The physical phenomenon of coherent scattering of X-rays on the crystal is called diffraction, even if it occurs at a low Angle. For example, if a phase has a d value of 31.5A and a corresponding diffraction of 2.80° (Cu-Kα), the 31.5A peak is still very sharp if the phase has a high degree of crystallinity. Thin films can also produce X-ray diffraction concentrated in a small Angle range depending on the thickness and microstructure of the film. In these cases, the small-angle X-ray scattering intensity of the sample comes mainly from the diffraction of the sample, which is called angular X-ray diffraction.

X-ray irradiation to ultrafine powder particles also causes coherent scattering phenomenon, which also occurs in the low-angle region. However, the features of the coherent scattering pattern produced by the fine particles are completely different from the features of the small-angle X-ray diffraction pattern produced by the large crystal face spacing or thin film above.

Small Angle diffraction is generally used to determine the spacing of very large crystal faces or the thickness of thin films, as well as the micro-periodic structure of thin films and the distribution of periodic holes.