  Structured materials like photonic crystals require for optimal use a high
precision both on position and optical characteristics of the components which
they are made of. Here, we present a simple tomographic algorithm, based on a
specific Green's function together with a first-order Born approximation, which
enables us to localize and characterize identical defects in finite-size
photonic crystals. This algorithm is proposed as a first step to the monitoring
of such materials. Illustrative numerical results show in particular some
possibility of focalization beyond the Rayleigh criterion.
