The final properties of a semi-crystalline thermoplastic polymer material are directly related to its crystallinity. Therefore, the study of polymer crystallization is of major interest for industrial as well as for fundamental research. The present study deals with the simulation of a rheo-optic technique for measuring crystallization kinetics. Rheo-optic measurements enable to measure the transmitted light intensity during the crystallization process. The relative crystallinity is generally said to be equal to the relative decrease of the transmitted light intensity. In other words, the relative crystallinity is said to be an affine function of the transmitted light intensity. In the present work this affine relationship is subjected to a tough examination. The aim of the present work is to estimate the light transmittance of a polyethylene sample using simulations based on the Avrami theory for 3D growth (spherulite) with instantaneous nucleation. The OCTA software enables to compute the turbidity spectra against light wavelength based on the Maxwell equations. These spectra is then integrated over the visible spectrum. The global turbidity of the material can thus be estimated for various spherulite average diameters. These calculations enable to determine the domain of validity of the affine relationship between turbidity and crystallinity. Moreover, these results enable to estimate the discrepancy at both low and high values of the relative crystallinity.