  Recent observations, especially by the Fermi satellite, point out the
importance of the thermal component in GRB spectra. This fact revives strong
interest in photospheric emission from relativistic outflows. Early studies
already suggested that the observed spectrum of photospheric emission from
relativistically moving objects differs in shape from the Planck spectrum.
However, this component appears to be subdominant in many GRBs and the origin
of the dominant component is still unclear. One of the popular ideas is that
energy dissipation near the photosphere may produce a non-thermal spectrum and
account for such emission. Before considering such models, though, one has to
determine precise spectral and timing characteristics of the photospheric
emission in the simplest possible case. Hence this paper focuses on various
physical effects which make the photospheric emission spectrum different from
the black body spectrum and quantifies them.
