Exact determination of the phase in timeresolved X-ray reflectometry

An exact solution of the phase retrieval problem is described as applied to in-situ X-ray reflectometry of a growing layered film. The following statement is proved: if the reflectivity R and the derivative dR/dt are known at the time t, then the real and the imaginary parts of the amplitude reflectivity r(t) are found uniquely at this point t.