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Diffuse emission in the mid-infrared shows a wealth of structure, that lendsitself to high-resolution structure analysis of the interstellar gas. A largepart of the emission comes from polycyclic aromatic hydrocarbons, excited bynearby ultra-violet sources. Can the observed diffuse emission structure beinterpreted as column density structure? We discuss this question with the helpof a set of model molecular clouds bathed in the radiation field of a nearbyO-star. The correlation strength between column density and ``observed'' fluxdensity strongly depends on the absolute volume density range in the region.Shadowing and irradiation effects may completely alter the appearance of anobject. Irradiation introduces additional small-scale structure and it cangenerate structures resembling shells around HII-regions in objects that do notpossess any shell-like structures whatsoever. Nevertheless, structuralinformation about the underlying interstellar medium can be retrieved. In themore diffuse regime ($n({HI})\lesssim 100$cm$^{-3}$), flux density maps may beused to trace the 3D density structure of the cloud via density gradients.Thus, while caution definitely is in order, mid-infrared surveys such asGLIMPSE will provide quantitative insight into the turbulent structure of theinterstellar medium.

Structure Generation by Irradiation: What Can GLIMPSE Teach Us about the ISM Structure?