Browsing by Subject "Correlated color temperature"

Now showing 1 - 8 of 8

Open Access
Implementation of high-quality warm-white light-emitting diodes by a model-experimental feedback approach using quantum dot-salt mixed crystals
(American Chemical Society, 2015) Adam, M.; Erdem, T.; Stachowski, G.M.; Soran-Erdem Z.; Lox, J. F. L.; Bauer, C.; Poppe, J.; Demir, Hilmi Volkan; Gaponik N.; Eychmüller A.
In this work, a model-experimental feedback approach is developed and applied to fabricate high-quality, warm-white light-emitting diodes based on quantum dots (QDs) as color-conversion materials. Owing to their unique chemical and physical properties, QDs offer huge potential for lighting applications. Nevertheless, both emission stability and processability of the QDs are limited upon usage from solution. Incorporating them into a solid ionic matrix overcomes both of these drawbacks, while preserving the initial optical properties. Here borax (Na2B4O7·10H2O) is used as a host matrix because of its lower solubility and thereby reduced ionic strength in water in comparison with NaCl. This guarantees the stability of high-quality CdSe/ZnS QDs in the aqueous phase during crystallization and results in a 3.4 times higher loading amount of QDs within the borax crystals compared to NaCl. All steps from the synthesis via mixed crystal preparation to the warm-white LED preparation are verified by applying the model-experimental feedback, in which experimental data and numerical results provide feedback to each other recursively. These measures are taken to ensure a high luminous efficacy of optical radiation (LER) and a high color rendering index (CRI) of the final device as well as a correlated color temperature (CCT) comparable to an incandescent bulb. By doing so, a warm-white LED with a LER of 341 lm/Wopt, a CCT of 2720 K and a CRI of 91.1 is produced. Finally, we show that the emission stability of the QDs within the borax crystals on LEDs driven at high currents is significantly improved. These findings indicate that the proposed warm-white light-emitting diodes based on QDs-in-borax hold great promise for quality lighting. © 2015 American Chemical Society.
Open Access
Nanocrystal integrated light emitting diodes based on radiative and nonradiative energy transfer for the green gap
(IEEE, 2009) Nizamoğlu, Sedat; Sarı, Emre; Baek J.-H.; Lee I.-H.; Demir, Hilmi Volkan
Recently the photometric conditions for ultra-efficient solid-state lighting have been discussed [1-2]. These studies show that a luminous efficacy of optical radiation at 408 lm/Wopt and a color rendering index (CRI) of 90 at a correlated color temperature (CCT) of 3000 K are achievable at the same time. For this purpose light emitting diodes (LEDs) emitting in blue, green, yellow, and red colors at 463, 530, 573, and 614 nm with relative optical power levels of 1/8, 2/8, 2/8, and 3/8, are required, respectively [1-2]. Although InxGa1-xN material system is capable to cover the whole visible by changing the In composition (x), it is technically extremely challenging to obtain efficient green/yellow light emitting diodes especially at those wavelengths (i.e., at 530 nm and 573 nm, respectively) due to reduced internal quantum efficiency [2-4]. Furthermore, by using the (Al xGa1-x)1-yInyP quaternary alloy it is also possible to cover from 650 nm to 580 nm. However, the efficiencies significantly decrease towards green. Therefore, there exists a significant gap in the green-yellow spectral regions (known as "the green gap") to make efficient light emitting diodes. To address this green gap problem, we propose and demonstrate proof-of-concept nanocrystal (NCs) hybridized green/yellow light emitting diodes that rely on both radiative energy transfer and nonradiative energy transfer (i.e., FRET-Förster resonance energy transfer) for color conversion on near-ultraviolet (near-UV) LEDs.
Open Access
Neural dynamics of light temperature under attentional load
(2024-07) Kılıç, Rabia Nur
This thesis investigates the neural dynamics of light temperature under varying levels of attentional load using EEG. The study involved twenty healthy adult participants (13 males, 7 females, mean age = 22.35) who performed rapid vi-sual detection tasks under different correlated color temperatures (CCT) of light: 2000K, 6000K, and 10000K. The experimental design comprised 288 trials per participant, with tasks administered under low and high attentional loads. The results indicated significant variations in EEG time-frequency responses in the alpha (8-13 Hz) frequency band based on attentional load, CCT conditions, and hemisphere. The analysis revealed significant main effects of Load, CCT, and Hemisphere on mean ERSP values. Mean ERSP values were higher under low load conditions compared to high load conditions, indicating that increased at-tentional load enhances alpha suppression and heightens attention. Mean ERSP values were lower in the CCT conditions compared to the control condition, re-flecting greater alpha suppression in the presence of distractor. Additionally, the right hemisphere exhibited significantly lower mean ERSP values compared to the left hemisphere and the midline, indicating lateralization of alpha band activity with greater alpha suppression in the right hemisphere. Behavioral data analysis showed significant differences in response times and accuracy across conditions. Participants demonstrated faster response times under cooler CCTs (6000K and 10000K) compared to the warm colors (2000K). These findings reveal the differ-ential impacts of light temperature and attentional load on behavioral and neural activity, in which our cognitive systems respond to external stimuli.
Open Access
Power conversion and luminous efficiency performance of nanophosphor quantum dots on color-conversion LEDs for high-quality general lighting
(SPIE, 2012) Erdem, Talha; Nizamoglu, Sedat; Demir, Hilmi Volkan
For high-quality general lighting, a white light source is required to exhibit good photometric and colorimetric performance along with a high level of electrical efficiency. For example, a warm white shade is desirable for indoors, corresponding to correlated color temperatures ≥4000 K, together with color rendering indices ≥90. Additionally, the luminous efficacy of optical radiation (LER) should be high, preferably ≥380 lm/W opt. Conventional white LEDs cannot currently satisfy these requirements simultaneously. On the other hand, color-conversion white LEDs (WLEDs) integrated with quantum dots (QDs) can simultaneously reach such high levels of photometric and colorimetric performance. However, their electrical efficiency performance and limits have been unknown. To understand their potential of luminous efficiency (lm/Welect), we modeled and studied different QD-WLED architectures based on layered QD films and QD blends, all integrated on blue LED chips. The architecture of red, yellow and green emitting QD films (in this order from the chip outwards) is demonstrated to outperform the rest. In this case, for photometrically efficient spectra, the maximum achievable LE is predicted to be 327 lm/W elect. Using a state-of-the-art blue LED reported with a power conversion efficiency (PCE) of 81.3%, the overall WLED PCE is shown to be 69%. To achieve LEs of 100, 150 and 200 lm/Welect, the required minimum quantum efficiencies of the color-converting QDs are found to be 39, 58 and 79%, respectively. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
Open Access
Superior warm-White light-emitting diodes integrated with quantum dot nanophosphors for high luminous efficacy and color rendering
(Optical Society of America, 2011) Nizamoğlu, Sedat; Erdem, Talha; Sun, X. W.; Demir, Hilmi Volkan
Quantum dot nanophoshor hybridized warm-white LEDs are reported to exhibit high photometric performance of luminous efficacy exceeding 350 lm/Wopt and color rendering index close to 90 at correlated color temperatures <3000 K. ©2011 Optical Society of America.
Open Access
Synthesis of fluorescent core-shell nanomaterials and strategies to generate white light
(American Institute of Physics Inc., 2015) Singh, A.; Kaur, R.; Pandey, O. P.; Wei, X.; Sharma, M.
In this work, cadmium free core-shell ZnS:X/ZnS (X-=-Mn, Cu) nanoparticles have been synthesized and used for white light generation. First, the doping concentration of Manganese (Mn) was varied from 1% to 4% to optimize the dopant related emission and its optimal value was found to be 1%. Then, ZnS shell was grown over ZnS:Mn(1%) core to passivate the surface defects. Similarly, the optimal concentration of Copper (Cu) was found to be 0.8% in the range varied from 0.6% to 1.2%. In order to obtain an emission in the whole visible spectrum, dual doping of Mn and Cu was done in the core and the shell, respectively. A solid-solid mixing in different ratios of separately doped quantum dots (QDs) emitting in the blue green and the orange region was performed. Results show that the optimum mixture of QDs excited at 300-nm gives Commission Internationale del'Éclairage color coordinates of (0.35, 0.36), high color rendering index of 88, and correlated color temperature of 4704-K with minimum self-absorption. © 2015 AIP Publishing LLC.
Open Access
Warm-white light-emitting diodes integrated with colloidal quantum dots for high luminous efficacy and color rendering
(2010) Nizamoglu, S.; Erdem, T.; Sun X.W.; Demir, Hilmi Volkan
Warm-white LEDs (WLEDs) with high spectral quality and efficiency are required for lighting applications, but current experimental performances are limited. We report on nanocrystal quantum dot (NQD) hybridized WLEDs with high performance that exhibit a high luminous efficacy of optical radiation exceeding 350 lm/Wopt and a high color rendering index close to 90 at a low correlated color temperature <3000 K. These spectrally engineered WLEDs are obtained using a combination of CdSe/ZnS core/shell NQD nanophosphors integrated on blue InGaN/GaN LEDs. © 2010 Optical Society of America.
Open Access
Warm-White light-Emitting diodes integrated with colloidal quantum dots for high luminous efficacy and color rendering: Reply to comment
(2011) Nizamoglu, S.; Erdem, T.; Sun X.W.; Demir, Hilmi Volkan
The correlated color temperatures and the corresponding color rendering indices calculated using actual experimental data (and not any prediction) in the original Letter [Opt. Lett. 35, 3372 (2010)] are correct. In addition, here the color rendering of our white LEDs integrated with nanocrystal quantum dots (NQDs) is provided for all test samples. Also, a new NQD-LED design with both high luminous efficacy of optical radiation and CRI is presented to have a chromaticity point in the quadrangle stated in the comment Letter [Opt. Lett. 36, 2851 (2011)]. The points made in the original Letter and all the calculation results provided therein are valid. © 2011 Optical Society of America.