  We present deep two-colour photometry of two rich clusters at z=0.18, A665
and A1689. We use these data to construct number counts as a function of
magnitude in the two fields. By combining these counts with similar
observations from a large area field survey we subtract the field contamination
statistically to produce luminosity functions for the two clusters. Great care
has been taken to achieve agreement between the photometry of these two
samples. The cluster data are complete to a limiting magnitude of I=22.5 or an
absolute magnitude in the cluster of I= -18.0 ($M^\star+5$). The luminosity
functions of both clusters are well described by a Gaussian function for the
bright galaxies, combined with a Schechter function at the faint end, similar
to that required to fit the luminosity function in local clusters. The slope at
the faint end of the Schechter function in both clusters is extremely steep in
V, alpha ~ -2. A shallower slope is seen to the limit of the I data, indicating
that the cluster population is rapidly blueing as we reach fainter. The
excellent agreement between the form of the luminosity function in our two
distant clusters, as well as agreement with the luminosity function given by
Driver et al (1994) for a single z=0.21 cluster, indicates that this faint blue
population is a general constituent of distant clusters. We compare our results
with those from studies of local clusters. Depending upon the degree of fading
(or disruption) of these faint blue galaxies,we tentatively identify their
remnants with the low surface brightness dwarf galaxies which are the dominant
population in local clusters. We discuss the possible role of this population
as the source of most of the X-ray gas in rich clusters.
