Dirac materials are systems of high interest as they show relativistic-like properties thanks to linear band dispersion in the three directions of the momentum space. In terms of examining the optical properties of these materials, we studied the reflectivity response of a TlBiSSe sample by using infrared spectroscopy. After conducting a series of reflectance measurements at various temperatures, we explained our TlBiSSe results based on its real part optical conductivity. Finally, the comparison of our data with an ideal optical response of Dirac systems allows us to extract the asymptotic velocity and the bandgap of the Dirac cone.