Man's physical universe

xanabras

LIGHT MAY BE PRODUCED BY LUMINESCENCE 405

glow. A capacitor is often added to bring up the power factor, and a

choke coil called the ballast must always be present to keep the current

from increasing to a point where the lamp would melt. A complete line

of high-power-factor ballasts is available, and, when such ballasts are

used, no additional capacitor is required.

Common fluorescent lamps have an average life of 2500 hours ;

most

filament lamps are rated at 750 to 1000 hours.

The 30-watt fluorescent lamp produces from 30 to 75 lumens of light

per watt, depending upon the color, while a standard 100-watt incandescent

lamp produces only 16 lumens per watt. In general, fluorescent

lamps are from 200 to 300 per cent more efficient in light output,

INSIDE OF TUBG COATED

WITH PLUOOCSCENT MATERIAL

SPACE PILLCO WITH A

_ RARE OAS AND

\ MERCUQY VAPOR

C.^ MCRCUBY

Fig. 177. The construction of the fluorescent lamp. (Courtesy of the General

Electric Company.)

color for color, than comparable incandescent sources. Incandescent

lights waste in the form of heat most of the energy furnished. Neon

lights are more efficient than incandescent lights because very little

energy is lost in the form of heat, but they are still much less efficient

than zeon or fluorescent lamps because they do not utilize the ultraviolet

light that is produced.

A great deal of research has been carried out on materials suitable

for producing fluorescence in fluorescent tubes. Calcium tungstate

has been selected to produce a blue color; magnesium tungstate, a bluewhite;

zinc silicate, green; zinc beryllium silicate, yellow-white; cadmium

silicate, yellow-pink; and cadmium borate, pink. These materials

are carefully purified and treated by heating them in electric

furnaces.

Pure zinc sulfide is not fluorescent; but a trace of radium, copper,

lead, or silver will make it fluorescent. The mechanism of the action

of these impurities is not understood.

The addition of a small amount of manganese is generally employed

to activate fluorescent powders.

Mixtures of fluoescent pigments are used in the daylight fluorescent

lamps that come closer to the color of natural daylight than any other

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