Linear dichroism spectra: measurement

There are two main methods for measuring LD spectra. The one requiring less specialized equipment is the two-spectra method. In this case, one inserts a polarizer into a normal absorbance spectrometer, measures a spectrum, and then rotates either the polarizer or the sample. If the sample is rotated, care must be taken to have the same light intensity through the same part of the sample for both polarizations. If the polarizer is rotated, care must be taken to ensure the same light intensity is present in both polarizations.

The second method, the differential method, where the instrument produces alternating beams of the two polarizations, is easier for the user to implement, and a wide range of sample orientation techniques may be used. However, much more sophisticated instrumentation is required with the differential method. The differential method usually involves using a circular dichroism spectropolarimeter either with a quarter-wave plate to convert the circularly polarized light to linearly polarized light or alternatively a doubling of the voltage across the photoelastic modulator that produces the required oscillating linear polarizations directly. Both options for collecting LD data provide the required polarized light beams and detector (see definition above). The differential method also automatically deals with the need to change the relative orientation of sample and light beam – as long as one knows which direction the instrument defines as parallel.