A number of rare-earth doped fluorides may be pumped by F2 laser radiation, offering the prospect of efficient and powerful tunable VUV lasers. At present the observation of VUV laser oscillation in many of these systems is hampered by the large absorption losses caused by even low levels of impurities, and so in order to ascertain the prospects for achieving laser oscillation it is useful to determine the net optical gain in the presence of such loss. The conventional technique is not availa...