Theory for Anisotropy

It is known that fluorescing fluorophores behave like radiating dipoles. The intensity of light radiated from a 

Now assume that we are observing a collection of fluoto-phores which are oriented relative to the z axis with a prd ilityy[0). In the following section we wilt consider the fcmn ofy(0) expected forexcitation photoselection. The measured flu escence intensities for this collection of molecules are 

the anisotropy is detennined by tbe average vdue of cos 0. where 0 is tbe angle of tbe emission dipole relative to the z-axis. This is becmise tbe observed intensi ties 7i and/j. are prc Kiftional to tbe square of tbe projection of the iodividud traisition moments onto tbe x- and tbe Z axis gure 10.4). 

It is instructive to consider tbe relationdiip b weoi r and 0. For a single fiuoio di(Me ( iented d( g tbe z axis, with cottinear transitions. 0=0 and r= t.O. However it is not possible to obtain a perfectly oriented exdted-state population nutb c tical excitation of homogeneous solu tions. Hence, tbe anisotropies are less dian 1.0. 

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