Investigation of the Optical Properties of YAG:Ce Phosphor

Recently, many studies have used optical ray-tracing analysis to investigate novel concepts of phosphor-converted white LEDs. Even though optical ray-tracing is a convenient tool, the accuracy of the results depends very much on the optical properties of the various components within the package used in the analysis. Presently, light transmission, reflection, and absorption properties of white LED phosphors are not very well quantified. Therefore, a laboratory study was conducted to quantify at different wavelengths of light the optical properties of a medium that has YAG:Ce phosphor mixed into epoxy. When short-wavelength radiation (blue light) strikes the epoxy-phosphor medium, some portion of the blue light is converted to longer wavelength radiation (yellow light). At a phosphor density suitable for creating a balanced white light, the amount of back-transferred and forward-transferred light, including blue and yellow light, are 53% and 47%, respectively. At a similar phosphor density, when green and red radiant energies strike the epoxyphosphor medium, most of the energy is not converted by the YAG:Ce phosphor because it is beyond the phosphor’s excitation region. In this case, nearly equal amounts of green and red radiant energy are transferred in the backward and forward directions. To demonstrate the usefulness of the results obtained in this study, an optical ray-tracing analysis of a remote phosphor white LED package was conducted. This analysis showed that the surface finish of the reflector cup of a reflective type remote phosphor white LED package does not affect extraction efficiency. However, the extraction depends very much on the effective reflectivity of the inside surfaces of the reflector cup.