High-speed imaging of short wind waves by shape from refraction

This paper introduces the first high-speed system for slope imaging of wind-induced short water waves. The imaging slope gauge method is used, which is based on the shape from refraction principle. The downward looking camera with a telecentric lens observes the refraction of light rays coming from a high power custom telecentric LED light source that is placed underneath the wind wave facility. The light source can be programmed to arbitrary intensity gradients in the x- and y-direction, so that the origin of a light ray is coded in intensity. Four gradient images (acquired at 6000 fps) are combined for one 2D slope image. By only using intensity ratios, the measurements become independent of lens effects from the curved water surface and inhomogeneities in the light source. Independence of wave height is guaranteed by using telecentric illumination and telecentric imaging. The system is capable to measure the slopes of a wind-driven water surface in the Heidelberg Aeolotron wind-wave facility on a footprint of 200 x 160 mm with a spatial resolution of 0.22 mm and a temporal resolution of more than 1500 fps. For the first time, it is now possible to investigate the structure of short wind-induced waves with sufficient spatial and temporal resolution to study their dynamic characteristics without aliasing effects. Example images and a video of a 3D reconstructed water surface are shown to illustrate the principle