Modern Light-emitting Diodes Research Articles

LED light can falsify pulse oximetry readings via the stroboscopic effect

Because of its simplicity, pulse oximetry plays a ubiquitous role in neonatology. Its measurements are based on the absorption of light by hemoglobin. Ambient light can affect these values, therefore algorithms are designed to compensate for constant ambient light. Modern light-emitting diodes often flicker at a very high frequency. Such flickering ambient light can lead to significant measurement errors in saturation. To present a novel way in which light-emitting diodes influence the function of pulse oximeters and to demonstrate mathematically that a stroboscopic effect may well be responsible for this disturbance. Using publicly available data, a mathematical model of a pulse oximeter with a calibration curve and a proprietary measurement algorithm was created. This was used to simulate saturation measurements in flickering ambient light. To do this, photopletysmograms for red and infrared light at 98% oxygen saturation were mathematically superimposed on the light emission from an examination lamp used in the intensive care unit. From these results, presumable saturation measurements from a pulse oximeter were extrapolated. The light-emitting diodes in the examination lamp flicker at 207 Hz. The pulsating light from the light-emitting diodes causes superimposition of the photoplethysmogram due to the stroboscopic effect. With increasing brightness, the saturation dropped to 85% and the pulse rate to 108 bpm. The pulsed light of light-emitting diodes can distort pulse oximetry measurements. The stroboscopic effect leads to low saturation values, which can lead to the risk of blindness in premature infants due to excessive oxygenation.

Choosing a Method for Approximating the Spectral Distribution of Colour LEDs and Comparing their Parameters and Characteristics in Nominal Mode

New technologies and new light sources make it possible to create fundamentally new approaches in the design and construction of lighting devices. Light emitting diodes (LEDs) are a relatively new, promising and energy-efficient light source with a few unique properties: long lifetime, high luminous efficiency, aesthetics, environmental friendliness, reliability, high durability, and possibility to create a variety of spectra easily. During the lighting devices development, it is convenient to use mathematical approximations of the spectral distributions of the used LEDs in the composition of the product to calculate the final spectrum of the device in the visible range. These calculation methods are particularly interesting in architectural lighting. They are also applicable to the development of modern LED greenhouse irradiators. The article considers two main methods for approximating the emission spectra of colour LEDs: via symmetric and asymmetric distribution. The spectral distributions of four main types of LEDs that are used in the lighting devices design based on colour LEDs were measured in a wide range of currents from 100 mA to 1000 mA: red, amber, green, and blue. The results were also compared with a model that considers the two-dimensional density of states and potential fluctuations in the active region of the heterostructure of LED. Based on the carried-out measurements and calculations, the optimal approximation method via the asymmetric spectral distribution of LED radiation was determined.

Features of Computer Design of LED Optical Systems

The paper considers the problems of computer simulation of modern lighting devices (LD) based on light emitting diodes (LED) and their optical systems (OS). LED OS provides developers with sufficient opportunities to implement a variety of problems of modern lighting. In the market of competing products, it is often necessary to calculate and manufacture LED optics for specific lighting problems, the main of which is the formation of a given distribution of illuminance over the surface of an object. It is shown that the most demanded services in LED lighting engineering are the development of LED optics according to the customer’s specifications, lighting engineering testing of a modelled product, development of 3D models for further production. The main stage in the context of such design is the calculation and simulation of the OS. The results of this process are basically determined by the complex of luminous, thermal, and functional characteristics of LD. The development of modern LED LD is inseparably linked with the use of specialized software. Modern technologies and approaches, CASE-tools in the design of the LD, the main requirements for the LD models, integrated complex AllFusion Modelling Suite are considered. Functional diagrams of the process of designing the LD have been developed. 3D models of single and block lenses and reflectors of various designs have been developed in the computer-aided design system KOMPAS‑3D. For given LED properties, selected materials, and ray tracing conditions the light distribution of LED modules was obtained and analysed in TracePro software. Acceptable results have been obtained, that is why this approach can be recommended for prototyping and studying the characteristics of experimental samples of LED modules and LD.

The VIIRS Day/Night Band: A Flicker Meter in Space?

The VIIRS day/night band (DNB) high gain stage (HGS) pixel effective dwell time is in the range of 2–3 milliseconds (ms), which is about one third of the flicker cycle present in lighting powered by alternating current. Thus, if flicker is present, it induces random fluctuations in nightly DNB radiances. This results in increased variance in DNB temporal profiles. A survey of flicker characteristics conducted with high-speed camera data collected on a wide range of individual luminaires found that the flicker is most pronounced in high-intensity discharge (HID) lamps, such as high- and low-pressure sodium and metal halides. Flicker is muted, but detectable, in incandescent luminaires. Modern light-emitting diodes (LEDs) and fluorescent lights are often nearly flicker-free, thanks to high-quality voltage smoothing. DNB pixel footprints are about half a square kilometer and can contain vast numbers of individual luminaires, some of which flicker, while others do not. If many of the flickering lights are drawing from a common AC supplier, the flicker can be synchronized and leave an imprint on the DNB temporal profile. In contrast, multiple power supplies will throw the flickering out of synchronization, resulting in a cacophony with less radiance fluctuation. The examination of DNB temporal profiles for locations before and after the conversion of high-intensity discharge (HID) to LED streetlight conversions shows a reduction in the index of dispersion, calculated by dividing the annual variance by the mean. There are a number of variables that contribute to radiance variations in the VIIRS DNB, including the view angle, cloud optical thickness, atmospheric variability, snow cover, lunar illuminance, and the compilation of temporal profiles using pixels whose footprints are not perfectly aligned. It makes sense to adjust the DNB radiance for as many of these extraneous effects as possible. However, none of these adjustments will reduce the radiance instability introduced by flicker. Because flicker is known to affect organisms, including humans, the development of methods to detect and rate the strength of flickering from space will open up new areas of research on the biologic impacts of artificial lighting. Over time, there is a trend towards the reduction of flicker in outdoor lighting through the replacement of HID with low-flicker LED sources. This study indicates that the effects of LED conversions on the brightness and steadiness of outdoor lighting can be analyzed with VIIRS DNB temporal profiles.

Spectral Design of Phosphor-Converted LED Lighting Guided by Color Theory.

In the race to develop new luminescent materials for the next generation of light-emitting-diode (LED)-based solid-state lighting and display applications, it is often forgotten that color theory and human perception should be some of the principal factors guiding materials design. In this Viewpoint, we explore some of the antiquated colorimetrics established originally for incandescent and fluorescent lighting and discuss how they are still widely applied in the literature today to interpret the color quality of luminescent materials, like inorganic phosphors and quantum dots, and to analyze prototype devices, despite their shortcomings. We then shift our analysis toward contemporary ideas in color theory that more accurately describe the color quality of modern LED light bulbs and flat-panel displays. Finally, the perspective examines the opportunities and challenges of applying these new concepts to guide the design of luminescent materials used in LED-based applications.

Comparative study of III-phosphide- and III-nitride-based light-emitting diodes: understanding the factors limiting efficiency

Modern light-emitting diodes (LEDs) require further improvements in the wall-plug efficiency (WPE). Thus, understanding and elucidating the physical factors limiting the WPE is of crucial importance. This study aims to understand and elucidate such factors via a comparative study of two different LED material systems, i.e. III-phosphide (AlGaInP) red and III-nitride (AlGaInN) blue LEDs. The WPE was decoupled into its component elements, which indicated that the dominant contribution to WPE reduction depends on the sample. To further investigate, thermodynamic and current component analyses were conducted. These analyses clearly demonstrated that the carrier dynamics is dependent of the material systems, i.e. internal quantum efficiency and voltage efficiency are simultaneously degraded due to the carrier accumulation in the blue sample. We found that the WPE can be improved using strategies dependent on the samples, such as improvement of the light extraction in the red sample and suppression of the carrier accumulation in the blue sample.

Explaining the Diffusion of Energy-Efficient Lighting in India: A Technology Innovation Systems Approach

Electricity consumption from lighting accounts for about 15% of total power demand and 5–6% of greenhouse gas emissions in developing countries. It is therefore a promising avenue to achieve considerable energy savings through technological innovation and upgrading. India has been very successful in recent years with a nationwide roll-out of modern light-emitting diode (LED) applications. This study uses the framework of technology innovation systems to identify the actors, institutions, and processes behind the diffusion of this technology. Our findings indicate that national innovation strategies, along with low-carbon technology (LCT) transfer policies, helped to bring down the cost of LED lamps in a rapidly expanding domestic market. Based on the findings, we further explore lessons for broader issues of low-carbon technology transfer and suggest an emerging intermediate step between north–south and south–south technology transfer.

Novel distortionless dimming in high power LED lighting using isolated SEPIC converter

A new low-cost energy-saving concept in the modern light-emitting diode (LED) illumination system is developed and implemented using an isolated single-ended primary inductance converter (SEPIC). The dimming scheme presented in this study is a single-step with negligible distortion in the AC mains current as compared to the traditional low-cost triode for alternating current (TRIAC) dimming system, which distorts the AC mains current wave-shape beyond sinusoidal nature due to the phase-cut techniques. This dimming concept is completely retrofitted, no additional arrangement is required to initiate the dimming as compared to a TRIAC dimmer or other controllers. The isolated SEPIC converter is modelled and analysed in detail using the state-space average technique. The stability of the designed converter and overall closed-loop control are verified using Bode-plot analysis. The full load converter efficiency is found around 93%, which is much higher than the conventional flyback topology-based LED driver. The concept presented in this study is tested with the help of a laboratory prototype with an LED load of 60 W.

Many-Body Effects in Strongly Disordered III-Nitride Quantum Wells: Interplay Between Carrier Localization and Coulomb Interaction

The joint impact of Anderson localization and many-body interaction is observed in the optical properties of strongly-disordered III-nitride quantum wells, a system where the Coulomb interaction and the fluctuating potential are pronounced effects with similar magnitude. A numerical method is introduced to solve the 6-dimensional coupled Schrodinger equation in the presence of disorder and Coulomb interaction, a challenging numerical task. It accurately reproduces the measured absorption and luminescence dynamics of InGaN quantum wells at room-temperature: absorption spectra reveal the existence of a broadened excitonic peak, and carrier lifetime measurements show that luminescence departs from a conventional bimolecular behavior. These results reveal that luminescence is governed by the interplay between localization and Coulomb interaction, and provide practical insight in the physics of modern light-emitting diodes.

Efficacy of light-emitting diode light polymerization units used in private practices in Toronto, Ontario, Canada

Efficacy of light-emitting diode light polymerization units used in private practices in Toronto, Ontario, Canada

Ocular Tolerance of Contemporary Electronic Display Devices.

Electronic displays have become an integral part of life in the developed world since the revolution of mobile computing a decade ago. With the release of multiple consumer-grade virtual reality (VR) and augmented reality (AR) products in the past 2 years utilizing head-mounted displays (HMDs), as well as the development of low-cost, smartphone-based HMDs, the ability to intimately interact with electronic screens is greater than ever. VR/AR HMDs also place the display at much closer ocular proximity than traditional electronic devices while also isolating the user from the ambient environment to create a "closed" system between the user's eyes and the display. Whether the increased interaction with these devices places the user's retina at higher risk of damage is currently unclear. Herein, the authors review the discovery of photochemical damage of the retina from visible light as well as summarize relevant clinical and preclinical data regarding the influence of modern display devices on retinal health. Multiple preclinical studies have been performed with modern light-emitting diode technology demonstrating damage to the retina at modest exposure levels, particularly from blue-light wavelengths. Unfortunately, high-quality in-human studies are lacking, and the small clinical investigations performed to date have failed to keep pace with the rapid evolutions in display technology. Clinical investigations assessing the effect of HMDs on human retinal function are also yet to be performed. From the available data, modern consumer electronic displays do not appear to pose any acute risk to vision with average use; however, future studies with well-defined clinical outcomes and illuminance metrics are needed to better understand the long-term risks of cumulative exposure to electronic displays in general and with "closed" VR/AR HMDs in particular. [Ophthalmic Surg Lasers Imaging Retina. 2018;49:346-354.].

Accounting for coherent effects in the ray-tracing of light-emitting diodes with interface gratings via mixed-level simulation

Ray-tracing (RT) has long been the workhorse technique for analyzing light-emitting diode (LED) dies and packages and has led to significant improvements in extraction efficiency and beam shaping. However, to achieve further enhancements, nano-/microscale features such as patterned substrates and surface textures have been explored. The coherent effects arising from these near/subwavelength features are difficult to include in the RT of the packaged device. We show that under certain conditions these effects can have a significant impact on LED performance, especially if back-reflectors are present. Furthermore, we demonstrate that coherence must be accounted for even in structures that would otherwise be considered as having relatively large feature sizes, such as gratings with periods many times the wavelength. We present comparisons between the optical responses of prototypical periodically patterned substrates modeled with RT alone and with a mixed-level approach that combines RT and rigorous electromagnetic simulation, such as rigorous coupled wave analysis and finite-difference time-domain. Several examples with varying lateral periods are computed with both methods. It is shown that these results may differ, and that these differences can be significant if back reflection is present. We conclude that a mixed-level approach is an efficient and accurate method to model light extraction in modern LEDs.

Experimental and Modeling Study of Breakup Behavior in Silicone Jet Dispensing for Light-Emitting Diode Packaging

Silicone jet dispensing has grown increasingly common in light-emitting diode (LED) packaging owing to its high speed and low cost. However, differences in breakup behavior result in different silicone jetting volumes that create challenges in maintaining dispensing volume consistency during silicone jet dispensing process; this severely affects the performance of LED packaging. To reveal the breakup behavior during the jet dispensing process, the experimental and modeling studies were conducted. First, the ultrafast breakup process was experimentally observed using a high-speed camera, which shows that this process only requires approximately 50 ms. Then, an analytical model was developed and verified through an experiment. Using this analytical model, the effects of initial disturbance amplitude, initial disturbance wavelength, and thread length on the thread breakup were studied. The simulation demonstrated that it is necessary to control the disturbance amplitude under 0.005 for volume consistency in jet dispensing; thus, a moving work piece rather than a moving nozzle is recommended in jet dispensing. A disturbance wavelength between 0.3 and 0.6 mm is beneficial for both thread breakup and high-volume consistency. Thread length has little effect on the breakup position but significant effects on the volume consistency and breakup time. A low substrate increases the volume and the consistency of thread breakup. This paper will provide a useful guide for dispenser designs of modern LED packages.

Occupancy distribution estimation for smart light delivery with perturbation-modulated light sensing

The advent of modern light-emitting diode (LED) techniques enables us to develop novel lighting systems with numerous previously unavailable features. Specifically, by using the fixtures for both illumination and to interrogate the space, source-to-sensor communication becomes possible at very low cost. In this paper, we present a novel framework to estimate the occupancy distribution in an indoor space using color-controllable LED fixtures (the same fixtures providing the illumination, simultaneously) and sparsely distributed non-imaging color sensors. By modulating randomly generated perturbation patterns onto the drive signals of the LED fixtures and measuring the changes in the color sensor responses, we are able to recover a light transport model for the room. Two approaches are proposed to estimate the spatial distribution of the occupancy, based on a light blockage model and a light reflection model, respectively. These two approaches, which can be combined, can faithfully reveal the occupancy scenario of the indoor space, while preserving the privacy of its occupants. An occupancy-sensitive lighting system can be designed based on this technique.

Thermal resistance of GaAs/AlAs superlattices used in modern light-emitting diodes

AbstractSuperlattices are used in modern light-emitting diodes to modify intentionally electron, phonon and/or photon transport within their volumes, which leads to their expected performance characteristics. In particular, superlattices may have a dramatic impact on device thermal properties. Superlattice thermal resistance is anisotropic and usually distinctly higher than its values in constituent bulk materials, which results from phonon reflections and/or phonon scatterings at numerous layer interfaces. In the present paper, thermal resistance of a typical superlattice of layer thicknesses neither much higher nor much lower than the phonon free path is discussed. Besides, as an important example, thermal resistance of the typical GaAs/AlAs superlattice is determined theoretically and compared with its measured values known from literature.

Effect of plasmonic losses on light emission enhancement in quantum-wells coupled to metallic gratings

Recent experimental work has shown significant luminescence enhancement from near-surface quantum-well (QW) structures using metallic grating to convert surface plasmon (SP) modes into radiative modes. This work introduces a detailed theoretical study of plasmonic losses and the role of SPs in improving light extraction from grated light-emitting QW structures, using the fluctuational electrodynamics method. The method explains experimental results demonstrating emission enhancement, light scattering, and plasmonic coupling in the structures. We study these effects in angle-resolved reflectometry and luminescence setups in InGaN QW structures with silver grating. In contrast to experiments, our model allows direct calculation of the optical losses. The model predicts that the plasmonic coupling and scattering increases light emission by a factor of up to three compared to a flat semiconductor structure. This corresponds to reducing the absorption losses from approximately 93% in the ungrated metallic structure to 75% in the grated structure. Lower losses are associated with a significant emission enhancement enabled by the SPs of silver/GaN interfaces, which are present in the blue/green wavelength range, and can be optimized by carefully nanostructuring the metal layer and by the positioning of the QW. In general, the enhancement results from the interplay of mode scattering, conversion of SP energy directly into light, and losses in the metallic grating. The reported losses are very high when compared to the losses present in modern light-emitting diodes (LEDs). Albeit, our work provides tools needed for further optimization of plasmonic light extraction, eventually leading to highly efficient LEDs.

Enhanced Optical Property of InGaN Light-Emitting Diodes with SiO2 Nano-Bowl Photonic Crystal by Nanosphere Lithography

Recently, nitride-based compound semiconductors have been attracting extensive attention in high performance electroluminescent devices due to their broad range of emission wavelength. InGaNbased Light-emitting diodes (LEDs) characterized by small size, high efficiency and long lifetime have been indispensable components in various applications including general lighting, traffic lights, automobile headlights, and backlights in liquid crystal displays. However, it is still necessary to further improve the light output power (LOP) of these modern LEDs to approach the next generation applications such as high-end solid-state lighting. The LOP of the LEDs depends on the external quantum efficiency (EQE) and is severely restricted by poor light extraction efficiency (LEE) due to the total internal reflection (TIR) effect caused by the large contrast in refractive index at the interface between the GaN film and the surrounding air. 1 Various approaches have been reported to improve the LEE of the LEDs by increasing the light scattering at the GaN/air interface including patterned sapphire substrate (PSS), 2,3 surface texturing, 4,5 and graded index structures. 6,7 Lately, many efforts have been taken to demonstrate the fabrication and application of the two-dimensional photonic crystal (2D-PC) structures to improve the LEE of the LEDs by suppressing the unwanted guiding modes. 8‐10 However, previous researches either require complicated process or expensive equipment 11 in the formation of wavelength periodic structures which greatly limited the application of 2D-PC structures to the LEDs. On the other hand, the 2D-PC structures can deteriorate the electrical properties 12 of the LED devices and affect the device lifetime. 13 To avoid these negative effects, 2D-PC structures can be formed onto the indium-tin oxide (ITO) current spreading layer Instead. During the fabrication of these structures in the characteristic length scales on the order of the wavelength, nano-patterning techniques are involved. Compared to nano-patterning using E-beam, 14 nano-imprint 15 lithographic techniques, nano-sphere lithography (NSL) 16‐18 has been demonstrated as a high-efficiency low-cost, and high throughput nano-fabrication method. In this paper, the fabrication and characterization of the InGaN LEDs with SiO2 nano-bowl photonic crystal (SNPC) atop the ITO current spreading layer patterned by self-assembled NSL were reported. The optical and electrical performances of the LEDs with SNPCs were evaluated in comparison with the conventional LEDs, together with an investigation of the mechanisms involved. The optical performances were also confirmed by the numerical simulations using the finite difference time domain (FDTD) method.

Diode ideality factor in modern light-emitting diodes

The diode ideality factor has been subjected to theoretical discussions from the viewpoint of light-emitting diodes (LEDs). As radiative recombination in LEDs is incongruent with the Sah–Noyce–Shockley (SNS) analysis, the SNS recombination plane was replaced with carrier confinement structures of modern LEDs, e.g., quantum wells (QWs). We calculated the voltage partitioning factors to indicate how carrier concentration changes as a function of terminal voltage. As local minority carriers determine the net recombination rate at QWs, we have deduced that the partitioning factor of the local minority carriers becomes the ideality factor of the LED. By applying incomplete ionization of acceptors in InGaN/GaN LEDs, a conclusion has been reached such that the partitioning factor of local minority carriers as well as the diode ideality factor can be nearly as large as 3.

Curing efficiency of modern LED units

Recent reports claim that modern light-emitting diode (LED) curing units improve curing efficiency by increasing the units' irradiance. In this context also, short polymerisation times up to 5 s are proposed. The aim of this study was to examine whether there are differences in the curing efficiency of modern LED curing units by assessing their effect on two different composite materials and by varying the irradiation time. A nano- and a micro-hybrid resin-based composite (RBC) were polymerised for 5, 10 and 20 s with three commercial and a Prototype LED unit (Elipar™ S10). Cylindrical specimens (6 mm in depth, 4 mm in diameter) were prepared in three increments, each 2-mm thick, and were consecutively cured. Degree of cure was measured for 20 min in real time at the bottom of the samples, starting with the photoinitiation. The micro-mechanical properties (modulus of elasticity, E and Vickers hardness, HV) were measured as a function of depth, in 100-μm steps, on the above described samples stored in distilled water for 24 h at 37°C. Data were analysed with multivariate ANOVA followed by Tukey's test, t test and partial eta-squared statistics. In descending order of the strength of their effect, the type of RBC, depth, polymerisation time and curing unit were significant factors affecting the micro-mechanical parameters (p < 0.05). The degree of cure at 6-mm depth was less but significantly influenced by the curing unit and curing time and was independent from the type of RBC. A 5-s irradiation time is not recommended for these units. Whereas a 5-s irradiation is acceptable at the sample's surface, a minimum of 20 s of irradiation is necessary for an adequate polymerisation 2 mm beyond the surface.

Standards and Performance Indicators for Surgical Luminaires

The illumination performance of surgical luminaires is quantified by performance indicators defined in an international standard (IEC 2000). The remaining maximum illuminance in relevant situations, the light-field size, and the spectral characteristics are performance indicators used by hospitals as input for luminaire opting processes. Industry however focuses on illuminance when communicating with health care professionals. The aim of this study is to evaluate whether these standards are sufficient to describe luminaire performance, especially for modern LED lighting technology. Illuminance distribution and spectrum measurements were performed on 5 different state-of-the-art (LED) surgical luminaires. The results showed that changing situations not only changed the maximum illuminance but also changed the light-field sizes and shapes, introducing substantial differences between luminaires. Moreover, colored cast shadows and light color variations across the light-field were observed for 3 luminaires using differently colored light emitting diodes (LEDs). Both the changing light-field sizes and shapes, and the cast shadows and light color variations for LED luminaires are not covered by the current standard. The standard should therefore be extended to incorporate these aspects, especially for such a high-end application as surgical lighting.