Linear dichroism

Linear dichroism (LD) is the difference in absorption of light linearly polarized parallel and perpendicular to an orientation axis: 
LD = A_// − A_⊥.
LD is related to the main subject of this book, namely circular dichroism (CD), in that both require the difference between the absorbances of different polarized light beams to be measured and CD spectropolarimeters can be adapted to produce the required alternating beams of polarized light for LD. The main practical differences between CD and LD (apart from the polarizations of light used) are that LD signals tend to be orders of magnitude larger than CD signals, so the data are easier to collect, and LD measurements are performed on systems that are either intrinsically oriented or are oriented during the experiment, so the samples are harder to prepare. Since oriented molecular systems are intrinsic features of the world in which we live, being key components of biological cells as well as leading to macroscopic effects such as crystals, liquid crystals, membranes, and muscles, LD is a useful structural probe. It is particularly useful for determining relative orientations of components of molecular systems. A selection of general references may be found in references 1–10