Optical energy storage materials can store energy when exposed to radiation and subsequently release it as light after thermal or optical stimulation. Such materials are thus employed in, e.g., detectors, dosimetry, self-lit signs, and imaging. In, e.g., dosimetry, the response of the material is correlated with the absorbed energy, but no distinction of different radiation energies can be achieved. In this work, Sr3MgSi2O8:Eu2+,Dy3+ was studied with thermoluminescence (TL) initiated by irradiating the material with photon energies between 2.6 (480) and 5.4 eV (230 nm). The TL glow curves revealed that the material has two main traps. Both the overall TL intensity and the TL intensity ratio between the two traps strongly depend on the photon energy of the irradiation. A mechanism of energy storage and charge carrier release in this material was constructed from the results obtained.