Degree to which linearly polarized incident electromagnetic radiation becomes elliptically polarized in an absorbing optically active medium. $$\tan \psi =(E_{\mathrm{R}}-E_{\mathrm{L}})/(E_{\mathrm{R}}+E_{\mathrm{L}})=\tanh (k^{\mathrm{L}}-k^{\mathrm{R}})\pi l/\lambda$$ where $E_{\mathrm{R}}$ and $E_{\mathrm{L}}$ are the electric vectors of right and left circularly polarized radiation, $k^{\mathrm{L}}$ and $k^{\mathrm{R}}$ are the absorption indices of the sample for left and right circularly polarized radiation, $\lambda$ is the wavelength of the radiation and $l$ is the path-length through the medium.