Visibility in water is limited by scattering. Indeed, there are many places in the world where a bather cannot see his own feet! Twenty feet in seawater could be regarded as the limit of practical seeing. Figure 12.1 gives the transmission of 10 m of seawater (from Smith and Baker 1981). Noting that most optical glasses show considerable absorption at wavelengths below 0.4 μm, a reasonable wavelength region for optimization would be 0.4–0.6 μm (this should also be modified by the illumination and detector system sensitivity). However, designs discussed in this section are for the normal visual region. The transmission of 10 m of seawater. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315222295/d6ecfdc7-e740-417e-bfc1-60d213f88f73/content/fig12_1.tif"/> (From Smith, R. and Baker, K., Applied Optics, 20, 177–184, 1981.)