Luminous Efficiency Of Radiation Research Articles

The design and synthesis of 1-phenylimidazo[1,5-a]pyridine – anthracene-based fluorophore for greenish-yellow organic light emitting diode and warm white LED

Solution-processed OLEDs offer a promising and affordable technology for solid-state lighting and superior self-luminous display panels. Herein, an imidazo[1,5-a]pyridine (ImPy)-based fluorophore decorated with an aromatic π-system anthracene is synthesized by a one-pot condensation approach. From the theoretical study, the energy difference between HOMO and LUMO is 3.1 eV. The molecule shows excellent thermal stability and electrochemical stability. It exhibits efficient intramolecular charge transfer and displays a positive solvatochromic photoluminescence emission band ranging from 528 nm in hexane to 586 nm in acetonitrile with large Stokes’ shift of ∼11,000 cm−1. The OLEDs made of this emitter displayed good device performance with luminous efficiency of 4.4 cd A–1, power efficiency of 2.2 lm W–1 and external quantum efficiency of 3.2%. Additionally, this OLED emitted greenish-yellow light with CIE coordinate (0.34, 0.45) and possessed a low turn-on voltage of 7 V. The material also used in white LED fabrication and the fabricated white LED displayed an excellent color rendering index (CRI), the luminous efficiency of radiation (LER), and correlated color temperatures (CCT) of 89328 lm W–1 , and 4907 K, respectively with CIE coordinate (0.35,0.32).

Efficient thermal and luminescent regulations of LuAG: Ce-PiG based remote LED/LD

The quantum loss in phosphor-in-glass (PiG) and energy loss in LD/LED exciting source could aggravate the thermal load of a glass-based luminescence device. In this study, luminous efficiency of radiation (LER) and thermal shock in LuAG: Ce PiG-based remote LD/LED prototypes were regulated effectively, by controlling the exciting distance between PiG and blue LD/LED. For remote LD, both the simulated temperature and the real working temperature of LuAG: Ce-PiG were initially increased and then declined with increase of the exciting distance (the numerical difference: 9.3%). The maximum working temperature of LuAG: Ce-PiG was 135.1 °C. Remarkably, LER of LuAG: Ce-PiG was initially reduced and then elevated with increase of the exciting distance, and the obtained maximum LER was 203.97 lm/W. Meanwhile, increasing the working distance leads to the decrease of both LER and working temperature of the PiG based LED, since the excitation loss was negatively correlated to the exciting distance. For the better application of LD/LED device in high power lighting and display, this work strictly controlling exciting distance is highly necessary.

Wide Range CCT Laser-Based Illuminant With High Efficiency and Excellent Optical Performances

A wide range correlated color temperature (CCT) white light laser-based illuminant (LBI) consisting of dual blue laser diodes (LDs), yellow phosphor color converters and red LDs has been proposed. In addition to the conventional 455 nm blue LDs-pumped phosphor color converters, another blue LD is utilized for the adjustment of CCT and improvement of the color rendering index (CRI). Considering systematic manipulation of the power ratio of LDs and peak wavelength of tuning blue LDs from 440 nm to 480 nm, different combinations of spectral power distributions of LBIs were simulated. It shows that an optimized 465 nm tuning blue LD is employed to achieve a balance between optical performances and blue light hazard efficiency of radiation (BLHER). The first practical LBI comprised of Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> :Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> (YAG:Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ) phosphor color converters and three monochromatic LDs of 455 nm, 465 nm and 635 nm was fabricated based on the stated simulations. In good agreement with the simulation results, the LBI has simultaneously achieved an excellent CRI beyond 85, a high luminous flux of approximately 2000 lm and a high luminous efficiency of radiation (LER) of over 300 lm/W at the fairly wide CCT range of 3300 K to 6000 K. The low BLHER was consistently below 0.237. The circadian action factor (CAF) was between 0.36 and 0.83, and its tunability was 2.31 above the CCT range, which favors the entrainment of circadian rhythm. Therefore, this work demonstrates a promising method for providing high-quality and human-oriented white light sources in applications requiring great brightness.

CsPbI3/N-GQDs dual layer phosphor-converted white-LEDs with ultrahigh luminous efficiency and color rendering index

Phosphor-converted LEDs or pc-LEDs, as a solid-state lighting source, are attractive for next-generation display technologies because of their energy savings, and green environmentally friendly nature. Recently, white LEDs are being produced commercially by coating blue LED (440–470 nm) chips with various yellow-emitting phosphors. However, the LEDs produced by this technique often exhibit high correlated color temperature (CCT) and low color rendering index (CRI) values, due to sufficient red spectral components not being present, and thus aren’t suitable for commercial grade white illumination. To circumvent this drawback, our work reports for the first time the use of blue and green-emitting nitrogen-functionalized graphene quantum dots (GQDs) coupled with red-emitting CsPbI3 NCs for phosphor-based LED applications. We deployed near-UV to visible excitable red-emitting perovskite CsPbI3 nanocrystals which contribute toward the red spectral component, thus greatly improving the CRI of the LEDs. CsPbI3 nanocrystals are optically excited by nitrogen-functionalized GQD with blue and green emissions in a remote double-layer phosphor stack technique. This double phosphor layer stacking greatly improves both the CRI and luminous efficiency of radiation (LER), which usually has a trade-off in previously reported phosphor stacks. A CCT of ∼5182 K providing daylight white tonality, with superior CRI (∼90%) and ultrahigh LER (∼250 lumens/watt) are reported, which are significantly higher than the established benchmarks.

Analysis of fluorescence characteristics of Sm3+-doped B2O3-rich glasses for Orange-light-emitting diodes

Optical absorption and visible luminescence aspects including visible emission decay times have been examined for six Sm3+ (1 mol%)-doped B2O3-rich glasses comprising single and mixed alkali oxides which were synthesized via melting-and-quenching approach. Upon intense λexci. = 409 nm, fluorescence spectra displayed four bands peaked at 562 nm, 598 nm, 645 nm, and 706 nm owing to 4G5/2 upper level to 6H5/2, 6H7/2, 6H9/2, and 6H11/2 lower levels transitions respectively. Among identified emissions, orange fluorescence (598 nm) is found to be strong in all samples. Moreover Sm3+: Li ions having sample exhibits the dominant visible emissions band intensity in all samples, so for this sample, Judd–Ofelt (JO) analysis was carried out to calculate JO parameters Ωt (t = 2, 4, 6), and from them, radiative transition probabilities, branching ratios (calculated and experimental), and radiative lifetimes of Sm3+: 4G5/2 level to distinct lower energy states were derived utilizing absorption and emission spectra. CIE (Commission Internationale de l′éclairage) coordinates, correlated color temperature (CCT), color purity, color rendering index, and luminous efficiency of radiation were determined from visible luminescence spectra of all studied glasses, and obtained CIE coordinates and CCT values lie in the reddish-orange light region of CIE diagram, indicating their potential for orange LEDs (light-emitting diodes) application. Here derived CCT values are lower than the typical warm light sources CCT limit (<3200 K). 4G5/2 state luminescence decay patterns revealed double exponential behavior for assessing the measured lifetimes. Effective bandwidth, stimulated emission cross-section, and gain bandwidth were computed for observed luminescence transitions of Sm3+: Li ions possessing sample.

Pr3+-doped B2O3-Bi2O3-ZnO-NaF glasses comprising alkali/mixed alkali oxides for potential warm white light generation, blue laser, and E-+S-+C-optical bands amplification applications

For six 1 mol% Pr 3+ -doped B 2 O 3 –Bi 2 O 3 –ZnO–NaF glasses consisting of single and mixed alkali oxides, optical absorption and visible and near-infrared (NIR) luminescence traits including visible fluorescence decay times were investigated. Judd–Ofelt (J‒O) analysis from absorption spectra was performed to calculate J‒O parameters Ω λ ( λ = 2, 4, 6) by including and omitting 3 H 4 → 3 P 2 transition. Utilizing Ω 2 , Ω 4 , and Ω 6 values and refractive indices, radiative transition probabilities ( A R ), branching ratios ( β R ), and radiative lifetimes for all Pr 3+ ion's luminescent levels were estimated. For observed intense blue fluorescence band upon 443 nm optical pumping, various parameters considered in designing visible laser systems were evaluated. Pr 3+ : Na + K ions comprising glass possesses large spectroscopic quality factor, high A R and β R , large stimulated emission cross-section ( σ em ), high quantum efficiency, large gain bandwidth, and significant optical gain for 3 P 0 → 3 H 4 transition for a potential blue lasing action among all samples. Further, Commission Internationale de l'éclairage (CIE) coordinates, correlated color temperature (CCT), color rendering index, and luminous efficiency of radiation were derived from visible luminescence spectra and deduced CIE and CCT values represent the neutral or warm white light region. Upon 3 H 4 → 3 P 2 excitation, NIR luminescence spectra show 3 P 1 + 1 I 6 → 1 G 4 , 1 D 2 → 3 F 4 , and 1 D 2 → 1 G 4 transitions in which broadband ( 1 D 2 → 1 G 4 ) at 1.3–1.65 μm range with effective bandwidth ~157 nm was obtained for Pr 3+ : Na + K ions constituting glass with high A R (1938 s −1 ), moderate β R (26%), and large σ em (2.64 × 10 −20 cm 2 ) at 1.472 μm, and its gain profile covers the E-+S-+C-optical communication bands' range for effective broadband amplification operation.

Survey of optical and fluorescence traits of Tm3+-doped alkali/mixed alkali oxides constituting B2O3-BaO-ZnO-LiF glasses for 0.45 μm laser and 1.46 μm fiber amplifier

For six 1 mol% Tm3+-doped B2O3-BaO-ZnO-LiF glasses containing single and mixed alkali oxides (fabricated by melt-cast approach), optical absorption, and visible and near-infrared (NIR) fluorescence features including visible luminescence decay times were explored. Optical band gaps, Urbach energy, and two-photon absorption coefficients were evaluated for all studied glasses. Judd–Ofelt (J–O) analysis from absorption spectra was performed to compute Tm3+: 4f–4f transitions J–O parameters Ωλ (λ = 2, 4, 6), and utilizing Ω2, Ω4 and Ω6 values radiative transition probabilities (AR), branching ratios (βR), and radiative lifetimes (τR) for all Tm3+ ion’s excited levels were assessed. For obtained intense blue emission band (454 nm) upon 358 nm excitation, various parameters considered in developing visible laser systems were calculated. Tm3+: Na ions having sample exhibits high AR, highest peak emission cross-section (σmaxem) (=5.89 × 10–21 cm2) and gain bandwidth(σmaxem × Δλeff) (=9.69 × 10–27 cm3) for 1D2→3F4 luminescence transition in all glasses for a favorable blue lasing process. All emission decay curves of the 1D2 upper level showed nonexponential nature. Commission Internationale de l'éclairage (CIE) coordinates, color purity, and luminous efficiency of radiation were derived from visible fluorescence spectra, and attained CIE (x,y) coordinates values reflect the purplish-blue light region. Under direct optical pumping of 3H6→3H4 transition using 808 nm laser diode, NIR fluorescence spectra exhibit a wideband within 1.3–1.6 μm spectral range peaked at 1.46 μm (3H4→3F4 transition). NIR emissions effective bandwidth (Δλeff) was varied relying on different alkali oxides. Δλeff ~ 121 nm was deduced for Tm3+: Li ions comprising glass with large σmaxem (=1.832 × 10–21 cm2), high(σmaxem × Δλeff) (=2.22 × 10–26 cm3), and optical gain (=12.608 × 10–25 cm2s) for 3H4→3F4 emission transition and its gain profile wraps the entire S-optical communication band range for efficient broadband amplification purpose in wavelength-division multiplexing systems.

Ce-doped YAG single-crystals prepared by continuous wave (CW)–CO2 laser combustion technique with attractive characteristics and moderate white LED performance

In this work, the influence of continuous wave –CO2 laser-induced solution combustion as a new and effective technique to prepare YAG:Ce phosphor was investigated. The as-prepared ceramic displayed a direct pure YAG phase within the laser exposure time of 1.5 min with an attractive characteristics and moderate white light-emitting diode (WLED) performance, suggesting that laser synthesis has a role to play in accelerating the diffusion of the cations and an increase in reaction rate. However, the thermal treatment step plays a key role to adjust efficient luminescence processes. In addition, the luminescence properties were strongly dependent on the sintering strategies due to the expulsion of defects (e.g. CeO2 and other impurity phases) from matrix. We showed experimentally that increasing the luminescence intensity has an effect observed on the CIE chromaticity parameters and the theoretically calculated luminous efficiency of radiation (LER). This work presents the effects of thermal treatment properties on the YAG host lattice and explains the mechanism of YAG formation under CO2 laser beam.

Weak thermal quenching and tunable luminescence in Ce:Y3(Al,Sc)5O12 transparent ceramics for high power white LEDs/LDs

Realizing the high brightness solid-state lighting with the emission spectra similar to that of sun light is significantly challenged by the inherent deficiency of the luminescence spectrum and thermal robustness in the single sturctured Ce3+:Y3Al5O12 (Ce:YAG) transparent ceramic (TC). In this study, based on the idea of energy band engineering, the energy splitting intensity of 5d level and the energy gap between the ground band and the 5d1 energy level of Ce3+ ion in Ce:YAG TC were effectively modified via Sc doping. A series of Ce:Y3Al5−xScxO12 (Ce:YASG) TCs were fabricated with impressive properties, showing a decreased stokes shift and a distinct blue-shift (541 → 520 nm) with broadened full width in emission spectra. Particularly, the prepared TCs exhibited a desired thermal stability (only ~6% luminescent intensity loss at 150 °C) and high internal quantum efficiency (IQE) of 84.2%. The optimal composition between Ce3+ and Sc3+ ions was investigated by constructing the Ce:YASG and (Y1−yCey)3Al4ScO12 (YCASG) TC-based white LEDs/LDs in a remote excitation mode. Both the luminous efficiency of radiation (LER) as high as 218 lm/W and tunable color from pale-green (pale-white) to yellowish white (pale-green) region were obtained. Therefore, the Ce:YASG TC is a highly promising color conversion material for high power lighting and displaying applications in the future.

Realizing white LEDs with high luminous efficiency and high color rendering index by using double green phosphors

The luminous efficiency of radiation (LER) and color rendering index (CRIs) are two key technological parameters of white light-emitting diodes (WLEDs), But there is always a trade-off between them, especially between LER and R9 (the 9th CRI of red color). Therefore, it is necessary to solve this problem by selecting suitable phosphors, mixing and coating it on the LED chip. In this work, two green phosphors and one red phosphor with less resorption of the two phosphors were selected by measuring the excitation spectra of seven red phosphors and emission spectra of two green phosphors. Then, different mass ratios of the phosphors were mixed with the AB glue and coated on the blue LED chips. After optimizing the ratio by measuring the luminescence spectra of the LED chips coated with different mass ratios of phosphors, high-quality white LEDs with LER = 142.74 lm/w, Ra = 97, R9 = 98.53, and the correlated color temperature of 3666 K were obtained. These LEDs have both high LER and high CRIs, further reducing the trade-off between luminous efficiency and color rendering index, which makes it can widely used for high quality solid state lighting.

Defect induced broadband visible to near-infrared luminescence in ZnAl2O4 nanocrystals

Materials emitting over a broad range of wavelengths have been gaining attention consistently owing to their potential applications. ZnAl2O4 has been an attractive material due to its robust luminescence properties. Herein, we report white and intense near-infrared emission, simultaneously, in undoped ZnAl2O4 nanocrystals at a relatively lower temperature (550 ∘C) than reported earlier. Emission is accompanied by various types of shallow and deep donor/acceptor defects, for instance, ionic vacancies and antisite defects existing in the crystal lattice. Switching of white to blue color is observed with annealing owing to rearrangement of defects. Rietveld refinement of X-ray diffraction patterns confirm the presence of multiple defects in the crystal lattice. Transmission electron microscopic analysis has revealed the change in morphology from spherical to polyhedron and increase in crystallite size on annealing. Suitable values of illuminating engineering society colorimetric parameters such as fidelity index, luminous efficiency of radiation, color gamut etc. predict its potential applications in lighting and bioimaging.

Enhancement of efficiency and CCT uniformity for red phosphor thin films, red LEDs and laminated white LEDs based on near-ultraviolet LEDs using MgO nanoparticles.

Red phosphor thin films (PTFs) with different MgO nanoparticle concentrations for near-ultraviolet (NUV) LEDs were prepared based on their strong scattering effect; red LEDs and laminated white LEDs were packaged further. SEM and XRD showed that CaAlSiN3:Eu2+ (CASN) and MgO nanoparticles were uniformly distributed in silicone resin and their crystal structure remained unchanged. The phosphor conversion efficiency (PCE) had a maximum value of 83.15% when the MgO nanoparticle concentration was 15 wt%. An increase of concentration can improve the spatial distribution uniformity of photonics for 410 nm, 627 nm and 660 nm. Fluorescence lifetime showed that the value is positively correlated with concentration change. The packaged red LED luminous flux reaches a maximum of 20.337 lm at a concentration of 15 wt%. The laminated white LED showed that the MgO nanoparticle concentration can be used to adjust the correlated color temperature (CCT) from 4322 K to 1987 K. Under similar CCT, the red phosphor concentration is only 1.83 wt%, the dosage is reduced by 56.12%, and the corresponding luminous efficiency of radiation (LER) and luminous efficiency (LE) are 296.03 lm W−1 and 73.72 lm W−1 respectively. The increase was 11.42%, the decrease was 10.14%, the color rendering index (CRI) increased from 90.6 to 91.8, and CCT uniformity increased from 82.04% to 89.27% with an increase of 8.81%. Research shows that MgO nanoparticles have potential application value in the preparation of high-quality white LEDs.

Realization of wide circadian variability by quantum dots-luminescent mesoporous silica-based white light-emitting diodes

Human comfort has become one of the most important criteria in modern lighting architecture. Here, we proposed a tuning strategy to enhance the non-image forming photobiological effect on the human circadian rhythm based on quantum-dots-converted white light-emitting diodes (QDs-WLEDs). We introduced the limiting variability of the circadian action factor (CAF), defined as the ratio of circadian efficiency and luminous efficiency of radiation. The CAF was deeply discussed and was found to be a function of constraining the color rendering index (CRI) and correlated color temperatures. The maximum CAF variability of QDs-WLEDs was found to be dependent on the QDs’ peak wavelength and full width at half maximum. With the optimized parameters, the packaging materials were synthesized and WLEDs were packaged. Experimental results show that at CRI > 90, the maximum CAF variability can be tuned by 3.83 times (from 0.251 at 2700 K to 0.961 at 6500 K), which implies that our approach could reduce the number of tunable channels, and could achieve wider CAF variability.

Directional sensitivity of the retina: 75 years of Stiles-Crawford effect.

The reduction of the brightness when a light beam's entry into the eye is shifted from the centre to the edge of the pupil has from the outset been shown to be due to a change in luminous efficiency of radiation when it is incident obliquely on the retina. The phenomenon is most prominent in photopic vision and this has concentrated attention on the properties of retinal cones, where responsibility has yet to be assigned to factors such as differences in shape, fine structure and configuration, and membrane anchoring of photopigment molecules. Geometrical optics and waveguide formulations have been applied to the question of how light is guided in receptors, but details of their geometry and optical parameters even if they become available will make calculations complex and of only moderate generality. In practice, the diminution of oblique light helps visual performance by reducing deleterious influence of ocular aberrations and of glare caused by light scattering when the pupil is wide. Receptor orientation can come into play in ocular conditions due to mechanical disturbance and has been shown to have potentiality as a tool for clinical diagnosis. Currently, open questions include microanatomical and molecular differences between rods and cones, the coupling of the optical image of the eye with the transducing apparatus in the photoreceptors, possible phototropism and more convincing methods of estimating the actual spatial distribution of photon events as it affects visual resolution.