  We investigate various probes of luminosity evolution in the rich cluster
environment, concentrating in particular on the spheroidal (E/S0) galaxies,
using a newly-constructed catalog of morphologically-classified Hubble Space
Telescope images of galaxies observed in the cores of 13 clusters with
redshifts 0.17<z<0.56. An important distinction of this study compared to
earlier work is the availability of new near-infrared ground-based photometry
for a substantial subset of our HST fields, which we have used to select and
study the various populations. We find no significant change in the
characteristic luminosity of the spheroidal populations at redshifts of 0.31
and 0.56. As a more sensitive probe of luminosity evolution, deriving effective
metric radii and mean effective surface brightnesses. At the standard condition
corresponding to R_e = 1 kpc, we find convincing evidence of evolutionary
brightening in both rest-frame B and K light, consistent with model predictions
based on the passive evolution of stellar populations.
