VISION TERMINOLOGY
Over 12 million Americans are visually impaired, but only about 10% of
those are fully blind. While the figure is overwhelming, it is worth
mentioning that the majority of non-blind people can experience at
least some vision improvement with treatment or visual aids which help
to bring their vision closer to average. Better technology continues
to offer more hope for those conditions previously untreatable.
Notice that the word average is used instead of the more accepted
term normal. In this document, normal vision is what each of us
has individually when all is well with our particular vision. Average is
the vision which a majority of people have during their normal periods.
It might sound trivial, but it is an important difference to consider.
Did you know that a vision rating of 20/20 only means that a person has
average eyesight? Yet some people brag about it like they are the
only ones who could achieve that lofty goal. The most common vision rating
system is the Snellen system, named after Hermann Snellen. The rating is
a combination of two factors; the distance from the eyes to a chart, and
the smallest size of the letters on the chart at that point where they can
be correctly identified.
The first number (usually 20/) is the distance (in feet) away from the
chart. The second number (/20) indicates the size of the type which can
be read on the chart. The letters on the chart are sized according to the
ability an average person would be able to read them at certain distances.
The 20 foot line is sized for reading from a distance of 20 feet away
(letters about .45 inches high). The 15 foot line is sized so the average
person can read it at 15 feet, etc.
A person with a 20/15 rating can read the 15 foot line from 20 feet away,
giving them better than average eyesight. A rating of 20/40 means the
person 20 feet from the chart can only read the line which should be
readable at 40 foot.
This rating system does not take into account such problems as nearsighted
or farsighted conditions nor does it measure many other very important
aspects of sight such as visual field or perceptions. All it does is
measure what a person can read from 20 feet away while looking straight
ahead. Even a person with a 20/20 (or better) rating can still experience
other visual limitations. While the rating system does have a useful
place in vision reports, it is far from the ultimate showing of perfect
vision.
Many doctors use additional charts or methods to measure vision ability
under different circumstances. However, the rating is almost always
expressed in the same format as described above.
Although the term blind is defined as meaning "without any
sight", the term legally blind is defined as "having a
vision rating of less than 20/200 in the better eye while using corrective
lenses (glasses or contacts). So the standard legal way of determining
sight limitation is based upon a incomplete vision rating system.
Fortunately, extremely reduced visual field also merits the legally blind
definition, covering persons having a visual field below 20 degrees.
The visual field is the area (measured in degrees, either horizontal
or vertical) of view a person can see without turning their head. The best
example of the degree measurements would be using the face of a clock. 180
degrees is the area from the 12 to the 6. 90 degrees is the area from the
12 to the 3. 30 degrees is the area from the 12 to the 1, so 20 degrees
would be two thirds of that distance.
Visual field includes both the central and peripheral vision. Central
vision is the area of attention, or what it is we are looking at.
Peripheral vision is the surrounding area that can be perceived but
is not being concentrated on. The eyes focus on the object(s) in the
central vision, but the brain can still detect and react to things
happening in the peripheral vision.
As this text is being read, the central vision will include the word being
read at any given time and the peripheral vision would be the other words,
the monitor (or paper) and whatever other objects are around. Reading can
be interrupted by any number of events occurring in the peripheral vision.
The visual field is determined by the characteristics of the eye and the
persons body. While the body does not normally impact central vision, it
can limit peripheral vision. Long hair in front of the face, the nose, and
eyelashes are the most common obstructions of the vision caused by the body.
Most of the impact on the visual field is due to the shape and quality of
the eye itself.
The term visual impairment refers to any visual condition that is of
less than expected quality. The term visual limitation involves any
visual condition that limits or restricts the view. For example, it is
possible to have a visual limitation which provides less than an average
visual field but still have good vision (not impaired) in the visual field
which is present. A visual impairment becomes a visual limitation if the
area affected provides less than a reasonable quality of vision.
People tend to think that most vision problems affect both eyes together
in that what impacts one automatically impacts the other. While some
vision problems are shared in both eyes (due to genetics, chemistry, or
injuries), most problems affect just one eye or affect each eye in
different ways. It is really not uncommon for a person to use visual aids
that have different prescriptions (properties) for the left and right eye.
Many people with minimal vision problems affecting just one eye often do
not realize it until a vision exam reveals it. This is because the brain
has two images (one from each eye) to work with and it usually compensates
for many minor problems in one eye with the information provided by the
other eye. In fact, these people often involuntarily rely on one eye for
much of their daily vision.
The two eyes work together not only to help cover each others problems,
but to provide for depth perception. Even though the angles are so
slightly different, there is enough difference for the brain to pick up on
and make us see images with depth. Visual depth problems can result from
differences between the images. Convergence errors can provide the
brain with two very different images to deal with. Refractive errors
can also unbalance the vision between the pair of eyes.
The eyes are hard at work from the moment we awake to the moment we fall
asleep. The eyes gather light from the world around us, convert it into
electrical impulses, and send those impulses to the brain where they are
interpreted into the images that we see. The two eyes gather images
independently, but the brain has the ability to control both eye positioning
together as well as merge the input from both eyes so that only one image
is perceived.
Light is reflected off of or absorbed by all of the objects in our world.
The more light that is reflected, the brighter the object appears to be.
Objects that absorb most of the light that impacts them appear to be very
dark or black. Different colors are created by different light rays in
the light spectrum being reflected or absorbed. Our eyes are
capable of processing a large part of the light spectrum but not all of it.
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