Abstract
The use of Video Display Terminals (VDT's) in modern
information processing environments is associated with a
variety of visual stress symptoms including eye strain,
headaches, blurry vision and neck/shoulder pain. This study
evaluates a method of prescribing computer specific glasses
to improve vision and reduce symptoms. Thirty-seven
computer users who suffer from VDT related visual
symptoms wore two pairs of prescription eyeglasses for
two weeks each in a double blind crossover experiment.
The experimental glasses were prescribed using a Gaussian
image testing device and the control glasses were prescribed
using a standard paper Snellen chart. Data was collected on
the frequency and severity of symptoms in their regular
work environment as well as in a controlled laboratory
setting. Results indicate that the experimental eyeglasses
were preferred overall (p<.05). Visual stress symptoms
were significantly reduced in frequency and severity for the
eyeglasses prescribed using the Gaussian image testing
device.
Keywords:
VDT, Eye strain, visual symptoms.
Introduction
In 1991, the National Institute of Occupational Safety &
Health reported that almost 60 million workers in the U.S.
suffer some visual stress symptoms while viewing VDT's
for three or more hours per day. Research comparing VDT
operators with workers performing similar tasks in a non-
VDT environment, found significantly increased symptoms
of visual discomfort and accommodative stress associated
with VDT use (Margach (1983), Ostberg (1985)).
This research evaluates a diagnostic tool for giving VDT
users corrective occupational eyeglasses which are designed
to reduce these visual stress symptoms. The technology is
based on the tendency of the human focusing system to
react differently to electronically generated characters than to
printed characters on a page. The brightness level of each
pixel is a bell shaped curve, bright in the center and dimmer
toward the edges (a Gaussian image). Our eyes have a very
hard time remaining focused on these images and loss of
accommodative accuracy and visual fatigue are the result
(Murch, 1982).
The diagnostic tool called the PRIOĻ System (Salibello,
1994) duplicates, in a doctor's office, the focusing demands
placed on a typical VDT user in their workplace. A backlit
panel is suspended from the reading rod of a typical
opthalmic chair. This device displays text which exactly
simulates the pixels on a VDT screen. The prescriptions
made using the PRIOĻ device are significantly different
than those made using the standard Snellen nearpoint acuity
card.
The current study compares the effectiveness of glasses
prescribed using the Gaussian PRIOĻ system vs. the paper
Snellen card method in reducing visual stress symptoms for
symptomatic computer users. Particular care is taken to
remove sources of experimenter and subject bias. Data is
collected from the subjects' normal working environment as
well as in a controlled laboratory setting to assess the
generalizability of the findings.
METHOD
Subjects
Thirty-seven, out of 44 subjects successfully completed the
study (24 women and 13 men). They were recruited
through an advertisement in a local newspaper. All of the
subjects used computers at least two hours daily in their
work and reported suffering from visual stress symptoms.
Subjects were selected from each of three age categories (20-
30 yrs., 35-45 yrs., and 50-65 yrs. old). These ages were
selected to look at the influence of the normal age-related
hardening of the lens and loss of accommodative ability.
All of the subjects wore corrective lens, either glasses or
contacts.
Procedure
The first step of the experiment was an eye exam by a
licensed optometrist. Two sets of single lens prescription
eyeglasses were made for each subject. The experimental
glasses were prescribed using the PRIO Gaussian Image
Testing Device and the control glasses were prescribed
using a standard Snellen chart.
For the week following the eye exam, the subjects were
asked to collect baseline data about their computer use and
frequency and severity of visual stress symptoms at work.
They were given a questionnaire to fill out at the end of
each working day. Subjects circled any symptoms that they
experienced during the day and rated the severity of the
symptom on a scale from 1 - 100. One was defined as
barely noticeable, and 100 as severe enough to force them
to stop working.
After the first week of wearing their normal eyewear and
recording their symptoms, subjects came back for a fitting
of their two new pairs of eyeglasses. The glasses were in
identical frames, selected by the subject. The subjects were
given one of the glasses to wear and a packet of daily
questionnaires for the next week.
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On the weekends, the subjects came to the laboratory to
complete a four hour battery of computer based information
processing tasks. The battery included both standard
laboratory tasks (e.g. signal detection, symbol coding) and
typical office tasks (e.g. text entry and editing). These
tasks were designed to cover a wide spectrum of visual,
cognitive and perceptual-motor skills. Along with
performance measures, subjects also filled out a symptom
questionnaire at the end of the experimental session.
At the completion of the study, subjects completed a final
questionnaire which asked them to specify the glasses they
liked best overall. They were also asked to guess which
glasses were the PRIOĻ glasses to ascertain if the study
was truly double blind.
RESULTS
Due to space limitations we report the overall preference
findings and the two most frequently reported visual stress
symptoms from the workplace and the laboratory. The
performance measures from the laboratory portion of the
experiment are not reported here except to say that not a
single task showed a significant difference between the two
eyeglasses. Performance was statistically equivalent for all
of the lab tasks.
Overall Preference
On the final questionnaire, 70% of the subjects preferred the
PRIOĻ prescribed glasses (c2 = 6.09; p<.02). The
PRIOĻ preference held true for all three age groups (c2 =
1.45; n.s.) and for both men and women (c2 = .43; n.s.).
These results were not due to subjects guessing which
glasses were which. Only 22 of the subjects correctly
identified the glasses (c2 = .42; n.s.).
Workplace Symptoms
The two most frequently reported symptoms at the
workplace were eye strain (78%) and headaches (64%). For
subjects reporting eye strain at the end of a week of wearing
the glasses, the severity was greater when wearing the
standard glasses (26.3 on the 100 point scale) than when
wearing the PRIOĻ glasses (18.2) (F = 3.97; p <.05).
Likewise, the severity of headaches was greater for the
standard glasses (22.1 vs. 13.4) (F = 2.39; p <.05).
Laboratory Symptoms
In the lab setting 95% of the subjects reported eye strain
and 57% reported headaches. The severity of eye strain was
not significantly different (31.5 for standard and 29 for
PRIOĻ). As in the workplace setting, the severity of
headaches was much greater with the standard glasses (29
vs. 17.4) (F = 5.56; p <.05).
The pattern of results for the other visual stress symptoms
was similar to the two reported here. For six of the seven
remaining symptoms, the subjects reported less severity
when wearing the PRIO glasses. Due to the lower
incidence of these symptoms, however, statistical
significance was not obtained.
DISCUSSION
Eyeglasses prescribed using the PRIOĻ System Gaussian
image tester are shown to be more effective at reducing
visual stress symptoms than eyeglasses prescribed using a
standard Snellen card. When computer users suffering from
visual stress symptoms are allowed to use both in their
work setting and in controlled experimental conditions they
prefer the PRIOĻ glasses by more than 2 to 1.
The two most common visual stress symptoms (eye strain,
and headache) are also significantly reduced when wearing
the PRIOĻ glasses. Similar patterns are found in both the
workplace and the laboratory for the self-reported severity of
symptoms. The advantage of the PRIOĻ glasses is not due
to subject bias.
Future research needs to be conducted to determine the
relationship between visual stress symptoms and
performance. No evidence was found for performance
differences for timed tasks conducted in a laboratory setting.
While it can be surmised that long term discomfort will
result in lower productivity, the current study does not
address this issue.
References
1. Margach, C. B. Prevention: Lessons from video display
installations. Journal of the American Optometric
Association, 1983; 4: 325-329.
2. Murch, G. M. How visible is your display? Electro-
Optical Systems Design, March 1982: 43-49.
3. Ostberg, O. CRT's pose health problems for operators.
Health & Safety, Nov/Dec, 1985.
4. Salibello, C. Comparing a printed image & a Gaussian
image diagnostic system for prescribing VDT eyewear.
Journal of Behavioral Optometry, 1994; 5(3), 59-62..
