Shang-Ping Ying

Investigation of remote-phosphor white light-emitting diodes with improved scattered photon extraction structure

In this study, an improved scattered photon extraction (SPE) package structure with a reflection plate over the remote phosphor layer is presented. The important factors for the remote phosphor layer in the conventional and improved SPE package structures, including the phosphor layer thickness and phosphor concentration, are also studied. The simulation results show that the yellow-blue ratio (YBR) of the conventional and improved SPE package structure increases with the phosphor concentration when the phosphor layer is thin and that the YBR is saturated when the layer is thick. The experimental results show that for both conventional and improved SPE package structure, the correlated color temperature (CCT) decreases as the phosphor concentration increases. However, the YBR of the improved SPE package structure is higher than that of the conventional SPE package structure for a given phosphor layer thickness and phosphor concentration. That is, in the conventional SPE package structure, it is necessary to increase the phosphor concentration and then increase the absorption of blue light to obtain more yellow light. Hence, for the improved SPE package structure, less phosphor is required in the phosphor layer for achieving the chosen CCT.

Electroluminescence enhancement in InGaN light-emitting diode during the electrical stressing process.

This study of the optoelectronic properties of blue light-emitting diodes under direct current stress. It is found that the electroluminescence intensity increases with duration of stress, and the efficiency droop curves illustrated that the peak-efficiency and the peak-efficiency-current increases and decreases, respectively. We hypothesize that these behaviors mainly result from the increased internal quantum efficiency.

Curved remote phosphor structure for phosphor-converted white LEDs

A phosphor-converted light emitting diode with curved remote phosphor layer was obtained in this study. By changing the geometry of the flat remote phosphor layer in a conventional remote phosphor structure, the package extraction efficiency and luminous efficacy of the curved remote phosphor structure was improved by reducing the total internal reflection at the interface of the air and phosphor layer. Ray tracing simulations were performed to analyze the package extraction efficiency of the curved remote phosphor structure. Experimental results showed that the convex remote phosphor structure had high luminous efficacy with high color uniformity. The convex remote phosphor structure would be a suitable structure if the remote phosphor structure is used in lighting applications.

Applying RGB LED in full-field optical coherence tomography for real-time full-color tissue imaging

A conventional handheld skin camera is suitable for 2D inspection of shallow skin. Due to its high resolution and noninvasiveness, optical coherence tomography (OCT) has become a popular medical-imaging technology. Among OCT schemes, full-field optical coherence tomography (FF-OCT) is suitable for rapid en face imaging, as it uses a 2D imaging device for pixel processing of a sample plane. Because of its wide bandwidth and long lifetime, an RGB LED was chosen in an FF-OCT system among three source candidates in this study. A full-color tissue image and real-time video were obtained from the system to demonstrate the potential of the RGB LED FF-OCT system in medical imaging. All devices used here can be integrated by micro-optoelectromechanical technology into a handheld model. Noninvasive, real-time, full-color handheld imaging capability contributes to advance dermatology and cosmetology.

The Modeling of Two Phosphors in Conversion White-Light LED

Because of the high color rendering property, the two phosphors that convert into white light in a light-emitting diode are essential in the general lighting applications. Predicting the spectrum after two phosphors mix is challenging because of the mutual interaction between the absorption of long wavelength phosphor and the emission of short wavelength phosphor. To efficiently model the target spectrum, this paper provides a systematic method for simulating the concentration and proportion of two phosphors by using empirical equations. The simulated and experimental spectra overlapped effectively. The results show that the maximum differences are about 0.0046 for CIE 1931 color coordinates (x, y), about 59 K for correlated color temperature, and about five for color rendering index, respectively. The results show that this spectral optimization is an effective approach.

Measurement of LED chips using a large-area silicon photodiode

We propose the output power measurement of bare-wafer/chip light-emitting diodes (LEDs) using a large-area silicon (Si) photodiode with a simple structure and high accuracy relative to the conventional partial flux measurement using an integrating sphere. To obtain the optical characteristics of the LED chips measured using the two methods, three-dimensional ray-trace simulations are used to perform the measurement deviations owing to the chip position offset or tilt angle. The ray-tracing simulation results demonstrate that the deviation of light remaining in the integrating sphere is approximately 65% for the vertical LED chip and 53% for the flip-chip LED chip if the measurement distance in partial flux method is set to be 5-40 mm. By contrast, the deviation of light hitting the photodiode is only 15% for the vertical LED chip and 23% for the flip-chip LED chip if the large-area Si photodiode is used to measure the output power with the same measurement distance. As a result, the large-area Si photodiode method practically reduces the output power measurement deviations of the bare-wafer/chip LED, so that a high-accuracy measurement can be achieved in the mass production of the bare-wafer/chip LED without the complicated integrating sphere structure.

Improving the color uniformity of multiple colored light-emitting diodes using a periodic microstructure surface.

This study looks at the performance of a periodic microstructure surface on multiple colored light-emitting diodes (LEDs). A periodic microstructure surface on the package was used to reshape the light from the different colored LED chips in the multiple colored LEDs, which decreased the color separation and color nonuniformity of the multiple colored LEDs. Experimental results show that the multiple colored LEDs with a periodic microstructure surface could achieve medium or high collimation as well as good color uniformity. Thus, this study provides a simple and cost-effective method to greatly improve the color uniformity of multiple colored LEDs with medium or high collimation.

Study of Different Testing Methods Used in UV-LED Optical Measurement

In this study, three different partial radiant flux testing methods including the large-area silicon (Si) photodiode method, the assembling Si photodiodes box method, and the integrating sphere method were used to measure the optical power of UV-LED die. The optical simulations and the experiments have been performed to study the influence of the varying LED die position and tilt angle. The optical simulations based on the real measurement geometries were used to confirm the gathering hitting optical power on Si photodiode through different methods. The simulation and measurement results in this study can be used as the future reference of different partial LED flux testing methods to measure the large number of UV-LED dies.