acuity is the ability to distinguish between two
distinctive points at a particular distance. This
requires the image formed on the retina to be
well circumscribed and distinct. If the image
focuses in front of or behind the retina, it will
strike the retina in an unfocused state, creating
the symptom of blur. This process is true for
all distances with the viewing range of the human
eye- which we routinely consider from within 20
feet to 16 inches.
Whenever we direct our gaze to some point within
20 feet, we must activate our focusing mechanism
to increase the focal power of the eye and regain
the clear image on the retina. The ability of
the eye to change its focal power is called accommodation
and our focusing ability is dependent upon age.
Therefore, we must consider many factors when
discussing the accommodative ability of the individual.
Blurred vision symptoms can result from refractive
error (e.g., farsighted, nearsighted, astigmatism),
improper prescription lenses, presbyopia (aging
changes) or other focusing disorders. It has been
found that small amounts of refractive error contribute
to the visual discomfort of computer users. Considering
the working environment, blurred images can also
arise from a dirty screen, poor viewing angle,
reflected glare or a poor quality or defective
monitor. All of these factors should be considered
when this symptom occurs.
A condition known as transient myopia
has been shown to be more prevalent in a population
of computer users. Research has found that 20%
of computer workers had a nearsighted tendency
toward the end of their work shift. Studies showing
permanent nearsighted changes have not shown this
to be a concern at this time. However, many of
those studies suffer from poor controls.
Glare is also a concern because of the eye attending
to the glare image rather than the screen image.
If a specular reflection is noticeable on the
screen, the eye will attempt to focus on it. The
image of the glare source will appear to be somewhere
behind the screen (much like your image is reflected
in a mirror) and the screen image can appear blurred.
This can become more noticeable as the usage time