Fluctuation microscopy is the enigmatic name given to an otherwise straightforward technique for studying medium range order in highly disordered materials. By medium range, we mean atomic ordering at length scales within the range 0.5-2.0 nm, where traditional imaging and diffraction techniques have the most difficulty detecting structural correlations in amorphous materials. Puzzlement over fluctuation microscopy generally arises not because of the "microscopy" part of the name, but because of the "fluctuation" part. What, exactly, is fluctuating? And, why does it fluctuate?

The fluctuations are simply the variations in scattering between small sub-volumes within a thin sample. These are usually not timevarying fluctuations (although they could be), but instead they are the position-varying fluctuations in local diffraction.