A new instrument for the optical measurement of the fine structure of the water surface in the field

The measurement of the fine structure of the ocean surface constitutes a difficult task. Here, we describe the technical details of a new instrument that allows the full reconstruction of the water surface with sub-millimeter spatial resolution in areas of up to of 15cm x 20cm The single camera system is based on the imaging slope gauge used in previous laboratory experiments to obtain a single slope component of the water surface. The light source was modified such that two arrays of light emitting diodes (LEDs) are used to produce intensity gradients at right anglesJn order to obtain both slope components of the water surface with a monochrome CCD camera, the LED arrays are illuminated successively within a few milliseconds. The whole system is mounted on a the frame of a buoy which follows the orbital motion of the long waves and is critically damped to prevent instability in rougher conditions. Even at high wind speeds, the ambient wave field is not disturbed by the structure. The onboard computer for data acquisition and image processing is remotely controlled from a ship. Battery power allows free-floating operation for a duration of approximately eight hours. In addition to the optical wave slope gauge, the instrument contains a digital gyro sensor which collects stabilized pitch, roll, and magnetic azimuth data, a Young wind speed anemometer, as well as global positioning and temperature sensors.