Page 56 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

44 CHAPTER 2 The Ocean Environment
Ultraviolet Visible light
0.3 0.4 0.5 0.6 0.7 0.8 Wavelength, μm
Infrared
0.9 1.0
FIGURE 2.13
Light transparency through water (by wavelength).
Disregarding scattering (which will be considered below), within 1 m of the surface, fully 60% of the visible light energy is absorbed, leaving only 40% of original surface levels available for light- ing and photosynthesis. By the 10-m depth range, only 20% of the total energy remains from that of the surface. By 100 m, fully 99% of the light energy is absorbed, leaving only 1% visible light penetration—practically all in the blue/green regime (Duxbury and Alison, 1997). Beginning with the first meter of depth, artificial lighting becomes increasingly necessary to bring out the true color of objects of interest below the surface.
Why not put infrared (IR) cameras on ROVs? The answer is simple—the visible light spectrum penetration in water favors the use of optical systems (Figure 2.13). IR cameras can certainly be mounted on ROV vehicles, but the effective range of the sensor suffers significantly due to absorp- tion. The sensor may be effective at determining reflective characteristics, but the sensor would be required to be placed at an extremely close range, negating practically all benefits from non-optical IR reflectance.
2.2.17.2 Refraction
Light travels at a much slower speed through water, effectively bending (refracting) the light energy as it passes through the medium. This phenomenon is apparent not only with the surface interaction of the seawater, but also with the air/water interface of the ROV’s camera system.
2.2.17.3 Scattering
Light bounces off water molecules and suspended particles in the water (scattering), further degrad- ing the light transmission capability (in addition to absorption) by blocking the light path. The scat- tering agents (other than water molecules) are termed “suspended solids” (e.g., silt, single-cell organisms, salt molecules, etc.) and are measured in milligrams per liter on an absolute scale. Modern electronic instruments have been developed that allow real-time measurement of water tur- bidity from the ROV submersible or other underwater platform. The traditional physical measure of
Relative transparency
Violet Blue Green Yellow Orange Red