Abstract
A particular video spot detector provides an irradiance time series from an arbitrarily chosen pixel in the video frame. The detector is applied to video records of the ocean surface, and the correspondence between the 1 -D irradiance frequency spectrum and the corresponding ocean surface elevation spectrum is studied. A major experimental result is that the frequency peak in the irradiance spectrum for a typical wind-driven sea is significantly enhanced compared to the surface slope spectrum. Video experiments from the oil rig Gullfaks A in the North Sea show an excellent agreement between the enhanced peak frequency in the irradiance spectrum and the peak frequency in the surface elevation spectrum measured by a microwave remote ocean surveillance wave radar. The enhancement of the peak in the irradiance spectrum is explained by a strong nonlinear geometrical projection effect. This is due to the rather small look angle with the horizontal, which is chosen so as to neglect irradiance contributions from beneath the sea surface. Based on a simple stochastic model that takes the geometric effect into account, irradiance spectra are simulated and are in good agreement with the experimental spectra.
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