A 3D semi-analytical model based on dyadic Green's functions is proposed to solve the forward problem for a magneto-optic imager with a simplified eddy-current excitation. The normal component of the magnetic field is evaluated in place of the ferrimagnetic garnet. A comparison between data provided by the model and data computed by a 3D finite element method is presented. Theoretical results are also validated with experimental data obtained by a Magneto-Optic Imager designed by Physical Research Incorporation.