Dépouillement par ordinateur des clichés de diffraction obtenus par la méthode de Laue

A computation method is presented which allows a rapid indexing of any unknown spot pattern obtained by back-reflection or transmission Laue methods. The Cartesian coordinates of n spots are measured in an orthonormal frame referred to the photographic film. Two spots 1 and 2 separated by a wide angular distance α^m are carefully chosen. Their indices are assumed to be less than 5. The set (E) of all the pairs of planes (h₁k₁l₁) and (h₂k₂l₂) making an angle α close to α^m is then computed. Since the pair of reflecting planes related to spots 1 and 2 belongs to (E), each computed pair of planes is tried, in order to determine the orientation of the crystal and to check whether the coordinates of the (n−2) other spots can be matched to dense planes of indices less than 8. If the uncertainty of the measurements is high or if n is too low, this method gives the possible orientations for the crystal. Plane indices less than 8 have been identified in cubic, tetragonal and orthorhombic crystals.