Figure 2
Configurations of the two phase plates, 1 and 2, as viewed when looking along the beam. The configuration of plate 1 is drawn, then four possible configurations of plate 2. Plate 1 is fixed. The difference in azimuths about the beam, χ2 − χ1, is set at 0°, 90°, 180° and 270° for configurations (I), (II), (III) and (IV), respectively. The angle θ2 of plate 2 lattice planes with the beam is set to θ1 in configurations (I) and (III) but is different in (II) and (IV) (see text). In these latter cases the σ and π directions are exchanged from one plate to the other, the diffraction plane being defined by the beam and the π direction. In these two cases the rays deviating by angles δα and δβ from the mean ray, in the planes bisecting the diffraction planes of the two plates, behave differently (see text). |