RGB Light-emitting Diode Research Articles

Enhanced performance of single-input multiple-output visible light communication system utilizing space diversity technology

Contradictions among receiver optical power, bandwidth of photodiode, and transmission distance are ubiquitous in visible light communication (VLC) systems. In order to moderate such contradictions and improve the performance of VLC systems, we proposed a space diversity receiver for a light-emitting diode (LED)-based VLC system and experimentally demonstrated the highest data rate for an RGB LED with a 1.4 Gbit/s data rate employing two detectors located in different space positions at the same time. The maximal ratio combining algorithm was used in the VLC system for the first time. As a result, the bit error ratios of the receivers can be reduced by 48.08 dB (red LED), 12.04 dB (green LED), and 27.48 dB (blue LED) compared with any separate detector. The performance of the VLC system was significantly improved. It turned out that it was highly desirable to use multiantennas in the VLC system.

Edge effect modeling and study for three-chip RGB light-emitting diodes

Subject of study. The paper deals with light quality improvement of multi–chip RGB light-emitting diodes (LEDs) and luminaries on their basis. In particular, we have studied the issues of the edge effect reducing, which is non–uniformity of color when observing the source of light under different angles as well as non-uniformity of color distribution on the illuminated surface. Methods. Experimental study of the edge effect has been performed, namely, the analysis of the halo at the periphery of the illuminated area and the non–uniformity of area at the surface of the screen illuminated with RGB LEDs with and without light concentrators. Modeling of illumination distribution at various distances from the source for the system containing four RGB LEDs with reflectors by ZEMAX software has been carried out. Assessment of the uniformity for light distribution via calculating the chromaticity coordinates has been performed. Main results. The possibility of modeling application at the stage of a luminary design is shown on the example of RGB LEDs for assessing the efficiency of light flux usage and colorimetric parameters. Suggested method simplifies significantly the design of luminaries and reduces associated costs. Practical relevance. The findings can be used in the design of luminaries based on RGB LEDs, including the ones with secondary optics elements.

An imperceptible plastic electronic wrap.

Extremely compliant sub-2-μm sensor films enable temperature mapping on complex 3D objects, like integrated circuits on printed circuit boards, food packages, and on human skin. In their stretchable form, these metal films withstand strains up to 275%. This imperceptible electronic foil technology platform offers new avenues for the design of complex, hybrid rigid-island stretchable-interconnect electronic devices such as RGB light-emitting diode (LED) strips that can be stretched and twisted without impairing their function.

Condenser-free contrast methods for transmitted-light microscopy.

Phase contrast microscopy allows the study of highly transparent yet detail-rich specimens by producing intensity contrast from phase objects within the sample. Presented here is a generalized phase contrast illumination schema in which condenser optics are entirely abrogated, yielding a condenser-free yet highly effective method of obtaining phase contrast in transmitted-light microscopy. A ring of light emitting diodes (LEDs) is positioned within the light-path such that observation of the objective back focal plane places the illuminating ring in appropriate conjunction with the phase ring. It is demonstrated that true Zernike phase contrast is obtained, whose geometry can be flexibly manipulated to provide an arbitrary working distance between illuminator and sample. Condenser-free phase contrast is demonstrated across a range of magnifications (4–100×), numerical apertures (0.13–1.65NA) and conventional phase positions. Also demonstrated is condenser-free darkfield microscopy as well as combinatorial contrast including Rheinberg illumination and simultaneous, colour-contrasted, brightfield, darkfield and Zernike phase contrast. By providing enhanced and arbitrary working space above the preparation, a range of concurrent imaging and electrophysiological techniques will be technically facilitated. Condenser-free phase contrast is demonstrated in conjunction with scanning ion conductance microscopy (SICM), using a notched ring to admit the scanned probe. The compact, versatile LED illumination schema will further lend itself to novel next-generation transmitted-light microscopy designs. The condenser-free illumination method, using rings of independent or radially-scanned emitters, may be exploited in future in other electromagnetic wavebands, including X-rays or the infrared.

Holographic head-mounted display with RGB light emitting diode light source

A compact head-mounted holographic three-dimensional display with an RGB light-emitting diode (LED) light source is developed. Issues regarding full-color holographic image design and the quality associated with the use of an LED light source are investigated. The accommodation effect and background noise in the proposed system are discussed based on experimental observation.

固态体积式真三维立体显示发光二极管投影光源

According to the needs of solid-state volumetric true-3Ddisplay for high brightness and adjustable color temperatures,aprojection light source with high power and single chip Light Emitting Diodes(LEDs)was designed.The brightness of the projection light source was computed theoretically.The results were then corrected according to the spectrum curve of a Liquid Crystal(LC)shutter. With the considerations of life and heat dissipation,the projection light source with high-power RGB LEDs and a white LED was put forward.Based on the optical characteristics of the chosen LEDs,the refraction-and-reflection mixed method was applied to the design of collimating lens.Both simulation and practical test were performed on the projection light source.The experimental results show thatthe divergence angle of the LED has been reduced from ±75°to ±20°after applying the collimating lens.A 3Ddisplay prototype was developed.The practical luminance on the LC shutter close to observers reaches 149cd/m2.The prototype presents the merits of no perceived flickers and strong 3D feeling,as well as the shortcoming of a smaller viewing angle.The proposed light source satisfies the requirements of solid-state volumetric true-3Ddisplay.

Noncontact monitoring of vascular lesion phototherapy efficiency by RGB multispectral imaging

A prototype low-cost RGB imaging system consisting of a commercial RGB CMOS sensor, RGB light-emitting diode ring light illuminator, and a set of polarizers was designed and tested for mapping the skin erythema index, in order to monitor skin recovery after phototherapy of vascular lesions, such as hemangiomas and telangiectasias. The contrast of erythema index (CEI) was proposed as a parameter for quantitative characterization of vascular lesions. Skin recovery was characterized as a decrease of the CEI value relative to the value before the treatment. This approach was clinically validated by examining 31 vascular lesions before and after phototherapy.

Color-Tunable and Phosphor-Free White-Light Multilayered Light-Emitting Diodes

A tightly integrated 3-D RGB light-emitting diode (LED) stack is demonstrated. Chips of identical dimensions are stacked on top of each other, with wire bonds embedded within. This is achieved by integrating laser-micromachined channels onto the sapphire face of InGaN LEDs, serving to accommodate wire bonds from the chip beneath. The resultant structure eliminates leakage of monochromatic light from individual chips, producing optimally mixed emission through the top aperture. The device can emit a wide range of colors and is an efficient phosphor-free white-light LED as well. When emitting at correlated color temperatures (CCTs) of 2362 K, 5999 K, and 7332 K, the device generates ~ 20 lm/W, exhibiting performance invariant of CCT. Thermal characteristics of this multilayered device are investigated via infrared thermometry.

Improving performance of 750-Mb/s visible light communication system using adaptive Nyquist windowing

We propose a configuration of a wavelength division multiplexing (WDM)-visible light communication (VLC) system using orthogonal frequency division multiplexing (OFDM) modulation and an adaptive Nyquist windowing of the OFDM signal in the receiver. Based on this configuration, we demonstrate a 750-Mb/s WDM-VLC transmission based on RGB light-emitting diode (LED) with a distance of 70 cm. The measured bit error rate (BER) for all channels are under the pre-forward error correction limit of 3.8 \times 10-3. The BER performances of all the channels of the proposed WDM-VLC system show considerable improvement compared with those of the system without Nyquist windowing.

A color local dimming algorithm for liquid crystals displays using color light emitting diode backlight systems

A color local dimming algorithm is proposed to reduce the image distortion and power consumption of liquid crystal displays (LCDs) by using color light emitting diode (LED) backlights. The proposed algorithm is demonstrated using a 55-in. LCD panel with a RGB LED backlight system. In the proposed algorithm, the dimming rate of local blocks is determined by calculating the image distortion of the dimming image, and this is used to achieve steady image quality. The image data of the LCD panel are modified to reduce color distortion by removing the differences in chromaticity. Simulated test results show that the average, standard deviation, and relative variation of PSNRs for the test video are 43.74dB, 3.57dB, and 0.082, respectively. The average color differences using the proposed algorithm are 9.6, 5.0, 5.6, and 4.2 for the test patterns with 2×2, 4×2, 4×4, and 8×4 white boxes, respectively. The measured average power saving ratio for the test video is 67.3%.

Specially Designed Driver Circuits to Stabilize LED Light Output Without a Photodetector

This study proposes a novel light-emitting diode (LED) driver circuit that stabilizes the lighting output without a photodiode. The LED driver circuit is composed of a self-alignment circuit to compensate for the photo-power deviation and a feedback control system instead of a conventional photodetector to maintain the electrical power of the LED. The LED characteristics, based on electrical and optical measurements that varied with temperature, were built to be a lookup reference of the generated LED photopower, which serves as a compensator. A commercial full color LED system, which comprises red, green, and blue (RGB) LED chips, was implemented according to the novel concept. The impact of a conventional current regulator, a proposed electrical-power stabilization circuit, and a self-aligned photo-power stabilization circuit was studied graphically, followed by experimental verification. The maximal and minimal rate of change of the radiant flux Φ driven by the proposed LED drivers are 3.467% in a red LED, and 2.848% in green LED, respectively, over the ambient temperature range of 0.5-85 °C. The experimental results verified the implementation of a lighting power stabilization control system without the use of a photodiode for RGB LEDs.

Enhancement of LED indoor communications using OPPM-PWM modulation and grouped bit-flipping decoding.

Combination of overlapping pulse position modulation and pulse width modulation at the transmitter and grouped bit-flipping algorithm for low-density parity-check decoding at the receiver are proposed for visible Light Emitting Diode (LED) indoor communication system in this paper. The results demonstrate that, with the same Photodetector, the bit rate can be increased and the performance of the communication system can be improved by the scheme we proposed. Compared with the standard bit-flipping algorithm, the grouped bit-flipping algorithm can achieve more than 2.0 dB coding gain at bit error rate of 10-5. By optimizing the encoding of overlapping pulse position modulation and pulse width modulation symbol, the performance can be further improved. It is reasonably expected that the bit rate can be upgraded to 400 Mbit/s with a single available LED, thus transmission rate beyond 1 Gbit/s is foreseen by RGB LEDs.

Improvement of Color and Luminance Uniformity of the Edge-Lit Backlight Using the RGB LEDs

The effect of the M-window color mixing bar on the characteristics of color mixing and hot spots in the edge-lit backlight employing red (R), green (G), blue (G) light emitting diodes (LED) were studied in terms of the structure of the M-window color mixing bar. The rays from RGB LEDs entering the M-window bar were mixed by internal reflection and scattering inside the M-window bar so that the hot spots and color separation were minimized. The M-window bar was designed and fabricated and the simulation results are matched quite well to experimental data.

Time-division color electroholography using one-chip RGB LED and synchronizing controller

We propose time-division based color electroholography with a one-chip RGB Light Emitting Diode (LED) and a low-priced synchronizing controller. In electroholography, although color reconstruction methods via time-division have already been proposed, the methods require an LCD with a high refresh rate and output signals from the LCD for synchronizing the RGB reference lights such as laser sources, which consequently increase the development cost. Instead of using such an LCD, the proposed method is capable of using a general LCD panel with a normal refresh rate of 60 Hz. In addition, the LCD panel used in the proposed method does not require the output signals from the LCD. Instead, we generated synchronized signals using an external controller developed by a low-priced one-chip microprocessor, and, use a one-chip RGB LED instead of lasers as the RGB reference lights. The one-chip LED allows us to decrease the development cost and to facilitate optical-axis alignment. Using this method, we observed a multi-color 3D reconstructed movie at a frame rate of 20 Hz.

Optical design of color light-emitting diode ring light for machine vision inspection

Uniform irradiance and color adjustability are the key features in the design of lighting for machine vision inspection systems. A simple and practical design tool of angled light-emitting-diode (LED) ring arrays for uniform near-field irradiance has been developed by introducing a simple model to simplify the complexity of nonrotational symmetric irradiance distribution of angled LEDs. The color distribution and color uniformity of the ring array assembled with RGB LEDs are analyzed based on the analytical model of color mixing. According to the theoretical analysis, the simulated results, and the design exemplifications, the practical design tool offers an easy way to estimate the performance of an RGB LED ring array and can be considered as a starting point to reduce the computation time for exact designs that must use a realistic LED model.

Eliminating hotspots in a multi-chip LED array direct backlight system with optimal patterned reflectors for uniform illuminance and minimal system thickness

We propose an optical design process that significantly reduces the time and costs in direct backlight unit (BLU) development. In it, the basic system specifications are derived from the optical characteristics of RGB light-emitting diodes (LEDs) comprising the BLU. The driving currents are estimated to determine the theoretical RGB flux ratio for a desired white point. The number of LEDs needed to produce the target luminance is then calculated from the combined optical efficiencies of the components. Last, an appropriate array configuration is sought based on the illuminance distribution function for meeting the target uniformity. To showcase the design process we built two 42-inch triangular cluster arrays of 40 x 16 LED elements. When a flat reflective sheet was used, the minimum thickness required of the system to satisfy the target uniformity was 30 mm. Introducing a patterned reflective sheet removed hotspots that resulted from reducing the system thickness without the aid of additional optical components. Using an optimized patterned reflective sheet, reduction in system thickness as much as 5 mm was possible.

LED 휘도의 디지털 제어 방식에 관한 연구

발광다이오드(LED)의 휘도를 제어하는 기존 방식은 펄스폭변조(PWM)방식이 주로 사용되었다. PWM 방식은 LED의 점등시간비와 휘도가 비례하는 아날로그적인 방식으로 통신 등의 디지털 방식과는 호환이 어려운 단점을 가지고 있다. 본 연구에서는 디지털 방식으로 적, 녹, 청색 LED의 휘도를 제어하는 실험을하였다. 이를 위해 LED 구동회로와 디지털 논리회로로 구성된 LED 디지털제어 장치를 개발하였다. 디지털 입력에 대한 적, 녹, 청색 LED의 점등 펄스 수를 제어하여 다양한 광색을 구현하고, 측정한 광색의 x, y 색도좌표가 원하는 색의 CIE 색도도 영역에 포함될 수 있도록 디지털 코드를 최적화시켰다. 본 연구 결과는 디지털 통신을 이용한 LED 램프의 원격 제어로 풀 컬러를 구현하는데 효과적으로 활용될 수 있다.

Design of an RGB LED Backlight Circuit for Liquid Crystal Display Panels

This paper presents a red, green, and blue (RGB) light-emitting diode (LED) backlight circuit for liquid crystal display (LCD) panels. Some studies have been conducted to devise dimming methods that regulate the RGB LED currents. In this paper, design considerations for the RGB LED backlight are presented. A laboratory prototype for 20-in panels is implemented and tested. The color gamut of an LCD panel with the designed RGB LED backlight is measured and discussed. The experimental results of the laboratory prototype are also shown to verify feasibility of the findings.

43.3: RGB LED Backlight of Color Sequential Display with Precise Current‐Balance Control Technique

AbstractThis paper proposes a high efficiency and precise current balance circuit in parallel RGB Light Emitting Diodes (LED) channels for color filter‐less panel display. The new technique not only enhances the current matching in each channel but also reduces the requirement of device numbers and external resistors. The error percentage of all channel LED currents can kept within 0.1% when the channel voltage has a voltage variation (Δ VCH) around 1. 1V. Furthermore, the proposed load dependent minimum drain voltage can effectively reduce the voltage drop across the precise current balance circuit. As a result, the efficiency of the LED driver can be improved.

Time-Stable Red, Green, and Blue Light-Emitting Diode Backlighting Control Using Time-Varying Transform Matrix

This paper proposes a driving current control method for a back light unit (BLU), consisting of red, green, and blue (RGB) light-emitting diodes (LEDs), whereby a RGB optical sensor is used to check the output color stimulus variation to enable a time-stable color stimulus for light emission by the RGB LED BLU. First, to obtain the present color stimulus information of the RGB LED BLU, an RGB to XYZ transform matrix is derived to enable CIEXYZ values to be calculated for the RGB LED BLU from the output values of a RGB optical sensor. The elements of the RGB to XYZ transform matrix are polynomial coefficients resulting from a polynomial regression. Next, to obtain the proper duty control values for the current supplied to the RGB LEDs, an XYZ to Duty transform matrix is derived to calculate the duty control values for the RGB LEDs from the target CIEXYZ values. The data used to derive the XYZ to Duty transform matrix are the CIEXYZ values for the RGB LED BLU estimated from the output values of the RGB optical sensor and corresponding duty control values applied to the RGB LEDs for the present, first preceding, and second preceding sequential check points. With every fixed-interval check of the color stimulus of the RGB LED BLU, the XYZ to Duty transform matrix changes adaptively according to the present lighting condition of the RGB LED BLU, thereby allowing the RGB LED BLU to emit the target color stimulus in a time-stable format, regardless of changes in the lighting condition of the RGB LEDs.