The understanding of plasma surface interactions, in order to assess the main processes which take place in the divertor region, requires knowledge of the plasma characteristics. In JET, the characterisation of the different divertor geometry for the successive Mark I, Mark IIA, Mark IIAP (the "plugged" version of Mark IIA) and finally Mark IIGB divertors entails knowledge of plasma parameters such as ion and electron density, ni and ne, ion and electron temperature, Ti and Te, and the recycling particle flux at the edge. Whereas the particle recycling can be estimated by measuring the intensity of Dα line emission, neither ni nor Ti can be easily and routinely obtained, compared to ne and Te, which are measured by Langmuir probes.