Comparative optical thermometry analysis using NaSrPO:Er/Yb phosphors: evaluation of LIR and LIR methods for high-resolution temperature sensing.

Optical thermometry is a valuable non-contact technique for temperature measurement, especially in environments where traditional methods are impractical. Despite its advantages, enhancing the precision of optical thermometers remains a significant challenge. In this study, we explored the thermometric properties of NaSrPO phosphors co-doped with Er/Yb, synthesized a solid-state reaction method, for temperature sensing within the 200-440 K range under 980 nm excitation. Upconversion (UC) luminescence, observed in the visible spectrum, was analyzed using the fluorescence intensity ratio (FIR) method, focusing on both thermally coupled (TCLs) and non-thermally coupled (NTCLs) energy levels of Er/Yb. Specifically, the transitions H → I, S → I, and F → I were examined to calculate thermometric parameters. The maximum absolute sensitivity ( ) and relative sensitivity ( ) for the H → I to S → I transition were 0.0009 K and 0.6% K, respectively, while for the H → I to F → I transition, was 0.004 K, with a maximum of 1.14% K. Furthermore, by employing a luminescence intensity ratio technique based on TCLs (LIR), the minimum temperature uncertainty (δ) was found to be 1.31 K at 320 K. In contrast, the luminescence intensity ratio method based on NTCLs (LIR) yielded a much lower minimum δ value of 0.34 K at 200 K, indicating superior performance in terms of temperature resolution. These findings demonstrate that the LIR technique provides more sensitive and accurate temperature measurement compared to LIR. The excellent temperature resolution and sensitivity of NaSrPO:Er/Yb phosphors highlight their potential for highly accurate optical thermometry applications in scientific and industrial contexts.