Experimental diffraction. Understanding Weissenberg diagrams
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A genuine Weissenberg diagram consists of isolating a reciprocal plane (therefore perpendicular to one direct axis of the crystal, which acts as the rotation axis), and collecting on a cylindrical photographic plate all the diffracted beams coming from the reciprocal points on that plane. The filtering of the diffracted beams from the selected reciprocal plane is carried out by including two internal metal cylinders that leave a slit allowing the diffracted beams from that plane to reach the photographic film.

The rotation of the crystal and the coupled translation of the photographic film during exposure to X-rays produce a geometric deformation of the reciprocal lattice when it is seen on the Weissenberg diagram.

This deformation is such that any reciprocal line, passing through the centre of the reciprocal lattice, appears in the diagram as a line of spots lying on a straight line with slope 2. Furthermore, the angle between two reciprocal lines that intersect each other in the reciprocal lattice is shown in the diagram as the distance between two parallel lines, as it is shown in the figure below:




Geometric distortion of  a reciprocal plane (figure on the left) when collected on a Weissenberg diagram (figure on the right).




Template drawn on a real Weissenberg diagram for the interpretation of the diffraction spots. These templates were used for the measurements of diffraction intensities in a manual photometer, so one could identify all spots with their Miller indices.


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