The geometry of the instrumental x-ray diffraction linewidth in a back-reflection focusing camera with the point source displaced from the focusing circle.

Equations are derived which relate the instrumental linewidth in a back-reflection focusing camera (used in the study of polycrystalline materials) to the displacement of the x-ray source from the focusing circle along a diameter passing through the specimen centre; the two-dimensional problem is solved by assuming that the source is negligibly small but that the sample is finite.