Relieving Visual Fatigue of Visual Display Terminal Operators

NON-INVASIVE METHOD TO RELIEVE VISUAL FATIGUE OF FREQUENT COMPUTER USERS

Vo Van Toi and B. Dumery
E-mail: [email protected]
Tufts University
Electrical Engineering and Computer Science Department
Biomedical Engineering Laboratory
Halligan Hall
Medford, MA 02155
Tel: (617) 628 5000 Ext. 5191
Fax: (617) 627 3220

Summary

The use of computers or devices equipped with video display terminals (VDTs) continues to gain in popularity. However, this has resulted in an increase in the number of complaints from visual fatigue associated with VDT use. Visual fatigue symptoms include dry eye, ocular muscle fatigue, blurred vision, and a heavy feeling in the eyes. The goal of this investigation is to develop a new therapeutic technique to relieve visual fatigue for computer operators.

I. Introduction

The use of computers or devices equipped with video display terminals (VDTs) such as Cathode Ray Tube (CRT) and Liquid Crystal Display (LCD) is rapidly increasing in such diverse areas as word processing systems, computer aided designs, and environments such as research, educational, medical and entertainment establishments. There has been an increasing number of complaints among VDT users ranging from visual discomfort and fatigue to neck pains and headaches (Ostberg, 1975; Smith, 1979). Results from an investigation involving 1,000 German office workers, a group of high-speed data acquisition operators, with 85% response indicated that, at least occasionally, their eyes were excessively strained at work (Cakir et al, 1978).

In the U.S. it is estimated that about 15 million VDTs were in use in 1988 and 70 million in 1990 (Foreman, 1988; Foreman, 1990). As a result, various recommendations have been proposed for the design of VDTs (color, shape, and position with respect to the operators) as well as the working environment. In 1990, San Francisco became the first city in the U.S. to pass an ordinance regulating the use of VDTs in the workplace. Part of the requirements correspond to the recommendations given above. An estimated 56,000 workers would be affected by the ordinance, and the San Francisco Board of Supervisors estimated that compliance with the ordinance costs about 80 million dollars to the businesses and city.

We propose here a new technique that is non-invasive to relieve visual fatigue for computer users who work with computers continuously for extended period of time. This study may lead to improvements in working conditions and job efficacy, and may also contribute to neurological studies to understand the mechanism of visual fatigue.

II. Visual fatigue

Due to the subjective nature of the symptoms related to visual fatigue, it is difficult to develop a clinical definition. Visual fatigue has been described in such terms as eye strain and asthenopia and covers a wide range of visual symptoms including burning, itching, tiredness, aching, and soreness of the eyes. The causes of visual fatigue are described in terms such as environmental; the visual status of the viewer; and constitutional. Environmental causes involve illumination, nature of visual task, and characteristics of the objects viewed. The visual status of the viewer refers to the prescription associated with the viewer’s vision. Constitutional causes involve both the physical health and emotional state of the viewer (Rinalducci, 1983). One of the main environmental effects of visual fatigue is dry eyes (Quaranta Leoni, 1994). Visual fatigue is related to stress placed on the accommodative faculties and the work required of extraocular muscles (Tsubota,1993). One of the suggested causes of visual fatigue is that VDT users blink much less than non-VDT users. Therefore, an appropriate method that is cost effective in relieving visual fatigue has long been sought and should be beneficial to the target population.

III. Dry eye symptoms

Dry eyes is the condition in which an insufficient amount of tear fluid is supplied to the surface of the cornea. This condition can be caused by a low blinking rate or by a disease called Keratoconjunctivitis Sicca. Tears keep the eyes moist, bring nutrients to the cornea, help ocular wounds heal, and protect against eye infections. The symptoms of dry eyes include a sensation of stinging, burning, and itching of the eyes. Vision may also be blurred. The standard test for dry eyes is the Schirmer test. This involves placing a strip of filter paper between the lower eyelid and the eyeball (conjunctival sac) for up to 5 minutes. The length of which the strip of paper absorbs moisture indicates tear production. Currently, there is no cure for dry eyes. To relieve pain, patients often need to administer artificial tears to keep the eyes moist and lubricated. The necessity of frequent tear application proves inconvenient for the patient and poor compliance on the patient's part results in an inefficient treatment of dry eye symptoms.

IV. Materials and Methods

It has been hypothesized that visual fatigue is caused either by reduction of the blinking rate or by dry eyes due to insufficient blinking. To test these hypotheses we proposed an investigation which consisted of:

Monitoring the blinking rate of VDT’s users before, during, and after each experimental series . For this purpose we developed a portable blink rate recorder which consists of an optical sensor that can be mounted on an eyeglass frame. The sensor is connected to an electronic circuit which continuously records the blinking rate. Subsequently, the data is read by a personal computer (Seigle, Dumery and Vo Van Toi, 1997). Unlike existing devices, ours is compact and self contained and does not interfere with a user’s activities and movements (Fig. 1).

Figure 1 : Subject wearing Blink Rate Recorder

Closely monitoring symptoms of visual fatigue using appropriate evaluation methods designed to be reliable, rapid, and not inclined to induce more visual fatigue for the subjects . Three methods were used: (1) measuring visual acuity, (2) examining data from rated questionnaires, and (3) establishing the flicker sensitivity of the subject’s visual system. Visual acuity was evaluated using standard charts. An appropriate questionnaire was devised to assess the comfort of the users. Flicker sensitivity was determined using a visual stimulator called a Papillometer (Fig. 2), described in detail in other publications (e.g., Vo Van Toi, 1978). The visual stimulator consists of a main box which generates a light stimulus, an eye piece for the subject to look through to observe the stimulus, and a remote control for varying stimulus parameters such as flicker frequency, sensitivity, etc.

Figure 2 : Visual Stimulator

Administering two therapeutic methods employing an eye dropper called the AMIE (Automatically Medicated Instrument for Eyedrops) (Fig. 3) . Normally, the AMIE is used to administer eyedrops into the patient’s eyes automatically or on demand (Vo Van Toi and Zhang, 1990). The AMIE requires no major effort or attention from the patient to administer eye drops. The device consists of a box containing a drug reservoir and a patented micropump, which transports medication through slender tubing to miniature nozzles (Vo Van Toi, 1992). The AMIE is battery powered and can be carried conveniently in the user’s pocket. Nozzles are mounted discreetly on the patient’s eyeglass frame (Vo Van Toi and Grounauer, 1992). Any low viscosity eyedrops currently on the market are compatible with the AMIE. The ejected amount of eyedrops, which is monitored by a single-chip microcomputer, is adjustable from 1m l to 5m l, and the ejection pace can range from 2 sec. to many hours. In the study presented here, the investigative therapeutic methods comprise: (a) using the AMIE which contains only air to stimulate the blinking rate, and (b) using the AMIE with a 0.9% saline solution to supply more moisture to the eyes.

Figure 3 : AMIE

Performing statistical analyses to infer: (a) the efficacy of an increased blinking rate, (b) the efficacy of supplemental liquid .

The investigations were conducted with the participation of a group of 6 subjects (2 females and 4 males, age: 26 ± 10 years old) who use computers frequently. They were not diagnosed with chronic dry eye syndromes or other eye diseases but have often experienced dry eye symptoms while working with computers. All but one were first-time subjects to our experiment. Each subject performed three tests conducted on different days: (1) the subject worked on the computer on a specific task for two consecutive hours, (2) the subject repeated test 1 while wearing the AMIE which ejected a jet of air to trigger supplemental blinking, or (3) the subject repeated test 1 while wearing the AMIE which delivered saline droplets. To prevent the air jet from causing the eyes to dry, only one eye received the jet which was aimed toward the nasal corner of the upper eye lid. In contrast, saline droplets were delivered to the caruncles the nasal corners of both eyes. Before and after each session, the subjects had their visual acuity tested and were asked to complete a questionnaire designed to reveal typical dry eye symptoms. Each question was rated from 1 to 5 (1:not at all; 5:extreme).

V. Results

We found that performing test 1 caused visual fatigue for all subjects. In particular, the blink rate is dramatically reduced (Fig. 4).

Figure 4 : Blink rate records of the same subject when performing a task without using VDT (left) and a task using VDT with particular visual attention (right).

We also found that the average blink rates of the subjects in test 1 was the lowest (about 8 bpm), in tests 2 about 13 bpm and in test 3 about 19 bpm (Fig. 5).

Figure 5 : The average blink rates of all subjects while performing a computer task (Baseline), while performing the same task in wearing the AMIE which generated air, and while performing the same task in wearing the AMIE which generated saline.

Further, in increasing the blinking rate alone the severity rating of visual fatigue was slightly reduced. In contrast, by periodically supplying saline droplets all visual fatigue symptoms are entirely eliminated (Fig. 6). The blink rates and the severity of the visual fatigue correlated well (r = -0.98) (Fig. 7).

Figure 6 : Performing the Baseline Test caused visual fatigue for all subjects. Periodically supplying saline droplets eliminated all visual fatigue.

Figure 7 : Increasing the blinking rate alone reduced the severity rating of visual fatigue. Blink rate and visual fatigue were correlated by -0.98.

In addition, in comparing the visual acuity before and after each test we found that it decreased in test 1, remained constant in test 2, and increased in test 3 (Fig. 8) and there is a good correlation between the visual acuity and visual fatigue (r = 0.98). In contrast, the changes of flicker sensitivity in all tests were not statistically significant.

Figure 8 : Comparing the visual acuity before and after each test we found that it was highly correlated to visual fatigue. Correlation = 0.98

VI. Conclusion

In the present investigation we found that using computers to perform tasks which require particular visual attention may cause a dramatic decrease of the blink rate. This occurred as soon as the task was started. After an experimental period of two consecutive hours, all subjects experienced various symptoms of visual fatigue. Results suggests that the blink rate is task dependent and highly correlates to visual fatigue. On the other hand, although visual fatigue can be efficiently quantified by the measurements of visual acuity and the responses on the questionnaire, it cannot be quantified by flicker sensitivity measurements. Further, it was unclear how visual fatigue progressed during the experimental period.

By artificially increasing the blink rate and by supplying saline solution to the subject's eyes while they perform computer tasks, we found that increasing the blink rate partially relieved the visual fatigue experienced by computer users. Furthermore, supplying saline drops periodically can effectively prevent visual fatigue from recurring. These results suggest that visual fatigue correlates highly to dry eye symptoms. In other words, using a computer continuously, especially when performing tasks requiring particular visual attention, the user does not blink often enough. A decrease of the blink rate causes dry eyes which, in turn, triggers visual fatigue. We also found that increasing the blink rate alone was not enough to relieve visual fatigue; it is necessary to keep the eyes moist by periodically supplying them with saline to prevent visual fatigue from happening.

Some investigators have reported that using VDT causes visual fatigue that in turn reduces blink rate. Our results do not support this statement. The question of whether reduction of blink rate is a cause or a consequence of visual fatigue needs further investigation.

VII. References

Cakir, A., Reuter, H., von Schmude, L., and Armbruster, A. (1978). Research into the effects of video display working places on the physical and psychological function of persons. Bonn, West Germany: Federal Ministry for Work and Social Order.

Foreman, J. (1988). VDTs put strain on county legislature. Arch Ophthalmol, 106, 1173.

Foreman, J. (1991). San Francisco passes ordinance regulating VDT use. Arch Ophthalmol, 109, 477.

Ostberg, O. (1975) CRTs pose health problems for operators. International Journal of Occupational Health and Safety, 44 (6), 24-26.

Quaranta Leoni, F.M., Molle, F., Scavino, G., Dickmann, A. (1994). Identification of the preferential gaze position thorugh elevation of visual fatigue in a selected group of VDU operators. Documenta Ophthalmologica, 87, 189-97.

Rinalducci, E.J., et. al. (1983). Video Displays, Work, and Vision. National Academy Press: Washington, D.C.

Siegel, A., Dumery, B., & Vo Van Toi (1997). Portable blink rate recorder. Investigative Ophthalmology & Visual Science, 38, 338.

Smith, W. J. A. (1979). A review of literature relating to visual fatigue. Proceeding of the Human Factors Society 23^rd Annual Meeting.

Tsubota, K., and Nakamori, K. (1993). Dry eyes and video display terminals [letter]. New England Journal of Medicine, 328, 584.

Vo Van Toi and Grounauer, P.A. (1978). Visual Stimulator. The Review of Scientific Instruments, 49, 1403-1406.

Vo Van Toi. (1982). Sensibilité de la Vision de L'oeil Humain au Papillotement et Certains Mécanismes de la Vision (Flicker Sensitivity of the Human Eye and Visual Mechanisms). Ph.D. thesis No. 449, EPFL, 263 pages.

Vo Van Toi and Zhang X.D. (1990). Eye Medication Delivery System. Investigative Ophthalmology and Visual Science (supp.), 31, 404.

Vo Van Toi and Grounauer, P.A.(1992) Automatic delivery of eye medication by droplet ejection. Investigative Ophthalmology and Visual Science, 33, 1012.

Vo Van Toi (1992). Eye medication delivery system, US patent No. 5, 171, 306.

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