Simple spectral technique to identify the ordinary and extraordinary axes of a liquid crystal retarder

•We demonstrate a technique to discernate between ordinary and extraordinary axes of a linear retarder.•The technique is very simple and can be aplied directly with a compact spectrometer system.•The technique is useful for fixed retarders (like quartz retarders) or dynamic retarders (like liquid crystal retarders).•We provide a theorethical explanation and analysis in terms of propagation in anisotropic materials.•Experimental results are included that verify the theory.

We present a very simple method to distinguish between ordinary and extraordinary axes in an optical retarder. The method is based on inserting the retarder in between two crossed linear polarizers, oriented at 45° to the neutral axes. By tilting the retarder to obtain oblique-incidence illumination, a different behavior is observed depending on the orientation of the ordinary/extraordinary axes relative to the tilt direction. Simply using white light illumination from a tungsten lamp and spectral analysis by means of a portable spectrometer, it is possible to differentiate between ordinary and extraordinary axes. Theoretical analysis is provided, as well as the experimental verification with a liquid crystal variable retarder (LCR). A significant difference of the LCR retardance variation is obtained for different orientation of the LC director relative to the tilt direction.