Double pattern improves the Schlieren methods for measuring liquid–air interface topography

The Schlieren method intends to reveal the elevation of a refractive fluid–fluid interface. The method is based on a comparison of images of a single pattern placed at the bottom of the container. Accurate measurements can be obtained with a simple and low-cost optical setup. However, it is restricted to weak interface deformations, weak slopes and weak paraxial angles. To overcome these limitations, we propose an enhanced optical setup that uses a bitelecentric objective and a double pattern. Thanks to this new setup, we avoid geometrical approximations and we extend the method to moderate/large deformations. Moreover, the proposed method does not depend on the liquid depth and could be used in various applications.