Man's physical universe



newer types of floor and table lamps combine direct and indirect

lighting, but they still have the disadvantage that the light sources

are in direct line of vision for general illumination and that they

seldom provide adequate general illumination. One of the newer

methods of providing the intensity of light required for special operations

and still keeping the light source concealed is to place lights

in the walls or ceiling behind small lenses which will focus the Hght

where it is wanted.

High-wattage Incandescent Lamps Are the Most Efficient Incandescent


An important fact that is unknown to many people is that more

light is obtained from one 100-watt incandescent lamp than is obtained

from two 50-watt incandescent lamps. For example, a 40-watt lamp

will give about 25 candle power, whereas a 1000-watt lamp may give

as much as 1500 candle power. Twenty-five 40-watt lamps would

cost as much to operate as one 1000-watt lamp, but they would produce

only 625 candle power.

It is important that lamps designed for a given voltage be used with

that voltage. A 110- volt lamp used with a 130-volt circuit has a very

short life, although greater illumination is obtained as long as it lasts

because a higher temperature is attained. On the other hand, the same

lamp used with a 100-volt circuit would last a long time, but it would

give less illumination for a given expenditure of money. The maximum

illumination per amount of money spent for both power and lamps is

obtained when the lamps are properly matched w^ith the voltage of the

circuit. This is a problem worth investigating because the voltage in a

circuit may not be what you think it



1. What is meant by the candle power of a lamp?

2. Define foot-candle.

3. How is light intensity measured and expressed?

4. How and why does light intensity fall off with an increase in distance?

5. What is the intensity of illumination due to a 100-candle-power lamp 10 feet


6. How is the candle power of a lamp measured?

7. What is the principle of the type of photographic exposure meter described

in this Section?

8. What intensity of light is considered desirable for studying?

9. What is the value of diffusing light for illumination?

10. How is light diffused for illumination?

11. Give a general rule concerning the position of light sources.

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