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An optical model of sea-surface imaging based on a two-scale presentation of sea roughness is developed. Using this model, an exact expression for the sea-surface image spectrum in diffuse sky light is obtained, which made it possible to estimate the nonlinear contribution of waves of different scales to the image spectrum. In particular, it is shown that the image spectrum is proportional to the wave tilt spectrum and the coefficient before the tilt spectrum is determined by the modulation of the short-wave contrast by long waves. Accuracy of determination of spectral contrasts of waves in surface-roughness anomalies (film slicks, internal-wave field, etc.) and accuracy of measurement of two-dimensional wave spectra from the sea-surface image spectrum are estimated. Examples of sea-roughness variability in the internal-wave field and examples of two-dimensional sea-roughness spectra are presented. All data were obtained by the optical method under full-scale conditions.

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