High-speed degree-of-polarization imaging with a ferroelectric liquid-crystal modulator

Images describing polarimetric characteristics of objects are more and more used when studying complex scenes. In some cases, only the depolarization phenomenon, mostly due to scattering, proves of interest and estimating the degree of polarization (DOP) is sufficient. For a linearly polarized beam under normal incidence, when the materials in the scene exhibit neither birefringence nor dichroism, this comes down to an intensity measurement in two crossed directions. Nevertheless, the classical optical setup, consisting mainly of a rotating polarizer used as a polarization state analyzer, gives good-quality images, but cannot provide dynamic information. In this paper, we propose to use a ferroelectric liquid-crystal modulator in an imaging polarimeter. We demonstrate the use of this modulator as a polarization state analyzer for frame rates up to about 400 Hz. Provided a fast camera is used, we show that the polarimeter allows us to surpass the classical video frame rate. We propose a setup using a rapid CCD camera (up to 233 Hz). Images in DOP are of very high quality and hold their own against images obtained with a classical polarizer. With a faster camera, we were able to obtain images at up to 400 Hz.