Appendix 2
2. Appendix II: Practical Factors In Illumination
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The physical aspects of illumination are of practical interest to the physician in evaluating the environmental setting of patients of all ages. Children in school, younger and older adults in the workplace, and the older person experience demands on their vision with little knowledge of correct illumination, often working under less than ideal circumstances.
Proper illumination increases the speed and efficiency of reading. It also encourages correct posture. Reflected glare, for example, may cause discomfort as the reader shifts position to avoid the reflection. With increasing age also comes increasing need for greater illumination to maintain reading proficiency. Corrective lenses are not the complete answer to seeing and reading at an acceptable level. Increasing the luminous intensity may be more effective than an increase in the prescription. Many eye conditions-macular degeneration, glaucoma, and diabetic retinopathy-are associated with reduced contrast sensitivity as well as acuity and may require not only magnification but increased illumination as well. Conditions that affect the optical media (corneal dystrophies, cataracts) may be worsened by increased lighting-indeed, in some cases the light source may become a glare source rather than a help.
The commonest sources of light are daylight, incandescent light (standard frosted, reflector, halogen), and fluorescent light. Daylight is composed of the entire spectrum of color; it is "white light." Incandescent light is formed by heating a tungsten filament and is the closest to the sun's spectrum. However, incandescent bulbs produce more heat than fluorescent tubes, which may become a safety issue particularly with halogen lamps. With high-efficacy fluorescent tubes coated with halophosphors (cool white), there are peaks particularly in the blue-violet range that may increase scatter and therefore glare unless the lamp is designed to reduce the blue and fill in with warmer red and yellow (warm white, yellow). The new compact bent or folded fluorescent lamps are coated with rare earth phosphors with greater efficacy and improved color rendering. A 15-watt compact lamp may produce as much as 75 watts of light energy.
DEFINITIONS RELATED TO ILLUMINATION
- Color rendering: The effect of the light from the light source on the color of objects.
- Color temperature: Indicates the hue of light (how white it looks). Incandescent lamps have low color temperature; sunlight has a high temperature.
- Luminous efficacy: The luminous efficacy of a lamp is expressed in lumens per watt (lm/W). The fluorescent lamp is more efficient than the incandescent lamp.
- Luminous intensity: The amount of light falling on a surface expressed in lux. For example, an 80 lux source for reading would be inefficient, whereas 500 lux would be efficient for reading and for desk or bench work.
- Luminance: The light reflected from an area expressed in candelas per square meter (cd/m2).
CHOOSING LAMPS
The basic requirement of any light source is that it be appropriate for the task. The wattage, color temperature, and luminous intensity are important considerations. The style of the luminaire (fixture), its placement, the shade, and the type of lamp (bulb) all have to be considered. Since intensity is inversely proportionate to the square of the distance from the light source, a reading luminaire should be flexible (gooseneck, spring arm) so that the source may be placed at the optimal distance from the task. The shade should be opaque for minimal lateral dispersion of light and maximum concentration on the area of the task. An incandescent flood bulb of 50 or 75 watts provides double the illumination of a standard incandescent bulb for the wattage; for example, a 50-watt reflector bulb provides 100 watts of energy. This lamp is therefore cooler than the nonreflector type.
There is a relationship between aging and light requirement. As long as accommodation is normal-up to the mid 40s-light intensity is not crucial to reading efficiency. After the onset of presbyopia, however, reading glasses must be augmented with stronger light. A rule of thumb is that a postpresbyope needs 1% more light each year.
CONTRAST
Contrast is another important factor in efficient reading. Highest contrast is achieved by black letters on a white background. Color contrast is also important in visibility of print. There are standards of color contrast that should be observed by all printers of pamphlets, brochures, and advertisements. One of the guidelines is to exaggerate differences between foreground and background colors. Another is to avoid using contrasting hues from adjacent colors of the hue spectrum-eg, a bad choice is orange against yellow rather than blue against yellow. As the normal eye ages, color discrimination is reduced, particularly if the lens has become yellow or amber, blocking some of the blue light rays: yellow may appear white or beige; blue may appear dark, almost black; and pale blue may appear aqua.
SOME FACTORS IN LIGHTING SELECTION
Younger children and young people in general do not specifically need bright light to read or perform tasks. The comfort factor, however, indicates that as a rule, incandescent lamps are best. Fluorescent light may cause glare unless it is modified with a warm white lamp or a tube with a higher proportion of red and yellow. An incandescent desk light can reduce the discomfort of fluorescent ceiling fixtures. In general, accent lighting should not be a glare source, and indirect lighting is best for ambient light. Halls and stairs should be well-lighted. And daylight is the best light, particularly when used in conjunction with artificial light sources.
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10.1036/1535-8860.appendix2