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Investigation of Light Extraction by Refractive Index of an Encapsulant, a Package Structure, and Phosphor

The effects of the refractive index of an encapsulant on the light-extraction efficiency (LEE) of white light-emitting diodes (LEDs) are studied. A dome-type blue LED with a high-refractive-index encapsulant (HRE) shows a higher LEE compared to that of a low-refractive-index encapsulant (LRE). In a flat-type surface-mounted device (SMD) package, however, an LED with LRE shows a higher LEE than an LED with the HRE. These results stem from differences in the total internal reflection at the interfaces between the encapsulant and the air. In an SMD package, the LEE at the interface between an LED chip and an HRE is higher than that between an LED chip and an LRE, whereas the LEE at the interface between an HRE and air is lower than that between an LRE and air. The simulation results based on Monte Carlo ray tracing agree well with the experimental results. On the contrary, when a flat-type white LED is fabricated with a high concentration of phosphors, the optical power with LRE is lower than that with HRE because the optical paths within an LED package are modified by phosphor.