We have developed a wave-height sensor that can be used to measure sur
face height directly and simultaneously over a two-dimensional imaged
area of an air-liquid interface, thus permitting real-time two-dimensi
onal wave-height spectra to be obtained directly by fast Fourier trans
form. The absolute interface height (or surface-wave amplitude) is obt
ained directly from the measured volume attenuation of the transmitted
Light through the medium rather than by tedious reconstruction from s
urface 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-dime
nsional fast Fourier transforms. Line scans across the images indicate
the ready utility of the instrument to obtain surface-height informat
ion at any location on the imaged area. Applications of this diagnosti
c and design issues of a two-dimensional wave-height spectrum analyzer
are presented. (C) 1997 Optical Society of America.