Schematic diagram showing the principles behind the DCT technique. A sample is aligned with its axis of rotation perpendicular to the beam. The beam illuminates the whole width of the sample, so that a transmission projection is recorded at the centre of the detector, whilst the rest of the detector area is left for detecting diffraction spots. The sample is then rotated (around the z-axis) and when a grain ‘comes into diffraction’ i.e. its alignment  to the beam results in braggs diffraction from one of its crystallographic planes, an extinction spot will appear in the transmitted projection, matched by a complimentary diffraction spot outside of the projection. Upon 180o rotation, the same plane should again come into diffraction, making a Freidel pair of spots (hkl & hkl), which when combined can provide high accuracy orientation data.