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Elaine Kitchel, M.Ed., Research Scientist

The American Printing House for the Blind

For years it has been known that persons with visual impairments need three times as much light, in general, to do the same task as a person with normal vision.  Some research was done between 1923 and 1965 by the United States Post Office to document that difference.  Since that time many new types of lighting have been developed for private and commercial use. Some of those new developments have fortunate applications for persons with low vision.

First, one ought to know how much light is needed by the visually-impaired individual.  There are a few exceptions but as a general rule, where a 50 watt bulb will do for a person with normal vision, a person with low-vision will need 150 watts.  That rule of thumb can generally be applied to all except persons suffering from retinitis pigmentosa, albinism, achromatopsia and photophobia.

Old research studied light in footcandles.  A footcandle is the amount of light at one foot from a one candela lamp.  This is equal to one lumen/foot of light today.  Light today is spoken about in lumens.  A lumen is the amount of light energy per second radiated from a one candela source and falling on a one foot square area at a distance of one foot from the source. You can probably see why it is easier to talk about light in terms of wattages.

In layman's terms, a person with normal vision can function quite nicely in a 12 x l2 foot room lit by 2, 40-watt fluorescent tubes. It is quite a different story for a person with low vision.  According to the formula, this person would need 6, 40-watt fluorescent tubes to light the same area.  However it is not that easy.  Persons with eye pathology are especially sensitive to the type of light emitted by regular cool-white fluorescent tubes. 

Blue light wavelengths and part of the blue spectrum are focused in front of the retina, while green and yellow are focused on the retina, and some red spectrum is focused behind.  Thus blue light contributes little to visual acuity and visual perception loses sharpness as the blue light component adds significantly to the eye's energy expenditure for focusing, and in reduced can greatly reduce eyestrain without loss of acuity.

There is mounting medical evidence that prolonged exposure to blue light may permanently damage the eyes, contribute to the formation of cataracts and to the destruction of cells in the center of the retina." (Quinn, 1998)

In spite of the problems with blue light, this is the type of tube most commonly found in schools, stores and nearly all public places.  For many persons with low vision, this is problematic. Not only do they need brighter light, they need a different kind of light.  Fortunately special tubes which do not emit the ultraviolet and blue end-spectrum light which is so plentiful in cool white tubes, are available to replace regular cool white fluorescent tubes use by so many businesses and public places. These are called SPX30 (General Electric)  or SP30 (Sylvania and Phillips) tubes.  These can make a significant difference for those persons with low vision by reducing photostress and discomfort.

For persons with low vision then, brightness and type of light are important.  Additionally, the directionality and the diffusion of the light are also important. These can be regulated by dimmers, diffusers and light filters. The peracube, a silver egg-crate type of grid which replaced the acrylic lenses on many fluorescent tubes, has made a positive difference for many persons with visual impairment.

 Much must be taken into account when designing a room or workspace for use by persons who are light sensitive. (Migraine sufferers, persons with multiple sclerosis, lupus and epilepsy often fall into this category as well as persons with ocular conditions.) For example, access to natural light is a consideration. Persons with macular degeneration often benefit from strong natural light, while those with retinitis pigmentosa perform better in a dim environment with the only light falling on the task at hand.

 The color and reflecting qualities of the walls are another important consideration.  Many institutions like to paint walls a glossy white or blue.  But studies show that most persons who are sensitive see their best when they have walls with a non-reflective finish of a warm, pinkish hue. This is because light from the blue end of the spectrum, also called short wavelength light, becomes focused in front of the retina instead of upon it.  This causes the eye to work much harder. 

For persons with normal vision and optimal ocular health, a hard-working eye is not a problem.  But for people whose eyes are already compromised by disease or injury, whenever their eyes are worked hard as they do under the short wavelength blue light, such as that emitted by cool white tubes, the eyes cannot carry away the products of photoreception fast enough to keep up.  This build-up of waste products in the eye is often interpreted by the brain as glare, pain, or light blindness.

Care then, should always be taken, whenever possible, to promote optimum visual performance in the person with low vision, by providing lighting without ultraviolet and blue wavelengths, and a visual environment which has been carefully selected to meet his needs.

References

Creech, L. L., & Mayer, J. A. (1997).  Ultraviolet radiation exposure in children: a review of measurement strategies.  Annals of Behavioral Medicine, 19(4), 399-407.

Fedorovich, I. B., Zak, P. P., & Ostrovskii, M. A. (1994).  Enhanced transmission of UV light by human eye lens in early childhood and age-related yellowing of the lens.  Doklady Biological Sciences, 336(1), 204-206

Gorgels, T. G., & van Norren, D.  (1995).  Ultraviolet and green light cause different types of damage in rat retina.  Investigative ophthalmology & visual science, 36(5), 851-863.

Hall-Lueck, A. (1986, May).  Facts and fads: what works and what doesn't.  Paper presented at the National Forum on Critical Issues in Infant and Preschool  Education of Blind and Visually Impaired Children, American Foundation for the Blind.

Ham, W. T., Jr. (1983).  Ocular hazards of light sources: review of current knowledge.  Journal of Occupational Medicine, 25(2), 101-103.

Ham, W. T., Jr., Ruffolo, J. J., Jr., Mueller, H. A., & Guerry, D., III.  (1980).  The nature of retinal radiation damage: dependence on wavelength, power level and exposure time; the quantitative dimensions of intense light damage as obtained from animal studies, Section II.  Applied Research, 20, 1005-1111.

Kitchel, E., (2000) The effects of blue light on ocular health. Journal of Visual Impairment and Blindness, 94(6) 399-403.

Quinn, N., (1998) Research  into the effects of video display terminals use and office environmental (fluorescent/neon) lighting. The computer filter.