  It is shown that homogeneous line-broadening in a diffuse cold atom cloud is
proportional to the resonant optical depth of the cloud. Further, it is
demonstrated how the strong directionality of the coherent interactions causes
the cloud's spectra to depend strongly on its shape, even when the cloud is
held at constant densities. These two numerical observations can be predicted
analytically by extending the single photon wave-function model. Lastly,
elongating a cloud along the line of laser propagation causes the excitation
probability distribution to deviate from the exponential decay predicted by the
Beer-Lambert law to the extent where the atoms in the back of the cloud are
more excited than the atoms in the front. These calculations are conducted at
low densities relevant to recent experiments.
