We present in this paper some results of numerical investigation into two-dimensional and one-dimensional spectra of strong scintillations on atmospheric inhomogeneities generated by the internal gravity waves. The calculations are based on the model of statistically homogeneous phase screen. Conditions for applicability of the perturbation theory in calculating spectra of weak scintillations are formulated. It is shown that the perturbation theory works well in describing large-scale scintillations even if the scintillations are strong. It is shown that, depending on the product of one-dimensional spectral density and the wave number, a plateau is formed, whose level is only determined by the value of 0, provided that it does not exceed one third of the anisotropy factor squared. Conditions of normalization of a small-scale part of the spectrum have been found, under which the scintillation spectrum coincides with the spectrum of squared coherence function on the phase screen.