Editing Fluorescence (section)

=== Fluorescence anisotropy ===
Fluorophores are more likely to be excited by photons if the transition moment of the fluorophore is parallel to the electric vector of the photon.<ref name=Lakowicz-1999/>{{rp|style=ama|pp= 12–13}} The polarization of the emitted light will also depend on the transition moment. The transition moment is dependent on the physical orientation of the fluorophore molecule. For fluorophores in solution, the intensity and polarization of the emitted light is dependent on rotational diffusion. Therefore, anisotropy measurements can be used to investigate how freely a fluorescent molecule moves in a particular environment.

Fluorescence anisotropy can be defined quantitatively as
: <math>r = {I_\parallel - I_\perp \over I_\parallel + 2I_\perp}</math>
where <math>I_\parallel</math> is the emitted intensity parallel to the polarization of the excitation light and <math>I_\perp</math> is the emitted intensity perpendicular to the polarization of the excitation light.<ref name="Berberan-Santos, Mario-2012"/>

Anisotropy is independent of the intensity of the absorbed or emitted light, it is the property of the light, so photobleaching of the dye will not affect the anisotropy value as long as the signal is detectable.