<p>Spectral brightness in a Terahertz (THz) free electron laser (FEL) is a critical parameter that measures the brilliance of the emitted radiation within a defined wavelength range. In this article, we present a detailed numerical and analytical investigation of key FEL parameters on THz radiation, considering a longitudinally polarized harmonic undulator. The study examines the dynamics of the ponderomotive force and stimulated emission mechanisms in the context of THz radiation generation. Our numerical results offer comprehensive insights into the on-axis and off-axis spectral brightness, average power, and total power of the harmonic undulator, highlighting its efficiency in producing THz radiation. LU-based FEL emerges as a promising alternative, offering a tunable and high-intensity platform for advancing high-power laser systems and applications in the THz regime.</p>