We consider a radiation solution ψ for the Helmholtz equation in an exterior region in R^2 . We show that ψ in the exterior region is uniquely determined by its imaginary part Im(ψ) on an interval of a line L lying in the exterior region. This result has holographic prototype in the recent work [Nair, Novikov, arXiv:2408.08326]. Some other curves for measurements instead of the lines L are also considered. Applications to the Gelfand-Krein-Levitan inverse problem and passive imaging are also indicated.