The variance as a criterion for defocusing in powder diffractometry

The focusing condition in a powder diffractometer has been examined by considering the variance, due to geometrical factors, of a diffraction line profile as a function of R, the distance of the receiving slit from the diffractometer axis. The value of R corresponding to minimum variance is taken as the point of focus. The method has been exemplified in the case of a flat sample and diffractometer geometry used in stress measurement (which is similar to Seeman-Bohlin geometry).