PHOTOGRAPHY BY ULTRAVIOLET AND INFRARED 611 



darker than the hardwoods, and dead trees can also be distmguished by their darker 

 rendering. These facts make infrared photography of interest to the forest survej'or. 

 The sky is rendered as very dark, because blue skylight is relativelj^ devoid of infrared. 

 Clouds appear white, because they reflect sunlight which contains infrared. Shadows 

 are very dark, especially if the sky is clear and the only light in them comes from the 

 blue sky. Water generally reproduces as black. Buildings will be reproduced in 

 tones which depend on the manner in which their materials of construction reflect 

 the infrared, and soil and rock may also appear in tones which are different from their 

 visual appearance. These effects all assist in determining the characteristic appear- 

 ance of infrared landscape photographs. They may be attractive to the pictorial and 

 commercial photographer and maj^ offer advantages or otherwise to the surveyor 

 whose prime interest is in rendering and identifying detail. 



Criminology. — In the field of criminology, infrared photography has found many 

 applications which include the following: detection and (deciphering of erasures and 

 forgeries; deciphering of charred documents, or those _which have become illegible 

 as the result of age or abuse; differentiation between inks, dyes, and pi|;ments which 

 are visually identical; distinguishing between cloths which are visuallj^ identical but 

 dyed with different dyes; detection of stains and irregularities in cloth; examination of 

 oloth, fibers and hair which are dyed too darkly to be easy of study by visible light; 

 study of fingerprints; examination of the contents of sealed envelopes; detection of 

 certain kinds of secret writing; detection and demonstration of blood stains on cloth; 

 determination of carbon monoxide impregnation of victims of gas poisoning; photog- 

 raphy in the dark. All these applications rely on the fact that the reflection and 

 transmission of infrared by materials is frequently very different from tlje behavior to 

 visible light. Straightforward infrared photographic methods are used, due regard 

 being paid to the region of spectral sensitivity and the contrast desired. 



Documentary Photography. — Some of the most elegant uses of infrared photography 

 are to be found in the field of the examination of documents. The most important 

 application is in the deciphering of writing made illegible by charring, deterioration as 

 a result of age or the accumulation of dirt, obliteration by application of ink as by a 

 censor or forger, invisible inks, and deliberate chemical bleaching or mechanical 

 erasure and subsequent overwriting. In these fields, the infrared method forms a 

 valuable adjunct to ultraviolet photography. Inks, pigments, and other materials 

 which may appear identical to the eye are frequently rendered quite differently in an 

 infrared photograph. If an ink transparent to the infrared is applied over one opaque 

 to it, the underlying ink will show up in an infrared photograph. The original writing 

 on charred documents may be revealed, although success will depend on the degree of 

 charring of the paper. Writing which has been mechanically erased may be revealed 

 by virtue of traces of carbon or other pigment left embedded in the paper fibers. 

 Chemically bleached writing is often deciphered if the product resulting from the 

 reaction of the bleach with the ink absorbs infrared radiation more fully than the 

 surrounding paper. Dyes and pigments visually identical can be distinguished if they 

 differ in their transparency to the infrared. 



Medical Infrared Photography. — Infrared photography shows promise of being a 

 valuable means of diagnosis. By the use of infrared-sensitive plates and films it is 

 possible to make pictures showing the superficial veins, some of which are not dis- 

 cernible either visually or in ordinary photographs. Distinct changes from the normal 

 venous pattern have been recorded in connection with several diseases: axillary 

 thrombosis, cirrhosis, and some tumors, for instance, show a marked disturbance of the 

 adjacent venous circulation. On the basis of this, it has been suggested that in 

 surgery for carcinoma of the breast, an infrared photograph made before the operation 

 might reveal very useful information. Changes in the superficial venous pattern 



