ARTIFICIAL LIGHTING
Artificial lighting does not come even close to the broad spectrum of sunlight. Nor its brightness.
Old-fashioned incandescent lamps, the kind that create light by heating a filament until it glows, give the broadest spectral range. Incandescent lamps typically glow in wavelengths from roughly 250 to beyond nine hundred nanometers, but most of their brightness falls in the yellow-red end of the spectrum, from 650 nanometers up.
Fluorescent lamps emit light in much narrower wavelengths, since they glow from the light given off by specific atomic elements, usually either mercury or sodium. Some of the fluorescents used in homes and offices are mercury lamps, which give off a bluish light that can make skin look ghastly and all colors seem eerily strange.
When I was a magazine editor, my Manhattan office was illuminated with mercury-vapor fluorescents. Artists who worked as illustrators for the magazine constantly complained that the office lighting distorted the colors of their paintings. Sure enough, if we went into an area lit by old-fashioned incandescents or, better yet, went out into the sunlight, the colors looked much better.
Today, broad-spectrum fluorescent lamps produce a light that is much closer to natural sunlight, making artists (and editors) much happier.
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Fluorescent lights flicker. Electrical current stimulates the gas atoms inside them glow; the current switches back and forth sixty times per second. This is too fast to bother most people. But when your field of vision places a fluorescent lamp "in the corner of the eye," where the very sensitive rods are looking at it, you probably notice the flicker. Stare straight at the lamp, so that the cone-rich foveae are pointed at it, and the flicker disappears. Look slightly away again, and that annoying flicker can again be seen.
The glass envelopes around each kind of lamp cut off virtually all the ultraviolet, but you can feel the infrared pouring out of an incandescent lamp. After all, the gadget only emits light because the filament inside it is heated by electricity until it glows. Fluorescents are much cooler; the electrical current flowing in a fluorescent lamp is exciting individual atoms of mercury or sodium vapor to emit light without heat.
Compared with the sixty thousand to 114,000 lux brightness of sunlight, artificial lighting is quite dim. Fifty lux is considered bright enough for areas in business offices in which computer display terminals are being used. To read penciled handwriting, lighting engineers say two thousand lux is sufficient. Most business offices are lit at a brightness of somewhere between two hundred to one thousand lux - the brightness typical of twilight outdoors!
This may explain a phenomenon discovered by researchers at the City College of the City University of New York. In 1987, a research team led by Josh Wallman came to the conclusion that reading can lead to myopia - nearsightness. Studies done on Eskimos who recently were subjected to compulsory education, and experiments performed with chicks, led Wallman's team to conclude that the task of reading stimulates the foveal region of the retina but not the broader area surrounding the fovea, and thus results in myopia.
Considering the light levels under which we do most of our reading, however, an alternative explanation might be that people become myopic because they are attempting to read in conditions that are too dim.
LIGHT FROM "THE TUBE"
More and more people are watching television and computer screens every day. The cathode-ray tubes that form these screens emit not only visible light, but also a good deal of ultraviolet and even "soft" (i.e., relatively low energy) X-rays. Distance lends some safety. The X-rays and most of the UV are absorbed by a couple of feet of air. Any kind of eyeglasses, even those with nothing but windowpane glass in them, will also block out most of the UV. Although low-emission screens are now generally available, opticians still recommend lightly tinted eyeglasses for persons who work all day at computer terminals.
Indoor lighting is very different from the natural sunlight that was humankind's only illumination, except for fire, until little more than a century ago. Yet even though we may spend most of our time indoors under artificial lighting, the natural rhythms of day and night, season following season, have profound effects upon all life on Earth - including humans. These effects come about through a sort of "third eye" in the human visual system, an optical tract that "sees" without vision.