604 HANDBOOK OF PHOTOGRAPHY 



proportion of their energy in this region. It so happens that the types of incandescent 

 tungsten-filament electric lamps most used for illumination and for photography by 

 visible light possess this characteristic. The wavelength of maximum energy and the 

 actual amount of energy in the infrared varies with the kind of lamp. For lamps 

 used for general lighting, the maximum ranges in position from 9500 to 10,500 A. In 

 the case of lamps for special purposes it may be at still shorter wavelengths. For 

 instance it is at 8000 to 8500 A. for the photoflood types of lamp, and at 9000 A. for 

 the 500-watt projection lamps. In general, the higher the wattage of the lamp, the 

 greater is the energy at the maximum. It will be clear that any of the higher wattage 

 lamps and those of the overvolted photoflood types will be suitable for infrared photog- 

 raphy. Those in common use are the studio types of lamp, the 500-watt projector 

 lamps, and photofloods. Since carbon-filament lamps and certain types of electric 

 radiant heaters have been much recommended for infrared therapeutical treatment, it 

 has often been supposed that they would be very suitable as sources for infrared 

 photography. They are not nearly so useful as the common incandescent tungsten- 

 filament lamps, however, because they emit very inefficiently in the spectral region in 

 which photographic plates can be sensitized. 



Carbon-arc Lamps. — The spectrum of the radiation from the carbon arcs consists 

 of a continuous background extending from the visible far into the infrared, and on 

 this are superposed lines and bands caused by the arc vapors. By modifying the 

 composition of the carbons, it is possible to modify the characteristics of the radiation 

 so that the spectrum consists mainly of tines. In the plain carbon arcs the chief source 

 of the radiation is the electrically positive carbon, while, in the case of those arcs using 

 carbons which are cored or impregnated with salts to give the flame arcs, the chief 

 source is the luminous flame between the electrodes. The carbon arcs provide the 

 highest available artificial temperatures, and they are very suitable sources for the 

 infrared. As was mentioned in the section on Ultraviolet Photography, the inclusion 

 of certain metals in the cores produces arcs having a very strong emission in the ultra- 

 violet. The carbons may be similarly modified, by incorporation of other metal salts, 

 to give high emission in the red and infrared. All carbon arcs may be used for infrared 

 photography, the most suitable being those with carbons of the following types: 

 National Carbon Co. Sunshine and Motion Picture Studio carbons; White Flame arc 

 carbons; Low Intensity Projector arc carbons; Suprex positive carbons; High Intensity 

 Projector carbons; High Intensity Sun Arc carbons. The Pointolite and Tungsarc 

 lamps are convenient sources of infrared of moderate efficiency. 



Gaseous-discharge Lamps. — The mercury- vapor lamps, described under Ultraviolet 

 Photography, are not very useful sources for general infrared photography. The 

 greatest emissions in the photographic infrared are at the 10,140 and 11,289 A. lines. 

 This is at too long a wavelength for general purposes, although, for cases where it might 

 be desired to work at these wavelengths, the mercury arcs would be very suitable. 

 None of the other gaseous-discharge lamps are of importance for practical infrared 

 photography. 



Photoflash Lamps. — The photofiash lamp emits its maximum energy at about 

 9000 A. ; it is a very good source of infrared radiation and is very convenient. 



Special applications of infrared photography will be surveyed in a later section 

 of this chapter. At the risk of anticipating this somewhat, we shall include here a 

 table of the sources most useful for a variety of purposes of infrared photography. 



Filters. — Infrared plates and films are sensitive to the ultraviolet, to the violet 

 and blue, to part of the green, and to a particular region in the infrared. To confine 

 the exposures to the infrared, it is therefore necessary to use a filter which does not 

 transmit the shorter wavelengths to which the material responds. The filters are 

 made of dyed sheets of gelatin used alone or cemented in glass, or of plates of glass 



