Imaging surface-wave analyzer for liquid surfaces

Abstract

We have developed a wave-height sensor that can be used to measure surface height directly and simultaneously over a two-dimensional imaged area of an air–liquid interface, thus permitting real-time two-dimensional wave-height spectra to be obtained directly by fast Fourier transform. The absolute interface height (or surface-wave amplitude) is obtained directly from the measured volume attenuation of the transmitted light through the medium rather than by tedious reconstruction from surface slope data obtained by light refraction or reflection methods. A surface-height resolution of tens of micrometers has been achieved. For this demonstration, two-dimensional gray-scale wave-height images of cylindrical surface waves are presented with corresponding two-dimensional fast Fourier transforms. Line scans across the images indicate the ready utility of the instrument to obtain surface-height information at any location on the imaged area. Applications of this diagnostic and design issues of a two-dimensional wave-height spectrum analyzer are presented.