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Internal magnetic measurements are essential to obtaining reliable and accurate magnetic reconstructions in the interior of the plasma column in Tokamaks. In the last years, polarimetry has been increasingly used to provide global constraints to equilibrium codes. Joint European Torus (JET) polarimeter has four lateral channels, whose arrangement is similar to the topology of the diagnostic foreseen in ITER. A statistical analysis of JET polarimetry measurements have been provided in the past for only one vertical channel (CH3) using a polarimetry propagation code based on the Stokes vector formalism. A new propagation code has therefore been developed for the lateral channels to simulate and interpret the measurements of the Faraday rotation (FR) and Cotton-Mouton (CM) phase shift in JET. In this paper, a complete analysis of the integral form of the Stokes equation for the lateral channels is presented. In particular, the analysis shows that there is a strong interaction between the FR and CM effect even at low currents and low magnetic fields. The code results of both FR and CM have been used to estimate the line-integrated density and a proper benchmarking with experimental data has been performed. Two approximations to obtain the electron density in real time, using only plasma current in case of FR measurements and also toroidal magnetic fields in case of CM measurements, are proposed.