A spectroscopic Mueller matrix polarimeter with four photoelastic modulators (PEMs) and no moving parts is introduced. In the 4-PEM polarimeter, all the elements of the Mueller matrix are simultaneously determined from the analysis of the frequencies of the time-dependent intensity of the light beam.

This article describes a sensitive method for measuring both the magnitude and the angle of the fast axis of low level linear birefringence in optical materials. Several different approaches have been investigated and one of them has been chosen as the basis for a linear birefringence measurement instrument. The instrument employs a low birefringence photoelastic modulator for modulating the polarization states of a He–Ne laser beam. After the modulated laser beam passes through the sample, two detecting channels analyze the polarization change caused by the sample. An algorithm has been developed to calculate the magnitude and angle of the retardance in a sample. A computer program implements the algorithm and displays the calculated values. Using this instrument, selected samples with different levels of linear birefringence have been studied. The resulting instrument achieves high precision and sensitivity for the final measurements. The sensitivity of the magnitude of linear birefringence is better than 0.005 nm (∼0.003 ° with a He–Ne laser at 632.8 nm), and the sensitivity of the fast axis angle of the sample is <1 ° (for retardance ⩾0.5 nm).

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Extensive tests to determine an optimal method for using a Bio-Rad FTS-60A spectrometer for measurement of vibrational circular dichroism (VCD) in both step-scan and rapid-scan modes are reported. In the latter case, results of “fast-scan” and “slow-scan” experiments are also presented for comparison. In step-scan mode, phase modulation (PM) during the polarization-modulation measurements is found to be detrimental to the signal-to-noise ratio (S/N), but is very useful for the normal IR measurements at slow step speeds. VCD S/N is improved by doing sequential spectral collection of the polarization-modulated spectrum without PM and the instrument response spectra with phase modulation. Systematic comparative studies are used to identify optimal parameter sets for doing the step-scan VCD measurements. These conditions use more rapid data collection with faster steps than previously suggested, which permits a more realistic evaluation of S/N and reproducibility of the spectra through comparison of spectra collected in blocks. Example VCD spectra are presented for small molecules in nonaqueous solution measured in short times and for proteins in aqueous solution measured over longer times scales. Near-IR FT-VCD measured in step-scan mode at both moderate resolution and high S/N are presented. At this stage, fast-scan, slow-scan, and step-scan techniques are shown to be virtually equivalent in the mid-IR, but the reduction in frequency dependence of the phase correction and the new capability for VCD measurement in the near-IR favor continued development of step-scan techniques.

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