The wavelength dependence of the optical absorption of solids, liquids and solutions has long been recognised as an indicator of electronic transitions and hence the associated electronic structure of these materials. With solids and highly ordered systems, a useful extension is the study of the absorption at a given wavelength (λ) as a function of the state of polarisation of incident, linearly polarised light. Such ‘dichroism’ data enable one to locate the principal directions of the transition moments in the crystal should a single absorbing chromophore be the sole origin of the effect. This may be done by evaluating the principal components11with i = 11, 2 or 3 of the extinction coefficient tensor which describes the system. For cylindrically symmetric particles,11=22. Dispersion data of dichroism is thus of double value.