We describe a method for the direct, absolute and noninvasive measurem
ent of surface height profiles on air-liquid interfaces. Wave amplitud
e spectra are directly obtained from electronically imaged surface hei
ght profiles without resorting to problematic reconstructions from wav
e slope measurements. This measurement principle relies on the volume
attenuation of light in the liquid rather than the refraction of light
at the wave-supporting interface. Surface height resolution of tens o
f micrometers has been demonstrated. A suitable photodiode detector an
d an appropriate laser provide the two parameters which are sufficient
to optimize system spatial and temporal resolution and spatial dynami
c range.