These relations give a mathematical formulation for the causality principle, viz. It follows that the Dispersion Curves for refraction and absorption are interconvertable quantities and have essentially the same information content. Similar relationships exist for the dispersions in Birefringence and Dichroism , eliminating the need to do both types of measurement. Dichroic measurements have become the preferred technique for practical reasons. The conditions for its occurrence are similar to those described for Linear Dichroism .
Atoms are strictly achroic as a result of their spherical symmetry.
A certain degree of anisotropy is required to generate the effect. In general, the electronic transitions in single molecules are Dichroic, with the possible exception of transitions induced by polarized light incident along certain molecular symmetry planes. The effect is also referred to as Natural Linear Dichroism to emphasize that no applied field is required for the effect to exist. If we consider a thin slab of Dichroic material, the direction along which the chromophores are aligned in the plane of the slab together with its normal define the optical anisotropy axes . Given the direction of the electric transition dipole with respect to the molecular frame, LD can be used to determine molecular alignment, or vice versa.