Schematic description of a powder scan in asymmetric diffraction conditions. The sample frame is determined by the three orthonormal vectors , and . The vector is normal to the surface of the sample. The scattering plane is defined by the incident wavevector and the vector. The incidence angle is the angle between and the vector lying on the surface of the sample. In asymmetric scattering geometry, the scattering vector associated with the hkl reflection is in general not parallel to . The angle between these two vectors, lying in the scattering plane, is equal to , where is the Bragg angle associated with the hkl reflection. Only a small part of the diffracted intensity along the Debye–Scherrer ring (solid circle; red in the electronic version of the journal) is collected by the detector.