Multiple Lead Research Articles

Optical thermal model for LED heating in thermoset-automated fiber placement

Control of material temperature distribution and governing phenomena during automated fiber placement is an important factor. Numerical modeling of the radiative heat transfer for a newly presented LED-based heating unit is developed and analyzed in theory. An optical model allows taking into account the radiative energy output of every individual LED. By adjusting the electrical input to the multiple LED arrays on the heating unit, the irradiance distribution on the substrate’s surface can be controlled. To investigate the capability to adjust the surface temperature distribution resulting from this feature, thermal models for two and three dimensions are developed and employed for the calculated irradiance distributions. The resulting temperature distributions show that temperature gradients can be avoided or created, depending on the input to the heating unit. The results from the two models are compared and a method to select an appropriate model in general is proposed.

A novel PAPR reduction scheme for VLC DCO-OFDM systems

In this study, we aim to reduce the peak-to-average power ratio (PAPR) for indoor visible light communication (VLC) orthogonal frequency division multiplexing (OFDM) systems and improve the system performance. By utilizing the spatial multiplexing (SMP) technique, we propose a subcarrier spatial grouping (SSG) scheme to address the high PAPR and LED restricted linear range problems. In this scheme, the high-PAPR wideband OFDM signals are partitioned into several low-PAPR narrowband independent signals that are simultaneously transmitted from multiple LED light sources. At the receiver, zero forcing (ZF) is used to reverse the impairments of the channel transform. The computer simulation results demonstrate that the proposed approach achieves lower PAPR and more robustness to LED nonlinearities leading to better bit error rate performance for the same spectral efficiency than that of the conventional DCO-OFDM scheme does in the whole signal-to-noise ratios range.

Effective freeform TIR lens designed for LEDs with high angular color uniformity.

Most LED luminaires face the problem of color nonuniformity in illumination fields, which seriously degrades their lighting quality. To address this issue, we proposed a freeform TIR lens, which could be applied to the most widely adopted phosphor-converted LEDs and multicolor LED arrays. Based on light pattern overlap and energy mapping, the novel TIR lens comprised the freeform panel and the collimator. Analysis of angular color uniformity (ACU), based on a phosphor-converted white LED model, proved that the normalized standard deviation (NSD) of the LED integrated with the freeform TIR lens significantly decreased from 0.792 to 0.232, compared with the NSD of the traditional TIR lens. Moreover, the proposed TIR lens was also applied to a multiple LED array for color mixing. Both the theoretical analysis and simulation results demonstrated their capability to enhance the ACU. These results demonstrated the effectiveness of the proposed method to improve the uniformity of the LED sources regardless of single or multiple LEDs. The improved lighting quality guarantees good employment of LEDs, especially in the fields of entertainment and architectural lighting.

Securing Visible Light Communication Systems by Beamforming in the Presence of Randomly Distributed Eavesdroppers

This paper considers secrecy enhancement mechanisms in visible light communication (VLC) systems with spatially distributed passive eavesdroppers (EDs) under the assumption that there are multiple LED transmitters and one legitimate receiver (UE). Based on certain amplitude constraints, we propose an optimal beamforming scheme to optimize secrecy performance. Contrary to the case where null-steering is made possible by using knowledge of the ED locations, we show that the optimal solution when only statistical information about ED locations is available directs the transmission along a particular eigenmode related to the intensity of the ED process and the intended channel. Then, a sub-optimal LED selection scheme is provided to reduce the secrecy outage probability (SOP). An approximate closed-form for the SOP is derived by using secrecy capacity bounds. All analysis is numerically verified by Monte Carlo simulations. The analysis shows that the optimal beamformer yields superior performance to LED selection. However, LED selection is still a highly efficient suboptimal scheme due to the complexity associated with the use of multiple transmitters in the full beamforming approach. These performance trends and exact relations between system parameters can be used to develop a secure VLC system in the presence of randomly distributed EDs.

Constellation design for color space-based modulation in visible light communications

In this paper, we present a new strategy to design the constellation symbols in a visible light communication (VLC) system by considering multiple LED transmitters at different wavelengths. For this purpose, the LED transmitters are modeled as fixed points in a color space, and the constellation symbols are obtained by convex combinations of these points. In our formulation, the resulting constellation symbols have to reproduce on average a desired white tonality. Hence, the constellation design strategy is defined as a constrained optimization problem that seeks to maximize the minimum distance among the resulting symbols, but at the same time, to yield a desired white tonality on average. For evaluation purposes, we consider the CIE 1931 color space with 3 and 5 LED transmitters, and constellations with 4, 8 and 16 symbols. Our proposed methodology is compared with a previous work in the state of the art, where an important improvement is achieved with respect to the resulting Euclidean distance among constellation symbols.

P‐2.1: Micro‐LED eye mask

Nowadays, with the development of the pace of society, people's daily life will be affected by many pressures, and everyone is quite concerned about their health. Eye fatigue, as a common and important sub ‐ health problem, many people have no time to consider it or have no appropriate way to solve this problem. This paper proposes a therapeutic method: the phototherapy eye mask based on Micro‐LEDs. This eye mask has the features of portability and high comfort, which can effectively relieve eye fatigue and relax the eye. Combined with the characteristics of Micro‐LED structure, dense arrangement, independent driving, and the principle of light medical treatment, we can achieve the desired effect for different skin and eye fatigue. This study will include a cell phone APP to assist Micro‐LED eye mask, which can individualize the phototherapy eye mask and customize the individual light therapy for different eye fatigue. LED technology is developing rapidly, besides mainstream lighting and display, it can also play a unique role in healthcare. There are LED figures in medical beauty center and scalded skin repair instrument. Micro‐LED is composed of multiple micro LED arrays. It has many characteristics such as small size, dense distribution, compact structure and each pixel can be independently driven. It also has many advantages such as high luminous efficiency, power saving and fast reaction speed. Therefore, based on the current situation of LED eye mask, it can be improved to Micro‐LED eye mask, which can be applied to every point of eye and omni‐directional coverage. Besides, this Micro‐LED eye mask can be combined with the actual situation of everyone to light anywhere they need to be treated, which can be the best effect on people's health.

Counting via LED sensing: Inferring occupancy using lighting infrastructure

As a key component of building management and security, occupancy inference through smart sensing has attracted a lot of research attention for nearly two decades. Nevertheless, existing solutions mostly rely on either pre-deployed infrastructures or user device participation, thus hampering their wide adoption. This paper presents CeilingSee, a dedicated occupancy inference system free of heavy infrastructure deployments and user involvements. Building upon existing LED lighting systems, CeilingSee converts part of the ceiling-mounted LED luminaires to act as sensors, sensing the variances in diffuse reflection caused by occupants. In realizing CeilingSee, we first re-design the LED driver to leverage LED’s photoelectric effect so as to transform a light emitter to a light sensor. In order to produce accurate occupancy inference, we then engineer efficient learning algorithms to fuse sensing information gathered by multiple LED luminaires. We build a testbed covering a 30 m2 office area; extensive experiments show that CeilingSee is able to achieve very high accuracy in occupancy inference.

Interpixel Interference Mitigation in Visible Light Communication Using Image Sensor

To improve the data rate in a camera-based visible light communication system, multiple LEDs in an LED array are used to transmit multiple bits in one frame. The problem is that LEDs are always captured in the image with some degree of blooming effect. This effect increases the intensity of pixels surrounding the origin LEDs in the image. When multiple LEDs are used, the blooming effect from all LEDs causes the interpixel interference (IPI) problem that greatly degrades the quality of the received signal. This paper analyzes the blooming effect and finds that the IPI problem increases when the number of “ON” LEDs in the array increases. Through the analysis, this paper proposes a differential coding to minimize the number of “ON” LEDs in every transmitted frame and thus minimize the IPI effect. The proposed scheme is then verified through simulations. The results show that a lower bit error rate, which indicates a better signal quality, can be achieved by applying the proposed scheme.

Multi-Element VLC Networks: LED Assignment, Power Control, and Optimum Combining

Visible light communications (VLCs) are a promising technology to address the spectrum crunch problem in radio frequency networks. A major advantage of VLC networks is that they can use the existing lighting infrastructure in indoor environments, which may have large number of LEDs for illumination. While LEDs used for lighting typically have limited bandwidth, presence of many LEDs can be exploited for indoor VLC networks, to serve each user by multiple LEDs for improving link quality and throughput. In this paper, LEDs are grouped and assigned to the users based on received signal strength from each LED, for which different solutions are proposed to achieve maximum throughput, proportional fairness, and quality of service. Additionally, power optimization of LEDs for a given assignment is investigated, and the Jacobian and Hessian matrices of the corresponding optimization problem are derived. Moreover, for multi-element receivers with LED grouping at the transmitter, an improved optimal combining method is proposed. This method suppresses interference caused by simultaneous data transfer of LEDs and improves the overall signal-to-interference-plus-noise-ratio by 2–5 dB. Lastly, an efficient calculation of channel response is presented to simulate multipath VLC channel with low computational complexity.

The Development of a System for Measuring the Junction Temperatures of Multiple LEDs on a Package Simultaneously by Using Pulsed-laser Raman Scattering

The Development of a System for Measuring the Junction Temperatures of Multiple LEDs on a Package Simultaneously by Using Pulsed-laser Raman Scattering

Multi-LED light source for hyperspectral imaging

We investigate a novel light source design for efficient collection of UV light from multiple LEDs. The design is based on a truncated cone with a large circular lid incorporating LEDs on one side, and a small circular exit aperture (diameter of 9 mm) on the other side. The exit aperture size allows a simple coupling with secondary optics of a microscope for hyperspectral imaging that requires a wide spectrum of frequencies of illuminating light. The light collection efficiency was optimized with respect to cone length and diameter of the LED lid. In all simulations, we use a highly UV-reflecting aluminum coating on the inner surfaces of the cone. The influence of the LED positions on the cone efficiency is determined by varying the LED distance from the central axis as well as the interLED distance. We found the maximum efficiency of the cone is realized for LEDs positioned at the center, and the shorter is the inter-LED distance, the better is the performance of the light source.

Multiple String LED Driver With Flexible and High-Performance PWM Dimming Control

The main objectives in driving multiple LED strings include achieving uniform current control and high performance PWM dimming for all strings. This work proposes a new multiple string LED driver to achieve not only current balance, but also flexible and wide range PWM dimming ratio for each string. A compact single-inductor multiple-output topology is adopted in the driver, accompanied by synchronous integrators and variable dimming frequency, to achieve both high efficiency and high performance dimming. By using the proposed variable dimming frequency scheme, high dimming frequency is applied to a string with high dimming ratio, which helps to maintain the deviation of LED string current in an acceptable range, while low dimming frequency is applied to a string with low dimming ratio, which helps to achieve rectangular LED current waveform. Meanwhile, the new time multiplexing control scheme automatically optimizes the LED string's bus voltages, thus minimizes each string's power loss. A three string LED driver prototype is constructed to validate the effectiveness of the peoposed control scheme, where the three strings can have different dimming ratios between 4% and 100%.

Combined effect of the indium content and well width on electroluminescence in InGaN/GaN multiple quantum well-based LEDs

Combined effect of the indium content and well width on electroluminescence in InGaN/GaN multiple quantum well-based LEDs

CeilingTalk: Lightweight Indoor Broadcast Through LED-Camera Communication

Although Visible Light Communication (VLC) is gaining increasing attention in research, developing a practical VLC system to harness its immediate benefits using Commercial Off-The-Shelf (COTS) devices is still an open issue. To this end, we develop and deploy CeilingTalk as a lightweight wireless broadcast system using COTS LED luminaries as transmitters and smartphone cameras as receivers so that it can be fully hosted in a smartphone and is feasible for all possible indoor environments. CeilingTalk innovates in both encoding and decoding to achieve an adequate throughput for realistic applications. On one hand, it employs Raptor coding to allow multiple LED luminaries to transmit collaboratively so as to benefit both throughput and reliability. On the other hand, it involves a lightweight decoding scheme to handle the asynchrony (both spatial and temporal) in transmissions. Moreover, we analyze the impact of various parameters on the performance of CeilingTalk, in order to derive a model for such VLC systems enabled by COTS devices and hence provide general guidance for future VLC deployments in larger scales. Finally, we conduct extensive field experiments to validate the effectiveness of our LED-camera VLC model, as well as to demonstrate the promising performance of CeilingTalk: up to 1.0 kb/s at a distance of 5 m.

Multiuser MIMO Indoor Visible Light Communication System Using Spatial Multiplexing

Visible light communications is an energy efficient and cost-effective solution for indoor wireless access. In this paper, we propose a multiple input multiple output system using centralized or decentralized transmitted power allocation algorithms with multiple LEDs and photodetectors. The proposed system uses an optical code division multiple-access technique to support multiple users. Time-space minimum mean squared error filters at the receivers are designed to diminish the effect of multiple-access interference. In the centralized power allocation algorithm, all the LED lamps in the room are coordinated and controlled by a central controller; each LED lamp supports all the users within the indoor area. The decentralized power allocation algorithms we propose have similar bit error rate performance yet less computational burden compared to the centralized algorithm. In our decentralized algorithms, users are supported by a subset of the LEDs, and so the optimization problem size can be reduced by as much as $93\%$ . For each receiver, multiple photodetectors with different orientations are employed to improve the signal to interference plus noise ratio. In addition, some practical considerations such as shadowing effects, illumination requirements, dimming control, and transmitted power quantization are taken into account.

A Single LED Positioning System Based on Circle Projection

A simple and efficient visible light positioning (VLP) system based on a single LED lamp is proposed and demonstrated. In our system, a common circular LED lamp with a point marker is used as a transmitter and a smartphone camera serves as a receiver. The LED image is no longer treated as a point as in the existing works, but as an image whose geometric features are exploited to determine the receiver's orientation and location relative to the reference LED lamp. The expressions for determining the receiver's orientation and location are derived in terms of the geometric parameters. Our algorithm enhances the system robustness by overcoming the possible interruptions in conventional VLP systems, which require multiple LEDs. Meanwhile, it avoids the usage of angular sensors, which are unable to provide reliable and accurate measurements on receiver's orientation in mobile indoor environment. We experimentally evaluate and analyze the performance of the proposed VLP system. A centimeter-level positioning accuracy is achieved in an area of 3 m × 3 m.

Automated surface inspection of small customer-specific optical elements

Abstract In industry, manual visual inspection is typically applied to assess surface imperfections on basic optical elements according to the standard DIN ISO 14997. This article proposes a machine vision setup to mimic the human tester's inspection process. It consists of multiple cameras and LED light sources. Both are arranged on the surface of a hemisphere with the optical element to be inspected at its center. By enabling individual LED sources on the hemisphere, any movement during acquisition can be omitted. Thus, the system is capable of acquiring a sparse pseudo BRDF (Bidirectional Reflectance Distribution Function) representation of imperfections. It is shown by experiments that this representation allows to discriminate between certain imperfections. Besides the mechanical setup, the image processing methodology and classification results are discussed. A comparison to results from manual inspection for 20 optical elements of the same geometry is also presented. Results indicate that a good agreement with the de-facto standard manual inspection method from industry can be obtained by the system.

Investigation Into the Use of Single Inductor for Driving Multiple Series-Connected LED Channels

An investigation into the use of single inductor for driving N series-connected LED channels is presented. Each channel has a parallel-connected switch for controlling the LED channel current, as well as charging the inductor. Compared with the conventional structure having all LED channels connected in parallel and (N + 1) switches, the proposed structure has the merits of 1) requiring N switches, 2) allowing all channels to be driven concurrently in every switching cycle, and 3) being insensitive to the duration of the transition switching from one channel to another. Detailed mathematical analysis on the topological operations will be discussed. The proposed concept will be demonstrated on a two-channel structure. A prototype with two 7 W LED modules has been built and evaluated. A control mechanism that can regulate the output luminous flux and correlated color temperature without sensing the inductor current and channel currents will be discussed. Experimental results are favorably compared with theoretical predictions.

车内多LED光源阵列分布通信系统信道研究

车内多LED光源阵列分布通信系统信道研究

Generating selected color using RGB, auxiliary lights, and simplex search

ABSTRACTA mixed light source generates various colors with the potential to adjust the intensities of multiple LEDs, which makes it possible to generate arbitrary colors. Currently, PCs and OSs provide color selection windows that can obtain the red, green, and blue (RGB) or hue, saturation, and lightness (HSL) color coordinates of a user’s selection. Mixed light sources are usually composed of primary-colored LEDs with LEDs in auxiliary colors such as white and yellow used in a few cases. The number of LED inputs and the dimming levels are greater than the number of elements in the color coordinate when using auxiliary-colored LEDs, which causes an under-determined problem. This study proposes a method to determine the dimming levels of LEDs based on a simplex search method (SSM). Commercial LEDs have different optical power values and impure color elements, even if they are RGB. Hence, the characteristics of the LEDs were described using a linear model derived from the tri-stimulus values (an XYZ color coordinate model) and dimming levels. Color mixing models were derived for an arbitrary number of auxiliary-colored LEDs. The under-determined problem was solved using an SSM without an inverse matrix operation. The proposed method can be applied to a machine vision system and an RGBW light mixer for semiconductor inspection. The dimming levels, obtained using the proposed method showed better optical responses than those derived using other methods.