Infrared emissivity of seawater and foam at large incidence angles in the 3-14 μm wavelength range

The emissivity of the sea surface is an important parameter for infrared measurements at large incidence angles, and the increased radiance of foam from breaking wave crests, attributed to the higher emissivity of foam over foam-free water, has not been confirmed experimentally at angles above 65°. Here we report on outdoor laboratory experiments performed to confirm model predictions of a dramatic decrease in seawater emissivity at large incidence angles and to provide the first measurements of foam emissivity in this regime. A method is presented using Fourier transform infrared (FTIR) spectroscopy to measure the spectral emissivity of seawater and foam at incidence angles from 60° to 85° and wavelengths from 3.5-5.5 to 8-14 μm. The emissivity of water and foam are found to decrease dramatically for incidence angles above 65°, with the decrease being less dramatic for foam than for water. The emissivity of foam is found to be higher than that of water for all wavelengths and incidence angles above 65° where the difference is statistically significant. The difference between the emissivity of foam and the emissivity of water increases with incidence angle and reaches a maximum of 0.23 at 8.9 μm and 85° incidence. The significant difference between the emissivity of foam and water should be accounted for in infrared models of the sea surface at large incidence angles.