Conventional White Light-emitting Diodes Research Articles

Preparation of BaAl2-xGaxSi2O8:Ce3+, M+ (Li, Na, K) blue-cyan phosphors based on charge compensation and cation substitution strategies and luminescence performance study

Highly efficient blue-cyan phosphors can fill the cyan gap in conventional white light-emitting diode (LED) devices to improve the spectral continuity while effectively increasing the white LED colour rendering index (Ra). In this study, Ba0·94Al2-xGaxSi2O8:0.06Ce3+, M+ (Li, Na, K, x = 0–0.8) blue-cyan phosphors with a feldspar structure were prepared by a high-temperature solid-phase method using charge compensation and cation substitution strategies. The barium (Ba) vacancy due to the charge-imbalance substitution of Ce3+ is compensated by the different charge compensators Li+, Na+, and K+. The charge compensators affect the luminescence performance of the samples, with the K+-compensated samples showing the greatest enhancement, with intensities up to 1.8 times that of the uncompensated samples. The cation substitution strategy using Ga3+ to partially replace Al3+ further improves the luminescence performance of the Ba0·94Al2Si2O8:0.06Ce3+, 0.04 K+ sample, and the excitation band is also improved. In this case, the luminescence intensity of the Ba0·94Al2Si2O8:0.06Ce3+ luminescent material is increased by a factor of 6, and a broadband blue-cyan phosphor with a full-width at half-maximum of 118 nm containing blue, cyan and green is obtained. The co-doping of charge compensators and partial cation substitution improved the luminescence performance of phosphors. The representative sample Ba0·94Al1.6Ga0.4Si2O8:0.06Ce3+, 0.04 K+ has a luminescence intensity at a temperature of 150 °C that is 83.2% of that at room temperature and good thermal stability. The combination of 365 nm near-ultraviolet (NUV) LED chips with Ba0·94Al1.6Ga0.4Si2O8:0.06Ce3+, 0.04 K+ and the commercial red powder (Ca, Sr)AlSiN3:Eu2+ yields a white LED device with an Ra of up to 93 and a correlated colour temperature (CCT) of 4825 K. These results show that the blue-cyan phosphor complements the cyan component while simplifying the manufacturing process of white LEDs.

Color Rendering Index over 95 Achieved by Using Light Recycling Process Based on Hybrid Remote-Type Red Quantum-Dot Components Applied to Conventional LED Lighting Devices.

Red color conversion materials have often been used in conventional white LEDs (light-emitting diodes) to enhance the insufficient deep-red component and thus improve the color-rendering property. Quantum dots (QDs) are one of the candidates for this due to their flexibility in controlling the emission wavelength, which is attributed to the quantum confinement effect. Two types of remote QD components, i.e., QD films and QD caps, were prepared and applied to conventional white LED illumination to improve the color-rendering properties. Thanks to the red component near 630 nm caused by the QD components, the color rendering indices (CRIs) of both Ra and R9 could be increased to over 95. It was found that both the diffusing nature of the reflector and the light recycling process in the vertical cavity between the bottom reflector and the top optical films play important roles in improving the color conversion efficiency of remote QD components. The present study showed that the proper application of remote QDs combined with a suitable optical cavity can control the correlated color temperature of the illumination over a wide range, thus realizing different color appearances of white LED illumination. In addition, a high CRI of over 95 could be achieved due to the sufficient excitation from fewer QDs, due to the strong optical cavity effect.

More light components and less light damage on rats' eyes: evidence for the photobiomodulation and spectral opponency.

The blue-light hazard (BLH) has raised concerns with the increasing applications of white light-emitting diodes (LEDs). Many researchers believed that the shorter wavelength or more light components generally resulted in more severe retinal damage. In this study, based on the conventional phosphor-coated white LED, we added azure (484nm), cyan (511nm), and red (664nm) light to fabricate the low-hazard light source. The low-hazard light sources and conventional white LED illuminated 68 Sprague-Dawley (SD) rats for 7 days. Before and after light exposure, we measured the retinal function, thickness of retinal layers, and fundus photographs. The expression levels of autophagy-related proteins and the activities of oxidation-related biochemical indicators were also measured to investigate the mechanisms of damaging or protecting the retina. With the same correlated color temperature (CCT), the low-hazard light source results in significantly less damage on the retinal function and photoreceptors, even if it has two times illuminance and blue-light hazard-weighted irradiance ([Formula: see text]) than conventional white LED. The results illustrated that [Formula: see text] proposed by IEC 62471 could not exactly evaluate the light damage on rats' retinas. We also figured out that more light components could result in less light damage, which provided evidence for the photobiomodulation (PBM) and spectral opponency on light damage.

Substantial Improvement of Color-Rendering Properties of Conventional White LEDs Using Remote-Type Red Quantum-Dot Caps.

A new type of remote red quantum-dot (QD) component was designed and fabricated to improve the color-rendering properties of conventional white LED (light-emitting diode) lightings. Based on an optical simulation, the rectangular cavity-type QD cap was designed with an opening window on the top surface. Red QD caps were fabricated using a typical injection molding technique and CdSe/ZnS QDs with a core/shell structure whose average size was ~6 nm. Red QD caps were applied to conventional 6-inch, 15-W white LED downlighting consisting of 72 LEDs arrayed concentrically. The red QD caps placed over white LEDs enhanced the red components in the long-wavelength range resulting in the increase of the color rendering index (CRI) from 82.9 to 94.5. The correlated color temperature was tuned easily in a wide range by adopting various configurations consisting of different QD caps. The spatial and angular homogeneities were secured on the emitting area because QD caps placed over the white LEDs did not exhibit any substantial optical path length difference. The present study demonstrates that adopting QD caps in conventional LED lightings provides a flexible and efficient method to realize a high color-rendering property and to adjust correlated color temperature appropriately for a specific application.

Ultrathin, transparent, flexible, and dual-side white light-responsive two-dimensional molybdenum disulfide quantum disk light-emitting diodes

Ultrathin, transparent, and flexible devices with multifunctionalities are indispensable for emerging technologies. However, the related reports are very few, and the efficiency is rather limited owing to material restriction and complicated fabrication processes. In this work, a light-responsive white light-emitting diode (LED) with ultrathin, transparency, dual-side emission, and flexible characteristics based on graphene-insulator-semiconductor (GIS) structure is demonstrated. The GIS light-responsive white LED consists of highly luminescent histidine-doped two-dimensional molybdenum disulfide (MoS2) quantum disks, an Al2O3 insulating layer, and a graphene transparent electrode. By applying an external bias, it enables to generate white light emission with a high emission efficiency of around 9%. The underlying mechanism can be well understood by quantum tunneling. In addition, the GIS structure also shows good durability under a bending test of 200 cycles. Unlike conventional white LEDs, which always required multiple emissive layers, our GIS light-responsive LED is only based on a single emissive material of histidine-doped MoS2 quantum disks, which possess a broadband emission property and low toxicity. Therefore, with the superior multiple functionalities, our study shown here is very useful for the future development of next-generation optoelectronic devices.

Shape optimization of quantum-dot caps for high color-rendering white light-emitting diodes studied by optical simulation

Conventional white lightings based on light-emitting diodes (LEDs) lack deep red, which is responsible for their low color rendering index (CRI). To apply red quantum-dot (QD) caps is one way to improve the color-rendering property of white LEDs. Red QD caps were fabricated using CdSe/ZnS QDs in the acrylic resin matrix and applied to conventional white LED lighting, which clearly showed a significant increase in the CRI, especially R9 associated with the strong red. Optical simulation was carried out to find out the optimized structure of the QD caps. The CRI increased while the optical efficiency decreased as the color conversion efficiency increased, which suggested that there should be a compromise between the two properties. It was found that to form a non-absorbing optical cavity in the QD cap was an effective way to achieve high CRI values while maintaining a moderately good optical efficiency.

The effect of artificial solar spectrum on growth of cucumber and lettuce under controlled environment

Light-emitting diodes (LEDs) have been widely applied in the controlled environment agriculture, which are characterized by relatively narrow-band spectra and energetical efficiency. Most recently, the spectrum of Sunlike LEDs has been engineered and it closely resembles solar spectrum in the range of photosynthetic active radiation (PAR, 400–700 nm). To investigate how plant growth responses to the spectrum of Sunlike LEDs, cucumber and lettuce plants were cultivated and their responses were compared with the conventional white LEDs as well as composite of red and blue LEDs (RB, R/B ratio was 9:1). We observed that although Sunlike LEDs resulted in a longer stem in cucumber, dry weight and leaf area were similar as those under RB LEDs, and significantly higher than those under white LEDs. Moreover, cucumber leaves grown under Sunlike and white LEDs showed higher photosynthetic capacity than those grown under RB LEDs. For lettuce, plants grown under Sunlike LEDs showed larger leaf area and higher dry weight than the other two treatments. However, the leaf photosynthetic capacity of lettuce grown under Sunlike LEDs was the lowest. In this context, the spectrum induced plant functions are species-dependent. Furthermore, the three types of LEDs show distinct light spectra and they are different in many aspects. Therefore, it is difficult to attribute the different plant responses to certain specific light spectra. We conclude that plants grown under Sunlike LEDs exhibit larger leaf area, which may be due to some specific spectrum distributions (such as more far-red radiation), and consequently are favorable for light interception and therefore result in greater production.

Recent Advances in the Hardware of Visible Light Communication

Visible light communication (VLC) is an emerging technology that can transform lighting facilities into access points for wireless communication. Its development is bottlenecked by the cut-off frequencies (e.g., -6-dB electrical cut-off frequency f c and -3-dB electrical cut-off frequency f -3 dB ) of hardware, such as light-emitting diodes (LEDs), color converters, and photodetectors (PDs). In the past decade, especially in 2018, many materials and devices have been tested for properties related to VLC, such as f c , response time, and transmission data rates, which are listed and analyzed here to get an updated overall picture. The f -3 dB for the latest blue LED and color converter can reach up to 1485 and 470 MHz, respectively, which are great improvements from <;5 MHz for conventional white LEDs. Among the new PDs, the largest characterized f c is 4.2 MHz. Moreover, the smallest response time of these PDs is 0.95 ns, which is promising to have f c of hundreds of megahertz. The limitation in the field of view when the active area is reduced to enlarge f c of the PD can be partially overcome with novel fluorescent antenna, whose f c is more than 40 MHz. These increments in the f c or f -3 dB of hardware not only enlarge the channel capacity of the VLC but also facilitate the channel impulse response measurement, and the system becomes more susceptible to changes in the environment. These advances will shed light on the future development of the VLC with faster speed, more portable devices, and more applications for the Internet of Things.

Stable White Light‐Emitting Biocomposite Films

AbstractThe demonstration of reliable and stable white light‐emitting diodes (LEDs) is one of the main technological challenges of the LED industry. This is usually accomplished by incorporation of light‐emitting rare‐earth elements (REEs) compounds within an external polymeric coating of a blue LED allowing the generation of white light. However, due to both environmental and cost issues, the development of low‐cost REE‐free coatings, which exhibit competitive performance compared to conventional white LED is of great importance. In this work, the formation of an REE‐free white LED coating is demonstrated. This biocomposite material, composed of biological (crystalline nanocellulose and porcine gastric mucin) and organic (light‐emitting dyes) compounds, exhibits excellent optical and mechanical properties as well as resistance to heat, humidity, and UV radiation. The coating is further used to demonstrate a working white LED by incorporating it within a commercial blue LED.

Improving the modulation bandwidth of LED by CdSe/ZnS quantum dots for visible light communication.

Visible light communication (VLC) is an advanced and high-efficiency wireless communication technology. As one of the most important light sources in VLC, conventional white light emitting diode (WLED) based on Y3Al5O12:Ce3+ (YAG:Ce) phosphor limits the system transmitting rate severely due to its narrow modulation bandwidth. Considering the short fluorescent lifetime of quantum dots (QDs), QD-LEDs with wide modulation bandwidths were designed here to improve the transmitting rate of VLC. CdSe/ZnS core/shell QDs and related luminescent microspheres (LMS) were implemented as light conversion materials for the QD-LEDs. Compared with conventional phosphor WLED, the proposed QD-LED and QD-WLED reached maximum improvement on modulation bandwidth at 74.19% and 67.75% respectively. Furthermore, mathematical modeling of smearing was analyzed to establish the relationship between fluorescent lifetime and modulation bandwidth. Our findings will provide an effective solution of white LEDs for high speed VLC.

Effect of the Spectral Quality and Intensity of Light-emitting Diodes on Several Horticultural Crops

Light-emitting diode (LED) lamps signify one of the most important advances in artificial lighting for horticulture over the last few decades. The objective of this study was to compare the cultivation of four horticultural plants using a conventional white LED tube (T0) light against one with a good spectral fit to the maximum photosynthetic response (T1) at two intensities. The experiment was carried out with two types of young lettuce, tomato, and bell pepper plants. In a controlled environment chamber, six and four lamps per square meter were used to achieve high (H) and low (L) intensity, respectively. We measured the lighting parameters illuminance (lux) and photosynthetic photon flux (PPF) intensity (µmol·m−2·s−1). The dry and fresh weight, leaf area (LA), and specific index were measured to gauge plant growth. The photosynthetic activity and energy efficiency (EE) were recorded for each species over 60 days of cultivation. The results clearly demonstrate that, compared with conventional LED lamps, the specific horticultural LED lamps with an improved light spectrum increased the EE of the evaluated vegetables by 26%. At both the studied light intensities, plant growth was clearly more closely linked to the spectral fit of the light to the maximum photosynthetic response recorded by McCree (1972) than to PPF or illuminance (lux). We therefore suggest that a specific, detailed spectral distribution study be conducted to predict the effect of the specific quantity and quality of light used in this study on a single parameter of plant growth.

Glass‐Ceramics and Solid‐State Lighting

Yttrium aluminum garnet (YAG) doped with Ce3+ ion is known as an excellent phosphor for light‐emitting diode (LED), usually used as a powder form dispersed in organic resins. We have developed translucent glass‐ceramics (GC) of YAG: Ce3+ microcrystals in 2004. The GC sheet with half millimeter thick can work efficiently to make identical emission spectra with conventional white LED when combined with a blue LED. This report reviews the development history of the GC materials and impact for all inorganic solution for solid‐state lighting.

Visible Light Communications towards 5G

5G networks have to offer extremely high capacity for novel streaming applications. One of the most promising approaches is to embed large numbers of cooperating small cells into the macro-cell coverage area. Alternatively, optical wireless based technologies can be adopted as an alternative physical layer offering higher data rates. Visible light communications (VLC) is an emerging technology for future high capacity communication links (it has been accepted to 5GPP) in the visible range of the electromagnetic spectrum (~370–780 nm) utilizing light-emitting diodes (LEDs) simultaneously provide data transmission and room illumination. A major challenge in VLC is the LED modulation bandwidths, which are limited to a few MHz. However, myriad gigabit speed transmission links have already been demonstrated. Non line-of-sight (NLOS) optical wireless is resistant to blocking by people and obstacles and is capable of adapting its’ throughput according to the current channel state information. Concurrently, organic polymer LEDs (PLEDs) have become the focus of enormous attention for solid-state lighting applications due to their advantages over conventional white LEDs such as ultra-low costs, low heating temperature, mechanical flexibility and large photoactive areas when produced with wet processing methods. This paper discusses development of such VLC links with a view to implementing ubiquitous broadcasting networks featuring advanced modulation formats such as orthogonal frequency division multiplexing (OFDM) or carrier-less amplitude and phase modulation (CAP) in conjunction with equalization techniques. Finally, this paper will also summarize the results of the European project ICT COST IC1101 OPTICWISE (Optical Wireless Communications - An Emerging Technology) dealing VLC and OLEDs towards 5G networks.

이격 형광체 구조가 적용된 백색 LED 광원의 온도변화에 따른 발광 특성 분석

본 논문에서는 일반적인 코팅형 형광체와 이격 형광체 등 두 가지의 형광체 구조가 적용된 백색 LED를 제작한 후 온도변화에 따라 각 백색 LED의 발광 특성이 어떻게 변하는지 측정, 비교하였다. 상온에서 80oC 사이의 구간에서 측정된 두 백색 LED의 발광 스펙트럼을 분석하기 위해 Gaussian Lorentz-cross product 함수와 Asymmectic double sigmoidal 함수를 각각 청색 피크와 황색 피크의 곡선맞춤에 활용하였다. 이로부터 각 피크의 중심파장과 진폭, 반치폭 및 비대칭성을 온도의 함수로 구할 수 있었다. 온도에 따른 휘도와 색좌표를 측정한 결과 이격형광체 구조의 백색 LED의 온도에 따른 휘도저하율 및 색좌표 변화율이 훨씬 적었다. 이는 청색 LED 칩과 형광체가 분리됨에 따라 형광체의 온도가 일반 형광체 도포형 LED의 경우에 비해 낮아져서 효율이 상승했고 아울러 형광체에서 발생한 빛의 칩에 의한 흡수가 감소했기 때문인 것으로 해석된다. Two types of white light-emitting diodes (LEDs) with different phosphor structures were fabricated and compared in terms of their optical characteristics. Their spectroscopic properties were analyzed as a function of temperature, from room temperature to <TEX>$80^{\circ}C$</TEX>. The temperature dependence of the luminance and the color coordinates showed that the decrease in luminance and change in the color coordinates of the remote-phosphor type LED were much smaller compared to the conventional white LED. These improvements were attributed to the decrease in phosphor temperature, due to the distance between the LED chip and the phosphor layer, as well as to the reduced absorption by the LED chip of the light emitted from the phosphor layer.

Fabrication and optical characteristics of phosphor-free InGaN nanopyramid white light emitting diodes by nanospherical-lens photolithography

A novel nanopattern technique of nanospherical-lens photolithography is introduced to fabricate the InGaN nanopyramid white (NPW) light-emitting diodes (LEDs) by selective area growth. Highly ordered NPW LED arrays are achieved after optimizing the growth conditions. It is found that the NPW LEDs vary from warm white light to cool with the increase in growth temperature. For the cool white NPW LEDs, the spectrum is similar to the conventional white LEDs obtained from the blue LEDs combined with yellow phosphors. The blue emission originates from the upper sidewalls of nanopyramids, and yellow light is mainly emitted from the lower ridges with respect to the base of nanopyramids. Furthermore, simulation shows that the light extraction efficiency of NPW LEDs is about 4 times higher compared with conventional ones, and the escape cone is as much as 85° due to their three-dimensional nanopyramid structures. These observations suggest that the proposed phosphor-free NPW LEDs may have great potential for highly efficient white lighting.

Color Degradation of Textiles with Natural Dyes and of Blue Scale Standards Exposed to White LED Lamps: Evaluation of White LED Lamps for Effectiveness as Museum Lighting

White light-emitting diodes (LED) are well suited for museum lighting because they emit neither UV nor IR radiation, which damage artifacts. The color degradation of natural dyes and blue scale standards (JIS L 0841) by white LED lamps are examined, and the performance of white LED lamps for museum lighting is evaluated. Blue scale standard grades 1-6 and silk fabrics dyed with 22 types of natural dyes classified as mid to highly responsive in a CIE technical report (CIE157:2004) were exposed to five types of white LED lamps using different luminescence methods and color temperatures. Color changes were measured at each 15000 lx·hr (500 lx at fabric surface × 300 hr) interval ten times. The accumulated exposure totaled 150000 lx·hr. The data on conventional white LED lamps and previously reported white fluorescent (W) and museum fluorescent (NU) lamps was evaluated. All the white LED lamps showed lower fading rates compared with a W lamp on a blue scale grade 1. The fading rate of natural dyes in total was the same between an NU lamp (3000 K) and a white LED lamp (2869 K). However, yellow natural dyes showed higher fading rates with the white LED lamp. This tendency is due to the high power characteristic of the LED lamp around 400-500 nm, which possibly contributes to the photo-fading action on the dyes. The most faded yellow dyes were Ukon (Curcuma longa L.) and Kihada (Phellodendron amurense Rupr.), and these are frequently used in historic artifacts such as kimono, wood-block prints, and scrolls. From a conservation point of view, we need to continue research on white LED lamps for use in museum lighting.

Wide Color Gamut Backlight from Three-band White LED

A Wide Color Gamut Backlight system was studied using a three-band white Light-Emitting Diode. A three-band white light-emitting diode (LED) was fabricated using an InGaN-based blue LED chip that emits 445-nm blue peak, and a green phosphor and red phosphor that emit 535-nm green and 621-nm red peak emissions, respectively, when excited by 450-nm blue light. Using for this three-band white LED, wide color gamut backlight unit (BLU) was attained. The luminance of BLU and CIE 1931 chromaticity coordinates was <TEX>$1,700Cd/m^2$</TEX> and (0.337, 0.346). Color filter matching simulations for this configuration show that the three-band white LED backlight can be enhanced by up to 16% over conventional white LED backlight color gamut.

White Complementary Solid-State Lamp

A complementary source of white light composed of a conventional, white, phosphor-conversion light-emitting diode (LED) complemented with red-orange (625 nm) and cyan (505 nm) colored LEDs was modeled and optimized. In comparison with the conventional white LED, the optimal complementary device features a reduction of color temperature from 6725 K to 5400 K and improvement of the general color rendering index (CRI) from 73 to 90 with the minimal special CRI increased from −18 to 77. We demonstrated that the complementary source can be employed as a versatile white lamp with a trade-off between luminous efficiency and color rendering implemented by variation of contributions from the primary LEDs.

Theory of Illuminance Properties of Multi-point Light Sources with Individual Phosphor-coated White Light-emitting Diodes under Direct Illumination Using Radiation Intensity Distribution Function

We described a basic theory of multi-point light sources that comprise white light-emitting diode (LED) arrays for lighting applications. Evaluating the luminous intensity distribution is very important to understand the illuminance properties in the multi-point LED sources. We thoroughly studied each property of conventional white LEDs using blue LED excitation and a novel type of white LED using near ultraviolet (UV) LED excitation, making use of our theory. Individual quaternary multi-point light sources composed of orange, yellow, green and blue (OYGB) phosphors, combined with near UV LEDs were fabricated. A theoretical method using the radiation intensity distribution function was carried out to determine chromaticity and luminous properties of the light from these OYGB white LEDs under direct illumination. This shows that OYGB white LEDs have some characteristics that are superior to those of the conventional white LEDs.

Development of the White Light-Emitting Diode-Illuminated Digestive Endoscope

Aim: Videoendoscope system for digestive tract has been well developed and widely distributed throughout the world. In a conventional videoendoscope system, large light source equipment and light-guide fiber bundle are essential, which impose some limitations on the reduction of the size of the system and the design of the scope. Capsule endoscope is useful mainly for small intestine, but it is not suitable for routine esophago-gastro-duodenoscopy and colonoscopy. Recently, light-emitting diode (LED) has been rapidly developed as a new strong light source. So, we have started to develop the new LED-illuminated digestive endoscope system. Methods: We have applied newly developed white LED as the light source of prototype LED-illuminated digestive endoscope. The new ultra-violet LED-based one-tip type white LED has been developed using unique epitaxial growth techniques and advanced optical characterization techniques (developed by Professor Tsunemasa Taguchi, Department of Electricity and Electronics, Yamaguchi University Faculty of Engineering; and Mitsubishi Cable Industries, Ltd.). The new white LED has the highest external quantum efficiency (43%). It also has better color rendering properties at the red region than conventional white LED. The new white LED was located at the distal end of the prototype scope, which was based on the conventional videoendoscope system (Fuji Photo Optical, Co., Ltd.). We observed the esophagus and stomach of a beagle dog under painless state of general anesthesia by well-trained veterinarian (Yamamoto Animal Hospital). This study was conducted under the approval of institutional review board. Support for the study was received from Ministry of Education, Culture, Sports, Science and Technology, Japan. Results: Esophageal mucosa and gastric mucosa of beagle dog was clearly observed under illumination of new white LED. The intensity of luminous flux of new white LED was smaller than light-source of conventional system, but enough for observation. Color rendering properties at red region for actual esophagus and stomach were good, and reddish area and submucosal blood vessels were well recognized. Conclusions: Our results suggested that the new white LED is suitable for the light source of digestive endoscope from the point of color rendering properties and quantity of luminous flux. We will continue further development of this advanced prototype LED-illuminated digestive endoscope.