e. the decrease in light output upon increasing temperature, is also a pressing issue. Both result in an efficiency decrease and in a color change of the LED. In this work, a method to determine the thermal diffusivity and the thermal conductivity of phosphors is presented using lanthanide-doped glasses as test samples. In the quest for temperature-stable phosphors with a high thermal conductivity, such luminescent glasses might represent an attractive alternative. Thermal waves are introduced to the samples by periodical heating with a laser. Lock-in infrared thermography, a non-destructive method, is used to monitor even small changes in surface temperature. The phase delay of the thermal wave passing through the sample is analyzed at different lock-in frequencies and a mathematical fitting of the phase data then provides the coefficients required to determine the thermal diffusivity. Subsequently, the thermal conductivity is calculated from the thermal diffusivity, the mass density, and the specific heat capacity.

Alkali earth co-doping effects on luminescence and scintillation properties of Ce doped Gd3Al2Ga3O12 scintillator

The Mg and Ca co-doped Ce:Gd3Al2Ga3O12 single crystals were prepared by micro pulling down method with a wide concentration range 0–1000ppm of the codopants. Absorption and luminescence spectra were measured together with several other scintillation characteristics, namely the scintillation decay and light yield to reveal the effect of Mg and Ca co-doping. The scintillation decays were accelerated by both Mg and Ca codopants. Comparing to Ca co-doping, the Mg co-doped samples showed much faster decay and comparatively smaller light output decrease with increasing Mg dopant concentration.

SU‐E‐T‐526: On the Linearity, Stability and Beam Energy Dependence of CdSe Quantum Dots as Scintillating Probes

Purpose:Structure and energy transfer mechanisms confer colloidal quantum dots (cQDs) interesting properties, among them their potential as scintillators. CdSe multi‐shell cQDs in powder were investigated under photons irradiation. The purpose of this work is to characterize signal to dose linearity, stability with time and to quantify the dependence of their light output with beam energy.Methods:The cQDs are placed at the extremity of a non‐scintillating plastic collecting fiber, with the other extremity connected to an Apogee U2000C CCD camera. The CCD camera collects the fluorescence light from irradiated cQDs from which the delivered dose is extracted. This signal is corrected for Cerenkov contamination at MV energies using the chromatic technique. The detector was irradiated with two devices: Xstrahl 200 orthovoltage unit for 120, 180 and 220 kVp and a Varian Clinac iX for 6 and 23 MV.Results:Linear output response with varying dose is observed for all beam energies with R2 factors &gt; 0,999. Reproducibility measurements were performed at 120 kVp: the same set‐up was irradiated at different time intervals (one week and three months). The results showed only a small relative decrease of light output of 3,2 % after a combine deposited dose of approximately 95 Gy. CdSe nanocrystals response has been studied as a function of beam energy. The output increases with decreasing energy from 120 kVp to 6 MV and increase again for 23 MV. This behavior could be explained in part by the cQDs high‐Z composition.Conclusion:The fluorescence light output of CdSe cQDs was found to be linear as a function of dose. The results suggest stability of the scintillation output of cQDs over time. The specific composition of cQDs is the main cause of the observed energy dependence. We will further look into particle beam dependence of the cQDs. Bourse d' excellence aux etudes graduees du CRC (Centre de Recherche sur le Cancer, Universite Laval)Bourse d' excellence aux etudes graduees du CRC (Centre de Recherche sur le Cancer, Universite Laval)

Ce-doped LuAG single-crystal fibers grown from the melt for high-energy physics

Under a stationary stable regime undoped and Ce-doped LuAG (Lu3Al5O12) single-crystal fibers were grown by a micro-pulling-down technique. The meniscus length corresponding to the equilibrium state was <200μm. Fluctuations in the fiber composition and pulling rate were found to have a significant effect on the properties of the fibers grown. A great improvement in the performance was found in samples containing low Ce concentrations (⩽0.1at.%) and produced using pulling rates <0.5mmmin−1. Under such conditions a good lateral surface fiber quality was obtained and light propagation was significantly improved. Conversely, a high Ce concentration and a high pulling rate resulted in a strong degradation of the fiber surface quality causing defects to appear and a decrease in light output.

Current status and performance of the BESIII electromagnetic calorimeter

The design and construction of the BESIII electromagnetic calorimeter is introduced briefly. Radiation dose of CsI(Tl) crystals is monitored and history graph of integral dose of crystals is showed. LED-fiber system is used for monitoring the EMC light output, and large decrease of light output of several crystals is discussed. BESIII electromagnetic calorimeter works very well and its performance reach the design value.

Energy Saving of Large-Scale High-Intensity-Discharge Lamp Lighting Networks Using a Central Reactive Power Control System

This paper summarizes the test results on several large-scale lighting systems based on a range of high-intensity-discharge (HID) lamps. The study focuses on the energy-saving, energy-efficiency, and dimming characteristics of magnetic-ballast-driven HID lamp systems that are suitable for public lighting networks. A central energy-saving unit that can change the input mains voltage of 220 V to a variable voltage within the range from 220 to 170 V is used to control a large lighting network. This central dimming unit offers a new freedom for users to use lighting energy when and where it is needed and to an appropriate power level. Lamp characteristics under full and reduced voltage operation for six types of HID lamps have been measured. Six large-scaled networks of ballast-lamp sets have been set up and dimmed with a central energy-saving unit with a rated power of 13.2 kVA per phase. The full load current under test was set at about 40 A per phase. The loss of the energy-saving unit on a per-lamp-set basis was measured at different voltage levels. It is found that the energy-saving unit can reduce overall system power at least 40% without causing lamp flickering. Through the controllable voltage range, there is no obvious change in color temperature. The lamp power and light output decrease almost linearly with input voltage within the range from 170 to 220 V, and the power loss of the energy-saving unit is less than 2.4% of the full power in all tests. For these large-scaled lighting networks, it is envisaged that 25%-35% of energy saving can be achieved in large public lighting applications.

Technological Aspects of Solid-State and Incandescent Sources for Miner Cap Lamps

Light-emitting diodes (LEDs) are emerging as viable replacements for incandescent (INC)-based cap lamps used in mining. The photometric and energy characteristics of these light sources differ in important ways. This paper describes the performance of LED and INC sources in cap lamps in terms of correlated color temperature, color rendering, light output, electric power, ambient temperature and air flow, and light source aging. Importantly, these characteristics can influence a miner's ability to spot mining hazards thus impacting safety. Second, some of these characteristics interact with the operating life of the cap lamp's battery power, such that differences between LED and INC sources can be magnified toward the end of a 10-h battery discharge cycle. Empirically, we have determined that after 8 h at an ambient temperature of 25degC , the average light output of an INC cap lamp can decrease to about 69% of its initial value when powered by a lead-acid battery, and it can decrease to about 65% of its initial value when powered by a nickel-hydride battery. An LED-based cap lamp using a constant current drive circuit can maintain about 96% of its initial value when powered by a nickel-hydride battery. Real-world tests addressing the effects of ambient temperature and air flow on the light output of an LED and INC cap lamp were conducted in the National Institute for Occupational Safety and Health Safety Research Coal Mine. The LED cap lamp yielded a vertical average illuminance improvement of approximately 9.5%, and the INC cap lamp yielded a vertical average illuminance degradation of approximately 4%. The differences between LED and INC cap lamps were further quantified by the calculation of ldquomesopic luminancerdquo data that indicated for the same photopic luminance (i.e., as measured using a conventional light meter) the LED cap lamp could be up to 38% more efficient than the INC cap lamp with a lead-acid battery at the end of the 10-h driving cycle. Lastly, accelerated life tests were used to empirically determine light output depreciation as the INC light source age approached its useful life. There was about a 35% decrease in light output. This is quite considerable, particularly given that the light output will decrease an additional 30% to 45% over the period of a 10-h shift. The implications of the differences between LED and INC sources are discussed. This information is crucial in determining how visual performance could be affected for real-world conditions where batteries discharge during the work shift and as the light source ages. To date, only idealized conditions have been used for LED and INC cap lamp visual performance research.

Long-wave (10 μm) infrared light emitting diode device performance

Electroluminescence in the range of 6–12μm is observed from an Sb-based type-II interband quantum cascade structure. The LED structure has 30 active/injection periods. We have studied both top-emitting and flip-chip mount bottom emitting LED devices. For room temperature operation, an increase, saturation and decrease in light output occur at successively higher injection currents. An increase of about 10 times in light output occurs when device is operated at 77K compared to room temperature operation. This increase is attributed to reduced Auger non-radiative recombination at lower temperatures. The peak-emission wavelengths at room temperature and 80K operation are 7 and 10μm, respectively. These devices can be used for high-temperature simulation in an infrared scene generation experiment.

Fluorescence and absorption of a polystyrene-based scintillator exposed to UV laser radiation

We have established that exposure of polystyrene-based scintillator samples to UV laser radiation (248 nm) leads to a significant decrease in the fluorescence intensity. We have carried out a spectral analysis of the luminescent and absorption properties of the scintillator, which allowed us to determine the major factor in the decrease in luminescence intensity of the samples exposed to UV radiation. We propose a new hypothesis for the mechanism of the processes leading to the decrease in light output of the scintillator during operation.

Characterization of scintillators for lost alpha diagnostics on burning plasma experiments

The characteristics of light output by ion beam irradiations under high ion fluxes have been measured for three kinds of scintillators: ZnS:Ag deposited on the glass plate, Y3Al5O12:Ce powder stiffened with a binder, and Y3Al5O12:Ce ceramics sintered at high temperature. The ion beam flux in the range from 1012to1013ions∕(cm2s) is irradiated to simulate the burning plasma experiments. The decrease of light output has been observed by long time ion irradiation. The deterioration of ZnS:Ag deposited scintillator is most serious. The deterioration has been improved for the scintillators of Y3Al5O12:Ce with a binder and that sintered. Their applications to ITER lost alpha diagnostics are discussed.

Monte Carlo and Lambertian light guide models of the light output from scintillation crystals at megavoltage energies

A new model of the light output from single-crystal scintillators in megavoltage energy x-ray beams has been developed, based on the concept of a Lambertian light guide model (LLG). This was evaluated in comparison with a Monte Carlo (MC) model of optical photon transport, previously developed and reported in the literature, which was used as a gold standard. The LLG model was developed to enable optimization of scintillator detector design. In both models the dose deposition and light propagation were decoupled, the scintillators were cuboids, split into a series of cells as a function of depth, with Lambertian side and entrance faces, and a specular exit face. The signal in a sensor placed 1 and 1000 mm beyond the exit face was calculated. Cesium iodide (CSI) crystals of 1.5 and 3 mm square cross section and 1, 5, and 10 mm depth were modeled. Both models were also used to determine detector signal and optical gain factor as a function of CsI scintillator thickness, from 2 to 10 mm. Results showed a variation in light output with position of dose deposition of a factor of up to approximately 5, for long, thin scintillators (such as 10 X 1.5 x 1.5 mm3). For short, fat scintillators (such as 1 X 3 X 3 mm3) the light output was more uniform with depth. MC and LLG generally agreed to within 5%. Results for a sensor distance of 1 mm showed an increase in light output the closer the light originates to the exit face, while a distance of 1000 mm showed a decrease in light output the closer the light originates to the exit face. For a sensor distance of 1 mm, the ratio of signal for a 10 mm scintillator to that for a 2 mm scintillator was 1.98, whereas for the 1000 mm distance the ratio was 3.00. The ratio of quantum efficiency (QE) between 10 and 2 mm thicknesses was 4.62. We conclude that these models may be used for detector optimization, with the light guide model suitable for parametric study.

Cationic polymerization of hydrogenated bisphenol‐A glycidyl ether with cycloaliphatic epoxy resin and its thermal discoloration

AbstractOne of the key issues associated with epoxy resin–based LED encapsulant is discoloration under operating conditions that caused a decrease in light output. In this article, thermal cationic polymerization of hydrogenated bisphenol‐A glycidyl ether using 3‐methyl‐2‐butenyltetramethylenesulfonium hexafluoroantimonate and its discoloration are reported. Polymerization was followed by thermal analysis, FT‐IR spectrum, and hardness measurements. Thermal discoloration was measured by yellowness index based on transmittance spectrum. Increased catalyst concentration causes acceleration in polymerization rates, but this also increased discoloration after thermal aging. In low catalyst concentration under 0.3 wt %, the hardness of plaque was low and required a high temperature to complete polymerization and this resulted in a highly colored plaque. Formulation with cycloaliphatic epoxy (3′‐4′‐epoxycyclohexane)methyl 3′‐4′‐epoxycyclohexyl‐carboxylate (CEL2021P) enhanced the polymerization even at low catalyst concentration and reduced thermal discoloration. With addition of di‐t‐butylphenol (BHT) as the oxidation inhibitor, the thermal discoloration was extremely improved but this also caused inhibition of polymerization as seen by thermal analysis. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1395–1400, 2005

Comparison of the Electroluminescence of Blue and Deep-UV Light-Emitting Diodes at Elevated Temperatures

AbstractThe performance of InGaN and AlGaN-based blue (465nm) and deep ultraviolet (UV) (280 nm) light-emitting diodes (LEDs) at elevated temperatures (25-175 °C) were investigated. As a result of uniform high-Al content AlGaN alloys yielded by migration-enhanced metalorganic chemical vapor deposition, the deep-UV LED showed dominant band-edge emission, much smaller alloy broadening and weaker localization effects as compared to the InGaN LED. Strong carrier localization was retained in the blue LED up to 175 °C, leading to temperature-independent emission intensity at low-energy tails. The UV LED, however, showed a much more rapid decrease in light output with increasing temperature. The characteristic temperature was 37 K, compared to 270 K for the blue LED. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the deep UV LED and suggest that increasing carrier confining potentials will provide a critical means to improve its thermal stability.