Grazing-Incidence Small Angle X-Ray scattering (GI-SAXS) was first introduced in 1989  as a novel technique for investigating structures on or close to the surface. At grazing incidence conditions, the incident beam undergoes total external reflection if the angle is below the critical angle. Scanning the angle of incidence from below to above the critical angle is therefore a kind of non-destructive depth profiling. This is used for conventional X-ray reflectivity (XRR) measurements, which are sensitive for electron density differences along the surface normal. XRR is widely used for determining thickness, roughness, and densities of thin amorphous, poly-crystalline, or crystalline films.
GI-SAXS instead is sensitive to in-plane correlations in surface and the interfaces. Therefore, periodic distributed electron density variations (height-height correlations e.g.) on or slightly below the surface can be investigated. Additionally the GI-SAXS signal is very sensitive to surface roughness.