The Ewald sphere can intersect with a relrod even when it misses
the actual reciprocal lattice point. Diffraction, at reduced intensity, can then still
occur. The deviation parameter, s,
defines how close a particular relrod is to the Ewald sphere. If we allow streaking, the
diffraction vector K is then given by
vectorially adding the deviation parameter s
to the reciprocal vector g, so:
K = g + s
Click on the animation
opposite to show the deviation parameter s
graphically.
The deviation parameter is defined to be positive in the direction of the beam
(downwards in this animation) and negative if it points upwards (as here).
In reciprocal space, if we represent the incident beam by
vector kI and the
diffracted beam by kD,
then the diffraction vector K is
given by:
K = kD - kI
A diffracted beam only arises when K = g
i.e. it is a vector between reciprocal lattice points. If we allow streaking of reciprocal
lattice points, then the diffraction vector is given by:
K = g + s
In a thin crystal, diffraction may be thus be seen from a particular set of
incident beam angles close together (not just a single angle), and/or a range of crystal
orientations.
The effect of streaking is that lattice points which do not touch Ewald's
sphere but are close, can still give diffracted beams. However, they will have a reduced
beam intensity. The intensity of the diffracted beam varies with the value of the
deviation parameter s as shown here.
Click on the animation
opposite to show the deviation parameter s
graphically. 
In reciprocal space, if we represent the incident beam by
vector kI and the
diffracted beam by kD,
then the diffraction vector K is
given by: 
The effect of streaking is that lattice points which do not touch Ewald's
sphere but are close, can still give diffracted beams. However, they will have a reduced
beam intensity. The intensity of the diffracted beam varies with the value of the
deviation parameter s as shown here.
