Conditions for equality of intensities of electron beams diffracted by twinned crystals

A diffracted beam h will maintain a constant intensity on passing between the two parts of a twinned crystal if there exists at least one point operator R in the set of equivalent operators describing the boundary such that: Rh = h (1) and Rk₀ = k₀ (2), where k₀ is the wave vector of the incident beam. For the zero beam, relation (2) becomes: Rk₀ = ± k₀ (2').