In this study, we present a through-focus re-radiation simulation aimed at detecting scattering from semiconductor structures. We employ the beam synthesis propagation (BSP) module within the finite-difference time-domain (FDTD) method, optimizing the simulation of optical systems by reducing time and computational resources typically required for imaging and illumination. To validate the approach, we simulated scattering from Silicon nitride (Si3N4) lines on a silicon (Si) substrate with various defect sizes and types at a 193 nm wavelength. The results demonstrated the detection of specific defect signals and identified the limitations of detectable defect sizes. These findings are intended to serve as pre-processing data for predicting outcomes in through-focus scanning optical microscopy (TSOM) imaging.