  The spectrum of exponents of the transfer matrix provides the localization
lengths of Anderson's model for a particle in a lattice with disordered
potential. I show that a duality identity for determinants and Jensen's
identity for subharmonic functions, give a formula for the spectrum in terms of
eigenvalues of the Hamiltonian with non-Hermitian boundary conditions. The
formula is exact; it involves an average over a Bloch phase, rather than
disorder. A preliminary investigation of non-Hermitian spectra of Anderson's
model in D=1,2 and on the smallest exponent is presented.
