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.
Electron spin resonance uses rotational correlation time of spin probes to understand how much the membrane restricts the probe's movement. Fluorescence employs steady-state anisotropy and rotation correlation time to study the probe's interaction with its surroundings.