Quantitative thermal imaging requires reliable knowledge of the emissivity of the observed surface and of any thermographic phosphors in the field of view. Emissivity is, however, notoriously difficult to determine with sufficiently low uncertainty, so the agreement between independent measurement techniques must be assessed before results can be trusted. Collaborative work within the international project ThermoSI has developed and optimized a virtual source method for determining the emissivity of phosphor-coated materials at elevated temperatures. The method is based on Kirchhoff’s law, which links the emissivity of an opaque surface to its reflectance. The reflected contribution of a controlled virtual radiation source is used to obtain the emissivity of the heated sample, providing a traceability chain independent of conventional direct radiometric methods.This contribution will elaborate on the optimization of the virtual source method for phosphor-coated surfaces. Specific samples were prepared by applying a phosphor coating to an aluminium substrate and were measured in an oxidizing atmosphere at temperatures above 523 K. The measurement setup and evaluation procedure were adapted to the coated samples, and the evaluation yields their emissivity together with the uncertainty of the method. The results will be put into contrast with measured emissivity values obtained by traditional techniques of identical samples and the in-situ approach of adjusting the thermal imager’s emissivity setting while viewing an object of known temperature. This comparison assesses the consistency between the methods and supports the use of in-field-of-view reference artefacts of known emissivity for quantitative thermal imaging. The project 23IND11 ThermoSI has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States.