Small scale fluctuations in refractive index can affect visibility and image quality in ocean optics. Such fluctuations are a result of temperature and salinity microstructure. Ocean mixing proceeds by the stirring together of dissimilar water types at finer and finer scales until diffusion creates a water type intermediate to the original components. Optically, the most important scale in the mixing cascade is microstructure because it consists of the highest gradient and smallest scale structures. Two classes of mixing process have been distinguished by shadowgraph images made in conjunction with profiles of temperature, salinity, and velocity shear. One class is diffusive and depends on the vertical distribution of temperature and salinity. The other class is turbulent and depends on velocity shear.