The much lower quantum efficiency of the red phosphor (59% vs. 91% for YAG:Ce) could possibly explain the lower light output of the R4-Y4 phosphor layer than the Y4-R4. In a follow-up step, we studied how the quantum efficiency of each phosphor affects the results by analyzing the configuration performances assuming equal quantum efficiencies. The results showed that the light output in radiometric units (W) is very close for the three configurations, while the light output in photometric units (lm) is different. This difference is because the spectral power distributions are different for the two phosphors, and when weighted by the eye sensitivity function, the lumen values are different.
Since the goal is to achieve white light with chromaticity values on or very close to the blackbody locus, an optical ray-tracing analysis was carried out with the two-phosphor SPE white LED package. Comparing Y14-R2.5 with R2.5- Y14, the results showed Y14-R2.5 was on the blackbody locus and had 80% more lumens than R2.5-Y14, which had chromaticity values far below the blackbody locus.