  We consider the optical generation and verification of entanglement in atomic
ensembles under non-uniform interaction between the ensemble and an optical
mode. We show that for a wide range of parameters a system of non-uniformly
coupled atomic spins can be described as an ensemble of uniformly coupled spins
with a reduced effective atom-light coupling and a reduced effective atom
number, with a reduction factor of order unity given by the ensemble-mode
geometry. This description is valid even for complex entangled states with
arbitrary phase-space distribution functions as long as the detection does not
resolve single spins. Furthermore, we derive an analytic formula for the
observable entanglement in the case, of relevance in practice, where the
ensemble-mode coupling differs between state generation and measurement.
