Lighting recommendations are typically written for the “average eye” i.e., the eyes of young adults who are in good health. Many eyes, however, are not average and there are populations of people who because of age, eye diseases, injury or other conditions that limit their vision have special lighting requirements. The largest of these groups is the elderly, a significant number of whom are partially sighted.
As the number of elderly people continues to grow attention has become focused on understanding their visual problems and developing specialized lighting recommendations.
Lighting appropriate for the average eye is not likely to be appropriate for the 70 or even 60-year old eye. The absorption of light during passage through the eye increases 3-fold from age 20 to age 70 (IESNA, 2000). In addition, as the eye ages, it becomes less responsive. The scanning function slows down and adaptation to different light levels occurs more slowly. The lens becomes increasingly rigid and loses some ability to adjust focus, especially in the near field. The lens typically becomes more yellowed and more light is scattered within the eye, causing reduced visual acuity, contrast sensitivityand colour perception. The result is that, beyond the age of 40, people typically start needing more light to see small details, and often need glasses to help them focus in the near field. Because of the increased scattering of light in the eye, glare sources also cause a greater loss in visual function and more discomfort.
In addition to normal aging, older people are more likely to be subject to diseases of the eye that lead to partial sight. Macular degeneration has become increasingly common in recent decades. It involves a loss of function of the fovea and greater reliance on peripheral vision. Cataracts that make the lens more opaque are also common, as are diabetic retinopathy and glaucoma. The effects of some of these conditions are shown in the figure below. While all of this may sound depressing, problems such as cataracts can be corrected with surgery and a growing number of treatments for retinal problems and diseases such as glaucoma have improved or maintained functional vision for countless numbers of people.