  The prospect of coherent dynamics and excitonic delocalization across several
light-harvesting structures in photosynthetic membranes is of considerable
interest, but challenging to explore experimentally. Here we demonstrate
theoretically that the excitonic delocalization across extended domains
involving several light-harvesting complexes can lead to unambiguous signatures
in the optical response, specifically, linear absorption spectra. We
characterize, under experimentally established conditions of molecular assembly
and protein-induced inhomogeneities, the optical absorption in these arrays
from polarized and unpolarized excitation, and demonstrate that it can be used
as a diagnostic tool to determine the coherent coupling among iso-energetic
light-harvesting structures. The knowledge of these couplings would then
provide further insight into the dynamical properties of transfer, such as
facilitating the accurate determination of F\"orster rates.
