Man's physical universe

xanabras

UNIT VI

SECTION 6

ILLUMINATION IS THE CHIEF APPLICATION OF

ARTIFICIAL LIGHT

Introduction.

Illumination deals with the problems of what kind of light sources

to use for a given purpose, how much light is required, how the amount

of light present can be measured, and where light

sources should be

placed.

Tremendous strides have been made in the science of illumination,

and yet few homes are properly illuminated. Proper illumination is

desirable because it saves eyestrain. In this age, when the eyes are

used so much in reading and close work, it is very important that

everything possible be done to relieve the eyes from unnecessary strain.

Illuminating Power Is Now Measured in Terms of Lumens.

The value of any light source for illumination depends upon how

much light it gives out. The unit of intensity is called the candle,

which is equal to the intensity of a standard candle burning under

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Fig. 215.

Light intensity is inversely proportional to the square of the distance

from the source.

specified conditions. The international candle is now defined to be the

light with an illuminating power equal to one tenth of that of a standard

gas light produced by a Harcourt pentane lamp. Obviously the

intensity of the light from a burning candle depends upon the distance

one is from it, so that one must incorporate the idea of distance into

the measurement of illuminating power. Afoot-candle is the most con-

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