The dependence of x‐ray or neutron specular reflectance on the scattering density has been linearized by modifying the Born approximation. This makes it possible to analyze the reflectivity curves by the Fourier transform method by using the box refinement technique. Thus, the phases of the scattered waves are iteratively obtained, by which the scattering density profile in layered systems can be directly evaluated. The validity of these modifications is demonstrated by some numerical examples. The box refinement technique requires fewer constraints to obtain the physically realistic scattering density than the least‐squares‐fitting method does. ©1996 American Institute of Physics.