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Liquid crystal variable retarders (LCVRs) are often used in Stokes polarimeters as they allow the measurement of different polarization components by applying an electric field that manipulates the induced retardance. However, the optical retardance introduced by these devices is in general not homogenous across the aperture. Another problem with this type of devices is that the fast-axis orientation is not homogenous, and it changes with the applied voltage. For the optimization of polarimeters, in terms of the noise amplification from the intensity measurements to the polarimetric data, the condition number (CN) is often used, but the effects of LCVR spatial variations are not considered. This paper analyzes the impact of errors in LCVRs in a set of optimized Stokes polarimeters simulated by adding errors in the induced retardance and fast-axis orientation. Then, the CN is calculated to observe the effect of these errors on the optimization. We show how errors in the LCVRs lead to different impacts in the polarimetric measurements for different optimized polarimeters, depending on their experimental parameters. Furthermore, we present the propagation error theory to choose the best experimental parameters to reduce the nonideal effects in optimized polarimeters.

We present a comparison of two experimental methods to measure retardance as a function of applied voltage and as a function of position over the aperture of liquid-crystal variable retarders. These measurements are required for many applications, particularly in polarimetry. One method involves the scan of an unexpanded laser beam over the aperture, and the other uses an expanded beam from a LED and a CCD camera to measure the full aperture with a single measurement. The first method is time consuming, is limited in the measured spatial resolution, and requires more expensive equipment to perform the scan, whereas the second method is low cost, with the spatial resolution of the CCD, and fast, but in principle has variations of the incident beam over the aperture that affect the measured retardance values. The results obtained show good agreement for the average values of retardance for the two methods, but the expanded-beam method shows more noise, particularly close to the voltage values at which the variable-retarder retardance versus voltage curves are unwrapped. These retardance variations can be reduced by smoothing the retardance image, which makes the expanded-beam method an attractive method for polarimetry applications since it gives the complete information in the full aperture of the device with the additional advantages of low cost, simplicity, and being less time consuming.

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