Electron Backscatter Diffraction - Pattern Centre

Pattern Centre

In order to relate the orientation of a crystal, much like in X-ray diffraction, the geometry of the system must be known. In particular the pattern centre, which describes both the distance of the interaction volume to the detector and the location of the nearest point between the phosphor and the sample on the phosphor screen. Early work utilised a single crystal of known orientation being inserted into the SEM chamber and a particular feature of the EBSP was known to correspond to the pattern centre. Later developments involved exploiting various geometric relationships between the generation of an EBSP and the chamber geometry (shadow casting and phosphor movement).

Unfortunately each of these methods are cumbersome, prone to some systematic errors for a general operator and can not be easily utilised in modern SEMs with multiple designated uses. Thus most commercial EBSD systems utilise the indexing algorithm combined with an iterative movement of both crystal orientation and suggested pattern centre location. Minimising the fit between bands located within experimental patterns and those in look up tables tends to converge on the pattern centre location to an accuracy of ~0.5–1% of the pattern width.