PHOTOGRAPHY BY ULTRAVIOLET AND INFRARED 613 



to pass freely. These photographs are primarily of interest for amusement, although 

 the attempt has been made to use them in the detection of criminals at work and in 

 the observation of spiritualist seances. Of more interest are the photographs of hot 

 bodies made in total darkness by the infrared radiation which they emit. By using 

 infrared plates it is possible to obtain an idea of the distribution of temperature over 

 heated materials, such as electric heaters, hot metal ingots, radio tubes, and cylinder 

 heads and exhaust manifolds of internal-combustion engines. 



Portraiture. — Infrared portraits can readily be made in the manner described in 

 the section on Methods of Infrared Photography. They are, however, of little interest 

 except for amusement and medical purposes. They differ entirely from those made 

 by visible light. The flesh has a white translucent appearance, the lips are light, the 

 eyes dark, all lines in the face are strongly emphasized, and the beard appears as if it 



Fig. 5B. — Infrared photograph of the same subject made on infrared plates using heat of 

 the flatirons as the only source of "illumination." 



were a stubbly growth, even on a clean-shaven face. The skin of negroes is repro- 

 duced light in tone. 



Scientific and Technical Applications. — Infrared photography has proved of impor- 

 tance in many fields of investigation, and its value is increasing as it becomes more 

 applied. It is not possible to mention more than a few special uses in this chapter, 

 but the imaginative photographer should have no difficulty in widening its field. 

 Useful references will be found in the attached bibliography. In plant pathology 

 the infrared has provided a valuable means of studying and diagnosing plant diseases 

 in which there is change in the pigment or cellular material. Different kinds of wood 

 show marked variation in their transparency to the infrared, and photographs have 

 shown that the infrared may be of much value in coal petrology and other fields of 

 paleobotany. In the textile industry infrared photography has been successfully 

 applied to the detection of irregularities in the dyeing and weaving of cloth and 

 damage to the fibers, particularly where the material is dyed in such a manner as to 

 render visual observation difficult. The graphic-arts industry has used infrared- 

 sensitive plates to differentiate between light and dark blues in multicolor printing 

 and in preparing the negative of the black printing plate from originals prepared in 

 specially selected colors. In the field of technology other applications are in the study 



