Wavelength LED Research Articles

1λ Ronchi tester to obtain the wave front aberration in converging optical systems using phase shifting interferometry

We implement a device to verify in a quantitative way the quality of any converging optical system by making an improvement to the source illumination of the Ronchi test and by implementing the phase shifting interferometry technique (PSI). We describe the design, implementation and verification of this specific device which has the capability to obtain the transversal aberration in two perpendicular directions. Illumination source was a set of LEDs powered with constant current to eliminate variations in the irradiance collected by the camera (CCD), and varying the voltage to increase the intensity of each different wavelength LED we test surfaces with different roughness. The PSI technique was developed by giving movements to the Ronchi ruling with step motors. The device has the capability to test any converging optical systems of either small or large f numbers, and working distances until 5000 mm. Aberrated wavefront with a precision of 1λ was computed when the light source is off axis for any rotationally symmetric mirror.

LED wavelength determination sensor system with BiCMOS triple junction photodetector

A sensor system for the wavelength determination of LEDs is presented. The used sensor is a triple junction photodiode which needs no external filters. The sensor is implemented in a standard BiCMOS process without any process modifications. The sensor system is optimised for low analogue complexity and compatibility with mass production in a cost-sensitive environment. A proof of concept sensor system is presented which demonstrates the ability to determine the wavelength of LEDs. The measurements evaluate the linearity of the system and the possibility for a fast calibration in the production environment.

Visibility measurement using multi-angle forward scattering by liquid droplets

A multi-angle forward scattering method is presented, which can be used to detect the extinction coefficient of liquid droplets in atmosphere visibility measurement. The wavelength of LED in the transmitter is 850 nm, and the forward scattering signals are collected by three receivers which are arranged at angles of 20°, 35° and 50°, respectively. Atmosphere visibility is measured by the receiver which is arranged at 35° angle. It can determine the type of weather by the ratio between scattering phase function P(20°) and P(50°), and also find the different ranges of these ratios in different weather. Compared to other visibility meters, the meter proposed not only measures the visibility accurately, but also distinguishes weather types such as fog, haze and mist.

Luminescence and energy transfer of Eu3+ or/and Dy3+ co-doped in Sr3AlO4F phosphors with NUV excitation for WLEDs

Eu3+/Dy3+ single doped and codoped Sr3AlO4F phosphors were prepared in a reducing atmosphere by the conventional solid-state reaction method. The XRD was used to confirm the crystal structure of the phosphors and their photoluminescence (PL) properties were studied by the excitation and emission spectra. These phosphors have intense absorption in NUV region, which is suitable for the emission wavelength of NUV LED. The dependence of luminescence intensity on doping concentration is investigated with respect to the energy transfer process in between Eu3+ and Dy3+ ions. The CIE color coordinates of the prepared phosphors were calculated. The obtained results have been rationalized in order to provide useful information for the development of a suitable material for the NUV based white-light emitting phosphors technology.

GaN계 청색 발광 다이오드에서 저전류 스트레스 후의 광 및 전기적 특성 변화

We analyzed the changes in electrical and optical characteristics of 1 multiple-quantum-well (MQW) blue LEDs grown on a c-plane sapphire substrate after a stress test. Experiments were performed by injecting 50 mA current for 200 hours to TO-CAN packaged sample chips. We selected the value of injection current for stress through the junction-temperature measurement by using the forward-voltage characteristics of a diode to maintain a sufficiently low junction temperature during the test. The junction temperature at the selected injection current of 50 mA was 308 K. Experiments were performed under the assumption that the average junction temperature of 308 K did not affect the characteristics of the ohmic contact and the GaN-based materials. Before and after the stress test, we measured and analyzed current-voltage, light-current, light distribution on the LED surface, wavelength spectrum and relative external quantum efficiency (EQE). After the stress test, it was observed experimentally that the optical power and the relative EQE decreased. We theoretically investigated and experimentally proved that these phenomena are due to the increased nonradiative recombination rate caused by the increased defect density.

Experimental Investigation of an Integrated Optical Interface for Power MOSFET Drivers

To solve the galvanic isolation challenges in drivers related to gate signal transfer to power transistors, an optical detector was monolithically integrated within a 600 V vertical power transistor without any modifications in the fabrication process. After fabricating an initial prototype, preliminary static and dynamic characterization results have been investigated. The fabricated devices showed responsivities of 0.046 A/W at 0 V bias and 0.15 A/W at 15 V reverse bias and a bandwidth of at least 800 kHz when triggered with a 525 nm wavelength LED at an optical power in the microwatt range.

黄色LED点滅光によるオオタバコガの飛翔抑制

We aimed to clarify the pulse conditions of a yellow LED that can be pulsed at the high speed required to control noctuid moths without suppressing the blooming of chrysanthemum. We used a lemon-yellow LED (central wavelength: 571 nm) of similar wavelength to yellow fluorescent light, with an intensity of 20 mW/m2 and pulsed-LED light period of 20 ms. We measured the flight activity of adult Helicoverpa armigera (Hübner) under various light to dark period ratios of the LED pulse: 20 ms : 0 ms (continuous lighting), 20 ms : 40 ms, 20 ms : 80 ms, 20 ms : 160 ms, 20 ms : 400 ms and these were compared with dark conditions 0 ms : 0 ms without illumination, using an actograph system. As a result, Helicoverpa armigera became accustomed to the LED lighting conditions over time and the number of flight activities gradually increased in unmated females in 20 : 0, 20 : 160 and 20 : 400 groups and in unmated males in 20 : 0 and 20 : 400 groups over one day. However, in the 20 : 40 and 20 : 80 groups, Helicoverpa armigera adults, either unmated females or males, did not become accustomed, but flight activity was clearly controlled over five continuous days. It was found that the flight of adult moths was strongly suppressed under the conditions of 20 : 40 ms and 20 : 80 ms pulsed-LED light.

Strong enhancement of 420∼425nm optical wavelength of blue LED by using CaF2 scintillating crystals as a cap

ABSTRACTIt has been found that the blue light emission of LED is remarkably enhanced by using optical window of CaF2 disk doped with Eu2O3. The CaF2 crystals doped with Eu2O3 strongly absorb UV light with 330~420nm optical wavelength as a cutting filter. Instead, CaF2 crystals emit 420~425nm wavelength visible light. As a result, the UV light shifts to a light of around 420nm wavelength, and the LED light emission intensity markedly increases. In our experiments, the UV components were absorbed, while, around 420nm emitting light has been strengthened by more than 30%. The FWHM value is improved by around 30%.

OPL volume 1424 Cover and Front matter

Recently, GaN has attracted much attention for short wavelength LEDs, LDs and future opto-electronic integrated devices. In our study, Boronmonophosphide(BP) grown on Si(100) by MOCVD is used for growing cubic-GaN (c-GaN). The epitaxial growth of GaN/BP/Si has been carried out. We observed the crystal defects by scanning electron microscope(SEM), transmission electron microscopy(TEM), and surface X-ray diffraction(XRD). The dislocation density in the BP layer markedly decreased with increasing the thickness. However the difference of the lattice constant between BP and Si leads to high dislocation densities in GaN layer. Therefore, the epitaxial growth of Si doped BP on Si has been carried out, in order to observe the effect of Si doping on BP crystalline quality. We observed the crystal defects by XRD and cross-sectional TEM. The dislocation of interfaces will be discussed.

Research Progress on Packaging Thermal Management Techniques of High Power LED

In order to dominate the lighting market, LED needs more electrical power to be driven for higher brightness, thereby increasing thermal power dissipation, which contributes to a high heat flux of 85W/cm2 within a recent typical high power LED chip. And the junction temperature has direct influence upon the light output efficiency, device life time, emitting wavelength and reliability of LED. Therefore, effective removal of heat to maintain a safe junction temperature is the key to meet the future flux per LED requirements. Compared with other individual thermal resistances along the thermal path, thermal design for much lower packaging thermal resistance is more critical to improve the performances of LED. In this paper, major present technical researches on packaging thermal management were analyzed for high power LED, and the advantages and shortcomings of these techniques were respectively summarized. Besides, some suggestions were provided for further research in this area.

The Role of the Schottky Metal in Optical Responses of Optical Position Sensitive Detector

The role of the Schottky barrier metal in position sensitive detector (PSD) formation is more complex than just acting to form a pseudo p-n junction and, in fact, it appears that it is the optical properties of this material that play a role in determining device performance. In this work titanium-silicon Schottky barrier PSDs were tested under 4 different wavelength LEDs and it was found that device performance was much better under long wavelength light. Along with the usual mechanisms, it is conjectured that this is related to the refractive index of the Schottky metal. PSDs with high sensitivities and strongly linear characteristics are much desired and many very high sensitivities have been reported upon by others, however, it is also important to consider the size of the linear region. The highest output measured here in response to 1 mW of LED light was 26 mV/mm over 9 mm surface path length. Shorter wavelength light has resulted in sensitivities which are significantly lower than the sensitivities obtained from longer wavelength light, but with higher linearities. The higher sensitivities are considered to be due to the absorption characteristics of the Schottky metal and this is attributed to recombination effects and differing transport paths that carriers take when illuminated by different wavelength light.

Pulsed multi-wavelength excitation using fiber-in-capillary light emitting diode induced fluorescence detection in capillary electrophoresis

A novel instrument was developed using a multi-wavelength pulsed LED array with in-column optic-fiber induced fluorescence detection by capillary electrophoresis. The light from 2 different wavelength LEDs (450 nm and 480 nm) was pulsed for short intervals at high intensity. The beam from each LED was collimated and reshaped with the gradient index (GRIN) lens group to achieve a highly effective coupling between LED light source and an optical fiber. The optical fiber was placed inside the capillary for in-capillary LED-induced fluorescence detection. The advantages of this system were validated by the simultaneous determination of vitamin B2 and fluorescein. Detection limits for vitamin B2 and fluorescein were estimated to be 5 nM and 0.29 nM ( S/ N = 3), respectively. The relative standard deviations (RSDs, n = 6) of the both compounds for migration time and peak area were better than 0.83%, 2.20% and 1.21%, 2.75%, respectively. The method was applied to the determination of vitamin B2 in commercial tablets and fluorescein in fluorescein sodium injection and the recoveries obtained were in the range of 96.6–102.0% and 99.9–102.8%, respectively. It was also applied to human serum, where the recoveries were found to be in the range of 94.4–97.0% and 92.6–96.4%, respectively. The system has been successfully applied in separation and determination of the both biological samples with acceptable analytical performance.

Varying ratios of wavelengths in dual wavelength LED photomodulation alters gene expression profiles in human skin fibroblasts

LED photomodulation has been shown to profoundly influence cellular behavior. A variety of parameters with LED photomodulation can alter cellular response in vitro. The effects of one visible and one infrared wavelength were evaluated to determine the optimal ratio to produce a net increase in dermal collagen by altering the ratio of total energy output of each wavelength. The ratio between the two wavelengths (590 and 870 nm) was shifted in 25% increments. Human skin fibroblasts in culture were exposed to a 590/870 nm LED array with total combined energy density fixed at 4.0 mW/cm.. The ratio of 590/870 nm tested parameters were: 100/0%, 75/25%, 50/50%, 25/75%, and 0/100%. These ratios were delivered using pulsed duty cycle of exposure (250 milliseconds "on" time/100 milliseconds "off" time/100 pulses) for a total energy fluence of 0.1 J/cm.. Gene expression was examined using commercially available extra cellular matrix and adhesion molecule RT PCR Arrays (SA Biosciences, Frederick, MD) at 24 hours post-exposure. Different expression profiles were noticed for each of the ratios studied. Overall, there was an average (in an 80 gene array) of 6% expression difference in up or downregulation between the arrays. The greatest increase in collagen I and decrease in collagenase (MMP-1) was observed with 75/25% ratio of 590/870 nm. The addition of increasing proportions of IR wavelengths causes alteration in gene expression profile. The ratios of the wavelengths caused variation in magnitude of expression. Cell metabolism and gene expression can be altered by simultaneous exposure to multiple wavelengths of low energy light. Varying the ratios of specific wavelength intensity in both visible and near infrared light therapy can strongly influence resulting fibroblast gene expression patterns.

The effect of coloured light on Ipomoea purpurea growth

Ipomoea purpurea is a climbing ornamental plant native to Mexico. The paper is describing the experimental setup and results for indoor growing plants exposed to white LED light (inside a reference chamber) and four different wavelength LED lights (inside a measure chamber). Four growing experiments of 12–15 days, took place in identical environmental conditions (identical temperature and relative humidity inside the reference and measure chambers, similar lighting conditions and soil moisture). At the end of the experiments, the plant chlorophyll and xanthophylls content have been measured and the plant aspect (vegetal mass, leaves colour and robustness) has been observed. The smallest content in chlorophyll (a and b) was developed by the plants growth in blue light (480 nm), however those plants where 10% taller than plants growth in white light, but less robust. The higher content in carotenoids and xanthophylls was observed in plants which growth in white and red light.

Phosphor-Free Monolithic White-Light LED

Phosphor-free monolithic InGaN-based white-light LED has the advantages of simpler device process and potentially higher efficiency. Several techniques have been developed for implementing such white-light LEDs. Among them, the key issue is the growth of a high-quality high-indium InGaN/GaN quantum well (QW). An underlying InGaN layer growth technique is introduced for enhancing the crystal quality of a high-indium QW. To demonstrate the superior properties of a QW grown with this technique, a green LED is fabricated based on the underlying growth technique to compare with another LED of the same emission wavelength based on the conventional growth method. Then, the underlying growth technique is used to grow three yellow-emitting QWs of high efficiency. The yellow photons mix with blue light from an overgrown blue-emitting QW to produce white light. The improved properties of the phosphor-free monolithic white-light LED are discussed in detail.

Triple wavelength LED driver for optical brain–computer interfaces

A dedicated triple wavelength LED driver is presented for optical brain–computer interfacing (BCI). The solution caters for the constraints of a common-anode grounded case and modulation up to several kilohertz that allows source separation of light that has backscattered from the brain. With total harmonic distortion of 0.95% and a frequency range of ~40 kHz, the driver has application in a continuous wave optical BCI. Other modulation strategies such as time division multiplexing (TDM) are catered for, owing to input DC coupling. Linearity in the optical output is maintained by the ‘load sensing’ differential op-amp on the LED’s current limiting resistor, which is the basis for the V-I conversion.

Device design and simulation for GaN based dual wavelength LEDs

A comprehensive optical model for dual wavelength LEDs is developed using optical ray tracing programs. Optical dispersion of GaN, InGaN, and AlGaN is also included in this numerical model. The light extraction efficiency of LEDs can be calculated based on LED structure and material properties. Moreover, a LED device structure can be optimized to improve the light extraction efficiency.

Cost-Effective Fabrication of Self-Made 1x12 Polymer Optical Fiber-Based Optical Splitters for Automotive Application

Problem statement: Multimode fiber cables can conduct many light rays and can operate free of disruption and with a greater bandwidth than a wireless connection. However, due to a slight variation in the speed of the light rays through the multimode fiber, a signal transmitted by all of these rays becomes spread out. Consequently, the signals become broader and therefore fewer signals fit in the fiber, limiting the transmission capacity. These demands grow almost daily. Hence new ways of splitting methods must be found to satisfy all application demands, especially related to automotive application. Approach: Home-made 1×12 optical splitter based on polymer optical fiber material base is one of the most innovative technologies on optical component which can be applied on some useful application. A perform technique had been used to fabricate kind of splitter. In order to develop such an efficient optical device which can be integrated into the body of automotive field, research with a good specification-oriented study tent to be conduct and it must be passed through a well-planned fabrication technique together with a proper characterization process. Multimode Step-Index Polymer Optical Fiber (SI-POF) type made of polymethyl methacrylate (PMMA) with Ocore = 1 mm and NA: 0.50 fully utilized in this research, as PMMA is one of the most commonly used optical materials. This material has been chosen as a base of splitter body in a fused-taper-twisted shape, produced by a unique fabrication stages. PMMA-POF can easily be used near it operating temperature between -40°C to +115°C. Results: By injecting 650 nm wavelength of red LED, characterization process start plays an important role in investigating level of efficiency of the device. Some parameters, such as optical output power and power losses on the devices were observed. Although the maximum output power efficiency of the splitter is about 40% but it can be improved gradually through experience and practice. Conclusion: The fabrication process is simple, easy and suitable to be used by household. Moreover, the users can determine the size of the fabricated device themselves. The POF-based optical 1×12 splitter had been suggested to be applied into automobile application to overcome blind spot area tracking problem as a one of low-cost solution in the future and also in home networking to avoid the bottleneck occurs between ONU and electronic appliances, resulting increase the speedy of data communication.

UV Ratiometric Imaging Of Isolated Ventricular Cardiomyocytes Using An LED Based Illuminator

Ratiometric fluorescence microscopy methods allow researchers to obtain calibrated images of dynamic changes in the physical properties of cells and tissues independent of dye concentration.The wavelength changes required for ratiometric imaging are routinely achieved using a short-arc source in combination either with a diffraction grating or interference filters mounted in a filter wheel. Filter wheels typically switch positions in around 50ms, scanning monochromators can achieve wavelength changes within a ms. These approaches generally limit ratio imaging rates to a little over 10Hz, and have the inherent drawbacks of short lamp life and high thermal emissions. An LED based system has much higher stability than a short-arc source, and with sub-microsecond wavelength switching times allows the very highest speeds to be obtained. Here we made use of the popular Ca2+ probe Fura-2 to record images of intracellular [Ca2+] in isolated ventricular cardiomyocytes at frame rates in excess of 100Hz using a simple and inexpensive LED based system.Transient spatial gradients of Ca2+ can exist in single ventricular cardiomyocytes during spontaneous release of Ca2+ from the sarcoplasmic reticulum. This can result intracellular Ca2+ waves traveling at 100-200microns/s along the length of the cell. Rapid imaging is required to resolve the time course and pattern of intracellular Ca2+ release.Conventionally Fura-2 ratios are calculated by monitoring the fluorescence signal elicited from excitation at 340 and 380nm, however short wavelength LEDs (340nm) are not available currently. Alternatively, reliable ratiometric measurements can be made by exciting Fura-2 at its isobestic point (360nm) and 380nm. We have followed fast spatial changes in Ca2+ by switching wavelength in the microsecond time domain using commercial LEDs emitting at peak wavelengths of 365nm and 385nm.

Well width dependent luminescence characteristics of UV‐violet emitting GaInN QW LED structures

AbstractTemperature and excitation power dependent photoluminescence spectroscopy on GaInN UV‐violet LED structures allows to separate the influence of the quantum confined stark effect (QCSE) from carrier localization in In‐induced band tail states as well as to determine the internal quantum efficiency (IQE). A LED series with different quantum well widths (1‐3.7 nm) was grown by MOVPE on sapphire and low defect density (LDD) GaN templates. The photoluminescence data reveal, that the QCSE cannot be neglected even for low In‐content (<10%) ultra violet LED structures. From temperature and laser power density dependent photoluminescence, the internal QE is evaluated. The data shows, that upon growth on LDD substrates, the internal QE is considerably increased for short wavelength (375‐400 nm) LEDs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)