
The
simple definition of a prism is a solid object that has the same
cross-section along the entire length of its shape. When you cut
straight across an object, the cut portion is called a cross-section.
Draw any shape using straight lines. Now use your imagination to see it
extend upward and that is a prism.
In
eyeglasses, prisms are used primarily for double vision, positional correction, or convergence correction.
Prism
eyeglasses are enough to correct these abnormalities if
the person has nearly normal vision in their level of nearsightedness
or farsightedness. However, if one of the eyes has greatly reduced
vision or blindness, there could be physical or neurological issues
causing the diplopia. This would then require surgery to repair the eye
muscles in addition to the use of the corrective prism lenses.
Standard eyeglasses
usually correct two types of eyesight errors, or a combination of
both: focal distance (near or farsightedness), or unusual curving of
the cornea (astigmatism and cylinder).
Prism lenses are necessary
to aid in getting the eyes to work together. One or both eyes tend to
pull up, down, to the left, or to the right. The use of prism in the
lenses tricks the brain into thinking the eyes are working together by
shifting the image only slightly up, down, left, or right.
Recently,
prisms have been helping people with hemianopia. This condition causes
blindness in half of the visual field in both eyes—either the left or
the right field. It is most common to brain injury and stroke victims,
and sometimes each eye is defective in exactly the same way. Other
times, each eye is affected differently. Less common is the lack of
sight in the upper or lower fields of vision.
Quadrantanopia is
the same condition, but one quarter of the eye is blind. The particular
field affected can be left, right, upper, or lower, and can also be the
same or different in each eye.
Since peripheral vision is most
often affected, prisms are ground into the lenses to show an image
outside the range of sight, simply by placing the prisms within the
viewer’s sighted area. This alerts them to something which would
normally not be seen and they can then turn to observe it more closely,
thereby avoiding running into people or objects simply from lack of
seeing them.
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