Cited Passage
2-6 U.S. Navy Diving Manual — Volume 1
2-6.2 Turbidity of Water. Water turbidity can also profoundly influence underwater
vision and distance perception. The more turbid the water, the shorter the distance
at which the reversal from underestimation to overestimation occurs. For example,
in highly turbid water, the distance of objects at 3 or 4 feet may be overestimated;
in moder ately turbid water, the change might occur at 20 to 25 feet and in very
clear water, objects as far away as 50 to 70 feet might appear closer than they
actually are. Generally speaking, the closer the object, the more it will appear to be
too close, and the more turbid the water, the greater the tendency to see it as too far
away.
2-6.3 Diffusion. Light scattering is intensified underwater. Light rays are diffused
and scattered by the water molecules and particulate matter. At times diffusion
is helpful because it scatters light into areas that otherwise would be in shadow
or have no illumination. Normally, however, diffusion interferes with vision and
underwater photography because the backscatter reduces the contrast between an
object and its background. The loss of contrast is the major reason why vision
underwater is so much more restricted than it is in air. Similar degrees of scattering
occur in air only in unusual conditions such as heavy fog or smoke.
2-6.4 Color Visibility. Object size and distance are not the only characteristics distorted
underwater. A variety of factors may combine to alter a diver’s color perception.
Painting objects different colors is an obvious means of changing their visibility
by enhancing their contrast with the surroundings, or by camouflaging them to
merge with the back ground. Determining the most and least visible colors is much
more complicated underwater than in air.
Colors are filtered out of light as it enters the water and travels to depth. Red light
is filtered out at relatively shallow depths. Orange is filtered out next, followed
by yellow, green, and then blue. Water depth is not the only factor affecting the
filtering of colors. Salinity, turbidity, size of the particles suspended in the water,
and pollution all affect the color-filtering properties of water. Color changes vary
from one body of water to another, and become more pronounced as the amount of
water between the observer and the object increases.
The components of any underwater scene, such as weeds, rocks, and encrusting
animals, generally appear to be the same color as the depth or viewing range
increases. Objects become distinguishable only by differences in brightness and
not color. Contrast becomes the most important factor in visibility; even very large
objects may be undetectable if their brightness is similar to that of the background.
2-7 MECHANICAL ENERGY IN DIVING
Mechanical energy mostly affects divers in the form of sound. Sound is a periodic
motion or pressure change transmitted through a gas, a liquid, or a solid. Because
liquid is denser than gas, more energy is required to disturb its equilibrium. Once
this disturbance takes place, sound travels farther and faster in the denser medium.
Several aspects of sound underwater are of interest to the working diver.
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