Linear dichroism (LD) or diattenuation is the difference between absorption of light polarized parallel and polarized perpendicular to an orientation axis. It is the property of a material whose transmittance depends on the orientation of linearly polarized light incident upon it. As a technique, it is primarily used to study the functionality and structure of molecules. LD measurements are based on the interaction between matter and light and thus are a form of electromagnetic spectroscopy.
This effect has been applied across the EM spectrum, where different wavelengths of light can probe a host of chemical systems.  The predominant use of LD currently is in the study of bio-macromolecules (e.g. DNA) as well as synthetic polymers.