A method and system for calorimetrically measuring the temperature-dependent absorptivity of a homogeneous material dimensioned to be thin and flat with a predetermined uniform thickness and a predetermined porosity. The system includes a material holder adapted to support and thermally isolate the material to be measured, an irradiation source adapted to uniformly irradiate the material with a beam of electromagnetic radiation, and an irradiation source controller adapted to control the irradiation source to uniformly heat the material during a heating period, followed by a cooling period when the material is not irradiated. A thermal sensor measures temperature of the material during the heating and cooling periods, and a computing system first calculates temperature-dependent convective and radiative thermal losses of the material based on the measured temperature of the material during the cooling period when beam intensity is zero, followed by calculation of the temperature-dependent absorptivity of the material based on the temperature-dependent convective and radiative thermal losses determined from the cooling period.