This paper aims to model Parasitic Light Sensitivity (PLS) in Global Shutter CMOS Image Sensors (GS-CIS) through separation of the optical problem from the carriers motion one, reducing required simulation time. The optical problem is solved thanks to Finite-Differences Time-Domain (FDTD) simulations. Solution to the carriers motion problem is shown in two ways: through use of Boltzmann Transport Equation (BTE) and through a straight line type of motion. The results show good reproduction of experimental data behavior, though fitting has still to be improved.