Yu-Yu Chang

Precise Simulation of Spectrum for Green Emitting Phosphors Pumped by a Blue LED Die

This paper proposes an effective approach for spectrum shaping in the optical modeling of phosphor-converted white light-emitting diodes (LEDs) in which overlapping of the absorption and emission spectra of the phosphor are considered. The spectrum shaping method was applied to explore the wavelength-dependent absorption effect and reabsorption by the green phosphor. The experimental results indicated that the adjustment factor for blue light can enable the blue light spectrum to fit well with the measured spectrum. The adjustment factor was linearly related to the transmission ratio of the blue light. The blue light emitted by the blue die and the green light emitted by the phosphor were simulated and predicted in an accurate way as compared with the experimental measurements. The enhanced accuracy in assessing the spectra resulted in enhanced precision of chromaticities. In the experiments, the color differences were one order smaller in CIE 1931 chromaticity (Δx, Δy) than in models without spectrum shaping and were almost imperceptible to the human eye. The novel optical modeling of green phosphor pumped by a blue die facilitates the application of these materials in high-color rendering in white LEDs and projection displays.

Study of Temperature Distribution Within pc-WLEDs Using the Remote-Dome Phosphor Package

Both theoretical and simulation approaches were used to obtain the thermal power and temperature distribution in phosphor-converted white LEDs (pc-WLEDs) using a remote-dome phosphor package (RDPP). Different material and geometric parameters were systematically and thoroughly studied. An experiment was performed to measure the temperature distribution in an RDPP pc-WLED, which confirmed the simulation results. Given practical limitations of the material parameters, the most feasible method to reduce the phosphor temperature is to extend the dome radius and the thickness of the phosphor region.

Diode-pumped, actively internal-Q-switched Nd:MgO:PPLN laser.

We demonstrated a diode-pumped, electro-optically internal-Q-switched laser system fabricated using a Nd:MgO:PPLN. We obtained laser pulses of pulse energy > 2.45 muJ and pulse width ~28 ns from this internal-Q-switched laser system with 2% output coupling.

Precise Spatial-Color Optical Modeling in Phosphor-Converted White LEDs

White light LEDs are playing a more and more important role in display in various structures. Here, a new modeling algorithm is proposed for use in phosphor-converted white light-emitting diodes (pcW-LEDs); the algorithm can be used to accurately simulate color appearances, potentially enabling optical designers to remove yellowish/bluish spots in LED lighting. The proposed modeling method was applied to simulate a pcW-LED without an encapsulation lens, and subsequently, a pcW-LED with a hemispherical lens. Both simulations accurately predicted the blue and yellow light distribution. The model was further verified by applying a total internal reflector lens to the pcW-LED. In the midfield region, the blue and yellow light distribution exhibited large variations as the observation distance changed; this varying light pattern for both the blue and yellow lights can be accurately predicted using the proposed model. The developed optical model should facilitate designing a pcW-LED that features high-quality illumination and increased color uniformity.

Stabilizing CCT in pcW-LEDs by self-compensation between excitation efficiency and conversion efficiency of phosphors

A novel method to stabilize the correlated color temperature in pcW-LEDs from their initial turn-on state to thermal equilibrium is proposed and demonstrated. Under the normal operation condition, it can stabilize the CCT of a pcW-LED by the positive matching of the blue LED peak wavelength to the phosphor excitation spectrum. When the operating temperature unavoidably becomes higher in the LED die quickly after the initial turn-on, the phosphor conversion efficiency degrades and the LED blue light performs red shift. With the positive matching, the red shift actually helps enhance the excitation efficiency of the phosphor to compensate the thermal quenching and efficiency degradation. Therefore, the ratio of the blue light to the yellow light can keep almost constant, as well as the CCTs. In the experiments, the CCT variation could be as small as from 7 K to 83 K in different cases. Finally, we introduce a new factor, the so-called guide number, which is used to count the total change of the enhancement in equivalent excitation efficiency and the relative reduction of the phosphor light emission. The guide number essentially helps in designing the matching blue LED die and phosphor pair for good CCT stabilization.

The optical properties of phosphor converted white LED with adding Zirconium dioxide particles (Conference Presentation)

We performed the simulation and experiment to investigate the influence of Zirconium dioxide (Zirconia, ZrO2) particles on the optical properties of phosphor converted white LED (pcW-LED). An efficient optical model was developed and applied to the incorporation diffuse particle of ZrO2 into a hemisphere package containing YAG phosphors. The optical properties (chromaticity, packaging efficiency) were estimated as a function of phosphor and ZrO2 particles, through the calculation of effective radius, refractive index, and absorption and conversion efficiency, in a range of correlated color temperature 4500 K to 6500 K. In the same way, the amount of phosphor and ZrO2 can be calculated accurately to obtain a targeted optical property in a hemisphere LED design. Especially, the angular distribution of CCT was also diminished, and even almost inexistent for low CCT design. In addition, the adding of ZrO2 particles allows clearly decreasing the amount of phosphor for an identical target CCT. It is really suitable in the context of decreasing the amount of phosphor or in some applications where the color uniformity is an important parameter, like indoor down-lighting.

Phosphor chessboard packaging for white LEDs in high efficiency and high color performance

We performed the simulation of white LEDs packaging with different chessboard structures of white light converting phosphor layer covered on GaN die chip. Three different types of chessboard structures are called type 1, type 2 and type 3, respectively. The result of investigation according to the phosphor thickness show the increasing of thickness of phosphor layer are, the decreasing of output blue light power are. Meanwhile, the changes of yellow light are neglect. Type 3 shows highest packaging efficiency of 74.3 % compares with packaging efficiency of type 2 and type 1 (72.5 % and 71.3 %, respectively). Type 3 also shows the most effect of forward light. Attention that the type 3 chessboard structure gets packaging efficiency of 74.3 % at color temperature of daylight as well as high saving of phosphor amount. The color temperatures of three types of chessboard structure are higher than 5000 K, so they are suitable for lighting purpose. The angular correlate color temperature deviation (ACCTD) of type 1, type 2 and type 3 are 6500K, 11500K and 17000K, respectively.

A study of optical modeling and evaluation of color rendering property of a dual-phosphor system(Conference Presentation)

In this thesis, on the basis of the phosphor optical models, green and red phosphor mixture optical model has been well established. Under some specific green to red phosphor doping proportions, this model can be utilized to simulate the chromatic properties, spatial CCT distributions, and packaging efficiency. There are some benefits of applying the phosphor optical model, one is that the confusion about mixture or layer phosphor configuration can perform better could be solved. Another is that the comparison and analysis of these phosphor configurations can be made not only in experiment but also in simulation, and will be more details to be discuss in the simulation. There are several types of packaging structures in high color quality applications. Consequently, the importance of phosphor optical model cannot be overestimated. After few steps above and with the help of experimental analysis and optimized in simulation, a packaging structure with high color quality and high efficiency has been approved. Finally, this light source with high performance will be utilized in the luminaire to improve the color and energy saving properties.

Optical modeling with precise spatial-chromatic light distribution in phosphor-converted white LEDs

White light LEDs become more and more important in display and lighting in various structures. Here a new modeling algorithm for phosphor-converted white light-emitting diodes (pcW-LEDs) is proposed, aiming to perform accurate simulation for color appearance, where potentially enabling optical designers to remove yellowish/bluish spots in LED lighting. The proposed modeling method is applied to simulate a pcW-LED with a hemi spherical lens. The simulation accurately predicts the blue and yellow light distribution. The model is further verified by applying a total internal reflector lens to the pcW-LED. In the midfield region, the blue and yellow light distribution exhibit large variations as the observation distance changed; this varying light pattern for both the blue and yellow lights can be accurately predicted by using the proposed model. The well-established optical model should facilitate designing a pcW-LED that features high-quality illumination and enhances color uniformity.

Optical design of dental light with high performance and low power based on white LEDs.

This paper presents an optical design of a dental light that meets the regulation of ISO 9680. The designed light pattern on the target is an elliptical shape with uniform illumination. Moreover, the real module contains four optical modules, and is operated at 9 W and performs a maximum illuminance of 42,010 lx. In order to reduce the correlated color temperature (CCT) variation, we replace the original white light-emitting diode with a new one, which has an extremely low angular CCT derivation. Accordingly, the CCT variation is reduced to 232 K from 1323 K of the original module.