We discuss the importance of the probe coherence volume in the interpr
etation of diffraction experiments. The availability of highly monochr
omatic radiation, neutron or x-ray, calls into question the analysis o
f data by deconvolution at the level of intensities. When the probe co
herence volume approaches that of microscopically ordered regions in t
he sample, new effects may be anticipated. For instance, under strong
absorption the intensity dependence on the incident energy can be used
to locate the source of the scattering below the sample surface. Two
examples, based on experiments at an x-ray synchrotron source are disc
ussed. [S0031-9007(98)07438-9].