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Lattice planes
Electron is inelastically scattered by an atom in the sample.

In 3D the lines of different intensity become cones. The cone of intensity lower than its background is called the 'defect cone'; the cone of intensity higher than its background is called the 'excess cone'. Both cones are separated from the plane of reflection by the Bragg angle θ.
Defect cone
Excess cone
Film

These large cones project as lines on the diffraction pattern. The excess cone projects as a line of higher intensity than the background, the defect cone as a line of lower intensity than the background. They are separated by a distance that corresponds to an angle of 2θ for the plane from which reflected.