Also why the line nearer to the central beam is dark and the one far away from the central beam is Bright?
When electron scatter the second time following bragg diffraction, they scatter of in two directions, of the front face and the back face of lattice planes, which is the reason for intesity difference.
The Kikuchi diffraction in TEM is generated via bragg diffraction by inelastic scattering electron, and the diffraction intensities from the front crystal plane and the back plane are different, therefore, one of the Kikuchi lines is bright and the other is dark. The inelastic scattering electron beam density nearer to the central beam is stronger than others far away from the central beam, however, much of the electron beam is diffracted to the dark line direction, thus, the total intensity of the Kikuchi line nearer to the central beam is weaker than the other one.
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