When conservation of number of particles holds true during scattering, it leads to a unitary condition for the scattering amplitude. In the general case, we have
Optical theorem follows from here by setting 
In the centrally symmetric field, the unitary condition becomes
where  and  are the angles between  and  and some direction . This condition puts a constraint on the allowed form for , i.e., the real and imaginary part of the scattering amplitude are not independent in this case. For example, if  in  is known (say, from the measurement of the cross section), then  can be determined such that  is uniquely determined within the alternative .