  The light curve of Supernova (SN) 1993J is calculated using two approaches to
radiation transport as exemplified by the two computer codes, STELLA and
EDDINGTON. Particular attention is paid to shock breakout and the photometry in
the U, B, and V bands during the first 120 days. The hydrodynamical model, the
explosion of a 13 Msun star which had lost most of its hydrogenic envelope to a
companion, is the same in each calculation. The comparison elucidates
differences between the approaches and also serves to validate the results of
both. STELLA includes implicit hydrodynamics and is able to model supernova
evolution at early times, before the expansion is homologous. STELLA also
employs multi-group photonics and is able to follow the radiation as it
decouples from the matter. EDDINGTON uses a different algorithm for integrating
the transport equation, assumes homologous expansion, and uses a finer
frequency resolution. Good agreement is achieved between the two codes only
when compatible physical assumptions are made about the opacity. A new result
for SN 1993J is a prediction of the continuum spectrum near the shock breakout
(calculated by STELLA) which is superior to the results of other standard
single energy group hydrocodes such as VISPHOT or TITAN. Based on the results
of our independent codes, we discuss the uncertainties involved in the current
time dependent models of supernova light curves.
