High Power LED Research Articles

The Photometric Test Distance in Luminance Measurement of Light-Emitting Diodes in Road Lighting

Over the last few years, light-emitting diodes have completely dominated the lighting field. In road lighting, high-power LEDs have replaced traditional light sources. That is why various technical aspects of LEDs have been researched extensively worldwide. However, little research has been conducted in the area of luminance measurement. This paper reviews the methods for measuring the luminance of high-power LEDs. Particular attention is paid to the influence of the measurement distance on the measurement results. Next, the results of the tests using a modern image luminance measuring device (ILMD) for luminance measurements are presented. It is concluded that it is necessary to redefine the photometric test distance. The conducted research demonstrates that an incorrectly selected test distance can lead to the huge errors of several hundred per cent or more. In addition, the possible impact of the incorrect measurements on the design of road lighting installations is presented. It is shown that a road lighting installation can use over 300% more electrical energy compared to the installation based on the correct luminance measurements of single LEDs. In the final stage of the research, the definition of the photometric test distance for LED measurements using ILMD is proposed. The results of the research can also be useful for the luminance measurements of other types of LEDs.

Luminescence Temperature Quenching in Mn2+ Phosphors

AbstractNarrower band red and green emission in phosphor‐converted white light‐emitting diodes (wLEDs) can improve the efficacy and color gamut in lighting and display applications. A promising luminescent ion is Mn2+ that can have both narrowband green (tetrahedral coordination) and red (octahedral coordination) emission. Unlike in earlier lighting applications of Mn2+ phosphors, temperature quenching is important in wLEDs. Insight into the thermal quenching behavior of Mn2+ luminescence is lacking. Here systematic research is reported for a variety of Mn2+‐doped phosphors; a huge variation in the luminescence quenching temperature T50, ranging from 50 K for Mn2+ in ZnTe to 1200 K in MgAl2O4, is revealed. The value T50 shows a positive correlation with the bandgap of the host, but no correlation with the full width half maximum (FWHM) of the emission band, indicating that thermally activated photoionization, not thermal crossover, is the operative quenching mechanism. This is confirmed by thermally stimulated luminescence (TSL) measurements that show a rise in TSL signal following photoexcitation at temperatures around T50 providing evidence that quenching is correlated with generation of free charge carriers. Based on these findings, as a design rule is obtained that for temperature‐stable Mn2+ luminescence in (high power) LEDs a wide‐bandgap host material is required.

Tailoring Dense, Orientation-Tunable, and Interleavedly Structured Carbon-Based Heat Dissipation Plates.

The controllability of the microstructure of a compressed hierarchical building block is essential for optimizing a variety of performance parameters, such as thermal management. However, owing to the strong orientation effect during compression molding, optimizing the alignment of materials perpendicular to the direction of pressure is challenging. Herein, to illustrate the effect of the ordered microstructure on heat dissipation, thermally conductive carbon-based materials are fabricated by tailoring dense, orientation-tunable, and interleaved structures. Vertically aligned carbon nanotube arrays (VACNTs) interconnected with graphene films (GF) are prepared as a 3D core-ordered material to fabricate compressed building blocks of O-VA-GF and S-VA-GF. Leveraging the densified interleaved structure offered by VACNTs, the hierarchical O-VA-GF achieves excellent through-plane (41.7W m-1 K-1 ) and in-plane (397.9W m-1 K-1 ) thermal conductivities, outperforming similar composites of S-VA-GF (through-plane: 10.3W m-1 K-1 and in-plane: 240.9W m-1 K-1 ) with horizontally collapsed carbon nanotubes. As heat dissipation plates, these orderly assembled composites yield a 144% and 44% enhancement in the cooling coefficient compared with conventional Si3 N4 for cooling high-power light-emitting diode chips.

Optimal design of heat dissipation modules for high-power LED based on the Taguchi method

The semiconductor component of InGaN-based blue light-emitting diodes (LED) emits white light when combined with a yellow phosphor mixture. However, owing to the lattice dislocations and defect points in GaN, it exhibits a high thermal resistance, which leads to heat accumulation and an increase in temperature. This is problematic as overheating causes LED to produce dark spots and lines and reduces the luminous flux and optical power of high-power LED. In this study, we propose a variety of optimal structures for heat-transfer modules and apply the proposed architectures in the assembly of high-power LED. First, the high-power LED substrate was coated with a film of aluminum nitride. Then, copper fins were connected to the vacant spaces in the circuit boards to increase the surface area of the heat-transfer region. The Taguchi method was used to identify the optimal substrate thickness, fin arrangement, and fin depth for the effective heat dissipation in a 12 W high-power LED. A dielectric layer was grown on the surface of the aluminum nitride film to serve as a passivation layer to insulate the patient. The passivation layer reduces the physical damage caused by thermal stress, thereby improving the service life and characteristics of heat-transfer modules. The proposed design not only yields a stable LED substrate (with low thermal stress) but also induces reliable heat transfer.

Lifetime Prediction of Current-and Temperature-Induced Degradation in Silicone-Encapsulated 365 nm High-Power Light-Emitting Diodes

We report on the degradation mechanisms and dynamics of silicone encapsulated ultraviolet A (UV-A) high-power light-emitting diodes (LEDs), with a peak wavelength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {365~ \text {n} \text {m} }}$ </tex-math></inline-formula> . The stress tests were carried out for a period of 8665 hours with forward currents between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {350~ \text {m} \text {A} }}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {700~ \text {m} \text {A} }}$ </tex-math></inline-formula> and junction temperatures up to 132°C. Depending on stress condition, a significant decrease in optical power could be observed, being accelerated with higher operating conditions. Devices stressed at a case temperature of 55 °C indicate a decrease in radiant flux between 10- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {40~\%}}$ </tex-math></inline-formula> varying with measurement current, whereas samples stressed at higher case temperatures exhibit crack formation in the silicone encapsulant accompanied by electromigration shorting the active region. The analyzed current and temperature dependency of the degradation mechanisms allows to propose a degradation model to determine the device lifetime at different operating parameters. Additional stress test data collected at different aging conditions is used to validate the model’s lifetime predictions.

LED Systems Applications and LED Driver Topologies: A Review

High-power light-emitting diodes (LEDs) and high-brightness LEDs have revolutionized the lighting industry. The availability of cost-effective LEDs with high luminous flux, efficacy, and reliability has expanded their application to replace incandescent and fluorescent light sources. They have also enabled many new applications that were considered infeasible before, due to the inherent limitations of traditional sources of light. This survey provides a comprehensive overview of ac- and dc-supplied LED lighting systems and their applications, corresponding specifications and their similarities to/differences from regular power supplies. Restrictions imposed by LEDs characteristics, safety considerations, and dimmers are discussed. Also, LED driving approaches are categorized and their suitability for different applications is provided.

Thermal Resistance and Luminescence Performance of Nano-Silver Sintered Interface in High-Power LED

Thermal Resistance and Luminescence Performance of Nano-Silver Sintered Interface in High-Power LED

Highly efficient Mn–Mn dimer activated phosphors for high-power near-infrared LED application

Broadband NIR phosphors as light sources are receiving increasing attention.

The electro-thermo-optical characteristics and limiting energy capabilities of high-power deep ultraviolet light emitting diodes (λ~ 270 nm)

The current and temperature dependences of the electrical, power and spectral characteristics of high-power deep-UV LEDs (λ~ 270 nm) have been studied. The main parameters of LED (internal quantum efficiency and light extraction efficiency) which determine output power capacity of the UV LED have been calculated using the ABC-model. The influence of current distribution, electrical losses and thermal resistance as factors limiting the energy capabilities was estimated. Keywords: AlGaN, UV LED, quantum efficiency, light extraction efficiency, temperature dependence, ABC-model.

Enhanced photoluminescence of a HF free novel NIR phosphor of K2ZnF4:Cr3+by lithium ion doping

Using Li+co-doping for charge compensation, the broadband NIR luminescence intensity was enhanced. Compared with K2ZnF4:Cr3+, the emission intensity of K2ZnF4:Cr3+,Li+increased 2.58 times. A high-power NIR LED device was realized.

Электротермооптические характеристики и предельные энергетические возможности мощных AlGaN-светодиодов глубокого ультрафиолетового диапазона (&amp;lambda;&amp;#126; 270 nm)

The current and temperature dependences of the electrical, power and spectral characteristics of high-power deep UV LEDs (λ≈ 270 nm) have been studied. The main parameters of LED (internal quantum efficiency and light extraction efficiency) which determine output power capacity of the UV LED are calculated using the ABC-model. The influence of current distribution, electrical losses and thermal resistance as factors limiting the energy possibilities were estimated.

Review on the study of potential health hazards caused by blue light due to Melanopsin activity

Humans live in a 24-hour environment, in which light and darkness follow a diurnal pattern. In humans, the known effects of light on circadian rhythms and sleep are all, without exception, mediated by the retina. The retina is a fine layer of nerve tissue at the back of our eyes, containing specialized photoreceptors. Melanopsin RGCs are extremely sensitive to blue light and even exposure to light levels as low as the one from a smart-phone or light emitting e-readers are associated with disruptions of circadian rhythm. Melanopsin absorbs light in the short-wavelength range of the visible spectrum, with λmax at or near 480 nm. High-intensity blue light from any source is potentially hazardous to the eye. Industry sources of blue light are purposely filtered or shielded to protect users. However, it may be harmful to look directly at many high-power consumer LEDs simply because they are very bright. When Melanopsin is activated by the short wavelength component of light, it suppresses Melatonin synthesis. This action is controlled mainly by light could affect several functions including the regulation of intraocular pressure. Blue light or blue-violet light can be damaging to delicate retinal cells deep in the eye. No one knows exactly what causes dry macular degeneration. Research indicates that it may be a combination of family genes and environmental factors, including smoking, obesity and diet. The condition develops as the eye ages. Additionally, our studies have shown blue light is a risk factor for the onset of age-related macular degeneration, which is a progressive condition that can lead to vision loss over.

High-power NIR-II LED of Nd3+ doped glass ceramics towards portable imaging

Based on the NIR luminescence performance of LiGa5O8:Nd3+ GCs, a portable NIR LED has been developed for imaging, nondestructive analysis, and other applications.

Heat dissipation design and optimization of high-power LED lamps

Heat dissipation design and optimization of high-power LED lamps

СПЕКТРАЛЬНІ ПАРАМЕТРИ КВАЗІМОНОХРОМАТИЧНИХ СВІТЛОДІОДІВ ДЛЯ СИСТЕМ ОСВІТЛЕННЯ З РЕГУЛЬОВАНИМ СПЕКТРАЛЬНИМ СКЛАДОМ

In this paper, the influence on the parameters of the synthesized light of the full width at the half-height level of the spectra of additional quasi-monochromatic LEDs for LED clusters with adjustable correlated color temperature built on the basis of a combination of white broadband high-power LEDs and quasi-monochromatic LEDs with peak wavelengths of 474 and 600 nm is considered. It was shown that the construction of LED clusters with adjustable CCT with an increase in the full width at half the height of the spectrum of quasi-monochromatic LEDs increases the CIE Ra of the resulting radiation. At the same time, the spectrum of quasi-monochromatic LEDs with a smaller full width at half the height of the spectrum has a greater influence on the change in the CCT of the resulting light, and therefore the contribution of the luminous flux of such LEDs to the resulting radiation will be less. Given that the parameters of the light sources used to create the light environment in which a person is, depends on the secretion of the hormones melatonin and cortisol, which significantly affect the well-being and psychophysiological state of a person, it is becoming increasingly common to build lighting systems with adjustable correlated color temperature. At the same time, achieving high luminous efficiency of modern LED lighting systems often negatively affects the quality parameters of the light environment they create. Non-compliance of the spectral composition of light with human needs at a certain time of day can adversely affect the natural cycles of human biorhythms. The results of the work contribute to the development of further research in the direction of finding new ways to build modern powerful energy-efficient LED lighting systems.

A DNA biosensors-based microfluidic platform for attomolar real-time detection of unamplified SARS-CoV-2 virus

The emergence of the coronavirus 2019 (COVID-19) arose the need for rapid, accurate and massive virus detection methods to control the spread of infectious diseases. In this work, a device, deployable in non-medical environments, has been developed for the detection of non-amplified SARS-CoV-2 RNA. A SARS-CoV-2 specific probe was designed and covalently immobilized at the surface of glass slides to fabricate a DNA biosensor. The resulting system was integrated in a microfluidic platform, in which viral RNA was extracted from non-treated human saliva, before hybridizing at the surface of the sensor. The formed DNA/RNA duplex was detected in presence of SYBR Green I using an opto-electronic system, based on a high-power LED and a photo multiplier tube, which convert the emitted fluorescence into an electrical signal that can be processed in less than 10 min. The limit of detection of the resulting microfluidic platform reached six copies of viral RNA per microliter of sample (equal to 10 aM) and satisfied the safety margin. The absence of non-specific adsorption and the selectivity for SARS-CoV-2 RNA were established. In addition, the designed device could be applicable for the detection of a variety of viruses by simple modification of the immobilized probe.

Photocatalytic Oxidative Cleavage of Alkenes by Molecular Oxygen: Reaction Scope, Mechanistic Insights, and Flow Application.

The oxidative cleavage of C═C bonds with molecular oxygen was promoted effectively by a catalytic amount of a decatungstate photocatalyst using black light irradiation (365 nm). Not only aromatic ketones but also aliphatic ketones were obtained by the photocatalytic protocol. The continuous flow reaction of α-methylstyrene using a high-power ultraviolet light-emitting diode (365 nm) dramatically decreased the reaction time.

Oxygen-Free Compound Casting of Aluminum and Copper in a Silane-Doped Inert Gas Atmosphere: A New Approach to Increase Thermal Conductivity

Novel aluminum-copper compound castings devoid of oxide layers at the interface between the joining partners were developed in order to increase the thermal conductivity of the hybrid component. Due to the natural oxide layers of both aluminum and copper, metallurgical bonds between such bi-metal castings cannot be easily achieved in conventional processes. However, in an atmosphere comparable to extreme high vacuum created by using silane-doped inert gas, metallurgical bonds between the active surfaces of both aluminum and copper can be realized without additional coatings or fluxes. An intermetallic was created between aluminum and copper. Thus, very high thermal conductivities could be obtained for these hybrid castings, exceeding those of conventionally joined samples considerably. The intermetallic phase seams emerging between the joining partners were investigated using scanning electron microscopy and X-ray diffraction. The reduction of casting temperatures resulted in narrower intermetallic phase seams and these in turn in a much lower contact resistance between the two joining partners. This effect can be utilized for increasing the heat transfer capabilities of compound casting components employed for cooling heat sources such as high-power light-emitting diodes.

Film Thickness and Glycerol Concentration Mapping of Falling Films Based on Fluorescence and Near-Infrared Technique.

Falling film evaporation processes involve high fluid velocities with continuous variations in local film thickness, fluid composition, and viscosity. This contribution presents a parallel and complementary film thickness and concentration mapping distribution in falling films using a non-invasive fluorescence and near-infrared imaging technique. The experiments were performed with a mixture of glycerol/water with a mass fraction from 0 to 0.65 gglycgtotal-1 and operating ranges similar to evaporation processes. The measurement system was designed by integrating two optical measurement methods for experimental image analysis. The film thickness was evaluated using a VIS camera and high-power LEDs at 470 nm. The local glycerol concentration gglycgtotal-1 was determined using a NIR camera and high-power LEDs at 1050, 1300, 1450 and 1550 nm. A multiwavelength analysis with all NIR wavelengths was implemented with a better correlation for falling films at low flow velocity. The results show an improvement in the analysis of falling films with high flow velocities up to almost 500 mm/s by using only the 1450 nm wavelength and the fluorescence measurement. Simultaneous imaging analysis of film thickness and concentration in falling films provides further insight into understanding mass and heat transport and thus supports the optimization of falling film evaporators.

Heat dissipation performance improvement of a solid-state fan using copper foams as collecting electrode

Heat dissipation for electronic devices is a rising issue today. In the present work, a solid-state fan (SSF) using copper foam as a collecting electrode has been developed. Experimental comparison and evaluation of the discharge characteristics and heat dissipation performance led to the optimization of the SSF's structure. The cooling of a high-power LED chip using the proposed SSF is followed by an experimental investigation into the luminous characteristics of the cooled LED chip. SSF with a mesh collector is compared with it as well. The results indicate that the 'needle-to-copper foam' SSF has a broader operating voltage range and a higher self-maintained discharge voltage than the 'needle-to-mesh' SSF, which leads to superior cooling performance at high applied voltage. The cooling impact is worse as the porosity increases but gets better with higher PPI values. The high-power LED chip's case temperature decreased by an additional 2.3 °C when the optimized SSF was used to cool it, and the output luminous properties is improved. Due to the benefits of comprehensive performance, a 'needle-to-copper foam' SSF has greater application potential in the heat dissipation of electronic devices than a 'needle-to-mesh' SSF.