Maps on matrix algebras preserving idempotents

Let Mn be the algebra of all n×n complex matrices and Pn the set of all idempotents in Mn. Suppose φ:Mn→Mn is a surjective map satisfying A−λB Pn if and only if φ(A)−λφ(B) Pn, A,B Mn, . Then either φ is of the form φ(A)=TAT−1, A Mn, or φ is of the form φ(A)=TAtT−1, A Mn, where T Mn is a nonsingular matrix.