Refraction, turbidity of the water, salinity, and pollution all contribute to the
distance, size, shape, and color perception of underwater objects. Divers must
understand the factors affecting underwater visual perception, and must realize
that distance perception is very likely to be inaccurate.
Light passing from an object
bends as it passes through the diver’s
faceplate and the air in his mask (Figure
2-5). This phenomenon is called refraction,
and occurs because light travels
faster in air than in water. Although the
refraction that occurs between the water
and the air in the diver’s face mask
produces undesirable perceptual inaccuracies,
air is essential for vision. When a
diver loses his face mask, his eyes are
immersed in water, which has about the
same refractive index as the eye. Consequently,
the light is not focused normally
and the diver’s vision is reduced to a level
that would be classified as legally blind
on the surface.
Refraction can make objects appear closer than they really are. A distant object
will appear to be approximately three-quarters of its actual distance. At greater
distances, the effects of refraction may be reversed, making objects appear farther
away than they actually are. Reduced brightness and contrast combine with refraction
to affect visual distance relationships.
Refraction can also affect perception of size and shape. Generally, underwater
objects appear to be about 30 percent larger than they actually are. Refraction
effects are greater for objects off to the side in the field of view. This distortion
interferes with hand-eye coordination, and explains why grasping objects underwater
is sometimes difficult for a diver. Experience and training can help a diver
learn to compensate for the misinterpretation of size, distance, and shape caused
by refraction.
Figure 2-5. Objects Underwater
Appear Closer.
|
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 moderately 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.
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.
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 background. 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 effecting the
filtering of colors. Salinity, turbidity, size of the particles suspended in the water,
and pollution all effect 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.
