It is desirable, in constructing an algorithm for real-time control or identification of free surfaces, to avoid representations of the surface requiring mesh refinement at corners or special logic for topological transitions. Level set methods provide a promising framework for such algorithms. In this paper we present: 1) A mathematical representation of free surface motion that is particularly well-suited to real-time implementation; 2) a technique for estimating an isotropic and homogeneous normal velocity based on a simple measurement; and 3) an application to a semiconductor etching problem.