  We describe a method that uses total internal reflection at the water-air
interface inside a large, transparent tank, to measure the interface's
deflections. Using this configuration, we obtain an optical set-up where the
liquid surface acts as a deformable mirror. The set-up is shown to be extremely
sensitive to very small disturbances of the reflecting water surface, which are
detected by means of visualising the reflections of a reference pattern. When
the water surface is deformed, it reflects a distorted image of the reference
pattern, similar to a synthetic Schlieren set-up. The distortions of the
pattern are analysed using a suitable image correlation method. The
displacement fields thus obtained correlate to the local spatial gradients of
the water surface. The gradient fields are integrated in a least-squares sense
to obtain a full instantaneous reconstruction of the water surface. This method
is particularly useful when a solid object is placed just above water surface,
whose presence makes the liquid surface otherwise optically inaccessible.
