ISO 25498:2018 specifies the method of selected area electron diffraction (SAED) analysis using a transmission electron microscope (TEM) to analyse thin crystalline specimens. This document applies to test areas of micrometres and sub-micrometres in size. The minimum diameter of the selected area in a specimen which can be analysed by this method is restricted by the spherical aberration coefficient of the objective lens of the microscope and approaches several hundred nanometres for a modern TEM.
When the size of an analysed specimen area is smaller than that restriction, this document can also be used for the analysis procedure. But, because of the effect of spherical aberration, some of the diffraction information in the pattern can be generated from outside of the area defined by the selected area aperture. In such cases, the use of microdiffraction (nano-beam diffraction) or convergent beam electron diffraction, where available, might be preferred.
ISO 25498:2018 is applicable to the acquisition of SAED patterns from crystalline specimens, indexing the patterns and calibration of the diffraction constant.