PHOTOGRAPHY BY ULTRAVIOLET AND INFRARED 603 



dyes belong to the classes known to the organic chemist as the cyanines, the most 

 important for recording the longer wavelengths being the carbocyanines and the 

 di-, tri-, tetra-, and pentacarbocyanines. By proper selection of the sensitizing dyes 

 during the manufacture of the emulsions, it is possible to make plates and films which 

 respond to bands of wavelengths from the visible red to the infrared beyond 13,000 A. 

 Those which sensitize in the longer wavelength regions are mainly of interest for scien- 

 tific purposes, while those which permit photography in the region from 7000 to 9000 A. 

 are most used for general applications. 



There is no fundamental difference between the practice of infrared photography 

 and that in which visible light is used. Any photographer equipped for work with 

 panchromatic plates and films can make infrared photographs with no extra expendi- 

 ture other than that necessary to purchase the proper kinds of plates and films and a 

 filter for use on the camera lens. The light sources commonly found in photographic 

 studios for ordinary practice happen to be the most suited for infrared photography. 

 Some precautions necessary in handling infrared materials differ slightly from those 

 necessary in common practice, but they present no difficulties and will be mentioned 

 in the following text. The value of photography by infrared lies in the fact that the 

 radiations are very often transmitted and reflected by materials to an extent which 

 bears no relation to the way in which they behave toward ^dsible light. Generally 

 they penetrate more freely. 



Sources of Infrared. — All sources which depend on incandescence for the produc- 

 tion of light emit infrared radiation freely. They include sunlight, arcs, electric-fila- 

 ment lamps, oil lamps, gas burners, candles, and other sources which rely on a material 

 being raised to a high temperature. They are not all equally efficient, however, and 

 the discussion here will be limited to those which might be of value. In addition 

 to the incandescent sources, there are some gaseous-discharge lamps which produce 

 spectra containing lines in the infrared, but they are little used in practice. 



Sunlight. — About one-half of the energy in sunlight lies in the infrared, and the 

 sun is therefore a very satisfactory source. It cannot be conveniently used in the 

 studio, but it is the natural source for outdoor photography and permits very impor- 

 tant work to be done in the infrared. There is much variation in the intensity and 

 quality of the sunlight which reaches the surface of the earth, and it is necessary for 

 the photographer to adapt his exposure to meet the changing conditions. The chief 

 causes of the variation are changes in the amount of heat energy radiated from the sun; 

 variations in the distance of the earth from the sun; variations in the amount of water 

 vapor, dust, etc., in the atmosphere of the earth; the zenith distance of the sun; and 

 the altitude of the photographic station above sea level. In a clear atmosphere at 

 sea level at midday in midsummer, about one-third of the energy from the sun is in 

 the part of the infrared which can be photographed. This figure can be considered 

 as a maximum and will be decreased as the factors above mentioned come into play. 

 There is no strict relationship between the intensity of infrared and of visible light from 

 the sun, a fact which makes ordinary exposure meters unreliable for estimating infra- 

 red exposures. The intensity of infrared in sunlight is much higher than that in any 

 other source. 



Incandescent- filament Electric Lamps. — ^The earlier types of electric lamp consisted 

 of filaments of carbon or metals, particularly tungsten, in an evacuated glass bulb. 

 Later lamps have filaments made exclusively of tungsten wire and contain an inert, gas, 

 usually a mixture of 80 per cent of argon and 20 per cent of nitrogen, and the filament 

 is in the form of a coil. The gas-filled coiled-filament lamp is very much more efficient 

 than the older types of vacuum lamp. For general infrared photography it is not 

 necessary to use more of the spectrum than the region from aboiit 7000 to 10,000 A. 

 It is desirable, therefore, to select sources of infrared which have as great as possible a 



