In Kinematical theory an approximation is made that the electrons are only scattered once. For transmission electron diffraction it is common to assume a constant thickness t , and also what is called the Column Approximation (e.g. references and further reading). For a perfect crystal the intensity for each diffraction spot g is then: I g = | ϕ ϕ ( k ) | 2 ∝ ∝ | F g sin ⁡ ⁡ ( π π t s z ) π π s z | 2 where s z is the magnitude of the excitation error | s z | along z, the distance along the beam direction (z-axis by convention) from the diffraction spot to the Ewald sphere, and F g is the structure factor : F g = ∑ ∑ j = 1 N f j exp ⁡ ⁡ ( 2 π π i g ⋅ ⋅ r j − − T j g 2 ) the sum being over all the atoms in the unit cell with f j the form factors, g the reciprocal lattice vector, T j is a simplified form of the Debye–Waller factor, and k is the wavevector for the diffraction beam which is: k = k 0 + g + s z for an incident wavevector of k 0 , as in Figure 6 and above. The excitation error comes in as the outgoing wavevector k has to have the same modulus (i.e. energy) as the incoming wavevector k 0 . The intensity in transmission electron diffraction oscillates as a function of thickness, which can be confusing; there can similarly be intensity changes due to variations in orientation and also structural defects such as dislocations. If a diffraction spot is strong it could be because it has a larger structure factor, or it could be because the combination of thickness and excitation error is "right". Similarly the observed intensity can be small, even though the structure factor is large. This can complicate interpretation of the intensities. By comparison, these effects are much smaller in x-ray diffraction or neutron diffraction because they interact with matter far less and often Bragg's law is adequate.