LET THERE BE LIGHT
Margaret M. Loos
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LIGHT
TECHNOLOGY
Before starting on the advancements in the use of artificial light, let’s examine some of human responses to light. Our eyes can respond well to one to one thousand foot-candles.
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A foot-candle is the international unit of illumination. It is the illuminance of a surface located one foot from a one candle source. A standard candle is said to have an intensity of one candle power (cp). Therefore if a light source emits 120 times the power of one standard candle it would have a cp of 120. A surface one foot away would receive an illuminance of 120 foot-candles. We usually experience only certain ranges of foot-candles at any given time, although in an ordinary room there may be as many as fifty light sources. Our eyes are rapid in adjusting to brightness, and slow in adjusting to dimness. Our eyes prefer varying stimuli, monotony is very bad causing emotional rejection. Excessive contrast is also unacceptable. Color is important for an agreeable environment. Warm illumination is preferred and incandescent light is rated high.
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Certain kinds of lighting such as mercury or sodium lighting can be horrible on the human complexion, making people appear ghoulish. Natural light has many subtle variations. The standard for white light in artificial illumination was decided to be that of the sunny midday in June, strangely enough, in Washington, D.C.
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When we experience low level light (under 30 foot-candles) we prefer our light slightly pink or orange. In the higher ranges we are more comfortable with cooler light, with the bluish cast of skylight at noon.
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These areas have been researched now that we have artificial light always with us, but choices were not always available. The illumination in use by the average person in even the more progressive countries has been with us for less than a century.