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HANDBOOK OF PHOTOGRAPHY 



Photoflash lamps cannot, of course, be tested on line voltage or on batteries which 

 pass sufficient current to heat the filament to incandescence. By passing very small 

 current through, the lamp, insufficient to produce incandescence, a sensitive micro- 

 ammeter or milliammeter in series with the filament will show a deflection if the fila- 

 ment circuit is continuous and the bulb is suitable for use. Another method which 

 has been used is to use a battery circuit with a high resistance relay in series with the 

 lamp filament. The relay contacts can be made to operate a lamp in another circuit 

 if the filament of the photoflash lamp is intact. 



When using photoflash lamps, the camera shutter may be opened temporarily 

 while the lamps are flashed, after which the shutter is closed. A much more con- 

 venient arrangement, and one which may be used where the general level of illumina- 



Table VIII. — Characteristics of General Electric Photoflash Lamps^ 



1 FoRSYTHE, W. F. and M. A. Easley, Characteristics of General Electric Photoflash Lamps, J. Opti- 

 cal Soc. Am., 21, 685 (1934). 



tion might fog the photographic plate, is to synchronize the action of the shutter with 

 the switch or flashing mechanism of the photoflash lamp. Several such synchronizing 

 devices are available commercially. In using such synchronizing devices, it is impor- 

 tant that the peak of illumination of the photoflash lamp occur when the shutter is 

 completely open. Because photoflash lamps require about 0.020 to 0.040 sec. to 

 reach their peak of illumination after current is turned on, a shutter speed of about 

 Hs sec. is usually used. The synchronizer is usually designed so that the shutter 

 action may be adjusted to the characteristics of the photoflash lamps for optimum 

 performance. It should be noted that the time of travel of focal-plane shutters 

 across the film area is slow compared with the duration of the flash. For this reason 

 photoflash lamps operate most satisfactorily with between-the-lens shutters. 



Flashlight Powders. — Rapid oxidizartion of metals such as magnesium and aluminum 

 was formerly used to provide an intense source of light of short duration, but these 

 flashlight powders have been almost entirely replaced by photoflash lamps. Flash- 

 light powders usually consist of a mixture of magnesium powder and an oxidizing 

 material like potassium chlorate. Since these mixtures oxidize rapidly with almost 

 explosive violence, they must be handled with care. They may be ignited by means 

 of an open flame, a fuse, or an electrically heated wire. 



Special Light Sources. — Sometimes special or unusual light sources are required for 

 some particular purpose. These may frequently be of considerable importance for 

 certain applications of photography but are not of sufficient general interest to warrant 

 extensive description. For further details on such light sources, reference should be 

 made to the Uterature.^ 



1 FoRSYTHE, W. E. editor, "Measurement of Radiant Energy," McGraw (1937), Chap. II, gives a 

 good summary of the various light sources which are useful for scientific purposes, as well as a good 

 bibliographical reference to the technical literature. 



