Long-wavelength coherent transition radiation is commonly used in electron beam diagnostics for the determination of bunch lengths. Typically the spectrum of coherent transition radiation for a short bunch accelerator is settled in the low or sub-THz regime. Hereby, we present a theoretical model based on physical optical techniques in order to calculate emission characteristics for transition radiation, for both the radiating near-field and the far-field. This approach yields analytic solutions for the emitted electromagnetic fields without the need to solve integral equations. The simulated intensity distribution is compared with measurements showing good agreement.