We consider the interaction between a single-mode atomic trap and a se
t of external modes that are decribed by a continuum of eigenstates. T
his theory is designed to model output coupling from current and propo
sed Bose-Einstein condensation experiments and proposed models for ato
m lasers. This work generalizes the optical input-output formalism [M.
J. Collett and C. Gardiner, Phys. Rev. A 30, 1386 (1984)].