MEASUREMENTS OF LINEAR DIATTENUATION AND LINEAR RETARDATION SPECTRA WITH A ROTATING SAMPLE SPECTROPOLARIMETER

A technique to measure linear diattenuation and retardance spectra of infrared materials in transmission is described. A sample is rotated b etween two stationary linear polarizers in the sample compartment of a Fourier transform infrared spectrometer. The intensity modulation tha t results from the rotation of the sample is Fourier analyzed, and the linear diattenuation and linear retardance of the sample are calculat ed from the Fourier series coefficients for each wavelength. The advan tages of this technique include immunity of the measurement to instrum ental polarization, to circular diattenuation, and to circular retarda nce. The rotating sample polarimeter does not require retarders. Compe nsation for systematic errors from polarizers with diattenuation less than one is included in the data reduction. This technique is useful f or the calibration of retarders and polarizers and hence for the boots trap calibration of more elaborate polarimeters such as Mueller matrix polarimeters. We present as an example of the technique the calibrati on spectra 3-14 mum of an infrared quasi-achromatic retarder whose fas t axis orientation oscillates with wavelength.