Oh say, can you see?
Our eyes are simple
tools. They are designed to simply catch light and send it to the brain. However,
the method by which they gather, filter and guide the light, as well as how
our brain processes the information received by the eye, makes for the wonder
of vision. In our interaction with our environment, there is little to compare
with the contribution the eyes play. How we use our eyes and visual system dictates
how well we survive in our environment.
this process of eyesight and vision doesnt always work as well as it should.
This is especially true today since many of our viewing tasks are done at a
close working distance. This requires the eyes to maintain an active focusing
status, which can cause stress and strain on the eyes and the muscles that control
them. This can in turn lead to a deficiency in the way the eyes focus the light
so that the images are not clear. The conditions that exist when light doesnt
focus properly in the eye are called refractive errors.
The three types
of refractive errors are nearsightedness, farsightedness and astigmatism. Nearsightedness,
or myopia (my-OH-pee-ah) occurs when the light entering the eye focuses too
soon. Nearsightedness results when an eye is too long, when the cornea is too
steeply curved, when the eyes lens is unable to relax enough to provide accurate
distance vision, or from some combination of these and other factors. Farsightedness,
also called hyperopia (hi-per-OH-pee-ah), is not exactly the opposite of myopia.
For the hyperopic person, an object that is twenty feet or more away (so that
the internal lens is relaxed) is directed past the retina, so that it looks
blurred because it hasnt yet focused. Farsightedness results when an eye
is too short or the cornea too flat, or from some combination of these and other
factors. The main difference between these two conditions is that they eye can
increase its focal power (to some degree) to compensate for farsightedness,
where it cant reduce its power to compensate for nearsightedness.
the front surface of the eye (called the cornea) should be almost spherical
in shape, like the surface of a ball, so that when light passes through it,
it can be focused at a single point. However, nature isn't always perfect and
the cornea is often warped so that it more closely resembles a barrel
than a ball. The lens too can be irregular in shape. These distortions can be
significant enough so that the light that passes through the cornea and lens
in the vertical orientation will focus at a different spot from the light that
passes through in the horizontal orientation. Now you have two points of focus
with a blur between them. This is known as astigmatism (a-STIG-ma-tism).
For the most part,
glasses can correct or compensate for these conditions. Glasses
for myopia will weaken the light focusing power so that the light
will fall further back into the eye and strike the retina. Glasses for hyperopia
will increase the focusing power of the light and make it strike the retina
closer toward the front of the eye. For astigmatism, the glasses are ground
so that the two major directions of light have the power focused together and
at the same point. On a simple level, this is the way glasses allow us to see
There are several
types of lenses available that can correct for hyperopia, myopia or astigmatism
to enable us to see at different distances. First there are single vision (or
all purpose) glasses. These have just one focal power and are designed
to refocus the light to the proper point. Next there are bifocals, which are
basically a single vision lens with a small segment of a higher power imbedded
into the lower portion of the lens. This higher power allows the wearer to see
at a near distance without having to use the focusing lens within the eye. The
trifocal lens is an extension of this concept with two additional segments imbedded
in the lens- each being a different power so that near and intermediate viewing
targets can be seen.
A more recent advance
is that of progressive lenses (also mistakenly called no-line bifocals).
This lens has a different power in each spot of the lens that is viewed so that
all distances can be seen clearly. From the distance to the near viewing range,
there are almost an infinite number of focal points.
The above lenses
are considered general-purpose glasses because they are usually the only type
of lens a person wears. However, there are also task-specific lenses
that are prescribed for specific viewing requirements. One example of this is
sunglasses-lenses designed for bright viewing situations (darkly tinted). Another
task specific lens is the reading lens. These are worn for near-point viewing
tasks. If one attempts to look at a distant object while wearing these lenses,
everything will appear blurry.
can come in several designs as well. The option include single vision, bifocal,
trifocal and progressive lenses. One of the more common visual tasks today is
that of computer viewing, so there have been several types of computer-specific
lenses developed. So when someone says, I use computer glasses,
it doesnt say much about what type of lens the person is using.
A single vision
lens for computer use is just that- a single vision lens of a prescription that
will allow the person to see the computer screen clearly. However, other areas
of viewing might be blurred, either at a near point or distant point. If one
wears a task-specific bifocal lens, the upper portion, which is normally designed
for distance viewing, is set for the intermediate distance (monitor distance)
and the lower portion is for close work (paper reading). These can be effective
but are unsightly (with a visible line) and have a limited range of view.
One of the first
task-specific lenses designed specifically for computer use was called the CRT
lens. This is a trifocal design with lines that go all the way across the lens.
The very top portion of the lens is for distance viewing; the middle segment
for intermediate viewing, and the bottom for near-point reading. Although quite
effective, this lens design is very bulky and the divisions in the lens are
very obvious. When looking from one portion to the next, there is a significant
jump of the image.
The most recent
advancement in task-specific lenses is the progressive lens. Although the general-purpose
progressive lens rarely works for a full-time computer user (the intermediate
zone is too narrow), there are no-line types of lenses that work
very well. There are several designs for this purpose and most of them have
a wide viewing zone to allow you to see your entire workspace clearly. Your
eye doctor should be able to make suggestions as to which version might be the
most appropriate for your viewing tasks.
Although we are
spending more time viewing computers and using our eyes to their maximum ability,
it is still possible to be comfortable and productive. Wearing glasses for computer
work is just a specific task that requires a specific lens- one that just might
help save your eyesight.