Page 290 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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Refraction
10.9 Underwater optics and visibility 279
FIGURE 10.32
Light refraction in water.
10.9.1 Focus error
Rays diverging from a point at a distance from the window appear, after they have passed through
the window, to come from a shorter distance.
10.9.2 FOV errors
A ray coming to the window at any angle of incidence other than zero will leave it at a wider angle. Thus, an optical system such as a camera, which has a given angular FOV in air, will have a small angular field in water. In addition, an object, which would produce an image of a certain size if it were in air, produces a larger image if it is in water. The lens appears to have a longer focal length, approximately 4:3, than that in air. This is why subjects appear magnified when using a div- ing mask underwater.
10.9.3 Distortion
Rays from points forming a rectangular grid in water will not seem to come from a rectangular grid after they have passed into air. There will be distortion in a lens that is not corrected for underwater use. Several types of distortion are possible due to the airwater interface distorting the image unevenly across the entire FOV. Nature took this into account by designing the human eye to ren- der light through the lens of our camera (the eyeball) evenly with the capture device (the retina) located on a curved surface. Unfortunately, both the camera lens (at the airwater interface as well as on the actual camera) and the capture device (the CCD or CMOS sensor) are typically both flat (and sometimes distorted due to water pressure), introducing a host of image aberrations. As depicted in Figure 10.33, various possible distortions are likely based upon the lens/capture config- uration. Careful consideration should be made while selecting both the housing lens and the FOV for proper matching of lens/camera optical characteristics.
About 4% of the natural light striking the surface at a normal angle of incidence is reflected away. The rest is quickly attenuated by a combination of scattering and absorption (discussed below).
