Combined optical slope/height measurements of short wind waves: principles and calibration

A novel short wave imaging technique is described. For the first time, it is capable of measuring the wave height and wave slope simultaneously with unprecedented accuracy. A telecentric optical system is used to image the waves so that the image magnification does not change with the wave height and the slope calibration is much less dependent on the position of the image. A telecentric illumination system contains an area-extended LED light source that is placed in the focal plane of a second lens below the water channel. In this way, the wave slope can be coded by the position-dependent intensity of the light source. LEDs with two different wavelengths in the red and near infrared part of the spectrum are used. Because the water column absorbs the two wavelengths differently, the difference in the observed intensities gives the wave height. The paper details the principle of the technique and the calibration procedures.