In electron spin resonance, rotational correlation time of spin probes is used to characterize how much restriction is imposed on the probe by the membrane. In fluorescence, steady-state anisotropy of the probe can be used, in addition to the rotation correlation time of the fluorescent probe.
In electron spin resonance, rotational correlation time of spin probes characterizes the level of constraint imposed by the membrane. In fluorescence, steady-state anisotropy and rotation correlation time are used to assess the probe's interaction with its environment.