570 ENERGY FOR MOVING PICTURES [Cn. XIII 



L/W The ratio of the luminous energy (L) and the total energy getting 

 through the water-cell (W) (Water-cell 8 cm. thick). 



W/R The ratio of the energy getting through the water-cell (W) and the 

 total energy (R) radiated by the light source. 



L/R Ratio of light energy (L) and the total energy (R) radiated by the 

 source. 



R Total energy radiated by the source. 



L The light energy radiated by the source (fig. 307). 



W. P. C. Number of watts required for each candle-power with the different 

 sources. 



C. P. W. Number of candle-power given by each watt with the different 

 sources. 



In the right-hand column are given the meter candles or lumens for each watt 

 of energy in the luminous part of the spectrum with the different sources. 



CALCULATION OF THE ENERGY REQUIRED FOR THE PROJECTION OP 

 MOVING PICTURES 



779. It is interesting to calculate, from the data on radiant 

 efficiency, how much energy is required to project a moving picture. 

 This has an important bearing on the fire risk with such projection. 

 Suppose, for example, the picture is to be 3.7 x 5 meters in size 

 (12 x 16.5 ft.), a suitable size for a 30 meter (90 ft.) hall. Its area 

 will be 18.5 square meters (298 sq. ft.). A suitable average illumi- 

 nation of the screen would be 100 meter candles or about 10 foot 

 candles. As the revolving shutter removes half the light, the 

 actual momentary illumination of the screen must be 200 meter 

 candles or 200 lumens per square meter. 



Basing the calculations on this, it is seen that 18.5 x 200 or 3700 

 lumens will be required. When using the right-angle carbon arc 

 with direct current the light represented by one watt when radiated 

 in the visible part of the spectrum is 377 lumens ( 778). In order 

 to get 3700 lumens it requires 3700/377 =9.8 watts of light energy. 

 This energy must get through the aperture plate which is 2.5 cm. x 

 1.75 cm. and which has an area of 4.2 square centimeters (i in. x ^ 

 in., area ^ square inch) hence the light energy per square centi- 

 meter of film area is 9. 8 74. 2 = 2.34 watts per square centimeter 

 ( 779 a )- When, however, the entire radiation from the arc is 

 used, only 10% of which is light, the energy is 10 times as great, and 

 even when a water-cell is used where 43% of the energy is light, the 

 energy is 2.3 times as great. These results are shown in tabular 



