LIGHT/DARK-ADAPTING EYEWEAR FOR PERSONS WITH LOW VISION
David A. Ross, M.S.E.E., M.Ed. Gary L. Mancil, O.D., F.A.A.O.
Atlanta VA Rehabilitation Research and Development Center Decatur, Georgia 30033
Abstract
The purpose of this project is to research, develop and evaluate eyewear that dynamically and "instantly" darkens/lightens to control the amount of light reaching the wearer's eyes. Final engineering prototypes will be evaluated by 104 subjects in a side-by-side comparison with each subject's preferred sunwear. Both clinical trials and outdoor mobility trials will be employed. The results of these trials (differences in acuity and contrast sensitivity under glare conditions, differences in walking pace and obstacle avoidance, and differences in subjects comments) will be evaluated relative to each of four low vision populations, along with recommendations for optimal device construction and manufacture. This will be presented in major conferences and distributed to potential manufacturers. An initial retail price of about $200 is expected.
Background
Persons with ocular diseases such as retinitis pigmen-tosa (RP), albinism, aniridia and achromatopsia have extreme problems with varying light conditions and can usually function effectively only under controlled lighting conditions. Other ocular diseases such as macular degeneration and conditions affecting the ocular media (e.g., cataracts, corneal dystro-phy) have varying effects on retinal adaptation. Dark adaptation times as long as 30 minutes are not unusual [1].
For all persons (fully sighted as well as persons with low vision), a fairly narrow range of overall illumination is optimal. Too much or too little light results in dramatic reductions of visual acuity and a corresponding reduction of visual function [2]. However, the unimpaired person has a type of visual reserve (i.e., more acuity, more field of view, more contrast sensitivity, more light/dark adaptation, etc. than the minimum required by the visual task) [3] that gives this person the ability to maintain functional performance in less than optimal conditions.
A fully sighted person exiting a darkened theater into bright sunlight may squint, shade the eyes, look down, etc. until the eyes have adequately adapted (only a few minutes); however, this person can continue to function while adapting to the sudden change in lighting and will be able to avoid obstacles and negotiate curbs and stairs. But the low vision traveler, who does not possess this reserve, cannot function under these circumstances.
The low vision traveler experiences two primary functional vision problems: detection of changes in terrain (such as curbs), and adapting to changing lighting conditions [4]. In a recent national survey, low vision consumers and their mobility instructors rank-ordered their most serious orientation and mobility problems. "Changing environmental lighting conditions"was considered the most difficult mobility situation by both consumers and instructors [4]. "Drop offs," down curbs and steps, were reported as second most difficult. Persons with low vision were also found to confuse shadows of buildings and mail- boxes with curbs and potholes. In terms of functional mobility, this confusion resulted in reduced travel speed and gait changes based on reactions to shadows [5].
Restricting the amount of light reaching the low vision person's eyes to a set upper limit might, in and of itself, provide a sufficient increase in acuity and contrast sensitivity to distinguish between a shadow and a "drop-off."However, while "static"sun lenses or individually prescribed illumination- restriction devices may alleviate some of this difficulty, this type of eyewear does not eliminate the variability in visual functioning brought on by ever-changing [rapidly changing] lighting conditions" [4].
Light-absorbing lenses are available and prescribed in a variety of styles, colors and levels of light transmission. But in order to adapt to a variety of conditions (bright sun, cloudy, indoor bright, indoor dim, fluorescent or incandescent lighting) it is necessary for low vision consumers to find a variety of absorptive lenses and illumination controls, and to change back and forth among them-a rather cumbersome task. Photo-darkening lens coatings have been available and do provide a degree of accommodation to changing lighting conditions. However, the photo-chemical processes currently employed do not adapt "instantly" to changing lighting conditions, especially when going from bright sunlight into shadow-a particularly hazardous situation for persons with low vision. Further, the wearer cannot control this photo-process to effect a light level that best suits his/her individual needs.
The concept of "ideal retinal illumination level" emerges from the physical property of optimum stimulus threshold for the eye, which is dependent upon a variety of factors (such as ocular health status, age, and task). Therefore, the development of eyewear that dynamically and "instantly" adjusts to restrict the amount of light reaching the eye to a brightness close to the "ideal retinal illumination level"should significantly contribute to visual functioning.
Research Questions
Two research questions are to be answered by this research:
1. If dark-adapting sunwear were available, under "typical" indoor and outdoor ambient lighting conditions, what light to dark transmission ranges would optimize the visual performance of the following low vision populations: (a) persons with central vision loss from age-related macular degeneration, (b) persons with cloudy ocular media and (c) persons with rod/cone dystrophy; where visual performance is defined in terms of (i) acuity and contrast sensitivity measured under both glare and non-glare conditions, (ii) outdoor mobility performance, and (iii) subject comments?
2. To what degree can existing technologies be employed to produce adaptive eyewear that effects these light to dark transmission ranges (a) in less than 300 milliseconds, (b) for prescription plastic eyewear, (c) without increasing weight placed on the nose by more than one ounce, and (d) at a reasonable cost (less than $250); and, does this significantly increase the functional vision of the above populations?
Methodology
This research has and is being conducted in six stages:
1. An initial evaluation of available light-control technology materials/ techniques has been performed.
2. Two initial prototypes were designed and constructed employing the most promising of the available materials at that time. To meet the weight requirements and minimize the cost of these initial prototypes, the electronics and batteries were placed in a small box that clipped to the wearer's belt.
3. The two initial prototypes were compared to the preferred sunwear of 22 subjects in (a) a battery of clinical vision tests (acuity and contrast sensitivity under glare and non-glare conditions), and (b) outdoor functional mobility trials. The purpose of these tests was to informally begin to answer the two research questions. Subject observations and comments were solicited and recorded.
4. The informal test results (including subject observations and comments) were analyzed and sorted by disability category and frequency. Design of the final prototypes is based on these results.
5. The final prototypes will be evaluated by a population of 104 subjects in a comparison test among each subject's preferred sunwear and each of the two designed prototypes . The subjects range in age from 55 to 75 years and are comprised of four sub- groups: (a) 26 "normal" subjects, (b) 26 subjects with central vision loss from age-related macular degeneration, (c) 26 subjects with cloudy ocular media, and (d) 26 subjects with rod/cone dystrophy.
6. Results will be analyzed relative to each subject grouping. Qualitative elements of the above tests will be descriptively presented through measures of central tendency and dispersion. Tabulation and graphic representations will be employed when appropriate. The objective data will be evaluated with multiple paired t-tests or repeated measures ANOVA to identify main effects and interactions of condition by device. Subjective data will form the basis for spe-cific case studies that will complement the objective findings by identifying specific examples of advan-tages and disadvantages of the eyewear. The 104 subjects employed for the final testing will provide more than adequate power (>.99) to differentiate among factors, and will give adequate power (>.85) for differentiating among the four sub-populations.
Significant results will be published and presented at low vision conferences. Design recommendations along with population information will be provided to interested manufacturers.
Results
Two technologies were initially investigated for light control: Liquid Crystal (LC) and Electro-Chromic (EC). However, the first prototypes employed only LC technology, because EC technology at that time could not darken below 15% transmission Ñ our initial goal was 1% transmission. As of this writing, though, new EC materials have been developed that are much more competitive. Two final prototypes are thus being developed at this time for final subject testing: one employing LC technology and one employing EC technology.
Initial subject testing yielded two important results. First, from subject responses it became evident that the equipment employed to measure prototype light transmission ranges in the laboratory was not adequately indicating percent transmission as perceived by the user. This lead investigators to analyze spectral issues and angular field of view issues. As a result, a means of measuring the amount of perceived light at the eye was developed and is now employed to evaluate prototypes.
Second, subjects complained that when going out of doors, the prototypes darkened immediately and completely, and then did not respond to further increases in ambient light intensity. The investigators discovered that this phenomenon was caused by the controlling electronic circuit. This circuit was designed to maintain a constant level of light at the user's eyes. Most subjects preferred a light level at their eyes in the range of 6 to 12 relative units when engaged in indoor testing. On a bright day, outdoor light levels can exceed 1000 units. When the circuit was adjusted to admit no more than 10 units of light, because the lenses can darken only to 5% transmission, they will darken completely and immediately on going outdoors whenever the light intensity is above 200 units, and thus any additional brightness will not be mediated. The investigators thus changed their original goal of maintaining a constant level of light at the wearer's eyes. The modified goal stated that the desired eyewear would restrict the intensity of light at the wearer's eyes to a specific range of approximately 10 to 40 units, and that changes in light intensity at the wearer's eyes would always represent much larger changes in ambient light intensity.
DISCUSSION
The planned final LC prototype (currently being constructed) employs a nematic LC material with a guest-host dye added to it. This guest-host dye, when aligned in rows by the nematic LC molecules, acts as a polarizer. When a voltage is applied the molecules are pulled out of alignment, and polarization is lost. A "flip-down"polarizer is employed with these lenses that will effect an overall light to dark range of 72% to 3.7% transmission in two sub-ranges. First, with the polarizer "flipped up," an "indoor" range of 72% to 38% will be effected. Second, with the polarizer "flipped down,Ó"an outdoor range of 26% to 3.7% will be obtained. Tests of this neutral gray LC material shows that it darkens very quickly (12 milliseconds), produces no visible optical distortions, and reduces ultraviolet radiation by almost two orders of magnitude.
An EC prototype is also to be constructed. The characteristics of the EC material to be employed have not been tested by the investigators as of this writing. The company devising these materials claims they can achieve a light to dark range of approximately 80% to 3% and a darkening time of 1 second or less. Optical clarity is also an issue that must be resolved by the investigators.
The control circuit for the final prototypes will be modified to implement the new goal stated above. The new circuit will allow the amount of light reaching the wearers eyes to increase slowly with the actual light increase, but in much smaller increments. Outdoors, as light varies in a range from 50 lumens to 1000 lumens (a 20 times increase in intensity), the wearer will perceive a change from 10 to 35 lumens (merely a 3.5 times increase in intensity).
Final subject testing is expected to commence in mid May, 1996.
References
1. Fraser, K.E. (1992) Training the low vision patient. Problems in Optometry. Philadelphia: J.B. Lippincott Co. Vol.4, No.1, pp. 72-87.
2. Luria, S.M. Vision with Chromatic Filters. American Journal of Optometry and Archives of American Academy of Optometry, 49(10), 1972m 818-829.
3. Whittaker, S., Lovie-Kitchin, J. (l993). The Visual Requirements for Reading. Journal of Optometry and Visual Science, 70, pp.54-65.
4. Smith, A.J.; De l'Aune, W.; Geruschat, D.R. (1992). Low Vision Mobility Problems: Perceptions of O&M Specialists and Persons with Low Vision. Journal of Visual Impairment & Blindness, 86 (1) 58-62.
5. Barber, A. (l985). The Effects of Low Vision Aids and Traditional versus Non-Traditional methods in the Mobility Performance and Stress Levels of Low Vision Individuals. Final Report: The Orientation and Mobility of Low Vision Pedestrians. Philadelphia: Pennsylvania College of Optometry.
ACKNOWLEDGEMENTS
Funding for this project was provided by the Department of Veterans Affairs, Rehabilitation Research and Development Service.
The Atlanta VA Rehabilitation R& D Center is part of the Atlanta VA Medical Center Research Service, which provided laboratory space and equipment, administrative support and staff resources for this research.
David A. Ross Senior Biomedical Engineer Rehabilitation R&D Center Atlanta VA Medical Center 1670 Clairmont Road (MS151-R) Decatur, GA 30033