Search Results (6)

Artificial lighting is ubiquitous in modern society, with detrimental effects on sleep and health. The reason for this is that light is responsible not only for vision but also for non-visual functions, such as the regulation of the circadian system. To avoid circadian disruption, artificial lighting should be dynamic, changing throughout the day in a manner comparable to natural light in terms of both light intensity and associated color temperature. This is one of the main goals of human-centric lighting. Regarding the type of materials, the majority of white light-emitting diodes (WLEDs) make use of rare-earth photoluminescent materials; therefore, WLED development is at serious risk due to the explosive growth in demand for these materials and a monopoly on sources of supply. Photoluminescent organic compounds are a considerable and promising alternative. In this article, we present several WLEDs that were manufactured using a blue LED chip as the excitation source and two photoluminescent organic dyes (Coumarin 6 and Nile Red) embedded in flexible layers, which function as spectral converters in a multilayer remote phosphor arrangement. The correlated color temperature (CCT) values range from 2975 K to 6261 K, while light quality is preserved with chromatic reproduction index (CRI) values superior to 80. Our findings illustrate for the first time the enormous potential of organic materials for supporting human-centric lighting. Full article

A remote-type white light-emitting diode (LED) consisting of a red quantum-dot (QD) film and a yellow phosphor plate was studied by both experiment and optical simulation. The sequence of the two color-conversion films had a substantial effect on the color-rendering properties of the vertically-stacked white LED, and the optimized configuration exhibited a high color rendering index of more than 90 thanks to the enhanced red component via the QD film. For the design of high-power white LED devices of a remote type, it was necessary to locate the color-conversion films below the diffuser plate to remove the substantial color dispersion depending on the viewing angle. The present study shows that high power and high color-rendering white LED devices can be realized in terms of two vertically-stacked color-conversion materials, which would provide long-term stability due to the remote design. Full article

Solid-state lighting (SSL) sources based on light-emitting diodes represent the new generation of highly efficient illumination systems that significantly impact energy-saving. The development of white light-emitting diodes (WLEDs) with a combination of high color rendering index (CRI) and high deep-red color rendering R₉ is an important challenge in the field of solid-state lighting. On the other hand, most WLEDs use rare-earth inorganic luminescent materials. The annual demand for rare-earth metals has doubled to 125,000 tons in 15 years, and the demand is projected to reach 315,000 tons in 2030. The explosion in demand for these materials, combined with a monopolistic supply source, represents a real risk for the development of WLEDs in the next few years. Luminescent organic materials are a relevant and promising alternative. Here, we report a WLED with a very high CRI of 95.7 and R₉ of 78.7, obtained using a combination of a blue LED chip (excitation source) and two organic luminescent dyes (Coumarin 6 and Lumogen Red) acting as spectral converters in a multilayer remote phosphor configuration. To the best of our knowledge, this is the first rare-earth-free WLED with such high values of CRI and R₉. Full article

With the development of high-efficiency and high-power LEDs, they have become the most energy-efficient and environmentally friendly artificial light source. Phosphor-converted white LEDs are currently mainstream in the market. The remote phosphor is an effective way to enhance the conversion efficiency and lifetime of phosphor-converted LEDs. For applications of high-quality lighting and LCD backlights, the uniformity of angular correlated color temperature (CCT) is very important. This report explored a remote phosphor white LED with low angular CCT variance and high luminous efficiency by using TiO₂ diffuser-loaded encapsulation. Experimental results revealed that for the TiO₂ diffuser-loaded encapsulation remote phosphor white LED, the angular color uniformity could be improved by 31.82% and the luminous flux by 8.65%. Moreover, the mean CCTs of the TiO₂ diffuser-loaded encapsulation and non-diffuser remote phosphor white LEDs were similar at a driving current of 350 mA. Finally, we showed that incorporating the TiO₂ diffuser into the phosphor layer of the remote phosphor white LEDs, does not influence the reliability of the LED. Full article

The Scattered Photon Extraction (SPE™) based on the concept of TIR lens combined with remote phosphor is proven to be one of the effective solutions for improving white LED efficiency, and it provides the omnidirectional light distribution for luminaire design. Not only the light extraction efficiency (LEE) is important, but also the angular uniformity of correlated color temperature (CCT) is a critical index in the evaluation of high-quality white LEDs. A non-optimized SPE™ will cause an increase in the angular CCT deviation (ACCTD) and ultimately affect lighting quality. Two possible ways using lens design are proposed to reduce the ACCTD and even improve its efficiency. Among them, using the concept of light guiding to design the lens can minimum the deviation of forward and backward CCT from 2720 K to 657 K, and the overall efficiency can be further enhanced by 12% compared to typical SPE™ lens. Full article

In this work, we report on the characterization and reliability/stability study of phosphorescent materials for lighting applications. More specifically, we investigated (a) phosphors directly deposited over light-emitting diodes (LED) chip, (b) remote phosphor (RP) solutions encapsulated in plastic medium for LED lighting, and (c) phosphors without binder for extreme high-intensity laser diode white lighting. The optical and thermal properties of phosphors were studied to develop a sample based on a mix of phosphor compounds in order to achieve different correlated color temperatures (CCT) and high color rendering index (CRI) LEDs. Thermal properties of cerium-doped YAG (Yttrium Aluminum Garnet) phosphor materials were evaluated in order to study thermal quenching. A maximum phosphor operating temperature of 190–200 °C was found to cause a sensible efficiency degeneration. Reduced efficiency and Stokes shift also caused a localized temperature increase in the photoluminescent materials. In the case of remote phosphors, heat did not find a low thermal resistance path to the heatsink (as occurred through the GaN LED chip for direct phosphor-converted devices) and thermal analysis indicated that material temperature might therefore increase to values in excess of 60 °C when a radiation of 435 mW/cm² hit the sample template. Reliability was also investigated for both plastic-encapsulated materials and binder-free depositions. Pure thermal reliability study indicated that phosphors encapsulated in polycarbonate material were stable up to temperature of approximately 100 °C, while binder-free phosphor did not show any sensible degradation up to temperatures of 525 °C. Full article