This article describes the development of a 600 GHz gyrotron-based optical system for field exposure assessment to investigate the biological effects of terahertz radiation. Triaxial elliptical mirrors are employed to shape the elliptical beam emitted from the gyrotron. An external power control module consisting of three wire grids in the beamline is used to maintain stable irradiation power at the sample. Beam radii of 5mm in both the x- and y-directions were measured at the sample with less than 1 mm deviation due to thermal diffusion. Irradiation power control achieved less than 1% relative standard deviation (RSD) of average incident beam power.