Progress towards absolute intensity measurements of emissions from high temperature thermographic phosphors

Phosphor thermometry has been successfully used in a number of applications ranging from turbo-machinery, pyrolysis, supersonic and hypersonic studies in the past few decades. There are a number of issues related to high temperature, which include faster decays, decreasing emission intensity and increasing blackbody radiation. Although absolute lifetime decay values are readily available, there has been no known work presenting absolute intensity measurements throughout the phosphors operating temperature range. This additional information could help design engineers facilitate phosphor and instrument selection, optimise system setup, and help estimate the performance of the technique at higher temperatures, for any given optical setup. A number of well known high temperature thermographic phosphors were investigated including YAG:Tm, YAG:Tb and Y₂O₃:Eu from 20 °C in an excess of 1000 °C. Both 355 and 266 nm excitation wavelengths from a Q-switched Nd:YAG laser were used. The subsequent emissions were passed through a narrowband interference filter to isolate the peak emission wavelengths, and were collected using PMT. The methodology for an absolute measurement, which requires a sound understanding of the PMT, including solid angle, collection efficiency, dynode gain, calibration and electronic temporal response for intensity measurements is presented and discussed. The results clearly indicate a variation in phosphor intensity with an increasing temperature, which is considerably different amongst different phosphors under different excitation wavelengths. The combined standard uncertainty of measurement was estimated to be approximately ±10.7%. The existing system was able to monitor intensity values up to 900 °C for Mg₃F₂GeO₄:Mn phosphors, 1100 °C for Y2O3:Eu, 1150 °C for YAG:Tb and up to 1400 °C for YAG:Tm thermographic phosphors. Y₂O₃:Eu using 266 nm excitation was found to exhibit the highest peak intensity per mJ of laser excitation from all the phosphors investigated at 20 °C. However, at high temperatures (900 °C+) YAG:Tm using 355 nm excitation was found to exhibit the highest peak intensity per mJ of an excitation energy.