The applications of white light-emitting diodes (LEDs)have become more and more wide but the light-extractione±ciency of white LED is always the main factor thatlimits its development. Further applications need higherlight-extraction e±ciency. Some ways of improving thelight-extraction e±ciency of white LED have been re-ported. For example, it can be improved by an omni-directional re°ector consisting of GaN, a quarter-wavelayer of SnO2 nanorod and a Ag layer[1] or by the surfacegratings and the encapsulation of a polymer[2]: The pack-aging technology is another way to improve the e±ciency.The combination of a phosphor wavelength converterwith a short-wavelength primary emitter is a commonmethod for LED-based white-light sources[3]: Typicalstructures of the phosphor arrangement in dichromaticwhite LED are shown in Figs. 1 and 2. Figure 1, whosephosphor layer replicates the contour of the LED chip,shows a conformal phosphor distribution[4] and Fig. 2shows a uniform distribution of phosphor within there°ector cup, which we refer to as \phosphor-in-cup".However, they both limit the light-extraction e±ciencyof LEDs.

　　The placement and arrangement of phosphor have sig-ni¯cant e®ects on the light-extraction e±ciency of whiteLEDs. A large portion of light emitted by the phosphorimpinges on the LED chip and can be absorbed severely.So the conformal and phosphor-in-cup distributions limitthe e±ciency of white LED. The structure whose phos-phor is placed at a su±ciently large distance from theLED chip is called remote phosphor con¯guration[5;6];as shown in Fig. 3. This kind of structure improves notonly the e±ciency but also the lifetime of LEDs. For thephosphor layer away from the LED chip, the heat gen-erated by the LED chip only transfer to the substrate[7]instead of the phosphor. The probability of a light rayemitted by the phosphor hitting the low re°ective LEDchip directly is small. However, there is still a largeprobability of a light ray re°ected by the re°ector cup tobe re-absorbed by the LED chip, as shown by rays 1 and3 of Fig. 3.

　　package, as illustrated in Fig. 4, is presented. The multi-layer phosphor con¯guration can increase the transmit-ted light (ray 3) and reduce the re°ected light thatdirectly impinges on the absorptive LED chip, thus im-prove the LEDs e±ciency.

　　The absorption coe±cient, the scattering coe±cientand the thickness of the phosphor layer all a®ect thetransmission of light. The ¯rst parameter has a muchlarger e®ect than the second one. Increasing the thicknesshas the same e®ect as increasing both of the absorptionFig. 1. Conformal distribution directly on LED chip.Fig. 2. Uniform distribution in re°ector cup (phosphor-in-cup).