An extensive period of deuterium/tritium operation (the DTE1 experiment) took place in the JET tokamak during 1997 [1]. The diagnostic capabilities at JET permitted the relative densities of deuterium and tritium to be determined sufficiently well that the fusion performance of the discharges could be interpreted unambiguously. However, a serious problem was found to be the darkening of optical windows (due to carbon/beryllium deposits) which has impeded the measurement of the effective charge of the plasma (Zeff) and, hence, the determination of absolute densities.