Transition and diffraction radiation from charged particles is commonly used for diagnostics purposes in accelerator facilities as well as THz sources for spectroscopy applications. Therefore, an accurate analysis of the emission process and the transport optics is crucial to properly characterize the source and precisely retrieve beam parameters. In this regard, we have developed a new algorithm, based on Zemax, to simulate both transition and diffraction radiation as generated by relativistic electron bunches, therefore considering collective effects. In particular, unlike other previous works, we take into account electron beam physical size and transverse momentum, reproducing some effects visible on the produced radiation, not observable in a single electron analysis. The simulation results have been compared with two experiments showing an excellent agreement.