286 HANDBOOK OF PHOTOGRAPHY 



region of 2700 to 3000°K. In spectral distribution, the carbon arc resembles the 

 photoflood lamp. Since the light is a closer approach to white than that produced by 

 incandescent lamps, no filters are required unless they are also required with a source 

 of sunlight. The photographic effects of carbon arcs are similar to those of sunlight as 

 shown in Fig. 17. 



In order to modify their spectral characteristics, the carbons are frequently drilled 

 lengthwise and are subsequently filled with a core of inorganic salts. Thus the white- 

 flame arc used for photographic purposes has a core of rare-earth fluorides. Because 

 this core material modifies the spectral radiation of the arc and increases the intensity 

 of the line and band spectra as compared with the continuous spectra due to incan- 

 descence, it is difficult to make any definite statements as to the photographic effec- 

 tiveness of various types of carbon arcs. Spectra produced by specially processed 

 carbon electrodes may usually be obtained from the manufacturer. ^ 



Enclosed Metallic Arc. — -Enclosed arc lamps with metallic electrodes and known as 

 Pointolite or Tungsarc are commercially available and are suitable for photographic 

 , purposes. The lamp consists of tungsten electrodes in an 

 ° "^ " evacuated bulb containing a pool of metallic mercury 



vapor. A filament bridges the tungsten electrodes, and, 

 when the voltage of about 40 volts is applied to the tube, 

 the filament becomes incandescent, thereby vaporizing 

 the mercurj^ so that within a few seconds an arc forms 

 between the two tungsten electrodes and the current rises 

 to about 30 amp. The characteristic of the transformer 

 jjj^g supplying the lamp must be such that as the arc current 



j?jQ_ Ig Light output ^^ established the voltage across the lamp will fall to 



of typical photoflash lamp about 10 volts. This voltage is sufficiently low as to 

 as a function of time. The practically extinguish the filament. The spectrum pro- 

 total duration of the flash ^^^^^ -^ continuous and is largely that of the incandes- 

 is of the order of about >2o . ^ , ■, ■, i- 



gg(._ cence of the tungsten electrodes, but a discontmuous 



spectrum of mercury is also present. 



Transient Light Sources. — Under the heading of transient light sources are included 

 those devices which are intended to provide an intense flash of light for short duration 

 for making photographic snapshots. Usually these transient light sources can be 

 used only once, but experimental work is in progress to provide a "flash-flood" lamp 

 which may be flashed as often as desired. The most common of these transient light 

 sources for general photography are photoflash lamps and flashlight powders, although 

 for special purposes, especiallj^ scientific photography, luminous gaseous-discharge 

 lamps and spark discharges are also of interest. 



Photoflash Lamps. — A very convenient and effective source of luminous intensity 

 for making photographs with an exposure of about }4 5 sec. is the photoflash lamp, 

 which is available in several sizes. These lamps consist of a glass bulb containing a 

 filament, metallic aluminum, and oxj'gen. The metallic aluminum maj' be in the 

 form of thin (0.00004 cm.) foil or fine ribbons or wires, crumpled inside the glass envel- 

 ope. The lamps have a standard Edison screw base, and operate at a voltage from 3 

 to 115 volts. Thus the lamps may be used in the ordinary house lighting circuit or 

 may be flashed bj- means of several dr_v cells. When the current is turned on, the 

 filament becomes incandescent and ignites the aluminum in the oxygen, producing a 

 quick, brilliant flash lasting from about 0.03 to 0.06 sec. The characteristics of the 

 luminous output as a function of time are shown in Fig. 18. The temperature of the 

 flash is approximately 3o00°K., although at the peak this temperature may be 



' The spectral characteristics of various carbon arcs have been given in the National Bureau of 

 Standards Scientific Paper 539. 



