Directional spectra of surface waves from photographs

Abstract

A method is developed in which the spectrum of ocean surface waves may be deduced from oblique photographs. The method consists of determining the change of surface brightness with variations of surface slope and measuring the diffraction pattern of the photographic negative. The diffraction pattern, obtained by a coherent optical processor, is then related to the two-dimensional wave number spectrum of the sea surface photographed. The conditions under which the optically derived spectrum is an accurate estimator of the ocean spectrum are discussed. The experimental method is applied to a nonstationary wave field exhibiting anomalously large spectral magnitudes. The two-dimensional spectrum of this wave field is displayed over a wavelength range of from 5 to 20 cm and an azimuth interval of about 90°.