There are a variety of techniques to determine the quality of optical surfaces, which provide quantitative information of the deformation of the shape of the surface under test. This work proposes to use the deflectometry technique using a Hartmann screen to test a spherical surface with a local deformation. In order to perform the theoretical analysis, a model of experimental setup in which the input parameters: the position of the Hartmann type screen and the location for each of its holes, the distance of the observation plane and the positions of the reflected rays, are known. With this model, based on the diameter of the deformation and deviation of the incident and reflected rays in the observation plane, we determine the theoretical sensitivity of the technique proposed.