Bichromatic Synthetic Schlieren for surface measurement

Abstract

We introduce Bichromatic Synthetic Schlieren (BiCSS) for instantaneous surface measurements, in where we use two simultaneous images taken at blue and near infrared wavelengths, making use of that refractive index variation depends on wavelength, rather than using a reference/dynamic image pair to measure the surface gradient. Experiments on solitary and periodic waves were used to test the method, showing that new method can produce good results. Using this method it might be possible to extend the wave steepness range of Synthetic Schlieren surface measurements by overcoming the Invertibility Condition caused by light ray crossing, but due to compromises in design of real lenses, it might be difficult to achieve in practice. It might also serve as a first step towards a synthetic Schlieren method without a reference image, making it possible to use these methods in experiments where taking the reference image is difficult or impossible.