We present, to the best of our knowledge, a new method for differential temperature measurement based on thermal sensitivity of the fluorescence lifetime of thermographic phosphors. Pairs of thermographic phosphors are excited with intensity-modulated light at frequencies ω and ω+Δω. The phase shift Δθ of the summary fluorescence intensity beat signal envelope is measured. A prototype of a fluorometric differential temperature sensor is developed, and feasibility of the method is experimentally demonstrated with a Sm2+:SrFCl crystal and the D15->F70 transition for high thermal sensitivity. The observed linear dependence between envelope phase shift Δθ and temperature difference ΔT agrees with the theoretical prediction. Sensitivity of S=-0.97°/°C was achieved. This method could also be applied to differential measurements of any parameter affecting fluorescence lifetime.