Even when the Bragg condition is not exactly satisfied, there is still intensity in the diffracted beams. However, the actual intensity depends on how far the detected diffraction beams are away from the exact Bragg condition. This distance is measured by excitation error or the deviation parameter (a vector s). When the Bragg condition is not exactly satisfied the excitation error is either negative (Figure 4137a) or positive (Figure 4137c), while when the Bragg condition is exactly satisfied the excitation error is zero (Figure 4137b).
Figure 4137a. Schematic illustration showing negative excitation error.
Figure 4137b. Schematic illustration showing zero excitation error.
Figure 4137c. Schematic illustration showing positive excitation error.
With the illumination geometry of CBED, the incident beam directions vary continuously within the cone. The continuous variation of incident beam directions induces a continuous variation of the excitation error of the Bragg reflection and the observed intensity distribution in the CBED disks corresponds to a two-dimensional rocking curve (pendellösung) of the dynamical theory of electron diffraction.