Relativistic f <-> f transitions in crystal fields

A relativistic model of f <-> f transitions based on the transformation of all intershell tensor operators to effective relativistic double tensor ope rators is developed. The transition amplitude is expressed in terms of effe ctive operators acting within the f(N) shell due to a partial closure perfo rmed upon the relativistic intershell operators. The final expression is di scussed in the light of its reduction to the non-relativistic case that des cribes f <-> f transitions in the language of the standard Judd-Ofelt theor y.