The development of an SI-traceable calibration sphere for low-temperature phosphor thermometry

Phosphor Thermometry (PT) is a promising optical and non-contact technique for surface temperature measurement, yet its adoption is often limited by calibration procedures that rely on industrial thermocouples which are prone to drift. This work presents the development of an SI-unit traceable calibration sphere which was designed to establish a documented chain of calibrations to the International Temperature Scale of 1990 (ITS-90). A bespoke 3D-printed sphere was developed, its circular excursion coated with Manganese-activated Magnesium Fluorogermanate (MFG), and subsequently calibrated between 20 and 60 °C using the intensity ratio technique. The calibration achieved a combined expanded uncertainty (k = 2) ranging from ±2.70 to ±2.88 °C, with the uncertainty budget dominated by the standard error within the reference thermocouple. Validation tests on an MFG-coated copper disc yielded residuals from 0.04 and -0.89 ◦C across the aforementioned temperature range. This proof-of-concept work demonstrates a viable methodology for practical SI-unit traceable PT calibration.