부산대학교 도서관

Lumen flux of white light-emitting diodes (LEDs) can be increased by increasing phosphor layer thickness and blue LED’s drive current. Higher LED currents will lead to an increase in blue light in the output lumen along with higher temperatures which is not acceptable, and a corresponding decrease in the yellow light emission. The physical reason for this decrease is attributed to the finite phosphor particles in the layer and re-emission occurring due to scattered or back reflected blue light rays. A parameter called blue to yellow ratio (BYR) is used which can lead to a current with maximum phosphor utilization while preventing the LED output color to shift from white toward blue. As the degree of impact on reliability from drive current and thickness is different, a careful adjustment of the drive current and phosphor layer thickness shall be made for a given lumen. Response surface method is employed to obtain an optimal combination of the current and thickness where the temperature is the minimum for a given lumen level. The temperature difference for different combinations of current and thickness for a given lumen can be as high as 15 °C as found in this paper.