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Ultraviolet light-emitting diodes (UV LEDs) have been widely applied in various fields, such as sewage treatment, sterilization, and disinfection medical treatment. However, the remained problems of UV LEDs mainly concern poor photoelectric stability, low efficiency, safety protection, and among others. In order to evaluate and predict the stability and lifetime of UV LEDs safely and efficiently, a comprehensive UV LEDs two-step aging test system has been designed in this work. Two steps of different temperature stresses with total aging time up to 720 h are applied to UV LEDs. The junction temperature derived from spectroscopy has been taken under consideration during the test to evaluate lifetime accurately. A smartphone-based remote control and test scheme has been developed to monitor and control the online testing and therefore ensure the safety and convenience during the aging experiment. more...

This letter proposes a multichannel optical fiber spectral and imaging system, aiming at evaluating the pixel-level optical properties of high definition display efficiently and precisely. The system makes use of 16-channel optical fibers to form a detecting array so that 16 points of a display under test can be measured cyclically by multiplexer and spectrometer and monitored by camera simultaneously. The optical path is reversible for both the light source and the laser positioning, and adjustable along a rail driven by a motor to enable the system to work in either magnification mode or reduction mode. The feasibility and accuracy of the system are verified by comparative experiments of standard light source, LED display, and OLED display, identifying both merits of a single-spot spectroradiometer and an imaging photometer/colorimeter and even more functions like pitch size and color gamut. more...

We propose a non-contact method for determining phosphor/silicone temperature (phosphor temperature) in phosphor-coated light-emitting diode (pc-LED). The pc-LED with a surface mounted device (SMD) structure is primarily consisted of InGaN/GaN blue LED chip, (Sr,Ca)AlSiN3:Eu2+ red rare-earth phosphor, and transparent dimethyl silicone. This proposed method is basically based on a linear temperature dependence of divisional normalized emission power for phosphor/silicone mixture. Four types of pc-LED corresponding to four different phosphor concentrations are examined by proposed method. A comparison of measured results between micro-thermocouple device and proposed method has also been carried out, showing a fairly good agreement between them, except for pc-LED with 4.8 wt% phosphor concentration. We suggest that this proposed method can provide a useful tool for further study of optical and thermal characteristics in pc-LED. more...

In this letter, we propose an experimental method for determining two-dimensional (2D) junction temperature distribution of light-emitting diodes (LEDs) using microscopic hyperspectral-based centroid wavelength. The 2D centroid wavelength matrix is extracted from the hyperspectral data cube as temperature sensitive optical parameters (TSOP), which is linearly calibrated by short pulse voltage signal in order to eliminate the influence of self-heating effect. The 2D junction-temperature distributions of LEDs under working conditions are derived by TSOP matrix. The comparative experiments by micro-thermocouple ( $\mu $ -TC) and IR imaging reveal good coincidence in both averaged temperature and temperature distribution trends. more...

A majority of luminance measurement systems are currently focused on measuring the total luminance of light-emitting diodes (LEDs). It has largely limit its application on micro-LEDs, which require capturing the microscopic surface luminance. In order to study the microscopic characteristics of micro-LED displays, in this paper we propose a practical method that can accurately measure the luminance of a whole micro-LED array and even the surface luminance distribution of any single micro-led chip on the array. The essentials of this method are a high-precision complementary metal oxide semiconductor camera and a self-made data processing algorithm. Results show that the maximum error of the luminance measurement is less than 8%, which can prove the accuracy of the method. The metrical wavelength range and the accuracy of system can be dynamically set according to the camera and microscope. Endowed with ultrahigh dynamic ranges, this system proves its competency for evaluating the spatially resolved distribution of luminance for high brightness micro-LEDs. more...

With the emerging market for multichip module based high-power LED systems, the demand for advanced thermal management techniques has been steadily growing. This paper presents a model developed by the authors for thermal coupling matrix that can calculate temperature distribution at a given heat power of each chip. The model significantly simplifies the task of measuring thermal coupling effect on multichip light-emitting diodes with symmetrical structure, while simultaneously preserving temperature accuracy. The modeling technique was validated experimentally on three-, four-, and six-chip rectangular modules, and a ten-chip circular module, by infrared thermography and transient thermal measuring system, respectively. The modeling technique was found to be more efficient especially for large systems. The model offers a rapid way to predict thermal coupling behaviors of multichip LED systems. more...

We introduce a new method for measuring the junction temperature of alternating current light-emitting diodes. This method employs a periodic bipolar voltage pulse signal as the input, and utilizes the amplitude of corresponding output current as the temperature sensitive parameter (TSP), of which a linear temperature dependence is proven in a prior experiment in this paper. On the basis of the TSP, we devise a detailed procedure—first, measure the thermal resistance of the package of ac-LEDs, which further contributes to the calculation of junction temperatures under various power inputs. Comparisons between values calculated by using this method and those obtained directly from a thermocouple indicate a decent accuracy of the former. The advantage of this method benefits from the stability of thermal resistance under different driving conditions that one could immediately obtain the junction temperature by measuring the electric and optical powers. more...

Relative reflected intensity of the incident exciting light is proposed to measure the junction temperature of light-emitting diodes (LEDs) under test. Reflectance spectra at a wide junction temperature range are acquired. Multichannel optical fibers greatly increase the collecting efficiency of the reflected light. Lock-in technique is utilized to exclude the interference of the emitting light from LEDs under test and to increase the dynamic range greatly. The results are in good agreement with those directly tested by a microthermocouple. To avoid extra carrier absorption and modulation effect, the incident exciting light should harbor smaller bandgap than that of LEDs under test. more...

We propose an experimental method that determines junction temperatures in light-emitting diodes by measuring currents while holding the low forward voltages constant. In this procedure, we first calibrate current-temperature-rela- tionship parameters under the condition of negligible thermal generation. With one of the two parametric values, we discover the existence of a forward voltage peak that yields most sensitive measurements of the junction temperature. Results show a nearly linear relationship between the algorithmic currents and temperature reciprocals with high testing precision. more...

We investigated the comparative structural and optical properties of semipolar InGaN/GaN multiple quantum wells (MQWs) grown on the (1101) facet GaN/sapphire substrate by metal-organic chemical vapor deposition using lateral epitaxial overgrowth. The scanning electron microscopy (SEM), photoluminescence (PL), and temperature-varying time-resolved photoluminescence measurement were performed to investigate the structure and optical properties. The cross-sectional SEM image shows that the stripe triangular structure of the QW with semipolar (1101) planes is obtained as sidewall facets with the mask stripes aligned along the GaN α-axis. The structural and optical advantages of semipolar orientations were confirmed by a modurate shift of the PL peak energy, higher internal quantum efficiency, and lower radiative recombination lifetime than the MQWs on (0001) GaN grown by conventional methods. The results were obtained because of the reduced polarization fields in semipolar InGaN/GaN MQWs comparing with that in polar (0001) MQWs. more...

A method is presented in this study to determine the junction temperature (Tj) of LED in terms of the relationship between the diode reverse current (IR) and Tj . A theoretical model for the dependence of IR on Tj is derived on the basis of the Shockley equation and is validated by our experimental results. The method is compared with the conventional forward voltage method, and its advantages have been identified. more...

This paper reports the measurement of the junction temperature and the determination of the thermal resistance of the single-crystal-silicon solar cell under the dark and illuminating conditions, respectively. Under the dark condition, the solar cell is considered as a conventional p-n junction and is subject to a reverse current in order to measure its junction temperature and determine the thermal resistance. A white LED array is used as the light source to operate the solar cell in order to avoid the heating effect of the infrared light by the solar simulator. Furthermore, we thoroughly calculate the thermal dissipation power. The result demonstrates that the thermal resistance drops from 3.7 to 2.0 K/W with the increase of the irradiance from 89.6 to 194.3 $\hbox{W/m}^{2}$ . It is also found that the thermal resistance under the dark condition is much higher than that under the illuminating condition, which is attributed to the light effect on the thermal resistance. more...

Three- and four-hump InGaN-based white light-emitting diodes (LEDs) have been computationally investigated. The investigation includes three-hump LEDs precoated with green- and red-emitting quantum dots (QDs) and four-hump LEDs precoated with green-, yellow-, and red-emitting QDs. Results show scotopic/photopic (S/P) ratios $>$ 3.80 and color rendering indices (CRIs) $\geq$ 70 for three-hump LEDs and S/P ratios $>$ 3.90 and CRIs $\geq$ 70 for four-hump LEDs under a correlated color temperature (CCT) of 45 000 K. Furthermore, for both three- and four-hump LEDs under the condition of CCT $>$ 5000 K, optimal results in this paper exhibit capabilities of exceeding $\hbox{S/P} = 2.50$ and simultaneously satisfying the criterion of CRI $\geq$ 70. Our simulation procedure is conducted under variations of nine CCTs, several wavelengths, and peak heights. Optimized results of S/P ratios and CRIs are identified and retained through filtering off a great number of unqualified candidates. Finally, present findings may help improve the quality of eye visions pertaining to LED illumination and help reduce the electrical energy consumption related to LED usage. more...

Three-hump InGaN-based white light-emitting diodes (LEDs) precoated with traditional yellow/green phosphors and red-emitting quantum dots (QDs), have been numerically investigated. Under variations of eight correlated color temperatures (CCTs), three wavelengths, two bandwidths, and two peak heights, optimal results of luminous efficacy radiation (LER) and color rendering index (CRI) are identified and retained through filtering off billions of unqualified candidates. These results include LER = 390 lm/W and CRI = 90 [chromaticity difference (Duv)Duv on LED spectral parameters affected by operating temperatures. Finally, we have discovered that higher instabilities may be induced for cool white LEDs (CCT = 6500 K) than for warm white LEDs (CCT = 3000 K) within the analysis of CCT versus spectral parameters. more...

InGaN/GaN light-emitting diodes (LEDs) with a graded-composition multiple quantum well (GQW) were designed not only to investigate the effect of polarization field on the efficiency droop in InGaN/GaN multiple quantum wells (MQWs) but to find out possible solutions to prevent or reduce the efficiency droop in GaN-based LEDs as well. Pulsed electroluminescence measurement, to avoid heating effects partly, revealed that the output intensity peak occurs at ~280A/cm2 current density for the GQW structure, whereas for a conventional structure, it occurs at a much lower current density of ~150A/cm2. The relative quantum efficiency indicates that the efficiency droop in GQW LEDs was reduced effectively due to the reduction in polarization field in the active layer. In the test GQW structure with inverse indium composition variation direction, electrons escape from the active region, which is in company with rapid saturation of output power and decrease in efficiency observed at high current density. It indicates that at high current density, the electron leakage is obvious. These results suggest that the polarization field in the active layer and the consequent electron leakage are probably the main mechanism responsible for the efficiency droop at high current injection levels. more...