 Astronomical polarimetry observations are carried out either as imaging polarimetry, where polarization is measured as a function of position in imaging data, or spectropolarimetry, where polarization is measured as a function of wavelength of light, or broad-band aperture polarimetry.Optically active samples, such as solutions of chiral molecules, often exhibit circular birefringence.Circular birefringence causes rotation of the polarization of plane polarized light as it passes through the sample.