A general photo-electro-thermo-temporal theory for light-emitting diode (LED) systems

With the emergence of new color-mixing LED systems based on LED devices of different color temperatures, the need for a new modeling technique for LED systems with nonidentical LED devices becomes imminent. This paper presents a modeling tech- nique for LED systems with LED arrays comprising nonidenti- cal LED devices that have nonidentical optical-thermal-electrical properties. Based on a general 3-D photo-electro-thermal LED node model, LED devices of different kinds can be arranged in var- ious array forms according to their system construction and design. By linking the system matrix to the correlated-color-temperature prediction, the proposed modeling technique provides an accu- rate prediction of the temperature distribution, luminous flux, and correlated color temperature of the LED systems. The tempera- ture distribution and light output of the LED systems have been measured using an infrared imaging system and a spectrophoto- colorimeter with an integrating sphere. The modeling technique has been successfully demonstrated and experimentally verified on several LED systems comprising nonidentical LED devices. It is particularly useful as a modeling tool to study new color-mixing LED systems based on different types of LED devices. Index Terms—Correlated color temperature (CCT), light- emitting diodes (LED), lighting system, nonidentical LEDs, photo- electro-thermal (PET), thermal model.

Current imbalance among parallel light-emitting diode (LED) strings could put excessive current and thermal stress on some of the LEDs in the systems, resulting in reduction in system lifetime. For LED road lighting systems, reliability is the paramount factor. This paper first explains how existing current-mirror (CM) circuits cannot cope with LED open-circuit faults and then describes a self-configurable CM circuit that can withstand open-circuit faults in LED systems with parallel LED strings. The ability to withstand open-circuit faults means that the LED systems can still function with reduced luminous output even if one LED string is cut off. The proposed circuit, which retains the feature of not requiring an auxiliary dc power supply, has been practically implemented and successfully tested in a 70-W LED system with three parallel strings.

This paper proposes a closed-loop nonlinear scheme for precisely controlling the luminosity and correlated color temperature (CCT) of a bicolor adjustable light-emitting diode (LED) lamp. The main objective is to achieve a precise and fully independent dimming and CCT control of the light mixture emitted from a two-string LED lamp comprising warm-white and cool-white color LEDs, regardless of the operating conditions and throughout the long operating lifetime of the LED lamp. The proposed control method is formulated using the nonlinear empirical LED model of the bicolor white LED system. Experimental results show that with the proposed closed-loop nonlinear approach, both CCT and dimming control of the bicolor lamp is significantly more accurate and robust to ambient temperature variations, ambient light interference, and LED aging than the conventional linear approach used in existing products. The maximum error in luminous flux employing the proposed closed-loop nonlinear approach is 3%, compared with 20% using the closed-loop linear approach. The maximum deviation in CCT is only 1.78%, compared with 27.5% with its linear counterpart.

In some well-established laser applications where large spot sizes are used, an array of high-intensity light emitting diodes (LED) emitting at similar wavelength could potentially replace the laser. This situation applies for the photodynamic bleaching of stains in teeth. This study compared the relative efficacy of an array of visible green LED (535 nm ± 15 nm) with a KTP laser in photodynamic bleaching of tetracycline-stained dentine in human tooth roots. After establishing consistent staining in 96 roots using a validated method, the roots were sectioned into 2-3-mm thick horizontal slices that were treated with gels containing rhodamine B (Smartbleach® or Smartbleach® 3LT). Colour changes were tracked up to 1 month after treatment. While both systems were effective in bleaching the tetracycline-stained dentine, KTP laser activation gave greater bleaching efficacy than LED activation, enhancing the action of the gel. Use of the KTP laser would be preferable over an LED system when confronted with tetracycline staining. Use of this photodynamic bleaching method offers valuable means to reduce the severity of tetracycline staining.

Objective: Otitis media (OM) is an infectious and inflammatory disease of the middle ear (ME) that often recurs and requires long-term antibiotic treatment. Light emitting diode (LED)-based devices have shown therapeutic efficacy in reducing inflammation. This study aimed to investigate the anti-inflammatory effects of red and near-infrared (NIR) LED irradiation on lipopolysaccharide (LPS)-induced OM in rats, human middle ear epithelial cells (HMEECs), and murine macrophage cells (RAW 264.7).Methods: An animal model was established by LPS injection (2.0 mg/mL) into the ME of rats via the tympanic membrane. A red/NIR LED system was used to irradiate the rats (655/842 nm, intensity: 102 mW/m2, time: 30 min/day for 3 days and cells (653/842 nm, intensity: 49.4 mW/m2, time: 3 h) after LPS exposure. Hematoxylin and eosin staining was performed to examine pathomorphological changes in the tympanic cavity of the ME of the rats. Enzyme-linked immunosorbent assay, immunoblotting, and RT-qPCR analyses were used to determine the mRNA and protein expression levels of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α). Mitogen-activated protein kinases (MAPKs) signaling was examined to elucidate the molecular mechanism underlying the reduction of LPS-induced pro-inflammatory cytokines following LED irradiation.Results: The ME mucosal thickness and inflammatory cell deposits were increased by LPS injection, which were reduced by LED irradiation. The protein expression levels of IL-1β, IL-6, and TNF-α were significantly reduced in the LED-irradiated OM group. LED irradiation strongly inhibited the production of LPS-stimulated IL-1β, IL-6, and TNF-α in HMEECs and RAW 264.7 cells without cytotoxicity in vitro. Furthermore, the phosphorylation of ERK, p38, and JNK was inhibited by LED irradiation.Conclusion: This study demonstrated that red/NIR LED irradiation effectively suppressed inflammation caused by OM. Moreover, red/NIR LED irradiation reduced pro-inflammatory cytokine production in HMEECs and RAW 264.7 cells through the blockade of MAPK signaling.

The light-emitting diodes (LEDs) are used widely around the world which have long life and are eco-friendly.In the past few decades, LEDs has made major breakthroughs in materials, packaging, and design.LEDs are developing in the direction of miniaturization and integration to meet the different occasions.At the same time, the heating problem of LEDs has become increasingly prominent.The heating power of high-power LEDs accounts for 60%-70% of the input electrical power due to the nonradiative recombination with the multiple quantum well layer and the light confinement within the LEDs packaging, which will cause sharply temperature rising of LEDs [1][2].High temperature will cause luminous flux degradation, wavelength drift, lifetime reduction, and even irreversible damage to the LEDs.Therefore, thermal management and thermal prediction for high-power LEDs are very important which ensure the stable operation of LEDs.At present, the cooling of LEDs is achieved by forced air cooling and liquid cooling.However, forced air cooling is usually bulky and noisy, and the low heat dissipation efficiency of forced air convection limits its application.Therefore, high-performance liquidcoolant-based thermal management technology is a competitive choice for LEDs due to its high heat dissipation efficiency.Spray cooling is a promising liquid-coolant-based thermal management technology for high-heat-flux electronic devices [3][4].It has the advantages of high critical heat flux (CHF), uniform surface temperature distribution, low surface overheating, and less working fluid demand.In the past, a large number of researchers conducted experiments and simulations on it.Although many of studies have shown that spray cooling is an effective way to dissipate heat from electronic devices with high heat flux, there are relatively few researches on spray cooling for LEDs, especially for high-power LEDs [5-6], which are not enough to provide guide for designing and application.In this work, a spray cooling system is proposed and it is used to verify the feasibility, advancement, and effectiveness of spray cooling on high-power LEDs of which the nominal power is 100 W. At the same time, in order to accurately explore the temperature distribution of LEDs, a model predictive inverse method is proposed and carried out.The results show that as the flow rate increases, the LEDs backside substrate temperature and target surface temperature will decrease, and the average heat transfer coefficient of the target surface will increase.The maximum heat transfer coefficient is as high as 26400 W/m 2 K.The Sauter mean diameter is one of the most important parameters of spray cooling.In this work, it is found that under the condition of high pressure drop and high flow rate, the Sauter mean diameter is smaller, which will enhance heat dissipation.At the same time, it is also discovered that as long as the proper thermal management is carried out, the LEDs can work normally when it exceeds 2.5 times of the nominal power.Three-dimensional simulation effectively supplements the experimental results, confirming that the closer to the junction point, the more uneven the temperature distribution.And spray cooling can keep the temperature of the LEDs chip below the temperature of destruction.Moreover, the prediction algorithm enables efficient estimation of the future temperature field of LEDs and early measures to prevent it from damage.According to the research results, spray cooling has been proven as an effective method for the thermal management of highpower LEDs.The design of the spray cooling system and prediction model may aid in solving the dilemma of overheating electronics.

One of the methods for creating white light with light-emitting diodes (LEDs) is mixing radiations from several different colored LEDs. Mixed-color LEDs are expected to have greater luminous efficacy because they do not undergo down-conversion losses like phosphor-converted white LEDs. However, in reality mixed-color LED systems require extra optical elements to reduce spatial color variation and create uniform white light. Optical diffusing techniques commonly used for these purposes cause light loss, because some portion of the light is scattered back toward the LEDs where it is absorbed and lost. In 2004, a technique known as scattered photon extraction (SPE) was used to extract backscattered light from the phosphor layer of phosphor-converted white LEDs to increase overall light output. In this study, it was hypothesized that by using similar SPE optics with optical diffusers, the spatial color uniformity of mixed-color white LED systems could be improved without sacrificing the overall luminous efficiency. With this new approach, both a microsphere-doped diffuser and SPE optics were utilized. The experiments showed that the proposed setup did increase the spatial color uniformity of the mixed-color LED system with more than 79% overall optical efficiency. This study also demonstrated the effects of microsphere size, concentration, and diffuser thickness on spatial color uniformity and optical efficiency.

Light-emitting diode (LED) technology is becoming the choice for many lighting applications that require monochromatic light. However, one potential problem with LED-based lighting systems is uneven luminance patterns. Having a uniform luminance distribution is more important in some applications. One example where LEDs are becoming a viable alternative and luminance uniformity is an important criterion is backlighted monochromatic signage. The question is how much uniformity is required for these applications. Presently, there is no accepted metric that quantifies luminance uniformity. A recent publication proposed a method based on digital image analysis to quantify beam quality of reflectorized halogen lamps. To be able to employ such a technique to analyze colored beams generated by LED systems, it is necessary to have contrast sensitivity functions (CSFs) for monochromatic light produced by LEDs. Several factors including the luminance, visual field size, and spectral power distribution of the light affect the CSFs. Although CSFs exist for a variety of light sources at visual fields ranging from 2 degrees to 20 degrees, CSFs do not exist for red, green, and blue light produced by high-brightness LEDs at 2-degree and 10-degree visual fields and at luminances typical for backlighted signage. Therefore, the goal of the study was to develop a family of CSFs for 2-degree and 10-degree visual fields illuminated by narrow-band LEDs at typical luminances seen in backlighted signs. The details of the experiment and the results are presented in this manuscript.

Engineering design and assessment of a demand-sensitive LED streetlighting system

High-energy ultraviolet (UV) light is an environmental stress that can be used to stimulate the biosynthesis of bioactive compounds in plants. This study aimed to comparatively determine the effects of UV-A, UV-B, and UV-C lamps or light-emitting diodes (LEDs) on the growth of dropwort (Oenanthe stolonifera) plants, and their contents of bioactive compounds. Dropwort seedlings with 2–3 offshoots were transplanted in a plant factory equipped with white LED and deep flow technique systems, and cultivated under standard growth conditions for 36 days. Thereafter, the dropwort plants were supplementally exposed to one of five UV treatments with energy equivalent to 10 W m−2: UV-C lamps for 2 days, UV-B lamps for 3 days, and UV-A lamps and LEDs with 370 nm or 385 nm peak wavelengths for 14 days. The variable fluorescence (Fv) to maximum fluorescence (Fm) ratio (Fv/Fm) of dropwort leaves began to significantly decrease 3 h after exposure to UV-C, and 6 h after UV-B exposure. Fluorescence in UV-C and UV-B-treated plants was lower than in control and UV-A-treated plants during the entire period of UV irradiation. The fresh weight of the shoots of plants treated with UV was not significantly different to those of the control plants during the entire UV irradiation period. The total phenolic content of dropwort shoots exposed to UV-A and UV-B treatments significantly increased compared to that of the control 1 day after treatment. The total phenolic content was highest in plants treated with the 370 nm UV-A LED, and this was significantly higher (33%) than the control. Plants treated with the 385 nm UV-A LED on day 3 of treatment had the highest total phenolic content compared to the other treatments. A similar trend was observed in contents of flavonoids and persicarin. UV light induced higher anthocyanin content than the control. The activity of phenylalanine ammonia-lyase after UV treatments was significantly higher than the control, supporting the findings of our bioactive compound assays. In conclusion, the results of this study suggest that irradiating vegetables with UV-A LEDs would be useful in plant factories with artificial light for improving vegetable quality without inhibiting growth.

Light emitting diodes (LEDs) have become an effective lighting solution because of the characteristics of energy efficiency, flexible controllability and extended lifetime. They find use in numerous lighting systems for residents, industries, enterprises and street lighting applications. The efficiency and trustworthiness of the LED systems considerably based on the thermal mechanical loading improved several degradation schemes and respective interfaces. The complication of the LED systems limits the theoretic interpretation of the core reasons for the luminous variation or the formation of the direct correlation among the thermal aging loading and the luminous output. Therefore, this paper designs a new hybrid Henry gas solubility optimization with deep learning (HHGSO-DL) algorithm for LED driver system design. The presented HHGSO-DL technique mainly concentrates on the derivation of empirical relationships among the design parameters, thermal aging loading and luminous outcomes of the LED product. In the presented HHGSO-DL technique, bidirectional long short-term memory (BiLSTM) algorithm is executed for examining the empirical relationship and its hyperparameters can be tuned by the HHGSO algorithm. In this work, the HHGSO algorithm is derived by the integration of traditional HGSO algorithm with oppositional-based learning (OBL) concept. The performance of the HHGSO-DL technique can be investigated on LED chip packaging and LED luminaire with thermal aging loading. The extensive results demonstrate the promising performance of the HHGSO-DL technique over other state-of-the-art approaches.

With a long experience in optoelectronics, CEA-LETI has focused on Light Emitting Diode (LED) lighting since 2006. Today, all the technical challenges in the implementation of GaN LED based solid state lighting (SSL) are addressed at CEA-LETI who is now an RandD player throughout the entire value chain of LED lighting. The SSL Line at CEA-LETI first deals with the simulation of the active structures and LED devices. Then the growth is addressed in particular 2D growth on 200 mm silicon substrates. Then, technological steps are developed for the fabrication of LED dies with innovative architectures. For instance, Versatile LED Array Devices are currently being developed with a dedicated μLED technology. The objective in this case is to achieve monolithical LED arrays reported and interconnected through a silicon submount. In addition to the required bonding and 3D integration technologies, new solutions for LED chip packaging, thermal management of LED lamps and luminaires are also addressed. LETI is also active in Smart Lighting concepts which offer the possibility of new application fields for SSL technologies. An example is the recent development at CEA LETI of Visible Light Communication Technology also called LiFi. With this technology, we demonstrated a transmission rate up to 10 Mb/s and real time HD-Video transmission.

The light-emitting diode (LED) is the major source of nonlinearity in LED communication systems, which needs to be carefully handled to avoid significant performance degradation. Post-distortion is an effective and convenient way to compensate the nonlinear distortion at the receiver side. In this letter, we propose a soft-in-soft-out (SISO) post distorter, which works with an SISO decoder iteratively by exchanging log-likelihood ratios of the coded bits at the receiver. It is demonstrated that the proposed iterative technique can achieve a remarkable performance gain compared to the conventional post-distortion technique in the literature.

EOL lighting equipment has been regarded as hazardous household waste (HHW) which is often disposed together with general waste in Thailand. Such improper management of EOL has caused increasing risk of environmental and health impact to Thai society. This study has two main objectives: (1) To estimate an EOL inventory of FLs and LEDs in Thailand and (2) To evaluate and compare environment impacts of FLs and LEDs throughout their life cycle and identify the appropriate approach for handling discarded FLs and LEDs at the end of life stage in Thailand. The study presents life cycle assessment (LCA) of two lighting technologies, fluorescent lamp (FL) and light emitting diode (LED) that are used widely in Thailand. The gate to grave investigation for FL and LED includes manufacturing, distribution, use and end-of-life (EOL) scenarios. The analysis used the SimaPro 7.1.8 program under the CML2001 assessment method for the midpoint and the Eco-indicator 99 assessment method for the endpoint. Furthermore, this research evaluated the potential future impact from different management schemes including: (1) 100% landfilling, (2) 10% recycling and 90% landfilling, (3) 30% recycling and 70% landfilling. The results showed that (1) the quantity of EOL FL has been continuously decreased from 60.15 million units in 2011 to 27.23 million units in 2025. The EOL LED will be increased in 2021 (365.62 million units) and decreased in 2024. Then, the quantity of EOL LED will be still increased in 2030 (250.1 million units). The finding is based on the expectation that the government will continue promoting the fluorescent replacement with LEDs as one of energy saving measures of the country. (2) the environmental impact of LED has lower in major impact categories than that of FL about 70%. The use phase caused the majority of the environmental impacts: 98.94% in FL and 96.46% in LED, followed by the manufacturing and EOL stage. For the EOL stage, FL and LED landfilling could contribute highest negative burdens to the human health, ecosystem quality and resource depletion impact compared to recycling approach. The results found that the recycling would be better than landfilling about 90.44 fold in FL and 296.36 fold in LED, respectively. Additionally, it revealed that the most benefits come from aluminum scrap recycling which can reduce 94.4% in the FL and the magnetic ballast. On the other hand, the LED recycling also distributes the profits to resources sustainability by recycling the aluminum from the heatsink and polycarbonate from tube cover of LED lamp set. Based on this study finding and leaching test result from the literature, this study recommended that LED can be considered as a non-hazardous waste which can be disposed normally with other non-hazardous waste. Given the potential of recycling LED and the hazardous composition in FL, lighting equipment should be included as one of priority products in the draft Thai WEEE management law.

The efficiency of light-emitting diode (LED) devices is a significant factor that reflects the capability of these devices to convert electrical power into optical power. In this study, a method for estimating the efficiency of LED devices is proposed. An efficiency model and a heat power model are established as convenient tools for LED performance evaluation. Such models can aid in the design of LED drivers and in the reliability evaluation of LED devices. The proposed estimation method for the efficiency and heat power of LED devices is verified by experimentally testing two types of commercial LED devices.