686 HANDBOOK OF PHOTOGRAPHY 



tubes, adapted from industrial lighting service, were also used. As a rule the lamps 

 were fixed in position, and the sets were built to conform to the lighting installations, 

 in contrast to the modern practice of adjusting the lighting to the sets. 



At this time arc and mercury-vapor lamps were not only the sole types available 

 in the sizes required, but with the "ordinary" and orthochromatic emulsions of the 

 period they were the most suitable from the standpoint of quality of light. The wedge 

 spectrograms of Fig. 30 show the spectral characteiistics of sunlight and of the five 

 principal electrical illuminants. The peculiarity of gaseous-conductor lamps is that 

 their spectrum consists of a few bright lines, which, especially in the blue and violet, 

 are adapted to exposures on blvie-sensitive emulsions. This is likewise true of sunlight 

 and of carbon arcs, which closelj^ reseinble siuilight in that they contain fairly equal 

 proportions of blue, green, and red. Incandescent lamps, being richer in the red- 

 j'ellow region, did not become a serious factor in motion-picture lighting until after 

 panchromatic-sensitive materials were introduced. The general adoption of incan- 

 descent lighting at this point was greatly accelerated b^^ the requirements of sound, for 

 the arc lamps of 1928 wei-e by no means as quiet as the types available today. 



Spotlighting lamps of the condensing-lens type were first adapted for motion- 

 picture service from the theater stage. The parabolic-mirror spotlight originated in 

 lighthouse and military searchlight service. A spotlight is distinguished from a 

 floodlight by its sharply concentrated beam and directivity, but at the beginning these 

 qualities were used in motion-picture lighting principally to project light into deep sets 

 where the overhead units left shadows. Modeling, the purposeful creation of high 

 lights and shadows by means of spotlights, did not come until years later. 



Light sources vary in intensity as well as in spectral and directional properties. 

 Mercury arcs of the Cooper-Hewitt type, as used in early motion-picture practice, 

 gave a feeble light of the order of 15 foot-candles per sq. in. This degree of brightness 

 was increased in the quartz mercury arc to 500 to 1000 foot-candles per sq. in. The 

 tungsten-filament lamp is capable of 10,000 foot-candles, while the carbon arc yields as 

 much as 100,000 foot-candles per sq. in. in the positive crater. At present there is 

 some speculation relative to the possibilities of water-cooled high-pressure vapor-type 

 lamps, but the commercial sources remain the incandescent lamp and the carbon arc. 



The basic characteristics of a good lamp for motion-picture photographj'^ may be 

 summarized as follows: 



1. Electrical efficiency, i.e., relatively high ratio of emitted light to power input. 



2. Spectral characteristics matching those of the film being exposed. 



3. Accurately adjustable directivity and beam width, without dark centers, hot 

 rings, hot spots, or other nonuniformities, whether in the form of shadows of the lamp 

 mechanism or optical aberrations. 



4. Freedom from "spill light" or random radiation outside the useful beam. 

 Factors 3 and 4 are correlated somewhat with 1, in that electrical and optical 



efficiency are of equal importance in making a sufficient level of illumination available 

 in the places where it is needed. 



In respect to factor 1, even the best modern light sources are only relatively 

 efficient. For example, incandescent lamps are burned at 21 lumens per watt or at 

 33 lumens per watt. (The latter figure entails operating the bulb at a temperature 

 of about 3380°K. bj^ overvolt aging, i.e., burning 90- or 105-volt filaments at 115 volts, 

 the voltage prevailing on motion-picture sets. This necessarily shortens the life of the 

 filament but gives a whiter light at greater efficiency.) At 21 lumens per watt the 

 efficiency of the lamp (light output /electrical input) is about 11 per cent. At 33 

 lumens per watt the efficiency is about 17 per cent. The arc lamp, operating at 45 

 lumens per watt, has an efficiency approximating 23 per cent. Of the total energy 

 radiated by lighting units, only about a fourth in the case of high-temperature incan- 



