  Diffuse emission in the mid-infrared shows a wealth of structure, that lends
itself to high-resolution structure analysis of the interstellar gas. A large
part of the emission comes from polycyclic aromatic hydrocarbons, excited by
nearby ultra-violet sources. Can the observed diffuse emission structure be
interpreted as column density structure? We discuss this question with the help
of a set of model molecular clouds bathed in the radiation field of a nearby
O-star. The correlation strength between column density and ``observed'' flux
density strongly depends on the absolute volume density range in the region.
Shadowing and irradiation effects may completely alter the appearance of an
object. Irradiation introduces additional small-scale structure and it can
generate structures resembling shells around HII-regions in objects that do not
possess any shell-like structures whatsoever. Nevertheless, structural
information about the underlying interstellar medium can be retrieved. In the
more diffuse regime ($n({HI})\lesssim 100$cm$^{-3}$), flux density maps may be
used to trace the 3D density structure of the cloud via density gradients.
Thus, while caution definitely is in order, mid-infrared surveys such as
GLIMPSE will provide quantitative insight into the turbulent structure of the
interstellar medium.
