446 ENERGY MAY BE PROPAGATED BY VIBRATIONS 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 Lamps. 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 is. STUDY QUESTIONS 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 awa> 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.
ILLUMINATION 447 12. Why are table lamps and floor lamps used more widely than flush-type lights, cove-lighting, and lens-lighting? 13. If one 100-watt light bulb provides adequate light intensity for reading in a floor lamp placed so that the light bulb is three feet from a book, how many 100-watt light bulbs would be required in a flush-type ceiling fixture, eight feet from the book, provided that the same types of reflectors were used in each case and no difi^using globes or plates were placed on the lamps? 14. Why should one select different types of glass blocks for diff^erent exposures? 15. Why are electric-light bulbs frosted? 16. What changes would inexpensive fluorescent lamps or zeon lights probably make possible in the illumination of houses? 17. Why is indirect lighting more costly than direct lighting? 18. Compare direct, indirect, and semi-indirect lighting with outdoor natural lighting. 19. Would you prefer to read out-of-doors in direct sunlight or diff^used sunlight? Why? 20. Why is the glare out-of-doors worse on a cloudy day than on a clear day? What application of this fact can you make to indoor illumination? 21. Why is the light transmitted through a window with a north exposure preferred for drawing and artwork? What type of artificial illumination most closely resembles the light from a north window?