Multiple Light-emitting Diodes Research Articles

Improved energy harvesting by enhanced nonlinearities: New phenomena and experimental demonstration

This paper investigates a nonlinear piezoelectric energy harvesting system that enhances conversion of mechanical energy to electrical energy by integrating piezoelectric materials with a nonlinear system. The research simplifies a flexible beam with macro fiber composite (MFC) piezoelectric patches using a lumped-parameter approach and derives the dynamic equations of the system using the Lagrange equations. The incremental harmonic balance method (IHBM) is then employed to solve these equations, simulating the nonlinear dynamics involved in the harvested voltage. Key findings include the initial discovery of multiple voltage islands which are voltage jumps that improve energy conversion efficiency, confirmed by experimental data validating the IHBM simulations. The study reveals that these phenomena are due to the enhanced nonlinearities of the system, particularly the geometric nonlinearity caused by the spring k1 and the nonlinear magnetic force between magnets. Adjusting k1 allows the observation of multiple voltage islands, and increasing d0 results in a broad-frequency constant voltage, offering new insights. The research concludes that the nonlinear piezoelectric energy harvester can effectively convert vibration energy into electrical one, as demonstrated by successfully powering multiple light-emitting diodes (LEDs). Bifurcation diagrams illustrate the adaptability of the system, showing transitions among periodic, quasi-periodic, and chaotic states for different parameters.

Thermophotonic cells in self-sustaining parallel circuits

Thermophotonic (TPX) cells, which integrate a light-emitting diode (LED) and a photovoltaic (PV) cell, offer potential advantages over thermophotovoltaic (TPV) cells for low-grade waste heat utilization. While TPX cells traditionally operate in an independent circuit, which requires an external battery storage unit, the self-sustaining circuit presents an appealing alternative that eliminates the need for the storage unit. In this study, we conduct a comprehensive analysis of TPX cells operating in the self-sustaining circuit. We provide clear expressions for the electrical currents flowing through the LED and PV cell, considering the different bandgaps of the LED and the PV cell. Our results reveal that the bandgap energy of the LED must exceed that of the PV cell for the TPX cell to function in a self-sustaining parallel circuit. Furthermore, we demonstrate that the performance of the TPX cell in the self-sustaining circuit improves with a narrower bandgap energy for the PV cell and a wider bandgap energy for the LED, regardless of the far-field or near-field configurations. While near-field TPX cells exhibit higher power density, they also experience decreased energy efficiency compared to far-field TPX cells. Additionally, we explore the scenario where the bandgaps of the LED and PV cell are the same, analyzing the performance of a closed-structured self-sustaining parallel circuit with multiple LEDs and PV cells in the near-field configuration. We find that the near-field effect has a marginal impact on TPX cells with identical bandgaps, which means that the near-field TPX cell in the self-sustaining circuit will not be useful in practice. The insights gained from this work serve as valuable design guidelines for TPX cells operating in the self-sustaining parallel circuit, enabling practical operation without the need for an external battery storage unit.

HCM‐based indoor optical wireless communications: Timing and data recovery

AbstractLight‐fidelity (Li‐Fi) is a promising solution to provide high‐rate, secure, and green communications to be used in the next generation of wireless networks. Since visible or infrared (IR) light‐emitting diodes (LEDs) are used as the optical source and have a non‐linear transfer function, the transmitted modulated signal can be distorted if the signal has a high peak‐to‐average power ratio (PAPR). Recently, a new modulation scheme called Hadamard‐coded modulation (HCM) is proposed which has no PAPR concern since it produces symbols with discrete levels. In the HCM technique, the symbol levels can be created using multiple LEDs. Therefore, each LED operates in its linear region and just switches on or off. A low‐complexity transceiver architecture for the HCM‐based communication links is proposed and a complete synchronisation procedure based on the spread‐spectrum techniques is presented. Finally, the bit error rate of the system is evaluated by Monte Carlo simulations, and effects of system parameters such as preamble length, fall and rise time of optical devices, and timing jitter on the bit error rate of the link are discussed.

Titania Nanoparticle-Stimulated Ultralow Frequency Detection and High-Pass Filter Behavior of a Flexible Piezoelectric Nanogenerator: A Self-Sustaining Energy Harvester for Active Motion Tracking.

A significant driving force for the fabrication of IoT-compatible smart health gear integrated with multifunctional sensors is the growing trend in fitness and the overall wellness of the human body. In this work, we present an autonomous motion and activity-sensing device based on the efficacious nucleation of the polar β-phase in an electroactive polymer. Representatively, we investigate the nucleating effect of TiO2 nanoparticles on weight-modulated PVDF-HFP films (PT-5, PT-10, and PT-15) and subsequently prototype a sensing device with the film that demonstrates superior β-phase nucleation. The PT-10 film, with an optimal polar β-phase, shows the highest remnant polarization (2Pr) and energy density of 0.36 μC/cm2 and 22.3 mJ/cm3, respectively, at 60 kV/cm. The films mimic a high pass filter at frequencies above 10 KHz with very low impedance and high ac conductivity values. The frequency-dependent impedance studies reveal an effective interfacial polarization between TiO2 nanoparticles and PVDF-HFP, explicitly observed in the low-frequency region. Consequently, the sensor fabricated with PT-10 as the sensing layer exhibits ultralow frequency detection (25 Hz) resulting from the blood flow muscle oxygenation. The device successfully senses voluntary joint movements of the human body and actively tracks a range of motions, from brisk walking to running. Additionally, through repetitive human finger-tapping motion, the nanogenerator lights up multiple light-emitting diodes in series and charges capacitors of varying magnitudes under 50 s. The real-time human motion sensing and movement tracking modalities of the sensor hold promise in the arena of smart wearables, sports biomechanics, and contact-based medical devices.

Comparative analysis of triboelectric behavior of natural waste biomaterials and green energy harvesting

This study examines the triboelectric behavior of fibers that are readily available and were obtained from the bombax ceiba tree (BOM) and the calotropis plant (CALO). A triboelectric nanogenerator (TENG) is fabricated from these fibers as charge-generating layers that produce a charge by contact electrification. The properties of these fibers which include surface morphology and crystallographic nature are examined using different characterization techniques. Scanning electron microscopy images reflect more roughness in CALO as compared to BOM whereas both materials show an almost amorphous nature in X-ray diffraction data. Further, both materials show tribopositive nature when tested against polytetrafluoroethylene (PTFE) and nylon. While examining their electrical performance, the CALO–PTFE pair produced 11.1 V while 10.7 V is generated from the BOM–PTFE pair. These combinations are capable of illuminating multiple light-emitting diodes of the green spectrum and can derive one digital calculator. Lastly, the dependency of TENG performance on the contact area of active layers is analyzed. From the results obtained, it is concluded that these materials can potentially power small electronic gadgets and can contribute to the growth of a sustainable civilization.

Flexible topographical design of light-emitting diodes realizing electrically controllable multi-wavelength spectra

Multi-wavelength visible light emitters play a crucial role in current solid-state lighting. Although they can be realized by combining semiconductor light-emitting diodes (LEDs) and phosphors or by assembling multiple LED chips with different wavelengths, these design approaches suffer from phosphor-related issues or complex assembly processes. These challenges are significant drawbacks for emerging applications such as visible light communication and micro-LED displays. Herein we present a platform for tailored emission wavelength integration on a single chip utilizing epitaxial growth on flexibly-designed three-dimensional topographies. This approach spontaneously arranges the local emission wavelengths of InGaN-based LED structures through the local In composition variations. As a result, we demonstrate monolithic integration of three different emission colors (violet, blue, and green) on a single chip. Furthermore, we achieve flexible spectral control via independent electrical control of each component. Our integration scheme opens the possibility for tailored spectral control in an arbitrary spectral range through monolithic multi-wavelength LEDs.

Numerical Modeling and Simulation of Non-Invasive Acupuncture Therapy Utilizing Near-Infrared Light-Emitting Diode.

Acupuncture is one of the most extensively used complementary and alternative medicine therapies worldwide. In this study, we explore the use of near-infrared light-emitting diodes (LEDs) to provide acupuncture-like physical stimulus to the skin tissue, but in a completely non-invasive way. A computational modeling framework has been developed to investigate the light-tissue interaction within a three-dimensional multi-layer model of skin tissue. Finite element-based analysis has been conducted, to obtain the spatiotemporal temperature distribution within the skin tissue, by solving Pennes' bioheat transfer equation, coupled with the Beer-Lambert law. The irradiation profile of the LED has been experimentally characterized and imposed in the numerical model. The experimental validation of the developed model has been conducted through comparing the numerical model predictions with those obtained experimentally on the agar phantom. The effects of the LED power, treatment duration, LED distance from the skin surface, and usage of multiple LEDs on the temperature distribution attained within the skin tissue have been systematically investigated, highlighting the safe operating power of the selected LEDs. The presented information about the spatiotemporal temperature distribution, and critical factors affecting it, would assist in better optimizing the desired thermal dosage, thereby enabling a safe and effective LED-based photothermal therapy.

Experimental Determination of Thermal Couplings in Packages Containing Multiple LEDs

Light Emitting Diodes (LEDs) are the most commonly used light sources. Temperature strongly affects their operation. Considering that multiple devices are often placed in a single housing, thermal couplings between devices become important. This problem is illustrated here based on the example of a light source containing four LEDs in a single package. Thermal analyses are carried out based on measured transient temperature responses. The measurement results are processed employing the Network Identification by Deconvolution method. The obtained results demonstrate clearly that depending on the device mounting manner and applied cooling condition the temperature rise value induced in neighboring devices can exceed 70% of the rise in the heating diode. Consequently, thermal models of such LEDs should consider not only self-heating effects, but also thermal interactions with the other diodes.

Joint SIC-Based Precoding and Sub-Connected Architecture Design for MIMO VLC Systems

Densely deployed light emitting diodes (LEDs) typically lead to high spatial correlation in multiple-input multiple-output (MIMO) visible light communication (VLC) systems. Although precoding can effectively alleviate the spatial correlation issue, most existing precoding algorithms require a dedicated baseband chain for each LED, leading to high energy consumption and hardware complexity when a large number of LEDs are used. In this paper, a successive interference cancellation (SIC)-based precoding scheme with sub-connected architecture (SIC-SA) is proposed. In the considered model, each baseband chain is connected to an LED sub-array containing multiple LEDs to reduce the complexity. Since, in this case, SIC-based precoding can only determine the signal of each baseband chain for an LED sub-array, while its target is to mitigate the spatial correlation between individual LEDs, the electrical/optical power of each LED must also be jointly optimized to accomplish the target. This joint SIC-based precoding, power allocation, and direct current offset design problem is formulated as an achievable sum rate maximization problem under dimming control and electrical power constraints. A two-step iterative algorithm is proposed to solve this problem. In the first step, the SIC-based precoding is designed to alleviate the multi-user interference. In the second step, the power allocation of LEDs is optimized by matrix decomposition and convex optimization, and a closed-form solution of DC offset is derived. Furthermore, considering the dynamic scenarios, a SIC-based precoding scheme with dynamic sub-connected architecture (SIC-DSA) is proposed, in which a switch network is used to adaptively adjust the LED sub-array structure based on the channel state information. Simulation results show that the proposed SIC-SA and SIC-DSA respectively achieve 0.1340 bps/Hz/W and 0.1305 bps/Hz/W energy efficiency gains over the zero-forcing precoding scheme with SA, when the signal-to-noise ratio is 30 dB.

Radiology blues: Comparing occupational blue-light exposure to recommended safety standards.

The blue-light hazard is a well-documented entity addressing the detrimental health effects of high-energy visible light photons in the range of 305 nm - 450 nm. Radiologists spend long hours in front of multiple light-emitting diode (LED)-based diagnostic monitors emitting blue light, predisposing them to potentially higher blue-light dosages than other health professionals. The authors aimed to quantify the blue light that radiology registrars are exposed to in daily viewing of diagnostic monitors and compared this with international occupational safety standards. A limited cross-sectional observational study was conducted. Four radiology registrars at two academic hospitals in Bloemfontein from 01 October 2021 to 30 November 2021 participated. Diagnostic monitor viewing times on a standard workday were determined. Different image modalities obtained from 01 June 2019 to 30 November 2019 were assessed, and blue-light radiance was determined using a spectroscope and image analysis software. Blue-light radiance values were compared with international safety standards. Radiology registrars spent on average 380 min in front of a diagnostic display unit daily. Blue-light radiance from diagnostic monitors was elevated in higher-intensity images such as chest radiographs and lower for darker images like MRI brain studies. The total blue-light radiance from diagnostic display units was more than 10 000 times below the recommended threshold value for blue-light exposure. Blue-light radiance from diagnostic displays measured well below the recommended values for occupational safety. Hence, blue-light exposure from diagnostic monitors does not significantly add to the occupational health burden of radiologists. Despite spending long hours in front of diagnostic monitors, radiologists' exposure to effective blue-light radiance from monitors was far below hazardous values. This suggests that blue-light exposure from diagnostic monitors does not increase the occupational health burden of radiologists.

Synthetic wavelength scanning interferometry for 3D surface profilometry with extended range of height measurement using multi-colour LED light sources

We report three-dimensional surface profilometry with extended range of height measurements using synthetic wavelength scanning interferometry without tunable filters, wavelength-tuning lasers, grating elements. We have used inexpensive multiple colour light emitting diodes (LEDs) and operate them sequentially one by one or combination of two or more colours simultaneously to visualize synthetic wavelength light source. Multiple colour LED light source was synthesized and entire visible range from violet to deep red colour was covered. A wide range of synthetic wavelengths were obtained. Five step phase shifting algorithm was used to recover phase maps with Mirau type interferometer which can be further utilized for 3D height measurements. A simple phase subtraction method was used to reconstruct the phase map at synthetic wavelength. The present system provides extended range of height measurements from sub-wavelength to tens of wavelengths. Experimental results of 3D-surface profile measurements of Si-IC chip and standard step object are presented.

Optimal Resource Management for NOMA-Based Visible Light Communication Systems With Shot Noise

In this paper, the problem of power allocation is investigated for downlink visible light communication (VLC) systems with non-orthogonal multiple access. In the system, both input-dependent shot noise and input-independent Gaussian noise are considered due to the properties of practical VLC channels. We consider two scenarios for the transmitter with a single light emitting diode (LED) and multiple LEDs, respectively. For single-LED scenarios, the problem is posed as a joint optimization problem of alternating current and direct current power control, whose goal is to maximize the minimum signal-to-interference-plus-noise-ratio (SINR) of all users. Although the original problem is non-convex, a geometric programming based algorithm is proposed to acquire the global optimal solution. For multi-LED scenarios, the transmit and dimming beamforming vectors are jointly optimized to maximize the minimum SINR of all users. A joint optimization algorithm that alternately optimizes transmit and dimming beamforming vectors is proposed. Particularly, the transmit beamforming problem is converted into a second-order cone programming problem through the successive convex approximation method. The dimming beamforming problem is transformed into a feasibility problem utilizing a low-complexity bisection algorithm. Numerical results reveal the superior performance of the proposed algorithm compared with its orthogonal multiple access counterpart and conventional schemes without shot noise.

Performance Analysis for DF Relay-Aided Visible Light Communication System With NOMA

Visible light communication (VLC) has been deemed to be one promising technique for the sixth-generation wireless communication network. However, due to the attenuation properties, the VLC ranges are usually limited. To unlock the advantage of multiple light-emitting diodes (LEDs) and extend the coverage of VLC systems, a relay-aided VLC system with non-orthogonal multiple access (NOMA) is proposed in this paper. Specifically, a decode-and-forward (DF) relay protocol is introduced to construct the relay-aided VLC system with NOMA. Moreover, the closed-form expression of the outage probability (OP) in the DF relay-aided VLC system with NOMA is derived to accurately characterize the theoretical performance of the system. Thereafter, the effects of the corresponding parameters on the OP are investigated. Simulation results verify the accuracy of the derived theoretical expression and illustrate that the DF relay-aided VLC system with NOMA can effectively improve system outage performance. Compared to the direct transmission NOMA VLC system without relay link, our scheme can obtain <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{15}\,\text{dB}$</tex-math></inline-formula> transmit signal-to-noise ratio gains in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-3}$</tex-math></inline-formula> OP level. Last but not least, the optimal transmit semi-angle and deployment height of the relay can be determined for achieving the optimal system outage performance.

Secure NOMA-Assisted Multi-LED Underwater Visible Light Communication

Security issue in underwater visible light communication (UVLC) arises mainly due to the scattering effect wherein numerous photons are statistically generated when a light beam strikes a water molecule. This paper considers an underwater communication scenario wherein a floating vehicle (FV) transmitter that is equipped with multiple light-emitting diodes (LEDs) communicates with the two legitimate near-end and far-end underwater vehicles (UVs) in presence of an eavesdropper. In particular, two non-orthogonal multiple access (NOMA) technology-based optimal LED selection (OLS) and suboptimal LED selection (SLS) schemes are proposed to select a LED that can transmit the information with the highest secrecy rate against active/passive eavesdropping attacks. Furthermore, the FVT transmits the information to both UVs with the selected LED only. Utilizing the successive interference cancellation (SIC) characteristic, this paper derives the closed-form secrecy outage probability expressions for both single-LED and multi-LED transmission strategies for both known and unknown CSI. The security performance of the proposed multi-LED NOMA-UVLC is compared with the conventional single-LED NOMA-UVLC under the effects of air bubbles for both fresh and salty water types. In addition, the validity of the numerical results is verified through Monte-carlo simulation analysis.

Design and performance study of a rotating piezoelectric wind energy harvesting device with wind turbine structure

This paper proposes a rotating piezoelectric ambient wind energy harvesting device with wind turbine structure (R-PWEH), which mainly consists of wind turbine, support structure, excitation module, and power generation unit. This paper mainly investigates the effect of wind turbines with a different number of fan blades and units with different shapes of power generation on the performance output of R-PWEH. A series of experimental studies were conducted on the R-PWEH in the wind speed range of 0–20 m/s. In the experimental study at a wind speed of 16 m/s, the maximum voltage output of the R-PWEH was 114.24 V when the wind turbine with 11 fan blades and the rectangular cantilever beam power generation unit was installed. The optimal external load at this point is 8 kΩ and the maximum output power is 36.89 mW. The fastest charging rate for the capacitor at this parameter allows for easy powering of multiple light-emitting diodes for illumination. It has good prospects for development in practical applications, such as the ability to power micro and small sensors.

Combinatorial wound healing therapy using adhesive nanofibrous membrane equipped with wearable LED patches for photobiomodulation.

Wound healing is the dynamic tissue regeneration process replacing devitalized and missing tissue layers. With the development of photomedicine techniques in wound healing, safe and noninvasive photobiomodulation therapy is receiving attention. Effective wound management in photobiomodulation is challenged, however, by limited control of the geometrical mismatches on the injured skin surface. Here, adhesive hyaluronic acid–based gelatin nanofibrous membranes integrated with multiple light-emitting diode (LED) arrays are developed as a skin-attachable patch. The nanofibrous wound dressing is expected to mimic the three-dimensional structure of the extracellular matrix, and its adhesiveness allows tight coupling between the wound sites and the flexible LED patch. Experimental results demonstrate that our medical device accelerates the initial wound healing process by the synergetic effects of the wound dressing and LED irradiation. Our proposed technology promises progress for wound healing management and other biomedical applications.

Secrecy Enhancement for SSK-Based Visible Light Communication Systems

Visible light communication (VLC) is a technology that uses unlicensed light spectrum resources and high spatial reuse rates for communication. Because it does not occupy any resource allocation in wireless communication, it has fully alleviated the problem of spectrum scarcity in radio frequency (RF) communication and gradually become a new development direction. However, owing to the inherent broadcasting nature of the VLC channel, the VLC link is vulnerable to eavesdropping by unexpected or unauthorized users in spacious public places. Therefore, enhancing the security of the VLC system has attracted extensive attention. This paper studies the security optimization scheme of the VLC system based on the space shift keying (SSK) technology in the free space optical environment called the SSK-VLC system. The antenna selection (AS) technology and artificial noise (AN) cancellation method are adopted to enhance the confidentiality of the SSK-VLC system. In this paper, we presume that the SSK-VLC system includes three parts: a transmitter containing multiple light-emitting diodes, a legitimate receiver, and an eavesdropper, respectively. By using the designed AS and AN method, the transmitted valid information can be demodulated at the legitimate receiver, and at the same time, the received information by the eavesdropper will be disturbed. The simulation results prove that the proposed optimization scheme can further improve the security performance, including the secrecy rate (SR) and the bit error ratio (BER), compared with the traditional SSK-VLC scheme.

Surface Structure-Controlled Monolithic Multiple Color Semipolar GaN-based Light-Emitting Diodes

Surface Structure-Controlled Monolithic Multiple Color Semipolar GaN-based Light-Emitting Diodes

Effect of the violet light from polywave light-polymerizing units on two resin cements that use different photoinitiators

Statement of problemSome light-emitting diode polymerization lights have been promoted as multiple peak or polywave lights that use multiple light-emitting diodes to produce both violet and blue light. However, whether the addition of violet light is required to light-activate resin cements that use bis(4-methoxybenzoyl)diethylgermane (Ivocerin) as the photoinitiator is unclear. PurposeThis in vitro study evaluated the effect of violet, blue, or a combination of violet and blue light through ceramic on the degree of conversion of 2 resin cements that use either camphorquinone or Ivocerin as the photoinitiator. Material and methodsA Bluephase Style polywave light-emitting diode polymerizing unit delivering 6.4 J/cm2 of violet and blue light was used. This comprised 1.4 J/cm2 of violet (385 to 420 nm) and 5.0 J/cm2 of blue light (420 to 515 nm). The light-emitting diode emitters in a second modified Bluephase Style were connected directly to a power supply so that either just violet (1.4 J/cm2) or just blue (5.0 J/cm2) light was emitted. RelyX Veneer and Variolink Esthetic LC resin cements were either directly light-activated or through 0.5 or 1.5 mm of lithium disilicate ceramic (IPS e.max CAD). The degree of conversion was monitored by using Fourier-transform infrared spectroscopy. Data were subject to a 3-way analysis of variance followed by the Tukey honest significant difference multiple comparison tests (α=.05). ResultsAll factors had a significant effect (P<.001). Increasing the ceramic thickness decreased the degree of conversion only for RelyX Veneer cement (P<.001). The effect of the thickness of ceramic was most noticeable when just violet light was delivered to RelyX Veneer. A significant reduction (P<.001) was found in the degree of conversion of RelyX Veneer when just violet light was delivered. Variolink Esthetic LC had significantly higher degree of conversion values than RelyX Veneer, irrespective of the light type used (P<.001). ConclusionsA multiple-peak light is not required to photopolymerize a resin cement that uses either camphorquinone or Ivocerin as its photoinitiator. Adding the violet light produced no significant increase in the degree of conversion of the Variolink Esthetic LC cement.

Cutaneous Photorejuvenation of Light Emitting Diodes via the Melatonin Membrane Receptor Pathway.

Melatonin receptors are present in the human skin and retina. These receptors can be stimulated by light emitting diodes (LEDs) at specific wavelengths, thereby inducing cutaneous photorejuvenation. However, the underlying mechanism remains unclear. To evaluate the influence of LEDs at specific wavelengths on melatonin membrane receptor (MT1) and cutaneous photorejuvenation via the MT1 pathway in vitro. Normal human dermal fibroblasts (HDFs) were irradiated using LEDs at different wavelengths (410~940 nm) at a dose of 1 J/cm². MT1 activity was evaluated after melatonin stimulation and LED irradiation. Thereafter, the expressions of collagen (COL) and matrix metalloproteinases (MMPs), with and without luzindole (MT1/2 receptor antagonist), were investigated via semi-quantitative reverse transcription polymerase chain reaction (PCR), real-time PCR, western blotting, and enzyme-linked immunosorbent assay. In HDFs, the MT1 mRNA and protein levels increased significantly in response to melatonin (dose, 50 nM) (p<0.01) and LED irradiation at 595, 630, 850, and 940 nm (p<0.01). LED irradiation up-regulated COL type I and down-regulated MMP-1. Compared to LED irradiation without luzindole, LED irradiation with luzindole produced no significant increase in COL type I mRNA and protein levels (p<0.01). We found that LED irradiation induces collagen synthesis and MMP-1 inhibition in HDFs via MT1 activation. Additionally, multiple LED wavelengths (595, 630, 850, and 940 nm) stimulated MT1 in HDFs, unlike in the eyes, where only blue light induced plasma melatonin suppression. This suggests the possibility of the melatoninergic pathway in photorejuvenation.