Photoselected molecules hindered in their rotation (e.g., in lipid bilayers or liquid crystals) do not become randomly oriented even after long time periods. Thus, the emission anisotropy does not decay to zero but to a steady value, r ∞, called residual emission anisotropy. In the case of a single rotational correlation time, τ c or θ, the decay of emission anisotropy following δ-pulse excitation is given by: \[r\left(t\right)=(r_{0}- r_{\mathrm{\infty }})\ \exp (- \frac{t}{\tau _{\mathrm{c}}})+r_{\mathrm{\infty }}\] where r 0 is the fundamental emission anisotropy.