The Motional Stark Effect (MSE) diagnostic is widely applied to determine the current distribution in high temperature plasmas. The MSE measurement is based on the detection of the polarised Balmer-a light, emitted by the NBI (hydrogen or deuterium) atoms, which is related to the direction of the magnetic field in the plasma. The 25 channels MSE diagnostic on JET covers the center and outer side of the plasma (major radius 2.68-3.88m) with a spatial resolution of 0.05-0.07m and channel-to-channel separation of ~0.05m. The safety factor, q, is determined from the most Doppler shifted p+ component (selected by interference filters with ~0.4nm bandpass) measured with avalanche photo diodes. The detection of the polarisation of the measured light is made by two photo-elastic modulators and a linear polariser. The measured light is analysed using a Stokes vector (derived from the amplitudes of the APD harmonics): the total intensity (FDC), two linearly polarised light components (40kHz, 46kHz) and a circular component (23kHz).