Optics Near the Snell Angle in a Water-to-Air Change of Medium

Underwater imaging of the air environment is described with particular attention given to the optical problems of near- horizon viewing. These include severe chromatic aberration from transmission through the seawater wedge, the horizon distortion effect, and anisoplanatism resulting from the change of refractive aberrations in different areas of view. Various optical designs or schemes are presented that correct or relieve the first two optical problems. Strategies to minimize or attempt to view through the varying refraction of the surface are also suggested, and one is investigated.