  TlBiSSe is a rare realization of a 3D semimetal with a conically dispersing
band that has an optical response which is well isolated from other
contributions in a broad range of photon eneries. We report optical and
magneto-optical spectroscopy on this material. When the compound is chemically
tuned into a state of the lowest carrier concentration, we find a nearly linear
frequency dependence of the optical conductivity below 0.5~eV. Landau level
spectroscopy allows us to describe the system with a massive Dirac model,
giving a gap $2\Delta =32$~meV and an in-plane velocity parameter $v= 4.0\times
10^5$~m/s. % Finally, we provide a theoretical recipe to extract all parameters
of the anisotropic Dirac band, including the Fermi energy and band degeneracy.
