In all cases, the curve passes through the central spot formed by the transmitted beam. When 0 = 45°, the curve becomes a parabola when 0 exceeds 45°, a hyperbola and when 0 = 90°, a straight line. For somewhat larger values of 0, the ellipse is incomplete because of the finite size of the film. When the film is in the transmission position, this curve is a complete ellipse for sufficiently small values of 0, the angle between the zone axis and the transmitted beam (Fig. In either Laue method, the diffraction spots on the film, due to the planes of a single zone in the crystal, always lie on a curve which is some kind of conic section. 5 = source, C = crystal, F = focal point. 5-10 Focusing of diffracted beam in the transmission Laue method. These facts account for the two main uses of the Laue methods the determination of crystal orientation and the assessment of crystal quality.

The positions of the spots on the film, for both the transmission and the back-reflection method, depend on the orientation of the crystal relative to the incident beam, and the spots themselves become distorted and smeared out if the crystal has been bent or twisted in any way. In the back-reflection Laue method the film is placed between the crystal and the x-ray source, the incident beam passing through a hole in the film, and the beams diffracted in a backward direction are recorded. This method is so called because the diffracted beams are partially transmitted through the crystal. The film in the transmission Laue method (the original Laue method) is placed behind the crystal so as to record the beams diffracted in the forward direction. In each, the film is flat and placed perpendicular to the incident beam. There are two variations of the Laue method, depending on the relative positions of source, crystal, and film (Fig.

Transmission laue method