Relative absorption correction for rotation film data

An improved method of implementing the absorption correction procedure of Stuart & Walker [Acta Cryst. (1979), A35, 925-933] is presented. This method scales measured reflections to a partial reference data set of corrected intensities by determining, for each crystal, a transmission surface representing the relative transmission as a function of secondary beam directions. By reformulating the observational equations and by introducing eigenvalue filtering of the least-squares normal equations, the problems of correlation between parameters defining the transmission surface are reduced.