  We demonstrate a method to experimentally calibrate the refractive index
modulation in photorefractive lattices, a task rarely addressed that is crucial
for quantitative comparisons of theories with experiments. We consider the
linear propagation of a normally incident plane wave through simple lattices
and its modulation amplitude at crystal output face. Finding no evidence of
longitudinal (Talbot-like) oscillations, we discard an ideal propagation theory
and construct a simple effective model that includes longitudinal relaxation.
We obtain calibrations of 1D and 2D lattices consistent with standard theory in
a high saturation regime. For 2D lattices, we find anisotropies $\chi=1.5-
2.5$, stronger for smaller lattice period, and refractive indexes larger than
for 1D lattices, also with more noise.
