However, the most general principle of light-production at the present time is the radiation of bodies by virtue of their temperature.  If a piece of wire be heated by electricity, it will become very hot before it becomes luminous.  At this temperature it is emitting only invisible infra-red energy and has an efficiency of zero as a producer of light.  As it becomes hotter it begins to appear red, but as its temperature is raised it appears orange, until if it could be heated to the temperature of the sun, about 10,000 deg.F., it would appear white.  All this time its luminous efficiency is increasing, because it is radiating not only an increasing percentage of visible radiant energy but an increasing amount of the most effective luminous energy.  But even when it appears white, a large amount of the energy which it radiates is invisible infra-red and ultra-violet, which are ineffective in producing light, so at best the substance at this high temperature is inefficient as a light-producer.

In this branch of the science of light-production substances are sought not only for their high melting-point, but for their ability to radiate selectively as much visible energy as possible and of the most luminous character.  However, at best the present method of utilizing the temperature radiation of hot bodies has limitations.

The luminous efficiencies of light-sources to-day are still very low, but great advances have been made in the past half-century.  There must be some radical departures if the efficiency of light-production is to reach a much higher figure.  A good deal has been said of the firefly and of phosphorescence.  These light-sources appear to emit only visible energy and, therefore, are efficient as radiators of luminous radiant energy.  But much remains to be unearthed concerning them before they will be generally applicable to lighting.  If ultra-violet radiation is allowed to impinge upon a phosphorescent material, it will glow with a considerable brightness but will be cool to the touch.  A substance of the same brightness by virtue of its temperature would be hot; hence phosphorescence is said to be “cold” light.

An acquaintance with certain terms is necessary if the reader is to understand certain parts of the text.  The early candle gradually became a standard, and uniform candles are still satisfactory standards where high accuracy is not required.  Their luminous intensity and illuminating value became units just as the foot was arbitrarily adopted as a unit of length.  The intensity of other light-sources was represented in terms of the number of candles or fraction of a candle which gave the same amount of light.  But the luminous intensity of the candle was taken only in the horizontal direction.  In the same manner the luminous intensities of light-sources until a short time ago were expressed in candles as measured in a certain direction.  Incandescent lamps were rated in terms of mean horizontal candles, which would be satisfactory if the luminous intensity were the same in all directions, but it is not.  Therefore, the candle-power in one direction does not give a measure of the total light-output.