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

Abstract

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 × 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.

Spectral characterization of fine‐scale wind waves using shipboard optical polarimetry

“…Additionally, and importantly, the camera is robust enough to allow acquisition aboard a moving vessel, allowing great latitude in the scientist's directed observational efforts. Other methods of fine-scale wave observation, including the laser slope/elevation gauge setup described in Donelan et al [2010] or the slope-from-refraction technique used in J€ ahne and Riemer [1990] (or more recently, Kiefhaber et al [2014]) offer high spatial and temporal resolution in the lab, but are not suitable for the field environment.…”

On the two-dimensional structure of short gravity waves in a wind wave tank

“…More recently, advances in digital imaging and processing accelerated development of various methods that enable the 3D reconstruction of the water surface. [28][29][30][31][32][33][34] Some of the implementations of this approach allow simultaneous measurements of both the wave slope and the surface elevation. 35 All those methods require a careful arrangement of illumination and thus are used exclusively in laboratory over relatively small areas; their application in field experiments is impractical.…”

High resolution 2-D fluorescence imaging of the mass boundary layer thickness at free water surfaces

“…The tank is annular, with an inner diameter of 9 m and a water channel width of 0.6 m. The mean water depth is 1 m, the closed, gas tight air space has a height of 1.4 m. A high resolution, high-speed and low noise scientific CMOS camera (pco.edge by PCO AG, Kelheim, Germany) is mounted in the focal point of a large planoconvex BK7 lens (diameter d = 0.32 m, focal length f = 2 m). This lens and the 135 mm standard camera lens form an object-space telecentric lens [21]. This ensures a constant magnification factor even at changing water height due to waves.…”

“…In addition to this boundary layer visualization technique, the 3-D water surface shape and thermal structures can be measured simultaneously at the same footprint with an imaging slope gauge [21] and a thermal camera. This gives the unique possibility to visualize the horizontal structures of gas and heat transfer and to study their effect on the wave field simultaneously.…”