  We discuss radiation fields in a compact space of finite size instead of that
in a cavity for investigating the coupled atom-radiation field system.
Representations of $T(1)\times SO(4)$ group are used to give a formulation for
kinematics of the radiation fields. The explicit geometrical parameter
dependence of statistical properties of radiation fields is obtained. Results
show remarkable differences from that of the black-body radiation system in
free space.
