Surface temperature measurements are crucial in various energy systems. This study investigates the potential of the surface phosphor thermometry implemented in the frequency domain. Being less documented than the temporal of ratio-based strategies, the frequency-based approach is characterized by means of commercially available Mg4FGeO6:Mn4+ thermographic phosphors. These particles are coated on a stainless-steel sample equipped with a K-type thermocouple, inserted into a temperature-controlled oven, and excited by a continuous wave diode laser operating at 395 nm. The laser power is modulated by means of a function generator that injects a sinusoidal signal. The temporal trace of the laser as well as the phosphorescence are acquired with two photomultipliers, subsequently processed to determine the phase shift between them as a function of temperature. Various calibration curves are established for different laser modulation frequencies, highlighting a high sensitivity to the selection of this frequency. This curves are well fitted with fourth-order polynomial functions, enabling to determine the sensitivity. The influence of the mean laser power has less effect on phase delay measurements at higher temperatures.