  We consider the Nambu-Goto bosonic string model as a description of the
physics of interfaces. By using the standard covariant quantization of the
bosonic string, we derive an exact expression for the partition function in
dependence of the geometry of the interface. Our expression, obtained by
operatorial methods, resums the loop expansion of the NG model in the "physical
gauge" computed perturbatively by functional integral methods in the
literature. Recently, very accurate Monte Carlo data for the interface free
energy in the 3d Ising model became avaliable. Our proposed expression compares
very well to the data for values of the area sufficiently large in terms of the
inverse string tension. This pattern is expected on theoretical grounds and
agrees with previous analyses of other observables in the Ising model.
