Illumination is the density of light falling onto a surface. It is a factor in visibility; and control of illumination may result in improved vision for those with impairments.
To properly control illumination, type of light source, intensity of the light source, distance from the light source, surface the lights fall on, diffuseness, the amount of glare, and environmental changes must all be considered.
The best lighting situations involve properly intense, diffuse light from overhead (or the side of usable vision if reading with only one eye) that causes the least amount of shadows and glare.
Light sources commonly include daylight, incandescent light, and fluorescent light. Daylight varies according to time of day and weather conditions. Incandescent light simulate daylight but in a constant, steady flow. Fluorescent lights may flicker but can be linked and adjusted to eliminate the flicker.
Specially designed lights may be used to maximize residual vision. The BLBS (Better Light Better Sight) lamp is a study lamp constructed according to the requirements of the Illuminating Engineering Society of America and incorporates ideal lighting components.
Tensor-type lamps are intense lights that can be adjusted for maximum illumination with limited glare. Small clip-on lamps are available that attach to the sides of spectacle frames.
The intensity of light is measured in footcandles. One footcandle equals the intensity of light that falls in one square foot of a surface located one foot from one international candle. The intensity of light on an object is controlled by the distance from the light to the object. A lamp two feet from the reading page has four times the light intensity as a lamp four feet away.
The surface on which the light falls affects the visibility. An uneven surface may contribute to uneven diffuseness of light, and a smooth, reflective surface may increase glare.
Diffuseness is the distribution of light. Shadows and spotlights may decrease visibility. Glare results from an excess of light, a concentration of light, or an uneven distribution of light, and may decrease visibility.
Illumination may be controlled or manipulated by making changes in the environment, as well as the light source. Light transmission, reflection control, and contrast enhancement may be used to maximize available illumination. Light transmission is improved through lenses, filters, and absorptive lenses that reduce glare and highlight contrast for those with light sensitivity, or photophobia. Lenses, filters, and absorptive lenses are available in a wide range of degrees of protection from simple filters that reduce glare to photochromic lenses that use gray, green, and amber filters to block all ultraviolet light.
Reflection is controlled by visors, side shields, specially treated lenses, and typoscopes. Visors may be supplied by the brim of a hat or can be filtering visor lens attached to the spectacle frame. Sideshields are filters that attach to the sides of spectacle lenses to control the light on the sides of the eyes.
A typoscope is a slit reading device that isolates one line of type at a time, thus reducing glare from the light reflected from the page. Typoscopes are especially helpful for those with cataracts.
Contrast is enhanced by using highly contrasting colors near one another, as black ink on white paper or fluorescent strips on stair risers. Contrast for reading pale or bluish print may be improved by using pale yellow tinted lenses or by placing a clear yellow or amber sheet over the page.
Lighting requirements vary according to the age of the individual and the cause of the visual impairment. As the eyes age, greater amounts of illumination are required to function properly. Sixty-year-old eyes require twice as much light to complete a task as twenty-year-old eyes.
Eye disorders and conditions may create a need for either reduced or increased light. Those who function best in low light include those with albinism, aniridia (lack of iris), cataracts, and corneal opacities. People who function best in maximum light include those with glaucoma, optic atrophy, surgical aphakia (lack of natural lens), myopia, coloboma, and macular disease.