Experiment Results and Discussion
First, we characterized multiple layers consisting of the same phosphor, YAG:Ce. A single layer of YAG:Ce was characterized at densities of 4 and 8 mg/cm2, and two layers were characterized with each layer at 2 and 4 mg/cm2. Results show the performance of a single layer is nearly equal to that of two layers when the amount of phosphor is the same. Results from the theoretical calculations, experiment, and optical ray-tracing agreed well. Second, we characterized multiple layers of two phosphors, YAG:Ce and a red phosphor, at 4 mg/cm2 each. Initially, YAG:Ce was the first layer and red as the second layer; then the order was switched. Then the two phosphors were mixed into one layer. Once again, the results from the experiments, optical ray-tracing, and theoretical calculations showed good agreement. Fig. 1 illustrates the chromaticity values (u’, v’) of the three phosphor configurations on the CIE 1976 UCS diagram. The chromaticity values of the three configurations are different. Table 1 shows the light output results. The first layer being yellow and the second layer being red with each at 4 mg/cm2 densities (Y4-R4) yielded 50% more light in photometric units (lm) than the single layer mixture of yellow and red phosphors.