Frequency resolved phosphor thermometry under high excitation density conditions

Frequency Resolved Phosphor Thermometry Under High Excitation Density Conditions Laser diodes (LDs) based lightning is expected to become the main high power lightning technology. It has higher light intensity per area which is needed in some quasi-point source applications such as mini projectors, car headlamps and narrow beam outdoor torches. The main challenge of LDs based lightning devices is stable operating at high temperatures. These devices usually operate with remote phosphors which convert part of LD’s emitted light to yellow and in this way generate white light. Temperature of phosphors rises because of energy loses due to nonradiative photoluminescence (PL) decay and Stokes shift. This can cause degradation of phosphor and changed colour of luminaire. For this reason it is important for a phosphor to have as good thermal stability as possible. In this work, temperatures of commercial phosphors’ powders in silicone and silicone gasket as well as YAG:Ce3+ phosphor ceramics at different excitation power densities were measured using phosphors’ photoluminescence decay time dependency on temperature. For calibration purposes samples were embedded in cryostat and their PL decay time’s dependency on temperature was measured using low power LED light. Later phosphors were withdrawn from controlled temperature environment, and the PL decay time was measured as a function of LD excitation power density. Temperature of commercial phosphors was determined on the basis of calibration measurement described above. Temperature of ceramic YAG:Ce3+ phosphor was not determined due to low FL characteristics dependency on excitation power density. Temperatures of three phosphors’ samples were compared with modelling results and infrared camera measurements. The results have shown that it is possible to determine temperatures of phosphors which have strong PL decay time dependency on temperature. An investigation also revealed that phosphors in silicone gasket have better thermal stability under high excitation density conditions than phosphors in silicone