Recently De Haan et al. described the phase-object approximation for small angle neutron scattering. This method can be extended towards neutron reflectometry. Prelimenary measurements have been done at the HOR reactor of Delft University of Technology. A theoretical framework has been developed for this. To test this framework and to show an example of an application of it, it is proposed to perform (off-specular) reflectivity measurements on a surface silicon grating below, close to and above the critical edge.